Principles of Insect Morphology 9781501717918

Table of contents :
CONTENTS
FOREWORD TO THE 1993 PRINTING
PREFACE
CHAPTER I. INTRODUCTION
CHAPTER II. GENERAL ORGANIZATION AND DEVELOPMENT
CHAPTER III. THE BODY WALL AND ITS DERIVATIVES
CHAPTER IV. BODY REGIONS, SCLERITES, AND SEGMENTATION
CHAPTER V. THE SEGMENTAL APPENDAGES OF ARTHROPODS
CHAPTER VI. THE HEAD
CHAPTER VII. THE HEAD APPENDAGES
CHAPTER VIII. THE THORAX
CHAPTER IX. THE THORACIC LEGS
CHAPTER X. THE WINGS
CHAPTER XI. THE ABDOMEN
CHAPTER XII. THE ORGANS OF INGESTION
CHAPTER XIII. THE ALIMENTARY CANAL
CHAPTER XIV. THE ORGANS OF DISTRIBUTION, CONSERVATION, AND ELIMINATION
CHAPTER XV. THE RESPIRATORY SYSTEM
CHAPTER XVI. THE NERVOUS SYSTEM
CHAPTER XVII. THE SENSE ORGANS
CHAPTER XVIII. THE INTERNAL ORGANS OF REPRODUCTION
CHAPTER XIX. THE ORGANS OF COPULATION AND OVIPOSITION
REFERENCES
INDEX

Citation preview

PRINCIPLES O F INSEC T MORPHOLOG Y

This page intentionally left blank

PRINCIPLES O F

INSECT MOEPHOLOGY BY

R. E. SNODGRASS With a New Foreword by George C. Eickwort

Cornell University Press ITHACA AN D LONDO N

Copyright © by Ellen Burden and Rut h Roach Foreword copyright © 199 3 by Cornell University All rights reserved. Except for brief quotations in a review, this book, or parts thereof, must not be reproduced in any form without permission in writing from th e publisher . For information, address Cornel l University Press, Sag e House, 512 East State Street, Ithaca , New York 14850 . First published 1935 by McGraw-Hill Book Company, Inc. First published 1993 by Cornell University Press Library of Congress Cataloging-in-Publication Data Snodgrass, R. E. (Robert E.), 1875-1962. Principles of insect morphology / by R. E. Snodgrass ; with a new foreword by George C. Eickwort. p. cm . Originally published: New York : McGraw-Hill Boo k Co., 1935, i n series: McGraw-Hill publication s i n the zoologica l sciences . Includes bibliographica l reference s (p . ) and index . ISBN 0-8014-2883-1. — ISBN 0-8014-8125- 2 1. Insects—Morphology. 2 . Insects—Anatomy. I . Title. II . Series : McGraw-Hill publication s i n the zoologica l sciences . QL494.S65 199 3 595.7'04—dc20 92-5471 0

Cornell Universit y Pres s strive s t o utiliz e environmentally responsible suppliers and materials to the fullest extent possible in the publishing of its books . Suc h material s includ e vegetable-based, low-VOC ink s an d acid-free paper s tha t ar e als o eithe r recycled , totally chlorine-free , o r partly composed of nonwood fibers . Paperback printing 1

09 8

7

6 5

4

CONTENTS FOREWORD TO 1993 EDITION B Y GEORGE C . EICKWOR T PREFACE xii INTRODUCTION 1

PAGE

ix i

CHAPTER I

CHAPTER I I GENERAL ORGANIZATIO N AN D DEVELOPMENT 1 Glossary o f Embryological Terms. 4

4 4

CHAPTER II I THE BOD Y WALL AN D ITS DERIVATIVE S 4 1. Th e Body Wall. 4 2. Externa l Processes of the Bod y Wall 5 3. Sens e Organs 5 4. Ectoderma l Glands , Corpor a Allata, Oenocytes. 6 5. Muscl e Attachments on the Bod y Wall 6 6. Moultin g . 6 Glossary of Terms Applied to th e Bod y Wall 6

8 8 5 9 0 2 4 8

CHAPTER I V Body Regions , Sclerites, an d Segmentatio n 7 Glossary o f General Terms Applie d to th e Bod y Segments an d th e Skeleta l Plates 8

1

CHAPTER V THE SEGMENTA L APPENDAGES OF ARTHROPODS 8 Glossary of Terms Applie d to th e Appendage s 9

3 8

CHAPTER V I THE HEAD . . 10 1. Genera l Morpholog y of the Arthropo d Head . 2. Structur e of the Definitiv e Insect Hea d 10 3. Specia l Modification s i n the Structure of the Hea d Glossary o f Terms Applie d to th e Hea d 12 CHAPTER VI I THE HEA D APPENDAGE S 13 1. Preantenna l Appendage s 13 2. Th e Antenna e 13 3. Th e Postantenna l Appendage s 13 4. Th e Mandibles. 5. Th e Superlingua e 13 6. Th e Maxilla e 14 7. TheLabiu m 14 8. Gland s of the Hea d Appendage s 15 Glossary of Terms Applied to th e Hea d Appendage s 15 v

0

0 10 0 4 ...118 7 0 0 1 3 33 9 1 5 3 5

V1

CONTENTS

CHAPTER VII I THE THORA X 15 1. Evolutio n o f the Thora x 15 2. Th e Nec k 15 3. Genera l Structure o f the Thora x 16 4. Th e Prothora x 17 5. Th e Pterothora x 17 6. Th e Thoraci c Muscle s 18 Glossary o f Terms Applie d to th e Thora x . 19

PAGE

7 7 9 0 2 3 6 0

CHAPTER I X THE THORACI C LEGS 19 1. Th e Structur e o f the Leg s 19 2. Muscle s an d Mechanis m of the Leg s 20 Glossary o f Terms Applie d to th e Part s of an Insect' s Leg 20

3 3 0 9

CHAPTER X THE WING S 21 1. Origi n and Evolutio n o f the Wing s 21 2. Developmen t o f the Wing s 21 3. Structur e o f th e Wings . 21 4. Th e Win g Muscles . 22 5. Th e Win g Movement s 23 6. Insec t Flight 24 Glossary o f Terms Applie d to th e Wing s 24

1 1 4 5 8 3 0 3

CHAPTER X I THE ABDOME N 24 1. Genera l Structure o f the Abdomina l Segments 24 2. Th e Abdomina l Musculature 25 3. Th e Abdomina l Appendages 26

6 7 7 7

CHAPTER XI I THE ORGAN S OF INGESTION 28 1. Th e Preora l Cavit y 28 2. Th e Cephali c Stomadaeu m 28 3. Th e Feedin g Mechanis m of Neuroptera an d Coleopter a 28 4. Th e Feedin g Mechanis m of Hymenoptera . 29 5. Th e Feedin g Mechanis m of Lepidoptera 30 6. Th e Feedin g Mechanis m of Diptera 31 7. Th e Mout h Parts of Siphonaptera 32 8. Th e Feedin g Mechanis m of Thysanoptera 32 9. Th e Feedin g Mechanis m of Hemiptera 32 10. Th e Feedin g Mechanis m of Anoplura 34

0 1 4 6 5 2 1 5 6 8 4

CHAPTER XII I THE ALIMENTAR Y CANAL 34 1. Developmen t o f the Alimentar y Cana l 34 2. Genera l Structure o f the Alimentar y Canal 34 3. Th e Stomodaeu m 34 4. Th e Mesentero n 5. Th e Proctodaeum . 37

7 7 8 9

359 4

CONTENTS 6. Th e Filte r Chambe r 38 Glossary o f Terms Applie d to th e Alimentar y Canal 38 CHAPTER XI V THE ORGAN S O F DISTRIBUTION, CONSERVATION , AN D ELIMINATION 38 1. Th e Bloo d 38 2. Th e Organ s of Circulation 39 3. Th e Fat-bod y 40 4. Th e Oenocyte s 5. Th e Corpor a allata 41 6. Th e Organ s of Elimination 41 Glossary of Terms Use d in Thi s Chapter 42 CHAPTER X V THE RESPIRATOR Y SYSTE M 42 1. Th e Integument a s a Respiratory Organ . 42 2. Bloo d Gill s 42 3. Th e Trachea l Syste m 42 4. Genera l Mechanis m of Tracheal Respiratio n 45 Glossary o f Terms Applied to th e Respirator y Syste m 46

vii

PAGE

3 7

9 9 7 7 410 1 3 0 2 3 4 5 2 1

CHAPTER XV I THE NERVOU S SYSTE M 46 4 1. Genera l Structure, Organization , and Functio n o f the Nervou s System. . 46 5 2. Th e Centra l Nervou s System 47 2 3. Th e Stomodaea l Nervou s System 50 1 4. Th e Periphera l Nervou s System 50 3 Glossary o f Terms Applie d to th e Nervou s System 50 7 CHAPTER XVI I THE SENS E ORGAN S 51 1. Genera l Structure an d Classificatio n of Insect Sens e Organs 51 2. Th e Hai r Organ s 51 3. Th e Campanifor m Organs 52 4. Th e Plat e Organs 52 5. Th e Scolopophorou s Organs 52 6. Th e Eye s 52 Glossary o f Terms Applie d to the Sense Organs 54

0 2 5 1 3 5 8 8

CHAPTER XVII I THE INTERNA L ORGAN S OF REPRODUCTION 55 1. Th e Femal e Organs . 55 2. Th e Mal e Organs 56 3. Genera l Morpholog y of the Reproductiv e Organ s 57 Glossary o f Terms Applie d to th e Interna l Reproductiv e Organs 57

0 2 7 3 8

CHAPTER XI X THE ORGAN S OF COPULATION AN D OVIPOSITION 1. Th e Mal e Genitalia 58 2. Th e Femal e Genitali a 60 Glossary o f Terms Applie d to th e Externa l Genitali a 62

581 2 7 0

REFERENCES 62

5

INDEX 64

7

This page intentionally left blank

FOREWORD TO THE 199 3 PRINTING George C. Eickwort This is the mos t important textbook of insect morphology ever written in English, by the foremos t arthropo d morphologist of this century (Thurman 1959a , Richard s 1973). Robert Evans Snodgrass (1875-1962) began his morphological research a s a freshman at Standfor d University. Afte r receiving his A.B. in 1901 , he held a succession of short-term positions in entomology whic h h e intersperse d wit h job s a s a n itineran t artist . I n 1917 he began work with the Burea u o f Entomology of the U.S . Department o f Agriculture a s a n illustrato r an d anatomist . H e remained wit h the US D A unti l hi s retiremen t i n 1945 , afte r whic h h e continue d re search a t th e U.S . National Museu m of Natural Histor y unti l hi s death . During hi s eighty-seve n year s h e publishe d abou t ninet y paper s an d books, totalin g abou t si x thousan d page s (Thurma n 1959b , Richard s 1973). H e gav e gues t lecture s a t Cornel l University an d publishe d hi s most importan t late r book s wit h Cornel l Universit y Pres s (Snodgras s 1951, 1952 , 1956) . It i s entirely fitting , therefore, that the Pres s shoul d reissue hi s classic textbook. Although Principles o f Insect Morphology i s considere d Snodgrass' s crowning achievement, it wa s by no means his last. The more important of his subsequen t paper s and books (cited in the reference s at th e en d of this foreword ) le d t o majo r revision s i n hi s understandin g o f the skele tomuscular syste m o f arthropods. I t i s our loss that Snodgras s neve r revised Principles; he claimed that his publisher wa s unwilling to reset th e type (Richards 1973). Snodgrass's fluenc y i n th e Englis h languag e illuminate s thi s text book, an d hi s knowledg e of French an d Germa n provide s access to th e vast Europea n literature in morphology. No other textboo k has sinc e at tempted suc h a thoroug h treatmen t o f comparative insect anatom y and development. Abov e all , Snodgrass' s beautifu l drawing s provide an un paralleled elucidatio n of structure; thes e figure s ar e reproduce d in mos t of today's entomolog y texts. (Abbreviation s for the figure s ar e provided by Bay and Elzinga [1980].) All modern studies i n comparative and func tional entomolog y begin with Snodgrass's Principles, an d no student ca n afford no t to have this book close at tarsus . Principles, however , now almost sixt y years old, in som e ways shows its age. Snodgrass believed in comparing structures fro m a n evolutionary perspective, but hi s tex t predate s th e developmen t of phylogenetic analysis. He was also a strong proponent of functional interpretatio n o f strucix

x FOREWORD

TO 1993 PRINTING

ture, althoug h no w functiona l analysi s involve s vide o recording , neuroethological investigation , an d interpretatio n fro m engineerin g an d physical perspectives. Because Principles wa s published befor e the invention o f the electro n microscope, details o f histological an d cellula r struc ture ar e inaccurate . Physiolog y ha s advance d eve n mor e rapidl y tha n morphology i n th e las t hal f century , makin g mos t physiologica l state ments obsolete . Eve n th e reference s t o genetic s an d behavio r ca n b e quaint: "Th e germ cells carry the determinant s o f heredity, calle d genes, whatever they may be" (p. 16) . A studen t shoul d therefor e see k a moder n text lik e Th e Insects b y Chapman (1982) for an up-to-date introduction to morphology. The multivolume set edited by Kerkut and Gilbert (1985 ) also provides the anatom ical basis fo r physiology. But a s much as the moder n evolutionary biologist i s stil l stimulate d b y Darwin' s Origin o f Species (1859) , today' s entomologist wil l be enlightened b y Snodgrass's Principles, th e "origin " of insect morphology.

REFERENCES BAY, D. E., and ELZINGA , R. J. (1980 ) An index to the figur e abbreviations appearin g i n Principles o f Insect Morphology b y R. E. Snodgrass. Bull. Ent. Soc. America, 26: 335-355. CHAPMAN, R . F. (1982 ) The insects, structure and function , 3 d ed. Cambridge , Mass . DARWIN, C. (1859) On the origi n o f species. London . KERKUT, G . A. , an d GILBERT , L . I. , eds. (1985) Comprehensiv e insec t physiolog y bio chemistry an d pharmacology . 1 3 vols. Oxford . RICHARDS, A. G. (1973) Anatomy and morphology. In History of entomology•, ed. R. F. Smith, T . E. Mittler, an d C. N. Smith, pp . 185-202. Pal o Alto. SNODGRASS, R. E. (1936) Morphology of the insec t abdomen , part III. The male genitalia (including arthropod s othe r tha n insects) . Smithsonian Misc. Coll., 95 (14): 1 96. . (1937) The male genitali a o f orthopteroid insects. Smithsonian Misc. Coll., 96 (5): 1-107. . (1938) Evolution of the Annelida, Onychophora, and Arthropoda. Smithsonian Misc. Coll., 97 (6): 1-159. . (1941) The male genitalia o f Hymenoptera. Smithsonian Misc. Coll., 99 (14): 1-86. . (1942) The skeleto-muscular mechanism s of the hone y bee. Smithsonian Misc. Coll., 103 (2): 1-120. . (1944 ) Th e feedin g apparatu s o f biting an d suckin g insect s affectin g ma n an d animals. Smithsonian Misc. Coll., 104 (7): 1-113 . . (1946) The skeletal anatom y of fleas (Siphonaptera). Smithsonian Misc. Coll., 104 (18): 1-89. -. (1947 ) The insect craniu m an d the "epicrania l suture." Smithsonian Misc. Coll., 107(7): 1-52.

FOREWORD T O 1993 PRINTING

xi

. (1948 ) Th e feedin g organ s o f Arachnida, including mites an d ticks . Smithsonian Misc. Coll., 110 (10): 1-93 . . (1950 ) Comparativ e studies o n th e jaw s o f mandibulate arthropods . Smithsonian Misc. Coll, 116 (1): 1-85 . . (1951) Comparative studies o n the hea d of mandibulate arthropods . Ithaca . . (1952) A textbook of arthropod anatomy. Ithaca . . (1954) Insect metamorphosis. Smithsonian Misc. Coll., 122 (9): 1-124. . (1956) Anatomy of the hone y bee. Ithaca . . (1957) A revised interpretation o f the externa l reproductive organs of male insects. Smithsonian Misc. Coll., 135 (6): 1-60 . . (1958 ) Evolution of arthropod mechanisms. Smithsonian Misc. Coll., 138 (2): 1-77.. . (1960) Facts and theories concerning the insect head. Smithsonian Misc. Coll., 142 (1): 1-61 . . (1963) A contribution toward an encyclopedia of insect anatomy. Smithsonian Misc. Coll., 146 (2): 1-48 . THURMAN, E . B . (1959a) Robert Evans Snodgrass, insect anatomis t an d morphologist. Smithsonian Misc. Coll., 137: 1-17 . . (1959b ) Bibliograph y o f R. E . Snodgras s betwee n th e year s 189 6 an d 1958 . Smithsonian Misc. Coll., 137 : 19-22.

This page intentionally left blank

PREFACE The principa l valu e o f facts i s that the y giv e us something t o thin k about. A scientific textbook , therefore, should contain a fair amoun t of reliable information, though it may be a matter of choice with the autho r whether h e leaves it t o th e reade r t o formulat e his own ideas a s to th e meaning o f th e facts , o r whethe r h e attempt s t o guid e th e reader' s thoughts alon g wha t see m t o hi m t o b e th e prope r channels . Th e writer of the presen t text , being convinced that generalizations are more important tha n mer e knowledg e o f facts , an d bein g als o somewha t partial t o hi s ow n way o f thinking abou t insects , ha s no t bee n abl e t o refrain entirel y fro m presentin g th e fact s o f insect anatom y i n a way t o suggest relation s betwee n them that possibly exis t onl y in his own mind. Each o f the severa l chapters o f this book, in othe r words , is an attempt to give a coherent morphological view of the fundamental nature an d th e apparent evolutio n o f a particula r grou p of organ s o r associate d struc tures. I t i s mor e tha n likely , practicall y certain , tha t man y o f th e generalizations here offered wil l soon be modified o r superseded by othe r generalizations, bu t the y wil l have serve d thei r purpos e i f the y induc e critical student s t o mak e a wider and mor e thorough study o f the prob lems of insect morphology. Two notabl e book s hav e appeare d recentl y i n entomology : one , "Lehrbuch de r Entomologie," by Dr . Herman n Webe r of Danzig, in it s first edition ; th e other , "A Genera l Textbook o f Entomology/' b y Dr . A. D . Imm s o f Cambridge, England, i n its thir d edition . I n preparin g the presen t tex t th e write r ha s made a specia l effor t t o concu r with th e authors o f these book s in th e matte r o f anatomical terms , i n orde r that students ma y a s far a s possible be spared confusio n i n turning fro m on e treatise to another. Unfortunately , however, there is still much unavoidable discrepanc y i n th e us e an d applicatio n o f anatomica l name s i n entomology. Th e trouble , i n larg e measure , ca n b e blame d o n th e insects themselves , sinc e the y wil l not entirel y confor m wit h an y pla n of nomenclatur e o r wit h an y schem e w e ca n devis e fo r namin g thei r parts consistently . T o make clear the meanin g of terms as used in this text, therefore , a glossar y o f definition s i s appende d t o eac h chapter , wherein, also , wil l b e foun d th e Germa n equivalent s o f man y o f ou r English an d Latinized technica l names . In makin g acknowledgments , the write r mus t firs t o f all declar e his indebtedness t o th e Burea u o f Entomology, Unite d State s Departmen t XIII

xiv PREFACE

of Agriculture , for the experienc e and information acquired in the course of hi s man y year s o f officia l service. Mos t o f the illustration s accom panying th e tex t tha t ar e no t accredite d t o particula r source s are th e property o f th e Burea u o f Entomolog y an d Plan t Quarantine , an d many o f them hav e been published in th e Miscellaneous Collections and the Annual Reports of the Smithsonian Institution of Washington, D.C. For th e us e o f these figures the write r hereb y expresses his thanks both to th e Burea u an d t o th e Smithsonio n Institution . Wit h regar d t o illustrations borrowe d fro m othe r works , th e write r i s particularl y indebted t o Professo r Herman n Weber , o f Danzig , fo r permissio n to us e figure s fro m hi s "Biologi e de r Hemipteren " an d "Lehrbuc h der Entomologie. " Th e res t o f the illustrations , eac h accredite d t o it s proper source in the scientifi c journals , have been freely draw n from th e common heritag e o f entomolog y contribute d b y th e man y worker s i n many land s wh o have devote d themselve s t o th e stud y o f insects. T o my wife , Rut h H . Snodgrass , credit i s du e for th e typin g o f the manu script an d fo r much of the wor k of indexing and proofreading. WASHINGTON, D . C . May, 1935 .

R. E . SNODGRASS .

PRINCIPLES O F INSEC T MORPHOLOG Y

This page intentionally left blank

PEINCIPLES O F INSECT MORPHOLOGY CHAPTER I INTRODUCTION Morphology, i n th e biologica l sense, i s th e scienc e o f for m i n living organisms. Anatom y is the determination of structural facts. Morphol ogy seeks to find the reason for structure, an d to understand the relation of differen t structura l form s t o on e another . Morphology , therefore , must be intimate with function, since it must see forms as plastic physical adaptations to the work to be performed. A few physiological functions are basic to all organisms; they are essential to the continuanc e of matter in a living state. Th e variou s structural type s of organisms ar e specia l ways o f accomplishing these functions , that is, for doing the sam e things in differen t way s o r unde r differen t circumstances . Som e represen t improvements i n the machiner y along established lines ; others represen t changes o r ne w idea s develope d alon g ne w an d divergen t lines . Th e morphologist, therefore , thoug h primarily a comparativ e anatomist , i n order properly to develop his subject, must giv e attention to the working of the physical mechanisms with which he deals in his anatomical studies , he must loo k for th e significanc e o f structural modification s and innovations, an d h e mus t understan d th e basi c physiologica l function s that underlie organic form . In the stud y of insect morpholog y we cannot confin e ourselve s to th e limits o f entomology . Th e fundamenta l organizatio n o f insect s wa s established lon g befor e insect s becam e a specialize d grou p withi n th e phylum o f the Arthropoda , an d th e basi c structur e o f the arthropod s i s much olde r tha n th e arthropod s themselves . A s organism s evolve , important structure s ar e often so modified that their true nature becomes obscured; but th e same structure i s not likely to be modified t o the sam e degree in all related groups , or in all members of the sam e group. Struc tural modification has been carried to a high degree in all the arthropods , affecting som e organs in one group, others in another group; and particu larly i s thi s tru e o f th e insects . Hence , i n th e discussio n o f th e mor phology of insec t organ s given in the followin g chapters , many reference s 1

PRINCIPLES OF INSECT MORPHOLOGY

will b e mad e t o correspondin g parts i n othe r arthropods , while , fo r a n understanding o f the mor e fundamental structures o f the arthropods , i t will be found necessar y to g o back to th e segmente d worms and to thos e wormlike creature s know n a s onychophorans . Befor e takin g u p th e particular subjec t i n hand , therefore , we mus t kno w somethin g o f th e distinctive structura l feature s o f th e Annelid a th e Onychophora , an d the majo r group s of the Arthropoda . THE ANNELID A

The typica l anneli d worm s are elongat e cylindrica l animal s divide d transversely int o a series of segments. Th e mout h i s situated ventrall y between th e firs t segmen t an d a preoral lob e (prostomium) ; the anu s is terminal i n th e las t segmen t (periproct) . Th e segment s betwee n th e prostomium an d the periproc t ar e true metameres , o r somites, produced by segmentatio n o f the primitiv e bod y regio n anterior t o th e periproct . Some o f th e annelid s ar e provide d wit h lateral segmenta l appendage s (parapodia), whic h are hollo w evaginations o f th e bod y wall , movabl e by muscles inserted o n or within their bases . The bod y cavit y o f the Annelid a is the coelome . I t i s often divide d transversely b y intersegmenta l sept a int o segmenta l coelomi c cavities . The alimentar y cana l i s a tub e extendin g throug h th e bod y fro m th e mouth, situate d ventrally betwee n the prostomiu m an d the firs t somite , to th e anus , whic h is terminal o n the periproct . A blood vascular sys tem i s well developed in some forms b y enclosur e of tracts of the haemo coele in mesodermal walls. Usuall y there is a median dorsal vessel and a median ventra l vesse l connected by lateral trunks, fro m whic h are given off branche s t o th e variou s organs o f the body . Th e excretor y syste m consists o f paired segmental tubes (nephridia ) opening , on the on e hand, into th e coelome , and, o n the other , t o th e exterior . Respiratio n takes place either directly through the body wall or by means of gills, which are evaginations o f the integument . The anneli d nervou s system include s a media n prostomia l ganglion , the brai n (archicerebrum) , lyin g dorsa l t o th e alimentar y canal , an d a ventral nerve cord consisting of double segmental ganglia united by paired connectives. Th e brai n an d th e firs t ventra l gangli a ar e unite d b y connectives embracin g the oesophagus . Th e brain innervate s whateve r sense organs , suc h a s palpi, tentacles , an d eyes , ma y b e locate d o n th e prostomium; it i s ofte n differentiate d into a forebrai n (protocerebrum) and a hindbrai n (deutocerebrum) . Th e forebrai n ma y contai n well developed associatio n center s i n th e for m o f stalked bodies , o r corpor a pedunculata. The ger m cells of the matur e annelid occur in groups imbedded in th e mesodermal linin g of the coelome , the simpl e organ s thus forme d bein g

INTRODUCTION

3

the gonad s (ovarie s and testes). Th e ripening ova and spermatozoa are discharged from th e gonad s either into the genera l coelomic cavity, fro m which the y escap e through th e nephridi a o r throug h pore s o f the bod y wall, o r int o specia l coelomi c receptacle s connecte d b y duct s wit h th e exterior. Th e youn g anneli d larv a ha s a characteristi c for m an d i s known as a trochophore . THE ONYCHOPHOR A

The onychophorans , includin g Peripatus an d relate d genera , ar e worm]ike animal s resemblin g th e annelid s i n man y respects . Thoug h segmentation i s no t eviden t i n th e cylindrica l bod y o r i n th e somati c musculature, th e presenc e of a series of paired lateroventra l ambulator y appendages give s th e anima l a segmente d appearance . Th e "legs " resemble the annelid parapodia in that each is a hollow evagination of the body wal l movable b y fou r set s o f muscles reflected int o th e appendag e from the somati c wall . The mouth is situated anteriorl y on the ventra l surface at the bas e of a prostomial lobe. Th e prostomium bears a pair of tentacles an d a pair o f simple eyes. A n extraoral mouth cavity contain s the tru e oral opening and a pair o f strongly musculate d mout h hooks . The body cavity of the Onychophora is continuous through the lengt h of th e animal . Th e circulator y syste m consist s o f a dorsa l vesse l only , which has paired opening s into the body cavity betwee n each pair of legs. The excretor y organs are nephridia simila r t o those of Annelida, opening externally on the bases of the legs. Delicat e internal ai r tubes (tracheae) , arising i n group s fro m pit s scattere d irregularl y ove r th e integument , probably subserv e respiration . Th e nervou s system consist s o f a dorsa l brain locate d i n th e head , an d o f two lon g lateral nerv e cords in which ganglia ar e but littl e differentiated. Th e brain innervate s th e tentacles , the eyes , an d th e mout h hooks . Th e reproductiv e organ s i n eac h sex are a pair o f long tubular sacs , the duct s o f which unite i n a median exi t tube tha t open s ventrall y nea r th e posterio r en d o f th e body . Mos t species o f Onychophor a ar e viviparous , th e embry o bein g develope d within th e oviduct s o f the female . Th e young animal takes on directl y the for m o f the adult . The Onychophor a have ofte n bee n regarded a s primitive arthropods , but ther e i s littl e i n thei r organizatio n tha t conform s wit h arthropo d structure. Thei r relationship s ar e undoubtedly wit h th e Annelida , bu t the fact that the young at no stage have any resemblance to a trochophore larva woul d seem to indicate that the Onychophor a are not derived fro m typical annelids . Th e Onychophora , in fact, hav e a n ancien t lineag e of their own; fossil forms are known from th e Middl e Cambrian that closely resemble modern species, excep t fo r the smalle r number of legs.

PRINCIPLES OF INSECT MORPHOLOGY THE ARTHROPOD A

The arthropod s hav e a n annulat e bod y an d segmenta l appendages . Their distinctive feature s are the jointing of the appendages and a grouping of the bod y segments, each appendage being composed of a number of limb segment s (podites ) individuall y movabl e b y muscles , whil e th e body segments are segregated to form more or less distinct trun k section s (tagmata). Th e integumen t i s usuall y hardene d b y th e depositio n of sclerotizing substance s i n definit e area s o f th e cuticula , formin g exo skeletal plate s (sclerites ) t o whic h mos t o f th e muscle s ar e attached . The intervenin g membranou s area s allo w o f movemen t betwee n th e plates. Thi s characte r ha s give n th e arthropod s a n unlimite d field for the developmen t an d evolutio n o f exoskeleta l mechanism s bot h i n th e trunk an d in the appendages . The compositio n and specialization of the trun k sections , or tagmata, are characteristi c o f each of the severa l major groups of arthropods. Th e most constan t an d distinctiv e tagm a i s the head . I n it s simples t for m the definitiv e head represents the embryoni c protocephalon, consisting of a larg e preora l regio n an d usuall y th e firs t pos t oral somite. Generally , however, i t includes a gnathal region (gnathocephalon) formed of at least three succeedin g somites . Th e bod y regio n followin g th e hea d ma y preserve a unifor m segmentatio n an d simpl e structure , o r i t ma y b e variously differentiate d into a thorax an d a n abdomen . I n som e forms the cephali c regio n an d a varyin g numbe r o f succeedin g somite s ar e combined in a "cephalothorax," or prosoma, distinct fro m th e abdomen . The primitiv e termina l segmen t (telson ) is probably not a true somite , but a n endpiece of the body bearing the anus , correspondin g to the peri proct o f the Annelida . The appendage s of the trun k includ e a pai r o f procephalic antenna e (antennules), an d a doubl e serie s o f segmented , postoral , ventrolatera l limbs, potentiall y a pai r o n eac h segmen t bu t th e last . Th e postora l appendages become variously modified in adaptation t o functional special izations. A typica l arthropo d lim b consist s o f a basi s (coxopodite ) movable anteroposteriorl y o n th e body , an d o f a six-segmente d shaf t (telopodite) movabl e in a vertical plan e on the basis . Endit e and exit e lobes o f th e lim b segment s ar e frequentl y develope d int o specialize d appendicular processes . The definitiv e alimentary cana l include s lon g anterior an d posterio r sections (stomodaeu m an d proctodaeum ) derive d fro m th e ectoderm . Typical segmenta l nephridi a ar e absent . Th e bloo d vascula r syste m is variously developed , but i n plan it conform s with that of the Annelida. Respiration take s place either through the general body integument o r by means of evaginations (gills ) or invaginations (tracheae ) of the bod y wall.

INTRODUCTION

5

The nervou s system o f the arthropod s has the sam e general structur e as that o f the Annelida . Th e primitiv e brai n consist s o f a preoral bod y of nerv e tissue lyin g above the stomodaeum , whic h is differentiated int o protocerebral (ocular ) and deutocerebra l (antennular ) regions , excep t in forms lackin g antennae. Th e definitiv e brain i n most groups , however, is a syncerebrum , sinc e i t include s als o th e firs t pai r o f gangli a o f th e central nerv e cord , whic h secondarily becom e tritocerebral brai n lobes . The protocerebrum contains ofte n highl y developed corpora pedunculat a and th e ocula r centers ; th e deutocerebru m innervate s th e antennule s (first antennae) ; and th e tritocerebru m th e first pair o f postoral append ages (chelicera e or second antennae). The visua l organ s o f th e arthropod s includ e dorsa l (median ) an d lateral eye s innervate d fro m th e protocerebrum . Th e dorsa l eye s ar e always simple ocelli, located usually on the upper or anterior surfac e of the head, bu t i n Xiphosura, an d possibl y i n th e trilobite s an d eurypterids , there i s a pair of rudimentary ventral eye s on the deflecte d under surfac e of the head before the mouth. Th e number of dorsal eyes varies from one to eight , bu t ofte n non e is present; primitivel y ther e wer e perhaps tw o pairs; a single median dorsal ocellu s probably represent s th e ocell i of one pair united . Th e lateral eye s ar e typicall y compound , being forme d of groups of simple optic units composin g a single organ, but ofte n the y ar e represented b y group s of distinc t ocelli . The reproductiv e organ s ar e mesoderma l sac s enclosin g th e ger m cells. Th e paire d mesoderma l exit duct s ope n eithe r separatel y t o th e exterior or into a common median outlet tube of ectodermal origin. Th e position o f the genita l apertur e is variable. The Arthropod a includ e three majo r groups , namely , th e Trilobita, the Chelicerata, and the Mandibulata. THE TRILOBIT A The trilobite s ar e extinc t creature s tha t flourished throughout th e Paleozoic er a bu t wer e mos t abundan t durin g th e Cambria n an d Ordo vician periods . The y ar e th e mos t generalize d o f know n arthropods . The bod y i s usuall y ova l an d flattene d an d carrie s ventrall y a doubl e series of jointed limbs . Th e trun k i s divided int o a head an d tw o bod y regions known as the thorax and the pygidium, but the name "trilobite" is derive d fro m th e apparen t tripl e divisio n o f th e trun k lengthwis e into a n elevate d media n are a (th e axis , o r rhachis ) an d tw o depresse d lateral area s (pleurae) . Th e head , whic h appear s t o includ e th e prostomium an d fou r somites , i s covere d b y a dorsa l carapace ; th e thorax consists of a variable number of free segments ; the pygidiu m contains several segments, which, except in certain earlie r forms , ar e unite d in a caudal shield. Eac h bod y segment, excep t the last, bears ventrall y

6 PRINCIPLES

OF INSECT MORPHOLOGY

a pair o f jointed appendages. O n the uppe r surface o f the hea d in most species are a pair o f compound lateral eyes , and i n some forms a median tubercle whic h appears t o b e a simpl e dorsal eye ; on the unde r surfac e is a pair of small spots which some writers believe to be ventral eyes. The distinctivel y generalize d feature o f th e trilobites , a s compared with th e othe r arthropods , i s th e lac k o f specializatio n an d structura l differentiation in the segmental appendages. Th e first pair of appendages, which probabl y ar e procephalic antennules, ar e filamentous and multiar ticulate. Th e rest ar e without doub t postora l limbs. The y are practically al l alik e excep t tha t som e of the mor e anterior one s may hav e a greater numbe r o f segment s tha n th e others . Th e trilobit e lim b pre serves th e typica l for m an d fundamenta l structur e o f al l arthropo d appendages. Th e basi s support s a larg e exit e (epipodite ) bearin g a series o f thin, closel y set plate s o r filaments, which probably functioned as gills. Th e telopodit e is usually six segmented. The trilobites appear to be related, on the one hand, to the Xiphosura , and, on the other , to the phyllopod crustaceans, since they have features characteristic of both these groups. The y ar e not literally th e ancestor s of th e othe r arthropods , however , since alon g wit h th e trilobite s ther e lived the highly specialized eurypterids and a large and varied crustacean fauna, but the trilobites are probably more closely related to the ancestra l arthropods than are any other known forms. THE CHELICERAT A In bot h th e chelicerat e an d th e mandibulat e arthropod s th e seg mental appendages are diversified in form and function, an d some of them are suppressed. Th e most generally distinctive features that separate the Chelicerata from th e Mandibulat a are the suppressio n of the antennules , and th e modificatio n of the firs t postora l appendage s to for m a pai r of cheliceraej whic h typically ar e pincerlike feeding organs . The body segments are grouped into two trunk regions, a prosoma and an abdomen . Th e firs t include s th e protocephalo n an d th e firs t si x postoral somites , whic h are alway s more or less united. Th e abdome n varies i n length an d ma y b e distinctly segmented , though i n the highe r forms i t i s usually shor t an d it s segmentatio n indistinc t o r suppressed . The prosoma bears six pairs of limbs, including the chelicerae , all of which, except th e chelicerae , are generall y leglike in form . Th e telopodite s of some o f the appendage s often contai n seve n segments instead o f six b y the interpolation o f a "patella" between the femur (meropodite ) and th e tibia (carpopodite) . Abdomina l appendages are usuall y absent , but in the mor e generalize d form s the y ar e retaine d i n modifie d shap e an d may hav e gill-bearing epipodites as in the Trilobita .

INTRODUCTION

7

The brai n o f the Chelicerat a i s a syncerebrum compose d of the primi tive cerebru m and the gangli a of the chelicera l segment , but , owin g to the los s o f th e procephali c antennules , th e deutocerebra l center s ar e suppressed, and the brain consist s of the protocerebrum, which innervates the eyes , an d o f th e tritocerebrum , whic h innervate s th e chelicerae . The ocula r organ s include median dorsa l eyes , lateral eyes , an d i n some cases ventra l eyes . Th e latera l eye s ar e compoun d i n mor e primitiv e forms; in others they are represented b y group s of simple eyes. The Chelicerata include the Eurypterida, the Xiphosura, the Pycnogonida, and th e Arachnida. Eurypterida.—The eurypterid s ar e extinc t Paleozoi c arthropod s that live d fro m th e Cambria n t o th e Carboniferou s period bu t attaine d their greates t developmen t i n th e Siluria n an d Devonian . The y wer e aquatic, mostl y fresh-water , o r mud-inhabitin g creatures . Whil e th e majority wer e relativel y small , les s tha n a foo t i n length , som e became the larges t o f al l know n arthropods , reachin g a lengt h o f 6 o r 7 feet . The segment s of the prosom a are united; th e abdome n consists o f 12 free segments, th e las t bearin g a telson , whic h i s usuall y a lon g taperin g spine bu t i s plate-like i n som e forms. Th e chelicera e are lon g or short . The nex t fou r pair s o f limbs are generall y small, an d th e sixt h pai r long and flat , evidentl y swimmin g organs; but i n some species the leg s are all long and slender. Th e first five segments of the abdome n have plate-lik e appendages overlappin g eac h othe r an d concealin g gills. O n the uppe r surface of the cephali c region are a pair o f compound lateral eye s and tw o small simpl e dorsa l eyes ; o n th e unde r surface , accordin g t o Storme r (1934), there appears to be in some forms a pair of ventral eyes . Xiphosura:—The modern members of this group, commonly known as horseshoe crab s o r kin g crabs , hav e s o man y point s o f resemblanc e with the ancient eurypterids that the two groups are often classed together as the Merostomata , and certain extinc t form s make the connectio n even closer. Th e body of the horseshoe crab is distinctly divided between th e prosoma an d th e abdomen ; i n eac h par t th e segment s ar e unite d an d covered by a large dorsal carapace , the secon d ending in a long spine-like telson. Th e si x thoracic appendage s are all chelate i n the female . Th e abdomen contain s onl y si x segments , whic h bea r plate-lik e appendage s very simila r t o thos e o f the anterio r par t o f the abdome n o f the euryp terids, eac h excep t th e firs t havin g a larg e gill-bearin g epipodite . O n the hea d ar e a pai r o f compoun d lateral eyes , a pai r o f simple media n dorsal eyes, and a pair of rudimentary median ventral eyes . Pycnogonida, o r Pantapoda.—The pycnogonid s ar e aberran t arthro pods commonly known as sea spiders. Fro m th e natur e o f their append ages they appea r t o belon g to the Chelicerata .

8 PRINCIPLES

OF INSECT MORPHOLOGY

Arachnida.—Here ar e include d th e scorpions , th e solpugids , th e phalangids, th e spiders , th e ticks , an d th e mites . I n mos t form s th e trunk is divided in the typical chelicerat e manner into a prosoma and an abdomen, bu t i n th e solpugid s the prosom a i s constricte d betwee n th e fourth and fifth pairs of limbs. Th e abdomen may be long and distinctl y segmented, in which case the distal part is narrowed and has the form of a jointed'"tail," bu t i n th e spider s an d mite s th e abdome n i s shor t an d rotund, wit h indistinc t o r suppresse d segmentation . Th e prosoma l appendages includ e the chelicerae , a pair o f pedipalps, an d fou r pair s of legs. Abdomina l appendages are either absen t o r represented b y modi fied rudimentar y structures . Th e cephali c regio n o f th e prosom a generally bears a group of simple dorsal eyes, and, in some forms, on each side one to five simple lateral eyes ; the latera l eye s are never compound. Paleontologically th e oldes t know n arachnids ar e contemporaneou s with the trilobites and eurypterids . THE MANDIBULAT A The mandibulat e arthropod s diffe r fro m th e Chelicerat a i n tw o characteristic features , namely , (1 ) th e retentio n o f th e procephali c antennules, an d (2 ) the modificatio n o f the base s o f the second postora l appendages to form a pair of biting, jawlike feeding organs, the mandibles. The appendage s o f th e firs t postora l segment , correspondin g t o th e chelicerae of the Chelicerata , are presen t i n most Crustace a a s a pai r of large biramous antennae (secon d antennae), but in the other mandibulate groups the y ar e suppresse d o r ar e represente d onl y by embryoni c rudi ments. Th e telopodites of the mandibles may be retained in Crustacea in the for m o f "palpi, " bu t otherwis e they ar e lost . Th e firs t tw o post mandibular appendage s ar e generall y modifie d a s accessor y feedin g organs (th e firs t an d secon d maxillae), though i n som e form s the y ar e reduced an d mor e or less rudimentary. Th e following appendage s ma y all have a uniform structure , or they may be variously modified o n differ ent region s of the body ; but a patellar segmen t is never present i n any of them. Tagmosis i s variabl e i n th e Mandibulata . Th e hea d ma y consis t of th e protocephalo n onl y (includin g the secon d antenna l somite) , bu t generally it contains a gnathocephalic section composed of the mandibula r and the tw o maxillary somites, t o whic h may be added the appendages , at least , o f the nex t trun k segment . I n th e malacostraca n Crustace a the protocephalon , th e gnatha l segments , an d a varyin g numbe r of succeeding segment s ar e mor e o r les s combine d in a "cephalothorax. " The Myriapod a and Hexapod a have a distinct hea d including four post oral somites ; th e bod y o f the firs t grou p shows no tagmosis , bu t i n th e hexapods it i s differentiated int o a thorax and an abdomen.

INTRODUCTION

9

The brai n o f th e Mandibulat a i s wel l differentiate d int o a proto cerebrum an d a deutocerebrum , innervatin g respectivel y th e eye s an d the antennules (first antennae); generally it includes also the first postoral ganglia o f the ventra l nerv e cord, innervating th e secon d antennae whe n these appendage s ar e present ; thes e gangli a becom e th e tritocerebra l lobes o f the definitiv e brain , thoug h the y retai n thei r primitiv e ventra l commissure. Th e mandibula r an d bot h maxillar y pair s o f gangli a ar e united in a composite suboesophageal ganglion contained in the head when the latter includes the gnatha l segments . A stomodaeal nervous syste m is usually present, havin g its principal connection s with the tritocerebra l ganglia. The majo r group s o f th e Mandibulat a ar e th e Crustacea, th e Myriapoda, and the Hexapoda. Crustacea.—The crustacean s includ e th e phyllopods , th e barnacles , the shrimps , crayfis h an d lobsters , th e crabs , an d relate d forms . Th e Crustacea ar e distinguished fro m al l the othe r arthropod s b y a biramous structure o f the limbs , each appendage typically havin g an oute r branc h (exopodite), arising from the basal segment (basipodite ) of the telopodite, and a n inne r branc h (endopodite) , which i s th e shaf t o f the telopodit e distal to the basipodite. Th e coxopodite, in many forms, supports a gillbearing epipodite , whic h apparentl y correspond s t o th e simila r basa l appendicular orga n of the limb s of Trilobita and th e abdomina l append ages o f Xiphosura . Tagmosis i s variable in the Crustacea . Th e protocephalon, including the tritocerebra l somite , ma y constitut e a smal l primitiv e hea d distinc t from th e res t o f th e trunk , o r i t ma y b e unite d wit h severa l somite s following t o for m a composit e head ; or , again , a variable numbe r of segments i n the thoraci c region are more or less united wit h the head in a cephalothorax distinc t fro m a n abdomen . The firs t antenna l appendage s (antennules ) ar e usuall y filamentous and multiarticulate ; the y ar e neve r biramous . Th e tritocerebra l appendages (secon d antennae) ar e typically biramou s and thus show that they belon g t o th e serie s o f postora l limbs , thoug h th e bod y segmen t bearing the m form s a par t o f th e protocephalon . Th e mandible s ar e always wel l developed ; th e tw o pair s o f maxillae ar e small , sometime s rudimentary. Th e next three pairs of appendages are termed maxillipeds; the followin g five are differentiated i n higher forms primarily a s ambula tory organ s (periopods) . Th e abdomina l appendage s (pleopods ) ar e usually differen t fro m th e thoraci c appendage s an d ar e ofte n speciall y modified. Mos t o f th e appendage s ma y b e modifie d fo r purpose s o f swimming. The brai n i n som e o f th e lowe r Crustace a i s a primitiv e cerebru m differentiated int o a protocerebrum and deutocerebrum containing respec-

10 PRINCIPLES

O F INSECT MORPHOLOGY

tively the ocular and first antennal centers. I n most forms, however, the definitive brai n i s a syncerebru m including the secon d antennal gangli a as tritocerebral lobes. Th e Crustacea have compound lateral eyes similar in structure t o thos e o f insects. Myriapoda.—The commo n myriapod s ar e easil y distinguishe d from th e othe r terrestria l arthropod s b y thei r slende r form s an d man y legs, but a more generally distinctive characte r is the division of the trun k into only two parts, a head and a body. Th e head bears a pair of preoral antennae (antennules ) and probably includes not more than four postora l somites. Th e appendage s o f th e firs t somit e ar e absent , thos e o f th e second ar e the mandibles , the othe r tw o pairs ar e variable i n structure . The bod y is typically lon g and uniforml y segmented , though some of th e segments may be reduced, and in two groups the segment s are united in pairs. Mos t o f the primitiv e segment s bea r eac h a pai r o f legs. Th e myriapods ar e terrestria l animals ; th e majorit y o f them hav e trachea l invaginations o f the bod y wal l fo r respiration . Eyes , whe n present i n modern forms , consis t o f group s o f simpl e lateral eyes , whic h approach the compoun d type i n Scutigera. Certai n Permia n diplopod s are said t o have had large compound eyes. The Myriapod a includ e tw o principa l groups . I n th e member s of one grou p (progoneat e forms ) th e reproductiv e organ s ope n nea r th e anterior en d o f th e body ; i n thos e o f th e othe r grou p (opisthogoneat e forms) th e genita l openin g i s a t th e posterio r en d o f th e body . Th e progoneate myriapod s includ e th e Diplopoda , th e Pauropoda , an d th e Symphyla; th e opisthogoneat e grou p consist s o f th e Chilopod a (centipedes). I n the diplopod s and pauropods most of the bod y somite s appear t o b e united i n successive pairs, a t leas t dorsally ; the leg s of th e diplopods occu r i n pairs o n each double segment. Th e pauropod s have branched antennae . Hexapoda.—The hexapod s ar e wel l characterize d b y th e featur e of their organization from which they get their name, which is the invariabl e specialization o f three pair s o f appendages as legs . Th e leg s are alway s the appendages of the first three postgnathal somites; the latter constitut e a definite locomotor center, the thorax, distinct from th e abdomen, which seldom bear s organ s of locomotion. Th e hea d has apparentl y th e sam e composition as in the Myriapoda , sinc e it alway s includes three pair s of gnathal somites . Th e abdome n neve r ha s mor e tha n 1 2 segment s (11 true somites) , o f which the las t is the periproct . The appendage s o f the hea d includ e a pai r o f procephalic antenna e (antennules), embryoni c rudiment s o f secon d antennae , a pai r o f man dibles, an d tw o pair s o f maxillae. Th e secon d maxillae are unite d i n a median composite organ known as the labium. Th e thoracic appendages are th e thre e pair s o f legs. Abdomina l appendages ma y b e presen t o n

INTRODUCTION

11

any o f the 1 1 true somite s of the abdomen , but the y ar e alway s greatl y modified o r rudimentar y an d whe n present assum e a variet y o f forms ; generally most o f them ar e absen t i n the adul t stage . Th e appendage s are neve r biramou s i n a manne r comparabl e wit h th e appendage s o f Crustacea, bu t epipodite s may be present o n the coxopodites. The brai n o f the hexapod s has distinc t protocerebral , deutocerebral , and tritocerebra l centers , th e las t forme d o f th e gangli a o f th e secon d antennal somite . Th e latera l eyes , whe n present , ar e typicall y com pound, but i n some adults an d in many larval form s they ar e replaced by groups o f simpl e eyes . Thre e dorsa l ocell i are commonl y present, th e unpaired ocellu s being median, anterio r (o r ventral) t o th e others , an d probably doubl e in its origin . The genital ducts open either separately o r through a median tub e of ectodermal origin . Th e paire d duct s o f the femal e ma y ope n between the eleventh and twelfth segment s of the abdome n (Protura), or between the seventh and eighth (Ephemerida) . Th e median oviduct opens on the seventh, eighth , o r nint h abdomina l segment , th e media n ejaculator y duct on the nint h segment , except in Collembola where the genita l open ing i n eac h sex is between the fift h an d sixt h definitiv e segment s o f th e abdomen. The Hexapoda include several more or less distinct groups, which are the Protura, the Collembola, the Diplura, th e Thysanura, an d th e Pterygota, bu t systematist s ar e no t agree d a s t o th e relationship s o f thes e groups. Sinc e the Pterygot a ar e the winge d insects, th e othe r form s are generally termed collectively the Apterygota. Or , again, the Protura ar e set apart fro m th e others , whic h are regarded a s the tru e Insecta . Th e Diplura are usually classed with the Thysanura, th e two groups then being distinguished a s Thysanura entotrophic a an d Thysanura ectotrophica . Protura.—The proturan s ar e minut e creature s resemblin g insect s except tha t the y lac k antennae . The y ar e mor e fundamentally distin guished from th e othe r hexapod s by the fac t that the body does not con tain th e definitiv e number o f segments a t hatching , ther e bein g adde d during growt h tw o segment s betwee n th e periproc t an d th e precedin g somite. Thi s postembryoni c developmen t o f segment s i s unknow n i n the othe r hexapod s bu t i s usua l i n th e myriapods . Th e reproductiv e ducts i n bot h sexe s of Protura ope n behind th e penultimat e segmen t of the adult , a s i n Chilopoda . Notwithstandin g thes e features , however, the Protur a appea r t o b e more closely allie d t o th e insect s tha n t o th e myriapods. Th e body is differentiated int o a principal locomotor center, the thorax , compose d of three segment s bearing three pair s o f legs, an d into a n abdome n o f 1 2 complete segmenta l annuli , whic h has smal l o r rudimentary tubula r appendage s o n its firs t thre e segment s only . Th e thoracic leg s en d eac h i n a simpl e clawlik e segment (dactylopodite) —

12 PRINCIPLES

O F INSECT MORPHOLOGY

another myriapo d feature , bu t on e occurrin g als o i n som e apterygot e insects an d i n many pterygot e larvae . Collembola.—The collembolan s ar e smal l creature s tha t constitut e a sharpl y define d grou p o f hexapod s wit h man y distinctiv e features . The abdome n never has more than six segments an d i s usually provide d with thre e highl y specialize d appendicula r organs . Th e firs t o f thes e appendages i s a larg e thic k tube , th e collophore , that project s from th e ventral side of the first abdominal segment. Th e third is a leaping organ, the furcula , arisin g fro m th e fifth segment, consistin g of a large base an d two lon g terminal prongs . Whe n bent forwar d i n repose, the furcul a i s held in place by the small forked second appendage, or tenaculum, arisin g from th e thir d abdomina l segment . Th e genita l openin g in bot h sexe s occurs between the fifth and sixth segments of the abdomen . Th e repro ductive organs closely resemble those o f Protura. Diplura (Thysanura entotrophicd). —The member s o f thi s grou p ar e characterized b y havin g th e mandible s an d maxilla e retracte d int o a pouch abov e th e labium . I n thi s respec t an d i n certai n othe r respect s they ar e mor e specialize d tha n th e tru e Thysanura , bu t th e latte r ar e more closel y relate d t o th e Pterygota . Th e commo n genera ar e Campodea an d Japyx. Thysanura (Thysanura ectotrophica). —The thysanuran s hav e th e mandibles an d maxilla e expose d i n th e usua l manner , an d the y retai n more complet e remnants o f abdominal appendage s than do the Diplura . The coxopodite s of the genita l appendage s i n mos t specie s ar e provide d with gonapophyses that for m a n ovipositor i n the female , whic h appear s to b e the prototyp e o f the egg-layin g organ of pterygote insects. O f the two principa l families , Machilidae an d Lepismatidae , th e secon d shows closer affinities wit h the Pterygot a in the structur e an d mechanism of the mandibles. Pterygota.—The Pterygot a includ e al l th e winge d insect s an d thei r wingless relatives; the latter presumably have lost their wings secondarily. The wing s are latera l extension s o f the dorsu m o f the secon d an d thir d thoracic segments and were fully developed as organs of flight in the oldes t known fossi l insects . Durin g th e Carboniferou s period , o r probabl y earlier, a group of winged insects evolve d a mechanism in the win g base for flexing the wing s horizontally ove r th e bac k whe n not i n use . Th e descendants o f this group (Neopterygota) includ e the majorit y of modern winged insects , whil e th e mor e primitiv e nonwing-flexin g insect s ar e represented toda y b y onl y tw o order s (Odonat a an d Ephemerida) , both o f whic h hav e descende d fro m Carboniferou s time s bu t ar e no t closely related t o eac h other . Ontogenetically th e wing s ar e develope d durin g postembryoni c growth, an d a postembryoni c metamorphosis , i n varyin g degrees , i s

INTRODUCTION

13

common in all the Pterygota . Amon g the wing-flexing insects, th e wings in one group are developed during the larval stages within closed pouches of th e bod y wall , an d th e larva e o f members of this group have become so specialized tha t thei r transformatio n t o th e adul t involve s th e inter polation o f a preimagina l instar , th e pupa , betwee n th e larv a an d th e fully forme d adult . Insect s o f this group are known as the Endoptery gota, becaus e of the internal developmen t o f the wings , or a s the Holo metabola, i n referenc e to thei r hig h degree of metamorphosis. Mos t of the othe r wing-flexing insects, a s well as those that do not flex the wings, develop the wing s externally in the usua l manner of growth of appendicular organs , an d the y hav e i n genera l a simple r metamorphosi s tha t allows the nymp h o r larva t o chang e directly int o th e for m o f the adult . These insects are distinguished, therefore , as the Exopterygota, o r Hemimetabola, but it should be observed that they d o not constitute a monophyletic assemblag e o f forms , sinc e the y includ e a larg e grou p o f wing-flexing insect s an d als o th e nonwing-flexin g Odonat a an d Ephemerida.

CHAPTER I I GENERAL ORGANIZATIO N AND DEVELOPMEN T To understand th e structura l organization o f any animal , i t i s necessary t o kno w that animaP s history, fo r n o living creatur e has arrived a t its present organization by a direct line of development from its beginning. Structure generall y is an adaptation to function; but man y of the organ s of complex animals have served a series of quite different function s durin g the cours e of their evolutio n and, as a consequence, have had their struc ture man y time s remodele d b y wa y o f adaptatio n t o thei r changin g function o r to ne w functions. The histor y o f a n anima l canno t b e know n fro m th e imperfectl y preserved record s o f its past . Embryonic developmen t ma y giv e u s a suggestion o f phylogenetic evolution , since in a general way the embry o repeats th e history o f its race ; but th e embryo , fo r purposes o f its own, usually digresses much from the ancestral story, and it commonly abbreviates its version of the earlier chapters or often omits these parts altogether . Fortunately, however , th e les s highl y evolve d th e fina l for m o f a n organism, the more of its early history i s it likely to retain in its ontogeny . Hence, we may approximate a reconstruction of the phylogeny of a species by filling out the obscure passages in its embryonic story, or by supplying those deleted , wit h materia l judiciousl y selecte d fro m th e fact s o f embryonic developmen t i n relate d form s successivel y lowe r in th e scal e of organization . T o th e evidenc e derive d fro m embryology , however, we mus t alway s ad d tha t t o b e deduce d fro m a stud y o f comparativ e anatomy, fo r i n th e structur e o f serially relate d adul t animal s w e ofte n get an insight int o th e cours e of evolution mor e reliable than that t o b e obtained fro m an y othe r source . In th e presen t chapter , therefore , we shall endeavo r t o buil d u p a concept of the fundamenta l organization of an insect o n evidence derived from embryolog y and from a study o f the adul t structur e o f other arthropods, of PeripatuSj and of the annelid worms. The Ger m Cell s and the Soma.—Th e generative cel l from whic h any individual o f th e metazoi c animal s i s produce d begin s developmen t b y division. Afte r successiv e repetitions o f division ther e ar e soo n forme d two distinc t group s of cells in the resultin g cel l mass; those o f one group continue t o b e germ cells t thos e o f the othe r grou p becom e the somatic cells. I t i s th e organizatio n o f this secon d an d alway s large r grou p of 14

GENERAL ORGANIZATION AN D DEVELOPMENT

15

cells formin g th e soma , or bod y o f the futur e animal , tha t w e study i n ordinary anatomy . The purpos e of the som a primarily i s to giv e protection t o th e ger m cells durin g th e perio d i n whic h thes e cell s develo p t o maturity . I t then become s obligatory upo n th e soma , i n mos t form s o f animals , t o bring th e ger m cells of opposite sexe s together i n such a manner that a sufficient numbe r of them may unite to insure perpetuation o f the species . In th e highe r form s o f animals, th e som a ha s als o taken upo n itself th e responsibility o f securing protection to the fertilized egg s and to the earl y developmental stage s o f the ne w somat a forme d b y them . Som e ani mals acqui t themselve s o f this dut y merel y b y depositin g thei r egg s in places wher e the developin g embryo s wil l hav e a reasonabl e guarant y against destruction ; others retain the fertilized eggs and give the embryos space for developmen t withi n thei r ow n bodies. Finally , assumin g stil l greater responsibility , th e som a o f th e mor e highl y endowe d types o f animals charge s itsel f wit h th e dut y o f protectin g an d nourishin g th e young durin g th e adolescen t perio d o f postembryoni c life . I n additio n to fulfilling it s many parental functions , and in order to accomplish these functions, th e som a must als o maintain itsel f a s a living unity . The method s tha t th e som a has adopte d fo r carrying on its variou s functions ar e the reasons for its structure; and the continual reorganizing of it s structur e i n adaptation to mor e efficient way s of accomplishing it s functions ha s resulted i n its evolution from a simple to a complex organization. Th e different methods that the somata of different animal s have adopted and perfected to meet their obligations are expressed in the many forms of life existing today and in those that have existed during the past. Structure, vegetativ e functions , sensitivity t o environmenta l condition s and changes , th e powe r o f automati c reactio n t o impingin g forces , instincts, consciousness, intelligence, and the faculty of making voluntar y adjustments to external conditions—all these things are properties devel oped and perfected in the som a for the benefi t o f the ger m cells or for th e somata tha t ar e to accompan y the succeedin g generations o f germ cells. The som a of the mor e complex animals cannot reproduc e itself; when its purposes are accomplished, its physical elements disintegrate , its vitality is reduced, and sooner or later its enemies or adverse circumstances bring about it s death . The ger m cell s appea r t o b e th e reproductiv e agent s o f th e soma ; but "reproduction/ 7 s o called , i s mor e trul y a repeate d productio n of somata an d ger m cells . Eac h ger m cell multiplies by division , an d th e resulting cell s underg o a developmen t int o spermatozoa o r ova, according t o thei r sex , o r som e o f the m ma y becom e merel y nutritiv e cell s for th e others . Th e persistin g ger m cell s remai n a s separat e entities , except i n th e unio n o f mal e an d femal e cell s t o for m a composit e cell

16

PRINCIPLES OF INSECT MORPHOLOGY

from which the development of both germ cells and soma usually proceeds. The ovu m alone , however , ma y produc e a complet e soma ; i n insect s parthenogenesis i s o f frequen t occurrence . Th e ger m cell s carr y th e determinants o f heredity , calle d genes, whateve r the y ma y be , an d apparently i n most cases receive no direct influence from th e accompany ing soma. Typical Earl y Stage s o f Development.—Th e egg , bein g a cell , ha s all the constituen t part s of an ordinary cell ; but, sinc e it i s destined t o a much greater activit y tha n any of the body cells, it is provisioned with a supply o f nutritive material, know n as yolk, or deutoplasm, store d i n th e meshes o f its cytoplasm . Th e know n facts o f comparativ e embryolog y lead us to believe that the early developmenta l stage s in all animals wer e once essentially th e same, though actually they may now be very differen t

FIG. 1.—Typica l earl y stage s i n genera l embryoni c development . A , th e blastul a (diagrammatic). B , C , D , thre e stage s i n th e differentiatio n an d invaginatio n o f th e endoderm (End), and the formation of the mesoderm (Msd) of a chiton. (From Kowalevsky, 1883. )

in differen t animals . Som e of the difference s ar e clearl y correlated wit h the quantit y of yolk contained i n the egg ; others ar e to b e attributed t o other causes, which may be termed embryonic expediency, for the embryo, as wel l a s th e postembryoni c instar s o f th e animal , depart s fro m th e ancestral lin e of development wherever an advantage is offered i n so doing. The developmen t o f th e insec t present s on e o f th e greates t puzzle s that the embryologist encounters , and it is certain that the insect embry o does no t structurall y reproduc e free-livin g stage s o f it s ancestry . I n order t o understan d insec t structur e a s presented b y the embryo , therefore, w e must discove r th e fundamenta l things i n it s developmen t tha t underlie thos e tha t hav e bee n buil t u p t o sui t th e convenienc e o f th e embryo. In th e typical , generalize d for m o f anima l development , whic h proceeds fro m egg s containin g a minimu m quantit y o f yolk , th e firs t division, o r cleavage, o f th e eg g cut s th e eg g int o approximatel y equa l halves, an d th e simila r succeedin g division s soo n produc e a globula r

GENERAL ORGANIZATION AN D DEVE LOPMENT

17

mass o f cells, o r blastomeres, called a morula. The n a cavity, th e blastocoele, appears in the cente r of the mass, and the cells become arranged in a single laye r a t th e surface . Thi s stag e i s th e blastula (Fig . 1 A) ; th e superficial layer of cells is the blastoderm (Bid). The term blast, so frequently recurrin g in embryologica l names, come s from the Gree k word /3Xa Internally the intersegmental grooves form folds , and o n thes e fold s ar e attache d the fiber s o f th e principa l longi tudinal band s o f somatic muscle s (LMcl). Animals having this type o f structur e ca n ben d th e body freel y i n an y direction , an d FIG. 36.—Type s o f bod y segmentation . they can shorte n it by a length primar y segmentation . B , C , secondar y wise contractio n o f the segments . A, segmentation. Ac , antecosta ; acs, ante In th e typica l adul t arthropo d costal suture ; ast, acrosternite ; dtg, acroter Isg, intersegmenta l fold ; LMcl, longi structure th e segmenta l area s of gite; tudinal muscles; Mb, conjunctival, secondar y the bod y ar e hardened by the de- intersegmental, membrane ; Rd, posterior fol d secondar y segment ; Seg', primary segmen t position o f sclerotizing substance s of(somite); Seg", secondar y segment . in th e cuticula , formin g usuall y tergal and sternal plates (B, T7, Stn). Th e areas of sclerotization, however, do not coincid e with the area s o f the origina l segments, since they do not cover th e posterio r part s of the latter and may extend anteriorl y a shor t distance befor e th e intersegmenta l groove s o n whic h th e muscle s ar e attached. Th e trunk thus becomes differentiated int o a series of sclerotic annuli, th e scleromata (B , Seg"), an d intervenin g membranou s conjunctivae (Mb). Th e forme r no t onl y are movable upon each other by reason

74 PRINCIPLES

O F INSECT MORPHOLOGY

of th e flexible conjunctivae bu t ca n be partially retracte d eac h into th e posterior en d o f th e on e precedin g b y th e contractio n o f th e longi tudinal muscle s attached upo n them (C) . On comparing the two kinds of organization described in the preceding paragraph, i t become s evident tha t w e must distinguis h tw o type s o f segmental structur e include d unde r th e genera l ter m "segmentation. " The first type (Fig. 36 A), which occurs in all soft-bodied arthropods an d in anneli d worms , i s th e embryoni c for m o f segmentation . I t is , therefore, th e mor e primitiv e one , an d w e ma y designat e i t primary segmentation. Th e other type (B, C) is clearly a secondary differentiation of th e body into successive parts by the formation of plates in the integu ment alternatin g wit h nonsclerotized areas. Thi s typ e o f body segmentation we may distinguis h a s secondary segmentation. Primary Segmentation.—In soft-bodied larval insects, as in the anneli d worms, th e principa l longitudina l muscles are attache d typicall y o n th e primary intersegmenta l folds . I t i s evident , therefore , tha t there i s a close relatio n betwee n thi s mor e primitiv e for m o f bod y segmentatio n and th e segmentatio n o f the muscle-formin g part s of the mesoder m into myotomes. W e may, then, defin e a s primary segmentation that form of segmentation (Fig . 3 6 A ) i n whic h the functiona l intersegmental line s of th e bod y wall (Isg) coincid e with the line s of attachment o f the princi pal longitudinal muscle fibers (LMcT). The segments in this type of segmentation (Seg') correspon d wit h th e tru e somites , o r embryoni c metameres. Secondary Segmentation. —The developmen t o f hardene d area s o r plates i n the bod y wall, a feature distinguishing mos t o f the arthropod s from th e annelids , wa s perhap s i n th e firs t plac e a protectiv e device . But, sinc e in the arthropod s th e muscle s have their attachment s o n th e body wall, an advantage i s gained if the muscles are affixed t o the integu mental plates, because the latter become, in this case, not only protective coverings bu t element s o f th e moto r mechanis m a s well . Henc e th e sclerotized area s o f th e arthropo d bod y wall , such a s th e majo r terga l ??? ??????? ?????? ????? ?? ?? ??? ?????? ??????? ??????? ??? ????? ?? ??? ??????? ?????????????? ????? ??? ???? ?? ????? ??? ???????????? ??????? are attached . I n orde r t o retai n th e powe r of motion, however , there must b e left a flexible nonsclerotized area (B , Mb) a t th e opposit e en d of the segment . Thes e flexible conjunctival areas o f th e integumen t no w become th e functiona l intersegmental membranes . Fo r thi s reaso n th e limits o f the definitiv e segments in arthropods that have body-wall plates (B, Seg") ar e no t coinciden t wit h th e primar y intersegmenta l fold s (A, Isg). Th e area s o f flexibilit y betwee n th e scleroti c part s o f th e segments (B, Mb) divid e the body in a new way, which is clearly secondary and therefor e constitute s a secondar y segmentation . I n primar y seg -

BODY REGIONS, SOLE RITES, AND SEGMENTATION

75

mentation th e longitudinal muscles are intrasegmental, inasmuch as they are coextensiv e with th e segmenta l area s o f the bod y (A) ; in secondar y segmentation th e muscle s becom e intersegmenta l (B) . Th e flexibl e areas of the bod y wall in secondary segmentation (B , Mb) are usually th e posterior part s o f the primar y segments . Thi s arrangemen t allow s th e

FIG. 37.—Th e bod y section s (tagmata ) o f a n insec t an d thei r typica l segmentation. Note free intersternite s (list, 2Ist) i n the thorax , and reversed overlapping of the thoracic sterna.

muscles of each segment to draw the followin g segmen t forward (C) , and the infoldin g o f th e conjunctiva l membrane s give s th e characteristi c posterior overlappin g to th e successive segmental plates . In adul t insects , secondar y segmentatio n i n its typica l for m prevail s throughout th e lengt h o f th e abdomen , wher e th e limit s o f th e terga l and sternal plates coincide morphologically with each other (Fig . 37, Ab).

FIG. 38.—Ventra l surface s o f tw o bod y segment s o f a chilopod , showin g alternatin g segmental sternal plates (Stri) an d intersegmental intersternites (1st).

In the thorax (Th), however , the terga, thoug h fundamentally secondary segmental plates o f the usual form, may undergo modifications that alte r this structure , an d the thoraci c sternal sclerotizations ar e characteristic ally o f a differen t typ e o f structure , whic h i s bes t illustrate d i n th e Chilopoda. The secondar y segmentatio n o f th e chilopod s ha s produce d typica l secondary segmenta l plate s onl y i n th e dorsum . I n mos t o f the centi -

76

PRINCIPLES O F INSECT MORPHOLOGY

pedes, exceptin g Scutigera, th e ventra l sclerotization s throughou t th e length o f the bod y hav e take n th e for m o f independent segmenta l an d intersegmental sclerites , th e forme r occupyin g th e primar y segmenta l regions, th e latte r bein g confine d t o th e intersegmenta l fold s (Fig . 38 , Stn, 1st). I n som e families th e ventra l longitudina l muscles retain thei r attachments o n the intersegmenta l plates , i n others they have migrate d to the segmenta l plates, o r to supportin g arms or ligaments of the latter . This sam e typ e o f sclerotizatio n occur s i n th e vente r o f th e thora x of many insects , i n which there i s a small intersternite (Fig . 37 , list, 21 st) situated posterio r t o th e principa l sternal plat e o f the prothora x and th e mesothorax (Stnij Striz). Structure o f a Typical Secondary Segment.— A typical segmen t of an adult arthropo d i s i n genera l on e o f th e secondar y annula r section s of the bod y defined by the lengthwise extent o f its dorsal and ventral plates (Fig. 3 6 C) . A considerable part o f the tru e segmenta l area , however, is formed b y th e posterio r conjunctiva l membrane (B , C, Mb), whic h is usually infolde d an d mor e o r les s conceale d withi n th e posterio r end s of th e terga l an d sterna l plate s (C) . Th e wal l of eac h segment , a s w e have seen , contain s typicall y a dorsa l scleroti c area , o r tergu m (B , T), ??? ? ??????? ????????? ????? ?? ??????? ??????? ????? ??? ??????? ?? ??????? walls may contain each one or more pleural sclerites. The Tergum.—In it s typica l for m th e bac k plate o f a secondary seg ment include s the sclerotizatio n o f the dorsu m of the primar y interseg mental area preceding (Fig. 36 B, T). Th e primitive intersegmental fol d (A, Isg)j therefore , become s a submargina l ridg e nea r th e anterio r edg e of th e inne r surfac e o f the tergu m (B , Ac). Thi s ridg e is the antecosta of th e tergum . Th e corresponding external groove , or transverse lin e of inflection formin g th e antecosta , i s th e antecostal suture (acs). Th e narrow precostal lip of the tergum is the acrotergite (atg). The postcostal tergal sclerotizatio n usuall y form s a simpl e plate , whic h i s subjec t t o modifications in various ways, especially in the wing-bearin g segments of pterygote insects , bu t i t ma y b e broke n u p int o smalle r sclerites , a s in many holometabolou s larvae . Th e dorsa l longitudina l muscles , pri marily attache d o n th e intersegmenta l fold s (A) , usuall y retai n thei r attachments o n the antecosta e i n the secondary segments (B , C), though some or all of their fibers may migrate to the precostal or postcostal area s of th e tergum . Tergal plate s ar e present i n the adul t stage s of nearly all arthropods . Their principa l variation in size with relation t o the shape of the segmen t is in a transvers e direction . The y ma y b e limite d t o th e media n par t of th e back , or their latera l area s may b e extended downward a varyin g distance i n the lateral wall s of the dorsum , and th e latera l margins may project a s fre e fold s eithe r horizontall y extende d o r ventrall y deflecte d

BODY REGIONS, SCLE RITES, AND SEGMENTATION

77

over the side s of the segment , sometimes concealing the base s of the legs . Frequently laterodorsa l sclerite s ar e distinc t fro m a principa l media n tergite. In the thoraci c segments of winged insects the typical structure o f the dorsal plates is generally obscured by a modification in the intersegmenta l sclerotization, correlate d with th e developmen t o f the wing s as efficien t organs o f flight . Th e interterga l part s o f the conjunctiva e between th e mesothorax an d metathora x an d betwee n th e metathora x an d firs t abdominal segmen t ar e almos t obliterate d b y a n anterio r extensio n of the acrotergite s of the metathoracic and first abdominal terga (Fig . 39 B). Furthermore a secondar y membranous suture ma y appea r i n the terga l

FIG. 39.—Diagram s showin g intersegmenta l relation s o f th e skeleta l plate s i n secondary segmentation . A , generalize d condition . B , specialize d conditio n i n th e dor sum o f the thora x of winged insects, in whic h th e enlarge d acrotergite s (A, atg) becom e the postnota l plates (B , PN) o f the secon d an d thir d segments.

region just behin d th e antecost a (Ac) of each of these segments . Ther e is thus formed posterior to each wing-bearing tergal plate (772, Ts) a postnotal plate (P7V 2, PNs)bearin g the intersegmenta l attachments o f th e dorsal muscles (DMcl). Since the dorsal muscles are greatly enlarged in th e thorax , th e antecosta e develo p larg e lobes , th e phragmata (Ph), for thei r accommodation . Th e postnota l plate s ar e usually regarded as belonging to the segmen t preceding in each case, but i t i s clear that the y are intersegmental structures analogous to the intersternites of the vente r between the prothorax and mesothorax and between the mesothorax and metathorax (A, Isf)j which are usually more closely associated with the preceding sterna (Fig . 37 , list, 21st). The Sternum.— Sternal plate s ar e no t s o constan t a featur e o f th e arthropod skeleto n a s ar e th e terga l plates . The y ma y b e presen t o r

78

PRINCIPLES O F INSECT MORPHOLOGY

absent withi n the sam e major group , and, where present, the y ar e ofte n highly variabl e i n bot h shap e an d siz e betwee n closel y relate d forms , and eve n in differen t bod y regions of .the same species. The ventra l plat e o f a segmen t usuall y ha s th e sam e structur e a s the tergu m (Fig . 3 6 B, Stri), a s in the abdome n of insects (Fig . 37 , A6), where the sternum generally is an inverted replica of the tergum. I n this case each sternum bear s anteriorl y a submarginal antecosta o n its inne r surface (Fig . 3 6 B, Ac), o n which the ventra l longitudina l muscle s ar e usually attached, an d presents a narrow precostal lip, or acrosternite (ast), corresponding t o th e acrotergit e o f the dorsa l plate . I n th e thora x of many insects , however , the intersegmenta l sclerotization s o f the vente r form smal l independen t plates , o r intersternite s (Figs . 37 , 3 9 A , 1st), as they do in the whol e length of the bod y in most of the chilopod s (Fig . 38). Th e intersternites of the thorax of insects, which may occur between the prothorax and mesothorax and between the latter and the metathorax, are known as spinasterna because each usually bears an internal spine-lik e process giving attachment t o som e of the ventra l muscles . Th e spinas terna ar e ofte n unite d wit h th e segmenta l stern a precedin g them, bu t never wit h thos e following . A definitiv e thoraci c sternum , therefore , never ha s a true antecosta . Mos t o f the ventra l muscle s of the thora x are attached (probabl y secondarily ) o n apodema l processe s o f th e seg mental plates ; a fe w fibers, however, usually retai n th e primitiv e con nections wit h th e media n processes , o r spinae, o f th e intersegmenta l spinasterna (Figs . 87, 103 A). The definitiv e stern a o f insect s ar e usuall y composit e plates , eac h comprising a media n region representing th e are a o f the primitiv e seg mental sternu m an d latera l part s derive d fro m th e lim b bases. I n th e thorax the adjoine d latera l element s are the ventra l arc s of the subcoxal parts of the leg bases; in the abdomen the sternal plates appear to contain, in mos t cases , the entir e basa l part s o f the otherwis e suppressed limbs . The term " sternum " is usually applied to the principal ventral plate of a segment regardless of the rea l or theoretical compositio n of the latter. The Pleural Sclerites. —The podial areas of arthropod body segments, that is , the so-calle d pleural areas in which the limb s are implanted, ar e usually membranou s an d seldo m contai n an y extensiv e sclerotizatio n that ca n b e attribute d t o th e wal l o f th e bod y itself . Th e proximal , or subcoxal, parts of the lim b bases, however, are ofte n expande d in th e pleural wal l an d separate d b y a n articula r rin g fro m th e coxa l part s of the limbs . Th e coxa e thus become the functiona l bases of the append ages, and the subcoxae serve as supports for the latter i n the podial areas of th e bod y wall . Whereve r sclerotizations occu r i n th e podia l areas , therefore, the y appea r generall y t o b e derive d fro m th e base s o f th e limbs, though it i s probable that certai n small sclerites may belon g also

BODY REGIONS, SOLE RITES, AN D SEGMENTA TION

79

to th e regio n o f th e peripodia l membranes . Al l sclerotization s o f th e podial area s o f th e bod y segments , however , ar e i n genera l terme d pleurites. I n th e thora x o f pterygote insect s th e subcoxa l sclerotization above, before, an d behind the cox a is known as the pleuron. Subcoxal pleurites occu r in mos t o f the bod y segments of the Chilo poda (Fig . 52-A , Scx)j wher e they hav e th e for m o f smal l sclerite s of various shapes more or less closely associated wit h the base s of the coxae. Similar sclerites are present in the pleural areas of the thorax in apterygote Hexapoda, but her e they frequently appear as two crescentic arches over the bases of the coxa e (Fig. 89). I n adul t pterygot e insect s the thoraci c pleural plates are much enlarged, especially in the wing-bearing segments, where they for m support s fo r the wing s as well a s giving articulation t o the coxa e of the legs . I n th e decapo d Crustacea th e inne r wall s o f th e gill chamber s are evidentl y o f subcoxal origin, bu t i n most o f the Crus tacea an d i n th e Arachnid a ther e i s littl e evidenc e o f th e presenc e of pleurites derive d from th e lim b bases. In th e abdomina l segment s of insects th e lim b base s ar e sometime s represented b y distinc t plate s occupyin g the pleura l area s betwee n th e terga an d sterna , a s i n certai n Thysanur a (Fig . 13 8 A, Cxpd) an d i n many larval form s (Fig . 15 0 A, Cxpd)} in th e genita l segment s the y ar e retained a s th e basa l plate s o f th e gonopod s (Fig . 3 5 C) . I n general , however, the abdomina l pleurites appea r t o b e fused wit h th e primitiv e sterna i n continuous plates, whic h are the definitiv e sterna. Intersegmental Relations.—The primary intersegmenta l grooves , we have seen, are the functional segmental limits only in soft-bodied arthro pods o r in forms wit h but a weak or partial sclerotization o f the integu ment (Fig . 3 6 A) . I n al l arthropod s wit h well-develope d body-wal l plates, th e definitiv e segmentation i s a secondary one; but th e limit s of the secondary segments differ accordin g to the relations o f the sclerotiza tion i n th e primar y intersegmenta l region s t o tha t o f th e segmenta l regions before an d behin d them. In the insect abdomen, where both the dorsal and the ventral primar y intersegmental area s o f sclerotization ar e continuou s with the segmenta l plates following, a typical secondary segmentation prevails (Fig . 37, A 6), and th e functiona l intersegmenta l ring s ar e th e membranou s posterio r parts o f th e primar y segment s (Fig . 4 0 A , Mb). Th e primitiv e inter segmental fol d (Isg) form s a n antecost a (Ac ) on both th e tergu m an d the sternum , and the precostal lip forms an acrotergite (atg) o f the dorsa l plate and an acrosternite (asf) of the ventral plate. Wherever there is a difference betwee n the dorsu m and the vente r in the sclerotizatio n of the intersegmental region , however , th e intersegmenta l relation s ar e les s simple. I f th e dorsa l plate s retai n th e for m typica l o f secondar y segmentation, while the ventral sclerotization takes the for m o f independent

80

PRINCIPLES OF

INSECT MORPHOLOGY

sternites an d intersternites , a s ofte n occur s i n th e thorax , th e ventra l half o f eac h intersegmenta l membran e wil l embrac e th e intersternit e (B, 1st) an d wil l include a part o f the tw o adjoinin g primary segmental regions. Again , if the intersternite i s united with the segmental sternum preceding (C) , whil e th e interterga l sclerotizatio n remain s continuou s with th e tergu m following , th e conjunctiva l membrane (Mb) will cros s obliquely o n the sid e of the bod y from th e posterio r par t o f the anterio r segment t o th e anterio r par t o f th e posterio r segment , an d th e ven tral postcosta l li p become s a poststernite (pst) o f th e anterio r segment . Finally, if both th e dorsa l and ventra l intersegmenta l sclerotizations ar e united o r closel y associate d wit h th e segmenta l plate s preceding , th e functional intersegmenta l membran e becomes the anterio r par t o f th e posterior segmen t (D , Mb'), an d th e postcosta l li p form s a posttergite (ptg) dorsally and a poststernite (pst) ventrally.

FIG. 40.—Four types of intersegmental relations, according to the position of the secon d ary inter segmental membrane, or conjunctiva (Mb, Mb'). Ac , antecosta; acs, antecosta suture; ast, acrosternite; Isg, primary intersegmental line; pst, poststernite; ptg, posttergite

Tagmosis.—In al l adul t arthropod s som e o f the segment s ar e mor e or les s unite d int o group s formin g distinc t trun k sections , o r tagmata. The number of tagmata, an d the number of segments in each tagma var y in different arthropods . Th e most constant tagmosi s of the trunk i s that which differentiates , in the embryo , the protocephali c head regio n fro m the primitive body (Fig. 23 A). Th e definitive head of most mandibulat e arthropods, however , contain s a secon d tagma , whic h i s tha t o f th e gnathal segment s (C , Gri). I n th e Hexapod a a thir d embryoni c tagma (Th) become s the thorax , o r second tagma o f the adult , whic h is usually composed o f three segments , thoug h i n mos t Hymenopter a i t contain s four. A fourt h embryoni c tagm a ( A b) become s th e adul t hexapo d abdomen, whic h ha s a t mos t 1 2 segments , includin g th e periproct . Tagmosis i s mor e variabl e i n th e Crustacea ; i n th e Chilopod a an d Diplopoda i t result s onl y i n th e formatio n o f a head , includin g th e gnathal segments , and a body; in the Chelicerata again it is variable, bu t the principa l divisio n o f the trun k i s int o a " cephalothorax" an d a n "abdomen."

BODY REGIONS, SCLERITES, AN D SE GMENTATION

81

GLOSSARY O F GENERA L TERM S APPLIE D T O TH E BOD Y SEGMENT S AND TH E SKELETA L PLATE S Acrosternite (ast). —The narro w margina l flang e anterio r t o th e antecost a o f a definitive sterna l plat e tha t includes the precedin g primary intersegmental selerotiza tion; characteristic o f abdominal sterna o f insects, bu t absen t o n thoracic sterna. Acrotergite (atg). —The anterio r precosta l par t o f the terga l plat e o f a secondary segment; usuall y a narro w flange, but sometime s greatl y enlarged , an d frequentl y reduced or obliterated . Antecosta (Ac). —The anterio r submargina l o r margina l ridg e o n th e inne r sur face o f a tergal o r sternal plat e correspondin g to th e primar y intersegmental fold , o n which typically th e longitudina l muscles are attached . Antecostal Sutur e (acs).—Th e external groove of the antecosta . Conjunctiva (Mb). —See Intersegmental membrane. (Gelenkhaut.) Dorso-pieural Lin e (a-a). —The lin e o f separatio n betwee n th e dorsu m an d th e pleural regio n of the body , ofte n marke d by a fold o r groove. Dorsum (D). —The entir e bac k o f a n anima l abov e th e pleura l regions ; o r spe cifically, whe n qualified b y the designatio n of a segment, the bac k region of a segment. Intersegmental Membran e (Mb). —The flexibl e conjunctiv a betwee n tw o sec ondary segments; usually the nonsclerotize d posterior part of a primary segment, bu t variable a s shown in Fig . 40. Intersternite (1st). —An intersegmenta l sclerotizatio n o f th e venter , suc h a s th e thoracic spinasterna . Laterotergite (%).— A lateral sclerotizatio n of the dorsum distinct fro m a principal median tergite. (Paratergite.) Limb Basis (Cxpd). —The primitiv e basal par t of a limb, implanted i n the pleura l area o f the bod y wall, bearing the telopodite . (Coxopodite.) Metamere.—An embryonic somite, o r primary body segment . Myotome.—A division o f the bod y muscles corresponding to a metamere. Notum.—See tergum. Paratergite.—See laterotergite. Pleural Regio n (P). —The podia l region , o r ventrolatera l part s o f th e bod y o n which the limb s are implanted, metamericall y divided int o segmenta l pleural areas. Pleurite (pi). —Any mino r sclerite o f the pleura l are a o f a segment, o r on e o f th e component sclerites o f a pleuron. Pleuron (PI). —The sclerotizatio n o f th e pleura l are a o f a segment , apparentl y derived from the proximal part of the limb basis, and usually subdivide d int o pleurites . Pleuro-ventral Line (b-b). —The lin e of separation betwee n the pleura l regio n and the venter; lyin g mesad of the limb bases, but obscure d when the latter are fused wit h the sterna . Postnotum (PN). —An intersegmenta l plat e o f th e dorsu m o f the thora x associ ated wit h th e tergu m o f the precedin g segment, bearing the antecost a an d usuall y a pair o f phragmatal lobes . (Phragmanotum.) Poststernite (pst). —The postcosta l li p o f a definitiv e sternal plat e tha t include s the intersegmenta l sclerotizatio n following . Posttergite (ptg). —The narro w postcostal li p of a postnotal thoraci c plate . Primary Segmentation.— A segmenta l divisio n o f the bod y correspondin g to th e embryonic metamerism . Sclerite.—Any sclerotized area o f the bod y wall , or of internal part s derived fro m the bod y wall. Scleroma.—The scleroti c annulu s o f a bod y segmen t i n distinctio n t o th e mem branous conjunctiva.

82 PRINCIPLES

O F INSECT MORPHOLOGY

Secondary Segmentation. —Any form o f body segmentation tha t doe s not strictl y conform wit h the embryoni c metamerism; the usua l segmentation o f arthropods hav ing a well-developed exoskeleton, i n which the membranou s intersegmental ring s ar e the posterio r part s of the primar y segments . Segment.—A body segmen t i s an y o f the successiv e annula r subdivision s o f th e arthropod trunk , whethe r correspondin g t o th e embryoni c metamere s o r produce d secondarily. Somite.—A primitive, o r primary , bod y segmen t correspondin g t o a n embryoni c metamere or myotome. Sternite.—A subdivision of a sternal plate , o r any on e of the scleroti c components of a definitive sternum . Sternum.—Either th e primar y ventra l plat e (Stn) o f a bod y segmen t o r a com posite definitiv e sternum (S). Subcoxa (Sex). —The proxima l par t o f th e lim b basi s whe n differentiate d fro m the coxa ; usually incorporate d int o the pleura l wal l of the bod y segment . Tagma.—A group of successive segment s formin g a distinct sectio n o f the trunk . Tergite.—A subdivisio n o f a definitiv e tergum, o r an y on e of several sclerites i n the dorsu m of a body segment . Tergum (T). —The dorsa l sclerotizatio n o f a bod y segment ; calle d als o notum, especially in the thorax. Trunk.—The entire series of body segments of an arthropod, including the cephalic, thoracic, an d abdomina l sections . Venter (F).—Th e entir e unde r surfac e o f th e anima l betwee n th e tw o serie s of limb bases, or , when qualified b y the designatio n o f a segment, the correspondin g sur face o f a singl e body segment.

CHAPTER V THE SEGMENTA L APPENDAGES OF ARTHROPOD S The Arthropod a are well named from th e fac t tha t they hav e jointed segmental appendages (fro m dpdpov, a "joint, " and TTOVS , 7ro).—Th e pocke t betwee n th e bas e o f th e hypopharyn x an d th e labium into whic h opens the salivary duct ; in higher insects converted into a salivar y pump or a spinning apparatus . Subgenal Areas (Sge). —The usuall y narrow lateral margina l areas of the craniu m set of f by th e subgena l suture s abov e the gnatha l appendages , including th e pleuro stomata an d hypostomata . Subgenal Suture s (sgs). —The latera l submargina l groove s o f th e craniu m jus t above the base s of the gnatha l appendages , forming internally a subgenal ridge (SgR) on eac h side , continuou s anteriorl y wit h th e epistoma l sutur e whe n th e latte r i s present. Suspensorium o f th e Hypopharynx.— A pai r o f bars o r group s of sclerites i n th e lateral walls of the adoral surface of the base of the hypopharynx. (Fulturae.) Tentorial Pits.—Th e externa l depression s i n th e crania l wal l at th e root s o f th e tentorial arms ; the anterior tentorial pits (at) locate d in the subgena l sutures or usually

THE HEAD

129

in the epistoma l suture , th e posterior tentorial pits (pt) i n the lowe r ends of the post occipital suture . Tentorium (Tnt). —The endoskeleta l brac e o f th e craniu m forme d o f unite d anterior an d posterio r pair s o f arms , bearin g primaril y th e origin s o f th e ventra l muscles of the gnatha l appendages , an d usuall y givin g attachment secondarily , o n a pair of dorsal arms , to th e antenna l muscles . Vertex (Fa;).—Th e top o f the craniu m between an d behind the compoun d eyes .

CHAPTER VI I THE HEA D APPENDAGES The usua l appendage s o f the insec t hea d includ e a pai r o f antennae, a pair of mandibles, a pair of maxillae, and the labium, the last representing a pai r o f unite d secon d maxillae . I n th e embry o o f th e walkingstic k insect, however , there has been observed a pair o f lobes lying anterior t o the antennae , whic h possibl y ar e rudiment s o f a pai r o f preantennae, and i n th e embryoni c stage s o f variou s insect s ther e ar e rudiment s o f postantennal appendages. The series of cephalic appendages appears to be th e sam e i n Crustace a an d Myriapod a a s in insects , excep t that th e postantennal appendage s ar e usuall y highl y develope d a s th e secon d antennae i n Crustacea an d appear t o be entirely absent , eve n as embryonic vestiges, i n Chilopoda . Th e homologie s of the hea d appendage s i n the several mandibulate groups are established b y their innervation fro m corresponding cerebra l an d pos t cerebral nerv e centers . I n man y Crustacea a pai r o f ventral hea d lobe s known as th e paragnatha occur s between the mandible s and the first maxillae, and a similar pair o f lobes, the superlinguae, i s present i n some insects a s lateral part s of the hypo pharynx. Sinc e th e superlingua e o f insect s hav e bee n suppose d t o represent a pair of head appendages, they will be discussed in the present chapter, thoug h it no w seems probable that neither th e paragnath a no r the superlinguae have the statu s of segmental limbs. Associated wit h th e hea d appendage s i s a serie s o f paire d glands , which appropriately ma y be described in connection with the appendages. 1. PREANTENNA L APPENDAGE S

The existenc e o f preantennal appendage s i n th e Arthropod a canno t as ye t b e regarde d a s established . Heymon s (1901) , however , ha s described an d figure d a pai r o f evanescent appendage-lik e lobe s i n th e embryo o f Scolopendra lyin g anterior t o th e antenna e (Fig . 7 0 A, Prnf), and Wiesmann (1926) reports the presence of a pair of similar preantennal rudiments i n the embryo of a phasmid insect, Dixippus morosus (B, Prnt). The stalk s bearin g th e compoun d eye s i n certai n Crustace a hav e a n appendage-like structure, sinc e they are movable, segmented, and amply provided with muscles ; but sinc e the compoun d eyes certainly belong to the prostomium, it seems most probable that the crustacean eye stalks are 130

131

THE HEAD APPENDAGES

of th e natur e o f th e sensor y tentacula r organ s o f th e prostomiu m i n Annelida.

b

a

FIG. 70.—Examples of the presenc e o f apparent rudiment s o f preantennal appendage s (Prnt) i n arthropo d embryos . A , Scolopendra. (From Heymons, 1901. ) B , Dixippus (Carausius) morosus. (From Wiesmann, 1926.) 2. TH E ANTENNA E

The antenna e ar e th e firs t o f th e appendicula r organ s o f th e hea d present i n the adult insect . The y are innervated fro m th e deutocerebral lobes of the brain and generally have been regarded as the appendages of a corresponding antenna l segment . Neithe r i n thei r segmentatio n no r in their musculature , however , do the insect antennae resemble the limbs of the postora l somites , an d th e homologou s organs o f the Crustacea , th e first antennae, or antennules, ar e never biramous in the manner characteristic o f th e secon d antenna e an d th e succeedin g appendages . I f th e antennae ar e not true segmenta l limbs, they mus t be regarded as organs analogous to th e prostomia l tentacle s o f the anneli d worms. Thoug h in the embryo s of some of the lowe r insects the antenna l rudiments arise a t the side s o f the mouth , o r eve n behin d th e latter , th e morphologicall y preoral positio n o f thei r nerv e center s i n th e brai n suggest s tha t th e antennae belon g to th e preora l par t o f the head . I n adul t insect s th e antennae ar e situate d o n th e anterio r part s o f th e parieta l region s of the cranium, usually on the facial aspect, but i n many larvae and in some adults the y ar e place d laterall y jus t abov e th e base s o f the mandibles . Antennae ar e absen t i n the Protura , an d the y ar e practically absen t i n most larva e o f th e highe r Hymenoptera , wher e the positio n o f eac h i s indicated onl y by a disc or a slight swellin g over the ti p o f the imagina l organ developin g beneath the larva l cuticula . The typical insec t antenn a i s a many-jointed filament, but generall y three principal parts ma y be distinguished i n its shaf t (Fig . 71 A). Th e first part , b y whic h th e antenna i s attache d t o th e head , i s usuall y larger tha n th e other s an d constitute s a basa l stal k o f the appendage , termed the scape (Scp). Th e second part, or pedicel (Pdc), i s short an d in nearly all insects contains a special sensory apparatus know n as the organ

132

PRINCIPLES O F INSECT MORPHOLOGY

of Johnston. Th e part of the antenna beyond the pedicel is the flagellum, or clavola (Fl). The flagellum is usually long and made up of many small subsegments, bu t i t ma y b e abbreviate d o r reduce d t o a singl e piece . Since th e flagella r division s i n orthopteroi d insect s increas e i n numbe r from on e instar to the next , the y appea r to be secondary subdivisions of one primar y antenna l segment . Th e antenna e ar e subjec t t o man y variations i n form, givin g rise to the severa l distinct type s recognize d in descriptive entomology , bu t th e basi c structur e o f th e appendage s i s remarkably uniform . The bas e o f the antenn a i s set int o a smal l membranou s area o f th e head called the antennal socket. Th e ri m of the socke t is often strength ened b y a n interna l submargina l ridg e formed b y a n externa l inflection , the antennal suture (Fig . 7 1 A, as). Usuall y a pivotlik e proces s on th e

???? ????????????? ?? ??? ???????? ?? ??????? ??????? ??? ??????? ???????????? of a n insec t antenna . B , hea d o f a chilopod , Scutigera forceps, wit h antenna l muscle s arising o n the cranium . as, antenna l suture; Fl, flagellum ; n, articula r point (antenni fer); Pdc, pedicel ; Scp, scape.

rim of the antenna l socke t (ri) form s a special support an d articular point for th e bas e of the scape , allowing the antenn a a free motio n in all directions. Th e pivot, o r antennifer, is generally ventral bu t i s not alway s so, and in some cases it is obsolete or absent. Each antenn a i s moved as a whole by muscles inserted o n the bas e of the scape . Th e origin of the antenna l muscle s in adult pterygot e insect s is commonly on the dorsa l or anterior arm s of the tentorium, bu t i n some larval insects the muscle s arise on the wall s of the craniu m (Fig . 64 B), as they d o in th e chilopod s (Fig . 7 1 B). Th e attachmen t o f the antenna l muscles o n the tentorium , therefore , is probably a secondar y conditio n resulting fro m a migration o f the crania l ends of the muscle s to the dorsa l tentorial arms after th e latter have made connections with the head wall. The pedice l and th e flagellum together ar e moved by muscle s arising in the scap e an d inserte d o n th e bas e o f th e pedice l (Fig . 7 1 A), bu t th e flagellum and its subsegments, so far as observed by the writer, are never provided with muscles in insects.

THE HEAD APPEN DAGES

133

3. TH E POSTANTENNA L APPENDAGE S

The appendage s o f th e tritocerebra l segmen t o f th e arthropod s undoubtedly belong to the series of true limbs, and it seems very probable that they represen t th e first pair o f appendages in this series. Morpho logically the postantenna l appendage s are postoral, sinc e they ar e innervated fro m th e tritocerebral lobe s of the brain, whic h are unquestionably postoral ganglia , sinc e the y preserv e thei r ventra l connection s b y a substomodaeal commissure . Th e postantenna l appendage s occu r a s functional adul t organ s only in Chelicerata an d Crustacea . I n th e first group the y for m th e chelicera e (Fig. 7 2 A); i n th e secon d they ar e th e highly developed second antennae (B) , which have distinctly the biramous leg type of structure, segmentation, and musculature.

FIG. 72.—Types of functional postantennal appendages. A , chelicera of a scorpion . B, secon d antenn a o f a decapo d crustacean , Spirontocharia, wit h biramou s structure lik e that of following appendages .

The postantenna l appendage s are a t bes t rudimentar y i n all insects . Embryonic vestige s o f the m occu r i n representative s o f severa l orders and are usually called " second antennal," " intercalary," or "premandib ular" appendages (Fig. 23 B, Put). In a few adult insects (Campodea, Machilis heteropus, Dissosteira) smal l lobes have been observed before th e mandibles, which may possibly be persist ing rudiments of the postantenna l appendages (Fig . 6 0 B , I). Th e occurrenc e of correspondin g structure s has no t bee n recorde d i n an y stag e o f th e myriapods . W e hav e n o evidence to sugges t what th e for m o f the postantenna l appendage s ma y have been when they wer e functional organs in insects, bu t i t i s perhaps reasonable to suppose that in terrestrial arthropods , in which the gnatha l appendages were not yet added to the head, the tritocerebral appendage s served i n some capacity connected with feeding . 4. TH E MANDIBLE S

The mandible s ar e th e appendage s of the firs t gnatha l segmen t an d are undoubtedl y homologous organs in al l the mandibulat e arthropods . The correspondin g appendage s o f th e Chilicerat a ar e th e pedipalps . The typica l mandibl e o f pterygot e insect s i s a stron g bitin g ja w hinged to the hea d by anterior an d posterior articulations , an d having a transverse movement of abduction and adductio n produced by abducto r

134

PRINCIPLES OF INSECT MORPHOLOGY

and adducto r muscle s arisin g o n th e dorsa l wal l o f th e cranium . I n most o f th e apterygot e insects , however , an d i n th e Mandibulat a gen erally, th e mandibl e ha s a singl e poin t o f articulation , an d dorsa l an d ventral muscles , suggestin g tha t i t ha s bee n evolve d fro m th e basi s of a leglik e appendage provide d wit h th e usua l terga l an d sterna l pro motor and remotor muscles. Th e presence of a well-developed telopodite in th e for m o f a segmente d palpu s o n the mandibl e o f many Crustacea , and th e limblik e structur e o f th e correspondin g appendag e i n certai n Arachnida amply confirm th e le g origin of the arthropo d jaw. To understand the mor e specialized, though simpler, mandible of th e Pterygota, we mus t first stud y the structur e and musculatur e of the organ as found in apterygote insects and in other mandibulate arthropods . The leglik e form o f th e appendag e i s wel l show n i n th e pedipal p o f a

FIG. 73.—Mandibles and mandibular musculature o f Myriapoda. A , right mandibl e of a diplopod , anterio r view . B , lef t mandibl e o f Scutigera, anterio r view . C , righ t mandible o f same, dorsa l view .

phalangid (Fig. 47, B), which consists of a basis (Cxpd) provided with a large endite lobe (Bnd\ and a long telopodite of six segments that are clearly identical with the segments of the legs. Amon g the Mandibulata , the mandibl e appear s t o b e i n som e respect s mos t generalize d i n th e Diplopoda, sinc e i n thi s grou p it ha s certai n feature s suggestiv e o f th e structure o f a n insec t maxilla . Th e ja w o f th e Chilopod a i s evidentl y derived fro m a n orga n simila r t o th e diplopo d mandible . I n th e mor e generalized Crustace a an d Hexapod a the appendag e is more generalized in certai n ways , thoug h i t i s onl y i n th e Crustace a tha t i t retain s th e telopodite; but i n the highe r forms of these group s the mandibl e present s numerous specializations in its structure, musculature , and mechanism. The Mandible s of Diplopoda. —The diplopo d mandibl e (Fig . 7 3 A ) consists o f a larg e basal par t (Cxpd) an d a movabl e terminal lob e (Lc) . The scleroti c wall of the basi s is distinctly divide d into a proximal plate, or cardo (Cd) , and a distal plate, o r stipes (St). Th e musculatur e o f th e basis consist s o f tw o group s of fibers . Th e fiber s o f on e grou p form a single anterior muscl e (/) arisin g dorsally on the hea d wal l and inserte d on th e uppe r (anterior ) margi n of the stipes . Th e fiber s o f the secon d

THE HEAD A PPENDAGES

135

group (KL) for m numerou s muscle bundles connected with eac h mandible, those from opposit e sides taking their origin s on a thick media n ligament (2 ) from which they diverge into the card o and stipes of each mandible, th e tw o set s formin g thu s a stron g zygomatic adductor betwee n th e two jaws . Th e media n ligamen t i s supporte d fro m th e dorsa l wal l of the hea d by two large vertical muscles. The movabl e dista l lob e o f th e diplopo d mandibl e (Fig . 7 3 A , Lc ) is o f particula r interes t becaus e o f it s resemblanc e to th e lacini a o f a n insect maxill a (Fig . 78,Lc). Th e lob e is hinged to th e inne r distal angle of the stipes and is provided with a short stipital flexor muscle (flcs) arising withi n th e stipes , an d wit h a larg e cranial flexor (flee) arisin g o n the hea d wall and inserte d o n a strong apodem e of the inne r basa l angl e of th e lobe . The Mandibles of Chilopoda.—The chilopod mandible (Fig . 7 3 B, C ) is similar to the jaw of the diplopod, but the basis (B, Cxpd) is not subdivided, an d th e dista l lob e (Lc) is les s movable , sinc e i t ha s n o tru e articulation wit h th e basis , thoug h it i s flexible on the latte r an d is provided with both stipita l and crania l flexor muscles (flcs, flee) . Th e basi s is rotated o n its long axis by an anterior dorsa l muscle (/) and a posterior dorsal muscl e (J); and it i s provided wit h a ventral adducto r (KL), th e fibers o f whic h take thei r origi n o n a media n ligamen t supporte d o n a pair o f ventral apophyse s arisin g a t th e bas e o f the hypopharyn x (Fig . 61 A, HA). The Mandible s o f Crustacea. —The crustacea n mandible s presen t a great variety of forms, wit h many types o f mechanism resulting fro m th e different way s i n whic h th e organ s ar e articulate d t o th e hea d o r th e mandibular segment. I n th e mor e generalized groups of both the Ento mostraca an d th e Malacostraca , however , they hav e a type of structur e very simila r t o tha t o f the chilopo d jaws. Th e mandible s o f Apus an d Anaspides ar e goo d representative s o f this apparentl y generalize d type of structure . The mandibl e o f Anaspides (Fig . 7 4 A) consist s o f a n elongat e basi s (Cxpd) with a large endite lobe (End) and of a small three-segmented telopodite, or palpus (Tlpd). The basis is broadly implanted by its entire inne r surfac e o n th e membranou s latera l wal l of the mandibula r segment and is provided with a single dorsal point of articulation (a' ) with the tergum . Th e broa d termina l lob e (End) i s entirel y immovabl e o n the basis , bu t i t i s differentiate d int o inciso r an d mola r areas . Th e musculature o f th e Anaspides mandible s i s ver y simple . Eac h ja w i s provided with an anterior rotato r muscl e (/) an d a posterior rotato r (J), both arisin g o n th e dorsa l wal l o f th e mandibula r segment , an d wit h strong ventral adductor muscles (KL). The fibers of the adductors are separated int o tw o groups ; thos e o f on e grou p tak e thei r origi n o n a

136

PRINCIPLES OF INSECT MORPHOLOGY

median ligament (fc. ) arisin g from the ventra l body wall, those of the othe r group ar e continuou s from on e jaw t o th e othe r an d for m a zygomatic adductor. The mandible s of Apus ar e simila r t o thos e o f Anaspides, but t y lack palpi , an d th e adducto r apparatu s consist s onl y o f a large , dumb bell-shaped zygomati c muscle between the tw o jaws. Thi s sam e type of mandible i s foun d i n man y othe r crustacean s an d i s evidentl y th e on e from whic h the mor e specialized types have been evolved. The Mandible s o f Apterygote Insects.—Amon g th e Apterygot a th e mandibles resemble those of the Chilopod a and th e simple r Crustacea i n all group s excep t Lepismatidae , i n whic h they tak e o n specia l features characteristic o f th e mandible s o f Pterygota . I n al l insect s th e mandibular telopodite i s entirely absent .

FIG. 74.—Generalize d typ e o f mandible i n Crustace a an d Hexapoda . A , Anaspides tasmaniae, posterior view . B , Heterojapyx gallardi, dorsa l view . C , Nesomachilis maoricus, posterior view .

In th e Japygida e th e mandible s ar e deepl y retracte d int o th e head , but eac h consists o f a slender basis (Fig . 74, B , Cxpd) articulate d t o th e head by a single point o f articulation (a') , and ending distally in an elon???? ???????? ???? ?????? ??? ????????? ?? ???????? ???? ???????? ??? posterior rotator muscle s (7, J) arising on the dorsa l wall of the hea d and is equipped with two ventra l adducto r muscle s (KL). Th e fiber s o f one ???? ?? ???????? ??????? ???? ??? ??? ????????? ????? ????? ???????? on a pair o f sternal apophyse s of the hea d springing from th e bas e of th e hypopharynx (Fig . 6 1 B , HA). Thes e apophyses ^ a s w e hav e see n (page 118), are evidently th e prototype s o f th e anterio r tentoria l arm s in ??? ?????????? ??? ?????? ?? ??? ????? ???? ?? ??????? ????? ???? ??? opposite jaws are united upon a median ligament (z) and constitute a common zygomati c adductor. In th e Machilida e th e mandible s (Fig . 74 C) are exserted , bu t the y have the sam e essential structur e a s in Japygidae. Th e fre e dista l lob e of eac h i s differentiate d int o a slende r inciso r poin t an d a thic k mola r process. Th e muscl e fibers of th e adducto r apparatu s ar e dispose d i n two distinct groups, those of one group (KLf) forming a wide flat muscle

THE HEAD APPEN DAGES

137

attached medially on the correspondin g hypopharyngeal apophysi s (HA), those o f the othe r (KLz) convergin g upon a narrow median ligamen t (z) to form, with the corresponding group from the opposite jaw , a zygomatic adductor betwee n the tw o mandibles . Morphology o f th e Arthropo d Mandibles. —From th e brie f revie w just give n of the basi c structure of the mandible s i n the principa l group s of mandibulat e arthropods, i t i s eviden t tha t th e mandibl e ha s bee n evolved fro m a limb of the ambulator y type , and that th e modification s that hav e produce d th e mor e generalized forms o f the ja w ar e o f a comparatively simpl e nature. The bod y of the mandibl e correspond s to the coxopodit e of a general ized appendage ; th e telopodit e i s retaine d a s a palpu s i n man y o f th e Crustacea, bu t i n other group s it ha s been completel y suppressed. Th e projecting termina l lob e is an endit e o f the basis ; in th e Diplopod a thi s lobe is freely movable , an d i n both th e diplopod s an d th e chilopod s it i s provided wit h muscle s correspondin g t o th e muscle s of th e lacini a o f a generalized insec t maxilla . I n othe r group s th e termina l lob e lose s it s mobility an d become s solidl y fuse d wit h th e basis , i n consequenc e of which it s muscle s have disappeared . Th e anterio r an d posterio r dorsa l muscles o f the mandibula r bas e correspon d t o th e dorsa l promoto r an d dorsal remotor of a generalized appendage (Fig. 41, 7, J). The ventral muscles evidently represen t th e ventra l promotor s an d remotor s (K , L), which, bein g grouped together, becom e functionally the adductor s (KL) of th e generalize d mandible . Th e somati c end s o f th e adductor s ar e usually supporte d o n sternal apophyse s o f the ventra l head wal l or on a membranous fol d o f th e latte r o r o f th e apophyses . Th e membran e between th e end s o f some of the fiber s fro m opposit e jaw s may becom e detached and form a ligament uniting the opposing fibers, thus producing a zygomati c adductor betwee n the mandible s having no connection with the bod y wall. The Mandible s o f Pterygot e Insects. —The typica l mandibl e o f th e biting typ e in pterygote insect s i s quite differen t i n bot h it s mechanis m and musculature from the mandible of most Apterygota an d other arthropods. Th e pterygot e typ e o f mandible , however , i s foun d i n Lepis matidae, and a similar form of mandibular mechanism has been developed in some of the higher Crustacea . The ja w o f the mor e generalized mandibulate arthropods , a s we have seen, i s hinged to th e lowe r edge of the cranium , o r the mandibula r seg ment o f the body , b y a single point o f articulation (Fig . 7 5 A, a'), which evidently corresponds to the dorsa l articulation o f the basis of an ambula tory limb (Fig . 4 3 A, B, a) . Th e mandibl e o f Lepismatidae an d Ptery gota differs fro m a generalized mandible in that it has a secondary anterior dorsal articulatio n o n th e hea d (Fig . 7 5 B, c ) an d thu s acquire s a long

138

PRINCIPLES OF

INSECT MORPHOLOGY

axis of attachment (a'-c ) with a definitely limited transverse movement of abduction and adduction. B y this chang e from a pivotal to a dicondylic hinge articulation , th e primitivel y anterio r an d posterio r dorsa l muscles of th e ja w (A , I, J ) becom e respectively a dorsal abductor (B , /) an d a ?????? ???????? ???? ??? ??????? ??????? ?????? ????????? ?? ????????? but, wit h th e increasin g siz e o f th e dorsa l adductors , the y becom e of secondary importanc e an d ar e usuall y reduce d (B , KL) o r absen t i n higher forms . I n man y o f th e lowe r Pterygot a an d i n Lepismatidae , however, the ventra l adductor s persist; they ar e highly developed in the larvae of Ephemerida and Odonata, where they arise on the anterior arm s of th e tentorium , an d the y ar e represente d i n th e adul t Isopter a an d most Orthoptera , excep t Acrididae.

???? ???????????? ?? ????? ?? ?????? ?????????? ?? ?????????? ???? ???? ??? ????????? tion (a') . B , pterygote typ e with tw o articulations (a' , c) .

The typica l biting jaw of pterygote insects, therefore , by the acquisi tion of a long hinge line on the head , with anterior an d posterior articula tions, come s to hav e a transverse movement of abduction an d adductio n (Fig. 7 5 B) , an d th e primitiv e dorsa l promoto r an d remoto r muscle s (7, J ) com e t o be , respectively , dorsa l abductor s an d adductors . Th e dorsal adducto r increase s i n size , while the ventra l adductor s (KL) ar e reduced, an d finally, with the disappearanc e o f the ventra l muscles , th e adductor function i s taken ove r entirely by the dorsa l muscle. I n addi tion t o these change s in the moto r apparatus , the actio n o f the mandibl e undergoes an alteration b y a change in the slope of its axis. I n the more primitive condition , retained i n Lepismatidae, th e axi s of the ja w slope s downward fro m th e posterio r articulatio n t o th e anterio r articulation ; in mos t Pterygot a i t i s oblique in the opposit e direction, thu s givin g the tip of the jaw a posterio r motio n accompanyin g the movemen t of adduction. The structur e an d mechanis m of the bitin g typ e o f mandibles in th e Pterygota ar e wel l represente d b y th e mandible s o f a n acridi d grass hopper (Fig . 76). Eac h mandibl e (D ) is a thick, stron g appendage with

THE HEAD APPENDAGES

139

a broa d triangula r base , havin g it s mesa l surfac e differentiate d int o a distal toothed incisor lobe (o) and a proximal molar lobe (p). The jaw is broadly hinge d t o th e pleurostoma l margi n o f the craniu m by th e oute r edge o f its triangula r bas e an d ha s a stron g articulation wit h th e hea d at eac h end of the hinge line (A, C, c, a'). I t shoul d be observed that th e articular surface s o f th e mandibl e li e outsid e th e basa l membranou s connection of the ja w wit h th e head ; they ar e merely specialized point s of contac t betwee n th e mandibl e an d th e craniu m (Fig . 2 7 D) . Th e musculature of the grasshoppe r mandible consists only of a dorsal abduc tor an d a dorsal adductor. Th e abductor i s relatively small; it arise s on

FIG. 76.—Endoskeleta l structure s o f th e cranium , an d mandible s o f a grasshopper , Dissosteira Carolina. A , interior view of right half o f cranium. B , the tentorium , ventral view. C , same as A, but wit h clypeus, labrum, and right mandible and muscle s in place. D, right mandible and its apodemes , posterior view.

the latera l wal l of the craniu m and i s inserted o n a small apodem e (Fig . 76 D , 8 A p) attache d t o a flange of the oute r margi n o f the mandibula r base sufficiently far outside the axis line to give effectiveness to the muscle. The adducto r i s a hug e muscl e compose d o f severa l bundle s o f fiber s (C, 9a , 96 ) arisin g o n th e dorsa l an d latera l wall s o f the cranium , an d inserted o n a large apodeme (D , 9Ap) attache d a t th e inne r angl e of the mandibular base . Th e widt h o f th e mandibl e betwee n th e hing e lin e and th e poin t of attachment o f the adducto r apodem e gives great powe r to the adducto r muscl e in closing the jaw . Further modifications of the pterygot e mandibl e by which it becomes adapted t o variou s specialize d mode s o f feedin g wil l b e describe d i n Chap. XII . 5. TH E SUPERLINGUA E

The paire d ventra l lobe s o f th e hea d know n a s th e superlingua e (or " paraglossae ") ar e best develope d in apterygote insect s and in some

140

PRINCIPLES OF INSECT MORPHOLOGY

of th e lowe r members o f th e Pterygota . I n adul t insect s th e super linguae, i f discernibl e a s such , alway s appea r a s lateral lobe s o f th e hypopharynx (Fig . 7 7 C , D , Sliri), th e media n par t o f whic h (Lin) i s designated th e lingu a (o r "glossa")- I n embryoni c stages o f apterygot e insects, however , th e superlingua e aris e a s independen t lobe s o f th e ventral wal l of the hea d in the neighborhoo d o f the mandible s (A , Sliri), and for this reason they have been regarded by some writers as representing a pair o f postmandibular appendages , equivalen t t o the first maxillae (maxillulae) o f Crustacea, tha t hav e secondarily united wit h the media n

FIG. 77.—The hypopharynx o f insects and a crustacean. A , ventral view of head of embryo of Anurida maritime,, showing rudiment s of lingua (Lin) an d superlingua e (Slin). (From Folsom, 1900. ) B , embryoni c superlinguae o f Tomocerus. (From Hoffmann, 1911. ) C, hypopharyn x o f ephemeri d nymph . D , hypopharyn x o f Nesomachilis, posterior view . E, detache d super-lingu a o f same. F , hypopharyn x o f an isopo d crustacean , compose d of median lingua and lateral paragnatha (Pgn), posterior view.

lingua t o for m th e definitiv e hypopharynx . Th e shap e o f th e super linguae i n certai n apterygot e insect s i s somewha t suggestiv e o f a rudi mentary limb appendage (D, E), but i n others the form is so variable that little significanc e ca n b e attache d t o i t i n an y case . Th e embryoni c superlinguae of Collembola have been said t o b e innervated fro m specia l centers i n the suboesophagea l ganglion , but differen t claimant s disagre e as t o th e positio n o f th e allege d centers , an d mos t investigator s fin d no evidence of the presenc e either of such nerve centers or of a corresponding hea d somite . Accordin g t o Hoffman n (1911 ) th e superlingua e of Tomocerus aris e a s lobe s a t th e base s o f th e mandible s (B . Sliri), an d Silvestri (1933) , i n a stud y o f the developmen t o f the hea d appendage s of Japyx, shows conclusively that the superlinguae are formed in connection wit h the media n part of the hypopharynx as lobes of the mandibula r somite.

THE HEAD APPEN DAGES

141

The supe r linguae o f some adult insect s hav e a clos e resemblance t o the paragnath a o f certai n Crustace a (Fig . 7 7 F , Pgri), and , whe n th e paragnatha ar e united with a median lingua (Lin), th e resulting structur e is very similar t o the insect hypopharynx (C) . Th e paragnatha ar e sai d to be innervated b y branches of the mandibula r nerv e trunks, an d ther e is no evidence that the organs are other than secondar y lobes of the head . If th e paragnath a an d th e superlingua e ar e no t homologous , they ar e entirely analogou s structures develope d in crustacean s an d insects , bu t not i n the myriapods . 6. TH E MAXILLA E

The maxill a o f insect s havin g typica l bitin g mout h part s closel y preserves th e structur e o f a lim b tha t ma y b e suppose d t o hav e two

FIG. 78.—Diagra m o f th e structur e an d musculatur e o f a typica l insec t maxilla , sug gesting that the card o and stipes represent th e coxopodit e (Cxpd) an d the palpus the telopodite (Tlpd) o f a leg.

movable basa l endite s provide d wit h muscle s arising i n the basis . Th e telopodite i s relativel y smal l an d palpiform , bu t i t ha s th e essentia l structure o f the shaf t o f a leg beyond the cox a both i n its segmentatio n and in its basal musculature . General Structur e o f a Maxilla.—Th e maxillar y basi s i s typicall y elongate (Fig . 78 , Cxpd) an d i s implanted b y it s entir e inne r surface on

142 PRINCIPLES

O F INSECT MORPHOLOGY

the pleura l region of the hea d (Fig . 54 B) just behind the mandible . O n its dorsa l extremit y i t bear s a singl e condyl e (Fig . 78 , a") b y whic h i t articulates wit h th e lowe r latera l margi n o f th e terga l regio n o f th e cranium (T). A lin e o f flexur e i n th e uppe r par t o f th e oute r wal l divides th e basi s int o a proxima l cardo (Cd) an d a dista l stipes (St). Usually th e card o is flexed mesally o n the uppe r en d of the stipe s (Fig . 80 A , C) . A t it s ventra l extremit y th e stipe s bear s tw o endit e lobes . The mesa l lobe is the lacinia (Fig . 78 , Lc), the oute r lobe the galea (Go). Laterad o f the gale a arises the palpus (Pip), o r telopodite. Th e relativ e size an d th e segmentatio n o f the palpu s var y muc h in differen t insects . In man y orthopteroid form s ther e ar e two small segments in the bas e of the palpus which appear to be trochanters (TV) , the second followed b y a longer femurlik e segmen t (Fm), whic h i s separate d b y a characteristi c ????????????? ????? ???? ???? ? ??????? ??????? ???? ? ?????? ???? ?????????? two o r three segments . Th e proxima l articulatio n o f the palpu s on th e basis (ct) has a dorsoventra l movemen t suggestiv e o f that o f th e coxo trochanteral join t of a leg. Th e region of the stipes supporting the palpu s ?? ????????? ?????????????? ?? ? ????????? ??? ??? ??????????? ?? ??? palpus give s no reason for believing that th e palpife r i n any wa y represents a primitive segment of the maxillar y limb. The Muscle s o f the Maxilla.—Th e musculatur e of a typica l maxill a in biting insect s comprises muscles of the basi s that move the appendag e as a whole, muscles arising within the basis that move the terminal lobes and th e palpus , an d muscle s of the palpu s segments. The basa l musculatur e o f th e maxill a i n al l bitin g pterygot e insect s (Fig. 78) is very similar to the musculature of the mandibl e in apterygot e insects an d othe r arthropods . I t include s anterior an d posterio r dorsa l muscles (7 , J ) takin g thei r origin s o n th e terga l wal l o f the head , an d ventral adductor s (KL) arisin g o n th e tentoriu m (Tut) i n pterygot e insects, o r on the hypopharyngea l apophyses in apterygote insect s (Fig . 79 B) . Th e dorsa l muscle s may compris e an anterior rotator (Fig . 78 , rtmxa) an d a posterior rotator (J ) attache d o n the cardo , though usuall y the secon d is absent; but nearl y alway s there i s a larg e anterior muscl e (flee) inserte d o n the inne r angle of the bas e of the lacinia , which is thus a cranial flexor of the lacinia, resembling in every way th e simila r muscle of ??? ???????? ?? ??? ????????? ??? ????????? ????? ??? ?????? ??? ?????? ?? the ventra l adducto r muscle s (Fig . 78 , KLt), takin g thei r origi n o n th e ????????? ?????? ??? ??????? ????????? ???? ??? ??????? ????? ?? ??? ????? being inserted i n the cardo , those of the othe r o n the posterio r margi n of the stipes. Thes e muscles are here termed " adductors " because morphologically they correspond to the ventral adductor s of a primitive append age; i n function , however , they ma y produc e various movement s of th e maxilla.

THE HEAD APPENDAGES

143

The muscle s o f th e termina l lobe s o f th e maxill a alway s hav e thei r origin in the stipes . A stipital flexor of the lacinia (Fig. 78, flcs) is inserte d on the basi s of the lacinia , and a flexor of the galea (fga) o n the bas e of th e galea. Th e lob e muscle s ma y b e branched , bu t the y neve r occu r i n antagonistic pairs . The palpu s i s moved by a levator muscle (Fig . 78 , 0) an d a depressor (Q), bot h arisin g i n the stipes , bu t neve r in the palpifer . Thes e muscles are evidently homologues of the levato r and depresso r of the telopodit e of a le g that arise s i n th e cox a an d ar e inserte d o n th e trochanter ; th e depressor o f the palpus , however , never ha s a branc h correspondin g t o the bod y muscle of the le g trochanter. Th e segments of the palpu s have usually each a single muscle inserted o n its bas e (Fig . 8 0 C). Structural Variation s o f th e Maxillae.—Th e maxilla e ma y 'becom e variously reduce d i n differen t group s o f insects , particularl y i n larva l

FIG. 79.—Maxill a o f an apterygot e insect, Heterojapyx gallardi. view. B , dorsa l (anterior ) view.

A, ventra l (posterior)

forms, b y a suppressio n o f one o r bot h o f th e termina l lobe s or b y th e loss o f th e palpus ; bu t other tha n this , excep t i n th e Hemipter a an d Diptera an d certai n othe r piercin g or sucking insects, the y d o not depar t radically fro m th e generalize d type o f structure , an d the y ar e funda mentally alik e i n bot h Apterygot a an d Pterygota . Th e crustacea n maxillae ar e i n genera l almost rudimentar y appendage s b y compariso n with th e typica l insec t maxilla , but a stud y o f such forms a s Anaspides brings ou t a fundamenta l similarity i n th e basa l structure . Th e post mandibular appendages of the Chilopoda are small leglike structures, each composed of a large basis and a reduced telopodite, but endit e lobes may be present o n the base s of the firs t pair .

144

PRINCIPLES OF

INSECT MORPHOLOGY

The apterygot e maxill a i s scarcel y mor e primitiv e tha n tha t o f the lower Pterygota. I n the Thysanura the maxillae are suspended fro m the hypostoma l margin s o f th e craniu m (Fig . 8 2 A , MX ) an d hav e th e usual maxillar y structure . I n Japygida e th e maxilla e ar e mostl y con cealed with the mandibles in deep pouches above the labium in which they have a horizonta l position . Th e basi s o f eac h appendage , however , i s composed of a small car do (Fig. 79 A, Cd) and a long stipes (St). The stipes bears a short palpu s (Plp)j a strongly sclerotize d lacinia (Z/c) , and a weak gale a (Go). Th e palpu s an d gale a ar e somewha t separate d fro m the stipes and connected with the base of the hypopharynx; this condition has given rise to the idea that they are not parts of the maxilla but repre sent th e superlinguae . Th e fact , however , that bot h structure s ar e well

FIG. 80.—Maxilla e of Periplaneta americana. A , left maxilla , posterio r view . B , inner surface o f cardo. C , right maxilla , anterio r view .

provided wit h th e usua l muscle s arising i n th e stipes (B ) show s conclusively tha t the y belon g to th e maxilla . Th e lacini a i s equipped wit h a ????? ???????? ?????? ?????? ??? ? ????? ??????? ?????? ?????? ??????? ?? ??? back o f the head . Th e basa l musculatur e o f the appendag e consist s of an anterio r dorsa l muscl e (/ ) an d o f tw o group s o f ventra l adductor s (aded, adst) arisin g o n the hypopharyngea l apophysi s (HA). The generalize d structure o f the pterygot e maxill a is well exemplifie d in the maxill a of the cockroac h (Fig. 80) . Th e card o an d th e stipe s (A, Cd, St) are flexible on each other by a distinct hinge, and their planes for m an abrup t angl e at th e union . Th e card o is articulated t o th e craniu m by a basal condyl e (a") ; its surfac e i s marked b y th e lin e o f an interna l ridge (B, r), but it is not otherwise "divided"as it is sometimes said to be. The elongat e stipe s has a sutural groov e (A , q) near its inner edg e which forms internall y a submarginal ridge on which muscles are attached. I n some insect s th e surfac e o f the stipe s i s marke d b y othe r ridge-formin g

THE HEAD APPE NDAGES

145

grooves o r b y sutures , but , a s wit h th e cardo , thes e feature s ar e no t evidence that the stipe s i s a composite sclerite. The larg e termina l lobe s o f th e cockroac h maxill a aris e fro m th e ?????? ??? ?? ??? ??????? ??? ????? ???? ????? ???????? ??? ??? ??????? (Lc) mesal , bu t th e gale a als o partl y overlap s th e lacini a anteriorly . The gale a i s a broad , sof t lobe , widene d distally ; th e lacini a i s mor e strongly sclerotize d and ends in an incisor point provided with two apical teeth curve d inward , an d it s inne r margi n i s fringe d wit h lon g hairs . Both lobes are movable on the end of the stipes; the galea can be deflexed, the lacinia can be flexed mesally. The galea has a single muscle (C, fga) arising in the stipes. The lacinia has both a stipital flexor (flcs) and ? ??????? ?????? ??????? ??? ??? ??? ??? ?????? ???? ???? ????? ?? ????? insertion o n the lacinia . I n som e insects th e are a of the stipe s support ing th e gale a i s differentiate d a s a distinc t lob e calle d th e subgalecij but th e bas e of the true gale a is to b e determined by the poin t o f attachment o f its muscle. The lon g maxillary palpus of the roac h is composed of five segments (Fig. 80 A, B, Pip). There is no palpifer lobe differentiated in the stipes, and th e smal l basa l segmen t o f th e palpu s canno t b e mistake n fo r a palpifer, sinc e th e palpu s muscle s (C , Iplp, dplp) ar e inserte d upo n it . There are only three muscle.s within the palpus, the first being a muscle of the second trochanteral segmen t arising in the first trochanter, th e second a lon g ventral muscl e (T ) o f the tibia l segmen t arisin g als o i n th e firs t trochanter, and the third (V ) a muscle of the terminal segment having its origin i n th e tibia . Betwee n the thir d an d fourt h segment s ther e i s a characteristic femoro-tibial flexur e (ft). The basa l musculature of the roac h maxilla (Fig. 80 C) is of the usua l type. There i s bu t a singl e anterio r dorsa l muscl e (I ) inserte d o n th e cardo. Th e ventral muscle s arising on the tentorium consist of two large groups of fibers, one (adcd) inserte d i n the cardo , the othe r (adst) o n th e mesal ridge of the posterio r surfac e o f the stipe s (A , q). Thes e muscles, though the y ar e evidentl y th e primar y sterna l adductor s of the append age, giv e a movement of protraction to the maxill a because of the angula tion between the cardo and stipes, and because the stipes rests and moves against th e sid e of the hypopharynx . 7. TH E LABIU M

The insect labium is a composite structure. It s majo r par t is formed by th e unio n o f a pai r o f gnatha l appendage s closel y resemblin g th e maxillae, but th e orga n perhaps includes in its base a part of the sterna l region o f th e labia l somite . Th e componen t labia l appendage s ar e termed the second maxillae o f insects, an d ther e i s little doub t that the y correspond to the second maxillary appendages of Crustacea. I n certai n

146

PRINCIPLES OF INSECT MORPHOLOGY

Crustacea th e first maxillipeds are united i n a labiumlike organ attached to the head. In it s generalize d form , th e labiu m (Fig . 8 1 A ) consist s o f a fla t median part , o f tw o latera l segmente d palp i (Pip), an d o f fou r unseg mented terminal lobes (Glj Pgl). Structurall y th e organ is divisible into a free distal prelabium (Prlb) bearing the palpi and the terminal lobes, and a proximal postlabium (Plb) largel y o r entirel y adnat e o n th e posterio r o r ventral wal l of the head . Th e lin e o f flexibility between th e tw o part s may b e terme d th e labial suture (Ibs). Al l the proxima l muscle s o f th e labium ar e inserted o n the movabl e prelabium . The body of the prelabium is commonly termed the prementum (Prmf) ; the postlabium, therefore, may be called the postmentum (Pmt). Some writers designat e th e tw o primar y part s o f the labiu m "mentum " an d

FIG. 81.—Diagram s illustratin g th e fundamenta l structure o f th e insec t labium , an d the correspondenc e of its parts with those of a pair o f united maxillae . Plb, postlabium, consisting o f the postmentu m (Pmt) ; Prlb, prelabium, including the prementum , or labiostipites (Prmt), palp i (Pip), an d termina l lobes (Gl, Pgl).

"submentum," respectively, but mor e generally these name s are given to subdivisions o r sclerite s o f the postlabium . Unfortunatel y th e curren t terms given to the parts of the labium cannot b e made to fi t consistentl y with th e morpholog y o f the organ . Th e prelabiu m functionall y i s th e under li p o f th e insect , an d i t ha s bee n terme d th e "eulabium " b y Crampton (1928) ; bu t commonl y th e nam e "labium " applie s t o th e entire organ , an d term s compose d wit h "mentum " ar e give n t o it s several parts or sclerites. Onl y by a radical chang e in the labia l nomen clature coul d it s numerou s inconsistencie s b e eliminate d (se e Walker , 1931). The Prelabium.—Th e prelabiu m (Fig . 8 1 A , Prlb) i s th e movabl e distal par t o f the labium . I t i s compose d o f a centra l body , th e prementum (Prmt), of the labial palpi (Pip), and of the terminal lobes (Gl, Pgl), th e last collectively constitutin g the ligula (Lig). The Prementum. —When th e labiu m i s compare d wit h a pai r o f maxillae, it becomes evident that the prementu m (Fig. 81 A, Prmt) repre-

THE HEAD APPENDAGES

147

sents the unite d stipites of a pair of maxilla-like appendages (B , Sf), sinc e it is the part of the labiu m in which arise the muscles of the palp i and th e ligular lobes . Th e prementum , therefore , i s appropriatel y designate d pars stipitalis Idbii, o r labiostipites. Th e paire d origin of the prementu m is suggested ofte n b y a distal clef t betwee n its stipital components (Fig . 83) or by th e presenc e of paired sclerites in its ventra l wal l (Fig . 15 8 C). Lateral lobe s of the prementu m bearing the palp i are frequentl y differ entiated fro m th e media n are a an d ar e termed th e palpigers (Fig . 8 1 A, Pig), since they are analogous with the palpifers of the maxillae (B, Plf). The siz e o f th e prementu m varie s muc h i n differen t group s o f insects . In adul t Coleoptera , for example, it is often a relatively smal l part of the labium (Figs. 67 C, 68 A, 158 C, 160 B, Prmf), while in the higher Hymenoptera i t become s the majo r piec e of the appendag e (Fig . 15 8 F), an d in

FIG. 82.—Articulation of the gnatha l appendages on the cranium . A , an apterygote insect, Nesomachilis, wit h single mandibular (a') , maxillar y (a") , an d labial (a" 7) articulation o n each side. B , head o f a myrmeleonid, posterior view.

the prehensil e labium o f odonate larva e i t i s a larg e spatulate lob e (E ) ??????? ??? ???????? ????? ????? ?? ??? ?????? ??????? ????? ??? ???? much confusio n a s t o th e identit y o f th e prementu m i n comparativ e studies o f the labium , bu t a n examinatio n o f the labia l musculatur e will seldom fai l t o giv e a positiv e determinatio n o f the limit s o f the stipita l region o f the labium , which is the prementum . The muscles inserted on the prementu m comprise two pairs of cranial adductors arisin g o n the tentoriu m (Fig . 8 4 A, ladlb, 2adlb), an d a pai r of media n retractors (o r flexors) arising on the postmentu m (Figs . 8 1 A, 84 A , rst). Th e muscle s that tak e thei r origi n withi n th e prementu m include the muscles of the palpi (Iplp, dplp) and the muscles of the terminal lobes (fgl,fpgl), togethe r usually with muscles associated with the duct o f the labia l gland s (Fig . 8 4 A, Is, 2s). The sclerotizatio n o f th e ventra l wal l o f th e prementu m i s highl y variable. Typicall y it form s a single premental plate (Figs . 5 9 B, 83 B, 158 E, F, Prmf), but often it is broken up into two or more sclerites.

148

PRINCIPLES O F INSECT MORPHOLOGY

In som e adul t Coleopter a ther e i s presen t a pai r o f latera l prementa l sclerites (Fig . 15 8 C, Prmt), whil e in adul t Neuropter a an d man y larva l Coleoptera th e prementa l sclerotizatio n i s characteristicall y subdivide d into a distal plate or group of sclerites (Figs . 67 B, 68 B, 82 B, 15 8 A, B, 159 A, B, db y o r a, 6) , giving insertion t o th e tentoria l adducto r muscle s (Fig. 15 9 B), an d int o a proximal plate or pair o f sclerites (c) , on which ??? ???????? ??? ?????? ????????? ??????? ????? ???? ??? ??????????? The surfac e o f the prementum , again, is sometimes entirely membranous . The Labial Palpi.—The palp i o f the labiu m ar e usuall y shorte r tha n the maxillar y palpi and are commonly three segmented (Fig . 83). Eac h is provide d wit h levato r an d depresso r muscle s takin g their origin s i n the prementum (Figs. 81 A, 83 B, 84 A, Iplp, dplp). Generally, therefore, the palp i ca n be distinguished fro m th e termina l lobe s of the labiu m b y their provisio n with antagonistic muscles. I n som e cases, as in odonat e larvae, an d possibl y i n adul t Diptera , th e ligula r lobe s ar e suppresse d and th e palp i becom e the movabl e termina l appendages o f the labiu m (Fig. 15 8 E, Pip). The Ligula. —The terminal lobe s of the labiu m vary much in relativ e size and shape in different insect s (Fig . 83) and ar e sometimes subdivide d (A); rarel y the y ar e absent , bu t the y ma y b e variousl y united . Col lectively th e lobe s (o r the dista l par t o f the labiu m includin g the lobes ) constitute th e ligul a (Fig. 81 A, Lig). The labiu m typicall y ha s fou r termina l lobe s born e o n th e dista l margin o f the prementu m (Fig . 8 1 A). Th e media n pai r ar e th e glossae (Gl), the lateral pair the paraglossae (Pgl). The labial lobes are clearly the laciniae an d galea e of the unite d labia l appendage s (B , Lc, Go). The y have th e sam e typ e o f musculatur e a s th e lobe s o f th e maxillae , eac h being provided with a single or branched flexor arising in the prementu m (Figs. 8 1 A, 83 B, 84 A), but neve r having a pair o f antagonistic muscles. The ligula r lobe s ar e sometime s confluen t a t thei r bases , sometimes th e pair o n each side are united, or , again , th e tw o glossae are combine d t o form a single median lobe (Fig. 15 8 D, GT), which , as in the bees , may b e prolonged i n a slende r tongue-lik e orga n (F , Gl)-, and , finally , th e fou r lobes ma y b e fuse d i n a singl e ligular flap terminating th e labiu m (Figs . 82 B, 158 B, Lig). O n the othe r hand, the labia l lobes are often reduced , and on e or both pair s ma y b e absent. I n man y holometabolou s larva e having the hypopharyn x adnat e upo n the anterio r (o r dorsal) surfac e of the prementum , the ligul a is fused wit h the en d of the hypopharyn x in a composite termina l lob e bearin g th e orific e o f th e labia l gland s a t it s extremity (Figs . 16 1 B, 16 4 C). The Postlabium.—Since the postlabium (Fig. 81 A, Plb) has no appendicular parts , i t consist s entirel y o f th e proxima l regio n o f th e bod y of th e labium . T o preserve uniformity in the nomenclatur e of the labia l

THE HEAD APPENDAGES

149

regions, therefore, the surface of the postlabium, as distinguished fro m th e distal prementu m o f th e prelabium , ma y b e terme d th e postmentum (Pmf). The postlabial sclerotization is so variable that the limits of the postmentum cannot be determined by a study of the labial sclerites alone. In practic e the postmentu m i s to be identified a s that part of the labiu m lying proxima l t o th e insertions o f al l th e labia l muscles . Th e onl y muscles having their origin s on the postmentu m ar e the media n muscles of th e labiu m (rsf) tha t extend from th e postmentu m t o th e prementum .

FIG. 83.—Types of generalized structure of the labium . A , Machilis. B , Termopsis, soldier form : bod y of labium compose d of prementum (Prmt) an d postmen-tu m (Pmt) only . C, Blatta orientalis, with small mental sclerites (Mt) i n distal part of postmental region . D , Scudderia, wit h well-develope d mentu m (Mt) an d submentu m (Sm£).

Though th e postmentu m i s usually broadl y adnat e o n the posterior , o r ventral, wal l o f th e head , it s dista l par t sometime s project s t o giv e support t o th e movabl e prelabium. I n th e larva e o f Odonata th e basa l stalk o f th e labiu m (Fig . 15 8 E) appear s superficiall y to b e th e post mentum, bu t th e musculatur e an d mechanis m o f the orga n ar e her e so different fro m thos e i n a labium of typical structur e tha t the homologies of it s part s becom e questionable. A recent paper by Munschei d (1933 ) gives a good account o f the musculatur e of the odonat e labium, bu t th e mechanism o f the larva l orga n is not satisfactoril y explained. In th e more generalized insects, th e labiu m joins the nec k membrane on a line between the posterio r tentoria l pit s (Figs . 5 9 B, 82 A), and th e proximal angle s of its basa l plat e (a'" ) ar e attache d t o th e postoccipita l

150

PRINCIPLES O F INSECT MORPHOLOGY

rim o f the hea d jus t behin d o r belo w th e tentoria l pits , i n lin e with th e articulations o f the maxilla e and mandible s o n th e subgena l margin s of the craniu m (a" , a'). Th e postmentu m thu s correspond s i n positio n to the cardine s of the maxilla e and would appear, therefore, to include in its composition the cardines of the primitive labial appendages (Fig. 81 B, Cd). Since , however, the secondar y median orific e o f the labia l glands , which belong s to th e vente r o f the labia l somite , move s forwar d durin g development until it come s to li e anterior (o r dorsal) to th e base s o f the labial appendages (Fig. 155, SIO), it seems probable that the postmentum contains in its median part also an element derived from th e venter of the labial segment . Th e postmentum thu s may be regarded as a composite structure forme d b y the unio n of the cardina l parts of the labia l append ages wit h th e primitiv e sternu m o f th e labia l segment , i n whic h character i t woul d resemble the definitiv e sterna o f most o f the succeedin g body segments. Som e writers regard the entir e postmentum as a sternal derivative, but in this case it must be assumed that the labial cardines are absent, an d that the sternum of the labial segment has become interposed between the tergum and the appendages in such a way that the latter are supported b y the sternu m alone . I n th e Machilida e th e lateral areas of the postmentu m (Fig . 8 3 A , Cd ) ar e separate d b y fain t line s fro m a triangular median area (stn) in a manner suggestive that the postmentum (Pmt) ha s a cardinosternal composition. The sclerotizatio n o f the postlabiu m form s i n man y insect s a singl e postmental plat e (Fig . 81 A, Pmt). Thi s plate may cover the entir e are a of th e postmentum , a s in Apterygota (Fig . 83 A), termites (B) , and some Neuroptera (Fig . 15 8 B), though again it may occupy only the basal part of th e postmentu m (Fig . 8 2 B) , or , a s i n mos t caterpillars , i t ma y b e reduced to a small sclerite (Fig. 164 C, pmt) in the otherwise membranous postmental wall . O n the othe r hand , th e postmenta l are a of the labiu m may b e entirel y membranous , as in hymenopterou s larvae (Fig . 161 , B , D, F, Pmt). In th e majorit y o f Orthopter a an d adul t Coleopter a th e postlabia l area, o r postmentum, contain s tw o distinc t plates . Th e dista l plat e i n such cases is generally called the mentum (Figs. 59 B, 67 C, 83 D, Mt), the proximal one the submentum (Smt). The mentum always lies proximal to the insertion s o f the media n muscle s o f the prementum , whic h arise o n the submentu m whe n th e postmentu m contain s tw o plates (Fig . 8 4 A, rst). Th e mentu m an d submentu m i n som e insects appea r t o b e differ entiations of a more primitive postmental plate; in others the mentu m is evidently a secondary sclerotization in the membranous distal par t of the postmentum. I n adul t Coleopter a th e mentu m i s typicall y larg e an d conspicuous (Fig . 15 8 C, Mt), bu t i n th e Orthopter a i t i s often reduce d (Fig. 8 3 C) an d i s entirely absen t i n Mantida e an d Acrididae.

THE HEAD APPE NDAGES

151

The proxima l angles of the postmentu m (o r of the submentum ) generally preserve the primitiv e clos e association o f the labia l bas e wit h th e posterior tentoria l pits; but the y may become far remove d from th e foramen magnu m i f th e postgena l region s o f th e craniu m ar e elongate , o r especially when a gular plate bridges the spac e between the postoccipita l margins proxima l t o th e labiu m (Figs . 6 7 C, 68 , Gu). Th e bas e o f th e labium lose s it s associatio n wit h th e posterio r tentoria l pit s onl y whe n mesal lobes of the hypostoma l area s o f the craniu m are develope d proximal to its base (Fig. 164 C, Hst) or form a complete bridge between the labium an d th e forame n magnum (Fig . 6 5 B, C) . Musculature o f th e Labium. —The muscle s o f th e labiu m ma y b e divided into four groups. Thos e of the firs t grou p are the muscle s of the palpi an d th e termina l lobes ; those o f the secon d include several pairs of muscles inserted nea r the orific e o f the duc t o f the labia l glands; those of the third group are the media n muscles extending from th e postmentu m to th e prementum ; an d thos e o f the fourt h ar e th e extrinsi c muscle s of the labium arising o n the tentorium an d inserted o n the prementum . The muscle s of the palp i an d th e termina l lobe s o f the labiu m (Fig . 81 A ) correspon d to muscle s of the palpi , laciniae , an d galea e of a pai r of maxilla e (B) . Eac h labia l palpu s i s provide d wit h a levato r an d a depressor muscle arising in the prementum (Fig. 84 A, Iplp, dplp). The glossae an d paraglossa e hav e eac h a flexo r muscl e takin g it s origi n in ??? ????????? ??????????? ??? ??? ??????? ???? ?? ??????? ??????? ?? ??? head wal l correspondin g to th e crania l flexors of the maxillar y lacinia e (Fig. 78, flee) . The labial muscles associated with the orific e of the labial, or salivary, glands hav e n o homologue s in th e maxillae , an d the y ar e no t alway s present in the labium. Generall y there are two pairs of them, which arise in the prementu m an d converg e to th e labia l wal l of the salivar y pocke t formed a t th e junctio n o f th e hypopharyn x wit h th e prementum , int o which open s th e salivar y duc t (Fig . 8 4 A , B , 2s , 3s) . Thes e labia l "salivary muscles " an d th e pai r o f opposin g muscle s fro m th e hypo pharynx (Is ) wil l be more fully describe d in Chap . XII . The media n muscles of the labium that exten d from th e pbstmentu m to the prementum als o have no homologues in the maxillae, and they ar e not alway s present i n the labium . The y aris e on the postmentum , o r on the submentum when there are two plates in the postlabial region, and are always inserted on the prementum (Fig. 84 A, B, rsf). They are, therefore, possibl y sternostipita l muscles, since there ar e never cardinostipita l muscles i n th e maxillae . Usuall y thes e muscle s ar e retractor s o f th e prementum, but i n some cases they serv e to flex the prementu m on th e postmentum (Fig . 15 9 E).

152

PRINCIPLES OF INSECT MORPHOLOGY

The extrinsi c muscle s of th e labiu m compris e tw o pair s o f muscles having thei r origin s o n th e tentoriu m an d thei r insertion s o n th e pre mentum (Fig . 8 4 A, B, ladlb, 2adlb). Thes e muscle s clearly correspond to th e tentorial adductor s of the maxillae , and i n a morphological sense, therefore, they may be termed the labial adductors, though in their actua l function it is probable that they produce various movements of the labium

FIG. 84.—Th e labiu m an d associate d structure s o f Orthoptera . A , labial muscula ture oiGryllus assimilis, dorsal (anterior ) view . B , diagram o f orthopteroid hypopharynx, salivarium, an d labium , lateral view. C , salivarium an d unde r surface of hypopharynx of Gryllus. D , salivariu m an d bas e o f hypopharyn x o f a mantis , Paratenodera cinensis, dorsal view. E , diagrammati c section o f salivarium o f Paratenodera.

besides tha t o f adduction . On e pai r o f the adductor s i s inserted ante riorly, o r dorsally , o n th e prementu m (ladlb), th e othe r posteriorly , o r ventrally (2adlb). I n som e o f th e highe r insects , a s i n th e bee s (Fig . 163 C), th e labia l adductor s ma y tak e thei r origi n on the -cranium , bu t this condition is evidently a secondary one resulting fro m a migration of the muscl e bases from th e tentoria l arm s to the adjacen t cranial walls in order to give the fibers greater length and increased effectiveness. Ther e are no head muscles inserted o n the postmentu m corresponding to eithe r the crania l muscle s or the tentorial adductor s o f the maxillar y cardines , but th e absenc e of these muscle s in th e labiu m i s evidently consequen t upon th e usua l immobility of the postmentum .

THE HEAD APPENDA GES

153

Associated wit h th e anterio r (o r dorsal ) adductor s o f th e labiu m there i s usuall y presen t i n generalize d insect s a pai r o f retractors o f the hypopharynx (Fig . 8 4 B , rhphy). Thes e muscle s tak e thei r origin s on the posterio r bridg e of the tentoriu m (Tnt) an d ar e inserted laterall y on the bas e o f the hypopharyn x (Hphy), wher e they ar e attache d t o th e plates (w) o f the latter , when these plate s ar e present . 8. GLANDS OF THE HEAD APPENDAGES

Associated with the mout h parts of insects is a series of paired glands, which perhap s ar e coxa l gland s o f th e gnatha l appendages . Som e writers hav e attempte d t o correlat e thes e gland s wit h th e nephridia l glands of Crustacea, bu t th e hea d glands of insects appea r t o b e entirel y of ectodermal origin. Gland s occur also in connectio n with the antennae , but i t i s doubtful if they belong to th e serie s of gnathal glands . Antennal Glands.—Gland s connecte d wit h th e antennae , s o fa r a s observed, are not o f common occurrence in insects. I n a n ant, Myrmicd rubrcij however , Jane t (1894 , 1898 ) ha s describe d a grou p of one-celled antennal glands , the duct s o f which open separately i n a small pit o n the rim o f the antenna l socket . I n th e roac h Periplaneta americana a smal l coiled tubular gland , mentione d b y Bugnio n (1921) , open s at th e bas e of each antenna . Perhap s a further searc h will show that antenna l gland s are more generally present i n insects than the fe w record s of their occurrence would indicate. Mandibular Glands.—Gland s associate d wit h th e mandible s ar e known t o occu r i n Apterygota , Isoptera , Orthoptera , Coleoptera , Tri choptera, larva l Lepidoptera , an d Hymenoptera . I n th e Apterygota , Willem (1900 ) report s th e presenc e of head gland s i n Orchesella opening on eac h sid e o f th e hypopharyn x nea r th e base s o f th e mandibles , an d Bruntz (1908 ) describe s mandibula r gland s i n Machilis maritima a s " anterior cephali c glands/ ' eac h o f whic h consist s o f a larg e racemose glandular mass with a principal lobe in the head and a smaller one in th e thorax, th e duc t extendin g ventrall y fro m th e forme r t o it s openin g in the preoral cavity at the base of the mandible. I n the Orthoptera, Suslo v (1912) foun d mandibula r gland s i n Mantida e an d Blattida e bu t dis covered non e i n Gryllidae , Tettigoniidae , o r Acrididae . Th e gland s of Mantis religiosa, he says, consis t eac h of a thick-walled glandular sa c an d a thin-walle d reservoir , th e secon d opening to th e exterio r mesa d o f the posterior angl e of the mandible . I n th e Hymenopter a als o the gland s of the mandible s ar e saclik e wit h thic k cellula r walls . Th e mandibula r glands attai n their highes t developmen t i n certai n lepidopterou s larvae , in which they hav e the for m o f long tubes extendin g often fa r bac k int o the body cavity. Th e secretion of the mandibula r gland s probably has a

154

PRINCIPLES O F INSECT MORPHOLOGY

"salivary" function i n mos t cases ; th e siz e o f the gland s i n som e cater pillars may be correlated with the transformation of the ordinary salivar y glands (labia l glands) into silk-formin g organs. Maxillary Glands.—Th e presenc e o f maxillar y gland s ha s bee n reported i n Protura, Collembola , Heteroptera, the larva e o f some Neuroptera an d Trichoptera , an d Hymenoptera ; the y occu r als o i n som e coleopterous larvae . Th e maxillar y gland s ar e usuall y smal l an d inconspicuous, but in certain prionid larvae (Orthosoma) they consist of long convoluted tubes opening mesad of the maxillary bases and extending far bac k in the bod y cavity . Labial Glands.—Th e gland s o f the hea d appendage s generall y mos t highly develope d in insect s ar e thos e o f the secon d maxillae, th e duct s of whic h ar e unite d i n a commo n median outle t tub e (Fig . 8 4 B , SID) that opens typically in the pocket of the ventral wall of the head betwee n the bas e of the fre e par t of the labiu m an d th e bas e of the hypopharyn x (Slv). Thes e gland s ar e commonl y know n a s th e " salivary glands" ; but sinc e their function i s variable and has not been definitely determine d in many cases , they ar e better termed the labial glands. In the embryo the labial glands originate as paired invaginations of the ectoderm just behind th e base s of the rudiment s o f the secon d maxillary appendages. A s development proceeds , the tw o orifice s approac h eac h other an d unite mediall y on the vente r o f the secon d maxillary segment . At the sam e time, the appendage s of this segment also come together an d unite b y thei r mesa l edges . Meanwhile , however, the media n apertur e of th e gland s has moved forward, s o that, when the labiu m is formed b y the fusion o f the second maxillary appendages, the outlet of the glands lies in the ventral wall of the head anterior to the base of the labium. Labia l glands ar e presen t i n al l th e principa l order s o f insect s excep t Coleoptera. The siz e and shape of the labia l glands are highly variable in differen t insects. Usuall y the gland s lie in the thorax , bu t the y ma y have a part in th e head , an d the y ofte n exten d int o th e abdomen . Typicall y the y are simple or convoluted tubes, bu t the y may be branched or take on th e form of dense racemose masses. A part o f each lateral duct is sometimes enlarged to form a reservoir. Th e secretion of the labial glands generally has some function connecte d with feeding, thoug h not necessaril y that of a digestiv e fluid, for in blood-sucking insects i t ma y hav e inflammatory and anticoagulator y properties . I n lepidopterou s an d hymenopterou s larvae the labia l glands are silk-producing organs. Severa l writers hav e attributed a n excretory function t o th e labia l glands of Apterygota based on thei r reactio n t o ammoniacarmin e and indigocarmin e injecte d int o the body, but as in the case of the so-called " nephrocytes "(see page 415) this test perhaps doe s not necessaril y indicate an excretor y function .

THE HEAD APPEND AGES

155

The salivar y pocket, or salivarium (Fig . 84 B, Slv), a t th e bas e of th e hypopharynx i n generalize d insects, int o whic h open s th e duc t o f th e labial glands , is of much interest becaus e of its variou s modifications i n the highe r order to for m a n orga n for activel y expelling th e secretio n of the glands. I t becomes the " salivary syringe "of Hymenoptera, Diptera, and Hemiptera , an d th e "sil k press " o f lepidopterou s larvae . Th e salivarium i n both it s generalize d and it s specialize d forms wil l b e mor e particularly describe d in Chap. XII . GLOSSARY OF TERMS APPLIED TO THE HEAD APPENDAGES Antennae (Ant). —The appendicula r organ s of the procephali c regio n of the hea d innervated fro m th e deutocerebra l lobes o f the brain ; called firs t antennae, or antennules, in Crustacea ; absen t i n Chelicerata . Cardo (Cd). —The proxima l subdivision o f a maxillary appendage. Chelicerae.—The firs t pai r o f appendage s o f adul t Chelicerata , innervate d fro m the tritocerebra l gangli a of the brain ; equivalent t o the second antennae of Crustacea. First Maxilla e (IMx). —The secon d pai r o f appendage s o f the gnatha l regio n of the head ; in insects calle d simply "th e maxillae." Flagellum (Fl). —The par t o f the antenn a dista l to th e pedicel , typically filamentous, bu t o f various forms, usuall y subsegmented o r multiarticulate. Galea (Ga). —The oute r endit e lob e o f a maxilla , provide d wit h a muscl e arising in the stipes . Glossae (Gl). —The tw o media n ligular lobe s of the labium , eac h provided wit h a muscle arising in the prementum . Gula (Gu).—A media n ventra l plat e o f the hea d o f some insects, develope d a s a sclerotization o f the gula r regio n of the nec k proximal to th e posterio r tentorial pits , continuous with the basa l plate of the labium . Labial Gland s (SIGl).— The usua l "salivar y glands" o f insects , openin g b y a media n duc t betwee n th e bas e o f the hypopharyn x an d th e labium , o r o n th e hypopharynx. Labial Sutur e (Z6s).—Th e sutur e o f the labiu m betwee n th e prementu m an d th e postmentum, alway s distal t o the mentu m when the latter is present . Labiostipites (Lst). —The prementum , o r that par t o f th e labiu m forme d b y th e stipites of the componen t labial (secon d maxillary) appendages. Labium (Lb). —The posterio r median appendag e o f the insect head forme d b y th e union o f the secon d maxillae. Lacinia (Lc). —The inne r endit e lob e of a maxilla, provided wit h a muscle arising in the stipes , an d often wit h a second muscle arising on the crania l wall. Ligula (Lig). —The termina l lobe s of the labiu m collectively , o r a terminal part of the labiu m forme d b y th e unio n of the lobes . Mandibles (Md). —The firs t pai r of appendages of the gnatha l regio n of the hea d in the Mandibulata ; bitin g jawlike organs in their generalize d form . Mandibular Glands.— A pai r o f gland s ofte n presen t i n insect s openin g mesall y at th e base s of the mandibles . Maxillae (Mx). —The firs t an d secon d maxillar y appendages , o r specificall y i n insects th e first maxillae. Maxillary Glands.—Gland s present in some insects opening mesally a t the bases of the maxillae .

156 PRINCIPLES

O F INSECT MORPHOLOGY

Maxillipeds.—The thre e pair s o f appendage s i n Crustace a followin g th e secon d maxillae; the first pair sometime s (Amphipoda ) unite d t o for m a labiumlike structure attached t o the head . Maxillulae.—TLe firs t maxilla e o f Crustacea . Mentum (Mt). —A dista l plate o f the postlabiu m betwee n the prementu m and th e submentum. (Secondary submental plate, Walker, 1931. ) Palpifer (Plf). —A lob e of the maxillar y stipes bearin g the palpus . Palpiger (Pig). —A lob e of the stipita l region of the labium , or prementum, bearing the palpus . Palpus (Pip). —The telopodit e o f a gnathal appendage . Paraglossae (Pgl). The lateral ligular lobes of the labium, each with a muscle arising in the prementum . Paragnatha (Pgn). —A pai r o f lobe s o f th e gnatha l regio n o f Crustace a situate d between the mandible s an d the first maxillae . Pedicel (Pdc). —The secon d segmen t o f the insec t antenna , containin g a specia l sense organ , th e orga n o f Johnston. Pedipalps.—The secon d appendage s o f adul t Chelicerata , correspondin g t o th e mandibles o f Mandibulata . Postantennal Appendage s (Pnt). —The appendage s o f th e tritocerebra l somite : the chelicera e of Chelicerata, the secon d antennae o f Crustacea, embryoni c rudiments in some Hexapoda, absen t i n Myriapoda . Postmentum (Pmt). —The postlabium , or basal part of the labium proximal to th e stipital region, or prementum; when sclerotized, containing eithe r a single postmenta l plate, o r a distal mental plate and a proximal submental plate. (Submentum, Walker , 1931.) Preantennae (Prnt). —Theoretically a pai r o f primitiv e procephali c appendage s anterior to the antennae; possibly represented in Scolopendra and Dixippus by a pair of embryoni c preantennal lobes ; absent i n all adult arthropods . Prelabium (Prlb). —The dista l part of the labium , comprisin g the prementum , th e ligula, and the palpi. (Eulabium.) Pfementum (Prmt). —The stipita l region of the labium , containin g the muscles of the palp i an d th e ligula r lobes , an d givin g insertio n t o th e crania l muscle s o f th e labium. (Mentum, Walker , 1931. ) Salivary Gland s (SIGl). —See labial glands. Scape (Scp). —The basa l segment o r stalk o f the insec t antenna . Second Antennae.—Th e appendage s o f th e tritocerebra l somit e o f Crustacea . (See postantennal appendages.) Second Maxilla e (2Mx). —The thir d pai r of gnathal appendages ; in insects unite d in the labium . Stipes (St). —The dista l subdivision o f a maxilla, bearing the endit e lobe s and th e palpus, an d containin g th e palpa l an d lobe muscles. (Plura l stipites.) Subgalea (Sga). —A lob e o r subdivision o f the maxillar y stipes bearin g the galea . Submentum (Smt). —A proxima l plate o f the postlabium ; when continuous with a gular plate the submentum lies distal to the posterior tentorial pits. (Primary submental plate, Walker, 1931. ) Superlinguae (Slin). —A pai r o f ventral lobe s o f the insec t hea d simila r i n som e respects t o th e paragnath a o f Crustacea, develope d fro m th e mandibula r somit e an d united wit h the lingu a in the hypopharyn x of adult insects .

CHAPTER VII I THE THORA X The thora x o f a winge d insec t i s a highl y perfecte d bi t o f anima l machinery. Insect s ar e unsurpasse d flyers , an d fe* v othe r creature s ca n make mor e effectiv e an d diversifie d uses o f their legs . A thorax o f th e insect typ e is exclusively a hexapod structure; it distinguishes the insect s and proturans from all their relatives. Othe r members of the Arthropoda may have a body section calle d th e "thorax, " bu t it s seg ments d o no t correspon d t o thos e o f th e insect thorax , an d it s function s ar e b y n o means a s centralize d o r specialized . Th e thorax o f the Hexapod a consist s o f the thre e body segment s followin g th e gnatha l seg ments (Fig . 2 3 C , Th)9 which are designated, respectively , th e prothorax (Fig . 85 , Thi), th e mesothorax (TA 2), an d th e metaihorax (Ths). Each segment bears a pair of legs (L) , and th e secon d an d thir d segment s carry th e wing s (Wz, Ws) i n alate Pterygota. The thora x contain s th e muscle s of the leg s and wings , and th e thoraci c gangli a ar e th e chief center s o f contro l fo r bot h set s o f appendages. Between th e thora x an d th e hea d i s a narrowed, mostl y membranou s par t o f th e trunk formin g th e neck , o r cervix (Cvx). 85.—Diagra m showin g Though th e nec k i s probabl y a composit e the FIG. contiguit y of the terga l sclerregion forme d fro m th e labia l an d th e pro - ites i n th e dorsu m o f th e wing bearing regio n o f th e bod y t o thoracic segments , i t i s mor e convenientl y prevent longitudina l movemen t of th e bac k plates. treated a s a part o f the thorax . 1. EVOLUTIO N O F TH E THORA X

The thora x mus t hav e bee n evolve d ver y earl y i n th e phylogeneti c history o f the Hexapod a a s a locomotor section o f th e bod y through th e specialization o f it s appendage s fo r mor e activ e progression . I n th e Apterygota as well as the Pterygota the thorax is distinctly differentiated 157

158

PRINCIPLES O F ItfSECT MORPHOLOGY

from th e abdome n i n th e structur e o f it s segments , showin g that th e inception of the thora x as a body region long antedated th e acquisitio n of wings. Th e thoraci c region of the embry o (Fig . 2 3 C, TK) i s well developed a s th e leg-bearin g par t o f the bod y a t a stag e whe n the gnatha l segments (Gri) ar e yet distinc t and show no evidence of their futur e union with the procephali c lobes (Prc). Concurren t wit h the specializatio n of the thoraci c appendages as organs of locomotion, the abdomina l appendages were lost (Fig . 2 4 D), an d th e futur e gnatha l appendage s assumed functions accessor y to feeding . Whe n the gnatha l segment s wer e then finally combined withihe protocephalon to become a part of the definitive head (E , H), th e hexapod s appeared i n their modern three-part form . The wings are acquisitions develope d comparatively late in the evolution o f insects, thoug h they are full y forme d i n the earlies t know n fossil forms. Th e wings are flat folds of the body wall extended from the latera l parts of the dorsu m of the mesothora x and metathorax; they ar e thus in a sense homodynamous with laterotergal lobe s that may occur on any of the bod y segment s i n bot h insect s an d othe r arthropods . Tha t th e wings belong to the dorsum is shown by the fact that the thoracic spiracles always lie below their bases . There i s no evidenc e that tru e wing s were ever presen t o n th e pro thorax, bu t i n many of the earlie r fossil insects small lateral lobe s project from th e margin s o f th e prothoraci c tergu m (Fig . 119 , pnZ) , suggesting that similar lobes on the mesothorax an d metathorax wer e the precursors of th e wings . Th e immediat e ancestor s o f the flyin g insects , therefore , probably had three pairs of laterodorsal, o r paranotal, flaps on the thorax , together formin g broa d extension s fro m th e dorsu m a t th e side s o f th e body. Evidently , then , in a second stage of their evolution , insects were enabled to depart fro m a strictly terrestrial or arboreal life by using their paranotal lobe s a s glider s o n whic h they coul d launc h themselve s int o the ai r fro m som e elevatio n o r sustai n themselve s afte r a preliminar y leap fro m th e ground . Later , a thir d stag e wa s inaugurated wit h th e transformation o f the paranota l lobe s of the mesothora x an d th e meta thorax int o movabl e organs of true flight . Each o f the thre e stages in the evolutio n of modern insects from thei r generalized polypo d ancestor s ha s lef t it s separat e impressio n o n th e structure of the thorax. Hence , in a study of the thorax, we may observe three group s of characters, aside from th e presenc e of the leg s and wings, that distinguish the thorax from the other body regions. First , there are features commo n to th e thora x o f apterygote an d pterygot e insect s that were probably evolved as direct adaptations t o a more efficient us e of the legs whe n th e functio n o f locomotio n became localize d in th e thorax . Second, ther e ar e character s distinctiv e o f th e pterygot e thora x no t evidently relate d t o th e legs , bu t whic h are repeate d i n eac h segment,

THE TH ORAX

159

and which , therefore, may b e suppose d t o b e cot-relate d i n thei r origi n with th e equa l developmen t o f paranotal lobe s o n the prothorax , meso thorax, an d metathora x t o for m a glide r apparatus . Third , ther e ar e characters peculia r t o th e mesothora x an d metathora x o f pterygot e insects whic h undoubtedl y hav e bee n acquire d i n connectio n wit h th e evolution o f the paranota l lobe s of these segments into organ s of flight . 2. TH E NEC K

The neck , o r cervix, o f insect s i s a narrowe d membranou s regio n of the trunk betwee n the head an d the thorax (Fig . 85, Cvx). I t i s usually short an d mostl y conceale d withi n overlappin g part s o f the prothora x (Fig. 87, Cvx), bu t i t i s generally of greater lengt h tha n it appear s t o b e and i s sometime s elongat e an d expose d (Fig . 99 , Cvx). Som e writer s have regarde d th e nec k a s a reduce d bod y segmen t ("microthorax") * but n o conclusiv e evidenc e ha s bee n adduce d i n favo r o f thi s view ; others have regarded it as a posterior par t of the labial segment, and still others as an anterior par t of the prothorax. Th e true morphology of the cervix i s still obscure , bu t man y structura l features associate d wit h th e neck suggest that it include s parts of both th e labial an d the prothoraci c segments (Fig . 87) , and that it contain s th e primar y intersegmenta l lin e between thes e segment s (17 sg). Thi s vie w is in par t substantiate d b y Smreczynski (1932) , wh o say s tha t i n th e embryoni c developmen t of Silpha obscura mos t o f th e secon d maxillar y segmen t enter s int o th e formation o f the neck . Both th e dorsa l an d th e ventra l serie s o f longitudinal trun k muscle s arise o n the bac k o f the hea d an d exten d throug h th e neck . Th e principal dorsa l muscle s (Fig . 87 , DMcl) ar e attache d anteriorl y o n th e postoccipital ridge of the cranium (PoK) and posteriorly on the antecosta, or phragma (IPh), of the mesothorax. The ventral muscles (VMcT) extend from the postoccipital ridge or the tentorial bridge (PT) to the apophyses o f th e prosternum . Neithe r th e dorsa l no r th e ventra l muscles, therefore , hav e connection s i n th e prothora x correspondin g t o the usua l antecosta l attachment s o f intersegmental muscles . W e hav e seen tha t th e postoccipita l ridg e o f the hea d mos t probabl y represent s the intersegmenta l fol d betwee n the maxillar y an d labia l segments . I t is evident , therefore , tha t th e intersegmenta l lin e betwee n th e labia l segment an d th e prothora x lie s somewher e i n th e membranou s nec k (1/sgr), and that the dorsal and ventral muscles of the neck and prothorax include the fibers normal to two segments; that is, the muscles of the labial segment hav e becom e continuou s wit h th e muscle s o f th e prothora x through the los s of their attachment s o n the intersegmenta l fold between these two segments. I t i s quite obvious that som e such structural modification a s thi s i s necessary t o giv e freedo m o f movement to th e head ;

160 PRINCIPLES

O F INSECT MORPHOLOGY

otherwise th e activitie s o f the hea d o n th e prothora x woul d be limite d to the restricted movement s of the ordinar y intersegmental mechanism. On eac h sid e o f the nec k ther e i s typicall y a pai r o f lateral cervical sclerites (Fig . 87 , lev, 2cv). Th e tw o sclerite s o f each pair ar e hinge d t o each other, th e first articulates anteriorl y wit h the bac k of the head , th e second posteriorl y o n th e prothoraci c episternum . Th e latera l nec k plates no t onl y link the hea d to the thorax , bu t th e anterio r end s of the first in each pair for m tw o fulcral point s o n which the hea d can be tilte d up and down by the dorsal and ventral muscles attached to it. Ther e are muscles also inserted on the neck sclerites, some arising on the bac k of the head, other s o n the pronotum . Th e cervica l plate s an d thei r muscles , therefore, whe n typicall y developed , constitut e a protracto r apparatu s of th e head , for the hea d is protruded when the angl e between the plate s is straightened b y the contractio n o f the muscles . Eithe r one or both of the latera l cervica l sclerites may be absent, however , and when only one is presen t i t i s sometime s fuse d wit h th e episternu m (Fig . 99 , cv). I n some insects ther e ar e also dorsal , lateral , an d ventral cervical sclerites , but thes e usuall y have n o muscles connected with them . Th e genera l mechanism of the insec t neck has been but littl e investigated; a review of the structur e o f the cervica l sclerites may b e found i n several papers b y Crampton (1917 , 1926 ) and i n one by Marti n (1916) . 3. GENERAL STRUCTURE OF THE THORA X

The thorax of an adult insect is in general easily recognized, since it is the sectio n o f the trun k bearin g th e legs , and th e wing s when wings are present. Ordinaril y th e thora x consist s o f th e thre e bod y segment s following th e head , but i n most o f the Hymenopter a th e fourt h segment is s o intimately associate d wit h th e thir d tha t i t virtuall y become s a thoracic rathe r tha n a n abdomina l segment . O n the othe r hand , th e thorax i s ofte n distinctl y divide d betwee n its firs t tw o segment s int o a prothoracic part an d a meso-metathoracic part . Th e second part, composed of the wing-bearin g segments more or less closely united with each other, may b e termecLthe pterothorax. In the presen t sectio n w e shall consider onl y those more fundamental features o f the thoraci c structure that presumably were developed before the paranota l lobe s evolved into movable organs of flight; the structura l modifications b y which the pterothora x ha s been evolve d into a mecha nism o f wing movement will be discussed separately . The Thoraci c Terga.—Th e terga l plate s o f th e thora x ar e usuall y modified i n variou s ways , bu t th e generalize d structur e i s preserve d in the mesothorax and metathorax of wingless insects. I n the Apterygota and i n nymphal and many larval Pterygota, the terg a of these segments are simpl e back plate s simila r t o thos e o f the abdome n where a typica l

THE THORAX

161

secondary segmentatio n ha s bee n establishe d (Fig . 37) . Eac h plat e comprises th e primar y segmenta l sclerotizatio n o f the dorsu m an d th e preceding intersegmenta l sclerotizatio n (Fig . 8 6 A) . Th e definitiv e tergum, therefore , is crossed anteriorly b y th e lin e o f the primar y inter segmental groove , which forms the antecosta l sutur e (acs) externall y an d a submargina l antecost a (B , Ac ) internally , an d i s thu s divide d int o a narrow precostal acrotergite (atg) an d a long postcostal area ending at th e secondary intersegmenta l membran e followin g (Mb) . Th e terga l ante costae o f generalize d thoraci c segment s giv e attachmen t t o th e dorsa l longitudinal muscles in the usual manner (Fig. 86 B, DMcl). In most winged insects, however , these muscle s are greatly enlarge d in the wingbearing segments , an d t o accommodat e the m ther e ar e develope d

FIG. 86.—Diagram s illustratin g intersegmenta l relation s betwee n th e terga l plate s o f generalized segments .

plate-like apodema l lobe s fro m th e antecosta e o f th e mesotergum , th e metatergum, an d th e firs t abdomina l tergum . Thes e antecosta l apo demes, which are usually paire d bu t sometime s single , are known as th e phragmata. Sinc e there ar e typically thre e o f them, th e phragmat a ma y be distinguished a s the first phragma (Fig . 98 , IPh), th e secon d phragma (2Ph), and the third phragma (3Ph). The thoraci c spiracle s ar e generall y situate d o n th e side s o f th e segments, bu t th e areas occupied by them mus t b e supposed to belong to the dorsum , thoug h the y ar e beneat h th e win g bases i n alat e segment s (Fig. 88, Sp). In Protura the spiracles are located in the lateral margins of th e mesothoraci c an d metathoraci c terga . Becaus e o f th e dorsa l extension o f th e thoraci c pleur a i n insects , th e thoraci c spiracle s ar e sometimes enclose d betwee n th e successiv e pleura l plates . Th e firs t spiracle i s mesothoracic , bu t i t i s ofte n displace d anteriorl y o n th e prothorax; the secon d is metathoracic an d i s also subjec t to a n anterio r migration. The Thoraci c Pleura. —The insect s resembl e th e chilopod s i n tha t there ar e associate d wit h th e functiona l le g bases on e o r mor e sclerite s

162

PRINCIPLES OF INSECT MORPHOLOGY

in the lateral walls of the leg-bearing segments. Evidenc e from ontogeny suggests that these so-called pleural sclerites belong to primitive subcoxal parts o f the le g bases, an d th e fac t tha t i n bot h th e chilopod s and th e insects some of the body muscles of the legs may be inserted on the pleural

FIG. 87.—Diagra m o f intersegmental relations betwee n th e hea d an d th e pr o thorax, an d between th e prothorax an d the mesothorax .

areas give s a furthe r reaso n fo r believin g tha t th e primar y lim b base s included not onl y the coxa e but als o the subcoxa l areas of the bod y wall containing th e pleura l sclerites . I n mos t othe r arthropod s th e coxopodites, or basal limb segments, ar e implanted in the pleura l walls of

FIG. 88.—Diagra m o f the theoreticall y primitiv e scleroti c element s o f a thoraci c segment, in whic h the subcoxa l part of the lim b basis (Sex) include s two supracoxal sclerotic arches, th e anapleurite (Apl) an d coxopleurite (Cxpl), an d a n infracoxa l arc , or sternopleurite (Spl).

the bod y segment s betwee n th e terga l an d sterna l plates , an d thu s no t only does each coxopodite include the cox a of the leg but it s base occupies the are a o f th e subcox a o f chilopod s an d insects . Th e leg s o f man y larval insect s ar e born e o n distinc t subcoxa l lobes o f the thoraci c bod y

THE THO RAX

163

segments, whic h contain th e pleura l sclerite s i n their dorsa l wall s (Figs . 152 A, 15 3 A, Sex). The primitiv e subcoxa l par t o f a thoraci c le g probabl y forme d a complete annulu s proxima l t o th e cox a (Fig . 88 , Sex), whic h becam e flattened ou t in the pleural area of the body wall (P) to form a support for the res t o f th e limb . Th e entir e subcoxa l elemen t i n th e bod y wall , therefore, include s not onl y the regio n of the pleura l sclerites abov e th e base of the cox a but als o a ventral arc below the coxa. The subcoxal sclerotization becomes variously broken up into sclerites, but a study o f the mor e primitive insects an d the chilopod s suggests that there wer e primarily thre e majo r scleroti c areas surroundin g the bas e of the coxa , two concentricall y place d abov e th e coxa , an d on e belo w it . These ar e respectivel y th e anapleurite (Fig . 88 , Apl) situate d dorsally , the coxopleurite (Cxpl) closely associated with the upper rim of the coxa, and th e sternopleurite (Spl) adjoinin g the sternum . Th e cox a is artic ulated betwee n the coxopleurit e and th e sternopleurit e (c , d). I n mos t pterygote insect s th e tw o supracoxa l arches unite t o for m th e so-calle d pleuron, bu t the y remai n quit e distinc t i n man y Apterygota ; th e infra coxal ar c usuall y become s a latera l elemen t o f th e definitiv e sternu m (Fig. 91 B, Ls). Th e pleuro-ventra l line of a thoracic segment (Fig . 88, 6-6), therefore , generally runs throug h th e latera l par t o f the definitiv e sternal plate . The Apterygote Pleurites. —In the Apterygot a the subcoxa l sclerites of the thora x ar e smal l an d variabl e an d d o no t for m definit e pleura l structures. A primitive conditio n in whic h each subcoxal area contain s two distinc t supracoxa l scleroti c arche s i s wel l show n i n som e o f th e Protura and Collembola (Fig. 89, Apl, Cxpl). If the coxa has a definite dorsal articulation i n these forms i t i s with the coxopleurit e (C, c). Th e presence o f a ventra l articulatio n (d ) probabl y mean s tha t th e sterno pleurite i s containe d i n th e definitiv e sterna l plate . I n Diplur a an d Thysamira th e pleurite s ar e variabl e an d mor e or les s degenerate , bu t in many cases there are distinct remnant s of both the anapleura l and th e coxopleural arches . Th e ventra l ar c o f th e subcox a mor e commonl y preserves its independenc e from th e sternu m in the chilopod s (Fig. 5 2 A, Spl) tha n it does in the insects. Th e thoracic pleurites of the Apterygot a in man y ways resemble the pleurites of the Chilopoda , an d i t i s evident that i n bot h group s the'sclerites ar e in a degenerativ e state , sinc e the y have n o ver y importan t functio n t o perform . Th e highl y develope d pterygote pleuron , however , a s w e shal l presentl y see , ha s apparentl y been derive d from a mor e primitive pleura l structure resemblin g that of the Apterygot a an d Chilopoda. The Pterygote Pleuron. —In the Pterygot a th e progressiv e evolution of the supracoxa l part of the subcox a into an important skeleta l part of the

164

PRINCIPLES OF INSECT MORPHOLOGY

body segment, and the union of the infracoxa l ar c with the sternu m have largely obscured the mor e primitive subcoxal structure exhibite d b y th e Apterygota. I t i s only in the prothorax of Plecoptera that the pterygot e

FIG. 89.—Example s o f th e presenc e o f two supracoxa l arche s i n th e thoraci c subcoxa l region, a s show n i n Fig . 88 . A , mesothora x o f Acerentomon doderoi. (From Berlese, 1910.) B , Isotoma. C , mesothora x o f Acerentulus barberi. (B, C from H. E . Ewing, 1928.)

pleuron retain s th e apterygot e conditio n i n whic h th e anapleurit e an d the coxopleurite are distinct sclerites (Fig. 90, Apl, Cxpl). In all other cases thes e sclerite s apparentl y ar e unite d i n th e singl e latera l plat e

FIG. 90.—Example s o f th e retentio n o f a distinc t anapleurit e an d coxopleurit e i n th e prothorax o f pterygote insects (Plecoptera) . A , larva o f Pteronarcys. B , larva o f Perla, external view . C , same, internal view .

supporting th e cox a (Fig . 9 1 A, B). Th e prearticula r par t o f the coxo pleurite, however, generally remains as a partl y o r entirel y fre e sclerite , the trochantin (Tri), th e ventra l extremit y o f whic h usuall y acquire s a n articulation wit h the anterio r margi n of the cox a (e).

THE THORAX

165

The usua l pterygot e thoraci c pleuron , forme d b y th e unio n o f th e two supracoxa l arche s o f th e subcoxa , i s typicall y a mor e o r les s con tinuous sclerotic area in the lateral wal l of the body segment, surrounding the bas e o f the cox a dorsally, anteriorly , an d posteriorl y (Fig . 91, A, B). Above th e cox a th e pleuro n i s reinforce d b y a stron g interna l pleural ridge (Fig. 92 A, PIK) extending upward from the coxal articulation (CxP), whic h i s forme d b y a linea r inflectio n of the oute r wall , know n as the pleural suture (Fig. 91, PIS). In a wing-bearing segment both the pleural ridg e and its sutur e are carrie d upwar d int o the pleura l win g process (B , WP), an d i n suc h case s th e ridg e brace s th e pleura l wal l

FIG. 91.—Diagrams illustrating the apparen t evolution of the pleural and sternopleural sclerotization o f a wing-bearin g segmen t fro m th e subcoxa . (Compar e wit h Fig . 88.) The anapleurit e an d it s ventra l extension s become the episternum (B , Eps)t the epimeronA (Epm), th e precoxale (Prcx) , an d th e postcoxale (Pcx) ; th e anterio r part of the coxopleurit e forms th e trochanti n (Tn)\ th e sternopleurit e unites with the primitive sternum (Stri) an d becomes a laterosternal element (Z/s ) o f the definitiv e sternum .

between the wing support and the coxal articulation. From each pleural ridge there project s inward an d downwar d an apodemal arm , th e pleural apophysis (Fig. 92 A, PIA), which is usually associated with a corresponding sternal apophysis (SA).The pleura l sutur e divide s th e uppe r par t o f the pleuro n int o a presutural episternum (Fig. 9 1 B , Eps) an d a postsutura l epimeron (Epm). The regio n o f th e pleuro n extendin g downwar d fro m th e episternu m anterior t o the cox a and the trochantin is the precoxal bridge, or precoxale (Prcx), generally united ventrally with the sternum; that behind the coxa, continuous fro m th e epimero n an d frequentl y unite d belo w wit h th e sternum, i s th e postcoxal bridge, o r postcoxale (Pcx). Th e precoxa l an d postcoxal sclerotization s ma y en d ventrall y i n a n infracoxa l fold , evi dently th e infracoxa l arc of the subcox a (A) ; when they ar e united wit h the sternu m i t woul d appear probabl e that th e ventra l subcoxa l arc has fused wit h th e primar y sternu m (Stn) an d form s a latera l par t o f th e

166

PRINCIPLES OF

INSECT MORPHOLOGY

definitive sternu m (B , Ls). Th e precoxa l regio n sometime s form s a distinct sclerit e separate d fro m bot h th e episternu m an d th e sternu m (Fig. 10 2 B , Prcx). Th e postcoxa l sclerotizatio n i s seldo m a n inde pendent sclerite , but i t is often suppressed . The anterio r remnan t o f th e coxopleurite , know n a s th e trochantin (Fig. 9 1 A , Tri), i s bes t preserve d i n th e mor e generalize d pterygote insects. Whe n well developed it bear s at it s anterio r o r ventral en d the anterior trochantinal articulation of the coxa (B, e); and usually the tergal promoter muscle of the le g is inserted on it. I n th e highe r insects the trochanti n become s reduced or obliterated, or it ma y be united with the lower margin of the episternum in such a way that its limits are ofte n difficult t o determine.

FIG. 92.—Diagrammati c cros s sectio n o f thoraci c segment s illustratin g th e evolutio n of th e furc a (B , Fu) fro m th e sterna l apophyse s (A , SA) an d th e inflecte d median part of the sternu m (*S) .

The thoraci c pleuron of the Pterygot a i s thus see n to diffe r consist ently from the variable and weakly developed pleural parts of Apterygota in that it forms , i n the adul t stag e of the insect , a definite an d elaborate structure i n the latera l wal l of the bod y segment betwee n the cox a an d the tergum . Moreover , the basi c features of the pterygot e pleuro n ar e the sam e in the wingles s prothorax as in the alat e mesothorax and meta thorax. W e cannot, therefore , attribute th e characteristi c structur e of the pterygot e pleuro n to th e developmen t of the wings . O n the othe r hand, we may suppose that the thoracic pleura of winged insects acquired their fundamenta l character s i n correlatio n wit h th e developmen t of paranotal lobes on all the thoraci c segments in the preflyin g glide r stage of insec t evolution. The Thoraci c Sterna.—Th e degre e o f sclerotizatio n i n th e ventra l walls of the bod y segments varies much in differen t arthropods ; in some the vente r i s entirel y membranous , in other s i t i s occupie d b y definit e sternal plates . Thoug h a typical sternu m (Fig . 36 B, Stri) include s the preceding intersegmental fol d (Ac) on whic h th e principa l ventral bod y

THE T HORAX

167

muscles are attached, the vente r ma y be occupied by a series of alternating segmenta l an d intersegmenta l sclerites , a s i n som e o f th e chilopod s (Fig. 38) . The sterna l plate s o f th e thora x i n adul t insect s diffe r generall y i n three respect s fro m thos e o f th e abdomen : first , i n th e independenc e of th e primar y segmenta l an d intersegmenta l sclerotizations , o r i n th e opposite relatio n o f th e secon d t o th e firs t whe n th e tw o ar e united ; second, i n the reverse d overlappin g of the plate s a t th e secondar y intersegmental lines; and, third, i n a transposition o f the attachment s o f most of the ventra l muscle s from th e intersegmenta l to the segmental sclerites . Basic Structure o f th e Thoracic Sterna. —In th e thora x th e interseg mental sclerite s o f the vente r betwee n the prothora x and the mesothora x and betwee n th e mesothora x an d th e metathora x ar e never unite d wit h the segmenta l plate s following ; eithe r the y remai n a s fre e intersternite s (Fig. 37 , list, 2Ist) o r the y unit e wit h th e segmenta l stern a preceding . The primar y intersegmenta l sclerotizatio n behin d th e metasternu m i s generally lost o r is united wit h the abdomina l sternum following . The segmenta l plat e o f th e vente r o f a thoraci c segmen t ma y b e designated the eusternum (Figs. 93 A, B, Stn, 96, ES). Th e intersternites of th e thora x (Fig . 9 3 A , 1st) ar e commonl y termed spinasterna (Figs . 93 B , 96 , Ss), becaus e eac h usuall y bear s a media n apodema l proces s called th e spina (Fig . 87 , Spn). Since bot h th e firs t an d th e secon d spinastern a ma y b e fre e inter sternal sclerites , o r th e second , o r als o th e first , ma y unit e wit h th e eusternum preceding , w e usuall y encounte r on e o f th e followin g thre e series o f sclerite s i n a stud y o f th e thoraci c sterna , excep t whe n th e eusternum itsel f i s secondaril y subdivided: (1 ) eusternu m o f prothorax , first spinasternum , eusternu m o f mesathorax , secon d spinasternum , eusternum of metathorax; (2) eusternum of prothorax, firs t spinasternum , composite mesosternum , eusternu m o f metathorax ; (3 ) composit e prosternum, composit e mesosternum , eusternu m o f metathorax . A fourth conditio n may aris e whe n the sterna l plate s of the wing-bearin g segments ar e al l united i n a larg e pterothoracic plastron . I t shoul d be observed that the metasternum never has a spinasternite, because the third intersternite eithe r i s suppressed o r become s the acrosternit e o f the firs t abdominal sternum . The eusternu m o f a thoraci c segment , a s w e have observed , usuall y comprises the primar y sterna l plat e and the subcoxa l sternopleurites , the latte r constitutin g th e laterosternites, or pleurosternites, of the defini tive sternu m (Fig . 9 3 D, Ls). I n som e insect s th e thoraci c stern a ar e bordered by distinct subcoxal folds continuous with the pleura before and behind th e coxa e (Fig . 9 5 A) ; i n other s th e limit s o f the laterosternite s are suggeste d b y submargina l sutures ; bu t i n genera l th e presenc e o f

168

PRINCIPLES O F INSECT MORPHOLOGY

lateral subcoxa l derivatives i s not eviden t i n the definitiv e sterna l plate s of adult insects (Fig. 96, ES) an d is only to be inferred from the continuit y of th e sternu m wit h th e precoxa l and postcoxa l bridges o f the pleuro n or from the presence of a ventral articulation of the coxa with the sternum , though th e latte r probabl y is secondary in some cases. The Reversed Overlapping o f th e Thoracic Sterna.—The sternal plates of the thora x characteristicall y overla p eac h other anteriorly (Fig . 37 ) an d thus presen t a relatio n jus t th e opposit e fro m tha t prevailin g i n th e dorsum, and in the venter of the abdomen . Thi s reversed overlapping of the thoraci c stern a i s particularl y strikin g i n som e o f th e Apterygot a and i n th e mor e generalize d Pterygota , bu t i t i s eviden t whereve r th e successive sternal plates are not united with each other. I t i s apparently correlated wit h th e reverse d relations o f the intersegmenta l spinastern a to th e eusterna l plates, and , whil e the reaso n for this peculiarly thoracic modification i s not clear , it mus t have some important significanc e i n th e mechanism o f th e thora x no t connecte d wit h th e wings . A s a conse quence, th e sternu m o f the metathora x (Fig . 36 , $3) stands usuall y a s a dividing plat e overlappin g i n bot h direction s betwee n th e mesothora x and th e abdomen , thoug h sometime s i t als o i s overlappe d b y th e firs t abdominal sternum . The Transposition o f th e Ventral Thoracic Muscles. —Associated wit h the reverse d overlappin g o f th e thoraci c sterna , bu t no t necessaril y correlated wit h it , ther e occur s a partia l o r complet e transfe r o f th e attachments o f the longitudina l sterna l muscle s from , th e intersternite s to th e precedin g segmental parts of the definitiv e sterna. Theoreticall y we mus t assum e tha t bot h th e dorsa l an d th e ventra l muscle s wer e originally attache d o n th e line s o f th e primar y intersegmenta l folds , which ar e preserve d as the antecosta e o f the terg a an d o f the abdomina l sterna, bu t whic h are reduced in the sterna l regio n of the thora x t o th e small, median, spinal processes of the spinasternites (Fig. 87, Spri). In adult pterygot e insect s th e principa l group s o f ventral muscl e fibers in the thora x exten d betwee n paire d apophyse s arisin g o n th e eusterna l plates (SAi, SA 2), thoug h a fe w fiber s usuall y preserv e th e origina l connections with the spinae (Spri). In most holometabolous larva, on the othe r hand , th e principa l ventra l muscle s throughout th e lengt h of the bod y are regularly attached o n the intersegmenta l folds . Th e adul t condition, then , i s evidently a secondary one. The Thoracic Sterna of Apterygota. —The sterna l sclerotization s o f th e thorax in the Apterygota are variously developed and show no progressive evolution within the group . I n th e Protura an d i n Japyx th e principa l sternal plat e i n each segment bears a n interna l median ridge which may be forked anteriorly , with the arms extending to the ventral articulation s of th e coxae . Endosterna l structure s ar e absen t i n Lepismatidae, where

THE THORAX

169

each sternu m i s produce d posteriorl y int o a large , scale-lik e lobe. I n Machilidae the thoraci c stern a ar e weakl y developed areas of sclerotization betwee n th e le g bases, separate d b y ampl e intersegmenta l spaces . From eac h intersterna l are a a pai r o f delicat e apodema l arm s project s inward fro m a commo n median base, formin g thu s a serie s of furca-lik e structures. Th e intersegmenta l positio n o f their bases , however , allie s these apodemal structures with the median processes of the spinasternite s in the Pterygota rathe r than wit h the true sternal apophyses . The Thoracic Sterna o f Pterygota. —The stern a o f pterygot e insect s are characterize d b y th e possessio n o f paire d apophyse s arisin g fro m the eusterna l plates . Th e sternal apophyses (Fig . 9 2 A , SA) ar e ofte n

FIG. 93.—Diagrams suggesting the evolution of a generalized definitive thoracic sternu m (D) b y unio n o f th e primitiv e sternu m (A , Stn) with th e infracoxa l arcs o f the subcoxa e (B, C , Sex), an d wit h th e followin g intersternite (A, 1st), which becomes the spinasternu m (B, C , D , Ss). Th e definitiv e eusternu m i s finall y divide d b y th e sternacosta l sutur e (C, D , k ) int o basisternum (Bs) an d sternellum (SI), an d ma y hav e a narro w presternum (Prs) se t of f by a n anterio r submargina l suture (j).

called the furcal arms, because in the higher pterygote orders the two apophyses i n eac h segment ar e supporte d o n a media n inflectio n o f th e sternum and thu s become the divergen t prong s of a forke d endoskeleta l structure known as the furca (B, Fu). The outer ends of the sternal apophyses ar e closel y associated wit h the inne r end s of the pleura l arms ????? ?? ??? ???? ???????? ??? ??? ????? ?? ????????? ????? ??????? ???? nected b y shor t muscl e fibers , o r i n som e cases fuse d wit h eac h other . When th e pleura l an d sterna l processe s ar e unite d o n eac h sid e o f th e segment, the y for m a buttresslike arc h acros s the coxa l cavit y fro m th e sternum t o the pleuron. The sterna l apophyse s suppor t th e principa l longitudina l ventra l muscles o f the thorax , an d the y giv e attachment t o som e of the ventra l muscles o f th e legs . Externall y thei r root s ar e marke d b y a pai r o f

170

PRINCIPLES OF

INSECT MORPHOLOGY

pits in the sternum betwee n the coxa e (Fig. 93 B, sa, so). Th e primitiv e position o f the apophyse s is doubtful; according to Webe r (1928 , 1928a) , the processe s are invaginations betwee n the latera l edge s of the primar y sternum an d the subcoxal laterosternites, bu t th e location of the externa l pits of the apophyse s does not alway s conform with this view. In the more generalized Pterygota, th e bases of the sternal apophyse s are ofte n connecte d by a n internal transvers e ridge , the sternacosta (Fig . 92 A , k) , th e lin e o f whic h appear s externall y a s a sternacostal suture through th e apophysea l pit s (Fig . 9 3 C , D , k) . Th e sterna l sutur e divides the surface of the eusternum into a presutural area, or basisternum

FIG. 94.—A n exampl e o f desclerotization in th e vente r of th e thorax . A , thoracic sterna o f Blatta orientalis. B , diagra m o f th e typica l sternal sclerotizatio n of a thoracic segment. C , resul t o f desclerotizatio n in th e pr o thorax o f Blatta. I> , resul t o f extreme desclerotization a s in th e mesothora x or metathorax of Blatta.

(Bs), an d a postsutura l area , th e sternellum (SI), o r "furcasternum. " When th e anterio r par t o f th e eusternu m i s reinforce d b y a n interna l submarginal ridge, there is formed externally a presternal suture (D, j), which set s of f a narro w margina l are a o f th e sternum , calle d th e presternum (Prs). Muscles are never attached on the presternal ridge, and the latte r shoul d not b e mistaken fo r a true antecosta ; the sterna l ante cost ae of the thora x ar e represente d b y th e spina e o f the spinasternites . In it s surfac e structur e th e thoraci c sternu m depart s i n man y way s from th e simpl e divisiona l patter n show n at D o f Fig. 93 . Th e sterna costal sutur e i s subjec t t o variation s i n form , bein g ofte n produce d forward an d variousl y branched, giving rise to a n endoskeleta l structure of diversifie d form. I n som e case s als o convergen t ridge s exten d pos teriorly fro m th e base s o f th e apophyse s an d ma y unit e i n a media n

THE THORAX

171

ridge, thus forming a Y-shaped endosternal ridge (Y-Leiste of Weber, 1933), th e externa l sutures o f which cut th e sternellu m into median an d lateral areas . Or , again, a confusing conditio n may aris e fro m a partial desclerotization o f the sterna l plates , a s i n th e mesothora x an d meta thorax o f Blattida e (Fig . 9 4 A) , wher e the eusternu m i n eac h o f thes e segments i s divide d int o on e o r tw o anterio r basisterna l sclerite s (Bs) and a posterio r sternella r sclerit e (SI) supportin g th e lon g apophyses . The prosternu m of the roac h (C) , however , retain s mor e nearly the generalized sterna l structur e (B) , an d i t i s not difficul t t o se e how th e sclerite pattern of the mesosternu m (D ) o r of the metasternu m ha s been derived from th e forme r (B ) by a loss of sclerotic continuity.

FIG. 95.—Example s o f highl y modifie d sterna l sclerotizatio n i n th e thorax . A , pterothorax o f Magicicada septendecim, ventra l view . B , thora x o f Calliphora, ventra l view. C, mesothoraci c furca of Calliphora, dorsal view .

In th e highe r pterygot e order s tn e sterna l apophyse s ar e carrie d inward upo n a media n inflectio n o f the sternu m t o for m th e Y-shape d endosternal apodem e known as the furca (Fig . 92 B, Fu). Th e stal k of the furc a ma y arise from a definite pi t markin g the divisio n between the basisternal an d sternella r region s o f the sternu m (Fig . 9 5 A , Bs 2, Sh), or the par t o f the sternu m bearin g the furc a ma y becom e detached a s a distinct furcasternum (B , Fs^). I n man y insects , however , the bas e of the furc a i s extended forwar d a s a long median ridge through the whole length o f the sternu m (C) , the sit e o f which is marked externall y b y a median sterna l groov e (B , fu%). I n thi s cas e ther e i s n o distinctio n between basisternum and sternellum or furcasternum, and it is impossible to say how much of the true sternum has been inflected to form the furca l base. In a study o f the thoraci c stern a o f the highe r insect s i t seem s more advisable to accept the facts as they are, unless identities wit h the sterna l regions o f mor e generalize d insect s ca n b e trace d throug h a serie s of

172

PRINCIPLES O F INSECT MORPHOLOGY

families. I n th e pterothora x o f th e highe r Diptera , fo r exampl e (Fig . 95 B) , th e mor e primitiv e suture s o f th e sterna l a s wel l a s th e pleura l areas hav e becom e almos t wholl y obliterated , an d secondar y groove s appear whic h divid e th e skeleta l surfac e int o part s tha t hav e littl e relation t o thos e i n mor e generalize d orders . Th e larg e ventra l plat e of th e mesothora x of Calliphora (Fig . 95 B) i s evidently compose d of th e sternum, the precoxal bridges, and parts of the episterna; the bridge (x) separating th e middl e an d hin d coxa e must includ e th e postcoxali a of the mesothorax , th e precoxali a of the metathorax , an d th e metathoraci c basisternum. The small sclerite (Fs$) between the hind coxae is a detached furcasternum , though th e furc a i s supported als o o n the plate (x) before it. The prosternum, on the other hand, retains the more ??????????? ????????? ?? ???? ??? ????? ?? ??? ??????? ????????? ????? separate a lon g basisterna l sclerit e (Bsi) fro m a smal l sternella r regio n (Sli), which is united with the mesosternum. A presternal sclerite (Prsi) is here entirely cut of f from th e basisternum . 4. TH E PROTHORA X

The prothora x differ s consistentl y fro m th e othe r bod y segment s in that its tergum and sternum alway s lack the antecosta l an d precostal elements of typical segmental plates, these parts apparently havin g been lost b y membranization in the neck . Th e prothoracic tergu m is a plate of th e primar y segmenta l regio n onl y (Fig . 87 , Ti). I t neve r bear s a phragma, since the first phragma (IPh) is never detached from the mesotergum, and the acrotergit e of the mesotergum is not sufficientl y enlarged to constitute a postnotum of the prothorax. Th e principal dorsal muscles of the prothorax (DMcl) exten d through the segment from the postoccipi??? ????? ?? ??? ???? ????? ?? ??? ?????????? ?? ??????? ?????? ?? ? mesotergum, but shorte r muscle s may connect the hea d with the proter gum or the latte r wit h the mesotergum . The siz e and for m of the prothoracic tergum ar e highl y variable. I n som e insects, a s in Orthop tera, Hemiptera , an d Coleoptera , th e protergu m ma y b e a larg e plate , sometimes greatl y expanded ; but since , in general, the bac k plate o f th e prothorax has little specific functio n asid e from givin g attachment t o th e dorsal muscle s o f the legs , i t frequentl y assume s strang e an d fantasti c shapes, or , o n th e othe r hand , i t i s reduce d i n siz e an d ma y b e bu t a narrow ban d betwee n th e hea d an d th e mesothorax . I n som e o f th e Hymenoptera th e protergu m is so intimately associate d wit h the meso tergum tha t th e isolate d pleurosterna l part s o f its segmen t for m a fre e suspensorium fo r th e firs t pai r o f legs . Whe n th e protergu m i s wel l developed it s surfac e ma y b e marke d b y sutures , whic h for m ridge s on th e inne r surfac e o f the plate ; bu t th e resultin g "divisions " o f th e protergum hav e n o relatio n t o thos e characteristi c o f the wing-bearin g

THE THO RAX

173

terga. Th e internal ridge s are usually found to have an intimate relatio n to muscle attachments . The prothoraci c sternu m ha s the sam e fundamental structure a s th e sterna o f th e pterothora x bu t i s commonl y mor e generalize d tha n th e latter. Th e eusternu m bear s a pai r o f apodema l apophyse s (Fig . 87 , SAi), and the spinasternum (ISs), which may be widely separated from the eusternu m o r fused wit h it , bear s a media n spina . Th e tru e inter segmental lin e betwee n th e prothora x an d th e mesothorax (21 sg) run s through th e spinasternit e ventrall y an d th e bas e o f th e firs t phragm a dorsally, bu t usuall y a wide membranous are a constitute s th e functiona l intersegmental conjunctiva . The pleuro n o f the adul t prothora x resemble s th e pleura l sclerotiza tion o f th e pterothora x o f nympha l an d larva l insect s i n tha t i t lack s the ala r developmen t characteristi c o f the adul t pterothoraci c pleuron . In it s genera l feature s i t ha s th e sam e typ e o f structure a s th e pleur a of th e wing-bearin g segments, an d onl y in th e Plecopter a (Fig . 90 ) doe s it sho w any suggestio n of the mor e primitive structur e o f the apterygot e pleuron. Th e episternu m an d epimero n are always wel l separated b y a pleural sutur e an d ridge , though th e epimero n is often muc h reduced or fused with the margin of the tergum. I n some Orthoptera the episternu m is largely conceale d within a lateral fol d o f the protergum , but , sinc e i t gives origi n t o th e abducto r muscl e of the coxa , it i s seldom reduced in proportion t o th e reductio n o f th e epimeron . Th e latera l sclerite s o f the nec k usuall y articulat e wit h th e prothoraci c episterna , bu t the y may b e fused wit h th e latte r t o for m a pair o f arms projecting from th e propleura t o suppor t th e head . Precoxa l an d postcoxa l extension s of the supracoxa l pleurites , o f which one o r bot h ma y b e continuou s wit h the sternum , ar e commonl y presen t i n th e prothora x a s i n th e ptero thorax. Th e entir e structur e o f th e prothoraci c pleuro n suggest s a n evolution homodynamou s wit h tha t o f th e pleur a o f the wing-bearin g segments t o a poin t wher e th e latte r becam e specialized as parts o f the wing mechanism. 5. TH E PTEROTHORA X

The wing-bearin g segment s diffe r structurall y fro m th e prothora x only in details tha t ar e clearl y adaptations t o th e functio n o f movement in th e wings . Th e modification s affec t chiefl y th e terga , i n a lesse r degree the pleura , an d least th e sterna . General Structur e o f th e Wing-bearin g Segments.—Th e typica l structure o f a wing-bearin g segment i s show n diagrammaticall y i n Fig . 96. Th e dorsu m o f the segmen t ma y b e occupie d entirely b y a singl e tergal plate (AN), which bears the wings; but usually the segment in which th e wing s are bette r develope d contain s als o a second , posterio r

174

PRINCIPLES OF INSECT MORPHOLOGY

plate (PN), whic h carrie s a phragm a (Pph). Sinc e entomologist s generally prefe r th e ter m notum for th e terga l plate s o f the thorax , w e may designat e th e wing-bearin g plate i n th e dorsu m o f a pterothoraci c segment th e alinotum (AN), an d th e phragma-bearin g plat e th e phragmanotum, or postnotum (PN). The alinotum i s ofte n supporte d on th e pleura b y preala r arm s (Pro) extending laterall y o r downwar d from it s anterio r angle s t o th e episterna; th e postnotu m i s generally firmly braced upo n th e pleura by latera l postala r extension s (Pa) united wit h th e epimera . Since th e phragmat a ar e inflec tions o f th e integumen t o n th e primary intersegmenta l groove s (Fig. 96 , acs) , th e phragma bearing postnota l plate s o f th e dorsum ar e i n ever y wa y compar able wit h th e spinastern a o f th e venter (Ss). Th e tru e interseg FIG. 96.—Diagra m o f th e typica l scler ites o f a wing-bearin g thoraci c segment , an d mental lines of the thorax (Isg) their subdivisions , latera l view . acs, run dorsall y throug h th e base s of antecostal suture ; AN , alinotum ; ANP, anterior nota l win g process ; Aph, anterio r the phragmata , an d ventrall y phragma, prephragma ; atg, acrotergite ; Ba, through th e base s o f th e spinae . basalare; Bs, basisternum; CxC, coxal cavity ; CxP, pleura l coxa l process; Epm, epimeron ; The phragma-bearin g plate s o f the Eps, episternum ; ES , eusternum ; Isg, dorsum, however, diffe r fro m th e primary intersegmenta l line ; k , sternacosta l suture; Mb , conjunctiva , secondar y inter - spina-bearing plate s o f the vente r segmental membrane ; Pa , postalare ; PCX, in tha t the y ma y b e more closely postcoxale; pla, roo t o f pleura l apophysis ; PIS, pleura l suture ; PN , postnotum , phrag - associated o r unite d wit h th e manotum; PNP, posterio r nota l win g proc - segmental plat e eithe r befor e o r ess; Pph, posterio r phragma , postphragma ; Pra, prealare ; Prcx, precoxale ; Prs, prester - behind them . Thu s th e segmen t num; Prsc, prescutum ; Rd , posterio r fol d carrying the principa l pair of wings or reduplicatio n o f alinotum ; Sa, subalare ; Scl, scutellum ; Set, scutum ; SI, sternellum ; may hav e a phragm a a t eac h end Ss, spinasternum ; Tn , trochantin ; WP , of it s terga l region . Th e anterio r pleural win g process . phragma i n thi s cas e ma y b e dis tinguished as a prephragma (Aph), an d the posterior one as a postphragma (Pph).. The wing s are flat fold s o f the bod y wall extending laterally fro m th e edges o f th e alinota l plate s (Fig . 85 , TP 2, W 9), thei r uppe r membrane s being continuou s with th e dorsa l integument, thei r ventra l membranes reflected int o th e latera l wall s o f th e segments . Th e posterio r borde r of each wing is continuous with the posterior marginal fold of the alinotum

THE THORAX

175

(Fig. 96 , Rd), bu t anteriorl y th e win g base end s behind th e preala r ar m of th e notum . The pleura l sclerotizatio n o f a wing-bearin g segmen t i s usually wel l developed and is almost always divided by a pleural suture (Fig. 96, PIS) into an episternum (Eps) and epimeron (Epm). At the upper end of the suture th e dorsa l margi n o f the pleuro n i s produced into a pleural wing process (TFP) , whic h serves a s a fulcru m fo r the movemen t o f the wing . Before an d behin d th e win g proces s i n th e uppe r membranou s part s of th e pleura l wall , there ar e situated two o r more epipleurites (Ba, So), usually smal l plate s upo n whic h are inserte d importan t muscle s o f th e wings. Ventrall y th e pleuro n i s generall y supporte d o n th e sternu m by the precoxal and postcoxal bridges (Prcx, PCX). The trochantin (Tri) i s variable an d i s usually suppresse d i n the highe r orders . The stern a o f th e pterothoraci c segment s hav e n o specia l feature s to distinguis h the m fro m th e prothoraci c sternum , excep t fo r th e siz e of th e basisterna l regions , whic h ar e usuall y enlarge d t o accommodat e the ventra l end s o f th e tergosterna l win g muscles , representative s o f which are absent i n the prothorax . The Terga l Plate s o f th e Pterothorax.—Th e terg a o f th e wing bearing segment not only support the wings but are themselves importan t elements i n th e mechanis m fo r moving the wings , since eac h acts as a n intermediary betwee n th e indirec t win g muscles o f its segmen t an d th e bases o f th e wings . Th e tergu m play s it s par t i n th e productio n o f wing movemen t b y respondin g t o th e contractio n o f th e longitudina l dorsal muscle s with a n upwar d curvature betwee n its tw o ends, an d b y a revers e actio n t o th e downwar d pull o f the antagonisti c tergosterna l muscles. Th e depressio n o f th e tergu m cause s th e upstrok e o f th e wings; th e dorsa l flexur e assist s i n th e downstroke . I t i s eviden t tha t the effec t o f the dorsa l muscles on the pterothoraci c terg a mus t depen d on a clos e connectio n betwee n th e mesotergu m an d th e metatergu m and between the metatergu m an d the first abdominal tergum; otherwise there woul d be muc h los t motion , since , wit h th e usua l intersegmenta l relations, th e contractio n o f the dorsa l muscle s simply pull s th e terga l plates together . To accommodat e the m t o thei r part s i n th e win g mechanism , th e alate terga have been modified in three principal ways. I n the first place, in orde r tha t th e terga l plate s ma y respon d b y change s in their dorsa l curvature t o the actio n of the longitudinal muscles attached o n them, th e intersegmental membrane s hav e bee n reduce d o r eliminated , usuall y by a redistributio n o f th e intersegmenta l sclerotizatio n betwee n th e mesotergum and metatergum an d between the metatergum an d the first abdominal tergum, which has given rise to the so-called postnotal plates . In th e secon d place, the wing-bearin g plates hav e been strengthened, i n

176

PRINCIPLES OF INSECT MORPHOLOGY

order to withstand th e strain of work imposed upon them, by the development o f various ridges on their inne r surfaces . Thes e ridge s ar e forme d by linea r inflections , o r "sutures," of the oute r surfaces . Consequently , a wing-bearin g tergum is subdivided b y it s ridge s and their suture s int o several distinc t area s characteristi c o f the terg a o f the pterothorax , bu t having n o morphologica l counterpart s i n th e terga l plate s o f other segments. Finally , sinc e the wing s are movable by definit e articulation s

FIG. 97.—Evolutio n o f th e postnota l plate s o f th e mesothora x and metathora x fro m the acrotergite s o f th e segment s following , an d th e developmen t of th e phragmat a an d dorsal muscles . A , generalize d condition . B , postnota l plate s forme d a s extension s of th e acrotergites . C , postnota l plates wit h phragmata cu t of f by secondar y membranes (Mb') from tergal plates following. D, section of Dissosteira, showing postnotum developed only i n metathorax , D , sectio n o f Calliphora, showin g postnotum i n mesothorax , an d metathoracic tergum almost obliterated o n median line.

on th e supportin g bac k plates , th e latera l margin s o f th e mesotergu m and the metatergu m presen t structura l feature s precisely adapte d t o th e hinging o f the win g bases o n the dorsum . Redistribution of the Intertergal Sclerotization in the Wing-bearing Segments.—The mesothoraci c an d metathoraci c terg a o f apterygot e insects an d o f nympha l an d larva l form s o f pterygote insect s havin g a well-developed thoraci c sclerotizatio n ar e th e sam e a s th e abdomina l terga i n tha t eac h terga l plat e comprise s the segmenta l an d precedin g intersegmental sclerotizatio n an d i s crossed anteriorly b y a su b marginal antecostal sutur e (Fig . 86). I t i s evident, therefore , that the immediat e ancestors o f th e winge d insect s ha d a typica l secondar y segmentatio n

THE T HORAX

177

throughout th e dorsu m o f the thora x an d abdome n (Fig . 9 7 A) . Thi s structure is retained i n the mesothoraci c and metathoraci c terga o f some adult winge d insects, suc h as the Isoptera , i n whic h the dorsa l thoraci c muscles are small and wea k and probabl y have little to d o with moving the wings . The pterothoracic and first abdominal terga of the Isoptera , however, are closel y attached t o eac h other. A similar structur e occur s in th e Blattidae , i n whic h als o th e smal l dorsa l muscle s ar e relativel y unimportant element s i n the win g mechanism by compariso n wit h those of mos t insects . Th e successiv e terga i n th e win g regio n o f Blattida e are connected by lateral expansions of the acrotergite s of the metatergum and th e firs t abdomina l tergum. I n bot h Isopter a an d Blattida e ther e may b e smal l phragmata l lobe s o n th e antecosta e o f th e mesotergu m and metatergum . With th e majorit y o f winged insects the dorsa l muscles of the ptero thoracic segment s ar e greatl y enlarged , an d thei r end s ar e attache d on well-develope d phragmat a dependin g fro m th e antecosta e o f th e mesotergum, the metatergum, and the first abdominal tergum (Fig. 97 B). The phragmata, as we have seen , are intersegmental inflections , and th e external grooves , o r antecosta l suture s (acs), throug h thei r base s mar k the primar y intersegmenta l lines . Th e acrotergit e (atg) o f th e meso tergum (r 2) retains the usual form of a narrow flange before the antecostal suture (A , B); but th e acrotergit e of the metatergum and the acrotergit e of the first abdominal tergum (B ) are each enlarged and extended forwar d to th e posterio r margi n o f the tergu m precedin g in eac h case . I n thi s way th e dorsa l intersegmenta l membrane s (A , M fe) ar e practicall y eliminated betwee n the mesotergu m and th e metatergum , an d between the latte r an d th e firs t abdomina l tergum , an d ar e replace d b y th e expanded acrotergites , whic h become postnotal plates of the mesothora x and th e metathorax , respectively (B , PNzj PNz). The obliteration o f th e dorsa l intersegmenta l membrane s b y th e development o f acroterga l postnotal plate s produce s a continuous sclerotization i n th e dorsu m of the pterothora x (Fig . 85 ) from th e bas e o f th e first phragma (Fig. 97 B, IPfi) to that of the third (3Ph). The contractile forc e o f th e dorsa l muscle s is thu s prevente d fro m pullin g th e successive terg a togethe r an d i s therefor e expende d agains t th e terga l plates themselves , whic h ma y no w respon d b y a n upwar d curvature , producing a depression of the wing s on the pleura l fulcra. Th e elimina tion o f th e secondar y intersegmenta l membrane s b y th e enlargemen t of th e acrotergite s virtuall y restore s th e pterothoraci c dorsu m t o a condition of primary segmentation, since the functiona l segmenta l limit s are no w marke d b y th e primaril y intersegmenta l phragmata . Th e postnotal plate s ar e thu s see n t o belon g morphologicall y each t o th e segment o f the terga l plate preceding it.

178

PRINCIPLES OF INSECT MORPHOLOGY

If th e hin d wing s are th e principal organ s of flight, a s in Orthopter a and Coleoptera , a postnotu m i s develope d i n th e metathora x onl y (Fig. 97 D, PNz)- With most of the higher insects, however, in which the for e wings are large and the hind wings small, there is usually present a postnota l plat e i n eac h o f th e alat e segments , thoug h th e secon d i s generally reduce d i n size , a s i s als o th e alinotu m o f the sam e segment . An extrem e adaptation t o the two-winge d condition occurs in the highe r Diptera (E) , in which the metatergu m i s a scarcely perceptible rudimen t (773), an d practicall y th e entir e dorsu m o f th e thora x i s forme d o f th e alinotum an d postnotu m o f th e meso thorax, betwee n which, or their respectiv e phragmata (IPh, 2Ph), exten d th e grea t dorsal muscles of the mesothorax . In man y o f the highe r insect s th e base of eac h phragm a become s separate d fro m the tergum behind it by a transverse lin e of membranization (Fig . 9 7 C , Mb'), an d i n such case s th e postnotu m (Fig . 98 , PN%, ???? ???????? ??? ???? ??? ??????????? ???? but als o th e narro w posterio r li p o f th e phragmatal inflection behind the antecostal suture. Wit h insect s havin g thi s typ e of structure, th e functiona l conjunct!va e o n the dorsum are the membranes (MV) behind th e base s of the phragmata , an d th e FIG. 98.—Diagram s showin g tergal plate s o f th e metathora x an d firs t the derivatio n o f th e phragma - abdominal segment s ar e incomplete by th e bearing postnotal plate (PN), associated wit h eac h wing-bearin g lack o f thei r usua l anterio r phragma alinotal plat e (AN), fro m th e bearing parts. Generall y th e posterio r li p tergum following i n eac h case . of a detache d postnotum i s very narrow or scarcely apparent , bu t i n som e cases it i s large, a s in the metathora x of Panorpa (Fig. 99, PNs), where it includes the major part of the first ????????? ?????? ????? Each phragma consists typically o f a pair o f thin, plate-lik e apodema l lobes (Fig . 98 , Ph) separate d b y a media n notc h givin g passag e t o th e dorsal blood vessel; but i n some cases the tw o lobes are united i n a single broad plate , an d th e bloo d vesse l the n dip s beneat h th e latter . I n certain Hymenopter a th e media n par t o f the secon d phragm a i s mem branous, giving the phragm a the appearanc e of being connected with th e tergum onl y by its lateral angles . The Sutures, Ridges, an d Surface Areas o f th e Alinotum. —The sur faces o f th e alinota l plate s ar e greatl y diversifie d i n differen t insect s by topographica l irregularitie s an d b y sutures . . Th e so-calle d suture s

THE THORAX

179

are mostly the external grooves of internal ridges, which are the important mechanical feature s o f the notum , but the suture s are the character s more generally used in descriptive works. Th e principal alinota l suture s and th e area s the y defin e ma y be described a s follows : The antecostal suture i s th e groov e throug h th e bas e o f a phragm a which marks the lin e of the antecost a (Fig . 10 0 A, acs). Th e acrotergit e before th e antecosta l sutur e i s usually a ver y narro w anterior li p of the alinotum (atg), excep t whe n i t i s enlarge d t o for m th e postnota l plate of th e precedin g segment (Figs . 96 , 98). The scutoscutellar, o r V-shaped, suture (Fig . 10 0 A , vs ) lie s i n th e posterior par t o f the alinotu m wit h its ape x directed forward ; i t divide s the notu m int o a n anterio r scutum (Set) an d a posterio r scutellum (Scl). Internally thi s sutur e form s usuall y a stron g V-shaped ridge (B , VR),

FIG. 99.—Head , thorax , an d bas e o f abdomen o f Panorpa consuetudinis.

which no t onl y strengthen s th e notu m bu t probably , i n it s typica l form, serve s als o a s a gradien t devic e to brin g th e pea k o f the upwar d flexure o f th e notu m durin g fligh t o n a lin e betwee n th e base s o f th e wings. Th e scutoscutella r ridg e an d it s suture , however , ar e subjec t to muc h variatio n i n for m an d degre e o f development , an d the y ar e sometimes obsolet e o r absent; but i n general the V-shape d ridg e an d it s suture ar e the mos t constan t feature s of the wing-bearin g plates and ar e present i n some form i n nearly al l winged insects (Fig . 101 , vs). A reversed notal suture occurs in some insects i n which the tru e scuto scutellar sutur e is obsolete or absent. I n the Acrididae , for example, th e usual V-ridg e an d it s sutur e ar e partiall y suppresse d (Fig . 10 1 C, vs), and the posterior part of the alinotum is marked by the line of a secondary ridge (rvs) o f simila r shap e bu t havin g th e apex directe d posteriorly . The tru e scutella r regio n is thus divide d int o a media n elevate d shield shaped are a (Scl) an d tw o latera l depresse d area s (scl, scl). A simila r

180

PRINCIPLES OF INSECT MORPHOLOGY

topographical conditio n i s even more strongly pronounce d in th e meso thorax o f Hemiptera an d Coleoptera . A transverse, or prescutal, suture (Fig . 10 0 A, ts), wit h its correspond ing interna l ridg e (B , TR), i s o f frequent recurrenc e in man y group s of insects. I t lie s i n th e anterio r par t o f the alinotu m an d set s of f a pre?????? ?????? ?? ??? ???? ??????????? ?????? ??? ?????????? ??????? prescutum is variable in size and shape . I t i s well developed in Plecop tera, i n som e Orthopter a (Fig . 10 1 B , C) , an d i n th e mesothora x of Lepidoptera (G ) and Coleoptera (H) , but i n other insects it i s frequently very narro w (D , E , I) ; in Dipter a i t end s i n a smal l lob e o n eac h sid e of th e notu m befor e th e win g base (D , E, e). Sinc e the prescuta l sutur e is often obsolet e o r absent, however , the prescutu m ma y b e bu t weakl y denned o r not distinguishabl e fro m th e scutu m (F , J).

FIG. 100.—Diagram s showin g th e principa l feature s o f a generalize d wing-bearin g ?????? ?????? ?? ?????? ??????? ???? ???????? ?? ??????? ??????? ???? ????????????? ridges. Ac, antecosta ; acs, antecosta l suture ; ANP, anterio r nota l win g process ; atg, acrotergite; AxC, axillary "cord"; Em, lateral emargination; Ph, phragma; PNP, posterio r notal win g process; Pra, prealare; Prsc, prescutum; Rd, posterio r margina l fol d o f alinotu m continuous wit h win g margins ; Scl, scutellum ; Set, scutum ; TR , transvers e nota l ridge ; ts, transvers e notal suture ; VR, V-shaped , o r scutoscutellar , ridge ; vs, suture o f V-shape d ridge; W, bas e o f wing.

A pai r o f convergent sutures, o r notaulices, sometime s occur s i n th e anterior par t o f the alinotum . Thes e suture s aris e anterolaterall y an d extend posteriorl y a varyin g distance , usuall y convergin g towar d the median line of the back (Fig. 101 I, no); but th e same sutures apparently, in som e cases, ma y continu e posteriorly t o th e transscuta l sutur e (F , no) and thus divide the scutum into a median area (g) and two lateral areas (h, h). Th e convergen t suture s ar e generall y know n a s th e "notauli " to systematists in Hymenoptera, which term is evidently a misspelling fo r notaulices (fro m aulix, auliciSj a furrow) , bu t th e sam e sutures ar e ofte n called "para.psida l furrows, " and , again , man y entomologist s hav e regarded them a s discontinuous median parts of the transvers e prescuta l suture turne d posteriorly . I f th e convergen t suture s ar e part s o f th e prescutal suture , th e are a betwee n them i s the prescutum , bu t i n som e insects, a s in Tenthredinidae (Fig . 10 1 I), bot h th e transvers e prescuta l

THE THORAX

181

suture (ts) an d the convergen t sutures (no) are present, showing tha t th e area between the latte r belongs t o the scutum . Th e developmen t of the convergent suture s an d thei r interna l ridge s i s correlate d wit h th e pos terior extensio n o f th e attachment s o f th e dorsa l win g muscle s o n th e scutum (Fig . 12 8 B). A pai r o f lateral sutures, o r parapsidal furrows, occur s frequentl y in th e scutu m o f th e mesothora x o f Hymenoptera ; th e suture s begi n

FIG. 101.—Examples of various sutural patterns i n the tergal plates of the thorax. A , mesonotum o f Japyx. B , metathoraci c alinotu m o f Gryllus. C , mesonotu m o f Melanoplus. D, pronotum (Ti) and mesothoracic alinotum of Holorusia. E, pronotum and meso thoracic alinotu m o f Tabanus. F , mes o thoracic alinotu m o f Cynips. G , meso thoracic alinotu m o f Phassus. H , mesothoraci c alinotu m o f Calosoma. I , mesothoraci c alinotum o f Pteronidea. J , mesothoraci c alinotu m o f Apis. (Fo r principa l suture s se e Fig. 100. ) a , b , c, d, intrascutal suture s o f Diptera; e , prescutal lobe ; /, tran s scutella r suture; g, median scutal area; h, h, lateral scuta l areas; i, median notal suture; j, j, transvers e intrascutal suture s o f Coleoptera ; -k, posterio r intrascuta l groove ; I, tran s scuta l suture ; TIO, notaulix ; pf , parapsida l furrow .

posteriorly usually at the transscutal suture and diverge forward a varying distance i n th e latera l area s o f th e scutu m (Fig . 10 1 F , pf). Thes e sutures, accordin g t o Tulloc h (1929) , ar e th e suture s properl y terme d parapsidal furrows . Th e part s o f the scutu m lyin g latera d o f them ar e the parapsides. I n commo n practice , however , th e anterio r convergen t sutures of Hymenoptera ar e ofte n calle d th e parapsida l furrows , and th e

182

PRINCIPLES O F INSECT MORPHOLOGY

areas laterad of them in Cynipoidea and Chalcidoidea are then designated the parapsides. I n general usage the two terms, notaulices and parapsidal sutures , therefore , are usuall y synonymou s since each ma y refe r t o the anterio r convergen t sutures. A median suture o f the notu m sometimes occurs i n th e anterio r par t of th e scutu m (Fig . 10 1 I, i ) o r extends through the entir e length of th e scutum (G , i). Internall y i t form s a media n carina , which, when fully developed, extend s lik e a ridgepol e from th e prescutu m t o th e ape x of the scutellum . A transscutal suture in most o f the clistogastrou s Hymenoptera , a s in the honey bee (Fig. 101 J, Z) , cuts completely through the posterior part of the scutum , settin g of f two posterolatera l area s o f th e latte r (set, set) from the major scutal area (Set). The parts of the alinotum separated by thi s sutur e ar e commonl y termed "scutum " an d "scutellum " b y students of Hymenoptera, but it is clear that these areas are not identica l with th e scutu m an d scutellu m o f more generalized insects. Th e tru e scutoscutellar sutur e is well preserved in the Tenthredinida e (Fig . 10 1 I, vs) an d ma y be present also in the higher Hymenoptera (J, vs) in conjunc tion wit h the transscuta l sutur e (Z) . A transscutellar suture cuts through the anterio r par t o f the scutellum in highe r Dipter a (Fig . 10 1 E, /) betwee n the latera l extremitie s o f th e scutoscutellar sutur e (vs). Various othe r suture s ma y occu r i n th e alinotum , whic h ar e ofte n characteristic o f differen t orders , bu t whic h ca n b e give n n o genera l names. I n th e metathora x of Coleoptera, for example, the latera l area s of th e irregula r scutu m ar e partiall y divide d b y obliqu e suture s (Fig . 101 H, jj j ) int o anterio r an d posterio r parts (Set, Set). I n th e Dipter a short latera l suture s or also longitudinal sutures occur in the mesoscutum (Z), E , a , b, c, d)y which give the sectional pattern characteristic of the wing-bearing notum in this order. A. comparative stud y of the suture s an d subdivision s o f the alinotu m brings ou t s o many differences i n these features betwee n different order s that i t become s questionabl e i f man y o f the m ar e trul y homologou s structures. Asid e fro m th e antecosta l suture , th e onl y fairl y constan t character o f a wing-bearin g tergum is its divisio n int o tw o majo r part s by th e sutur e o f the V-shape d endoterga l ridge, an d eve n thi s featur e is often obscure d by a partial suppressi6 n of the ridge. Th e other ridges and sutures, producin g various tergal subdivisions , ar e local adaptation s to mechanica l stresse s an d demand s fo r flexibilit y i n th e wing-bearin g plate, an d a carefu l stud y of the win g mechanism would probably reveal the reason for them in each particular case. Th e structure of the tergum is often quit e differen t i n the tw o segments of the pterothora x according to the relativ e development of the two pairs of wings, and in most insect s

THE TH ORAX

183

with smal l o r rudimentar y hin d wing s the metatergu m become s much reduced. The Alar Margins o f th e Alinotum. —The latera l margin s o f th e alinotum ar e specificall y modifie d i n adaptation t o the comple x articular and flexor mechanisms of the win g bases. Ver y frequently each anterior lateral angle of the postcostal region of the notum is produced in a prealar bridge, o r prealare (Fig . 96 , Pra), that extend s laterad o r ventrad t o th e episternum (Eps) and thus supports the notum anteriorly on the pleural wall o f th e segment . Th e wing s arise fro m th e scutoscutella r margin s of th e alinotu m (Fig . 10 0 A, W), whic h may b e long or much shortened, and the posterior thickened edges, or "axillary cords" (AxC), of the basal wing membranes are alway s continuous with the posterio r margina l fol d of the scutellum (Rd). Each ala r margi n o f the scutu m present s typicall y a n anterior notal wing process (Figs. 96, 100 A, ANP) and a posterior notal wing process (PNP). The anterior notal wing process is almost always present and supports th e nec k o f th e firs t axillar y sclerit e o f th e win g bas e (Fig . 101 B, I , I Ax). Immediatel y behin d i t i s a dee p emarginatio n i n th e edge of the scutu m (Fig. 10 0 A, Em). Th e posterior win g process usually gives suppor t t o th e thir d axillar y o f the win g base (Fig . 122 , 3Ax), bu t sometimes a fourth axillar y intervenes between the third axillar y and the notal margin, and in such cases a posterior win g process is usually absent. In th e metathora x o f som e Orthopter a a specia l ar m o f th e alinotu m supports th e vanna l vein s of the wings . The Pleuro n o f a Winged Segment.—Th e pleura of the wing-bearin g segments d o no t diffe r fundamentall y from th e pr o thoracic pleura , bu t secondary differences betwee n the tw o may be considerable on account of the degenerativ e tendenc y o f the prothoraci c pleura , an d becaus e of th e special development s tha t tak e plac e i n the pterothoraci c pleura . Th e pleura of the pterothora x ar e important element s of the win g mechanism, though, fo r the mos t part , their role is a passive one . Th e pleur a show many mino r variation s i n structure , an d thei r area s ma y b e variousl y broken up into secondary sclerites. I n most cases it is difficult t o discover the mechanica l significance o f these modifications , but thei r progressiv e development withi n a family o r order ofte n furnishe s a valuabl e clu e t o the relationship s o f genera and families . The principal alar functions of the pleuron in a winged segment are to furnish a fulcru m fo r th e win g an d t o giv e attachmen t t o th e pleura l wing muscles , thoug h usually , a s already observed , th e pleuro n als o supports th e terga l plate s on the preala r an d postala r arm s of the latte r (Fig. 96 , Pra , Pa) . Th e win g fulcrum , o r pleural wing process (WP), typically has the for m of a short, thick arm arising from the dorsal margin of th e pleuron . Th e win g proces s i s brace d internall y b y th e pleura l

184 PRINCIPLES

O F INSECT MORPHOLOGY

ridge, whic h in th e adul t insec t extend s dorsally o r obliquely from th e coxal process of the pleuro n to th e win g process . Th e pleura l suture i s thus to be identified as the groove on the external wall of the pleuron (PIS) that extend s betwee n th e articula r processe s o f th e win g an d th e leg . Usually the pleural suture takes a direct course between these two point s (Fig. 10 2 D, PIS2), but it may be irregular or angularly bent (E, PZS2). The episternu m an d epimero n o f the wing-bearin g segments exhibi t numerous variation s i n for m an d underg o variou s subdivision s int o secondary sclerites, bu t thei r modification s are in general easy to follow . The most frequent type of subdivision divides the episternal and epimeral regions int o dorsa l an d ventra l areas . Thes e ar e distinguishe d a s th e supraepisternum, or anepisternum, the infraepisternum, or katepisternum, the supraepimeron, or anepimeron, and the infraepimeron, or katepimeron. An anterio r subdivisio n o f th e episternu m i s a preepisternum; i f i t i s continuous wit h th e presternum , th e anterio r pleurosterna l piec e thu s set of f i s terme d th e prepectus. Th e precoxa l an d postcoxa l area s o f the pleuron (Fig. 96, Prcx, PCX) are usually sclerotized in the wingbearing segments, forming precoxal and postcoxal bridges to the sternum . Sometimes they ar e separated fro m th e episterna l an d epimera l regions, but usuall y i n adul t insect s the y ar e unite d wit h th e sternum . Th e postcoxal bridg e i s generall y narrowe r tha n th e precoxa l bridge an d i s more frequently absent. The trochanti n o f th e pterothoraci c segment s (Fig . 96 , Tri) i s bes t developed i n th e mor e generalize d Pterygota , bu t i t alway s show s a tendency toward reduction and is lost in the higher orders. The Epipleurites. —The chie f distinctiv e featur e of the pterothoraci c pleura, aside from th e presenc e of the win g processes, is the developmen t and individualization of small sclerites beneath th e win g bases, o n which important muscle s o f th e wing s ar e inserted . Thes e plate s ma y b e termed the epipleurites, since they lie above the principal pleurites from which they are derived, though they hav e often bee n called the "parap tera." Th e anterior , o r episternal , epipleurite s ar e th e basalares; th e posterior, o r epimeral , epipleurite s ar e th e subalares. Generall y ther e is but on e basalare i n each segment (Fig . 96, Ba) an d on e subalare (So), though eac h is sometimes double. The epipleurites appear to be derived by a secondary separation fro m the upper edges of the episternum and the epimeron. Whil e the subalar e is nearly alway s a distinc t sclerit e i n adul t insects , th e basalar e i s frequently but a n imperfectly separated lob e of the episternu m (Fig . 99, Ba) or merely an area of the latter on which the anterio r pleura l wing muscles are attached. I n nymphal Orthopter a neithe r th e basalar e no r the sub alare i s ye t differentiate d from th e res t o f the pleuro n (Fig . 10 2 A, B) , and bot h th e anterio r an d posterio r pleura l win g muscles arise fro m th e

??? ?????? ??? upper edge s o f th e latter , th e firs t o n th e episternum , th e secon d o n the epimero n (C , 3E', 3E"). The Mesopleuron o f Díptera. —In th e highe r Dípter a th e structur e of the pleuro n of the mesothora x becomes complicated by several unusua l modifications. I n the Tipulidae , however , the mesopleura l elements are relatively simpl e an d eas y t o identify . I n Holorusia (Fig . 10 2 D), fo r example, the pleural suture (PISz) takes the ordinary straight course from th e bas e o f th e cox a t o th e win g process , an d th e onl y unusua l features o f th e pleuro n ar e th e presenc e o f a n incomplet e sutur e (a ) separating the episternal region (Epsz) from the precoxal region (Prez),

FIG. 102.—Example s o f simpl e an d highl y modifie d pattern s o f th e pleura l sclerite s of wing-bearin g segments . A , nymp h o f Melanoplus. B , nymp h o f Gryllus, meso pleuron. C , inne r vie w o f sam e showin g muscles . D , Holorusia granáis, mesopleuron. E, Tabanus atratus, mesopleuron and metapleuron.

and the partial separation of the lower part of the epimeron (epm^) from the principa l epimera l area (Epm^) abov e it . Th e mero n of the middl e coxa (Mer%) i s conspicuou s by it s size , but i t i s no t detache d fro m th e rest o f the coxa . In th e highe r Diptera , a s exemplifie d by Tabanus (Fig . 10 2 E), th e pleural sutur e o f th e mesothora x (P£/S 2) i s sharpl y flexe d i n tw o rec tangular bends , an d a long , membranou s clef t (5 ) extend s downwar d in the episternum , befor e the dorsal part of the pleural suture, from the wing ??????? ???-? ?? ? ?????????? ????????????????? ?????? ???? ??? episternal are a i s divide d b y thi s clef t (& ) int o a larg e anterio r regio n (Eps%) separated from the precoxal area by the suture a, and into a smaller posterior region (eps%), most of which is ventral to the horizontal

186

PRINCIPLES OF INSECT MORPHOLOGY

???? ?? ??? ??????? ?????? ??? ?????????? ???? ??? ???????? ???? ??? ??? ???????? ?? ???? ??????? ???? ? ????? ????????????? ?????? ??? ? smaller infraepimero n (epm*).Finally , th e mero n o f th e mesocox a (Mer%) i s completel y detache d fro m th e res t o f th e cox a an d i s solidl y incorporated int o th e pleural wall , being unite d bot h wit h th e epimera l plate above it and with the narrow postcoxal bridge (Pcx%) behind it. The ventra l en d of the subala r muscl e of the wing , normally attached o n the meron , as it i s in Tipulidae, ha s migrate d upwar d to th e horizonta l part of the pleura l suture; but th e rem o tor muscl e of the cox a retains it s attachment o n the meron and becomes an adjunct to the indirect elevator s of th e wings . Thes e comple x changes in the mesopleuron , together wit h the reductio n o f th e metapleuro n an d th e unusua l modification s of th e sterna (Fig . 95 B), give the thorax of the higher Diptera a very specialized type o f structure . The Sternu m o f a Winged Segment.—Th e sterna of the wing-bearing segments sho w fewe r specia l modification s than d o eithe r th e terg a o r the pleura of these segments; their essential structure has been sufficientl y described i n th e genera l discussio n o f the thoraci c stern a (page s 16 6 to 172). Whil e th e pterothoraci c stern a diffe r i n n o importan t respec t from th e sternu m o f the prothorax , peculiaritie s o f structur e ar e likel y to b e mor e accentuate d i n them . Eac h i s characterize d principall y b y the greater siz e of the basisternal regio n on which the tergosternal muscle s are attached . I n th e highe r order s th e secon d spinasternu m i s usuall y consolidated wit h th e eusternu m o f th e mesothora x an d ma y becom e indistinguishable fro m th e latter. 6. TH E THORACI C MUSCLE S

The thoraci c muscle s of all pterygot e insects , exceptin g perhaps th e Odonata, confor m closel y t o on e genera l pla n o f arrangement . Th e potential numbe r o f muscle s i n eac h segmen t appear s t o b e limited , or, a t least , th e maximu m number o f muscles or o f functiona l unit s of fiber bundle s ca n b e prett y definitel y state d accordin g to ou r presen t knowledge of the thoracic musculature in the principal orders of pterygote insects, thoug h th e ful l complemen t doe s no t occu r i n an y on e group . Variations i n th e muscl e pattern , therefore , ar e th e result principall y of th e absenc e o f certain muscles , though a singl e muscle in on e specie s may b e represented i n another by two or more bundles of fibers having a common point of insertion. Th e leg musculature varies accordin g to th e different type s o f movemen t i n th e coxa e resultin g fro m alteration s in the coxa l articulation o n the body, the simpler types of leg musculature being evidentl y secondar y modifications correlated wit h a limitatio n o f the coxa l movement.

THE THORAX

187

Morphologically th e usua l thoraci c muscles , a s represente d i n a n álate segment , ma y b e classe d a s (1 ) dorsa l muscles , (2 ) tergopleura l muscles, (3 ) tergosterna l muscles , (4 ) tergocoxa l muscles , (5 ) pleuro sternal muscles, (6 ) pleurocoxal muscles, (7 ) ventral muscles, (8 ) sternocoxal muscles , (9 ) latera l intersegmenta l muscles , an d (10 ) spiracula r muscles. Th e musculature o f the prothora x differ s fro m that of a winged segment chiefl y i n th e lac k o f th e tergosterna l muscle s an d o f muscle s in other groups that function principally in connection with the movemen t of th e wing s in the pterothoraci c segments .

FIG. 103.—Th e thoraci c musculature , diagrammatic , showin g mos t o f th e muscle s known to occur in a wing-bearing segment. A , dorsal, ventral, tergosternal, and oblique muscles o f right side , inne r view . B , latera l an d le g muscle s o f righ t side . A , dorsal muscles (IA, obliqu e lateral dorsals; mA, longitudinal median dorsals); B, tergopleural muscles; C, tergosternal muscle; D, the wing flexor; E1', basalar muscles (IE' t pleurobasalar; 2E', sternobasalar ; 8E', coxobasalar); ' E" , subala r muscle s {IE", pleurosubalar ; 3E", coxosubalar); F, obliqu e intersegmental muscle ; G, pleurosternal muscle; H, ventra l muscles; /, J, tergal promotor and remoter of coxa (uppe r parts cu t off) ; K, L, sternal promotor and remote r o f coxa ; M, abductor s o f coxa ; JV , adducto r of coxa.

For a practica l stud y o f th e thoraci c musculatur e i t wil l b e foun d more convenien t t o classif y th e muscle s i n th e followin g groups , th e individual muscle s o f whic h ar e show n diagrammaticall y i n Fig . 103 , the mor e median muscle s on the righ t sid e a t A , the lateral muscles at B. The spiracula r muscles , omitte d here , wil l b e describe d i n connectio n with th e spiracles . A. Dorsal Muscles. —The muscle s o f thi s grou p compris e longi tudinal median muscle s and obliqu e lateral muscles (Fig . 103 A, mA, I A).. In th e prothora x th e principa l dorsa l muscle s exten d fro m th e firs t phragma t o th e postoccipita l ridg e o f the hea d (Fig . 87, DMcl), thoug h others ma y g o from th e tergu m t o th e hea d o r fro m th e tergu m t o th e

188

PRINCIPLES OF INSECT MORPHOLOGY

first phragma. I n the pterothorax the median dorsals are usually highly developed (Figs. 97 D, DMclj 103 A, mA), at least in the segment bearing the principal pair of wings (Fig. 97 E), and are attached on the phragmat a or also on the alinotal and postnotal plates. The y are reduced in wingless insects, o r in insects with weak powers of flight, as in Isoptera, Blattidae , and Gryllidae. Th e oblique lateral dorsals, when present (Fig . 103A, IA), extend fro m th e posterio r part o f the scutu m to th e followin g phragma . Though usuall y relativel y small , thes e muscle s ar e highl y develope d in th e mesothora x of higher Diptera an d ar e larg e and powerful , almos t vertical, muscle s in the mesothora x of some Homoptera (Fig . 12 8 B, IA). B. Tergopleural Muscles. —These muscles , foun d principall y i n th e wing-bearing segments , ar e highl y variabl e i n thei r development , an d some or all of them may be absent. Fou r muscles of this group have been recorded i n th e mesothora x o f differen t insects . On e goe s fro m th e prealar ar m o f the tergu m t o th e episternu m (no t show n in th e figure) , another (Fig . 10 3 B, 2B) fro m th e latera l terga l margi n to th e basalare , a thir d (35 ) fro m th e tergu m t o th e win g process. Th e fourt h muscle (4J5) extend s fro m th e posterio r par t o f the scutu m t o th e bas e o f th e pleural arm or the lowe r part of the pleura l ridge. Th e last i s an impor tant muscle in Ephemerida, Plecoptera, Sialidae, Mecoptera, Trichoptera, Aphididae an d is often tw o branched. C. Terg asternal Muscles.— Generally larg e muscle s o f th e ptero thorax i n flyin g insects ; attache d abov e o n th e scutum , belo w o n th e basisternum anterio r t o th e coxa e (Figs. 10 3 A, 128 B, 130). Absen t in weak-flying insects, and not represented in the prothorax. Thes e muscles are th e principa l levator s o f the wings , being functionall y antagonistic to the dorsa l muscles in their actio n on the tergum . D. Axillary Muscles. —The muscle s o f th e axillar y sclerite s o f th e wing bas e aris e o n th e pleuro n an d ar e probabl y i n origi n tergopleura l muscles sinc e th e wing s ar e expansion s o f th e dorsum . Tw o muscle s occur in this group. One , known only in Diptera, i s inserted o n the firs t axillary. Th e other , presen t i n al l winge d insects (Fig . 10 3 B, D) , i s inserted o n the thir d axillar y and is the usua l flexor of the win g (Fig. 128 C, D); i t i s a simple or branched muscle arising o n the uppe r part o f the pleuron. E. Epipleural Muscles. —The muscle s comprise d i n thi s grou p ar e the latera l muscles of the mesothora x and metathorax attache d dorsall y on th e epipleura l sclerite s (basalar e an d subalare) . I n th e adul t the y are importan t muscle s o f the wings , but i n nympha l stage s the y aris e on th e uppe r margin s o f the pleura , an d tw o o f them appea r t o b e pri marily coxa l muscles (Fig. 102 C). Th e basalar muscles (Fig. 103 B, E') include a muscle from the episternum (IE'), a muscle from the sternum before th e cox a (2E') t and a large muscle (3Ef) attached below on the

??? ?????? ??? outer margin of the coxa before the pleural articulation o f the latter. Th e first tw o o f these muscle s are variabl e i n occurrence , either on e o r bot h being often absent; the third is a constant feature of the thoracic muscula???? ?? ?????? ???????? ??? ??????? ??????? ???? ????????? ??? ??????? muscles i n revers e order , on e arisin g o n th e epimero n (1J£") > anothe r on th e sternum , an d th e thir d (32?") - o n th e cox a behin d th e pleura l articulation. Th e coxosubala r muscl e i s a n importan t elemen t o f th e wing mechanism ; th e postcoxa l sternosubala r i s highl y develope d i n Ephemerida bu t ha s no t bee n observe d i n othe r orders ; th e epimero subalar muscl e appears occasionally in various insects . ?? ??????? ?????????????? ?????????? ??????? ?????? ???????? below o n th e sterna l apophysis , dorsall y o n th e anterio r margi n o f th e following pleuro n o r tergu m (Fig . 10 3 A , F) . Thi s muscl e i s mor e commonly presen t i n generalize d insects an d i n larva l forms ; i n adult s it usuall y occur s only between the prothora x and the mesothorax , but a corresponding muscl e i s sometime s presen t betwee n th e mesothora x and the metathorax . G. Pleurosternal Muscles. —The muscl e mos t commonl y presen t i n this grou p consist s o f shor t fiber s connectin g th e oppose d end s o f th e pleural an d sterna l apophyse s o n eac h side o f the segmen t (Fig . 10 3 A, (?); absent when the apophyses are united. I n rare cases a muscle extends from th e lower end of the pleura l ridge to th e sterna l apophysis . H. Ventral Muscles. —Longitudinal o r obliqu e horizonta l muscle s stretched betwee n th e eusterna l apophyses , betwee n th e spinasterna l spinae, an d betwee n the apophyse s an d th e spina e (Fig . 10 3 A). Th e prothoracic muscle s of thi s grou p ar e attache d anteriorl y o n th e head , usually o n the tentoria l bridge , or some of them o n the cervica l sclerite s (Fig. 87) . /. Tergal Promotor o f th e Leg.—Usually a singl e large muscle, some times double , arisin g dorsall y o n th e tergum , inserte d belo w o n th e ventral en d o f th e trochanti n (Fig . 10 3 B, 7 ) o r o n th e anterio r angl e of th e cox a if the trochanti n i s absent . J. Tergal Remotor o f the Leg.—A single muscle, or a group of muscles, arising dorsall y o n th e posterio r par t o f the tergum , inserte d ventrall y on the posterio r rim of the cox a (Fig . 10 3 B, J). K. Sternal Promotor o f th e Leg. —Origin o n th e sternum ; insertio n on the anterio r part of the coxa l base (Fig . 103 B, -K). I f the cox a turns on the pleura l articulation alone , this muscle is an anterior rotato r of the coxa. L. Sternal Remotor o f th e Leg.—Origin o n th e sternum , th e sterna l apophysis, o r the spina ; insertion o n the posterio r par t o f the coxa l base (Fig. 10 3 B, L). Thi s muscle , as the last , is a rotator o f the cox a if th e latter has a free movemen t on the pleuro n only.

190

PRINCIPLES OF INSECT MORPHOLOGY

M. Pleurocoxal Muscles. —Usually tw o muscles , arisin g o n th e epi sternum, inserte d on the coxa l bas e anterior t o th e pleura l articulation (Fig. 10 3 B, IMj 2M). Thes e muscle s appea r t o b e abductor s o f th e coxa i f th e cox a ha s n o sterna l articulation ; otherwis e they ar e coxa l promotors. N. Adductor Muscle o f the Coxa.—A muscle present in insects lacking a sternal articulation of the coxa, arising on the sternal apophysis, inserted on th e mesa l margin of the cox a (Fig . 10 3 B, N) . P. Extracoxal Depressor o f th e Trochanter. —The depresso r o f th e trochanter usually has one or more branches arising in the bod y segment bearing th e le g (Fig . 117) ; generally there is a branc h from th e tergu m (Fig. 11 5 A , 133c) , anothe r fro m th e sterna l apophysi s (133d) , an d sometimes one from th e pleuro n (Fig . 102 C, P) . GLOSSARY O F TERM S APPLIE D T O TH E THORA X Names use d i n th e presen t chapter , bu t no t give n i n th e followin g list , ma y be found i n the glossarie s o f Chaps. Ill, V , IX, an d X . Alinotum (AN). —The wing-bearin g plat e o f th e dorsu m o f th e mesothora x o r metathorax o f pterygote insects . Anapleurite (Apl). —The dorsa l supracoxal sclerotization o f a generalized thoracic pleuron. (Eupleuron.) Anterior Nota l Wing Process (ANP). —The anterio r lob e of the latera l margin of the aliñ o turn supportin g th e nec k o f the firs t axillary . (V order er Tergalhebel.) Basalare (Ba). —The episterna l epipleurit e (sometime s double ) givin g insertio n to th e anterio r pleura l muscle s o f the wing ; often represente d b y a n undetache d o r partially detache d lob e o f th e episternu m befor e th e pleura l win g process . (Episternalgelenkstück, preparapteron.) Basisternum (Bs). —The principa l area o f the sternu m anterio r t o the root s o f the sternal apophyse s o r th e sternacosta l suture . (Sternannum.) Cervical Sclerites , Cervicali a (cv). —The sclerite s o f the neck , particularly on e o r two pair s o f latera l nec k plate s (Kehlplatten) joinin g th e hea d t o th e prothoraci c episterna. Cervix (Cvx). —The neck ; including probabl y th e posterio r nonsclerotize d part of the labia l somit e and th e anterio r par t o f the pr o thorax. Coxopleurite (Cxpl). —The sclerit e o f a generalize d thoracic pleuro n adjacen t t o the dorsa l margin o f the coxa , bearin g th e dorsa l coxa l articulation; it s anterio r par t becomes the definitive trochantin. (Eutrochantin, Trochintinopleura.) Epimeron (Epm). —The are a o f the pleuro n posterior t o th e pleura l suture, some ????? ??????? ???????????? ???? ? ?????????????? ?? ??????????? ??? ?? ?????????????? or katepimeron. Epipleurites (Ba, Sa). —The basala r (Ba) and subala r (Sa) sclerite s o f a wing-bearing segmen t differentiate d fro m th e uppe r end s o f th e episternum an d epimeron , ????????????? ??????????? ????????????????????? Episternum (Eps). —The are a o f the pleuro n befor e th e pleura l sutur e an d abov e the trochantin, sometimes divided horizontally into a supraepisternum, or anepisternum, and a n infraepisternum, o r katepisternum. Eupleuron (Apl). —See anapleurite. Eusternum (ES). —The intrasegmenta l ventra l plate of a thoracic segment , exclusive of the spinasternum , bu t usuall y includin g the sternopleurites . Eutrochantin.—See coxopleurite.

THE THORAX

191

First Thoraci c Spiracl e (Sp 2).—The spiracl e o f the mesothorax , ofte n displace d into th e posterio r par t o f the prothorax . Furca (Fu).- —The forke d endosterna l proces s o f highe r insects , forme d o f th e sternal apophyse s supported o n a median inflection o f the sternum . Furcasternum.—A distinct par t o f the sternu m i n some insects bearing the furca . (The term generall y applied t o the sternellum. ) Intersternites (1st). —Primary intersegmenta l sclerite s of the venter , becomin g the spinasterna of the thorax . Laterosternite (Ls). —The latera l par t o f a definitive thoracic sternum apparentl y derived fro m the ventra l arc (sternopleurite ) of the subcoxa. Mesothorax (Thz). —The secon d segment o f the thorax ; bearin g th e firs t pai r of wings in winged insects . Metathorax (Thi).~— The thir d segmen t o f the thorax ; bearin g the secon d pair of wings in winged insects. Notaulices (no). —Longitudinal furrow s convergen t posteriorl y i n th e anterio r part o f th e mesonotu m o f som e insects. (Incorrectl y spelle d "notauli, " an d some times mistake n fo r the parapsida l furrows . Singular , notaulix.) Notum (T). —The tergum , or particularly th e tergu m of a thoracic segment. Parapsidal Furrow s (pf). —Latera l groove s divergen t anteriorl y i n th e posterio r part o f the scutu m o f the mesothorax of some Hymenoptera. Parapsides.—Lateral areas of the mesoscutu m in some Hymenoptera laterad of the parapsidal furrows . (Singular , parapsis.) Paraptera.—See epipleurites. Phragmanotum (PN). —See postnotum. Phragmata (Ph). —Plate-like apodema l lobes of the antecosta e o f the mesonotum, metanotum, an d firs t abdomina l tergum; the secon d and thir d carrie d b y th e post notal plates o f the pterothora x when the latter are separated fro m th e followin g terg a to whic h they normall y belong. (Singula r phragma.) Pleural Apophysis (PIA). —The interna l ar m o f the pleura l ridge. Pleural Ridg e (PIR).—The endopleura l ridge formed by the pleural suture, bracing the pleuro n above the leg , or between the coxa l articulation and the win g support. Pleural Sutur e (PIS). —The externa l groov e o f the pleura l ridge , separatin g th e epistermim from th e epimeron. Pleural Wing Process (WP). —The win g support o f the pleuro n at th e uppe r end of th e pleura l ridge . Pleuron (PI). —The sclerotizatio n o f the pleura l area o f a body segment, probabl y derived fro m th e subcoxa l part o f the primitiv e limb basis. Postalar Bridge , Postalar e (Pa). —A latera l extensio n o f th e postnotu m o f a wing-bearing segmen t behin d th e win g base , generall y unite d wit h th e epimeron . (Later opostnotum.) Postcoxal Bridge , Postcoxal e (Pc). —The postcoxa l par t o f th e pleuron, ofte n united wit h the sternu m behin d the coxa. Posterior Nota l Win g Process (PNP). —A posterio r lob e of the latera l margi n of the alinotum supporting the third axillary sclerite of the wing base. (Hinterer Gelenkfortsatz.) Postnotum, Phragmanotu m (PN). —The postscutellar , phragma-bearin g plat e often presen t in the dorsu m of an álate segment , derived from th e anterio r part of the following tergum . (Postscutellum.) Prealar Bridge , Prealare (Pro).— A latera l extensio n o f the prescuta l are a o f th e alinotum befor e th e win g base, sometimes connected with the episternum. Precoxal Bridge , Precoxale (Prcx). —The precoxa l part o f the pleuro n anterior t o the trochantin , usuall y continuou s wit h th e episternum , frequentl y united wit h th e sternum, sometime s a distinct sclerite .

192

PRINCIPLES OF INSECT MORPHOLOGY

Prepectus (Prp). —An anterio r margina l sclerit e o f th e sternopleura l area s o f a segment, set off b y a transverse suture continuous through the sternum and episterna . Prescutal Sutur e (ts). —A transvers e groov e o f th e mesonotu m o r metanotu m behind th e antecosta l suture , settin g of f a prescutu m fro m th e scutum , an d formin g internally a prescutal ridge (TR). Prescutum (Prsc). —The anterio r are a o f the mesonotu m or metanotum betwee n the antecostal suture an d the prescutal suture, whe n the latter is present. Presternum (Prs). —A narro w anterior are a of the sternu m sometimes set of f from the basisternu m by a submarginal suture o f the eusternum . (No t the acrosternite. ) Propodeum.—The firs t abdomina l segment o f clistogastrous Hymenopter a incor porated into th e thorax . (Median segment.) Prothorax (Thi). —The firs t segmen t of the thorax . Pterothorax.—The two wing-bearin g segments, ofte n closel y connected or unite d with each other. Scutellum (Sd).— The are a of the alinotu m posterior to the sutur e of the V-shape d notal ridge, or the correspondin g area when the ridg e is incomplete or absent . Scutoscutellar Sutur e (vs). —The externa l sutur e o f the V-shape d nota l ridg e of the alinotum , th e arm s divergen t posteriorly , dividin g th e notu m int o scutu m an d scutellum. Scutum (Set). —The are a o f the alinotu m anterio r t o th e sutur e o f the V-shape d notal ridge, or between this suture an d the prescuta l suture i f the latte r is present . Second Spiracl e (Sp^). —The metathoraci c spiracle , locate d nea r 1h e anterio r margin of the metapleuron , between the mesopleuro n and the metapleuron , or in th e posterior margi n of the mesopleuron . Spina (Spn). —The media n apodemal process of a spinasternum. Spinasternum (Ss). —One o f th e spina-bearin g intersegmenta l sclerite s o f th e thoracic venter , associated , o r united , wit h th e sternu m preceding ; a spinasternu m may becom e a par t o f th e definitiv e prosternu m o r mesosternum , bu t no t o f th e metasternum. Sternacosta.—The transvers e interna l ridg e o f th e sterna l sutur e throug h th e bases of the sterna l apophyses . Steraacostal Suture.—Th e externa l sutur e o f th e sternacosta , separatin g th e basisternum fro m th e sternellum . Sternal Apophyse s (SA). —The latera l apodema l arm s o f th e eusternum ; i n higher insects united on a median base, the whol e structure forming th e furca. Sternellum (SI). —The are a of the eusternu m posterio r t o th e base s of the sterna l apophyses or the sternacosta l suture . Sternopleurite (Spl). —The infracoxa l sclerotizatio n o f a generalize d thoraci c pleuron, generally united wit h the primar y sternum in the definitiv e eusterna l plate. Sternum (S, Stn).—Primarily the primitive sternum (Stn), or sclerotization of the true venter o f a segment; secondarily the definitive sternum (S), whic h i n th e thora x usually include s th e sternopleurite s an d ma y includ e th e followin g intersegmenta l spinasternum. Subalare (Sa). —The epimera l epipleurite giving insertion to the posterio r pleura l muscle of the wing . (Epimeralgelenkstuck, postparapteron.) Trochantin (Tri).— The precoxa l sclerite o f a thoraci c pleuro n derive d fro m th e anterior par t o f the primitiv e coxopleurite ; usually articulate d a t it s ventra l en d to the anterio r margi n of the coxa , and giving insertion to the terga l promotor muscle of the leg. V-shaped Notal Ridge (VR).—The V-shape d endoskeletal ridg e of the mesonotum or metanotum, its arms divergent posteriorly , marke d externally by the scutoscutella r suture. (V-Leiste.)

CHAPTER I X THE THORACI C LEG S The appendage s of insects that ordinarily serve as organs of terrestrial locomotion ar e th e appendage s o f th e thre e thoraci c segments ; bu t i n some Apterygot a an d i n th e larva e o f variou s pterygot e insect s th e abdominal appendage s als o pla y a par t i n the locomoto r function. I n the presen t chapte r onl y th e genera l structur e an d th e musculatur e of the thoraci c leg s wil l b e considered ; th e leg s o f man y insect s assum e various othe r function s tha n tha t o f locomotio n an d ar e structurall y modified accordingly . 1. STRUCTUR E O F TH E LEG S

In describin g the structur e an d mechanis m of the leg s we shall limi t the ter m "leg " t o th e fre e par t o f the appendag e having the cox a a s its

FIG. 104.—Middl e leg of a grasshopper, anterio r view .

base, sinc e we need not b e concerne d here with th e questio n o f the sub coxal origi n o f th e pleuron . Th e surface s o f th e leg s ar e oriente d fo r descriptive purpose s whe n th e lim b i s extende d a t righ t angle s t o th e body; the preaxial surface is then anterior, th e postaxial surface posterior , the oute r surfac e dorsal , an d th e inne r surfac e ventral . The Le g Segments.—Th e typica l an d usua l segment s o f th e insec t leg (Fig . 104 ) ar e th e coxa (Cx), on e trochanter (TV) , th e femur (Fm), the tibia (Tb), the tarsus (Tar), and the pretarsus (Ptar). In the Odonata two trochanteral segment s are present (Fig . 10 9 C, 1 Tr, 2 2V), but the y are not movabl e on each other . The Le g Joints.—The joints o f the leg s are membranou s rings o f th e leg wal l betwee n th e cylindrica l sclerotize d area s tha t constitut e th e segments. Th e membran e o f th e join t i s th e articular corium. Some times ther e ar e n o contiguou s point s o f articulatio n betwee n adjoining 193

194

PRINCIPLES OF INSECT MORPHOLOGY

segments; but usuall y one or two pairs of opposed articular surfaces limi t the movement of the joint to that of a hinge. Hinge d joints are therefore either monocondyli c (Fig . 10 5 A) or dicondylic (B) . A single articula tion i s typicall y dorsal ; i n dicondyli c joint s on e articulatio n i s anterio r and the other posterior, except at the trochantero-femoral joint where the articulations i f presen t ar e usuall y dorsa l an d ventral . Th e coxo -

FIG. 105.—Diagram s of articular mechanisms at th e femoro-tibia l joint of a leg. A , monocondylic joint . B , C , dicondyli c joint, en d vie w an d sid e vie w wit h levato r an d depressor muscles.

trochanteral hing e i s alway s dicondyli c wit h a n anteroposterio r axis . In th e telopodite , dicondyli c hinges are characteristic of the leg s of adult insects; monocondyli c hinges ar e usua l i n th e leg s o f larvae (Fig . 106) , but.in th e larva e o f Neuroptera an d Trichopter a th e femoro-tibia l joint is dicondylic . The structur e o f th e articulation s betwee n th e le g segment s varie s much a t differen t joint s an d a t correspondin g joints in differen t insects . Sometimes th e opposin g surface s simply touc h a t thei r points . I n other cases the articulation is of the ball-and-socket type , a condyl e on one surfac e fittin g int o a socket on the other . I n dicondyli c hinge s of thi s kin d th e tw o articulation s are frequentl y reverse d i n struc ture. A n occasional, perhaps generalized, typ e o f articulatio n consists o f a flexibl e scleroti c ba r f~ .„ continuous from one segment to FIG. 106.—Thoraci c le g o f a caterpillar , & anterior view . th e othe r throug h th e articula r membrane. The Coxa.—In its mor e symmetrical form the cox a has the shap e of a short cylinde r or truncate con e (Figs. 104 , Cx, 107 A), though commonly it i s ovate an d ma y b e almos t spherical . Th e proxima l end of the cox a is girdled by a submarginal basicostal suture (Fig. 10 7 A, bcs), which forms internally a ridge , o r basicosta (J5c) , an d set s of f a margina l flange , th e coxomarginale, or basicoxite (Bex). The basicosta strengthens the base

THE THORACIC LEGS

195

of the coxa and is commonly enlarged on the outer wall to give insertion t o muscles (B , C); on the mesa l half o f the coxa , however, it i s usually weak and ofte n confluen t wit h th e coxa l margin. Th e trochantera l muscle s that tak e thei r origi n i n th e cox a ar e alway s attache d dista l t o th e basicosta. The coxa is attached t o the body by an articular membrane , the coxal corium, which surrounds its base. I t ha s almost alway s an outer articu lation with the pleuron of its segment, and it may have an inner articula tion wit h th e sternu m o r wit h a laterosternal sclerite , a s wa s observed in Chap. VIII. Thes e tw o articulations ar e perhaps the primar y dorsa l

FIG. 107.—Diagram s illustratin g structura l detail s o f th e coxa - A , latera l vie w of coxa . B , C , inne r view o f outer wall o f basicoxal region. D , a cox a wit h the mero n extended distally . Be, basicosta ; 6cs , basicosta l suture ; Bex, basicoxite ; c , pleural articular socket ; C# , coxa ; cxs, coxa l suture ; /, anterio r coxotrochantera l articulation ; Fm, femur; g, posterior coxotrochanteral articulation; Mer, meron ; Tr , trochanter.

and ventra l articula r point s o f th e subcoxo-coxa l hing e (Fig . 88 , c , d). In addition , th e insec t cox a has ofte n a n anterio r articulatio n wit h th e anterior, ventra l en d of th e trochantin (Fig . 91 B, e), but th e trochantina l articulation doe s no t coexis t wit h a sterna l articulatio n (A) . Th e pleural articular surfac e o f the cox a is borne o n a mesal inflection of th e coxal wall (Fig. 10 7 A, c). I f th e cox a is movable on the pleura l articu lation alone , the coxa l articular surfac e i s usually inflected t o a sufficien t depth t o giv e a leverage to th e abductor muscle s (Fig. 114 , M) inserte d on the oute r ri m o f the coxa l base. Distall y th e cox a bears a n anterio r and a posterior articulatio n wit h the trochanter (Fig . 10 7 A, /, g) . The outer wall of the cox a is often marke d by a suture extending fro m the bas e t o th e anterio r trochantera l articulatio n (Figs . 106 , 10 7 A, 108 A, cxs). I n some insects the coxa l suture falls in line with the pleura l suture (Fig . 10 8 B), and in such cases the cox a appears to be divided into two parts corresponding to th e episternu m and epimero n of the pleuron . The coxa l suture i s absent i n many insects (Fig . 10 8 C). The inflection o f the coxa l wall bearin g the pleura l articula r surfac e divides th e latera l wal l of the basicoxit e int o a prearticula r par t an d a

196

PRINCIPLES OF INSECT MORPHOLOGY

postarticular part, an d the tw o areas often appea r a s two marginal lobes on the bas e o f the coxa . Th e posterio r lob e is usually th e large r an d i s termed th e meron (Fig . 10 7 C, Mer). The mero n may be greatl y enlarge d b y a n extensio n distall y i n th e posterior wall of the coxa (Fig. 107 D, Mer); i n the Neuroptera, Mecoptera (Fig. 99) , Trichoptera, an d Lepidoptera , th e mero n is so large that th e coxa appear s t o b e divide d int o a n anterio r piece , th e so-calle d "cox a genuina," and the meron (Mer), but the meron never includes the region of th e posterio r trochantera l articulation, an d th e groov e delimiting it i s always a part of the basicosta l sutur e (Fig . 10 7 D, bcs). A coxa with a n

FIG. 108.—Coxa l structure s o f a grasshopper , Dissosteira Carolina. A , cox a an d trochanter of first leg, anterior view. B , same of middle leg, with adjoining part of pleuron . C, cox a an d reduce d trochante r o f hind leg , anterio r view . D , inne r vie w o f articulatio n of middl e cox a with pleuron .

enlarged meron has an appearance similar to one divided by a coxal suture falling in line with the pleural suture (Fig . 108 B), but th e two conditions are fundamentall y quite differen t an d shoul d no t b e confused . The mero n reaches the extrem e of its departure from th e usua l condition i n th e Diptera . I n som e o f th e mor e generalize d flies, as i n th e Tipulidae, th e mero n of the middl e leg appears as a large lobe of the coxa projecting upward and posteriorly from the coxa l base (Fig . 10 2 D, Mer) ; in higher members of the orde r it becomes completely separated fro m th e coxa and forms a plate of the lateral wall of the mesothorax (E, Mer). By thi s transpositio n o f th e meron , th e remoto r muscl e of - th e cox a attached o n it lose s its functio n a s a leg muscle and serves as a depressor of the tergum , thereby becoming an adjunct to the usua l elevator s of the

THE THORACIC LEGS

197

wings. Th e meral plate in the thorax of the Diptera was long a puzzle to entomologists until it s tru e natur e wa s shown by Crampto n and Hase y (1915) an d b y Crampto n (1925 , 1925a) . The Trochanter.—Th e trochanter (Fig . 104, Tr) i s the basa l segmen t of th e telopodite ; i t i s alway s a smal l segmen t i n th e insec t leg , freel y movable by a horizontal hinge on the coxa , but mor e or less fixed to th e base o f th e femur . Whe n movabl e o n th e femu r th e trochantero femoral hing e is usually vertical or obliqu e in a vertica l plane, givin g a slight movemen t of production and reduction at th e joint , though only a reductor muscl e i s presen t (Fig . 10 9 A , K). I n th e Odonata , bot h

FIG. 109.—Th e coxa l an d trochantera l musculature . A , diagra m o f inne r vie w of posterior wal l o f coxa , trochanter , an d bas e o f femur , wit h typica l musculature . B , trochanter an d bas e o f femu r o f a n ichneumoid , Megarhyssa, showin g basal subdivisio n (fm) o f femur . C , correspondin g par t o f th e le g o f a dragonfl y larva , showin g divide d trochanter ( 1 Tr, 2Tr) wit h reducto r muscle s of femur (R) i n secon d segment .

nymphs an d adults , ther e ar e two trochantera l segment s (C , ITr, 2Tr), but they are not movable on each other; the second contains the reductor muscle o f th e femu r (R). Th e usua l singl e trochantera l segmen t o f insects, therefore , probabl y represent s th e tw o trochanter s o f othe r arthropods fused int o one apparent segment , since it is not likely that the primary coxotrochantera l hinge has bee n lost fro m th e leg . I n som e of the Hymenopter a a basa l subdivisio n o f the femu r simulate s a second trochanter (Fig . 10 9 B, fm), bu t th e insertio n o f the reducto r muscl e (R) on its base attests that it belongs to the femora l segment , since as shown in th e odonat e le g (C) , th e reducto r ha s it s origi n i n th e tru e second trochanter. The Femur.—This, the thir d segmen t of the insec t leg (Fig. 104, Fm), is usually the longes t an d strongest par t o f the limb , but i t varie s in size from th e hug e hind femur o f leaping Orthoptera (Fig . 116, Fm ) to a very small segmen t such as is present i n man y larval forms . Th e volum e of the femu r i s generall y correlate d wit h th e siz e o f th e tibia l muscle s

198

PRINCIPLES OF INSECT MORPHOLOGY

contained withi n it , bu t i t i s sometime s enlarge d an d modifie d in shap e fo r othe r purpose s tha n tha t o f accommodatin g th e tibia l muscles. The Tibia.—Th e tibi a (Fig . 104 , Tb ) i s characteristicall y a slende r segment in adult insects , onl y a little shorter tha n th e femu r o r the com bined femu r an d trochanter . It s proxima l en d form s a mor e o r les s distinct hea d ben t towar d th e femur , a devic e allowin g the tibi a t o b e flexed close agains t th e unde r surfac e o f the femur . The Tarsus.—Th e tarsu s o f insect s correspond s t o th e penultimat e segment o f a generalized arthropod limb , which is the segmen t calle d the propodite in Crustacea (Fig . 50 A, Tar). I n adult insect s it is commonly subdivided int o fro m tw o t o fiv e subsegments , o r tarsomeres (Fig . 104 , Tar), bu t i n th e Protur a (Fig . 53A) , som e Collembola , an d mos t holo metabolous insect larva e (Fig . 106 ) i t preserve s th e primitiv e for m o f a simple segment. Th e subsegment s o f the adul t insec t tarsus are usuall y freely movabl e o n on e another b y inflecte d connectin g membranes , bu t the tarsu s neve r ha s intrinsi c muscles . Th e tarsu s o f adul t pterygot e insects havin g fewer than five subsegments is probably specialized by th e loss of one or more subsegments o r by a fusion o f adjoining subsegments . In th e tars i o f Acridida e the lon g basal piec e i s evidentl y compose d of three unite d tarsomeres, leaving the fourth and the fifth free (Fig . 11 8 A). The basa l tarsomer e i s sometimes conspicuousl y enlarge d an d i s distin guished a s th e basitarsus. O n th e unde r surface s o f th e tarsa l subseg ments i n certai n Orthopter a ther e ar e smal l pads , th e tarsal pulvilli, or euplantulae (Fig . 11 8 A, a) . Th e tarsu s i s occasionall y fuse d wit h th e tibia i n larva l insects , formin g a tibiotarsa l segment ; i n som e case s i t appears to be eliminated or reduced to a rudiment betwee n the tibia and the p r e tarsus. The Pretarsus.—Th e terminal part o f the insec t le g in its usua l for m departs s o far fro m th e simpl e structure o f a primitive en d segment of th e limb tha t entomologist s generall y hav e no t understoo d it s nature , though it s morphologica l status ha s lon g bee n clea r fro m th e wor k of de Meijer e (1901) . In th e majorit y o f arthropods th e le g end s i n a simpl e clawlik e segment, whic h i n th e Crustace a i s know n as th e dactylopodite (Fig . 5 0 A , Piar), Th e crustacea n dactylopodit e i s provide d wit h tw o muscles , a levator an d a depressor, both arisin g in the tarsus, or propodite. I n th e Hexapoda, a simpl e dactylopodite-lik e en d segmen t o f the le g occurs in the Protura , in some Collembola, in the larva e o f man y Coleóptera , an d in th e larva e o f Lepidopter a an d Tenthredinida e (Fig . 106 , Ptar). I n these forms it differ s fro m th e crustacea n dactylopodit e onl y in lacking a levator muscle and in having the fibers of the depressor muscle distributed in the tibia and the femur .

THE THORACIC LEGS

199

In mos t other adult , nymphal , and larval insects, the pretarsus bears a pai r o f movabl e latera l claw s situate d upo n it s bas e an d articulate d dorsally to th e en d of the tarsus , an d the bod y of the segmen t is reduced to a smal l media n cla w o r a lobe-lik e structure . Th e media n cla w i s well preserved in the Lepismatida e (Fig . 110 , G , D, dac) and th e tendo n of th e depresso r muscl e arises fro m th e ventra l li p o f its bas e (C , E , x). In Japyx th e bas e of the pretarsu s form s a large plate ventrally (B , Utr) upon whic h i s attache d th e depresso r "tendon " (x), whil e it s ti p i s reduced t o a minut e media n cla w (A , dac) lying dorsall y betwee n th e bases o f th e latera l claw s (Uri). I n th e so-calle d triunguli n larva e of meloid beetle s th e apparen t "latera l claws " o f th e fee t ar e probabl y spines arising from th e bas e of the media n pretarsal claw , as are als o th e "lateral claws " o f certain lampyrid larvae.

FIG. 110.—Example s o f th e retentio n o f th e media n pretarsa l cla w (dac) in insects . A, pretarsu s o f Japyx, dorsa l view . B , same , ventra l view . C , pretarsu s an d en d o f tarsus o f Lepisma, latera l view . D , same , en d view . E , media n claw , o r rudimentar y dactylopodite, o f Lepisma, wit h "tendon " (x) o f depressor muscl e attache d ventrally.

The typica l pretarsus , o r termina l foo t structure , i n insect s havin g true latera l claw s (Fig. Ill A , B) arises fro m th e en d of the tarsu s b y a membranous base , upo n whic h ar e supporte d th e pai r o f lateral claws (Un) an d a median lobe, the arolium (Ar). Th e claws are hollow multicellular organ s an d their cavitie s ar e continuou s wit h th e lume n o f th e pretarsus. Eac h cla w i s articulate d dorsall y t o th e unguifer (A , k), a median proces s of the dista l en d o f the las t tarsomer e (Tar). Th e aro lium, likewis e a hollow lobe, is a direct continuatio n o f the media n part of th e pretarsal base ; it may be entirely membranous, or its walls may be partly sclerotized . O n the ventra l surfac e o f the pretarsu s i s a median basal plate , th e unguitractor (B , Utr), whic h is partly invaginate d int o the en d of the tarsus (Tar). T o its proximal end is attached th e tendon like apódeme (x ) o f the depresso r muscle of the pretarsus , usuall y called the retractor o f th e claws. Th e unguitracto r plat e ma y b e divide d int o two sclerite s (C , Utr), o r sometimes there is a sclerite dista l t o i t distin guished as the planta (Fig. 118 C, Pin). Latera l plates beneath the base s of th e claw s are terme d auxiliae (Fig. Ill B, E, I). I n th e Dipter a tw o large lateral lobes of the foot , known as the pulvilli (D, E, Pv), arise fro m the auxiliar y plates , on e beneat h th e bas e o f eac h claw , an d ther e i s commonly als o presen t a media n process , o r empodium (Emp), arisin g

200

PRINCIPLES OF INSECT MORPHOLOGY

from th e dista l end of the unguitractor plate. Th e empodium may hav e the for m o f a spine , o r i t ma y b e lobe-lik e an d simila r i n for m t o th e pulvilli. Th e aroliu m is rudimentary o r absent i n most Dipter a excep t Tipulidae. All part s o f th e pretarsu s ar e subjec t t o muc h variation . Th e lateral claw s ar e sometime s o f unequa l size , on e becomin g reduce d o r

FIG. 111.—Example s o f "foot " structure s o f insects . A , Periplaneta americana, dorsal view . B , same , ventral view . C , Magicicada septendecim, ventra l view . D , asili d fly, lateral view . E , same , ventra l view . Ar , aroliu m (dorsa l lobe) ; Emp, empodiu m (median ventra l proces s o r lobe) ; k , unguife r proces s o f tarsus; I, auxilia ; m, arolia r pad ; n, accessory sclerite s betwee n claws ; Pv, pulvillus (latera l ventra l lobe); ,Tar, tarsus; Un, ungues (latera l pretarsa l claws) ; Utr, ungitracto r plate ; x , "tendon " o f depressor muscl e of pretarsu s (retracto r o f claws).

occasionally obliterated , th e resul t bein g a one-clawe d foot , a s i n th e Coccidae, Pediculidae, and mammal-infesting Mallophaga. Again , bot h claws ma y becom e ver y small , an d bot h ma y b e lacking . I n th e Thysanoptera th e claw s are minut e an d th e foo t consist s principall y of the bladderlike arolium. I n some insects the arolium is hollowed beneath and act s a s a vacuu m cu p to enabl e th e insec t t o wal k o n surfaces too smooth o r too har d fo r the claw s to grasp . Othe r insect s clin g to suc h surfaces by means of a gummy liquid exuded from th e ventra l sid e of th e foot. 2. MUSCLE S AN D MECHANIS M O F TH E LEG S

The muscle s o f th e legs , a s o f an y segmente d appendage , ar e com prised i n tw o sets , namely , (1 ) muscles o f the lim b basis , o r those tha t move th e appendag e a s a whole , an d (2 ) muscles o f the telopodit e seg -

THE THORACIC LEGS

201

ments, o r those that move individual parts of the limb . Muscle s of th e second clas s ar e usually named accordin g to th e lim b segmen t o n which they have their insertions, thoug h the y may be motors o f the entire part of th e appendag e distal t o the insertions . Mos t o f the muscle s of the le g segments of insects take their origi n in the segment immediately proxima l to the one on which they are inserted, but some of them arise in the second or third segmen t remove d from th e poin t o f insertion . Mechanism o f the Le g Base.—Th e possibl e movement s o f the cox a depend upo n th e natur e o f the coxa l articulation wit h th e body , whic h may hav e an y on e of three type s o f structure (Fig . 112) . I f th e cox a is articulated t o the pleuro n only (A , c), it i s free t o make any movement s that it s musculatur e wil l impart t o it ; if, however, it i s hinged between

FIG. 112.—Diagram s illustrating thre e types of coxa l articulation . A , with pleura l articulation (c ) only . B , wit h pleura l (c ) an d trochantina l (e ) articulations . C , wit h pleural (c) and sternal (d) articulations.

pleural an d trochantina l articulation s (B , c , e), it s movement s mayb e more limited , thoug h th e flexibilit y o f the trochanti n usuall y doe s no t impose a rigid hing e motion on the coxa ; but i f the cox a is articulated t o the pleuro n dorsall y an d t o th e sternu m ventrall y (C , c , d), it s move ments are strictly limite d to those of a hinge with the axi s in a transvers e plane. A typical insect coxa of the first or second types, having only a pleural articulation wit h the bod y or both pleura l and trochantinal articulation s (Fig. 113 A, GJ e)j is provided with muscles that arise on the tergum, muscles tha t aris e o n the sternum , an d muscle s havin g thei r origi n o n the pleuron . Th e dorsa l muscle s include tergal promotors (7 ) an d tergal remotors (J). The sternal muscles comprise sternal promoters and ????????? ??? ????????????? ???????? ??? ????????? ???????? ??? ??? ??? ???? an adductor (N). The pleural muscles include functional abductors (M) and, i n th e wing-bearin g segments o f adul t insects , th e muscle s o f th e basalar an d subala r sclerite s (3E' , 3E"), whic h serve a s wing muscles. The muscle s o f the le g base ar e no t necessaril y inserte d directl y o n the cox a itself. Th e terga l promotor, fo r example, is always inserted on

202

PRINCIPLES OF

INSECT MORPHOLOGY

the trochan tin (Fig . 11 3 A, Tri), excep t when this sclerite is much reduced or i s absent . Som e o f th e othe r muscle s ar e frequentl y inserte d o n apodemes that arise in the articular membran e betwee n the coxa and th e pleuron. I n specia l cases , certai n muscle s inserte d o n th e pleuro n ar e evidently coxa l muscles tha t hav e bee n transferre d t o th e pleura l wall . The presenc e o f a n articulatio n betwee n th e trochanti n an d th e cox a does no t usuall y resul t i n a n alteratio n o f the coxa l musculature. A cox a of the thir d type (Fig . 11 2 C), havin g it s movement s limite d to thos e o f a hing e o n a transvers e o r transversel y incline d axi s b y

FIG. 113.—Th e coxa l musculature . A , diagra m o f typica l musculatur e o f a cox a with pleura l and trocha n tinal articulations . B , muscle s o f the mes o thoracic le g o f a be e with pleural and sterna l articulations . (Se e Fig. 10 3 B.)

articulations wit h bot h th e pleuro n (c ) an d th e sternu m (d) , i s likely to have a more simple musculature than that of a coxa of the first or second type. I n th e middl e le g o f a bee , fo r exampl e (Fig . 11 3 B), th e usua l tergal musculatur e o f th e cox a is absent, thoug h th e cox a has anterio r and posterior sterna l muscle s (K , L), a large two-branched pleura l muscle (M), and a subalar muscl e (3E"). Thi s for m o f reduced coxal musculature i s probably a specialize d conditio n i n th e highe r insects , sinc e th e primitive musculatur e o f the le g bas e appear s t o compris e bot h terga l and sterna l muscles . I n th e prothora x o f the be e a posterior tergocoxa l muscle (terga l remotor, J) is present. If th e cox a has n o sterna l articulation , th e pleura l articula r surfac e on it s bas e i s usually mor e or less inflecte d mesally fro m th e oute r wal l (Fig. 114 , c) , occupyin g thus a point approximatel y centra l i n the plan e of th e le g base. B y this devic e a leverag e i s give n t o muscle s inserte d to an y sid e of the articula r point . Th e attachment s o f the coxa l muscles occur approximatel y a t th e opposit e end s o f two axia l line s throug h th e articulation, on e longitudinal (6-6) , the othe r transverse (c-c) . Th e coxa, therefore, ha s movement s o f promotion an d remotion o n a transvers e axis, an d movement s o f abduction an d adduction o n a longitudina l axis ;

THE THORACIC LEGS

203

while furthermore, becaus e of the singl e point of articulation, it is capable also o f a partial rotation o n a vertica l axi s (d-d) throug h th e articula r point (c). In comparativ e studie s o f insec t musculatur e w e mus t conside r muscles a s functiona l group s o f fiber s rathe r tha n a s individua l fibe r bundles, for it i s often foun d tha t a single muscle in one species is represented b y several muscles in another, and that, even in the sam e species, the numbe r o f muscle s i n a functiona l grou p varie s i n th e differen t segments o f th e thorax . Thu s while th e coxa l musculatur e ma y be represente d diagrammaticall y in a simplifie d form , a s i n Fig . 113 A , it mus t b e born e i n min d that a t eac h cardina l poin t o f th e coxal bas e ther e ma y b e attache d several distinct fiber groups consti tuting a functiona l unity . Ther e can b e littl e doub t als o tha t between different species, or between differen t segment s o f th e same species , homologou s muscles may hav e quit e differen t functions owing to difference s i n the relation s between the skeletal parts on which they ar e inserted. Fo r thi s reasond , Fig. 114.—Diagram of the possible axes it i s impossibl e t o nam e insec t of movement of a coxa having only a pleural muscles Consistentl y in al l Case s O n poin t o f articulatio n (c) . b-b, axis o f a basi s O f their functions . abductio n and adduction (muscle s M an d N); c-c, axi s o f promotio n an d remotio n (muscle s For a n elementar y Stud y o f th e J an d J); d-d, axis o f partial rotatio n (mus-

insect leg muscles it will be best cles K and L)to examin e som e particula r species , an d th e le g musculatur e o f th e acridid Dissosteira Carolina i s her e give n a s fairl y representativ e o f th e more generalize d typ e o f le g musculatur e i n insects . Th e muscle s of the leg s of the grasshoppe r are essentially th e sam e in each segment, with the differenc e onl y that a single group of fibers in one leg may b e repre sented by two or more groups in another, an d that the anterio r rotato r is absent i n th e prothoraci c leg . Th e followin g description s ar e base d specifically o n the musculatur e o f the hin d leg. Muscles o f th e Le g Bas e o f Dissosteira.—Th e coxa e o f th e grass hopper ar e attache d t o th e bod y b y th e pleura l articulation s onl y (Fig. 108 D, c) , thoug h a smal l trochantina l plat e i s presen t a t th e bas e o f each leg , the sclerit e bein g bes t develope d i n th e for e le g (A , Tn ) an d becoming successively smaller i n the othe r tw o (B , C, Tn).

204

PRINCIPLES OF INSECT MORPHOLOGY

Tergal Promotor o f the Coxa (Fig . 11 5 A , 118). —A larg e muscl e of the hin d le g lyin g immediatel y posterio r t o th e tergosterna l muscl e of th e metathorax . Origi n dorsally o n th e latera l are a o f th e scutum ; insertion ventrall y o n a stalke d apodema l dis c o f th e anterio r angl e of th e cox a (D , F) . I n th e prothora x thi s muscl e i s inserte d o n th e ventral end of the trochantin, a s it is in most cases in which the trochanti n is not rudimentary . Tergal Remotors o f th e COO T (Fig . 11 5 A , 119, 120).— Two bundle s of fibers attached on the posterio r par t of the coxa . Th e large r anterio r one (119) arises o n th e posterio r margi n of the scuta l are a o f the meta tergum an d i s inserted o n a n apodema l disc of the posterio r inne r angle of th e coxa l bas e (D , F , 119). Th e smalle r posterio r muscl e (A , 120) lies close behind 119 and is inserted on a slender apódeme attached t o th e extreme posterior angl e of the cox a (B , D, F , 120). The terga l promoto r an d remotor s ar e clearl y antagonist s t o eac h other becaus e o f thei r opposit e relation s t o th e pleura l fulcru m o f th e coxa. Anterior Rotator o f th e Coxa (Fig . 11 5 D, E , F , 121). —A large muscle with fibers arising in two groups, one from th e lateral part o f the sternu m before th e bas e o f the sterna l apophysis , th e othe r fro m th e sternella r lobe behin d th e apophysi s (E , 121)] al l fiber s convergin g to a common point o f insertio n o n th e mesa l sid e o f th e anterio r angl e o f th e coxa l base (D , E, 121). Posterior Rotators of the Coxa (Fig . 115 D, E , 122, 123, 124)—A. group of thre e muscle s arising on the posterio r surfac e o f the latera l ar m o f th e metasternal apophysi s (E) ; al l inserte d posteriorl y o n th e bas e o f th e coxa (D , E) . In th e mesothora x there i s but a singl e anterior rotato r an d a single posterior rotator , the first arising on the sternellar lobe , the second on the mesosternal spina. I n th e prothora x the anterio r rotato r i s absent, an d the posterior rotator includes two muscles, one from the sternal apophysis, the othe r fro m th e spina . Sinc e th e rotator s li e approximately i n th e plane o f the coxa l base, i t i s clear that they must serv e to giv e the cox a a partial rotatio n o n the pleural articulation . Abductors o f th e Cox a (Fig . 11 5 C , 125, 126). —Two muscle s arisin g on the episternu m of the metathora x and inserted on the oute r rim of the coxal bas e appea r t o belon g t o th e abducto r syste m o f th e hin d leg . The first , however , a very small muscle (125), i s inserted s o far forwar d on th e cox a tha t i t probabl y function s a s a n accessor y t o th e terga l promotor (A , 118). Th e secon d (C , 126) covers mos t o f the inne r fac e of th e episternu m an d it s fibers converge upon a slender apodemal stal k arising i n th e articula r membran e a t th e bas e o f th e cox a clos e befor e the pleura l articulation (C , D, F , 126). Becaus e of the mesa l inflectio n

THE THORACIC LEGS

205

of th e articula r surfac e o n th e bas e o f th e cox a (F , c ) thi s muscl e i s enabled t o functio n as an abducto r o f the leg .

FIG. 115.—Musculature of the hin d cox a of a grasshopper, Dissosteira Carolina. 113, tergosternal muscle; 114, wing flexor; 118, tergal promotor of coxa; 119,120, tergal remotors of coxa ; 121, sternal promoto r (anterio r rotator ) o f coxa ; 122, 123, 124, sterna l remotor s (posterior rotators ) o f coxa; 125, 126, abductors o f coxa; 127, basalar-sternal muscle; 128, basalar-coxal muscle ; 129, subalar-coxa l muscle ; 130, sterna l adducto r o f coxa ; 133c, 133d, bod y branches of depressor o f trochanter .

Adductor o f th e Coxa (Fig . 115 C, D, E , F , 130).— A fla t muscl e taking its origi n o n the posterio r surfac e o f the metasterna l apophysi s beneat h the posterio r rotator s (E) , an d extendin g posteriorl y an d ventrall y

206

PRINCIPLES OF INSECT MORPHOLOGY

to th e inne r margin of the coxa l base (C , D, E , F , 130). Thi s muscl e is evidently antagonisti c t o th e abducto r (126) since the tw o ar e inserte d at opposit e ends of a transverse axia l line through the pleural articulatio n (C, D, F) . Two other muscle s attached o n the oute r ri m o f the coxa , present i n the mesothorax and metathorax but absen t i n the prothorax, ar e muscles of th e epipleurite s an d functio n a s win g muscle s i n th e adult . Thes e muscles in the metathora x ar e the following : Second Pronator-extensor o f th e Hind Wing (Fig . 11 5 B , 128). —A large muscl e attache d dorsall y o n th e secon d basala r sclerit e (2Ba) and ventrall y o n th e bas e o f th e cox a anterio r t o th e pleura l fulcru m (D, F, 128). Depressor-extensor o f th e Hind Wing (Fig . 11 5 B , 129). —A ver y thick muscl e attached dorsall y on the subala r sclerit e (So) an d ventrall y on the coxa l base posterior t o the pleura l articulation (D , F, 129). In th e nymp h th e las t tw o muscle s tak e thei r origi n o n the dorsa l edge o f th e pleuron , on e o n th e episternu m (Fig . 10 2 C, 3Í7') > th e othe r (3.E") o n the epimeron , an d the y her e evidently belong to the abducto r system o f the coxa . Muscles o f th e Telopodite.—Th e par t o f th e le g beyon d th e cox a is th e principa l movabl e par t o f th e limb . Th e coxo-trochantera l joint a t it s bas e i s almost universall y a dicondyli c hing e with articula tions anterio r an d posterio r relativ e to th e norma l vertica l plan e o f th e shaft o f th e limb . It s musculature , therefore , consist s o f levato r an d depressor muscle s inserted on the basal segment, which is the trochanter . The levato r fiber s aris e entirel y withi n th e cox a (Fig . 10 9 A, 0) ; th e depressor fiber s includ e a coxa l grou p (Q ) an d usuall y on e o r mor e groups (P ) arising in the bod y segment supportin g th e le g (Fig . 117) . Muscles of th e Trochanter. —In th e hin d le g o f Dissosteira ther e ar e two levato r muscle s o f th e trochante r (Fig . 11 6 A , 131, 13-2) arisin g dorsally i n the bas e of the cox a and inserte d o n the dorsa l lip of the bas e of th e trochanter . Th e depresso r muscle s include tw o group s o f fiber s arising ventrally i n the bas e of the cox a (133d), an d thre e group s takin g their origi n i n th e bod y o f th e metathorax . O f th e latter , tw o aris e on th e tergu m (Fig . 11 5 A, 133c, the secon d not shown) , and th e thir d (133d) o n th e sterna l apophysi s (E , 133d). Th e fiber s o f all group s ar e inserted o n th e ventra l li p o f the trochante r an d togethe r constitut e a powerful depresso r o f the telopodit e (Fig . 117 , P , Q). Muscles o f the Femur. —In th e hin d le g o f Dissosteira ther e i s n o movement between the smal l trochanter (Fig . 11 5 D, Tr ) an d th e femu r (Fm), an d consequentl y the femu r ha s n o muscles . Whe n the femu r i s movable o n th e trochanter , i t i s provided wit h a shor t reductor muscl e arising in the trochante r an d inserted on the posterio r edge of the femora l

THE THORACIC LEGS

207

base (Fig . 10 9 A, R). Thi s muscl e is present i n the firs t an d secon d legs of Dissosteira an d impart s a slight rearwar d flexion to th e femu r o n th e dorso ventral trochantero-femora l hinge . Muscles o f th e Tibia. —The tibia l muscle s ar e th e mos t importan t muscles of the hind legs in the Acrididae, since it is the stron g and sudden movement o f the hin d tibia e o n th e femor a tha t give s th e grasshoppe r its powe r of leaping . Th e muscle s occupy almost th e entir e cavit y of each femu r (Fig . 116 ) an d determin e th e siz e and shap e o f th e latter ; they comprise levator and depressor groups of fibers. Most of the levator muscle consist s o f tw o larg e masse s o f shor t overlappin g fibe r bundle s occupying th e dorsa l three-fourth s o f th e femora l cavit y (A , B , 135a,

FIG. 116.—Muscle s of th e hin d le g o f a grasshopper , Dissosteira carotina* 131, 132, levators o f trochanter; 133a, coxal branch of depresso r of trochanter; 134, 136, levators of tibia; 136, depresso r o f tibia ; 137, 138, levator an d depresso r o f tarsus; 139, depresso r of pretarsus (retracto r o f claws).

135b). The y ar e attache d t o th e latera l femora l wall s o n th e space s between the " fishbone "ridges that form the external sculptured patter n of th e oute r an d inne r face s o f th e femur ; the y ar e inserte d o n a fla t apódeme tha t taper s distall y t o a thic k stal k arisin g fro m th e dorsa l margin o f th e tibia l bas e (A , 135 Ap).Tw o smal l branche s o f th e levator muscl e arise i n th e dista l part o f the femu r fro m th e dorsa l wall and ar e inserte d o n th e bas e o f th e apódem e (A , 135c, th e othe r no t shown). The depresso r o f the hin d tibia i s a relatively smal l muscle with long, slender fibers arising in the ventral part of the femur (Fig . 116 A, B, 136a) and convergin g to th e side s o f a tapering apódem e arising in the ventra l membrane of the knee joint. Th e terminal straplike part of this apódeme slides ove r a stron g interna l proces s (A , a) nea r th e en d o f the ventra l wall o f the femur . Tw o smal l accessor y band s o f depressor fiber s aris e

208

PRINCIPLES OF INSECT MORPHOLOGY

distally fro m th e dorsa l wal l o f the femu r an d ar e inserte d o n th e bas e of th e depresso r apódem e (136b, th e posterio r on e not shown) . In th e fore - an d middl e leg s o f Dissosteira the relativ e siz e o f th e tibial muscle s is the revers e o f that i n th e hin d leg , th e depresso r bein g

FIG. 117.—Mechanis m o f th e hin d le g o f a grasshopper . O , levator o f trochante r and femur ; P, Q, depressor s o f trochanter an d femur ; S, T, leyator an d depresso r of tibia; U, V, levato r an d depresso r o f tarsus; X, depresso r o f pretarsus.

the stronge r o f the two . I n thes e leg s there i s also a very smal l anterio r levator, whic h is reduced to a fibrous strand in the hin d le g (Fig. 11 6 A, 134). Th e mechanis m o f the tibia l muscle s o f the hin d le g is show n in Fig. 117 .

FIG. 118.—Pretarsu s of a grasshopper, Dissosteira carotina. A, tarsus and pretarsu s disjointed, showin g tendonlik e apódem e (HOAp) o f depresso r o f pretarsus . B , pretarsu s and en d of tarsus, dorsa l view. C , same, ventral view . Ar , arolium ; Pin, planta; Tar, tarsus; Uf , unguifer ; Un , ungues; Utr, unguitracto r plate.

Muscles o f th e Tarsics.—The tarsa l muscles , a levator an d a depressor, are both small and lie in the dista l part of the lon g slender tibia, (Fig . 116 A, 137, 138), the first inserted o n the dorsa l li p of the firs t tarsomere , th e second o n th e ventra l lip . Th e othe r subsegment s o f th e tarsu s ar e

THE THORACIC LEGS

209

never provide d wit h muscle s an d ar e consequentl y no t independentl y movable. Muscles ofthePretarsus. —The pretarsu s o f insects, a s in the Chilopod a and Diplopoda , alway s lack s a levato r muscle . Th e depresso r form s the flexor , o r retractor , o f the claws , s o calle d becaus e i t serve s t o flex the claws ventrally and proximally on the dorsal articulations o f the latter with th e dista l en d o f the tarsus . It s fiber s aris e i n th e tibi a an d th e femur an d ar e inserted o n a long tendonlike apódem e (Fig . 11 8 C, llOAp) arising fro m th e unguitractor plate (Utr) i n the base o f the pretarsus, and extending throug h th e tarsu s and tibia (A ) into the femur . I n eac h leg of Dissosteira the firs t branc h o f the flexo r o f the claw s arises posteriorl y in th e basa l par t o f the femur ; the secon d arise s i n th e proxima l ben d of th e tibi a (Fig . 11 6 A, 1391})} th e thir d (139c) o n th e inne r wal l of th e basal thir d o f th e tibia . Th e extensio n o f th e claw s i s cause d b y th e elasticity of the basa l parts of the pretarsu s supportin g them . GLOSSARY OF TERMS APPLIED TO THE PARTS OF AN INSECT'S LEG The following terms ar e here defined a s they are used in entomology; more general terms applied to th e appendage s are give n in the Glossar y of Chap. V. Arolium (Ar). —The usua l median lobe of the pretarsus , arisin g between the base s of th e claws . Auxiliae.—Small plate s beneat h th e base s o f th e pretarsa l claws , bearin g th e pulvilli whe n the latter ar e present . Basicosta (Be). —The proxima l submarginal ridge of the inner wall of a leg segment. Basicostal Sutur e (bcs). —The externa l groov e o f a le g segmen t formin g th e basicosta. Basicoxite (Bex). —The usuall y narro w basa l ri m o f th e cox a proxima l t o th e basicostal sutur e an d its internal ridge . (Coxomarginale.) Basitarsus.—The proximal segment of the tarsus . Coxa (Cx). —The basa l segmen t of the le g articulatin g wit h th e pleuron , or als o with the sternum . Coxal Corium.—The articular membran e surroundin g the bas e o f the coxa . Coxomarginale (Bex). —See basicoxite. Dicondylic Joint.— A joint wit h tw o point s o f articulation betwee n th e adjacen t leg segments. Empodium (Emp). —A media n lobe or spine-like process arising ventrally between the bases of the pretarsa l claws , usually fro m th e unguitracto r plate . Euplantulae.—Padlike structure s o n th e ventra l surface s o f th e tarsa l subseg ments. (Tarsal pulvilli.) Femur (Fm). —The thir d an d usually the larges t segmen t of the insec t leg. Meron (Mer). —The latera l postarticula r are a o f th e bas e o f th e coxa , i n som e insects greatl y enlarge d an d extende d distally i n the posterio r par t o f the coxa , bu t always proximal to the basicostal suture . Monocondylic Joint.— A join t wit h a singl e poin t o f articulatio n betwee n th e adjacent le g segments. Planta.—A media n ventra l sclerit e o f th e pretarsu s dista l t o th e unguitracto r plate.

210

PRINCIPLES OF

INSECT MORPHOLOGY

Pretarsus (Ptar). —The termina l part s o f the le g distal t o th e tarsus , includin g median remnant s o f th e dactylopodite , an d th e latera l claws , o r ungues ; in mos t larvae a simple clawlike segment. Pulvilli (Pv). —Lateral lobe s o f th e pretarsu s arisin g beneat h th e base s o f th e claws. Ventra l lobe s o f the tarsa l subsegment s (euplantulae ) ar e sometime s calle d tarsal pulvilli. Tarsus (Tar). —The fift h segmen t o f the leg , usually divide d into fro m tw o to five subsegments, o r tarsomeres. Tibia (Tb). —The fourt h segment o f the leg . Trochanter (Tr). —The usua l second segment of the insec t leg, probably composed of tw o unite d trochantera l segments ; i n som e case s (Odonata ) showin g a divisio n between its componen t segments . Ungues (Un). —The latera l claws of the pretarsus ; usually calle d "tarsal " claws . Unguifer.—The media n dorsa l process on the en d of the tarsu s t o whic h the pre tarsal claws are articulated . Unguitractor Plate (Utr). —The ventra l sclerit e of the pretarsu s fro m whic h arises the tendonlik e apódeme of the retracto r muscl e of the claws .

CHAPTER X THE WING S Insects diffe r fro m th e othe r flyin g animal s i n tha t thei r wing s are structures superadde d to th e primitiv e moto r equipmen t o f their ances tors. Th e birds an d th e bats , i n acquiring the powe r of flight, have lost the us e of a pair of limbs for other modes of locomotion, since their wings are th e foreleg s mad e ove r fo r purpose s o f flying . Insect s thu s see m related t o th e winge d creatures o f fiction, though th e latter , i t mus t b e observed, ar e give n wing s quit e irrespectiv e o f their anatomica l possi bilities of using them. An d yet the wing s of insects, as we shall presently see, when first acquired were probably outgrowths of the bac k incapable of movement. Certainl y their evolution into organs of flight has involved much reconstructio n i n th e thoraci c segment s t o contriv e a moto r mechanism for them . 1. ORIGI N AN D EVOLUTIO N O F TH E WING S

The oldest insects known from the fossil records lived in Carboniferou s times, their remains being found i n the lower beds of the Upper Carbonif-

FIG. 119.—Example s o f fossi l insect s wit h paranota l lobe s o n th e pr o thorax. A , Stenodictya lobata. (From Brongniart, 1890. ) B , Lemmatophora typica. (From Tillyard, 1928.)

erous o r Pennsylvania n period . Thes e ancien t insect s ha d tw o pair s of fully develope d wings , whic h differe d bu t littl e i n structur e fro m th e wings of modern insects (Fig . 119) . Man y o f th e Carboniferou s insects, however, ha d i n additio n t o th e wing s a pai r o f small , fla t lobe s (pnl) projecting laterall y fro m th e tergu m o f th e prothorax , an d thes e lobe s 211

212

PRINCIPLES OF

INSECT MORPHOLOGY

suggest tha t a t a n earlie r perio d th e wing s themselves wer e developed from simila r terga l lobe s o f the mesothora x an d th e metathorax . W e may visualiz e the immediat e ancestor s o f the winge d insects, therefore , as creature s having th e bod y alread y differentiate d int o head , thorax , and abdome n an d characterize d b y th e possessio n o f a serie s o f thre e partly overlapping , fanlik e extensions , o r paranotal lobes, projectin g o n each sid e o f the bod y fro m th e thoraci c terga . I f th e paranota l lobes , then, wer e the precursor s of the wings , it seem s most probabl e that the y served a s glidin g organs , allowin g their possessor s t o launc h themselve s into th e ai r fro m a n elevatio n an d t o sai l of f t o som e mor e distan t objective. The structur e an d developmen t o f the wing s of modern insects attes t the origi n o f the wings fro m latera l fold s o f the terga l margins , fo r eac h wing i s essentiall y a hollo w exten sion o f th e bod y wall , th e dorsa l lamina o f which is directl y contin uous with th e tergal plat e support ing it , whil e the ventra l lamin a i s reflected int o the latera l wal l of the segment (Fig . 120). Th e pleura l ,™ ~ , . . , plate s o f th e thoraci c segment s A FIG. 120.—Diagrammati c cros s section o f ^ f >

a thoracic segment with paranotal extensions mus t hav e bee n evolve d f Or th e

of th e tergum. purpos e o f supportin g th e base s of the paranota l lobe s fro m below . W e can thereb y understan d wh y th e pleura o f adult pterygot e insects have the sam e essential structur e i n all the thoraci c segments, and why they diffe r s o characteristically fro m th e primitive pleurite s of the Apterygota . Th e specia l features of the pleur a in the wing-bearing segments are final adaptations t o the later-develope d mechanism for moving the wings . The firs t ste p i n th e evolutio n o f the paranota l lobe s int o organ s of flight mus t hav e consiste d i n the acquisitio n o f a line of flexibility in th e base o f eac h lobe . Th e longitudina l dorsa l muscle s (Fig . 103 A , A ) could then , b y archin g th e terg a upwar d betwee n th e end s o f the seg ments, thro w th e win g flaps downward, since the latter , i n thei r recen t capacity o f glider lobes, were already substantially supporte d fro m below on the pleura . I n orde r t o giv e mechanical efficienc y t o th e muscle s in arching th e terga , however , movemen t betwee n th e terga l plate s ha d first t o b e eliminated . Scleroti c continuit y i n th e dorsu m o f the wing bearing region has been acquired either b y a union of the successiv e terga or b y a forwar d extensio n o f the acroterga l lip s o f the metatergu m an d first abdominal tergum into the regions of the intersegmenta l membrane s (Fig. 85).

THE WINGS

213

With the downstrok e of the wings produced by an upward bend of the tergum, th e upstrok e mus t depen d o n some antagonistic forc e tha t wil l flatten th e tergum . I t i s possible that , i n the firs t place , the elasticit y of th e terga l plate s suffice d t o restor e th e norma l contou r o f the latter ; but mos t moder n insect s hav e specia l terga l depresso r muscle s i n th e two wing-bearing segments. Thes e muscles lie laterad of the longitudinal muscles; they ar e attache d dorsall y o n the anterio r latera l part s o f th e tergum an d ventrall y o n th e sternu m befor e th e coxa e (Fig . 10 3 A, (7). They are perhaps derived from the primitive lateral body muscles. The wing s of insects ar e thus movable u p an d dow n by a relativel y simple mechanism. Flight , however , is not t o b e achieved b y th e mere flapping of a pair of flat appendages. Forwar d motion in the ai r depends upon a more comple x movement i n the moto r organs , involvin g a sligh t forward an d rearwar d action o f the wing s and a partial rotatio n o n their long axes . Wit h th e wing s sufficiently flexibl e a t thei r bases , an d o f a proper structur e i n their dista l parts, these movement s may result fro m the changin g air pressur e o n their surface s whe n they ar e vibrate d i n a vertical direction. Th e stiffenin g o f the anterio r part s o f the wing s by a forward crowdin g of the vein s (Fig . 131) , an d th e flexibilit y o f the mor e weakly supporte d posterio r areas , automaticall y give s a torsio n t o th e wing planes in motion. Bu t i f the wings had to depend upon air pressure alone for the slan t o f their plane s that give s the forwar d impuls e to th e insect in the air , it is evident that the wing s on opposite sides of the bod y would hav e alway s approximatel y th e sam e degree of movement. Con trolled o r differentia l actio n i n th e tw o wing s o f a pai r woul d then b e impossible, and the insec t would have no power of directing its flight or of changing its course. Mos t modern insects, however, do control efficientl y their motion s o n th e wing , an d man y o f them, beside s bein g wel l abl e to direc t their forward flight, can also fly sidewise and backward without changing the positio n o f the body , or they ca n hover at on e point i n th e air. I t i s possible tha t som e insects ma y shif t thei r cours e by alterin g the postur e whil e flying, but i t ha s bee n show n that steering , fo r th e most part , is a function o f the wings. The rotar y movemen t o f the wing s is produce d chiefl y b y powerfu l muscles lyin g agains t th e pleura l wall s o f th e wing-bearin g segment s (Fig. 129 , E', E"). Thes e muscles are inserted usuall y on small sclerites situated immediatel y beneat h th e wings , respectively befor e an d behin d the win g fulcrum (TFP) , o r i n som e cases o n lobe s o f the dorsa l margin of th e pleuron . Th e principa l pai r o f thes e so-calle d " direct win g muscles'7 i n eac h side o f the segmen t tak e thei r origin s ventrall y o n th e coxa (Cx), showing that they are primarily leg muscles that have been given over to the service of the wings. Whe n insects in this way acquired the direct action of muscles on the wing bases, they possessed a mechanism

214

PRINCIPLES OF INSECT MORPHOLOGY

capable o f controlling the movement s o f each win g separatel y an d thu s became endowed with the powe r o f directive flight . 2. DEVELOPMEN T O F TH E WING S

Since th e insec t win g is a flattened , double-layere d expansio n o f th e body wall , its ow n walls consist o f the sam e element s a s the bod y wall , namely, cutícula , epidermis , an d basemen t membrane , an d it s lume n contains nerves, tracheae, and the body fluid, or blood. The wing s of insects wit h incomplete metamorphosis gro w externall y in the sam e manner a s do the legs , the mouth parts, and other appendic ular organs . Th e win g bud s appea r firs t i n th e secon d o r third insta r of th e nymp h a s hollow, flattene d outgrowth s of the bod y wall along th e lateral margins o f the dorsum in the mesothorax and metathorax. The y increase i n siz e a t eac h moult , withou t muc h chang e in structur e unti l they assum e th e adul t for m a t th e transformatio n t o th e imago . I n insects with complete metamorphosis th e wings develop during the larval stage beneath the outer cuticula, usually within pouches of the epidermis . The tim e o f their firs t formatio n varie s i n differen t insect s fro m a lat e embryonic perio d t o th e las t larva l instar . Th e internal win g buds ar e normally everte d fro m thei r pouche s durin g th e prepupa l perio d o f th e last larval stage , bu t the y ar e then stil l covere d by the loosened cuticula of the larva. Whe n this last larval skin is shed, and the insect enters th e pupal stage , th e wing s are first exposed as external organs . Durin g th e pupal stag e the y develop rapidly an d the n quickl y take thei r fina l form , when the insect issues as an imago from the pupal skin. In the ver y young wing bud the epidermi s consists of upper and lower layers correspondin g t o th e dorsa l an d ventra l surfac e o f the win g fold ; but ver y soo n the epiderma l cell s become elongate, and th e inne r ends of those i n th e opposin g layer s mee t an d unite . Th e fuse d basemen t membranes the n becom e th e so-calle d middle membrane o f th e wing . Along certai n lines , however , th e basemen t membrane s d o no t com e together; the channels thus left open , which are remnants of the primitiv e wing cavity, determin e the course s of the futur e vein s of the wing . Th e channels contai n the wing nerves, tracheae, an d blood. I n later stages of development, th e epiderma l cell s condens e alon g th e vei n channel s an d form here the thick cuticular layers that are to constitute th e walls of the wing veins. Whe n the wing development is completed, the epidermis has largely disappeared , an d th e matur e win g is almost entirel y a cuticula r structure. Nevertheless , a n activ e circulatio n o f bloo d persist s i n th e adult wing , observed in insects o f most o f the principa l orders , an d sens e organs ar e o f frequent occurrenc e on the win g surfaces. The histologica l change s i n th e growt h o f th e win g ar e somewha t more complicated in the Holometabola , bu t th e developmenta l processes

THE WINGS

215

are essentially the same as in insects with a simpler metamorphosis. Th e most importan t account s of the developmen t o f the wings will b e foun d in th e paper s b y Weisman n (1864) , Goni n (1894) , Maye r (1896) , Corn stock an d Needha m (1898-1899) , Merce r (1900) , W . L . Towe r (1903) , Powell (1903) , Marshal l (1915) , Comstoc k (1918) , an d Kohle r (1932) . The origi n and growth of the trachea e of the wing s are of much importance in the stud y of the win g venation , because , in man y cases , the tracheation of the young wing serves as a key to the homology of the veins of th e adul t wing . Th e wing tracheae arise from a basal trachea, o r two united tracheae , a t th e bas e of the win g bud, and i n general are given off into the latter in two groups, one anterior, the other posterior. I n insects with incomplet e metamorphosi s th e trachea e tak e thei r place s i n th e wings befor e th e vein s ar e forme d an d thu s appea r t o determin e th e courses o f the veins . I n th e Holometabola , however , the vei n channels may be formed in advance of the tracheae, and, though in the Coleóptera, Neuroptera, an d Lepidopter a eac h trachea i s said t o penetrat e th e vei n corresponding t o th e on e its homologu e occupies in insect s wit h incom plete metamorphosis , i n th e Trichoptera , Hymenoptera , an d Dipter a the relation s betwee n th e trachea e an d th e vein s ar e no t s o clearl y preserved. The interna l win g bud s o f holometabolou s larva e ar e aerate d firs t by a few simple tracheae, but i n the later larva l stages they ar e supplied with numerou s bundle s o f tracheoles tha t gro w directly fro m th e wall s of th e primar y win g trachea. Th e definitiv e wing tracheae ar e finall y formed durin g th e las t larva l stag e an d becom e functional in th e pupa l stage, whe n the earlie r trachea e an d tracheole s degenerate . Fro m th e walls o f th e definitiv e tracheae , finally , a secon d se t o f tracheole s i s developed, an d thes e tracheole s become functional at th e chang e to th e imago. The development of the wing tracheoles and tracheae was first studied by Goni n (1894) ; the detail s o f the origi n of the win g trachea fro m th e basal trache a o r trachea e o f the win g in th e principa l group s o f insect s have bee n describe d b y Chapma n (1918) , an d th e developmen t o f th e wing veins of a cockroac h by Bec k (1920). 3. TH E STRUCTUR E O F TH E WING S

In studyin g th e wing s o f insect s w e mus t giv e specia l attentio n t o three features of their structure, namely , the articulation to the body, th e veins, and th e differentiation o f the ala r surfac e into wing regions. Th e veins serv e t o strengthe n th e win g an d t o adap t i t t o th e movement s demanded o f a n orga n o f flight . Th e articula r part s furnis h th e basa l structure i n the win g necessary for the movement s of flight in the dista l area an d constitut e als o the flexor apparatus i n the wing-flexin g insects .

216

PRINCIPLES OF INSECT MORPHOLOGY

The win g region s ar e loca l differentiation s o f the win g are a partl y sub serving the functio n o f flight, but largel y accessory to th e ac t o f flexion. The principal veins of the wing s spring from th e win g base, and mos t of them , except those of the posterio r area , branch in varying degrees in

FIG. 121.—Diagrams of wing venation. A , the archetype venation, with veins named according t o th e Comstock-Needha m system . (Adapted from Bradley, 1931 , t o include three branches o f cubitus.) B , th e usua l win g vein s an d axillarie s a s designate d i n th e accompanying text . A , ana l veins ; Ax , axillar y sclerite s (first , second , third , an d fourth); AxC, axillar y cord ; C , costa ; Cu, cubitus ; ft, .humeral cross-vein ; HP, humera l plate; /, jugal veins; jf, juga l fold; M , media ; m, m', median plates; MA , medi a anterior ; m-m, media n cross-vein ; m-cu, mediocubita l cross-vein ; MP, medi a posterior ; PC , pre costa; Pcu, postcubitu s (firs t anal) ; R , radius ; r , radia l cross-vein ; R s, radial sector ; s , sectorial cross-vein ; tg , rudiment o f tegula; V , vanna l vein s (ana l vein s except the first) ; vf, vannal fold.

the dista l part o f the wing . I t i s probable that al l the divers e patterns of win g venatio n foun d i n livin g an d extinc t insect s hav e bee n derive d from a single type of primitive venation; but th e tru e primitive venation is not actually known, because the oldest fossil insects yet discovered have a highl y comple x syste m o f wing veins . Th e venatio n patter n give n i n Fig. 12 1 A represents th e pla n o f venatio n whic h student s o f the win g

??? ???????? veins o f insect s regar d a s a n ancien t typ e fro m whic h th e venatio n o f modern insects has been derived, and it i s therefore termed th e archetype venation. The theoreticall y complet e archetype venatio n (Fig . 12 1 A) includes the followin g veins , name d accordin g to th e Comstock-Needha m syste m of vei n nomenclature: first, á small precosta (Pc ) a t th e bas e of the wing ; second, a costa (C) , which is usually margina l i n modern insects; third, a two-branched subcosta (Sc) ; fourth , a five-branche d radius (jR) ; fifth , ? ???????????? ????? ???? ?????? ? ?????????????? ??????? ????? ???? finally, a varying numbe r o f anal veins (A). The vei n nomenclatur e give n abov e i s adopte d i n th e presen t tex t with the exception that the anal veins are not recognized as a homogeneous group. Th e firs t ana l vei n (Fig . 12 1 A, I A) i n more generalized insects is alway s associate d a t it s bas e wit h th e cubitus , an d i n th e wing s of many nymphal insects it is represented by a distinct trachea (Fig . 12 5 A, Pen). Th e independenc e of the firs t ana l fro m th e othe r anal s becomes an importan t featur e in a stud y o f th e mechanis m o f th e wings . Fo r this reason it i s here designated th e postcubitus (Fig . 12 1 B, Pcu). Th e rest o f the anals , whic h constitute a definite functiona l group of veins in generalized insects , associate d wit h th e flexor sclerite (3 Ax) o f the wing base, ar e distinguishe d a s vannal veins (V) becaus e th e win g regio n containing the m ofte n form s a larg e fanlik e expansio n (vannus ) of the posterio r par t of the wing . A t the bas e of the wing , proximal to th e vannal region , there i s usually a small but variousl y develope d lobe, th e jugum, whic h may contain on e o r tw o jugal veins (B , J). Th e second principal branch of cubitus, the postcubitus, and the vena dividens sometimes presen t i n th e fol d betwee n the postcubita l an d vanna l vein s ar e the first , second , and thir d plica l vein s o f Forbes (1933) . In th e wing s o f moder n insect s (Fig . 12 1 B) th e precost a doe s no t appear, an d th e anterio r for k o f media (A , MA) i s usually absent . Th e remaining veins in the wing are subject to many modifications in differen t groups o f insects b y th e unio n o f adjacent vein s o r by a partia l o r even complete suppression of certain veins , and the venationa l patter n ma y be further complicate d b y th e additio n o f secondar y veins . I t ofte n becomes, therefore, a difficult matte r to identify with certaint y th e vein s that ar e present, an d th e proble m of determining th e wing-vei n homologies i n th e variou s order s o f insect s ha s bee n a majo r subjec t i n entomology. A help in the study o f the venatio n o f adult insects may b e derived fro m a n examinatio n o f th e basa l connection s o r associatio n o f the veins with smal l sclerites in the articula r regio n of the wing . Thes e sclerites, the pteralia , ar e present i n the wing s of all insects, bu t the y are particularly develope d in insects that flex the wing s over the bac k when at rest . Th e sclerite s hav e definit e and constan t relation s bot h t o one

218

PRINCIPLES OF INSECT MORPHOLOGY

another an d t o th e base s o f the veins . Thi s fac t ha s lon g been known, but fo r some reason students o f wing venation have made little use of the basal connection s o f th e vein s i n adul t insect s fo r determinin g vei n homologies. The Articulation of the Wings.—Th e various movements of the wings, especially i n insects that flex the wing s horizontally over the bac k when at rest, demand a more complicated articular structur e a t th e win g base than a mere hinge of the win g with the body . Eac h wing is attached t o the body by a membranous basal area, but th e articula r membran e contains a numbe r o f smal l articula r sclerites , collectivel y know n a s th e pteralia (Fig . 12 1 B). Th e pterali a includ e a n anterio r humeral plate ???? ?? ??? ???? ?? ??? ?????? ????? ? ????? ?? ?????????? ???? ?????????? with the subcostal , radial, an d vannal veins , and two less definite median plates (m? ??? ?? ??? ???? ?? ??? ???????????? ????? ??? ?????????? ??? specifically develope d onl y i n th e wing-flexin g insects , wher e they con stitute th e flexor mechanism of the win g operated b y th e flexor muscle arising o n the pleuro n (Fig . 12 8 C, D). Characteristi c o f the win g base is also a small lobe on the anterio r margi n of the articula r are a proximal ?? ??? ??????? ????? ????? ??? ?? ???? ?????? ?? ??? ???????? ?? ???? insects, i s develope d int o a large , flat , scale-lik e flap , th e tequia, over lapping th e bas e of the wing . Posteriorl y th e articula r membran e ofte n forms a n ampl e lobe between the win g and th e body , an d it s margi n is generally thickened and corrugated, giving the appearanc e of a ligament, the so-calle d axillary cord (Fig . 122 , AxC), continuou s mesally with th e posterior margina l scutellar fold o f the terga l plat e bearin g the wing. The articula r sclerites, or pteralia, o f the win g base of the wing-flexin g insects an d thei r relation s t o th e bod y an d th e win g veins , show n dia grammatically i n Fig. 122 , are a s follows : The Humeral Plate (HP). —Usually a smal l sclerit e o n th e anterio r margin o f the win g base, movably articulated wit h the bas e of the costa l vein; greatl y enlarged in Odonata (Fig . 12 3 B). The First Axillary (IAx). —This sclerit e i s th e anterio r hing e plat e of th e win g base. It s anterio r par t i s supporte d o n the anterio r nota l wing process of the tergum (ANP) ; it s posterior part articulates wit h th e tergal margin. Th e anterio r end of the sclerit e is generally produced as a slende r arm , th e ape x of which (e ) is alway s associated wit h th e bas e of th e subcosta l vei n (Sc), though i t i s not unite d wit h the latter . Th e body o f the sclerit e articulate s laterall y wit h the secon d axillary. The Second Axillary (2Ax). —This sclerit e i s mor e variable i n form , than th e firs t axillary , bu t it s mechanica l relations ar e n o less definite . It i s obliquely hinged to the oute r margin of the body of the first axillary, and th e radia l vein -(R) i s always flexibly attached t o it s anterio r en d (d). The secon d axillary presents bot h a dorsa l an d a ventra l sclerotization

THE WINGS

219

in the win g base; its ventra l surfac e rest s upon the fulcra l win g process of th e pleuro n (Figs . 12 8 C, 129 , 2Ax). Th e secon d axillary, therefore, is th e pivota l sclerit e o f the win g base , an d i t specificall y manipulate s the radial vein . The Third Axillary (3 Ax).—The thir d axillar y sclerit e lie s i n th e posterior par t o f th e articula r regio n o f th e wing . It s for m i s highl y variable and often irregular , but the third axillar y is the sclerit e on which is inserte d th e flexo r muscl e o f th e win g (D). Mesall y i t articulate s

FIG. 122.—Diagra m showin g th e articulatio n o f the win g wit h th e alinotum , an d th e basal relation s o f th e vein s to th e humera l plate an d th e axillar y sclerites. (Letterin g as on Fig. 121. )

anteriorly (/) with the posterio r end of the second axillary, and posteriorl y (b) with the posterior wing process of the tergum (PNP), or with a small fourth axillar y when the latter is present (Fig . 121 B, ±Ax). Distall y the third axillary is prolonged in a process which is always associated with the bases o f th e grou p o f veins i n th e ana l regio n o f the win g here terme d the vanna l vein s (V). Th e thir d axillary , therefore , i s usuall y th e posterior hing e plat e o f th e win g bas e an d i s th e activ e sclerit e o f the flexor mechanism, which directly manipulates the vannal veins. Th e contraction o f th e flexo r muscl e (D ) revolve s th e thir d axillar y o n it s mesal articulations (6 , /) an d thereby lift s it s dista l arm ; this movement produces the flexion of the wing. The Fourth Axillary (Fig . 121 B, 4Ax).—This sclerit e is not a constan t element o f th e win g base . Whe n presen t i t i s usuall y a smal l plat e intervening betwee n th e thir d axillar y an d th e posterio r nota l win g process and i s probably a detached piece of the latter . The Median Plates (m , m f).—These sclerite s ar e no t s o definitel y differentiated a s specifi c plate s a s ar e th e thre e principa l axillaries , bu t

220

PRINCIPLES OF

INSECT MORPHOLOGY

nevertheless they are important element s of the flexor apparatus. The y lie i n th e media n are a o f the win g base dista l t o th e secon d and thir d axillaries and are separated fro m eac h other by an oblique line (6/ ) which forms a prominent conve x fold durin g flexion of the wing . Th e proximal plate (m) i s usually attache d t o th e dista l ar m o f the thir d axillar y an d perhaps shoul d b e regarde d a s a par t o f th e latter . Th e dista l plat e (mf) is less constantly present as a distinct sclerite and may be represented b y a general sclerotization o f th e bas e of the mediocubita l field of the wing. Whe n the vein s of this region are distinct a t their bases, the y are associated wit h the oute r median plate . The Win g Bas e o f Ephemerida.—The mayflies , when at rest , brin g the wings together vertically over the back, but they do not flex the wings in the sens e of folding them horizontally. A flexor mechanism, therefore,

FIG. 123.—Th e wing articulation i n insects that do not flex the wings . A , wing base of a mayfly . B , win g bas e o f a dragonfly . Ax, axillar y region ; AxP, axillar y plate ; HP, humera l plate .

is not develope d in th e base s o f the wings ; and ye t th e structur e o f th e articular area s o f the wing s (Fig . 12 3 A) i s no t radicall y differen t fro m that of the wing-foldin g insects. A t th e bas e of each wing of the mayfl y there i s a small humeral plate (HP) intermediating betwee n the hea d of the costa l vein (C) and a small tergal lobe of the body segment supporting the wing . I n th e axillar y regio n ther e i s a grou p o f weakl y define d sclerites (Ax), whic h in their arrangemen t an d relations to the vein bases give a suggestio n o f th e axillarie s o f th e wing-foldin g insects . Th e posterior par t o f the axillar y membrane in the mayfl y has the usual for m of a fol d bordere d b y a corrugate d thickening, o r axillar y cor d (AxC), continuous wit h th e posterio r margi n o f the tergum . The Win g Bas e o f Odonata.—Th e articula r regio n of the win g of a dragonfly contain s tw o large , strongl y sclerotize d plate s (Fig . 12 3 B). The anterior plate (HP) support s the costa l vein by a small intermediate sclerite (c ) a t th e bas e of the latte r and thus correspond s to th e humeral

??? ????? ??? plate o f Ephemerida (A ) an d th e wing-flexin g insect s (Fig . 12 1 'B, HP). The grea t enlargemen t o f the humera l plat e i n Odonat a i s evidentl y a specialized feature of the flight mechanism in this group . Th e posterio r plate of the dragonfl y wing (Fig. 12 3 B, AxP) carrie s the fou r basal shafts of th e postcosta l vein s and hence may b e termed the axillary plate, since it correspond s in position to the grou p of sclerites in the ephemeri d wing (A, Ax) tha t appear to represent th e axillarie s of the wing-flexing insects . The humera l plate o f the odonat e win g is hinged to th e anterio r hal f of the lateral edge of the tergu m (T ) o f the segmen t supporting the wing , or in some species to a distinct sclerit e (a) of the tergum. Th e axillary plat e is articulated t o the posterio r hal f o f the latera l terga l margi n opposit e a deep membranous area o f the latter . Th e pleura l wing process support ing the wing has two arms, one applied to the humeral plate, th e other to the axillar y plate. Th e basal plates o f the dragonfly' s wing turn u p and down o n the fulcra l arm s whe n the wing s are lifte d o r depressed . Th e two plates , however , are slightl y movabl e on each other, and , sinc e th e costal vei n (C ) is doubly hinged t o th e humera l plate b y a smal l inter mediary piec e (c ) at it s base , th e costa l are a o f th e win g can b e quit e freely deflected independen t of the res t o f the win g area, which is solidly supported o n the axillar y plate by the veins attached t o the latter . The fligh t mechanis m of the Odonata , including the structur e o f th e wing bases and the attachments o f the wing muscles, appears to be a special development of a more generalized structure, retained by the Ephemerida , from whic h the win g mechanism of other insects has been evolved. The Win g Veins.—The usual veins of the wing , omitting th e precost a of certai n fossi l insects , ar e show n diagrammaticall y a t B o f Fig . 121 . Their characteristi c feature s an d basal connection s are as follows : Costa (C). —The usua l first vein of the wing , commonly marginal, bu t sometimes submarginal ; associate d a t it s bas e wit h th e humera l plat e (HP). The trachea of the costal vein is perhaps a branch of the subcostal trachea . Súbeosla (Se). —The secon d vein of the wing , typically forke d distall y into two short branche s (Sci , Sc z); associate d a t it s bas e with the dista l end of the neck of the first axillary (lAx)-. Radius (R). —The thir d an d generall y the stronges t vei n of the wing. Toward the middl e of the win g it fork s into a first undivided branch (JRi ) ??? ? ?????? ??????? ?????? ??? ?????? ?????? ????? ????? ?????????? dichotomously int o fou r dista l branche s (E 2, Rs, R±, R$). Basall y th e radius is flexibly united with the anterior en d of the second axillary (2Ax). Media (M). —The fourt h vein of the wing . I n th e archetype pattern (A) the media forks into two main branches, a media anterior (MA), whic h divides into two distal branches (MA\, MA%), and a median sector, or ????? ????????? ????? ????? ??? ???? ???????? ???????? ???? ??

222

PRINCIPLES OF INSECT MORPHOLOGY

Ms, Mi). I n mos t moder n insects (B ) the medi a anterior ha s been lost, and th e usua l "media " i s th e four-branche d media posterio r wit h th e common basa l stem . I n th e Ephemerida , accordin g t o presen t inter pretations o f the win g venation, both branches of the medi a are retained , while i n Odonat a th e persistin g medi a i s the primitiv e anterio r branch . The ste m o f the medi a is often unite d wit h the radius, but whe n it occurs as a distinct vei n its bas e is associated wit h the dista l median plate (m!) or is continuously sclerotized with the latter .

FIG. 124.—Base s of the for e an d hin d wings of Periplaneta americana.

Cubitus (Cu). —The fift h vei n o f th e wing , primaril y tw o branched . The primar y forking o f the cubitu s takes place near the bas e of the win g (Figs. 12 4 B, 12 5 A, B, C), forming the two principal branches (Cu it Cu 2). The anterio r branc h may break up into a number o f secondary branches (Figs. 12 4 B, 12 5 B), bu t commonl y it fork s int o tw o dista l branche s (Fig. 12 1 B , Cuia, Cuib). Th e secon d branc h o f th e cubitu s (Cu 2) i n Hymenoptera, Trichoptera, and Lepidoptera was mistaken b y Comstock and Needha m fo r th e firs t anal , a s ha s bee n show n by Tillyar d (1919) , Lameere (1922) , Tanaka (1926) , Imms (1931a, 1934), and others. Proxi mally th e mai n stem o f the cubitu s i s associated wit h th e dista l media n plate (m' ) o f the win g base.

??? ???????? ??????????? ??????????? ???? ?? ??? ????? ???? ?? ???????? ??? Needham (Fig . 12 1 A, 1A), except where these writers wrongly identified the secon d branc h o f cubitu s a s th e "firs t anal. " Th e postcubitus , however, has the statu s of an independent win g vein (B , Pcu) and shoul d be recognized as such. I n nymphal wings, as amply shown by Comstock (1918), it s trache a arise s betwee n the cubita l trache a an d th e grou p of vannal tracheae (Fig . 12 5 A, Pcu). I n th e matur e wing s of more generalized insect s th e postcubitu s i s alway s associate d proximall y wit h th e cubitus (Fig . 12 4 B) an d i s never intimatel y connecte d wit h th e flexo r ???????? ????? ?? ??? ???? ????? ?? ??????????? ?????????? ??? ???? choptera the postcubitu s ma y be more closely associated with the vanna l veins (Fig . 12 5 C, Pcu), but it s base is always free fro m th e latter . The ' postcubitus i s usuall y unbranched ; accordin g t o Lameer e (1922 ) i t i s primitively tw o branched . I n a forme r pape r th e write r (1930 ) calle d ???? ???? ??????? ??????? ?????? ??? ?????????? ???????? ?? ?? ?????? senting th e secon d branch o f cubitus i n Orthoptera . Vena Dividens. —This i s apparentl y a secondar y vei n presen t i n th e hind wing of some Orthoptera (Figs . 12 4 B, 13 4 B, vd), developed in th e ???? ???? ???? ???? ??? ??? ?????? ?????? ???? ??? ???? ?????? ?????? ??? Vannal Veins (IF to nV).—The vanna l vein s are the ana l vein s that are immediately associate d wit h the third axillary, and which are directly affected b y th e movemen t o f this sclerit e tha t bring s abou t th e flexion of th e wings . I n numbe r the vanna l vein s vary fro m 1 to 12 , according to th e expansio n of the vanna l are a o f the wing . Th e vanna l trachea e usually aris e fro m a commo n trachea l ste m i n nympha l insect s (Fig . 125 A, V) , an d th e vein s ar e regarde d b y Lameer e (1922 ) an d Tanak a (1926) a s branches of a single ana l vein . Distall y th e vanna l vein s ar e either simpl e or branched. Jugal Veins (J). —The juga l lob e of th e win g i s ofte n occupie d by a network of irregular veins, or it ma y be entirely membranous; but some times i t contain s on e or two distinc t smal l veins , th e first jugal vein, or vena arcuata (Fig . 12 1 B, 1«7), and th e second jugal vein, or vena cardinalis (2J). Cross-veins.—All th e vein s o f th e win g ar e subjec t t o secondar y forking an d to union by cross-veins . I n som e orders of insects the cross veins are so numerous that the whol e venational pattern becomes a close network o f branching veins and cross-veins . Ordinarily , however, there is a definit e number o f cross-veins havin g specifi c location s a s indicate d at B o f Fig. 121 . Th e mor e constant cross-vein s ar e th e humeral crossvein (h ) between costa an d subcosta , th e radial cross-vein (r ) between R\ and th e firs t for k o f R s, the sectorial cross-vein (s ) between the tw o fork s ?? ??? ??? ?????? ?????????? ????? ??????? ?? ??? ??? ??? ??? ?????? cubital cross-vein (m-cu) betwee n media an d cubitus .

224

PRINCIPLES OF INSECT MORPHOLOGY

The vein s of the win g appear to fall into an undulating serie s of convex veins and concave veins, according to whether they have a tendency t o fol d up o r down when the win g is relaxed. Th e basa l shaft s o f the vein s ar e convex, but accordin g to Lameere (1922 ) each vein fork s distall y into a n anterior conve x branch and a posterior concav e branch. Thu s th e cost a

FIG. 125.—Example s o f win g venation. A , hin d win g of nymp h o f Scudderia. (From Comstock, 1918. ) B , hin d wing o f a plecopteron, Isogenus. C , forewin g o f Panorpa.

and subeost a ar e regarded a s convex and concav e branches o f a primar y first vein, Rs is the concav e branch of the radius, MP th e concave branch of the media , Cu\ and Cu^ ar e respectivel y conve x and concave , while th e primitive postcubitu s an d the firs t vanna l hav e eac h a n anterior conve x branch an d a posterior concav e branch. Th e conve x or concave nature of th e vein s has been used as evidence in determining the identitie s o f the persisting dista l branche s o f the vein s o f modern insects , bu t i t ha s no t been demonstrate d t o b e consistent fo r all wings.

??? ????? ??? The Win g Regions.—In th e wing s of all insects w e must distinguis h a basa l articular area fro m th e tru e ala, or dista l expans e o f th e win g containing th e veins . Th e win g base o f Odonata, a s we have observed , contains tw o larg e plates , a n anterio r humera l plat e (Fig . 12 3 B, HP ) supporting th e costal vein , and a posterior axillar y plate (AxP) support ing the other veins. Th e structure here is probably a specialized development of a more generalized structure of the win g base in primitive insects. In th e Ephemerid a (A ) th e humera l plat e i s small , a s i t i s i n insect s generally, an d th e axillar y regio n (Ax) contain s a grou p o f indistinctl y differentiated sclerites . I n th e wing-flexin g insect s th e axillar y sclerite s are well defined an d individualized; the are a containin g them is a definit e feature o f th e win g base an d ma y b e terme d th e axillary region o f th e wing (Fig . 126 , Ax). The tru e ala r are a o f the win g is alway s mor e o r les s asymmetrica l in form . Th e contou r o f the fron t margi n i s differen t fro m tha t o f th e hind margin, and the pattern of the anterior venation never matches with that of the venatio n i n the posterio r par t o f the wing . Ther e i s a tend ency for the anterio r vein s to become thickened and crowded toward th e forward margi n in such a manner as to giv e greater rigidity t o th e fron t half o f the wing , while the weake r posterior vein s are more widely spaced and giv e flexibility to th e rea r half . Th e ala r are a thu s become s differ entiated into an anterior region (Fig. 126, Rm), which is actively effectiv e in flight , an d a posterio r mor e passive regio n (Vri). Th e anterio r rigi d part o f the win g may b e termed th e remigium (fro m Latin , a n oar). I n more generalize d slow-flyin g insects , th e posterio r flexibl e par t o f th e wing is often enlarge d to for m a fanlike expansio n of the win g and hence may b e termed th e vannus (from Latin, & fan). In th e more specialize d swift-flying insect s th e vannu s is reduced; but sinc e it contain s the vein s connected wit h th e flexo r sclerit e o f th e win g base an d i s therefor e an essential par t o f th e flexo r apparatus , th e vannu s i s seldo m entirel y obliterated. Finall y ther e i s ofte n develope d a t th e bas e o f th e wing proximal t o th e vannu s a membranou s lob e o f th e wing , th e neala of Martynov (1925) , commonl y calle d th e jugum (Fig . 126 , Ju) becaus e that of the forewin g in some insects serves to yoke the tw o wings on each side wit h eac h other . A t th e posterio r angl e o f th e win g bas e ther e sometimes occur s a membranou s lobe , o r pai r o f lobes , know n a s th e alula, or calypter. The three regions of the ala r surface ar e commonly separated b y line s of foldin g i n th e win g membrane . Thi s i s tru e particularl y whe n th e wings ar e wid e an d canno t b e place d fla t ove r th e bod y i n th e flexe d position. Ther e occur s then betwee n the remigiu m and th e vannus , o r approximately separatin g thes e regions , a plica vannalis, or vannal fold (Fig. 126, vf). This fold either allows the vannus to take a horizontal

226

PRINCIPLES OF INSECT MORPHOLOGY

position over the back , while the remigiu m slopes downward on the side , or it enable s the vannu s to be folded beneat h th e remigiu m in the flexed wing. Th e jugum , whe n well developed, is likewis e separated fro m th e vannus by a line of folding, th e plica jugalis, or jugalfold (jf), an d i n th e flexed wing the jugu m is usually turned u p o r down on the inne r edge of the vannus . Th e vanna l fold , calle d als o th e "ana l furrow/ ' doe s no t occur a t exactl y th e sam e plac e i n th e wing s o f al l insects , a s wil l b e noted i n specia l example s to b e describe d later , an d i n narrow-winge d insects it ma y be eliminated. Th e wing regions are particularly distinct

FIG. 126.—Diagra m o f the win g regions in wing-flexin g insects . Ax, axillary region; bf, basal fold ; jf, juga l fold ; Ju, jugum; Rm, remigium; Vn, vannus ; vf, vanna l fold.

in insect s havin g a larg e vannus, an d especiall y i n thos e tha t plai t th e vannus whe n the win g is flexed. The Axillary Region. —The regio n containin g th e axillar y sclerite s (Fig. 122 ) ha s i n genera l the for m o f a scalen e triangl e (Fig . 126 , Ax). The bas e o f th e triangl e (a-b) i s the hing e o f the win g with th e body ; the ape x (c ) is the dista l end of the third axillary sclerite (Fig . 122, c); th e longer sid e (Fig . 126 , o-c ) i s anterio r t o th e apex . Th e poin t d on th e anterior sid e of the triangl e mark s the articulatio n of the radia l vein with the secon d axillar y sclerit e (Fig . 122 , d). Th e lin e betwee n d an d c (Fig. 126) is the plica basalis (&/), or fold of the wing at the base of the mediocubital field (Fig. 122, bf). The Remigium. —The win g region anterio r t o th e vanna l fol d (Fig . 126, Rm) i s the par t o f the win g chiefl y productiv e of the movement s of flight, sinc e i t i s directl y affecte d b y th e moto r muscle s o f th e wing . When th e vanna l fol d ha s th e usua l positio n anterio r t o th e grou p of vannal vein s (Fig . 12 1 B, «;/) , th e remigiu m contain s th e costal , sub costal, radial , medial , cubital, an d postcubital veins. I n the flexed wing the remigiu m turns posteriorl y o n th e flexibl e basa l connectio n o f th e radius with the second axillary (Fig. 122, d), and the base of the mediocubital field is folded medially on the axillary region along the plica basalis (bf) between the median plates (m, m') of the wing base.

THE WINGS

227

The Vannus. —The vanna l fol d typicall y occur s betwee n th e post cubitus and the first vannal vein (Figs. 121 B, 122, vf). In Orthoptera it usually has this position (Fig. 124 B, 134 A, B, vf). In the forewing of Blattidae , however , the onl y fol d i n thi s part o f the win g lies imme diately befor e the postcubitus (Fig. 124 A, 2/). I n Plecoptera the vanna l fold i s posterio r t o th e postcubitu s (Fig . 12 5 B, vf), bu t proximall y i t crosses the bas e o f the firs t vanna l vein . I n th e cicad a (Fig . 12 7 A) th e vannal fol d lie s immediately behin d th e firs t vanna l vei n (IF) . Thes e small variation s i n th e actua l positio n o f the vanna l fold , however , d o not affec t th e unit y o f actio n o f th e vanna l veins , controlle d b y th e flexor sclerite (3Ax), i n th e flexio n o f th e wing . I n th e hin d wing s of most Orthopter a a secondar y ven a dividen s form s a ri b i n th e vanna l fold (Figs . 12 4 B, 13 4 B, D , vd). The vannu s i s usuall y triangula r i n shap e (Fig . 126 , Vn), an d it s veins typically sprea d ou t fro m th e thir d axillar y lik e th e rib s o f a fan . Some o f th e vanna l vein s ma y b e branched , an d secondar y vein s ma y alternate wit h the primar y veins (Fig . 13 4 B, a, b, c). Th e vannal region is usually bes t develope d in the hin d wing , in which it ma y be enlarge d to form a sustaining surface , as in Plecoptera an d Orthoptera. Th e great fanlike expansion s of the hin d wings of Acrididae (Fig. 13 4 B) ar e clearl y the vannal regions, since their veins are all supported on the third axillary sclerites of the win g bases, though Martynov (1925 ) ascribe s mos t o f th e fan area s in Acrididae to the juga l regions of the wings . The true jugum of th e acridi d win g i s represente d onl y b y th e smal l membran e (Ju) mesad o f the las t vanna l vein . Th e jugu m i s more highly develope d in some othe r Orthoptera , a s i n th e Mantidae . I n mos t o f th e highe r insects wit h narro w wing s th e vannu s become s reduce d (Figs . 12 5 C, 127 C), and the vanna l fold i s lost, but eve n in such cases the flexed wing may bend along a line between the postcubitus and the first vannal vein . The Jugal Region, or Neala.—The juga l region of the win g (Fig. 126 , Ju) i s usuall y a smal l membranou s are a proxima l t o th e bas e o f th e vannus strengthene d b y a fe w small, irregula r veinlik e thickenings; bu t when well developed it is a distinct section of the win g (Figs. 124 A, 125 C, 127 A, D, Ju) and may contain one or two jugal veins (Figs. 121 B, 127 D, U, 2J) . Whe n th e juga l are a o f the forewin g i s develope d a s a fre e lobe, i t project s beneath th e humera l angl e o f the hin d win g and thu s serves t o yok e the tw o wing s together. I n th e Jugata e grou p o f Lepidoptera i t bear s a lon g fingerlike lobe. Th e juga l regio n is termed th e neala ("ne w wing" ) b y Martyno v (1925) , becaus e i t i s evidentl y a secondary and recentl y developed part o f the wing . The Alula. —At th e posterio r angl e of the win g base in some Díptera there is a pair of membranous lobes (squamae, o r calypteres) know n as th e alula. Th e alul a i s wel l develope d i n th e hous e fl y (Fig . 12 7 C, c , d).

228

PRINCIPLES OF INSECT MORPHOLOGY

The oute r squama (c ) arise s fro m th e win g base behind the thir d axillar y sclerite (3Ax) an d evidentl y represent s th e juga l lob e o f othe r insect s (A, D) ; th e large r inne r squam a (d) arise s fro m th e posterio r scutella r margin of the tergum of the wing-bearing segment and forms a protective , hoodlike canopy over the halter. I n th e flexed wing the outer squama of the alul a i s turned upsid e dow n above th e inne r squama , th e latte r no t

FIG. 127.—Example s o f win g venation . A , Magicicada septendecim, hin d wing , extended. B , sectio n o f same alon g line a-b when folded. C , Musca domestica, wing and calypteres. D , Epicauta pennsylvanica.

being affecte d b y th e movemen t o f the wing . I n man y Dipter a a dee p incision o f the ana l are a o f the win g membrane behind th e singl e vanna l vein sets off a proximal ala r lob e distal to th e oute r squam a o f the alula . 4. TH E WIN G MUSCLE S

The movement s o f th e wing s in th e majorit y o f insect s ar e accom plished b y fiv e pair s o r paire d set s o f muscle s i n eac h álate segment . These muscles are the dorsal muscles (Fig. 103, A), the tergosternal muscles

THE WINGS

229

(C), th e axillary muscles (D) , th e basalar muscles (E') t and the subalar muscles (E"). The dorsal and tergosternal muscles are often called the " indirect wing muscles/7 and the axillary and epipleural muscles the " direct win g muscles/' but , strictly speaking , onl y the axillar y muscle s in most insect s ar e attached directl y o n the win g bases . In additio n t o th e muscle s liste d abov e a s specifi c win g muscles , i t is probable that most o f the segmenta l an d intersegmental muscles of the pterothorax tha t ar e not le g muscles have som e action i n relation t o th e wing movements. Particularl y th e tergopleura l muscles (Fig. 103 B, B) , which exten d fro m th e tergu m t o th e basalare , t o th e win g process, o r to the epimeron , must exer t som e controlling influence o n the movemen t of the tergum. Thos e inserted o n the basalare undoubtedly have a direct action o n the wings ; a large tergobasalar muscl e present i n some Dipter a sharply extend s th e win g i n a horizonta l plane . Sinc e however , th e tergopleural muscle s ar e highl y variabl e an d ar e no t constantl y presen t in the wing-bearing segments, they will not be considered in the followin g general discussion o f the win g muscles. The Dorsa l Muscles.—Thes e muscle s ar e th e ordinar y longitudina l muscles o f the back , which , in th e usua l secondar y segmentatio n o f th e body (Fig . 37), extend fro m th e antecost a o f one tergum t o tha t o f th e next. I n th e wing-bearin g segment s o f most insect s th e dorsa l muscle s are differentiate d into median longitudinal muscles (Fig . 128 A, mA) an d lateral oblique muscles (IA). The media n dorsa l muscle s are usually greatl y enlarge d in the wing bearing segments , an d their expansio n is accommodated by the develop ment o f phragmata l lobe s o n th e antecosta e o f th e mesotergum , th e metatergum, an d th e firs t abdomina l tergum , bu t frequentl y als o thei r dorsal fiber s encroac h upon th e postcosta l surfac e o f the alinotu m an d on the precostal surfac e of the postnotum (Fig . 128 A). Th e longitudina l dorsal muscle s ar e th e principa l depressor s o f the wings , since, b y thei r contraction, the y arc h th e wing-bearin g terga upwar d between the end s of th e segment s and thu s deflec t th e wing s on the pleura l fulcra (Fig . 131 C). Th e actio n o f th e dorsa l muscle s a s win g depressors , however , depends o n a n obliteratio n o f th e dorsa l intersegmenta l membranes , a condition that ha s been brought about eithe r b y a fusion o f the consecu tive terg a o r b y a forwar d extensio n o f the precosta l lip s o f th e meta thoracic an d first abdominal terga (Fig . 85) to form the phragma-bearin g postnotal plate s o f the mesothora x an d metathorax , respectively . Th e dorsal muscle s ar e mos t highl y develope d i n th e segmen t bearin g th e principal pai r o f wings; they ar e usually reduce d or absent i n a segmen t of whic h the wing s are smal l or are use d for othe r purpose s than that of flight. Wit h insect s suc h a s Isoptera , Blattidae , an d Gryllida e havin g weak powers of flight, the dorsa l muscles are very small in both segment s

230

PRINCIPLES OF INSECT MORPHOLOGY

of the pterothorax, but they are also reduced in the strong-flying Odonata, in which the wing s are moved by the latera l thoraci c muscles. The latera l oblique dorsal muscles (Fig . 12 8 A, I A) aris e o n the pos terior par t o f the scutu m an d ar e inserted posteriorl y o n the succeedin g phragma latera d o f th e base s o f th e media n dorsals . Usuall y thes e muscles are relatively small , and they are not alway s present; in functio n they probabl y supplemen t th e tergosternal s i n thei r downwar d pull o n the tergum . I n th e mesothora x o f the cicad a an d i n th e Dipter a th e

FIG. 128.—Th e win g muscles. A , dorsa l muscle s o f pterothorax o f Panorpa, latera l view. B , mesothora x o f Magicicada, media n dorsal s (mA) removed , showin g almos t vertical positio n o f larg e latera l olorsal s (IA}. C , mesothora x o f Dissosteira with basala r (Ef), subalar (E"), and wing flexor muscles (Z>) of right side, mesal view.

oblique dorsal muscles are unusually large (B , IA) an d assum e a positio n so nearly vertical, by reason of the great size of the second phragma (2P/¿), that the y becom e powerful adjunct s o f the tergosterna l muscle s (C ) a s depressors o f the tergum . The Tergosterna l Muscles. —These muscle s li e t o th e side s o f th e median dorsa l muscle s in the anterio r par t o f the segmen t (Figs . 10 3 A, 128 B, C). The y ar e attache d dorsall y o n th e anterio r latera l area s of the tergum, and ventrally on the basisternum befor e the coxae. Ther e may b e one or several pairs o f them i n each segment. Functionall y th e tergosternal muscle s ar e antagonist s o f th e longitudina l dorsals , sinc e by contraction they depress the tergum and thereby elevate the wings on the pleura l fulcr a (Fig . 13 1 A). Thes e muscle s have no representative s in th e prothorax , an d the y ma y b e absen t i n th e pterothora x o f insects of wea k flight .

THE WINGS

231

In th e Dípter a a thir d pai r o f muscles, lyin g betwee n th e anterio r tergosternals an d th e posterio r obliqu e dorsals , become s secondaril y levators o f the wings . Th e muscle s o f this pai r ar e th e norma l terga l remotors o f th e middl e leg s inserte d o n th e mera l lobe s o f th e coxae . In th e highe r Diptera , however , th e mesothoraci c mero n i s detache d from the rest of the coxa and becomes solidly incorporated into the latera l wall o f the thora x (Fig . 10 2 E, Mer). Th e terga l remoto r o f the cox a is thus anatomically transferre d fro m th e le g and give n over functionall y to the servic e of the wing, since, by the loss of movement at it s lower end, it become s a depressor o f the tergum . The Axillar y Muscles.—Th e onl y muscle s attache d directl y o n th e wing bases , i n insect s othe r tha n Odonata , ar e muscle s arisin g o n th e pleuron and inserted o n the first and third axillar y sclerites . A muscle of the firs t axillar y i s known to occu r only in Diptera. I n a syrphi d fl y thi s muscl e consist s o f two parts , on e arisin g o n th e epi sternum, the othe r behin d the pleural ridge , bot h inserte d on the inne r margin o f th e firs t axillary . A pul l o n th e muscl e turn s th e axillar y upward o n its terga l articulations , whic h is the usua l actio n o f the firs t axillary durin g flexion of the wings . The muscl e of the thir d axillar y (Fig . 10 3 B, D ) i s present i n al l th e wing-flexing insects , sinc e i t i s th e effecto r o f th e flexio n movement s of th e wing . Th e muscl e arises o n the pleuron , but i t i s variable i n size and distribution . Typicall y i t consist s o f a singl e bundl e o f fiber s attached o n th e pleura l ridg e (Fig . 12 8 C, D) , bu t i t ma y compris e several branche s arisin g o n th e episternum , th e pleura l ridge , an d th e epimeron. Distall y the flexor muscle is inserted o n the bas e of the thir d axillary sclerit e (Fig . 122 , 3Ax) . It s contractio n revolve s th e thir d axillary dorsall y an d inwar d o n the proxima l articulation s o f the latte r (6, /) an d thus turn s th e ala r are a of the win g posteriorly o n the axillar y region b y a flexure along the lin e of the plic a basalis (bf). A muscl e correspondin g to th e win g flexor is wel l develope d i n eac h wing-bearing segmen t o f zygopterous Odonata. I t arise s o n the pleura l ridge an d i s inserted posteriorl y o n the axillar y plate . The Basala r Muscles.—Th e muscle s o f th e basala r sclerites , o r of the basala r lob e o f th e episternum , usuall y includ e tw o muscle s o n each side , bu t sometime s thre e ar e present , o r agai n onl y one . Th e first o f th e potentia l three muscles of this group arises o n the episternu m (Fig. 103 B, IE'), the second (2Ef) arises on the sternum or the precoxal bridge of the pleuron or occasionally on the trochantin (Gryllus) , th e thir d (3Er) arises on the outer rim of the coxa anterior to the pleural coxal articulation. Th e las t muscl e appear s t o b e a pleura l le g muscle that has secondaril y becom e a win g muscl e i n th e adul t b y reaso n o f th e intimate connectio n of the basalar e wit h th e humera l angle of the win g

232

PRINCIPLES OF INSECT MORPHOLOGY

(Figs. 12 8 C, 129 , 130 , a) . Th e basala r muscle s o f th e adul t winge d insect functio n a s depressor s o f th e costa l margi n o f th e win g durin g flight and a s extensors o f the flexed wing, for whic h reasons they ma y b e termed the pronator-extensor muscles of the wing. In Odonat a ther e ar e two anterior win g muscles arising o n the lower edge of the episternu m an d inserted by long tendons directly on the larg e humeral plate of the wing base (Fig. 123 B, HP). Ther e are no epipleural sclerites i n th e dragonflies , an d n o pleurocoxa l muscles ar e associate d with the win g mechanism.

FIG. 129.—Diagra m o f the pleura l mechanism s o f the wing .

The Subala r Muscles.—I n mos t insect s ther e i s but a single subala r muscle, usually of large size (Fig. 12 8 C, 3E"), lyin g against th e epimera l wall o f th e pleuro n o n eac h sid e o f eac h wing-bearin g segment , whic h is attache d ventrall y o n th e mero n o f th e coxa . Associate d wit h thi s muscle, however , there i s sometimes , a s i n Gryllidae , Trichoptera , an d Lepidoptera, a secon d muscl e (Figs . 10 3 B, 12 9 !£"') arisin g o n th e epimeron and inserted on the posterior part of the subalare or on a distinct second subala r sclerit e (Gryllus). Thi s muscl e i s a counterpar t o f th e first basalar muscle (Fig. 103, IE') arising on the episternum. A subalar muscle correspondin g t o th e sterna l muscl e o f th e basalar e (2E') i s known t o occu r onl y i n Ephemerida ; i t i s her e a larg e muscl e o f th e mesothorax arisin g mediall y o n th e sternu m behin d th e coxa e an d inserted dorsally on the subalar region of the pleuron. I n the mesothorax

THE WINGS

233

of th e highe r Dípter a th e singl e large subalar muscle arises o n the lowe r part o f the epimero n dorsa l t o th e meron , bu t thi s muscl e is probabl y the usua l subalar-coxa l muscl e transpose d fro m th e displace d mero n to the pleura l wall. Th e subalar muscle s serve to exten d an d to depres s the win g because of the clos e connection of the subala r sclerit e wit h th e second axillar y sclerit e o f th e win g bas e (Figs . 12 8 C , 129 , &) . The y may b e called , therefore, the depressor-extensor muscle s of the wing . In th e Odonat a tw o posterior pleura l wing muscles take their origin s on the ventral edg e of the epimeron in each álate segment and are inserted directly o n the axillar y plat e o f the win g base (Fig . 12 3 B, AxP). 5. TH E WIN G MOVEMENT S The insec t win g i s movabl e o n th e body b y th e flexibilit y o f it s basa l con nections with the tergal plate and with the pleural wal l o f th e segment , bu t i t i s definitely hinged to the tergum by the first and third axillar y sclerites (Fig . 122 , \Ax, 3Ax) o r b y th e firs t an d fourt h (Fig . 121 B, lAxj 4A#) if a fourth axillary is present. Th e wing , therefore, is capabl e of respondin g onl y t o th e up-and-dow n movements o f fligh t o n it s extrem e bas e line. Mos t o f th e other movement s of flight, a s well as the movement s of flexion FIG. 130.—Diagrammati c cros s and extension , depen d o n th e flexibl e section of a winged segment, anterior connections of the veins with the articula r view, showing basalar mechanis m of an d anterio r deflectio n o f sclerites, an d o n th e interactio n o f th e extension the wing . articular sclerite s themselves . Th e mo tions o f insects ' wing s fal l int o tw o distinc t categories ; those o f on e include th e movements o f flight , thos e o f th e secon d embrac e the movements of flexion and extension. The Movement s o f Flight.—The movement s o f th e win g tha t mak e flight possibl e consis t o f a n upstroke, a downstroke, a forward movement , a rearward movement, and a partial rotation of each wing on its lon g axis. The Upstroke o f th e Wings. —The elevatio n o f th e wing s i n fligh t is produced , a s w e hav e seen , b y th e simpl e devic e o f depressin g th e tergum o f the segmen t bearin g the wing s (Fig . 13 1 A), th e actio n bein g the resul t o f a contractio n o f th e vertica l tergosterna l muscle s (C) , assisted i n som e case s b y th e obliqu e dorsa l muscle s an d i n Diptera b y the remotor s o f th e coxae . Th e mechanis m o f th e upstroke , therefore, is simply that o f a leve r o f the firs t order , the fulcru m bein g the pleura l

234

PRINCIPLES OF INSECT MORPHOLOGY

wing proces s (WP) upo n whic h the bas e o f the win g rests. Th e tergo sternal muscle s are often larg e and powerful, suggestin g that the upstrok e of the wings is an important contributan t t o the forc e of flight. The Downstroke o f th e Wings. —The depressio n o f th e wing s i s no t the work of a single set of muscles. I t result s i n part from the restoratio n of th e dorsa l curvature o f the bac k by th e contractio n o f the longitudina l dorsal muscle s (Fig . 13 1 C, A) , whic h are th e segmenta l antagonist s of the tergosterna l muscles ; but probabl y a n important effecto r o f the wing depression i n mos t insect s i s th e posterio r pleura l muscl e o r muscle s

FIG. 131.—Diagram s o f successiv e position s o f th e wing s in fligh t an d th e correspondin g movements of the tergum .

(Figs. 12 8 C, 129 3E") inserte d on the subala r sclerit e (Sa). Th e subala r sclerite bein g i n immediat e connectio n (6 ) wit h th e secon d axillar y o f the win g base (2Ax), a pul l upo n th e subala r muscl e strongly depresse s the wing. The Anteroposterior an d Rotary Movements o f th e Wings.—The partia l rotation of each wing on its long axis is a part of the anterio r an d posterio r movements an d i s accompanie d b y change s in th e positio n o f the plan e of th e win g surface durin g th e upstrok e an d th e downstrok e cause d b y pressure o f the air . I t wa s formerl y suppose d tha t th e torsio n o f th e wings, includin g th e horizonta l an d rotar y movements , i s entirel y th e result o f changing air pressur e on the flexible posterior area s o f the wing s as th e latte r ar e vibrate d i n a vertica l direction . Ther e i s n o doub t that th e wing s do respond b y a differentia l actio n i n their plane s t o ai r pressure alone , bu t i t i s also true tha t eac h wing is partially revolve d a t the bas e b y th e muscle s o f it s moto r mechanism . Th e muscle s tta t

THE WINGS

235

produce thi s movemen t ar e undoubtedl y th e muscle s o f th e basala r and subala r sclerites . Th e firs t (Fig . 130 , E')-, pulling downward on th e basalare (JBa) , turn this sclerite inwar d on the uppe r edg e of the epister num (PI), an d th e connectio n (a ) of the basalar e with the humera l angle of the wing base deflects the anterior part of the wing as it turns it slightl y forward. Th e mechanis m o f anterio r deflection , includin g th e basala r sclerite an d it s muscl e or muscles, has been called the pronalor apparatus of th e wing . Th e movemen t o f anterio r deflectio n accompanie s th e depression o f the win g (Fig . 13 1 C). The revers e movement , o r th e combine d rearwar d motio n an d pos terior deflectio n o f th e win g accompanyin g the upstrok e (Fig . 13 1 A), is probabl y cause d largel y b y ai r pressur e o n th e expanded , flexible posterior are a o f the win g surface; but i t i s likely that the tensio n o f th e

FIG. 132.—Curve s described on a moving recorder by the win g tip o f a stationary blo w fly making th e win g movements of flight . (From Ritter, 1911.)

subalar muscle s (Figs . 12 8 C, 129 , E"), exerte d o n th e secon d axillar y sclerite (2Ax) posterior to the pleural fulcrum, contribute s t o the posterior deflection o f the win g during the upstroke . The Wing Motion in Flight.—The motion o f each wing in flight is th e resultant o f its severa l elementa l movements . Durin g th e downstroke , the win g goes from abov e downward and forward ; it s anterio r margi n is deflected an d it s posterio r are a turn s upwar d (Fig . 13 1 C). Durin g th e upstroke, the wing goes upward and relatively backward, and its posterio r surface i s deflected (A) . As a result o f the compoun d motion o f the vibratin g insec t wing , th e tip o f the wing , if the insec t i s held stationary, describe s a curv e having the for m o f a figur e 8 . Thi s fac t ha s lon g bee n know n fro m direc t observation o n insect s i n whic h th e figur e describe d b y th e vibratin g wings is made visibl e i n strong ligh t b y bits of gold leaf attache d to th e wing tip s (Marey , 1869 , 1874) . Th e win g motion , however , has bee n studied mor e accurately by mechanical devices in which a graphic record of th e win g movement s i s obtained , a s i n th e experiment s o f Mare y (1869a) an d o f Ritte r (1911) , showin g that th e wing s o f a n insec t i n motion describe a series of open loops (Fig. 132), th e distanc e betwee n th e

236

PRINCIPLES OF INSECT MORPHOLOGY

loops depending on the speed at whic h the insec t flies . Th e win g movements hav e also been recorded by cinematographi c methods (se e Marey, 1901; vo n Lendenfeld , 1903; Bul l 1904 ; Voss , 1913 , 1914) . Th e rotar y movement o f the wing s i s most accentuate d i n swift-flyin g insects , suc h as th e dragonflies , bees , an d flies, which hav e relativel y narro w wings; in slowe r flying insects wit h broa d wings , such a s th e grasshopper s an d butterflies, th e up-and-dow n movemen t is the principa l one. The Rate o f th e Wing Vibration. —The rapidit y o f th e win g motio n varies much in different specie s of insects. Landoi s (1867 ) deduced fro m the pitc h o f the soun d made by insect s in flight that the house fly makes 352 wing strokes a second, a bumble bee 220, and the honey bee, when at its best , 440 , thoug h whe n tire d it s hu m indicate s a spee d o f only 330 beats a second . Mare y (1869a ) obtaine d graphi c record s o f th e win g beats on a revolving cylinder, and h e gives 330 wing strokes a second fo r the hous e fly, 240 for a bumble bee, 190 for the honey bee, 110 for a wasp, 28 for a libellulid, and 9 for the cabbage butterfly. Vos s (1914), however, calculating th e rat e o f th e win g motio n fro m serie s o f movin g pictur e photographs, obtaine d i n most case s lower figures; the honeybee , by his test, makin g 180 to 20 3 wing strokes a second, the hous e fly from 18 0 to 197, th e mosquit o fro m 27 8 t o 307 , whil e variou s othe r insect s hav e mostly a slowe r rate. I n genera l it ma y be said th e flies and bees have the highes t speed of wing movement, other insects , by comparison , being slow o f fligh t an d correspondingl y slo w i n win g motion . Th e lowes t records of speed are obtained from th e butterflie s and moths, the cabbag e butterfly makin g a t bes t abou t 9 stroke s a second , som e of the noctui d moths abou t 40 , though the sphin x moths, o n the othe r hand , ar e swif t fliers and mov e the wing s at a high rate of speed. Th e studen t wil l find summarized statement s o f th e recorde d rate s o f th e win g stroke s i n insects give n b y Vos s (1914 ) an d b y Prochno w (1924 , 1925) . I t mus t be recognized , however , tha t experimentall y obtaine d record s a t bes t tell only what the insect s di d under the conditions of the experiment . The Movement s o f Flexio n an d Extension.—Th e movement s b y which the wing s are folded afte r flight , o r extended preliminary to flight , are execute d to o rapidl y t o b e observe d closel y i n a livin g insect ; bu t the actio n o f a win g and th e operatio n o f the flexo r mechanis m ca n b e well studie d i n freshl y kille d specimens . A grasshopper , a bee , a fly , or almos t an y insec t sufficientl y larg e wil l answe r th e purpose , bu t th e grasshopper, o r particularly th e scorpionfl y Panorpa , wil l be found t o b e a ver y suitabl e subject. I f the win g of a fresh specime n i s slowly folde d posteriorly ove r th e bac k an d the n brough t forwar d int o th e positio n of flight, th e accompanyin g movement s o f the vei n base s o n th e articula r sclerites an d th e movement s o f th e sclerite s o n on e anothe r ca n b e observed. Fro m th e actio n o f th e part s i n a dea d specime n th e

THE WINGS

237

probable workin g o f th e flexo r mechanis m in th e livin g insec t ca n b e deduced. We hav e see n that th e axillar y sclerite s ar e containe d i n a n axillar y region o f the win g base , whic h i s approximatel y triangular (Fig . 13 3 A, Ax), th e ape x o f th e triangl e (c ) being formed b y th e oute r en d o f th e third axillary sclerit e (Fig . 122 , 3Ax). Th e costa l vein (C ) alone has no connection wit h th e axillar y triangle , it s bas e bein g associated wit h th e humeral plat e (HP). Th e subcost a '(Sc) ha s a loos e attachmen t (e ) with th e hea d o f the firs t axillary , and th e radiu s (R ) i s flexibly continuous b y it s bas e (d) wit h th e second axillary. Th e vannal veins (IV-nV) ar e closely associated with the oute r en d o f the thir d axillary . The media n an d cubita l vein s (M, Cu, Pcu) have no direct connection with the axillary sclerites, bu t thei r bases, whe n distinct , ar e eithe r associated wit h th e secon d median plate (rn!) o r mor e o r les s unite d in th e correspondin g are a o f th e wing whe n thi s plat e i s absent . The base of the mediocubita l field , therefore, abut s upon the basal fold of th e wing , o r plic a basali s (&/) , which forms the hing e line between the two median plates (m , m') whe n these plates ar e present a s distinct SclerotizationS. FlG

. 133.—Diagram s of the typical folding

The essentia l skeletal element of o f a win g durin g flexion . A , th e win g

i • j i ¿ i - i extende d an d flat . B , th e wing partly flexed a the flexor mechanism IS the third by dorsal revoiuti0n of axillary area on its axillary. Thi s SClerit e Í S typicall y bas e (0-6) . C , th e full y flexe d wing . (Fo r

Y-shaped in form (Fig. 122, 3Ax) lettering see Fig' 126° inasmuch a s i t consist s o f a dista l stalk , th e oute r en d o f whic h (c ) is associated wit h the base s of the vanna l veins, and presents two proximal arms, th e posterio r on e o f which articulates wit h th e tergu m (6) , while the anterio r usuall y articulate s wit h th e posterio r en d o f th e secon d axillary sclerite (/). Th e flexor muscle of the win g (D) is inserted on th e base o f the thir d axillar y i n th e crotc h betwee n th e tw o basa l arm s of the latter. Flexion of the Wings.—Flexion begin s with a relaxation of the extenso r muscles, whic h allow s eac h win g t o tur n a littl e posteriorly . Thi s automatic preliminar y movement of the win g produce s a stron g convex

238

PRINCIPLES OF INSECT MORPHOLOGY

fold at th e base of the mediocubita l field along the line of the plic a basalis (Fig. 122 , &/) , whic h is betwee n th e tw o media n plate s (m , ra' ) i f thes e plates ar e present. A t the sam e time, th e movemen t revolves the thir d axillary sclerit e (3 Ax) upwar d o n it s basa l articulation s (6 , /) . Th e insertion poin t o f th e flexo r muscl e (Z> ) o n th e thir d axillar y i s thus turned dorsad and mesad of the axi s of the basa l hinge line of the sclerite , and th e muscle , having now gained a purchas e on the latter , i s able b y contraction t o continu e the revolutio n o f the sclerite , turning i t dorsall y and mesall y unti l i t i s completel y inverte d an d reverse d i n position . The movemen t o f the thir d axillar y bring s wit h i t directl y th e vanna l region of the win g (Fig. 133 A, Vri), th e bas e of which is lifted an d carrie d horizontally agains t th e sid e o f th e bac k (B) , whil e indirectl y als o i t turns th e remigia l regio n (Rm) posteriorl y o n th e articulation s o f th e subcostal an d radial veins with the first and second axillaries, producing a convex fol d alon g th e plic a basali s (Fig . 122 , 6/) , a t th e bas e o f th e mediocubital field. Since the firs t media n sclerite of the win g base (Fig . 122, m) is usually attached to th e dista l arm of the third axillary , the rotation of the flexor sclerite (3Ax) has also a direct effect on the mediocubital field and brings about th e foldin g alon g th e plic a basali s (bf) betwee n th e tw o media n plates. Wit h insect s i n whic h th e vanna l are a o f the win g is reduced, the actio n o f the thir d axillar y i s principall y o n th e mediocubita l field through th e first median plate. B y the revolutio n o f the thir d axillary , the fol d o f the plic a basalis i s accentuated a s th e firs t media n plate (m ) is turne d verticall y o n it s hing e wit h th e secon d axillar y an d i s finally tilted mesally . Th e plic a basali s no w crosse s th e win g bas e obliquel y ???? ?? ????? ??????????? ??? ??????? ????? ??? ?? ???? The fina l pul l o f th e flexo r muscl e apparentl y i s expende d o n th e general win g base , for , in man y insects , whe n the win g i s full y flexed , the firs t axillar y i s revolved int o a vertica l plan e o n its hing e with th e tergum, an d th e secon d axillar y i s thereb y lifted , turne d int o a nearl y longitudinal position , an d brough t clos e agains t th e sid e o f th e back . A movement o f the firs t an d secon d axillaries, however , does not alway s accompany th e win g flexion, the essentia l change s i n th e basa l regio n being the revolutio n of the thir d axillar y and th e foldin g alon g the lin e of the plica basalis at the bases of the median and cubital veins. As the posterio r edge of the flexing wing comes against the sid e of th e body, the jugal lobe (Fig. 133 A, Ju) is deflected and turned beneath the vannu s alon g th e lin e o f the plic a jugali s (B) . I f a plic a vannali s (vf) i s present , th e remigia l region (Rm) ma y b e turne d downwar d (C) during th e flexio n o f th e win g thoug h man y insects , suc h a s th e flie s and bees , keep both the remigiu m and th e vannu s in a horizontal plane. If th e vannu s i s larg e i t als o ma y b e deflecte d beneat h th e remigium ,

THE WINGS

239

as i n the^hin d win g of the cicad a (Fig . 12 7 B) i n which both the jugu m and th e vannu s ar e turne d downwar d agains t th e sid e o f the abdome n beneath th e slopin g remigium . I n som e Lepidopter a th e flexe d hin d wing fold s als o alon g supplementar y line s o f plicatio n i n th e rea r par t of th e wing . The flexin g o f th e win g become s a stil l mor e complicate d proces s if the vannal region is particularly enlarged , as in Plecoptera, Orthoptera , and Dermaptera . I n mos t o f the Orthopter a th e vannu s o f each hin d

FIG. 134.—Wings of a grasshopper, Dissosteira Carolina. A , forewing. B , hind wing. C, position and plication of wings folded ove r the bod y as shown in transverse section. D , plications of flexed and folde d righ t hind win g in section , mor e enlarged.

wing i s so greatly expande d (Fig . 13 4 B) that , whe n the win g is flexed, it mus t b e plaited an d folde d togethe r lik e a fa n i n orde r t o giv e space for th e res t o f th e wing . Th e foldin g an d plaitin g o f th e full y flexe d wings of a grasshopper are shown at C and D of Fig. 134 . Th e narrowe r forewings, o r tegmin a (C , TF 2), overla p eac h othe r t o for m a rooflik e covering with steeply sloping sides completely enclosing the more delicate hind wing s (TF 3) folde d beneat h them . Th e membran e o f most o f the vannal regio n of each hind wing is deeply inflecte d betwee n the primar y vannal vein s (D) , an d th e secondar y vein s (a , 6 , c) li e i n th e trough s of th e folds . I n mos t o f the Orthopter a th e vanna l fol d o f the hin d wing lies betwee n the postcubitu s an d th e firs t vanna l vein ; the fol d usuall y contains a secondary vein, the ven a dividens (Figs . 12 4 B, 13 4 B, D, vd).

240

PRINCIPLES OF INSECT MORPHOLOGY

The hind wings of Dermaptera an d Coleópter a when flexed are shortened by fold s acros s the vein s in order that they ma y be covered by th e protective forewings , o r elytra. Th e fanlik e hin d wings of the Dermap tera consis t principall y o f the expande d vanna l regions . Whe n flexed, the fan s ar e plaited betwee n the vein s an d the n folde d twic e across th e veins. In the Coleóptera the large jugum (Fig. 127 D, Ju) is folded in the usual manner beneath the vannus, an d the transverse plication s take place i n th e dista l par t o f th e wing . Th e transvers e foldin g result s automatically fro m th e structur e an d flexibility of the veins . Extension o f th e Wings.—Extension involve s a reversa l o f the move ments o f flexion . Th e flexor muscles must firs t relax . I t i s probable , then, tha t a contractio n o f the basala r muscle s (Figs . 12 8 C, 129 , E'), pulling on the humera l angle of the win g base, extend s the win g directly in mos t insects , thoug h th e actio n o f these muscle s i n thi s capacit y i s often difficul t t o demonstrat e i n a dea d specimen . O n the othe r hand , with insects in which the secon d axillary sclerite is elevated o n the oute r edge of the upturne d first axillary in the full y flexed wing, it i s clear that the win g may b e extended b y the downwar d pull of the subala r muscle s (Efr) on the second axillary, for a pressure on this sclerite from above at once restores all the axillary elements to a horizontal plane and thereby spreads the wing. Som e insects may be seen to extend the wings deliberately befor e takin g flight , bu t wit h mos t specie s fligh t i s practicall y simultaneous with the win g expansion. 6. INSEC T FLIGH T

An objec t self-moved throug h the ai r must b e able to creat e a differ ence in the density , o r pressure, of the ai r o n opposite sides of it; motion takes place toward the region of lowered pressure. Fligh t by any heavier than ai r anima l o r machine that doe s not depen d upo n risin g column s of ai r for its suppor t mus t hav e a mechanism capable not onl y of producing horizonta l motio n bu t als o o f creatin g a liftin g forc e sufficien t t o overcome the pul l o f gravity. Mos t flying machines are s o constructed that the forc e o f the propelle r gives only a forward drive , the lifting forc e in horizontal flight being the resul t o f decreased pressure above the wings created secondaril y by th e motio n o f the plane . Th e wing s of insects , on th e othe r hand , furnis h directl y no t onl y the drivin g powe r but th e lifting forc e a s well; that i s to say , th e movemen t of the wing s creates a region of lowered pressure both befor e an d abov e the bod y of the insect . The possibilitie s o f a moto r mechanis m fo r aeria l locomotio n ca n be judge d b y studyin g th e ai r current s th e moto r wil l produc e if i t i s itself hel d stationary . Th e natur e o f the ai r current s produce d by th e wing vibration s o f insects , whe n th e insect s ar e secure d b y th e bod y in such a manner that the wing movements will not be hindered, has been

THE WINGS

241

studied b y Demol í (1918) . B y mean s o f a simpl e apparatus consistin g of a frame with horizontal bars on which were suspended fine owl feathers, Demolí, b y observin g the deflectio n o f the feather s when an insec t wit h its wings in rapid vibration wa s brought near them, was able to determine the directio n of the ai r current s created by the win g movements. Experimenting i n thi s wa y wit h insect s o f differen t orders , Demol í found tha t th e ai r current s draw n toward th e stationar y insec t b y th e vibrating wings come from i n front, fro m above , from th e sides , and fro m below, whil e the current s give n of f are al l thrown ou t t o th e rea r (Fig . 135). Th e strengt h o f the currents , however , is not th e sam e fro m al l directions, a s is indicated b y th e relativ e thicknes s o f the arrow s in th e diagrams. Th e air is drawn toward the insect most strongl y from befor e and abov e th e anterio r par t o f th e body ; th e outgoin g current s ar e strongest i n a horizonta l o r slightl y downwar d direction. Mos t o f th e oncoming currents, therefore, ar e turned to the rea r in the neighborhood of th e insect' s body and ar e condensed in a small region behind it .

FIG. 135.—Diagram showing direction an d relative strength of air currents produce d by th e vibrating wing s of a stationary insect. (From Demolí, 1918.)

If th e insec t i s fre e t o move , th e mechanica l effec t o f th e vibratin g wings o n the ai r wil l be the sam e as when the insec t i s held stationary ; but, instea d o f moving the air , o r instead o f moving the ai r to th e sam e extent a s before, th e greate r par t o f the win g force wil l propel the insec t through the air opposite the direction of the air currents created when the insect i s secured . I n term s o f mechanics , th e directio n fro m whic h a current i s drawn b y a stationary objec t is the directio n o f lowered pressure, whil e the opposit e i s that o f increased pressure. Accordin g to th e observations o f Demolí , therefore , whe n a n insec t launche s itsel f int o the ai r an d set s u p a vibratio n o f it s wings , ther e i s a t onc e create d before it an d above it a region of decreased pressure, and the convergence of al l th e current s behin d produce s her e a regio n o f greatl y increase d pressure. Th e lowere d pressur e abov e counteract s th e weigh t o f th e insect; th e increase d pressur e behin d drive s th e insec t forwar d int o th e low-pressure region in front . The drivin g forc e o f the insect' s win g movements probably depend s upon th e angl e a t whic h th e win g surface s cu t th e air . Slow-flyin g

242

PRINCIPLES OF INSECT MORPHOLOGY

insects wit h broad wings , such a s the butterflie s an d grasshoppers , kee p the win g surfaces almos t horizonta l an d fly more in the manne r o f small birds wit h comparativel y fe w strokes o f the wing s in an y uni t o f time; some o f th e larg e swallowtai l butterflie s eve n soa r fo r shor t distance s with th e wing s held stationary . Th e mor e swiftl y flyin g insects , how ever, havin g narro w wings , tur n th e win g surface mor e nearl y vertica l with eac h stroke, whethe r u p o r down, and, a s Hitter (1911 ) says , "th e insect flies fastest whe n the downstroke approaches a vertical direction," because th e curv e o f the upstrok e i s drawn forwar d i n th e directio n of

flight.

The spee d o f insec t fligh t ma y b e ver y hig h considerin g th e smal l size o f insects , bu t i t varie s greatl y wit h differen t species . Demol í (1918) ha s compute d th e flyin g rat e o f variou s specie s fro m th e tim e in whic h individuals traverse d a room , goin g direc t fro m th e dar k sid e to th e light . Th e haw k moth s (Sphingidae ) h e foun d ar e th e swiftes t flyers, making a speed up to 1 5 meters a second. A tabanid fly (Tábanus bovinus), however , i s a clos e second, goin g a t a rat e o f 1 4 meters. A dragonfly (Libellula depressa), doin g ordinaril y 4 meter s a second , i s capable of 6 to 1 0 meters in the sam e length of time. A house fly travels from 2 to 2. 3 meters a second ; a bumbl e bee (Bornbus) fro m 3 to 5; the honey bee, unladen, has a speed of 3.7 meters a second, but whe n weighted with pollen it make s only 2.5 meters i n the sam e unit g f time . Insects appea r t o hav e n o steerin g apparatu s othe r tha n th e wing s themselves. Ordinar y observation , a s wel l a s th e experimenta l test s made b y Stellwaa g (1916 ) o n the steerin g power s o f insects, sho w that little or no compensatory movements of the body or legs are made during flight. Stellwaa g showe d tha t livin g insect s impale d o n pin s tur n themselves t o th e righ t o r lef t b y a differentia l actio n o f th e wing s when th e latte r ar e rapidl y vibratin g wit h th e movement s o f flight . The muscles concerned in the differential , o r steering, actio n of the wings must b e the latera l muscle s of the ala r segments , whic h are those o f th e basalar and subalar sclerites (Fig. 129, Ef, E"), since these muscles alone hav e specifi c connection s wit h th e wings . Th e longitudina l an d vertical muscle s o f th e wing-bearin g segments, thoug h poten t effector s of win g movements, can not unequall y distribute thei r influenc e between the tw o sides of the segment . Not onl y ca n mos t insect s guid e thei r cours e adroitl y i n forwar d flight, bu t man y o f them ar e abl e t o fl y directl y backwar d o r sidewis e without alterin g th e positio n o f th e body . Th e dragonflie s ar e par ticularly adep t i n these modes of flight, but man y o f the smalle r insects , such a s th e flie s an d bees , ar e quit e equa l t o th e dragonflie s i n thei r ability t o dar t suddenl y t o on e sid e o r rearward , whil e th e hea d stil l points i n th e directio n o f th e arreste d forwar d flight . Reverse d an d

THE WINGS

243

lateral flyin g i s probabl y controlled also by th e latera l muscle s of th e flight mechanism, but i t is remarkable that organs so evidently fashioned for forwar d flight , a s ar e th e wing s o f insects , ca n functio n efficientl y for producin g motion in other directions. Still another feat that many insects perform on the wing with apparent ease i s hovering. Presumably , in maintaining one positio n i n th e air , the wings are vibrated approximately in a horizontal plane, thus creating a region of decreased ai r pressure above the bod y of the insect , but non e before it. The rate of the win g movements then must be just sufficien t to creat e a balance with the forc e o f gravity. GLOSSARY O F TERM S APPLIE D T O TH E WING S Alula, o r Calypter.— A pai r o f membranou s lobes a t th e posterio r angl e o f th e wing base, particularly develope d in some Díptera. Anal Fold (vf). —See plica vannalis. Anal Veins (A). —All th e vein s betwee n the cubitu s and th e juga l region, including, accordin g to the Comstock-Needham system, the veins here called postcubitus and vannals. Arcuate Vein (I/).—See vena arcuada. Axillary Cor d (AxC). —The thickened , corrugate d posterio r edg e of the articula r membrane o f th e win g base , continuou s wit h th e posterio r margina l fol d o f th e aliño turn. Axillary Plat e (AxP). —The posterio r sclerit e o f the win g base in Odonata , sup porting the subcostal , radial , medial , cubital, an d vannal veins. Axillary Regio n (Ax). —The regio n o f th e win g bas e containin g th e axillar y sclerites. Axillary Sclerites.—Th e sclerites of the axillar y region in the wing-flexin g insects , partly differentiated in Ephemerida, represente d b y the axillar y plate in Odonata . Basal Fol d (&/).—Se e plica basalis. Cells.—The areas of the win g membrane between the vein s and cross-veins . Costa (C). —The usua l firs t vei n o f th e wing , typicall y marginal , connecte d basally wit h the humeral plate . Cross-veins.—Short vein s betwee n th e lengthwis e vein s an d thei r branches ; numerous in net-veined wings , in others generally few and located in definite positions. Cubitus (Cu). —The usua l fifth vein of the wing . First Axillary (\Ax).—The anterio r hing e plate o f the win g base, associated wit h the bas e of the subcosta l vein . (V order e Tergalgelenkplatte.) First Media n Plat e (m).— A smal l sclerit e o f variable shap e lyin g i n th e angl e between the secon d axillary and the dista l arm of the third axillar y at the base of the mediocubital field; accessory to th e third axillar y i n function, and usuall y attache d to it. Fourth Axillar y (&Ax).—A posterio r hing e plate o f the win g base present i n some insects, intervenin g betwee n the thir d axillar y an d th e posterio r win g process of th e tergum. (Hintere Tergalgelenkplatte.) Frenulum.—The spine or group of bristles arising on the humera l angle of the hind wing of most moths, projecting beneath the for e wing, and often held here in SLfrenulum hook. Humeral Plat e (HP). —The anterio r preaxillary sclerite of the win g base supporting th e costal vein ; very large in Odonata.

244

PRINCIPLES OF

INSECT MORPHOLOGY

Jugal Regio n (Ju). —A posterio r basa l lob e o r are a o f the win g set of f from th e vannal regio n by th e plic a jugalis , containing th e ven a arcuat a an d ven a cardinali s when these veins are present . Media (M). —The usua l fourt h vei n o f the wing ; its base , whe n not unite d wit h radius,- associated wit h the media n plates of the win g base along the fol d o f the plic a basalis. Median Plates.—See first median plate and second median plate. Paranotal Lobes (pnl).—Lateral lobe s of the pronotum in certain fossil insects, and theoretical lobes of the mesonotu m an d metanotu m supposed to b e the precursor s of the wings. Plica basalis (bf). —The basa l fold o f the wing , or line of flection between the bas e of th e mediocubita l field and th e axillar y region , forming a prominent convex fol d i n the flexed wing extending between the median plates fro m th e articulatio n o f radius with the second axillary to the articulation o f the vannal veins with the third axillary. Plica jugali s (jf). —The juga l fol d o f th e win g o f som e insects, o r radia l lin e of folding settin g of f the juga l region fro m th e vanna l region . (Axillary furrow, plica ano-jugalis.) Plica vannalis (vf). —The vanna l fol d o f the wing , or radial line of folding usuall y between th e cubita l field and the first vannal vein, but somewha t variable in position (Anal furrow, plica analis.) Postcubitus (Pew).—Th e usua l sixt h vei n o f th e wing , represente d b y a n inde pendent trache a i n mos t nympha l wings, associated basall y wit h th e cubitu s i n th e adult. (First anal of Comstock and Needha m in most cases.) Precosta (Pc).— A small first vein of the win g in certai n fossi l insects . Pteralia.—The articula r sclerite s o f th e win g base, includin g the humera l plate and the axillar y plate or axillary sclerites. Radius (R). —The thir d vei n of the wing ; its bas e flexibly attached to the second axillary. Remigial Region, or Remigium (Rm). —The win g area anterior to the vanna l fold , containing the costal , subcostal, radial, medial, cubital, and post cubital veins. (Preanal region, preclavus.) Second Axillary (2Ax).—The pivota l plat e of the win g base resting on the pleura l wing process, connected with th e bas e of the radia l vein. Second Media n Plat e (m'). —A variabl e sclerotizatio n at th e bas e o f the medio cubital field , foldin g convexl y on th e oute r edg e of the firs t media n plate alon g th e plica basalis; often absent, o r represented by the united bases of the medial and cubita l veins. Subcosta (Sc). —The usua l secon d vei n o f th e wing , associate d basall y wit h th e anterior en d o f the firs t axillar y sclerite. Tegula (Tg). —A large , scale-like lobe overlapping the bas e of the forewin g in some insects; usually represented by a small setigerous pad or lobe (tg) at the anterior root of th e win g base. Third Axillar y (3Ax).—The flexo r sclerit e o f the win g base; the sclerit e on whic h the flexor muscle is inserted. Vannal Region, or Vannus (Vri). —The win g area containin g the vanna l veins, or veins directl y associate d wit h the thir d axillary ; when large, usuall y separated fro m the remigiu m by th e plic a vannalis ; ofte n formin g a n expande d fanlike are a o f th e wing. Vannal Veins (IF , 2V , etc.).—Th e veins associated a t thei r base s with th e thir d axillary sclerite , an d occupyin g the vanna l regio n o f the wing . (Th e "anal " vein s except th e first, or postcubitus.)

THE WINGS

245

Veins.—The tubula r thickening s o f the wing s springing fro m th e win g base an d branching distally . Vena arcuata (U).—The first jugal vein. Vena cardinali s (2J).—Th e secon d juga l vein , usuall y appearin g a s a basa l branch o f the ven a arcuata . Vena dividen s (vd). —A secondar y vei n presen t i n som e Orthopter a lyin g i n th e fold betwee n the remigiu m and vannus . Wing Base.—The proximal part of the wing between the bases of the veins and th e body, containin g th e humera l and axillary sclerites. Wing Regions.—The principal areas of the wing s differentiated in the wing-flexin g insects, and often separated by distinct lines of folding, including the axillary, remigial, vannal, an d juga l regions.

CHAPTER X I THE ABDOME N The third division o f the insect trunk , th e abdomen , differs character istically from th e hea d an d th e thora x b y it s simplicit y of structure an d general lac k o f segmente d appendages . Th e unio n wit h th e thora x may be broad o r constricted, but , excep t i n the aculeat e Hymenoptera , there i s seldo m an y questio n a s t o th e lin e o f separatio n betwee n th e thoracic an d abdomina l regions o f the body . Th e abdome n varies much in for m i n differen t insects . It s segment s usuall y remai n distinct , though som e of the posterio r segments ar e commonly reduced or absent.

FIG. 136.—Examples of the presence of twelve segments i n the hexapo d abdomen. A , end o f abdomen o f embryo o f Gryllotalpa. (From Heymons, 1895. ) B , termina l segment s of abdome n o f an adul t pro turan, Acerentulus confinis. (From Berlese, 1910.)

In certai n aberren t species , however, the entire abdome n may be greatl y reduced in size. The usual number of segments in the abdome n of adult insect s is 10 or 11, an d fro m embryologica l evidenc e i t appear s tha t th e primitiv e number wa s no greate r tha n 12 . Twelv e segment s ar e wel l develope d in adul t Protur a (Fig . 13 6 B), an d th e sam e number occur s in embryo s of certai n generalize d insects (A) , but i n postembryoni c stage s possibl e remnants o f a twelft h segmen t ar e rare . Th e twelft h segmen t o f th e Hexapoda appear s t o b e th e periproct , tha t is , the primitiv e endpiec e of th e bod y anterio r t o whic h th e tru e somite s ar e formed . I n th e Protura th e tent h an d elevent h segment s ar e sai d t o b e differentiate d during postembryoni c development ; bu t i n al l th e tru e Insect a th e definitive segmentation o f the bod y is established befor e hatching . A reduction i n the numbe r o f abdominal segments is the rul e in both immature an d adul t insect s generally . Eleve n segment s ar e distinc t i n 246

??? ??????????

many of the mor e generalized insects, bu t i n the highe r orders not mor e than 1 0 segments are usually present, an d sometimes only 9 are distinct . In th e Collembol a the numbe r is reduced to si x and th e limit s of some of these ar e obscured in certai n forms . Generall y reduction take s plac e a t the posterio r end of the body, but i n many of the highe r insects there is a tendency toward elimination o f the firs t abdomina l segment . In genera l the abdome n serves as a container o f the principa l viscera of th e insec t an d i s the chie f par t o f the bod y that produces movements of respiration . O n the ventra l surfac e o f its posterio r par t ar e situate d the aperture s of the genita l ducts, with which are associated the organ s of copulation an d oviposition ; the alimentar y cana l opens at th e en d of its terminal segment. Th e median female genital aperture varies in position; in a few insects it i s located just behind the sevent h abdomina l sternum , in others it is on or behind the eighth sternum, and in still others it is on or behind the venter of the ninth segment. Th e male aperture appears to be always on the posterio r par t of the nint h segment, except in Collembola, in which tiie gonopore in each sex is between the fifth and sixth segments. For convenienc e o f stud y th e segment s o f th e abdome n ma y b e ??????? ???? ??????????? ?? ????????? ????????? ??????? ????????? ??? postgenital segments. Th e genita l segment s ar e primaril y th e eight h and th e nint h i n th e female , an d th e nint h i n th e male , sinc e it i s th e appendages or other outgrowths of these segments that form the principal parts o f th e externa l genitalia . On e o r mor e segment s precedin g an d following the primary genital segments, however, are frequently involved in th e genita l modification s o f the abdomen , and i t i s ofte n foun d mor e expedient t o divid e th e abdome n accordingl y into a preabdomen an d a postabdomen. I n th e higher Diptera, fo r example, the first five segments form a distinc t preabdomen , while the remainin g segment s ar e mor e or less modifie d a s a par t o f th e genita l apparatus , includin g th e lon g telescopic "ovipositor'' of the femal e (Fig . 312 B, C) . Notwithstanding th e simplicit y o f appearance in the structur e o f th e abdomen and the retentio n o f individuality o f its segments, the abdome n is in many respects a highly modified an d specialize d region of the body . Though it s sclerotize d areas hav e usuall y th e for m o f simple segmenta l plates, th e stern a a t leas t ar e evidently composit e structures; and, while segmental appendage s ar e characteristicall y absent , suc h rudiment s of them a s d o persis t rais e question s i n morpholog y that ar e difficul t t o settle. 1. GENERA L STRUCTUR E O F TH E ABDOMINA L SEGMENT S

The abdomina l segments of adult insects for the mos t part are typica l secondary segments , the functiona l conjunctiva e being the membranou s posterior part s o f th e primitiv e somites . Th e primar y intersegmenta l

248

PRINCIPLES OF

INSECT MORPHOLOGY

folds usuall y for m internall y submargina l antecosta e o n th e definitiv e tergal and sternal plates, t o which the longitudinal muscles are attached, and the y ar e marke d externall y b y correspondin g antecosta l sutures . The terg a an d stern a regularl y overla p posteriorl y (Fig . 37) . I n soft bodied larva l insect s th e abdomina l segmentation i s more nearly o f th e primary type, though in holometabolous larvae there is a tendency for the longitudinal muscles to become separated into groups of fibers that do not all have intersegmental attachments . The generalize d for m o f a n abdomina l segmen t i s approximatel y retained i n larva l insect s tha t preserv e rudiment s o f th e abdomina l appendages. I n a n ephemeri d larva, fo r exampl e (Fig . 15 0 A, B), eac h gill-bearing segment is distinctly divide d into a dorsum and a venter b y large lateroventral lobes (Cxpd) supporting the gills, which evidently

FIG. 137.—Sclerotizatio n o f th e abdomen . A-E , example s o f variatio n i n th e abdominal selerotization above and belo w the dorso-pleura l line (a-a). F , typica l secondary segmentatio n of the abdomen . G , inner view of consecutive tergal plates, with muscl e attachments.

represent th e base s o f abdominal appendages . Generally , however, the limb bases are more or less united with the venter, an d in the adult insec t the sterna l plate o f each segment is usually a continuous selerotization of the ventra l an d pleural regions. The Abdominal Sclerotization.—The sclerotized parts of the abdominal integument usuall y take the for m of dorsal an d ventral segmental plates , separated b y membranou s area s o n the side s (Fig . 13 9 A). I n certai n larval an d adult insects, however, there ar e four distinc t series of abdominal plates , namely , dorsa l terga l sclerites , latera l pleural sclerites , an d ventral sterna l sclerites . Thu s i n som e of the Thysanur a (Fig . 13 8 A) each abdominal segment presents a broad tergal plate (T) above , a small median sternal plate (Sin) below , and, flanking the latter , a pair o f large pleural plates (Cxpd). The pleural plates of the Thysanura, it is gen-

THE ABDOMEN

249

erally conceded , represent th e base s o f abdominal limbs . A generalized abdominal segment, therefore, we may assume, had a tergum occupying at least th e majo r par t o f the dorsu m (Fig . 13 7 A, T) , a pleuro n (PI), o r a group of pleurites, on each side situated in the are a of the lim b base (P) , and a sternum (Stn) in the venter. In modern insects, however, the relation o f th e definitiv e abdomina l sclerotizatio n t o th e morphological regions o f th e bod y i s highl y variable , an d th e numerou s anatomica l inconsistencies that arise create many difficulties i n nomenclature. In th e usua l conditio n foun d i n adul t an d nympha l insect s th e primitive pleur a an d sternu m o f each segment (Fig . 13 7 A, PZ, Stn) ar e united i n a continuously sclerotized definitive sternal plate (B , S) oppose d to th e tergum . I f th e terga l sclerotizatio n extend s downwar d o n th e sides of the dorsu m so far a s to include the spiracula r areas , the spiracle s

FIG. 138.—Pleurosterna l plate s o f th e abdomen . A , unde r surfac e o f abdomina l segment of Nesomachilis, showin g true sternum (Stn) an d plate s of limb bases (Cxpd). B , abdominal sternu m o f Heterojapyx, with lim b base s unite d wit h sternum , an d a sterna l apotome (Apt} separate d fro m th e latter . C , definitiv e sterna l plate, o r coxosternum , of ninth abdomina l segment o f male termite, Termopsis.

will be enclosed in the latera l part s of the tergu m (C) . I n man y cases , however, th e dorsu m contain s latera l terga l sclerites , o r laterotergites (D, Itg), quite distinct from the principal median tergite (mtg). The laterotergites ofte n contai n the spiracles, but th e spiracles may be located in the membran e above or below the laterotergites . Wit h som e insects , again, the spiracle s occur in lateral parts of the ventral plates (E) , and in such cases it is evident that the definitive sterna are continuous sclerotizations o f th e primar y sternal , pleural , an d lateroterga l areas . Finally , as in th e larva e o f Plecoptera an d i n th e mal e genital segmen t o f many adult insects , th e tergal , pleural , an d sterna l plate s ma y becom e con fluent in a continuously sclerotize d annulus . The Abdominal Terga. —The dorsa l sclerotizatio n o f a n abdomina l segment usuall y ha s th e for m characteristi c o f a secondar y segmenta l plate (Fig . 13 7 F). A typical abdomina l tergum (T) , therefore, presents anteriorly a marginal o r submarginal ridge, the antecost a (F , G, Ac), on which the principa l longitudina l muscle s usually have their attachments . The antecosta l sutur e (F , aes) i s generall y but faintl y marked , an d th e precostal acrotergite (atg) varie s from a scarcely perceptible marginal rim to a fairly wide flange extending anterior to the muscl e attachments (G) .

250

PRINCIPLES OF INSECT MORPHOLOGY

In som e cases , however , the antecost a an d acrotergit e ar e los t an d th e muscles attac h simpl y o n th e anterio r edg e o f the tergum . Apodema l arms are sometimes developed from the anterior margins of the abdomina l terga, which give effectiveness particularl y t o protractor muscle s inserted upon them. Behin d the tergu m is the conjunctiva l membrane (F , Mb), and th e abdomina l terg a regularl y overla p posteriorly , excep t wher e successive segmental plates ar e united . In many insects, particularly i n larval forms, the dorsa l sclerotizatio n of th e abdome n ma y b e broke n u p int o group s o f segmenta l tergites . In simpl e case s w e may distinguis h i n eac h segmen t a media n tergit e (Fig. 139 J3,mtg) and one or more laterotergites (%); but of ten the sclerotization o f the media n area is again subdivided int o smalle r sclerites. Th e lower limi t o f th e dorsu m mus t b e determine d b y discovering , where possible, the position of the dorso-pleural line (a-a), which is often marked by a lateral groov e extending into the thorax above the subcoxal pleurites (Sex). Abdominal Pleurites. —Strictly defined , an abdomina l plat e properl y called a pleura l sclerit e i s a sclerotizatio n i n th e regio n of the abdome n corresponding t o tha t o f the subcoxa l pleura l plate s o f the thorax . A n abdominal pleurite, therefore, is presumably a derivative o f the primitiv e basis o f a n abdomina l appendage . Th e stylus-bearin g plate s o f th e abdomen o f som e Thysanur a (Fig . 13 8 A) , th e gill-bearin g lobe s o f ephemerid larvae (Fig . 150), the basal plates of the oviposito r (Fig. 35 C), or th e latera l sclerite s i n th e abdome n o f many holometabolou s larva e (Fig. 13 9 B , pi), lyin g betwee n th e dorso-pleura l an d pleuro-ventral grooves, ar e example s o f abdominal plate s tha t ma y ver y evidentl y b e referred t o th e tru e pleura l region . But , again , i t i s undoubtedly tru e in many case s that smal l sclerite s occurrin g in the pleura l regio n of th e abdomen ar e secondar y sclerotizations , o r latera l subdivision s o f th e definitive pleurosterna, and thus cannot be supposed to represent literall y the base s o f abdominal limbs . Suc h sclerites ar e sometime s designated laterosternites, though they ar e pleurites i n the sens e that the y li e in th e pleural region . Th e ter m "pleurite, " however , should no t b e give n t o laterotergites o r sclerite s tha t li e clearl y abov e th e dorso-pleura l line (Fig. 13 9 B, Itg), suc h as those ofte n calle d "epipleurites" in descriptiv e entomology. I n man y species of lepidopterous larvae the seria l identity of th e thoraci c an d abdomina l area s o r their sclerite s is shown by corre sponding seta e o r groups of setae locate d o n them (Fig . 15 3 A). The Abdominal Sterna.— The definitiv e sternal plates of the abdome n of adult insects are in general similar to the tergal plates, each including a primary intersegmenta l are a i n it s anterio r par t (Fig . 13 7 F) . Th e antecostae (Ac) may be coincident with the anterior margins of the plate s or precede d b y distinc t acrosterna l flanges . Th e stern a o f most adul t

??? ??????? ??? insects, however , a s w e have seen , ar e evidentl y composit e plate s (Fig . ??? ?? ??? ???? ?????? ?? ? ????? ?? ??? ??????? ??????? ??? ????? ???? the regions of the primitive limb bases (PI). The frequent occurrence of styli o n th e nint h abdomina l sternu m o f the mal e i n more generalize d pterygote insects (Fig . 13 8 C) attests the triple composition of the sterna l plate, sinc e in some of the apterygot e insects the stylus-bearin g plate s are either onl y partiall y fuse d wit h th e sternu m (B , Cxpd) o r entirel y fre e from it (A) . A definitive sternal plat e that includes the areas of the limb

FIG. 139.—Example s o f abdomina l sclerotizatio n wit h referenc e t o th e dorso-pleura l line (a-a) and the pie uro-ventral line (b-b). A, metathorax and abdomen of adult male Gryllus. B , metathora x an d abdome n o f larv a o f Calosoma. Itg, laterotergites ; mtg, mediotergite; pi, pleurites; stn, sternites.

bases is morphologically a coxosternum, or pleurosternum. Th e limb base elements of such a sternum (commonl y called "coxites") are coxosternites, or pleurosternites. The ventra l sclerotizatio n o f th e abdomen , a s tha t o f th e dorsum , may be broken up into a group of sternites, a s in various holometabolous larvae (Fig . 13 9 B, stn). I n th e Japygida e a shor t anterio r subdivisio n of eac h abdominal sternu m i s separated b y a membranous fold fro m th e rest o f the plate , forming a distinct sterna l apotome (Fig . 13 8 B, Apt). Characteristics o f th e Abdomina l Segments.—Adul t insect s havin g well-developed organ s o f copulatio n an d eg g laying usuall y sho w a dis tinct differentiatio n i n th e structur e o f th e segment s o f th e visceral , genital, an d postgenita l abdomina l regions . Th e modification s affec t principally th e genita l segments , whic h are structurally adapte d t o thei r special functions , an d th e postgenita l segments , whic h generall y suffe r

252

PRINCIPLES OF INSECT MORPHOLOGY

reduction in proportion to the hyperdevelopment of the genital segments. The Visceral Segments. —The segment s o f th e viscera l regio n o f th e abdomen ar e usuall y o f simpl e structure an d diffe r bu t littl e fro m on e another. I n adult pterygote insects they never bear appendicular organs. The first segment is either broadly joined to the thorax or separated fro m it by a constriction. I n winge d insects the antecost a o f the first abdom inal tergu m bear s th e thir d pai r o f phragmatal lobe s (Fig . 9 7 D , 3P/i) , and the acrotergit e is usually much enlarged, forming the postnotal plate of the metathorax (PNs), which, together with the base of the phragma, is frequently detached fro m th e res t o f the firs t abdomina l tergu m an d becomes virtuall y a par t o f the metathorax . Th e res t o f the firs t seg ment is often reduce d or fused with the second, and the sternal sclerotization i s sometime s obliterated . I n th e clistogastrou s Hymenopter a th e entire first abdominal segment is so intimately united with the metathora x that i t form s anatomicall y a part o f the thorax , terme d th e propodeum. In thes e insect s th e constrictio n betwee n th e apparen t thoraci c an d abdominal section s o f the bod y occurs between the firs t an d secon d segments o f the abdomen . I n som e of the ant s the secon d segment is small and a second constriction occurs between it and the third segment. The Genital Segments. —In som e o f th e simple r insect s ther e i s n o modification o f th e segment s associate d wit h th e genita l aperture s t o distinguish the m a s genital segments ; but usuall y th e nint h segmen t in the male , an d th e eight h an d nint h i n th e femal e sho w some structura l adaptation to the genital functions . Modifications o f th e eight h segmen t occu r principall y i n femal e insects havin g a well-develope d ovipositor , sinc e th e firs t valvula e of the oviposito r are developed from this segment. Th e valvulae are borne ???????? ?? ????? ??????? ??????? ??? ????? ????????? ????? ??? ?? ?? ????? which correspon d to th e stylus-bearin g plate s o f the Thysanura , thoug h styli themselve s ar e absen t fro m th e eight h segmen t i n al l pterygot e insects. Th e sternu m o f th e eight h segmen t ma y b e a simpl e plat e resembling the stern a precedin g it, bu t ofte n i t is enlarged and produced posteriorly beneat h th e bas e o f the ovipositor . I n suc h case s it form s the female subgenital plate. O n the othe r hand , th e eighth sternu m i s sometimes reduced, and in some insects it is practically obliterated. Thi s condition i s usuall y accompanie d b y a n enlargemen t o f th e sevent h sternum, whic h then become s the subgenita l plate . The secon d genital segment usually ha s les s of the typica l segmenta l form tha n doe s th e first . I t i s th e segmen t o f th e secon d an d thir d valvulae o f the oviposito r i n th e female . Thes e valvula e ar e born e b y pleural sclerite s o f the nint h segmen t (Fig . 31 4 A, B , 2FZ/) , th e second valvifers (commonl y called "coxites") > whic h correspond to the valvifers of th e eight h segment . Rudimentar y styl i occur on the secon d valvifers

THE ABDOMEN

253

in nymphal and adul t form s o f some of the lowe r Pterygota (E , Sty), bu t generally they are absent fro m th e ninth segment of the female , excep t in Thysanura. Th e vente r o f the nint h segmen t i n th e femal e i s usuall y inconspicuous, and , wher e a n oviposito r i s present , i t i s reduce d t o a narrow membranou s spac e betwee n th e valvulae , bu t i t ma y contai n intervalvular scleroti c remnants o f the nint h sternum . In th e mal e the nint h segmen t retains a generalized structure i n th e Thysanura, bu t i n pterygote insects i t i s subject to man y modification s and takes o n a great variet y o f forms. Th e dorsal and ventral area s ar e usually sclerotize d an d for m definit e terga l an d sterna l arc s o f the seg ment. I n some insects the bases of the male gonopods are distinct plate s having a normal pleural position on the sides of the ninth segment between the tergum and the sternum, and in such cases they usually bear movable lobes, serving generally as claspers, which apparently represen t th e styl i of generalized insects. Th e gonopod bases, however, may be united wit h the sternum , an d th e resultin g coxosterna l plat e the n carrie s th e styli , if th e styl i ar e preserved, whic h retain a typical stylifor m shape in some Orthoptera an d Isoptera . O n the othe r hand , th e basa l plate s o f th e gonopods may be displaced posteriorly a s free lobe s bearing the claspers , or, again, they becom e fused wit h both th e tergu m and the sternum in a continuous segmental annulus . The modification s of the genita l segment s an d th e structur e o f th e organs of copulation an d eg g laying will be more fully describe d in Chap . XIX. The Tenth Segment.—The tenth segment is present in the abdome n of nearly al l insects, bu t it s limit s ar e ofte n difficul t t o determin e becaus e of th e frequen t unio n between the tent h an d elevent h segments. Whe n only one postgenital segmen t is retained, as in the majorit y of holometabolous insects, both larval and adult, this segment is presumably the tenth . It sometime s bears a pair o f appendicular processes, such as the soci i of adult Trichopter a an d Lepidoptera , th e cercuslik e appendages o f adul t Tenthredinidae, an d th e postpede s o f larva l Neuroptera , Trichoptera , Lepidoptera, and Tenthredinidae. Th e tenth segment appendages may be termed collectivel y the pygopods, sinc e the tenth segmen t is the pygidial, or "rump, " segment . Th e tent h segmen t sometime s bear s lobe s o r processes that clearl y have no relation t o appendages . When tw o postgenital segment s ar e present , a s in many o f the mor e generalized insects , th e tent h segmen t i s frequentl y reduced an d mor e or less unite d wit h th e nint h o r the elevent h segment . I n non e o f th e exopterygote insect s doe s it hav e appendage s i n postembryoni c stages , though rudiments o f limbs may be present o n it i n the embry o (Fig . 136 A, XApd). The tenth, segment, accompanied by the eleventh, occurs as a complet e an d independen t annulu s amon g the Thysanur a (Fig . 140

254

PRINCIPLES OF INSECT MORPHOLOGY

A, X), Odonata (C), Ephemerida (D), Dermaptera (I), Homoptera (L), and i n female s of Panorpidae. I n th e Ephemerid a it s terga l plat e (D , XT) i s produced posteriorly in a small truncat e lob e between the base s of th e cerc i and thus resembles the supra-ana l plate o f some other insect s formed o f the elevent h tergum, bu t i n th e ephemeri d the dorsa l part of the elevent h segment, or true epiproct , lies beneath the lob e of the tent h

FIG. 140.—Postgenita l segment s o f the abdomen . A , B, Nesomachilis maoricus. C , Plathemis lydia, adul t male , ventral view . D , ephemerid, adult male . E , perlid larva. F , embiid. G , Gryllus assimilis. H , Blatta orientalis, ventra l view . I , Anisolabis marítima, female . J , Blatta orientalis, dorsa l view , segment s separated . K , Dissosteira Carolina,, female . L , Magicicada septendecim, male . An , anus ; Cer, cercus; cf , cauda l filament; cxpd, bas e of cercus (coxopodite); Eppt, epiproct ; Ovp, ovipositor; Papt, paraproct; paptl, lob e o f paraproct; sal, supra-anal lobe ; xmcls, muscles o f tenth segment .

tergum and carries the median caudal filament (cf). Likewise, in the Plecoptera (E) , Embiidae (F) , and Blattidae (J) , the tenth tergu m (XT) is the terminal dorsa l plate of the abdomen ; the epiproc t in these insect s is reduced to a supra-anal pad or membrane beneath the en d of the tent h tergum. I n th e Orthopter a th e ventra l par t o f th e tent h segmen t i s mostly membranou s and usually does not appea r i n the adul t a s a defi nitely define d segmenta l region , thoug h i t sometime s contain s a smal l sternal sclerotization. Th e tergum of the segmen t is generally a distinc t plate, as in Acrididae (K), but sometime s it is more or less united with the

THE ABDOMEN

255

epiproct t o for m a composit e supra-ana l plate , thoug h th e divisio n between the tw o parts may remain quit e evident , a s in Gryllus (G) . The Eleventh Segment.—The eleventh segmen t o f the abdome n represents th e las t tru e somit e o f the body . I t i s present i n the embryo s of lower insects a s a well-developed metamere bearing the rudiment s o f the terminal pair of appendages, which are the cerc i of the imago (Fig. 13 6 A, XI). I n adult Protur a (B) it is a normal annulus with tergal and sternal plates; but i n al l the tru e Insect a th e elevent h segmen t i s more o r less reduced, an d it s individualit y i s often los t b y union with th e tent h segment. I n mos t o f the Holometabol a i t i s suppressed entirely , an d th e body ends with the tenth segment. To generaliz e on the structur e o f the elevent h segment, we may sa y that, when present, i t form s a conical endpiece of the body , bearin g th e cerci laterally an d the anu s at it s apex ; its dorsal surface i s covered by a triangular o r shield-shaped terga l plate , th e epiproct (Fig . 14 0 K, Eppt), and its ventrolateral parts form two lobes, the paraprocts (Papf). The ventral margin s o f th e paraproct s ar e usuall y connecte d basall y b y a median membranou s are a (C , H), an d the posterio r margin of the latter is sometimes produced in a small subanal lobe , or hypoproct. Occasion ally th e paraproct s bea r termina l lobes , suc h a s th e small , soft , apica l parts in som e adul t Odonat a (C , paptl), th e stylus-lik e processe s of th e paraprocts i n tridactyli d Orthoptera , o r th e broad , tracheate d plate s forming the lateral gills of zygopterous odonate larvae (Fig. 141 C, papil). The cerc i ar e implanted typicall y i n membranous areas between th e bases o f the epiproc t an d th e paraproct s behin d th e tent h tergu m (Fig . 140 K , Cer). Thoug h the y ar e generall y closel y associate d wit h th e tenth segment, embryologists mostly agre e that they aris e in the embryo as limb rudiments on the eleventh segment (Fig. 136 A, XIApd). Their connection wit h th e tent h segmen t become s more pronounced with th e reduction of the elevent h segment or its union with the tenth. I n Cam podeidae an d Japygida e th e abdomina l segment s beyon d th e tent h are obliterated , bu t th e cerc i ar e retaine d an d ar e necessaril y born e directly o n the en d of the tent h segment. I n Machilida e each cercus is supported o n a larg e pleural lob e o f the elevent h segmen t (Fig . 14 0 A, Cxpd), an d i n man y o f the mor e generalized Pterygot a th e appendag e has a small , usually imperfec t basa l segmen t (G , Cxpd). Th e shaf t o f the cercu s i s sometime s distinctl y divide d int o segmentlik e section s (D, F) , bu t i t neve r contain s muscles . Mos t o f the muscle s that move the cerci , whic h ar e inserte d o n o r nea r th e cerea l bases, tak e thei r origins on the tent h tergu m an d ar e probabl y muscle s of the tent h seg ment (A , xmcls). I n som e insects one muscle of each cercus arises on th e epiproct, bu t th e cerc i neve r hav e muscle s from th e paraprocts . Th e latter, therefore , d o no t hav e th e relatio n o f lim b base s t o th e cerci .

256

PRINCIPLES OF INSECT MORPHOLOGY

The cerc i of insects apparently correspon d to th e uropods o f malacostracan Crustacea. The y are usually simple processes, conical or filamentous in form , an d o f a sensor y function, but sometime s the y ar e modifie d t o serve a s claspin g organs. The generalize d structure o f th e elevent h segmen t i s perhap s mos t fully retaine d i n th e Machilidae . I n Nesomachilis, fo r exampl e (Fig . 140 A), the eleventh segment, though normally concealed within the tenth, has th e for m o f a complet e ring wit h distinc t terga l an d sterna l region s separated o n the side s by the larg e lateral lobes (Cxpd) bearin g the cerci (Cer). Th e tergal region (XIT) i s produced into the media n caudal filament (cf). The ventral region presents anteriorly a narrow sternal bridge (B, XlStri) betwee n th e latera l cercus-bearin g lobes , an d posteriorl y a ???? ?? ????? ?????????? ?????? ?? ??? ????? ?? ??? ????? Among th e lowe r Pterygota , th e part s o f th e elevent h segmen t ar e entirely distinc t fro m th e tent h segmen t i n Odonata . I n a n adul t dragonfly (Fig. 140 C) the epiproct (Eppf) is a large free median lobe tapering t o a truncat e point . Th e cerc i aris e latera d o f th e epiproc t and are broadly hinged to the posterior margin of the tenth tergum. Th e paraprocts (Papt) are wide triangular ventral lobes at the sides of the anus; each contains a large basal plat e an d terminate s i n a smal l fleshy process (paptl). In the larvae of anisopterous Odonata the epiproct and paraprocts for m the three tapering valvula r processe s that close the large anal openin g (Fig . 14 1 A, B) . I n zygopterou s larva e eac h lob e o f th e eleventh segment bears a gill plate (C) , the median gill (cf) bein g a process of th e epiproct , an d th e latera l gill s (paptl) processe s o f the paraprocts . The smal l cerc i '(Cer) aris e i n th e usua l position . I n th e Ephemerid a ????? ?? ?? ??? ??????? ???????? ??????? ??? ?????? ???????? ???? ?? concealed beneat h th e overhangin g media n lob e o f th e tent h tergum ; and i n th e Plecopter a (E ) an d Embiida e (F ) th e epiproc t i s reduce d to a supra-ana l pa d adnat e t o th e ventra l surfac e o f the tent h tergum . In Dermapter a (I) , however , th e epiproc t (Eppf) i s a distinc t plat e between the bases of the cerci , movably hinged to the posterio r margin of the tent h tergum. I n mos t orthopteroid insects th e elevent h segment is distinct, thoug h often closel y united wit h the tenth (K) , and the epiproct may b e fuse d wit h th e tent h tergu m (G) ; bu t i n Blattidae (H , J ) th e epiproct i s practically obliterate d excep t for a membranous fold beneat h the tent h tergu m o n which the muscle s of the paraproct s (H , Papt) ar e attached. The Twelfth Segment. —The primitive termina l segmen t o f the arthro pod trunk i s the periproct , o r endpiece of the bod y containin g th e anus , anterior t o which the true appendage-bearing somites are formed. I n the malacostracan Crustace a th e periproc t form s th e telson, typicall y a broad terminal lobe of the abdomen having the anu s situated in the basal

??? ??????? ???

part o f it s ventra l surface . Th e periproc t appear s t o b e represente d in th e embryo s o f som e insect s b y a termina l twelft h segmen t o f th e ??????? ????? ??? ?? ?????? ????? ????? ??? ??????????? ??? ????? adult hexapod s a twelfth abdominal segment with tergal and sternal plates occurs onl y i n th e Protur a (B) . I n mos t insect s n o trac e o f a twelft h segment i s to b e found, an d the periproc t mus t be supposed to b e repre sented, i f at all , only by the circumana l membrane at th e en d of the elev enth segment . The bes t exampl e o f th e possibl e retentio n o f a twelft h abdomina l segment i n postembryoni c stage s o f insects i s furnished b y th e larva e of

???? ????????????? ???????? ?? ???????? ?? ???????? ?????? ??? ???? ?? ??????? showing possible rudiment of twelfth segment, or periproct (Prpt). B, same, lateral view, with part s in usual position . C, Archilestes granáis larva.

anisopterous Odonata , in which the anu s i s contained i n a small circular ???? ????? ??? ?? ????? ?????????? ????????? ??????? ??? ????? ?? ??? epiproct (Eppf) and the paraprocts (Papt). In the walls of this fold there is a small dorsal sclerite, or lamina supra-analis (sal), and two latero ven???? ?????????? ?? ??????? ???????????? ?????? ????? ????????? ??? ???? ?? adult Odonata , but a small supra-anal lobe , apparently a remnant o f the lamina supra-analis , project s from beneat h the epiproc t (Fig . 140 C, sal). A simila r lob e occur s i n larv a o f Ephemerid a an d i n som e adul t Thysanura (B , sal). Th e supra-ana l lob e o f thes e insects , therefore , might be regarded as a dorsal remnant of the telson . 2. TH E ABDOMINA L MUSCULATUR E

The abdomina l musculatur e o f adul t an d larva l insect s i n genera l conforms t o a rather simpl e fundamental pattern, which is repeated wit h only mino r variation s i n eac h of the pregenita l segments ; in th e genita l and postgenita l segment s th e basi c pla n o f musculature i s mor e o r les s obscured by special modifications. I n som e of the Apterygota , however, and i n larval form s o f holometabolous insects th e bod y musculature ma y

258

PRINCIPLES OF INSECT MORPHOLOGY

be highl y complex . Som e writers hav e regarde d th e comple x types of muscle arrangement a s representing a primitive condition; but since these types hav e no conformity among one another, an d since the musculatur e of holometabolous larvae shows in all orders a progressive evolution away from th e simpl e adul t type , it woul d seem that th e latte r mus t b e more nearly representativ e o f th e muscl e pattern o f primitiv e insects . Th e abdominal musculatur e of adult insect s i s more elaborate tha n th e bod y musculature of the thorax, but o n the whole it is simpler than the thoraci c musculature because of the absenc e of leg muscles. Since the muscles are derived from the walls of the embryonic coelomic sacs, o r a t leas t fro m th e metameri c division s o f the mesoderm , we may assume tha t th e primitiv e somati c fiber s o f arthropod s wer e al l intra segmental i n arrangemen t (Fig . 3 5 A), as they ar e in the Annelida ; bu t with th e acquisitio n o f secondar y segmentation , consequen t upo n th e development o f sclerotic plate s in th e bod y wall , the longitudinal fiber s become functionall y intersegmenta l (B) . Th e bod y o f th e anima l ca n thus be shortened by a telescoping of its segment s (C ) brought about b y contraction o f th e longitudina l muscles , an d i t ca n b e compresse d b y contraction o f th e latera l dors o ventral muscles . Th e opposit e move ments ma y resul t eithe r fro m th e elasticit y o f th e bod y wal l o r fro m pressure generated by contraction i n one part o f th e body transmitted t o another throug h th e mediu m of the bod y liqui d an d the viscera l organs; but i n th e abdome n o f th e highe r arthropod s protracto r an d dilato r apparatus ar e developed in which certain muscle s become antagonistic t o the retractor s an d compressors. General Pla n o f th e Abdomina l Musculature.—The muscle s o f th e insect abdome n ma y i n genera l b e classe d i n thre e groups , namely , dorsal muscleSj ventral muscles, and lateral muscles. Th e dorsa l muscle s include longitudinal dorsals an d transverse dorsals; the ventra l muscle s are similarly divide d int o longitudinal ventrals an d transverse ventrals; th e lateral muscle s compris e lateral muscles o f th e body wall an d spiracular muscles. Eac h of these sets of muscles is again often subdivided int o tw o or mor e mino r groups . Th e namin g o f th e muscle s accordin g t o thi s classification would , in a final analysis, lea d to the compoundin g of terms of unwield y length. Henc e the write r (1931 ) ha s proposed a scheme for simplifying th e nomenclatur e by limiting th e term s "dorsal " and "ven tral" to the longitudinal dorsal and ventral muscles only, and dividing the transverse muscle s into dorsa l and ventral sets . Accordin g to thi s pla n the majo r group s of muscles are a s follows : I. Dorsa l muscles (Fig. 14 2 A, d), th e fiber s o f which ar e typicall y longitudinal and attache d on the intersegmenta l folds o r on the antecosta e of successive terga. II. Ventra l muscles (v\ resemblin g th e dorsa l muscles in tha t thei r fiber s ar e typically longitudinal and attached on the intersegmental folds or on the antecostae of successive sterna.

THE ABDOMEN

259

III. Latera l muscles (Z) , typically dorsoventral, an d bot h intrasegmental (le) an d intersegmental (U) i n position . IV. Transvers e muscles (C , ¿), lying internal to th e longitudinals , including dorsal transverse muscles (td) and ventral transverse muscles (tv). V. Spiracula r muscles, generally not mor e tha n two connected wit h each spiracle, one an occlusor, the othe r a dilator.

Each o f the first three of these primary groups of muscles may undergo an endles s diversificatio n resultin g fro m a multiplicatio n o f fibers in th e group, a separation o f the fibers into subgroups, or a rearrangement o f the fibers brought abou t b y change s in the point s o f attachment . With respec t t o th e dorsa l an d ventra l muscle s th e mos t genera l departure fro m th e simpl e plan , i n whic h th e fiber s al l li e i n a singl e

FIG. 142.—Diagrams of abdominal musculature. A , B, simple types o f musculature, right hal f o f a segment , inne r view . C , cros s sectio n o f a segment . (Fo r letterin g se e page 260.)

plane agains t th e bod y wall , consist s o f a differentiatio n of th e fiber s in each group into external muscles and internal muscles. Thu s it is found in nearly all insects that the dorsal and ventral muscles comprise each two layers, there being , namely, internal dorsals (di) an d external dorsals (de), and internal ventrals (vi) an d external ventrals (vé). A secon d for m o f diversification affecting the same muscles consists of a more or less distinct grouping o f th e fiber s int o median an d lateral sets . I n mos t insects , therefore, w e ma y distinguis h fou r set s o f dorsa l fibers , an d fou r set s of ventral fibers . Th e several resulting muscle s or sets of fiber bundles the n may be designated as follows: median and lateral internal dorsals (Fig . 143 A, dim, dil), median an d lateral external dorsals (dem, del), median an d lateral internal ventrals (vim, vil), an d median and lateral external ventrals (vem, vel). In som e insects ther e i s a longitudinal muscl e or group of longitudina l fibers situated o n th e latera l par t o f th e dorsu m abov e th e lin e o f th e spiracles, externa l to the upper end s of the internal lateral muscles. Thi s muscle i s sometime s calle d a "pleural " muscle , bu t sinc e i t evidentl y belongs t o th e dorsu m i t i s mor e properl y terme d a paradorsal muscle (Figs. 14 2 B, C , 14 3 B, p).

260

PRINCIPLES OF INSECT MORPHOLOGY

The latera l muscle s are more subject to irregularitie s o f position tha n are the dorsal and ventral muscles , but they likewise are often divide d into internal laterals (Fig . 142 , If) an d external laterals (le). To express more concisely the majo r groups of abdominal muscles and their principa l subdivisions , w e ma y tabulat e th e severa l set s o f fiber bundles enumerate d abov e a s follows : I. Muscul i dorsales (d). 1. M . dorsales interni (di). a. M . dorsales interni mediales (dim). b. M . dorsales interni laterales (dil). 2. M. dorsales externi (de). a. M . dorsales externi mediales (dem). b. M . dorsale s externi laterales (del). ?? ?? ???????????? ???? II. Muscul i ventrales (v). 1. M . ventrales interni (vi). a. M . ventrales interni mediales (vim). b. M . ventrales interni laterales (vil). 2. M. ventrales externi (ve). a. M. ventrales externi mediales (vem). b. M. ventrales externi laterales (vel). ???? ??????? ????????? ???? 1. M . laterales interni (li). 2. M . laterales externi (le). IV. Muscul i transversales (t). 1. M . transversi dorsales (id). 2. M . transversi ventrales (¿v) . V. Muscul i spiraculorum 1. M . occlusore s spiraculorum (osp). ?? ?? ????????? ???????????? ???????

It i s ofte n difficul t t o defin e individua l muscle s o f th e bod y wal l of insects becaus e th e fibe r bundle s ar e no t surrounde d b y a commo n sheath; bu t generall y th e muscle s ar e distinc t becaus e o f th e groupin g of th e fiber s an d ma y b e give n individua l names . Fo r referenc e pur poses, however , i t wil l b e foun d mor e practica l t o indicat e individua l muscles o n drawing s wit h Arabi c numerals , sinc e i t i s ofte n difficul t or impossibl e t o identif y correspondin g muscle s throughou t th e serie s of segments . Though i n the viscera l regio n of the abdome n th e muscle s may b e segmentall y repeate d wit h fai r regularity , th e arrangemen t i s usually s o distorte d i n th e genita l an d postgenita l region s tha t th e muscle homologies become very doubtful . We ma y no w giv e a brief summar y o f the principa l modification s i n the arrangemen t o f th e muscle s o f th e severa l principa l muscl e group s in the viscera l segments of the abdomen. Th e musculatur e o f the genita l and postgenita l segment s require s a specia l stud y an d wil l not b e con sidered here .

THE ABDOMEN

261

The Dorsal Abdominal Muscles. —The muscle s o f th e dorsu m ar e composed primaril y o f longitudina l fiber s o f segmenta l lengt h attache d on the intersegmenta l folds (Fig . 142 A, d). I n many larvae the principa l dorsal fiber s retai n thi s primitiv e condition ; but i n insect s havin g full y developed terga l plate s th e dorsa l muscle s becom e functionall y inter segmental because the folds on which they are attached becom e the ante costae o f th e definitiv e terg a (Fig . 14 3 C). Sinc e the segmenta l plate s are pulle d forwar d b y th e contractio n o f th e longitudina l muscles , th e anterior en d of a longitudinal abdominal muscle may be termed its origin, and the posterio r en d its insertion. The interna l dorsal s commonl y retai n thei r longitudina l positio n and thei r segmenta l length (Fig . 14 3 C, D, di) ; bu t the y underg o many departures fro m thi s generalize d conditio n throug h becomin g obliqu e or by a shif t i n their origin s to th e postcosta l region s of the terga . Th e external dorsals , o n th e othe r hand , ar e seldo m o f segmenta l length ; typically the y ar e shor t muscle s lying i n th e posterio r part s o f the seg ments (C , dé), an d ofte n the y becom e strongl y oblique , sometime s actually transverse, givin g a movemen t of torsion t o th e segment s the y connect. Finally , thei r origin s may become transposed t o th e posterio r margins o f th e terga , i n whic h cas e th e externa l dorsal s ar e reverse d in position (D, dé); functionally they then become antagonistic to the internal dorsals (di) and ac t as abdominal protractors, since their contrac tion lengthen s the abdome n by decreasin g the overla p o f the segments . In some cases the anterio r end s of the protractors ar e attached o n apodemal arms of the anterior margins of the terga, thus increasing the effective ness of the muscles . Th e dorsa l muscles are often variously reduced, and some o f th e principa l group s of fiber s ma y b e entirel y suppressed . The paradorsa l muscl e (Figs . 14 2 B, C , 14 3 B, p ) i s no t commonl y present i n adult insects , or, at least , it s fibers are not generall y separate d from thos e o f the othe r latera l dorsa l muscles . I t i s well show n in th e Acrididae as a distinct muscl e (Fig. 144 A, 169), and it i s a characteristic feature o f the musculatur e of some larval insects . The Ventral Abdominal Muscles.—The ventral muscles of the abdomen undergo a n evolutio n paralle l i n mos t respect s t o tha t o f th e dorsa l muscles. Th e fiber s o f the interna l laye r ar e typicall y intersegmenta l wherever complet e sterna l plate s ar e presen t an d serv e a s retractors of th e ventra l arc s o f the segments . Th e externa l ventral s ar e usuall y short an d tak e thei r origin s o n the posterio r part s o f the sterna . Fre quently the y becom e sternal protractors b y a reversa l i n position , an d commonly thei r anterio r end s ar e the n carrie d forwar d o n anterio r apodemal arms of the stern a (Fig . 14 4 A, 174). Th e ventra l muscles , as the dorsa l muscles, however, are sometime s reduced, and on e or more of the principal groups may be lost.

262

PRINCIPLES OF INSECT MORPHOLOGY

The Lateral Abdominal Muscles. —The latera l muscles of the abdome n do not conform so closely to a general plan of arrangement as do the dorsal and ventra l muscles . Mos t o f the m ar e intrasegmenta l i n position , and tergosterna l in their attachment s (Fig . 14 2 A, le) ; bu t som e of them may li e o n th e intersegmenta l fold s (li), an d frequentl y som e o f the m are intersegmenta l in th e sens e that the y cros s obliquely from on e seg -

FIG. 143.—Diagram s of abdominal musculature . A , cross section, showing principal muscles differentiate d int o distinc t group s o f fibers , wit h latera l muscle s comprisin g tergosternal (£-s) , tergopleura l (£-p) , an d sternopleura l (s-p) muscles . B , illustratin g lateral muscle s differentiate d int o compressor s (cpr) an d dilator s (dlr). C , longitudina l section throug h consecutiv e terga, showing usual position o f external dorsal s (dé) an d internal dorsal s (di), whic h are bot h retractors . D , same , wit h oute r dorsal s (dé) reversed i n position t o functio n as protractors. (For othe r letterin g se e page 260.)

ment t o th e next . Furthermore , th e latera l muscle s ar e no t alway s strictly tergosterna l i n thei r attachments , fo r som e o f the m ma y b e attached a t on e end on small sclerites located in the pleura l areas of th e lateral integument between the tergal and sternal plates. Suc h muscles, therefore, ma y b e terme d tergopleural , or pleurosternal . A divisio n of the latera l muscle s into internal laterals and external laterals (Fig . 14 2 C, li, le) is not alway s apparent, ofte n becaus e of the absenc e of an interna l group, but i t i s of common occurrence. In som e insects there is a well-define d interna l se t o f lateral muscles lying mesad of the latera l longitudina l tracheal trunk , havin g the upper attachments o n the dorsu m above the paradorsa l muscle (Fig. 14 2 C, li),

THE ABDOMEN

263

when th e latte r muscl e i s present . Th e interna l lateral s ma y b e dis tributed alon g th e lengt h o f eac h segment , bu t i n som e case s the y ar e limited t o th e extrem e anterio r part s o f th e segments , an d i n certai n holometabolous larva e the y li e o n th e intersegmenta l fold s (A , li). The externa l lateral s (C , le) aris e dorsall y belo w th e paradorsa l muscl e (p), whe n this muscl e is present . Frequentl y som e o f them cros s each other obliquely , and i n their attachment s the y ar e ofte n diversifie d int o tergosternal, tergopleural , and pleurosternal groups. Functionally, mos t o f th e latera l muscle s ar e compressors o f th e abdomen (Fig . 14 3 B , cpr) , sinc e thei r contractio n approximate s th e tergal an d sterna l plates . Wit h som e insects , however , i n whic h the latera l parts of the abdomina l terg a overla p the edge s of the sterna , certain o f the latera l muscles are so situated a s to be antagonistic t o th e others. Thes e muscles , therefore , become dilators o f th e abdome n (B , dlr). Thei r dilato r actio n result s fro m th e fac t tha t thei r terga l attach ments are on the lowe r edges of the terg a ventra l t o their sterna l attachments. B y contraction , therefore , they separat e th e terga l an d sterna l plates. Th e effectivenes s o f thes e muscle s i s usuall y increase d b y th e elevation o f their sterna l end s o n latera l apodema l arm s o f the sterna l margins (Fig . 14 4 A, 177). Th e contractio n o f th e latera l abdomina l muscles most frequently produces a movement of the sternal plates; but if the sterna l arc s are larger and more rigid than th e terga l plates , i t i s the latter that respond to the actio n of the lateral muscles . Whe n a dilato r mechanism is absent, the expansio n of the abdome n following contraction results fro m th e genera l elasticit y o f the abdomina l integument . Soft skinned larval insects usually contract only a small part of the body at one time, an d this part is then expande d by pressure resulting fro m contrac tion in some other part . The Transverse Abdominal Muscles.—The transvers e muscle s o f th e abdomen ar e bes t know n as th e muscle s o f the dorsa l an d ventra l dia phragms (Fig . 14 2 C, tdj tv). Th e fiber s o f th e dorsa l diaphrag m aris e typically i n group s o n th e anterio r edge s o f th e latera l part s o f th e abdominal terga an d spread mesall y to their attachment s alon g the ven tral wall of the heart. I n a few insects they ar e evenly distributed alon g the entir e lengt h o f each tergum o r collected into anterio r an d posterio r groups. Th e ventral transvers e muscle s in some insects, a s in Acrididae and Hymenoptera, for m a continuou s shee t o f weblike tissue through out mos t o f th e lengt h o f th e viscera l regio n o f th e abdomen , whic h constitutes a ventra l diaphrag m stretche d betwee n th e edge s o f th e sterna over the ventral nerve cord; in others, however, as in Tettigoniida e and mos t Gryllidae , th e ventra l fiber s ar e aggregate d t o for m widel y separated compac t muscles crossing the anterio r part s of the abdomina l sterna. No t onl y ar e th e ventra l transvers e muscle s mor e variabl e i n

264

PRINCIPLES OF INSECT MORPHOLOGY

their arrangemen t than are their dorsal counterparts, bu t the y are of less constant occurrenc e and ar e generall y absent i n holometabolou s larvae. The Spiracular Muscles o f th e Abdomen. —The regulato r mechanis m at th e entranc e t o th e abdomina l trachea e usuall y include s one or two muscles associate d wit h eac h spiracle . Th e muscl e mos t generall y present i s an occlusor . Thi s is a short muscl e usually attache d a t bot h ends on apodemal processes of the spiracular atrium, where its contraction compresses the inne r end of the atriu m an d s o closes the entranc e to th e trachea; in the Acrididae the occlusor muscle arises dorsally on the tergum close behin d th e spiracle . A n antagonisti c muscle , o r dilato r o f th e spiracle, is absent i n many insects ; when present i t arise s ventra l t o th e spiracle, o n either th e tergu m o r the sternum , an d i s inserted o n on e of the processe s o f th e atriu m i n lin e wit h th e occlusor . Th e regulato r mechanism o f the spiracle s will be more fully describe d in Chap . X V o n the respirator y system . The Abdomina l Musculatur e o f a Grasshopper. —The abdomina l musculature wil l b e bes t understoo d b y studyin g th e muscle s o f some fairly generalized insect, and any of the larger grasshoppers will serve as a good subjec t for laboratory work . The abdomina l muscle s ar e wel l develope d i n th e Acrididae , sinc e the grasshopper s mak e dorsoventral expansion s an d contraction s o f th e abdomen during breathing an d execut e strong movements in this part of the bod y durin g th e act s o f copulatio n an d oviposition . Th e grea t extension o f the femal e abdome n during oviposition , however , is apparently caused by the action of the muscles connected with the ovipositor; as the latter organ automatically dig s into the earth, it stretches the viscera l region of the abdome n far beyond the capacit y of the protracto r muscles. The muscula r activitie s o f th e abdome n ar e al l accomplishe d b y th e abdominal muscles, there being no muscles in the grasshoppe r extending from th e thora x int o th e abdomen . Th e abdomina l musculature shows little variatio n i n th e severa l segment s o f th e viscera l region , excep t in the first and second segments (Fig. 14 4 B). Th e muscle pattern in the third segment (A ) may be taken a s typical of the genera l segmental plan of th e abdomina l musculature; but i n the genita l segment s the muscula ture is highly modified, and in the terminal segments it is reduced. Fo r a general review of the abdomina l musculature in orthopteroid insect s th e student i s referre d to th e wor k of Ford (1923) . Th e followin g specifi c descriptions ar e base d o n Dissosteira Carolina, the abdomina l muscles of which the write r has full y describe d elsewhere (1935). The Dorsal Muscles. —The dorsa l muscles of the grasshoppe r occupy the latera l area s o f the abdomina l terga, bu t the y d o not entirel y cover the terga l surfaces . Th e interna l dorsal s for m severa l broa d band s of fibers in each side of the body (Fig. 144 A, 167,168,169) an d are in general

THE ABDOMEN

265

longitudinal thoug h the y hav e a tendency to obliquity , whic h is accentuated i n th e mor e posterior segment s (B) . Th e mos t latera l grou p of dorsal fiber s o n eac h sid e (A , 169) i s a paradorsa l muscle , sinc e i t i s separated from the others by the upper ends of the internal lateral muscles (175, 176). Th e others , again , ar e divide d b y th e attachment s o f th e muscles of the dorsa l diaphragm o n the bod y wall (A , td) int o a median intrapericardial grou p of three o r fou r fla t band s o f fiber s (167a } 6 , c, d,), and int o a broad , latera l extrapericardia l muscle (168). Th e intraperi cardial fiber s ar e attache d anteriorl y o n a secondar y terga l ridg e (tr) ;

FIG. 144.—Abdomina l musculatur e o f a grasshopper , Dissosteira Carolina. A , muscle s of righ t half o f third segment . B , muscle s of right hal f o f segments I t o V .

posteriorly al l the dorsa l muscles are inserte d o n the anterio r margi n of the following tergum (IVT). The internal dorsal muscles are thus retractors o f the terga . Th e wide separation of the end s of corresponding groups o f th e longitudina l fiber s i n consecutiv e segments o f th e grass hopper presents a n atypical condition . The external dorsal s compris e tw o muscle s i n eac h segment , on e median (Fig . 14 4 A, 170), th e other latera l (171), whic h assume oblique or transvers e positions . I n th e thir d segmen t eac h o f th e externa l dorsals arise s o n the posterio r part o f the tergum , th e media n on e (170) extending dorsall y t o it s insertio n o n the anterio r edg e o f the followin g tergum, the latera l on e ventrally (171). I n the more posterior segment s the correspondin g muscles are longer and cros s each other o n the sid e of the tergum . Th e externa l dorsal s o f the grasshoppe r ar e thu s torsio n muscles servin g t o giv e a partia l transvers e rotatio n o f the abdomina l segments o n each other. The Ventral Muscles. —The ventra l muscle s form a unifor m serie s i n the firs t seve n segments o f the femal e an d i n th e firs t eigh t segment s of the male . Th e interna l ventral s ar e distinctl y divide d i n eac h segment into a broad median band of longitudinal fibers (Fig. 14 4 A, 172) reaching

266

PRINCIPLES OF INSECT MORPHOLOGY

from a submarginal sternal ridge (sr) t o the anterio r edge of the followin g sternum, an d into a smaller bundle of lateral fiber s (173) extending fro m the anterio r lateral area of the sternum to the anterio r end of the anterio r apophysis (aAp) o f the followin g sternum. Bot h sets of internal ventral s are sternal retractors . The externa l ventra l muscle s consist o f a singl e bundle of fiber s o n each side of each segment (Fig . 14 4 A, 174)- Eac h muscle arises on th e posterior lateral area of the sternu m of its segment and extends anteriorly to its insertion o n the overlapping under surface of the anterior apophysi s (aAp) o f the succeedin g sternum. Th e externa l ventral s ar e thu s pro tractor muscle s inasmuc h a s thei r contractio n serve s t o separat e th e sternal plates . Sinc e ther e ar e n o terga l protractors , th e sterna l pro tractors evidentl y may give an upward flexure to the extende d abdomen or serv e t o counterac t a deflexe d conditio n produce d b y th e interna l ventrals i n opposition to the internal dorsals . The Lateral Muscles. —The arrangemen t o f the latera l muscle s forms the sam e pattern in segment s III t o VI I (Fig . 14 4 B), in which all the lateral muscle s ar e tergosterna l i n their attachments . There ar e tw o internal laterals in each side of each of these segments (A , 17'5,176), bot h arising o n th e tergu m beneat h th e ventra l edg e o f th e latera l dorsa l muscle (168)] the first (175) is inserted on the base of the lateral sternal apódeme (lAp), th e secon d (176) on the latera l margi n o f the sternum . External t o th e secon d interna l latera l ar e tw o obliqu e oute r lateral s (178, 179) having thei r origin s o n the tergu m ventra l t o th e paradorsa l muscle (169) ; th e tw o cross each other, going respectively posteriorly and anteriorly t o thei r insertion s o n th e latera l edg e o f the sternum . Th e internal lateral s an d the two oblique external laterals ar e all compressors of th e abdome n and ar e therefor e expirator y muscles in respiration . A third externa l latera l arise s from th e lowe r anterior angl e of the tergu m (A, 177) and extend s dorsally to it s insertio n o n the uppe r outer surfac e of the lateral sternal apódeme (lAp). Thi s reversed lateral is antagonistic in its action t o the other laterals; it is therefore a dilator o f the abdome n and a n inspiratory muscl e in respiration . In th e firs t abdomina l segmen t th e latera l musculatur e i s reduced to a single slender muscle (Fig. 14 4 B, 146), which is apparently a tensor of th e tympanu m o f the " auditory" organ. I n th e second segment th e lateral muscles do not entirely confor m with those of the segments follow ing, and in addition t o the tergosternal muscles there are a pair o f tergopleural muscles and a single short sternopleura l muscle; these lie external to the tergosternal laterals and are therefore not seen in the figure. The Comple x Types of Abdominal Musculature.—The musculature of apterygote hexapod s i s no t wel l know n i n al l th e severa l groups , bu t it ha s been carefully studie d in Protura an d Japygidae. Th e abdominal

THE ABDOMEN

267

musculature o f Protura , a s describe d b y Berles e (1910) , i s somewha t more complex than that of adult pterygotes . I n the Japygida e the mus culature throughou t th e bod y present s a highl y complicate d pattern ; in each of the first eight abdominal segments of Heterojapyx, for example, there are at least 40 pairs of muscles having a most intricate arrangement, which, except that the fibers are comprised in dorsal, ventral, an d latera l sets, show s little t o sugges t tha t th e muscl e pattern o f adult pterygot e insects ha s bee n derived fro m it . Th e multiplicit y o f muscles in Heterojapyx would appear to be a specialized condition. Among holometabolous larvae the bod y musculature is elaborate in the maggot s of higher Diptera, but i t reaches its greatest degree of complexity in the caterpillars

FIG. 145.—Ventra l muscle s an d muscle s o f right hal f o f th e mesothora x an d metathora x of a caterpillar , Malacosoma americana, inner view .

(Fig. 145). I n more generalized forms, however, the abdominal musculature of the larva is not essentiall y different fro m that of the adult . 3. TH E ABDOMINA L APPENDAGE S

The usua l abdominal appendages of adult insects are th e gonopod s of the genita l segment s and th e uropods , or cerci , of the elevent h segment . In som e of the Apterygota , however , appendages occu r also o n the pre genital segments, and th e larva e of Pterygota present numerou s varieties of appendicula r structure s o n the abdomen , many o f which appear t o b e rudiments o f tru e segmenta l limbs . Th e cerc i hav e alread y bee n described in connection with the elevent h segmen t (pag e 255), the gonopods wil l b e discusse d i n Chap . XI X o n th e externa l genita l organs ; the present section , therefore, is limited to a brief revie w of the pregenita l appendages o f the Apterygot a an d th e appendicula r organs of pterygot e larvae.

268

PRINCIPLES OF INSECT MORPHOLOGY

Abdominal Appendage s o f Protura. —A pai r o f shor t cylindrica l appendages i s presen t o n eac h o f th e firs t thre e abdomina l segment s of adult Protura , arisin g fro m th e membranou s part s o f thes e segment s between th e posterio r angle s o f th e terga l an d sterna l plates . Th e appendages ar e bes t develope d in Eosentomidae , wher e the thre e pair s are alik e in size and structure ; each organ consist s o f two segments (Fig. 146) an d a smal l termina l vesicl e (v) , whic h is eversibl e an d retractile . In Acerentomida e the appendage s of the firs t pai r ar e lik e thos e o f th e Eosentomidae, bu t th e secon d an d third pair s ar e ver y small , simpl e tuberculiform protuberances , unseg mented an d lackin g th e vesicle . Each appendag e o f th e large r type in the tw o families , a s describe d b y Berlese (1910) , i s movabl e b y tw o tergal muscle s (B , /, /) inserte d o n the basa l segment, one anteriorly, th e FIG. 146—Abdomina l appendages of othe r posteriorly . Th e secon d seg Protura. A , abdomina l leg of Eosen- men t Í S provide d likewis e wit h tw o tomón germanicum. (From Prell, 1913. ) mi™lp« nrip arisjino- anfprinrlv thp

B, musculature of first abdominal leg of muscles> one arising anteriorly, tüe Acerentomon doderoi. 1910

(From

Berlese,

othe r posteriorl y i n th e proxima l seg-

-) ment, th e tw o crossin g eac h othe r medially to b e inserted o n opposite sides of the base of the distal segment. The terminal vesicle is retracted by a single large muscle (rv), which takes its origi n mesally in the bas e of the first segment and is inserted o n a central depressio n o f the ventra l fac e o f the vesicle . Th e extrusio n o f th e vesicle i s evidently brough t abou t b y bloo d pressur e fro m withi n th e body.

Abdominal Appendage s of Collembola. —The Collembol a have thre e characteristic appendicula r organ s o n th e abdomen , which , thoug h unpaired a t leas t basall y i n the adul t stag e an d locate d mediall y o n the ventral side of the body, are said to be formed in the embry o from paire d rudiments. Eac h retain s i n it s adul t structur e evidenc e o f it s doubl e origin. Th e first appendage is carried by the first abdominal segment and is known as the ventral tube, or collophore (Fig . 147 A, Coll); th e secon d is the clasp , or tenaculum (C), of the thir d segment; the thir d is the spring , called the furcula (A, Fur), apparentl y arisin g fro m the fifth abdomina l segment, thoug h its muscle s take thei r origi n in the fourth. The Collophore. —The ventra l tube , o r collophore , is a large , thic k cylindrical pouc h o f th e bod y wal l projectin g ventrall y an d somewha t anteriorly fro m th e sterna l regio n o f th e firs t abdomina l segmen t (Fig . 147 A, Coll). I n mos t species the tub e ends in a bilobed terminal vesicle

THE ABDOMEN

269

(B, v) 9 which is ordinarily retracted but is capable of being protruded by blood pressure . A pai r o f larg e latera l retracto r muscle s (rv), arisin g within the body, traverse th e collophor e to be inserted o n the lobes of the terminal vesicle . Th e structure of the collophor e thus suggests that th e organ i s formed b y th e fusio n o f a pair o f abdominal appendage s resem bling thos e o f the Protura (Fig . 146) , thoug h i n the latte r th e retracto r muscle o f th e vesicl e (TV) i s sai d t o aris e i n th e bas e o f th e appendage , while the appendage itself i s movable by two muscles (I, J) arisin g in the body an d inserte d o n its base . I n som e of the Collembola , a s in Smin thuridae, eac h lobe of the vesicl e i s produced into a long eversible tube . The anterio r surfac e o f th e collophor e present s a media n vertica l

FIG. 147.—Abdomina l appendage s o f a collembolan , Tomocerus vulgaris. A , entir e insect wit h furcula i n flexed position. B , collophore . C , tenaculum. D , furcula .

groove continuous ventrally wit h the depressio n between the lobe s of the terminal vesicle and communicating dorsally with a median channel of th e ventral wal l of the thorax . Anteriorl y the thoraci c channe l is continued upon th e oute r fac e o f the rudimentar y labiu m a s far a s the dista l clef t of the latter, where there open into it the ducts of two pairs of head glands. It ha s bee n suggested b y Wille m and Sabb e (1897 ) tha t th e secretio n of the head glands is conveyed to the collophore through the ventral thoracic channel and , whe n collecte d betwee n th e lobe s o f the termina l vesicle , enables th e tub e t o functio n a s a n adhesiv e organ . Henc e th e nam e collophore ("glu e bearer") - A n elaborat e descriptio n o f th e histolog y of th e collophore , the head glands, and the connectin g channel is given by Hoffmann (1905) , who concurs with Willem and Sabbe as to the function of the tube . A n adhesive function o f the organ , however, has apparentl y not been demonstrated, an d it i s quite possibl e that the channe l between the vesicl e an d th e labiu m migh t conve y liqui d fro m th e forme r t o th e mouth; an d yet , thoug h th e Collembol a inhabi t mois t places , mos t of them d o not ordinaril y com e in contac t wit h water .

270

PRINCIPLES OF INSECT MORPHOLOGY

The Tenaculum. —The tenaculum, or clasp, is a minute organ situate d medially on the concav e ventral surfac e o f the thir d abdomina l segment. It consist s of a conica l base an d o f two laterally divergent dista l prong s toothed o n thei r oute r margin s (Fig . 14 7 C). Eac h pron g i s provide d with an adductor muscle (ad). The tenaculum serves to hold the furcula in place when the latte r is flexed against th e ventral sid e of the body, th e prongs projectin g between the base s o f the furcula r arms . The Furcula.—The furcula i s the leapin g organ of the Collembol a an d is the featur e from whic h the insect s ge t their commo n name of " springtails." Th e furcul a consist s o f a larg e media n base , th e manubrium (Fig. 14 7 D, mn) , an d o f two slende r arms , eac h of which is subdivide d into a long proximal segment, the dens (d) , and a short terminal segment , or muero (ni). O n th e bas e o f th e manubriu m ar e inserte d a pai r o f flexor an d a pai r o f extenso r muscle s arisin g i n th e fourt h abdomina l segment. I n Tomocerus vulgaris eac h o f th e arm s i s provide d wit h a n abductor muscl e (ab) an d a n adducto r muscl e (ad ) havin g thei r origin s in the manubrium . Whe n the furcul a i s flexed in the positio n o f repose (A), its proxima l half i s concealed in a ventral concavit y of the abdomen , and the arms are closed upon the tenaculum, which fits into an oval space between th e base s o f the dente s (D , a ) havin g thin, har d edge s that ar e held b y th e teet h o n th e oute r margin s o f th e tenacula r prong s (C) . The sprin g evidently i s released by th e contractio n o f the adducto r mus cles of the prong s (C, ad). A t the sam e time the furcula r arm s are spread and the entire organ is forcibly extended, throwing the insect upward and forward. The furcul a varie s muc h in lengt h i n differen t specie s of Collembola, and i t i s absent i n the gener a Neanura and Anurida. I n som e species of Sminthurus tha t liv e o n th e surfac e o f wate r th e sprin g ha s a fanlik e structure, th e divergen t arms being fringed wit h long, stiff hairs . Abdominal Appendage s of Thysanura. —The abdomina l appendage s of Thysanur a ar e o f particula r interes t becaus e the y hav e bee n take n as a starting point fo r the study of the structure of the genital appendages of adul t pterygot e insects, an d they appea r als o to retain th e basic struc ture o f the abdomina l appendage s o f pterygote larvae . The thysanura n abdomina l appendages , not includin g the cerci , best preserve their individuality in Machilidae, where they are present on each of th e firs t nin e segments o f the abdome n excep t the first . Eac h o f th e pregenital appendage s i n thi s famil y consist s o f a larg e lateroventra l basal plate, or rather of a flat basal lobe having a wide plate in its ventral wall (Fig . 14 8 B, Cxpd), an d o f a dista l taperin g process , terme d th e stylus (Sty), whic h is freel y movabl e o n th e basis . Th e basa l plate s of each segmen t ar e intercalate d proximall y betwee n th e deflecte d latera l edges o f th e tergu m (Fig . 13 8 A, T ) an d th e smal l triangula r media n

THE ABDOMEN

271

sternum (Stn); the y ar e unite d wit h th e sternum , an d ankylose d wit h each othe r mediall y behin d th e sternum . Eac h plat e i s provide d wit h muscles arising on the tergum and has all the aspects of being the enlarged basis o f an otherwis e rudimentary limb . The abdomina l styli of Thysanura ar e equippe d with muscle s arisin g proximally in the basa l plate s (Fig . 14 8 B, smds). The y woul d appear, therefore, t o b e th e rudimentar y telopodite s o f the abdomina l append ages. I n th e Machilidae , however, similar stylifor m processes occu r on the coxa e of the secon d and thir d thoraci c leg s (A) , which, though the y lack muscles , sugges t b y thei r for m that the y ar e seriall y homologou s with the abdominal styli. I t is possible, therefore, that both the thoracic and abdominal styli are coxal epipodites; they are not "exopodites," as they ar e often suppose d to be, since the tru e exopodite of Crustacea (Fig .

FIG. 148.—Appendage s o f Apterygota . A , metathoraci c le g o f Nesomachilis wit h coxal stylus. B , abdominal appendag e of Nesomachilis bearing a stylus (Sty) an d retractile vesicle (Vs). C , abdominal appendag e o f Heterojapyx wit h bas e (Cxpd) unite d wit h sternum.

44 C) arises fro m th e first trochanter (basipodite) . Th e abdomina l styl i of Thysanura suppor t the abdome n of the insec t in life and play an activ e part i n locomotion . Whateve r ma y be thei r morphologica l nature, th e styli appea r t o b e represente d i n variou s form s o n th e abdomina l seg ments of pterygote larvae and on the mal e genital segment of many adult insects. Styli ar e presen t o n th e posterio r segment s o f som e Thysanur a i n which th e lim b bases ar e fuse d wit h th e sterna , an d the y occu r likewise on lateral lobes of the definitiv e pleurosternal plate s o f Diplura (Fig . 148 C). Ther e ca n b e littl e questio n tha t th e styl i presen t o n th e nint h sternum o f certai n mal e Pterygota , suc h a s Ephemeridae , Termitidae , Blattidae, Grylloblattidae , an d Tettigoniida e (Fig . 13 8 C), ar e homo logues o f th e thysanura n styli , an d i t seem s equall y certai n tha t th e movable genita l clasper s o f mal e holometabolou s insect s ar e organ s equivalent t o th e mor e typica l styl i o f thes e les s specialize d pterygot e insects.

272

PRINCIPLES OF

INSECT MORPHOLOGY

A secon d dista l structur e o f th e pregenita l appendages , presen t i n most o f the Thysanura , ha s th e for m o f a smal l eversibl e and retractil e vesicle (Fig. 148 B, Vs) located mesad of the base of the stylus, and provided wit h stron g retracto r muscle s (rvs) arisin g proximall y o n th e basal plate. I n some species there is a pair of vesicles on each appendage. The functio n o f thes e organ s i s no t known . Thei r structur e suggest s that the y represen t th e termina l vesicle s o f th e pro turan abdomina l appendage s (Fig . 146 , v); bu t th e presence o f tw o sac s o n eac h appendag e i n som e species o f Thysanur a preclude s th e ide a tha t the y may b e rudiments o f the dista l parts of the abdomina l limbs. Th e gill-bearin g tubercles o n the base s o f th e abdominal limb s o f th e larv a o f th e neuroptero n Corydalus an d th e termina l lobe s o f th e abdomina l legs o f caterpillar s hav e a structur e ver y simila r t o that o f th e retractil e vesicle s o f th e Thysanura . The gonopod s o f Machilida e an d Lepismatida e differ fro m th e pr e genita l appendage s i n tha t eac h may b e provide d wit h a gonapophysi s an d alway s lacks a retractil e vesicl e (Fig . 31 3 A , B , C) . Th e gonapophysis (B , C , Gon) i s a slende r process arising from th e mesa l proxima l angl e o f the coxopodit e an d is provide d wit h shor t muscle s arisin g i n th e latte r (gmcl). Its proximal position on the coxopodite shows tha t i t ha s n o relatio n t o th e vesicle s o f th e pregenital appendages and als o does not favor the ide a FlG ^g Larva that the gonapophyses are the telopodites of the geniof Siaiis, showing leg- ta l appendages . Th e fou r gonapophyses form the ovi abdoamenndaSeS °f ^e posito r of the female . Th e first pair i s usually absen t in the male, and in some species neither pai r is present. Abdominal Appendages of Pterygote Larvae.—The larvae of pterygote insects ar e remarkable fo r the variet y o f appendicular organ s they hav e on the abdomen . Morphologist s have not given much attention to these structures becaus e i t ha s bee n suppose d tha t the y ar e specia l develop ments servin g th e need s of/th e larvae ; but ther e i s no question a s to th e origin o f some of them fro m th e limblik e rudiment s o f the embryo , an d nearly al l of them sugges t by thei r structur e an d musculatur e that the y are parts at least of true segmental appendages. The mos t leglike in form o f the larval abdomina l appendages occur in the neuropterou s genus Siaiis and on certain aquatic coleopterous larvae, especially i n th e familie s Dytiscida e an d Gyrinidae . Th e larv a o f Siaiis (Fig . 149 ) has o n each of the firs t seve n segments of the abdome n a pai r o f long , tapering , six-segmente d appendage s projectin g laterall y

THE ABDOMEN

273

from th e side s of the body . Eac h appendag e (Fig . 15 1 D) i s supporte d on a lateral lobe (Cxpd) of the body wall of its segment, and within this lobe ther e aris e muscle s inserted anteriorl y an d posteriorl y o n the bas e of th e movabl e shaf t of the appendage . Mor e than this, there are mus cles within th e proxima l part of the appendag e itself. Th e developmen t of thes e organs in the embry o has not bee n studied, s o far as the write r is aware. Th e abdomina l appendage s o f th e coleopterou s larva e men tioned abov e ar e ver y simila r t o thos e o f Sialis. Th e appendage s in al l cases ar e penetrate d b y trachea e an d ar e suppose d t o functio n a s gills , but thi s assumptio n need s experimental evidence. The well-know n gills o f ephemeri d larva e ar e born e o n lobe s on th e sides of the abdomina l segments (Fig . 15 0 A, B, Cxpd) situate d between

FIG. 150.—Abdomina l appendage s of a n ephemeri d larva. A , diagrammati c cross section of an abdomina l segment, showing pleural lobes (Cxpd) o f the bod y bearing the gills (Brri). B , a gill and it s basi s (Cxpd) i n th e pleura l wall of the body . C , the gil l muscles.

the terga and the sterna, and each gill is provided with muscles inserted on its bas e which arise in the ventra l par t of the supportin g lobe (B, bmcls). There i s littl e doubt , therefore , tha t th e gill s ar e appendicula r part s of abdomina l limbs , o f which the supportin g lobe s ar e th e bases . Th e gill stalk or gill plate, by its positio n o n the basi s an d its basa l muscula ture, suggest s tha t i t i s a homologu e o f th e stylu s o f th e thysanura n abdominal appendages . Th e gil l basis i s very evidentl y th e equivalent of th e stylus-bearin g plate s o f Machilidae, though, since it i s immovable, there ar e no body muscles inserted upo n it. Returning agai n t o th e neuropterou s family Sialidae , w e find in th e genera Chauliodes and Corydalus long tapering appendages on the sides of the firs t eigh t abdomina l segments , an d a termina l pai r (pygopods ) on

274

PRINCIPLES OF INSECT MORPHOLOGY

the tent h segmen t (Fig . 15 1 A, B , C) . Eac h appendag e i s a hollo w process of the integumen t an d i s supported o n a lateral lob e of the bod y ???? ??? ?? ?????? ??? ????????? ????? ???? ?? ???? ???? ??? ???????? subcoxae, an d withi n the m aris e muscle s inserte d o n th e base s o f th e movable parts of the appendage s (F , smcls). I t woul d appear, therefore, that w e hav e her e als o a reduce d an d modifie d lim b consistin g o f th e ?????????? ?????? ??? ? ?????? ???? ?????? ??? ?????? ???????????? ??? ephemerid gill, or the thysanuran stylus. In Corydalus the basis of each of th e firs t seve n pairs o f appendages supports ventrall y a large tubercle bearing a thick tuft o f gill filaments (C, E, Vs). A long muscle (E, F, rvs) arising on the dorsu m of the bod y segment is inserted b y thre e branche s

???? ?????????????? ?????????? ?? ?????? ?? ??????????? ?? ??? ?? ??????? ?? Chauliodes, dorsa l view . B , same, ventral view. C , Corydalus cornutus, segments VII-X and appendages . D , appendag e o f Sialis. E , a n abdomina l segmen t o f Corydalus, pos terior view . F , righ t hal f o f sam e i n sectio n showin g muscle s o f appendage . G , tent h segment appendag e (pygopod ) of Corydalus, right , anteromesa l view . H , claw s of terminal appendage o f Corydalus an d thei r retractor muscle .

in the distal end of the tubercle and evidently serve s to retract th e latter. The gill-bearing tubercles of sialid larvae thus recall, in their structure and musculature, the retractil e vesicle s of the abdomina l limbs of Thysanura , though in the siali d th e tubercle muscles arise on the dorsu m of the bod y and not i n the appendag e bases. The appendage s o f the tent h larva l segment , o r pygopods , i n bot h ?????????? ??? ?????????? ?????? ???? ????? ?? ??? ????????? ???????? ?? that the basi s o f each projects from th e bod y a s a short, fre e cylindrica l lobe (Fig . 15 1 A, B, C , Ppd, G ) bearing the stylu s laterall y (C , G , Sty), and th e tubercl e i s provided wit h tw o larg e curved claw s (d) instea d o f gills. Th e claw s ar e se t o n the fla t dista l en d o f the tubercl e b y long ,

THE ABDOMEN

275

parallel bases , an d th e retracto r muscl e (H , TVS') i s inserte d posteriorl y at th e proxima l ends of the conve x margins of the claws. The larva e o f Trichoptera likewis e have a pai r o f large claw-bearing pygopods on the tenth abdominal segment, though they have no appendages o n th e othe r segment s o f th e abdomen . I n som e form s thes e terminal appendage s are short , eac h consisting o f a decurved distal cla w arising from tw o basal plates implanted on the sid e of the tent h segment (Fig. 152 B, Ppd, C). I n others the appendages are long, freely movable, cylindrical organs , projectin g posteriorly, eac h bearing a large decurved claw on its distal en d (E, F). Neithe r the structure nor the musculature (D, E ) o f th e pygopod s o f trichopterou s larva e i n an y wa y resemble s

FIG. 152.—Abdome n an d appendage s o f larva e o f Trichoptera . A , Platyphylax designatus, metathorax an d bas e o f abdomen. B , end o f abdomen. C , same, left pygopo d of tent h segment . D , same , righ t pygopo d an d muscles . E , Hydropsyche, righ t pygopo d and muscles. F , same , end of abdomen, with pygopods, an d intestinal filament s protrude d from th e anus .

that of the terminal appendage s of the sialid larvae, nor do they have any similarity t o th e termina l leg s of lepidopterous larvae; this fact i s somewhat surprisin g considerin g that i n man y respect s th e Trichopter a an d Lepidoptera appea r t o b e relate d orders . Clawlik e appendage s ar e present o n th e tent h segmen t o f certai n coleopterou s larvae , a s i n th e family Helmidae , bu t th e morphologica l statu s o f suc h structure s i s doubtful. The abdomina l leg s o f lepidopterous larva e ar e sai d b y student s o f embryology t o b e develope d fro m lim b rudiment s i n th e embry o tha t correspond t o th e rudiment s o f the thoraci c legs . The y appear , there fore, t o be true segmental appendages. Mos t caterpillar s hav e five pairs of thes e abdomina l legs (Fig. 15 3 A), four pair s being on segments III t o

276

PRINCIPLES OF

INSECT MORPHOLOGY

??? ?????????? ??? ????? ?? ??????? ?? ??? ??? ?????? ?? ????????? reduced, a s i n th e loopers , an d i n som e form s al l th e appendage s ar e suppressed. A typica l abdomina l le g of a caterpillar consist s o f three part s (Fig . 153 F). Proximall y ther e i s a rin g o f flexible integument (nib)] beyon d this i s a longe r cylindrical sectio n (Cx) formin g th e greate r par t o f th e appendage, an d usuall y havin g a scleroti c plat e i n it s oute r wall , ofte n marked b y a distinctiv e grou p o f seta e (A) ; distall y th e le g end s i n a ?????????? ???? ??? ???? ?????? ??? ??????? ????? ????? ??? ?????? ?? ????? (d). Functionally th e plant a i s the mos t importan t par t o f the abdomina l leg o f th e caterpillar , an d structurall y i t i s th e mos t variable . I n it s more generalize d condition th e plant a i s a shor t circula r pa d (Fig . 15 3 B, Vs ) with a central depression (é) o n which is inserted a group of retractor muscl e fibers (rvs). I n suc h cases the crochet s (d) ma y b e arrange d in a complet e circl e aroun d th e peripher y o f the dista l planta r surface , with their recurve d point s turne d outwar d an d upward . Wit h mos t caterpillars, however , the claw s are limited t o a semicircle or a small ar c usually o n the inne r margi n o f the plant a (C , D), an d i n suc h case s th e ?????? ?????? ???? ????????? ??????? ???????????? ?? ? ????????? ?? obliteration o f its oute r half . Th e plant a the n assume s th e for m o f a lobe projecting to the mesal side of the lim b axis, the latter being marked by the insertio n poin t o f the retracto r muscl e (e), an d th e crochet s curve mesally an d upwar d when the plant a i s protracted i n the usua l positio n (H). Immediately abov e eac h abdomina l le g o f th e caterpilla r ther e i s usually a prominen t lob e o r swellin g o f the bod y wal l (Fig . 15 3 A, F , Scx)j limite d above by a groove marking the dorso-pleura l line (a-a) o f the abdomen. Correspondin g lobes are present on the legless segments of the abdomen, an d also on the thora x abov e the base s of the legs . Th e serie s of suprapeda l lobes , therefore , appear s t o represen t th e latera l part s of the subcoxae on both the thorax and the abdome n (A , Sex). The musculatur e o f an abdomina l le g of a caterpillar consist s o f tw o sets o f fiber s (Fig . 15 3 I), thos e o f one set bein g inserted o n the bas e of the principal part of the le g (Cx), those of the othe r on the dista l surface of the plant a (Vs). Th e planta r muscles , in the specie s figured, consist of four fibers, three of which (5,6) arise in the upper part o f the subcoxa l lobe, while th e fourt h (4 ) arises on the latera l wal l of the bod y segment. Th e insertion o f the othe r muscles (1, 2, 3) on the base of the principa l segment of the le g (Cx) suggests that the latter is the coxa ; the musculatur e of the planta leave s littl e doub t tha t th e plant a i s a structur e equivalen t t o the gill-bearin g tubercle s o f th e neuropterou s larva e abov e describe d (Fig. 15 1 F), an d that i t i s therefore analogous at leas t t o th e retractil e

THE ABDOMEN

277

vesicles o f Thysanura. I n th e caterpillar s most o f the planta r muscle s arise i n th e lim b base , bu t ther e i s alway s a lon g fiber fro m th e bod y wall; th e planta r musculatur e is thus intermediat e between that o f th e vesicles of Thysanura an d th e gil l tubercles of Neuroptera. Representa tives o f styli are not presen t in any lepidopterous larvae. The appendage s of the tent h abdomina l segmen t o f the caterpillar , known a s the anal legs, or postpedes, resembl e the appendage s of the pre -

FIG. 153.—Abdomina l appendage s o f larva e o f Lepidoptera . A , Carpocapsa pornonella. B , same , lef t abdomina l leg, ventra l view . C , Hyphantria cunea, left leg , ventral view. D , Xylina, righ t leg , ventral view . E , diagrammati c section o f caterpillar graspin g a twig . F , Malacosoma americana, left leg , posterio r view . G , diagra m o f positio n o f planta o n a roug h surface . H , sam e o n a smoot h surface . I , Malacosoma americana, right leg and muscles , posterior view .

ceding segments i n structure ; their musculatur e differ s fro m tha t o f th e others in that the basal muscles are largely eliminated, while the muscles of the plant a ar e much larger and include both dorsal and ventral groups of fibers . When a caterpillar wit h lobate plantae cling s to a small twig or plant stem, the abdomina l fee t ar e turned mesall y an d clas p the suppor t wit h the incurve d claw s (Fig . 15 3 E). Th e closur e of eac h pai r o f leg s o n the suppor t must be caused by the contractio n of the media n muscles (1) inserted o n their bases , fo r the planta r muscle s (rvs) evidentl y serv e t o release th e gras p o f th e claws . I f th e caterpilla r walk s o n a fla t bu t

278

PRINCIPLES OF

INSECT MORPHOLOGY

rough surface, th e planta r lobe s are turned outward by their muscle s (G) and their inne r surfaces are applied to the support with the claws directed downward. If , however , th e caterpilla r find s itsel f o n a smooth , har d surface, suc h a s tha t o f glass , th e sole s o f th e planta e ar e presse d fla t against i t (H) , with th e claw s turned upward , and apparentl y a tensio n of th e planta r muscle s convert s th e sof t en d wall s o f the planta e int o vacuum cups by which the caterpilla r maintains it s foothold. The caterpillar s d o not mov e either thei r abdomina l o r their thoraci c appendages i n the wa y that adul t insect s mov e their thoraci c legs . I n

FIG. 154.—Terminal abdominal appendage s an d othe r appendicula r processes o f larvae of Hymenopter a an d Coleóptera . A , Pteronidea ribesii, end o f abdomen wit h pygopods . B, Cimbex americana. C , Cephaleia. D , Oodes helopiodes, en d o f abdome n wit h uro gomphi. (From Kemner, 1918. ) E , Thanatophilus. (From Kemner, 1918. ) F , Dytiscus circumcinctus, end o f abdomen wit h appendicula r processes .

regular forwar d progressio n th e las t pai r o f abdomina l leg s ar e firs t released from the support and brought forward by a contraction and slight humping o f th e posterio r par t o f th e body . The n i n tur n th e othe r abdominal legs are lifted an d advanced in the sam e manner, a s the wav e of bod y contractio n run s forwar d through th e segments . Finally , th e movement affect s th e thoraci c segment s an d thei r appendages . Thu s the crawlin g caterpilla r move s forwar d wit h eac h successiv e wav e o f contraction tha t runs through its body. I f the caterpilla r i s a "looper," the posterio r grou p o f appendage-bearin g segment s i s brought forwar d together an d th e bod y i s stretched ou t fo r a ne w grasp b y th e thoraci c legs. The larva e o f th e sawflie s (Tenthredinida e an d relate d families ) resemble caterpillar s i n th e possessio n o f appendage s o n th e abdomen , which are similar t o those of the caterpillar s bu t no t s o highly organized. The appendage s o f the tent h segment, however, differ i n differen t forms . Those o f species living in the ope n (Fig . 15 4 A, B) are much the sam e as the ana l leg s o f lepidopterou s larva e an d ar e adapte d t o graspin g th e

THE ABDOMEN

279

edges o f leaves; species that bore into the stem s o f plants o r that liv e in the protectio n o f web nests or curled leaves, however, such as the Cephi dae an d Pamphiliidae , hav e slender , jointe d appendage s o n th e tent h segment (C , Ppd). Abdomina l appendage s simila r i n appearanc e t o those of lepidopterous and tenthredini d larvae occur also on some larvae of Coleóptera . Finally, we may mention , in connection with the stud y of abdominal appendages, certai n fixe d o r mobil e processe s foun d o n th e termina l segments of certain larvae. Suc h structures are of frequent occurrenc e on the dorsu m o f the nint h segmen t i n th e larva e o f Coleópter a an d hav e been variousl y calle d styli , cerci , pseudocerci , an d corniculi , bu t th e term urogomphi (Bovin g an d Craighead , 1932 ) i s mor e specifi c an d descriptive. Th e urogomph i var y muc h i n siz e an d shap e fro m shor t spine-like points to long, thick processes or multiarticulate filaments, and they ar e sometime s distinctl y jointe d (Fig . 15 4 D, E , ug). I n som e species they are fixed outgrowths of the posterior end of the ninth tergum; in other s the y aris e fro m th e membran e behind the terga l plat e an d ar e then flexible at thei r bases. Evidentl y the urogomph i are simply cutic ular outgrowths of the dorsu m of the ninth segment having no relation t o segmental appendages. Th e terminal appendages of the larva of Dytiscus (F, ug?), however , ar e o f a mor e problematical nature ; the y appea r t o belong to the ninth segmen t and are provided with muscles arising on the tergum o f the eight h segmen t (Speyer , 1922 ; Korschelt, 1924) ; but i t i s possible that these appendage s also are urogomphi, and that the muscles that mov e them ar e the intersegmenta l muscles between the eight h an d ninth segments . The larvae of some chalastogastrous Hymenoptera have a pair of small processes arising on the tergum of the tenth segment (Fig . 15 4 A, a) which have sometime s bee n regarde d a s rudimentar y cerci , bu t whic h ar e evidently mer e cuticula r processe s comparabl e wit h th e urogomph i of coleopterous larvae . I n certai n form s ther e i s onl y a singl e media n process (C, 6). The cauda l horn of sphingid caterpillar s is an analogous structure.

CHAPTER XI I THE ORGAN S OF INGESTIÓ N The organ s primaril y concerne d with th e intak e o f foo d ar e th e gnathal appendage s an d lobes of the hea d surrounding the ora l apertur e of th e alimentar y canal , know n collectively a s th e mout h parts . Bu t the mout h parts do not constitut e th e entir e apparatu s o f ingestión, for when th e foo d ha s been delivered into th e mout h cavit y i t mus t ye t b e passed o n t o th e sectio n o f the alimentar y cana l where digestion take s place. Th e anterio r par t o f the stomodaeum , then, i s always an impor tant par t o f the ingestiv e system . I n th e suckin g insects, whos e foo d consists mainly of plant an d anima l juices, the pumpin g apparatus asso ciated with the mout h is principally a highly specialized development of the preoral cibarium; but, in its anatomical continuity, the pump Becomes virtually a par t o f th e alimentar y trac t an d i s usuall y calle d th e "pharynx." I n order to understand the true morphology of the ingestive organs, therefore, it wil l be necessary to refe r back , on the on e hand, t o the content s o f Chap . VI I fo r th e basi c structur e o f the mout h parts , and, o n the other, to anticipate somethin g from the subject matter of the following chapte r o n the alimentar y canal . So divers e i n for m ar e th e feedin g organ s i n th e variou s group s of insects specialize d for obtainin g particula r kind s of food tha t th e stud y of the mout h parts becomes a major subjec t in any course in entomology. A good system fo r classifyin g th e leadin g types o f mouth-part structure , therefore, wil l be of much assistance i n understandin g th e variou s func tional adaptation s o f the organs, and the student i s referred to the tabula tion o f insect mout h part s o n a functiona l basis give n b y Metcalf an d Flint (1928) , an d mor e full y elaborate d b y Metcal f (1929) . Fo r mor phological purposes, however, the mout h parts cannot b e studied fro m a physiological standpoint, sinc e very different type s of structure ar e ofte n adapted t o simila r mode s o f feeding , an d grea t discrepancie s in bot h structure an d functio n hav e bee n independentl y evolve d i n adul t an d larval forms of the same orders. Hence , in the following discussion of the more specialized feeding organ s of insects, th e leadin g types o f structur e will be described as they occu r in the ordina l groups. Since all the more generalized forms of modern insects have the mouth parts constructe d fo r feedin g o n so-calle d soli d substances , tha t is , o n the whol e tissues o f plant s an d animal s rathe r tha n o n thei r juice s or 280

??? ?????? ?? ????????? ???

liquid products , ther e i s no questio n tha t th e "orthopteroid, " o r bitin g and chewing , type o f mouth parts is the on e from whic h the other type s have bee n derived , a s th e mor e specialize d form s clearl y sho w i n mos t cases by their ow n structure and development . Th e fundamental structure o f mouth part s of the bitin g type , havin g bee n full y describe d an d illustrated i n Chap. VII, need be given little attention in the present chap ter, wherei n will be discusse d th e mor e important modification s characteristic of the principa l orders . 1. TH E PREORA L CAVIT Y

Since the mout h parts of insects are closely assembled in their attach ments o n th e head , the y enclos e between the m a spac e whic h is ofte n called th e "mout h cavity, " and whic h functionally deserve s thi s name; but inasmuc h as thi s regio n lies entirel y outsid e th e ora l aperture , i t i s more appropriately terme d th e preoral cavity (Fig . 155 , Pro). I n a strict sense, o f course , it i s not a cavit y a t al l bu t merel y a n externa l space bounded anteriorly b y the epipharyngea l wall of the labrum and clypeus, posteriorly by the labium , an d laterally by the mandible s an d maxillae. ?????? ??? ??????? ?????? ???? ??? ??????????? ??????????? ??????? which morphologically is a median lobe of the ventra l wall of the gnatha l region of the head . Th e tru e mouth of the insec t i s the anterio r opening of the stomodaeum (MtJi), which is located in the ventral wall of the head (in hypognathou s insects ) immediatel y behin d th e clypeu s an d i n fron t of th e hypopharynx , wher e it i s normall y conceale d between th e base s of th e mandibles . Correspondingl y situate d a t th e posterio r en d o f th e hypopharynx, between the latte r an d th e bas e o f the prementum , i s the ??????? ?? ??? ???????? ???? ?????? In th e orthopteroi d insect s th e preora l cavit y i s largely occupie d by the hypopharynx (Figs. 60 A, 155, Hphy). Anteriorly, however, there is an open food meatus (fm) betwee n the hypopharyn x and the epipharyn geal wall of the labru m and clypeus , which leads up to th e mout h (Mth) ; and posteriorl y ther e i s a broa d salivary meatus (sm) between the hypo pharynx an d th e labium , a t th e inne r en d of which is the openin g o f th e salivary duct (SIO). The food passage is closed laterally by the mandibles, an d its upper or inner part, lying proximal to the molar surfaces of the close d jaws, forms the preoral food chambe r here named the cibarium (Cb). Th e salivar y channe l (sm) terminates i n th e salivar y pocket , o r salivarium (Slv) y between the base of the hypopharynx and the base of the labia l prementum . Th e cibariu m and the salivariu m ar e importan t elements i n th e feedin g mechanis m o f nearl y al l insects ; i n th e highe r orders the y ar e variousl y modifie d t o for m specialize d organ s fo r th e ingestión o f liqui d food an d fo r the ejectio n of saliva or other products of the labial glands .

282

PRINCIPLES OF INSECT MORPHOLOGY

The Cibarium.—Th e cibarium of generalized insects (Fig . 155, Cb) i s a part of the intergnathal preoral cavity (PrC) of the feeding apparatus. Morphologically i t lie s outsid e th e tru e mout h (Mth); bu t functionall y it i s the "mout h cavity " o f the insec t an d is so defined (Mundhóhle) b y Weber (1933) . It s concav e floor is formed b y th e adora l surfac e o f th e base of the hypopharynx, flanked by the suspensorial sclerites of the latter (Figs. 60, 155, HS); it s roof , or anterior wall , is the epipharyngea l surfac e of the clypeus . Th e cibarium , in chewing insects, serves as a chamber in which the foo d material , pushe d upward through the foo d meatu s by th e adduction of the jaws , is held at th e base of the hypopharynx preparatory to bein g passe d int o th e mouth . I f a partiall y narcotize d cockroac h is offered a bit o f moistened bread, a particle is seized between the man dibles; after a few movements of the jaw s the particle may be seen neatly stowed in the cibaria l pocket at th e base of the hypopharynx, from whic h it presentl y disappear s int o th e mouth . Durin g feeding , a copiou s flo w of saliv a issue s fro m th e salivar y channe l o n th e labiu m an d flood s th e tips of the mout h parts. On th e inne r surfac e o f th e epipharyngea l wal l o f th e cibariu m i s inserted a pair of dilator muscles (Figs. 60 A, 155, dlcb) takin g thei r origin on th e clypeus . Th e cibariu m is compresse d by th e contractio n o f th e retractor muscles of the mout h angle s (rao) , which arise on the iron s an d are inserte d o n th e ora l branche s o f the suspensoria l bar s o f the hypo pharynx (HS). Th e contractio n o f thes e muscle s accompanyin g th e adduction o f the mandible s pull s the hypopharyn x forward an d upward, and i t i s this movemen t o f the hypopharyn x apparentl y tha t force s th e food fro m th e cibaria l chambe r through th e mouth into the buccal region of th e stomodaeum , whenc e it i s carrie d alon g b y th e peristalsi s o f th e stomodaeal wall . Th e opposit e movemen t o f th e hypopharyn x i s pro duced by th e contractio n o f the retracto r muscle s (Figs. 60 A, 84 B, 155 , rhphy) arisin g on the tentoriu m an d inserted on the latera l sclerite s (w) of the hypopharyngeal base . In mos t o f the suckin g insects, particularly in Dytiscidae, Thysanop tera, Hemiptera , an d Diptera , th e cibariu m undergoe s a remarkabl e transformation b y whic h i t i s converte d int o th e sucking pump o f th e feeding mechanism . B y a n extensio n an d closur e of th e latera l lip s of the tru e mout h aperture , th e cibariu m become s a chambe r partl y o r entirely enclose d within the hea d cavity; its dista l opening into the foo d meatus (Fig . 155, fm) i s then the functional mouth. Th e dilator muscles of the cibaria l pum p ar e alway s th e epipharyngea l muscle s arising o n th e clypeus (dlcb). O n the other hand, in Lepidoptera and Hymenoptera the sucking pum p include s th e pharynx , an d it s dilato r muscle s aris e o n the clypeal, frontal, an d postfrontal regions of the cranium. Th e relatio n of th e muscle s t o th e part s o f th e ingestiv e trac t wil l b e show n i n a following section .

THE ORGANS OF INGESTIÓN

283

The Salivarium.—I n it s simples t for m th e salivariu m i s merel y th e pocket where the posterior or ventral wall of the hypopharynx is reflected into th e anterio r o r dorsa l wall of the labia l prementu m (Fig . 155 , Slv), into which opens the duct of the salivary glands (SID). On its dorsal wall is inserted a pair of dorsal salivary muscles (Is) takin g their origin s on the suspensorial sclerites (HS) of the hypopharynx or on the lateral walls of the hypopharyn x when these sclerites are absent. O n its ventral wall ar e inserte d th e salivar y muscle s of the labium , usually tw o pair s (2s, 3s) arising in th e prementum .

FIG. 155.—Sectiona l diagram of the hea d o f an orthopteroi d insec t showin g the general ized stomodaea l an d hypopharyngea l musculature . Br , brain ; BuC, bucca l cavity ; Cb, Cibarium ; dp, clypeus ; cpZr , compresso r labri ; Cr , crop ; dlbc, dilato r buccalis ; dlcb, dilator cibarii ; Idlphy, 2dlphy, firs t an d secon d dilatores pharyngium; dlpphy, dilato r post pharyngialis; fm, foo d meatus ; Fr; irons ; FrGng, fronta l ganglion ; Hphy, hypopharynx ; HS, hypopharyngea l suspensorium; Lm, labrum; Mth, mouth ; Oe , oesophagus; Phy, pharynx; Pmt, postmentum ; PPhy, posteriorpharynx ; PrC, preoral (mouth ) cavity ; Prmt, pre mentum; rao, retractor angul i oris ; rhphy, retracto r hypopharyngis ; I s , 2s, 3s, muscles of salivarium; SID, salivar y duct ; SIO, salivar y orifice ; Slv, salivarium ; sm, salivary meatus ; SoeGng, suboesophagea l ganglion ; Tnt, tentorium ; w, basal sclerit e o f hypopharynx .

The primitiv e for m o f th e salivariu m i s wel l show n in som e o f th e Orthoptera. I n the Acrididae, for example, the organ is a simple salivary cup o n the bas e of the prementum , into whic h fits a prominent knob on the bas e o f th e hypopharynx . I n th e manti s th e pocke t i s produced into a long, flat, triangular pouc h (Fig. 84 D, Slv) with the orific e o f th e salivary duct at its apex. Th e lateral margins of the pouch are strengthened b y tw o weakly sclerotic bars (w ) connected distally with the basa l angles of the hypopharynx . O n these bars ar e inserted th e tw o pair s of salivary muscle s from th e labiu m (2s , 3s). Th e dorsal , or hypopharyngeal, wall of the pouc h is somewhat concave (E), with a median fold o n which is inserted a pair of wide dilator muscle s (D, E, Is ) tha t converge from th e latera l wall s o f th e hypopharynx . A simila r structur e i s

284

PRINCIPLES OF

INSECT MORPHOLOGY

described b y Walke r (1931 ) i n Grylloblatta, includin g the thre e pair s of muscles. I n Gryllus th e salivariu m i s narrowe d t o a shor t rigi d tub e (Fig. 84 C, Slv), with both the hypopharyngeal and labial salivary muscles inserted upo n it . Th e basa l bar s o f the hypopharyn x (w) diverg e fro m the mout h of the tub e int o th e latera l walls of the hypopharynx , where, as i n mos t Orthopter a (Fig . 60) , they giv e attachmen t t o th e tentoria l retractor muscle s of the hypopharyn x (rhphy). The salivar y ejectio n apparatu s become s highl y develope d i n th e larvae of Lepidoptera as the silk press (Fig . 165 , Pr). I n th e caterpillar , however, th e hypopharyn x an d th e prementu m ar e unite d i n a media n lobe supporte d b y th e postmentu m an d th e maxillar y stipites , o n th e extremity o f which the duc t o f the sil k gland s open s throug h a hollo w spine, th e spinneret (Sr). I t i s evident, therefore , that th e sil k pres s i s the salivariu m enclose d by the complet e unio n o f the hypopharyn x wit h the prementum . Bot h dorsa l an d ventra l salivar y muscle s are presen t in th e caterpilla r (Fig . 16 5 E) a s in Orthoptera. I n th e highe r Hymenoptera th e salivar y duc t terminates i n a cylindrical pouch opening above the dista l part of the prementu m (Fig. 16 3 C, Syr) just before the bas e of the rudimentary hypopharynx (Hphy). This pouch, known as the salivary syringe , ha s tw o pair s o f muscles inserted o n it. I n th e hone y bee one pair arise s on the hypopharyngea l region covering the ora l surface of the prementum , bu t i n Xylocopa thes e muscle s (Fig . 16 3 C, Is ) hav e migrated t o th e wal l o f th e prementum . Th e othe r muscle s (3s) aris e ventrally in the prementu m an d ar e inserted o n the side s of the syringe . The salivar y syring e o f Diptera (Fig . 17 2 D, Syr) an d o f Hemipter a (Fig. 179 , Syr) i s evidentl y als o a derivativ e o f th e salivarium , thoug h in thes e order s i t ha s a termina l outle t duc t (sm) tha t traverse s th e hypopharynx an d open s o n th e ti p o f thi s organ . Th e ventra l labia l muscles are absent i n both cases , but th e dorsa l dilators ar e present. I n the Hemipter a th e dilato r muscle s (dlsyr) aris e on the inne r face s o f long ????? ?????? ?? ??? ??????????? ????? ?? ?? ????? ?? ??? ??????? ???? take their origin on the posterio r wall of the suckin g pump of the feedin g apparatus (Fig . 172 D), but th e pump chamber is evidently the cibarium , the floor of which is formed b y th e basa l part of the hypopharynx. 2. TH E CEPHALI C STOMODAEM

The stomodaeu m i n it s generalize d form i s a simpl e tub e extendin g from th e mout h t o th e mesentero n (Fig . 189 , Stom). I n mos t insects , however, i t i s differentiate d into severa l mor e o r les s distinc t region s distinguished b y variation s i n the diamete r o f the tube , accompanie d by differences i n th e intim a an d i n th e muscula r sheat h (Fig . 190 , Stom). The stomodaea l region s ar e structura l adaptation s t o functiona l differ ences i n variou s section s o f th e tube , an d the y ar e no t strictl y homol ogous in all insects .

??? ?????? ?? ????????????

The par t o f th e stomodaeu m containe d i n th e hea d lie s abov e th e transverse ba r o f th e tentoriu m an d passe s int o th e thora x throug h the upper part of the foramen magnum (Fig. 155). I t i s embraced by the ????? ??????????? ???? ??? ????? ???? ?? ??? ?????????????? ???????? (SoeGng); the frontal ganglion (FrGng) lies on its dorsal wall anterior to the brain . Th e firs t par t o f the stomodaeu m lie s immediatel y withi n the mouth and may be termed the buccal cavity (BuC). Following the buccal cavit y i s the regio n of the pharynx (Phy) } usually apparent as a dilatation o f th e stomodaeu m betwee n th e fronta l ganglio n an d th e cerebral nerv e connectives . Posterio r t o th e brai n th e stomodaeu m may tak e th e for m o f a simpl e oesophagea l tube , bu t i n Orthoptera , Coleóptera, an d som e other insect s i t i s here differentiated int o a second ?????????? ??????? ?? ????????? ??????? ??????? ??? ??????????? ??????? ???? ???? ?? ????????????? ?? ??? ???????? ??????? ????????? 1925). Followin g th e posterio r pharyn x ther e ma y b e a n oesophagus (Oe), whic h generally enlarges into th e crop, o r ingluvies (Cr). All parts of the head stomodaeum, as well as the preoral epipharyngeal surface o f th e clypeu s an d labrum , ar e provide d wit h dilato r muscle s arising on the hea d walls and on the tentoriu m (Fig . 155) . Th e number of thes e muscles is not th e sam e in all insects, but thos e that arise on the head wall maintain definit e relations in their points of origin and insertion . They ar e therefor e o f muc h valu e fo r determinin g homologie s both i n the crania l areas of their attachments , an d in the parts o n which they are inserted. Th e dorsa l serie s o f these muscle s is consistentl y divide d b y the fronta l ganglio n connective s into a n anterio r se t o f muscles arisin g on the clypeu s and labrum, and a posterior set arising on the fronta l an d parietal areas of the cranium . Th e following anterior an d dorsal muscles are regularly present i n orthopteroid insects , an d representativ e muscle s recur i n most o f the othe r orders . ??????????? ????? ????? ???? ???????? ????? ?? ?????? ?????? ??? labrum, attache d on its anterio r an d posterior surfaces . Dilatores cibarii (dlcb). —A pai r o f muscles within the clypeus , arising on it s anterio r wal l an d inserte d o n th e epipharyngea l surfac e o f th e cibarium. Thes e muscle s become the principa l dilator s o f the suckin g pump in Dytiscidae, Thysanoptera , Hemiptera , and Diptera . Dilatores buccales (dlbc). —A pai r o f muscle s arisin g o n th e clypeu s and inserte d o n the stomodaeu m just within the mouth . The foregoin g muscle s li e anterior t o th e nerv e connective s o f th e frontal ganglio n (FrGng) ; th e followin g ar e inserte d posterior t o th e connectives. Retractores angulorum oris (rao). —A pai r o f larg e muscle s arisin g dorsally o n the irons , inserte d o n th e ora l branche s o f the suspensoria l sclerites o f the hypopharynx .

286

PRINCIPLES OF INSECT MORPHOLOGY

Dilatores pharyngis frontales (Idlphy). —One o r mor e pair s o f slender muscles arising on the irons, inserted o n the anterio r par t of the pharynx . Dilatores pharyngis postfrontales (2dlphy). —One o r mor e pair s o f muscles arising o n the postfronta l region of the cranium , inserted o n th e pharynx befor e th e brain . Dilatores postpharyngeales (dlpphy). —One o r mor e pair s o f muscle s arising on the vertex , inserted o n the stomodaeu m behin d the brain . Besides these muscles there are also lateral an d ventral dilator s o f the stomodaeum arising on the head walls and on the tentdrium, but the y ar e not s o constant a s the dorsa l muscles , an d thei r diagnosti c valu e i s les s important. From th e foregoin g revie w we should not e particularly th e followin g points: (1) The muscles of the clypeu s are distributed to the cibariu m and to the bucca l cavity; (2 ) the fronta l ganglion lies over the anterio r en d of the pharynx , an d it s connective s g o anterior t o th e retracto r muscle s of the mouth angle s and the first pharyngeal dilators; (3) the dorsal dilators of the pharyn x arise on the fronta l an d postfrontal regions of the cranium . 3. TH E FEEDIN G MECHANIS M O F NEUROPTER A AN D COLEÓPTER A

The feedin g organs of Neuroptera and Coleópter a ar e in general of the orthopteroid typ e o f structure, bu t i n som e members o f each order the y are specially modified fo r other purposes than those of biting and chewing, such a s thos e o f grasping, injecting , an d sucking . I n certai n features , particularly i n th e structur e o f the labium , a n interestin g interrelation ship i s foun d betwee n larva l an d adul t forms . Th e labiu m o f adul t Coleóptera, fo r example , i s a three-par t structur e resemblin g tha t o f many Orthopter a i n tha t th e postlabiu m contain s a distinc t mentu m and a submentum . I n th e Neuroptera , however , a tru e mentu m i s apparently neve r present , an d som e larval Coleópter a resembl e Neurop tera in the structur e o f the labium , while others hav e a labium lik e that of th e adult s of their ow n order. The Mandibles.—Th e jaw s are th e mos t importan t member s o f th e feeding organ s i n bitin g insects , an d i n th e Neuropter a an d Coleópter a they usuall y preserv e th e orthopteroi d structure . Wit h phytophagou s species there i s generally a well-marked differentiatio n i n eac h mandibl e between a dista l inciso r lob e with cuttin g edge s (Fig . 15 6 A, in) an d a basal mola r lob e (mol) provided wit h a n irregula r masticator y surface . In predaciou s species, however, the graspin g function o f the jaw s is more important tha n tha t o f chewing, and i n suc h specie s th e mandible s ar e usually simpl e bitin g organ s wit h stron g inciso r point s (B , E) , whic h may b e notche d o r toothed , bu t i n whic h effectiv e mola r surface s ar e generally absent. I n som e forms with greatly enlarge d jaws, as the mal e stage beetles (Lucanidae) , the huge mandibles have no function i n connec-

THE ORGANS OF INGESTIÓN

287

tion wit h feedin g an d ar e use d fo r holdin g th e femal e a t th e tim e of mating. Among bot h th e Neuropter a an d th e Coleóptera , predaciou s larva e of certain species feed only on the juices or liquefied body contents of their prey a s they hold the latte r in their jaws , and som e of these larvae , b y a special modificatio n o f th e feedin g mechanism , becom e tru e suckin g insects. The mos t familia r insects having the grasping-suckin g type of mouth parts ar e th e larva e o f th e divin g wate r beetles , Dytiscus an d relate d genera. Th e mandible s o f th e Dytiscus larv a ar e long , curve d fang s mol

FIG. 156.—Mandible s o f Coleópter a an d Neuroptera , an d suckin g apparatu s o f th e larva of Dytiscus. A, mandibles of scarabaeid larva, posterior view. B, right mandible of adul t Chrysopa. C , lef t mandibl e of Dytiscus larva, dorsa l view, showing inner canal . D, sectio n o f hea d o f Dytiscus larva showin g cibarial (Cb) an d pharyngea l (Phy) pumps . (From Burgess, 1883. ) E , mandible s of adult Pterosticus, posterior view , showing grooves.

(Fig. 156 C, Md) hinged to the anterior lateral angles of the head by dorsal an d ventral articulation s s o that they work in a horizontal plane . Each mandibl e i s traverse d b y a tubula r cana l (c) , reall y a groov e o n the inne r fac e o f the appendag e wit h confluen t edges , openin g distall y near the tip (x) and proximally near the base of the jaw (y). The labrum (D, Lm) i s sharply deflecte d agains t th e bas e of the labiu m (L6) , where a marginal ridge of the forme r i s securely held in a transverse groov e of th e labial surface . Jus t behin d th e closur e thu s forme d i s a transvers e preoral chamber (Cb), whic h is evidently the cibariu m (Fig . 155 , C&) , it s floor being the dorsal surface of the hypopharynx (Hphy). The lateral extremities o f the cibaria l chamber, o r "mout h cavity, " exten d t o th e bases o f the mandibles , where, on each side, there i s a small apertur e t o the exterior . Whe n the mandible s ar e flexed, the proxima l openings of

288

PRINCIPLES OF INSECT MORPHOLOGY

their canal s come into contact wit h the latera l aperture s of the cibarium and thus establish continuous passages into the latter from th e tips of the fangs. Th e dorsa l wal l of the cibariu m i s provided wit h stron g dilato r muscles (Fig . 15 6 D , dlcti) arisin g o n th e clypea l regio n (Clp) o f th e frontoclypeal plat e of the craniu m (C) . B y th e actio n o f these muscles the close d cibarial chamber becomes a preoral pump. Th e true mouth of the Dytiscus larv a (D , Mih) lie s i n th e posterio r wal l o f the cibarium . It leads into a large, strongly musculated anterior pharynx (Phy), which also apparentl y i s a part o f the pumpin g apparatus . When the Dytiscus larv a close s its mandibles i n the bod y of its prey , a poisonou s an d digestiv e flui d discharge d fro m th e stomac h i s ejected from th e cibariu m through the mandibular canals, which, as described by Blunck (1916a) , spread s quickl y throug h th e bod y o f th e victi m an d rapidly dissolve s the softe r tissues . Th e liquefied materia l i s then, b y a reversal i n the actio n o f the pumpin g mechanism, sucke d back int o th e pharynx an d passe d o n t o th e stomach . Detaile d description s o f th e feeding apparatu s an d th e metho d o f feedin g o f the Dytiscus larva ar e given b y Burges s (1883) , Rungius (1911) , Blunc k (1916a , 1918) , Kor schelt (1924) , and Webe r (1933) . The occurrenc e of grooves on the mandible s is not unusua l in Coleóptera (Fig . 15 6 E, g) ; an d othe r predaciou s species have taken advantag e of their presence in much the same way as has the Dytiscus larva. I n th e larvae o f certain Lampyridae, fo r example, the mandible s are perforated by channel s openin g at thei r bases , throug h whic h a liqui d i s injected into th e bod y o f the prey . Thi s liquid , accordin g to Bugnio n (1929a) , comes fro m th e stomac h an d convert s the tissue s o f the recipien t int o a " bouillon nutritif"; but i n the cas e of the lampyrids, Bugnio n observes, ingestión take s plac e directly throug h th e mout h an d not b y way of th e mandibular canals . I n som e othe r lampyri d larva e th e mandible s ar e simply grooved , but th e groove s are converted into tube s b y long acces sory lobe s applie d agains t them . I n stil l othe r specie s th e accessor y lobes are short and the mandibular grooves are open canals. Th e sucking apparatus o f th e lampyri d larvae , a s illustrate d b y Bugnio n (1929a , Fig. 21) , appear s t o b e principally, a s in the Dytiscus larva, th e cibaria l chamber o f the preora l cavity , wit h it s dorsa l dilato r muscle s arising on the clypea l area o f the hea d wall. A grasping-sucking feeding mechanism occurs also in many predacious larvae o f the Neuropter a that have long, fanglike jaws . Th e mandible s of suc h species are deeply grooved on their ventral surfaces , bu t her e th e closing lobes are long blades of the maxillae, which fit into the mandibula r grooves an d thu s for m tubula r channel s betwee n th e tw o appendages , through whic h th e larv a suck s ou t th e juice s of its victims . Familia r examples o f neuropterou s larva e thu s equippe d ar e th e aphislion s an d

THE ORGANS OF INGESTIÓN

289

antlions. Lozinsk i (1908) , in his study o f the latter , describes a group of glandular cell s in th e wal l of each maxillary blade , whic h discharge int o a cuticula r cana l openin g a t th e ti p o f the organ . Th e secretio n fro m these cells , he believes, is poisonous and accounts for the eas e with which the larv a overcome s a struggling ant hel d in its fangs . The Maxillae.—I t i s seldo m that an y difficult y i s encountere d i n a study o f th e maxill a i n adul t Neuropter a an d Coleóptera , sinc e th e appendage usuall y preserve s th e typica l generalize d form an d muscula ture (Fig . 15 7 A, B). Th e posterior surface s o f both the card o and stipes may b e conspicuousl y marked by th e line s of internal ridges , which give

FIG. 157.—Maxilla e of Coleóptera and Neuroptera . A , Chrysopa adult . B , Pterosticus adult. C , scarabaeid larva. D , carabi d larva, Scarites.

them a "divided" appearance, but eac h part preserves its unity, an d the stipes may alway s be identified as such by the origi n of the muscle s of the palpi and terminal lobe s within it (A) . Th e galea sometimes appear s t o be two segmented, bu t th e tru e galea is to be determined by the poin t of insertion of its flexor muscles (A, fga). In larval forms the maxilla often suffers a reduction , especiall y i n it s appendicula r part s (C , D) , an d i n such case s i t i s onl y b y a comparativ e stud y o f serially relate d specie s that th e persistin g lobe s can be identifie d (se e Blunck, 1918 ; Korschelt , 1924). The Labium.—It is in the study of the labium that students of Neuroptera and Coleóptera find themselves most often confronte d wit h problems concerning th e identitie s o f th e parts , an d wit h difficultie s i n makin g satisfactory comparison s betwee n divergen t forms . Discrepancie s o f interpretation ar e i n par t merel y th e us e o f common terms i n differen t senses by different writers , but i n a larger measure they are the result of a failure t o determin e th e fundamenta l morphology o f the labium , whic h in most case s is readily disclose d by a study of the labia l musculature . Neuroptera.—The part s o f the labiu m i n Neuropter a ar e likel y to b e misinterpreted because , in both larval and adult forms (Figs . 68, B, 82 B, 158 B), the labium contains a middle plate (c) , which at first sight appear s to b e a menta l sclerite . A n examinatio n o f th e labia l musculature ,

290

PRINCIPLES OF INSECT MORPHOLOGY

however, reveals that the media n muscles (rst) are inserted on this plate , which is thus shown to belong to the prementum. The tentorial adducto r muscles o f the labiu m ar e inserte d o n the dista l par t o f the prementum . The postmentum varies in size, but it consists of only one sclerite (Pmt). The labium of Neuroptera, therefore, is characterized by a differentiatio n of th e prementa l sclerotizatio n int o a distal plate , o r plates, bearin g th e insertions o f th e tentoria l adducto r muscle s o f th e labium , an d int o a proximal plat e givin g insertio n t o th e media n retracto r muscles . Th e postlabial sclerotizatio n i s not divided int o a mentum and a submentum , as in adult Coleóptera , bu t ther e may be a wide membranous area dista l to the singl e postlabial píate . The labiu m o f a n adul t myrmeleoni d (Fig . 8 2 B ) show s wel l th e typical structure o f the neuropterous labium. Th e postlabium contains a large proxima l sclerit e (Pm¿) , bu t it s dista l par t i s membranou s an d i s traversed by the retractor muscles (rsi) extending from the postmentum to th e proxima l sclerite (c ) of the prementum . Th e sclerotizatio n o f th e prementum i s differentiate d int o a pai r o f anterio r sclerite s (ab) giving insertion t o th e tentoria l adductor s (adlb), an d int o th e larg e proxima l plate (c ) on which the media n retractors (rst) are inserted . Eac h o f th e anterior sclerite s (ab) i s expanded on the latera l wall of the prementu m in a triangular plat e supporting the hypopharynx. Th e labium of Chrysopa (Fig. 15 8 B) is structurally th e sam e as that of the myrmeleonid , but th e postmental plate (Pmt) is very long, and the proximal sclerite (c) of the prementum (Prmt) is a narrow transverse bar giving attachment to the retractor muscles (rst). The distal sclerotization of the prementum (06), on which are inserted th e tentoria l adductor s of the labiu m (adlb), i s continuous wit h that o f the broa d spatulate lígula (Lig). In th e Sialida e th e prementu m i s relativel y large . I n th e larv a of Corydalus (Fig . 6 8 B, Prmt) it s sclerotizatio n include s a dista l plate (ab) supporting th e palp i an d givin g attachmen t t o th e tentoria l adducto r muscles, and a pair o f proximal plates (c , c) on which the media n muscles (rst) ar e inserted . Th e postmentu m (Pmt) i s broa d bu t shor t an d i s continuous proximall y wit h a well-develope d gula (Gu) posterio r t o th e tentorial pits (pt). Larval Coleóptera. —The labiu m o f man y coleopterou s larva e ha s a structure very similar to that of the labium of larval and adult Neuroptera . In a silphi d larva , fo r example (Fig. 15 8 A), the prementu m (Prmt) con tains tw o principa l sclerite s (a , c) , o n th e proxima l one o f which (c ) ar e inserted the median retractor muscles (rst), and on the distal one (a) the dorsa l adductors . A pair o f ver y smal l intermediate sclerite s (b ) is here present , however , whic h giv e insertio n t o th e ventra l adductor s (2adlb). The proximal premental sclerite of coleopterous larvae (c) is commonly mistaken for the mentum, but th e attachmen t o f the retracto r

THE ORGANS OF INGESTIÓN

291

muscles (rsf) o n its base shows clearly that it i s not th e homologu e of th e mentum o f a n adul t beetl e (C , Mf), whic h alway s lie s proxima l t o th e insertions of the median muscles (D). Th e plate in question, on the other hand, corresponds exactly to the proximal premental sclerite in the labium of Chrysopa (Fig. 158 B, c). The basal region of the silphid labium, lying proximal t o th e labia l sutur e (A , Ibs), contain s a well-develope d post mental plat e (Smf), whic h her e evidently corresponds to th e submentu m of th e adult , sinc e ther e i s a distinc t thoug h weakl y sclerotize d are a distal to it (Mf) in the position of the mentum of an adult beetle (C, Mí). In man y coleopterou s larvae , however , th e mentu m i s eithe r entirel y

FIG. 158.—Variou s types o f labia l structure. A , B , C , larv a of Silpha, adul t Chrysopa, an d adul t Pterosticus, showin g correspondin g division betwee n prementu m (Prmt) and postmentum (Pmt) as determined by the musculature. D, prementum and ligula of adult Pterosticus. E , labium of larval dragonfly. F , labium of adult Bremus, lateral view.

unrepresented, o r it s are a i s include d i n tha t o f th e singl e postmenta l plate. A simpl e conditio n o f thi s typ e o f structur e i n th e coleopterou s larval labiu m is shown in th e Scarabaeida e (Fig . 15 9 A, B). Th e bod y of the labium here consists of a movable prementum (A, B, Prmt) having the hypopharyn x (B , Hphy) adnat e o n its dorsa l surface , an d o f a broa d postmental plate (Pmt) in the ventral wall of the head. The median retractor muscles of the prementum (A, B, rsf) arise on the proximal margin o f th e postmentum . Th e ventra l wal l o f th e prementu m con tains a dista l sclerit e (a ) surroundin g th e base s o f the palpi , tw o smal l intermediate sclerite s (6 ) o n whic h ar e inserte d th e ventra l adducto r muscles (B , 2adlb), an d a large proximal plate (c ) giving insertion t o th e retractor muscles (rsf). The proximal plate is reflected dorsally on the

292

PRINCIPLES OF

INSECT MORPHOLOGY

sides o f the prementu m t o th e bas e o f the irregula r hypopharyn x (B , C, Hphy). Various other coelopterous larvae are found to have this same type of structure i n the labium, but a n extensive comparativ e study of the labial musculatur e wil l b e necessar y t o determin e it s prevalence . Th e median muscles of the labium (rst) function as retractors of the prementum whe n th e prementu m an d postmentu m ar e separate d b y a membranous area , but i f the adjacen t plates ar e hinged to each other th e muscles becom e flexor s (adductors ) o f th e prementum . Th e secon d function i s well exemplified i n th e larv a o f Dermestes. A secon d type o f labial structure , whic h is identical wit h that o f th e adult, als o occurs in the larva e o f Coleóptera. I n th e melandryi d larv a

FIG. 159.—Labiu m o f Coleóptera . A , searabaei d larva , Ochrosidia villosa, ventra l view. B , same , labiu m an d hypopharynx , latera l view . C , same , hypopharyn x an d mouth, dorsa l view . D , labiu m o f adul t Phyllophaga, showin g submentu m invaginate d between mentu m an d gula. E , same , longitudinal section .

(Fig. 16 0 A) th e middl e part o f the labiu m (Mt), thoug h weakl y sclerotized, i s a rigid extension fro m th e submentu m (Smt) an d has no muscles inserted upo n it . Thi s par t o f the larva l labiu m evidentl y become s th e mentum of the adult (B , Mi). Th e prementum is the small terminal part of the labium (A, Prmt) retractile within the mentum. The same type of structure occur s likewis e in som e larval Staphylinida e (C , D), i n which the mental region (Mi) may be largely membranous, but it is the area of th e labiu m containin g th e mentu m o f th e adul t beetl e (Fig . 6 8 A) . In carabi d larvae (Fig . 67 D) the labial plate lyin g between the maxillar y cardines is a part of the postmentum (Pmf), and apparently its distal part i s separated i n the adul t (Fig . 15 8 C) fro m th e proxima l submenta l area to form the mentum (Mí). A guia r plat e i s either presen t o r absen t i n th e larva e o f Coleópter a and when present i s variously developed. Frequently , however , the guiar region i s entirel y membranou s (Fig . 16 0 A , gu), an d i t i s ofte n almos t

THE ORGANS OF INGESTIÓN

293

obliterated b y a n approximatio n o f the postgena l area s o f the cranium, being reduce d in such cases to a median membranous line , o r " suture," proximal t o th e tentoria l pit s (D , gu). I n certai n coleopterou s larvae , especially in the Prionidae , th e bas e of the labiu m i s separated fro m th e neck by a sclerotization uniting th e postgenal areas of the cranium, which appears to be a hypostomal bridge. I n these larvae there is no true gula, since the tentorial pit s remain at th e posterior margi n of the head . Associated wit h th e labiu m i n man y coleopterou s larvae i s a pair of bars extendin g outwar d fro m th e side s o f the prementu m o r th e hypo pharynx t o th e posterio r articulation s o f the mandible s o r t o th e dista l extremities o f th e hypostoma l margin s o f th e craniu m (Fig . 6 7 B , d). These bars, often calle d "bracons," lie in the membranous ventral wal l of the head between the bases of the mandibles and the maxillae. Adult Coleóptera. —The labiu m o f adul t Coleópter a i s typicall y a three-part structur e (Fig . 15 8 C), there being in its ventral wal l a welldefined middle plate (Mf) which lies proximal to the insertions of all the labia l muscle s and is, therefore, a true mentum , that is, a distal plat e of the postlabium. The prementum (Prmt) is usually small, and its sclerotization i s variable , bu t i t alway s bear s th e insertion s o f al l th e stipital muscle s of the labiu m (D , stmds). Generall y the prementu m is retractile int o th e menta l region , sinc e th e bas e o f th e prementu m i s usually attache d t o th e mentu m b y a n infolde d membran e (C , D , /); but i n som e case s th e prementu m i s hinged to th e dista l margi n o f th e mentum, an d it s movemen t i s the n on e o f flexio n o n th e latte r (Fig . 159 E) . The ligul a i s generall y a distinc t par t o f th e adul t coleopterou s labium, wit h th e glossa e an d paraglossa e mor e o r les s separate d (Fig . 158 D, Glj Pgl), thoug h th e glossa e are usually unite d i n a median lobe; but th e entir e ligul a ma y b e a singl e broa d termina l fla p betwee n th e palpi (Fig . 15 9 D , Lig). Th e muscle s of the ligula , whe n present, a s well as those of the palp i arise in the prementum (Fig . 15 8 D). The basal region of the adult coleopterou s labium generally contain s a mentum an d a submentum (Fig . 15 8 C, Mt, Smt), thoug h the respectiv e areas of the two plates are sometimes confluent or are separated onl y by a groove or a transverse depressio n (Fig . 67 C). Th e wide anterior par t of the submentu m lie s betwee n the maxillar y cardine s (Figs . 6 7 C, 6 8 A); proximally the submentum extends to the posterior tentorial pits (pf), an d its length, therefore, varies according to the position of the pits. Proxima l to th e pit s i t i s continuou s wit h th e gul a (Gu). Th e mentu m projects forward fro m th e dista l margin o f the submentu m betwee n th e base s of the maxillary stipites and supports the prementum. I t i s usually a welldeveloped plate , bu t it s siz e is variable (Figs . 67 C, 68 A, 15 8 C, 16 0 B, Mf). When the entire postlabium is sclerotized in the larva, the mentum

294

PRINCIPLES OF INSECT MORPHOLOGY

and submentu m o f th e adul t ar e t o b e regarde d a s subdivision s o f th e postmental plate ; i f onl y th e proxima l par t o f the larva l postlabiu m i s sclerotized, the mentum appears to be developed in the distal membranous part. An unusua l labia l structur e i s foun d i n som e adul t Scarabaeidae , as i n Phyllophaga (Fig . 15 9 D), i n whic h th e labiu m appear s t o consis t only of a prementum (Prmt) and a mentum (Mi), projecting beyond the

Fig. 160.—Head and mouth parts of Coleóptera. A, larva of Melandrya with membranous guia r regio n (gu). B , adul t M striata wit h well-develope d gul a (Gú). C , larv a of Staphylinus wit h guiar area represented by a median suture (gu). D , adult staphylinid, Thinopinus pictus, with same type of structure, but retaining hypostomal sutures (hs). Cv, neck; Gu, gula; gu, guiar area, either wid e or reduced to a median "suture"; hi, hypostomal lobe; hs, hypostomal suture; Hst, hypostoma. Poc, postocciput; PoR, postoccipital ridge; pos, postoccipital suture ; ps, pleurostomal suture.

large gul a (Gu). A n examination o f the inne r surfac e of the labiu m (E) , however, show s tha t th e submentu m i s represente d b y a n interna l recurved plate (Smt) deeply inflected between the mentum and the gula, on which arise the median muscles of the prementum (D, E, rsf). These muscles here act as flexors of the prementum , sinc e the prementu m ha s a definite hing e on the dista l margin o f the mentum .

??? ?????? ?? ????????? ??? 4. TH E FEEDIN G MECHANIS M O F HYMENOPTER A

The Hymenopter a i n classificatio n are usuall y assigne d a plac e near the to p o f th e serie s o f insec t orders , bu t thei r structura l attainment s seem scarcely to warran t s o high a rank, an d the hymenopterou s mout h parts, though adapte d in the adul t stag e for feeding on liquids, are never so highly modifie d t o this end as are the organ s of Lepidoptera, Diptera , and Hemiptera . Th e essentia l feature s in the suckin g mechanism of the higher hymenopterous familie s are present likewis e in the lowe r members of th e group , showing that th e basi c structur e forme d i n th e latte r ha s been evolved into th e specialize d structure o f the former , an d suggesting also that the fundamental mechanism of the mouth parts must have been acquired i n th e firs t plac e a s a n adaptatio n t o th e feedin g habit s o f ancestral form s resembling the moder n sawflies, or members of the chalas togastrous families . The Larva l Mout h Parts. —The mout h part s o f al l hymenopterou s larvae ar e i n som e respect s degenerate , an d i n parasiti c specie s th e reduction is usually carried much further tha n i n others; but i n all form s the basi c structur e o f the mout h part s i s the same . Th e featur e char acteristic o f the m i s a clos e associatio n o r unio n o f th e maxillae , the labium, and the hypopharynx to form a n under-lip complex , in which the ligul a an d th e hypopharyn x are combined in a median lobe on which opens the duc t o f the labial glands. Thes e glands , at leas t i n the matur e stage o f th e larva , produc e th e silklik e materia l fro m whic h th e fabri c of th e cocoo n i s spun . Th e composit e feedin g an d spinnin g orga n of hymenopterous larva e i s i n man y respect s identica l wit h th e simila r organ o f lepidopterous larvae , an d th e likenes s i n the mout h part s onl y accentuates the general resemblance between the larvae of the two groups, so conspicuous in the body form of a sawfly larva an d a caterpillar. The hymenopterou s larva l mout h part s preserv e a mor e generalized condition in the chalastogastrous families , as shown in the larva of Cimbex (Fig. 16 1 A, B , C) . Th e mandible s (A ) ar e stron g bitin g jaw s o f th e ordinary typ e o f structure . Th e maxilla e ar e unite d basall y wit h th e labium (B) , bu t eac h consist s o f a card o an d a stipe s (C) , wit h tw o terminal lobe s and a segmented palpus. Th e labium is distinctly divide d into a wide, membranous postmentum (B, Pmt) and a distal prementum ?????? ??????? ? ???? ?? ????? ???????? ??? ?????? ????? ??? ??? ????? pharynx (Hphy) are united in a median distal lobe, on the apex of which is the spinneret (Sr) containing the orifice of the labial glands. In th e aculeat e Hymenopter a th e larva l mout h part s becom e more simplified, an d i n som e respect s mor e specialized . I n Apida e an d Vespidae (Fig . 16 1 D ) th e mandible s retai n th e ordinar y for m an d position, bu t th e maxilla e an d labiu m ar e reduced. Eac h maxill a ma y

296

PRINCIPLES OF

INSECT MORPHOLOGY

consist of a distinct cardo and stipes (Cd , St), but it always terminates i n a single smal l lobe , which , b y compariso n wit h Cimbex (C) , is apparentl y the galea , at the base of which is a small papilla possibl y representing th e palpus. Th e labium is a simple structure, compose d of a basal postmen ???? ????? ??? ? ?????? ????????? ??????? ??? ??? ???? ???????? ?? distinct sclerites .

???? ?????????? ????? ?? ?????? ???????????? ?? ?????? ?????????? ?????????? B, same , labium, hypopharynx , and maxillae , ventra l view . C , same , lef t maxilla . D , Vespa macúlala, hea d an d mout h parts, latera l view . E , Chrysis, mout h parts , latera l view. F , Sphecophaga, hea d and mouth parts, anterior view. G , Exetastes, mout h parts, ventral view. H , Itoplectis, mout h parts, ventral view.

It i s in parasitic specie s that the mouth part s of larval Hymenopter a become most specialized and acquire a distinctive structure. Th e weakly sclerotized hea d capsul e i s generall y strengthene d alon g it s subgena l regions b y stron g margina l ridge s (Fig . 16 1 E), formin g a prominen t bar o n eac h side , differentiate d into a hypostom a (Hst) supportin g th e labium and maxillae, and a pleurostoma (Plst) bearing the mandibular

THE ORGANS OF INGESTIÓN

297

articulations. Anterio r t o th e mandible s th e latera l ridge s ar e usuall y produced as a pair of epistomal bars (E , F, G , E sí) extendin g to the ante rior tentorial pits (at), bu t generall y the bar s ar e not connecte d between the pits, the irons and clypeus being thus continuous (F, FrClp). In some cases the epistomal bars are united by a transverse ridg e at the base of th e labru m (H) . The mandible s o f more generalize d parasiti c specie s hav e th e usua l form and position (Fig. 161 E, Md)', but very commonly they assume a horizonta l positio n an d becom e mor e o r les s conceale d behin d th e labrum (F , G, H). I n some forms they are very small or rudimentary . The maxilla e i n mos t parasiti c specie s ar e simpl e elongat e lobes wit h n o demarkatio n int o card o an d stipe s (Fig . 16 1 E , MX). Very commonly , however , a scleroti c spu r (s ) fro m th e hypostom a just behind th e mandibl e extend s int o th e wal l o f the maxilla , an d i t ma y completely divid e th e maxill a int o a proxima l part , unite d wit h th e postmentum, and a free distal part (F, G, MX). Generally the posterior (or ventral) margin of the maxilla is reinforced by a sclerotic bar (q), which i s sometime s unite d proximall y wit h th e hypostom a (E) , bu t which mor e commonly ends in a fre e basa l expansio n (F , G) , thoug h i t may b e more or less reduced or almost completel y suppressed (H , q) . The labiu m preserve s it s divisio n int o postmentu m an d prementu m (Fig. 16 1 E, F , Pmt, Prmt). Th e firs t i s alway s membranous , bu t th e prementum usuall y contain s a margina l sclerotization , wit h sometime s a centra l expansion . Th e shap e o f th e prementa l sclerit e i s highl y variable (F , G, H, t), but it s general form is often characteristi c o f genera or group s o f genera. I n som e species the prementa l sclerit e articulate s laterally wit h th e dista l extremitie s o f the maxillar y sclerite s (G , r), on which th e prementu m apparentl y i s movable . Th e structur e i s the n very simila r t o tha t characteristi c o f th e spinnin g apparatu s o f lepidopterous larvae (Fig . 16 5 D, E). A curious conformation i n the mout h region sometime s result s fro m a suppressio n o f th e hypostoma l ridge s and th e proxima l part s o f th e maxillar y sclerites , accompanie d b y a strong developmen t o f th e pleurostoma l an d epistoma l ridge s an d th e spurs (s) of the maxillary sclerites to form an oral framework (H) supporting the labrum , the mandibles , th e maxillar y lobes, and the prementum . The Feeding Mechanism of Adult Hymenoptera.—The mouth parts of adult Hymenoptera hav e the sam e fundamental characteristic as those of the larva , namely , th e unio n o f the maxilla e wit h th e labium , bu t th e terminal part s o f the appendage s ar e better develope d in the imag o and are readil y identifie d i n th e mor e generalize d forms, whil e the secretio n of th e labia l glands has the usua l "salivary " function. The Mouth Parts o f a Sawfly. —The basi c structur e o f th e mout h parts o f adult Hymenopter a i s well show n in th e Tenthredinidae . Th e

298

PRINCIPLES OF INSECT MORPHOLOGY

mandibles her e have the for m o f typical bitin g jaw s (Fig . 16 2 B). Th e maxillae an d labiu m ar e unite d i n a composit e structure (A ) suspended from the postgena l margin s of the craniu m by the basa l articulation s of th e maxillar y cardines . Eac h maxill a (C ) is composed of a triangula r cardo (Cd) and an elongate stipes (St); the stipes bears a five-segmented palpus, a broa d gale a ((?a) , an d a smal l lacini a (Le) . Th e labiu m i s somewhat compresse d between th e maxillar y stipite s an d cardine s (A) , to whic h it i s attached b y membrane. Th e body of the labiu m consist s of a sclerotized prementum (Prmt) and of a large, mostly membranous postmentum (Pmf) containing a small proximal sclerite (Smt). The prementum (Prmt) i s reinforce d b y a media n interna l ridg e continue d forward fro m a thickenin g o f it s posterio r margi n an d thu s appear s to b e compose d of a pai r o f united sclerites . Th e muscle s of the palp i

FIG. 162.—Mout h part s of adul t Hymenoptera . A, Pteronidea ribesii, labiu m and maxillae. B , same , mandibles . C , same, maxill a detached . D , Andrena carlini, labiu m and maxillae , posterio r view . E , same , lateral view. F , mandibl e o f Andrena.

arise o n it s inne r surface , an d th e crania l muscle s o f th e labiu m ar e inserted upon it. The proximal sclerite of the postmentum (Smf) has the positio n o f a submentum an d apparentl y i s to b e identified wit h th e lorum of the Apida e (Figs. 16 2 D, 16 3 A, Lr), since it ha s th e relatio n of the loru m in the basa l mechanis m o f the maxillolabia l complex , though it i s no t connecte d wit h th e cardines . Th e termina l lobe s o f the ten thredinid labiu m includ e a pai r o f fre e latera l paraglossa e (Fig . 16 2 A, Pgl) and a narrow median lobe (Gl), which is evidently the united glossae. Arising from th e dorsa l surface o f the labiu m is a median elevation whic h is probably th e hypopharynx . The maxillolabia l comple x o f Tenthredinida e i s attache d t o th e posterior wal l o f th e hea d betwee n th e postgena l margin s o f th e epi cranium b y ampl e membranes, whic h allo w i t a fre e movemen t o n th e suspensoria forme d o f th e maxillar y cardines . A line o f flexur e crosse s the posterio r par t o f the orga n through th e stipitocardina l suture s of the maxillae laterally, curvin g anterior to the submenta l plate of the labium.

??? ?????? ?? ????????????

In th e usua l position , th e par t dista l t o thi s fol d lie s paralle l wit h th e under surface of the head, while the cardosubmental part is bent abruptl y toward th e head , wher e it i s attached . Th e entir e orga n ca n thu s b e extended b y swingin g distall y o n th e maxillar y cardines . Th e dista l parts of the maxilla e lie dorsal (anterior ) to the base s of the labia l palpi , and th e maxillar y lobe s are turne d i n a plan e vertica l t o th e surfac e of the lígula, so that the terminal parts of the labium and maxillae form th e floor and sides of a wide troughlike channel leading upward to the mout h over the dorsa l surface o f the labium . The maxillolabia l orga n thu s simpl y forme d i n th e Tenthredinida e from th e usua l part s o f the maxillar y an d labia l appendage s i s retaine d with bu t sligh t modification s in the majorit y o f adult Hymenoptera , a s shown i n th e serie s of studies b y Bugnio n (1925 , 1927 , 1929 , 1930) , an d it furnishe s the basi s o f the mor e specialized lapping an d suckin g appa ratus o f the bees . Th e structur e an d mechanis m o f the mout h part s of Sphecidae are elaborately describe d by Ulric h (1924) . The Feeding Mechanism o f Bees. —In th e bee s th e mandible s los e the typica l bitin g for m an d becom e mor e o r les s flattene d o r spoo n shaped to for m tool s tha t may be use d for a variet y of purposes (Fig . 162 F). Th e maxillolabia l comple x is lengthene d (D , E) , an d it s fre e parts become modified a s accessories to a sucking pump developed fro m the buccopharyngea l region of the stomodaeum . A generalize d condition o f th e apoi d typ e o f mout h part s i s foun d in som e o f th e solitar y bees , suc h a s Andrena (Fig . 16 2 D , E) . Th e ????????? ?????? ?? ???? ????????? ??? ??? ?????? ??????? ???? ???? forms a short, conical, hairy "tongue," with the paraglossae (Pgl) diverging fro m it s base . I n th e maxilla e the lacinia e ar e lost , bu t th e galeae (Go) ar e enlarged ; the palp i (MxPlp) ar e reduce d in size , though they retai n a distinc t segmentation . Th e cardine s (Cd) ar e long , rod like suspensori a o f th e maxillolabia l complex , articulate d wit h th e postgenae, whic h are united i n a median hypostoma l bridg e behind th e base of the labium, as in the honey bee (Fig. 65 C). Betwee n the cardines ?? ? ????? ???????? ???????? ????? ??? ?? ??? ???????????? ????????? ???? the dista l end s o f the cardine s an d supportin g th e bas e o f the labium . This sclerite , known as the lorum, lies proximal to th e transvers e lin e of flexion passing throug h th e stipitocardina l joint s an d thu s correspond s in positio n t o th e submenta l sclerit e o f Pteronidea (A , Smt). A smal l triangular plat e i n Andrena unite d wit h th e bas e o f th e prementu m ??? ??? ?? ???????? ? ??????? In th e highe r bees, such as Xylocopa, Bonibus, and Apis, still furthe r modifications hav e take n plac e i n th e maxillolabia l apparatus . Th e galeae are larg e flat blades (Fig . 16 3 A, Go) muc h longer than th e maxil lary stipites ; th e lacinia e ar e rudimentar y o r absent ; th e maxillar y

300

PRINCIPLES OF INSECT MORPHOLOGY

?????? ?????? ???? ?? ????????? ??? ??????? ?? ????? ???? ?? ???? ??? MxPlp). The glossal tongue of the labium (Gl) is greatly lengthened and highl y mobile, being flexible in all directions and capable of an active movement o f protraction an d retraction . It s bas e i s closel y embraced ?? ??? ?????????? ????? ??????????? ?????? ??? ?????? ????? ??? ???? ???

???? ???????????? ????????? ?? ????? ????? ?? ???? ????????? ??????? ?????? ??? maxillae wit h attachmen t t o head . B , sectio n o f hea d o f Xylocopa virginica, showin g sucking pum p (Pmp). C , sectio n o f basa l par t o f labiu m o f Xylocopa, showin g salivary ??????? ????? ??? ??????? ?? ??? ??????????

???? ????????? ???????? ??? ??? ????? ???????? ?? ???? ??????? ? large flat blade tapering distall y t o th e tw o small distal segments . Th e ?????? ?? ??? ???? ?????? ????????? ?? ????????? ???????? ???????? ? large, strongl y sclerotize d plat e (commonl y called th e "mentum " b y students o f Hymenoptera) . Th e membranou s palpigers an d th e bas e of th e ligul a ar e partl y retractil e int o th e anterio r en d o f th e stipita l region. A t th e bas e o f th e prementu m i s a smal l triangula r sclerit e

THE ORGANS OF INGESTIÓN

301

(Mí), which appears to belong to the postlabium, since it lies proximal to the insertion s o f the posterio r cranial muscles of the labiu m (C , 2adlb), and may therefore be termed the mentum. Th e apex of this sclerite fits into th e concav e angle of the loru m (A , Lr), which , as in Andrena (Fig . 162 D), articulate s b y it s latera l extremitie s with th e dista l end s o f the maxillary cardines . Retracto r muscle s o f th e prementu m ar e absen t in Hymenoptera . The long glossal tongue of the bees is an organ of particular importanc e in the feedin g mechanism . I t i s densely clothed wit h hairs excep t at it s base an d terminate s i n a smal l lob e calle d th e "spoon/ ' o r flabellum (Fig. 16 3 A, Fbl). Whe n the be e feeds o n a liquid easil y accessible , th e broad maxillar y galea e an d th e labia l palp i ar e brough t togethe r ove r the tongue , thu s improvisin g a tubula r proboscis , th e en d o f whic h is thrust int o th e foo d liquid . B y a rapi d back-and-fort h movemen t of th e tongu e th e liqui d i s drawn up int o the tube , an d fro m th e latte r it is sucked up to the mouth by the stomodaeal pump. I f the food liquid is confine d i n a narrow space, as in the coroll a of a flower, however, the tongue ma y b e thrust ou t fa r beyon d th e end s o f the maxilla e in orde r to obtai n it . Bot h th e tongu e and the paraglossa e are deeply retractil e into the dista l part o f the prementu m by a n infoldin g o f the membran e at their bases caused by a contraction of the glossal muscles (C, fgl). In some o f th e shorte r tongue d bee s th e maxillar y galea e ar e stiff , sharp pointed blades and are used for cutting through the outer wall of a corolla in orde r to gain acces s to the necta r within . The posterio r surfac e o f the glossa l tongu e i s excavate d b y a dee p channel whic h extend s fro m th e flabellu m t o th e bas e o f th e tongue , where the latter is closely embraced by the paraglossa l lobes . Dorsally , the paraglossa e cove r th e salivar y orific e locate d o n th e ora l surfac e of the labiu m at the dista l en d of the prementu m (Fig . 163 C, SIO). Th e paraglossae thus evidently serve to conduct the salivary secretion around the bas e o f the tongu e into th e channe l on the ventra l sid e of the organ , through whic h it i s conveyed to th e ti p o f the latte r to-b e mixe d wit h the foo d durin g ingestión . The salivary duct (Fig. 163 C, SID) opens into the lumen of an expulsive apparatus known as the salivary syringe (Syr). This organ consists o f a n elongat e pouc h wit h tw o pair s o f muscle s inserte d o n its walls. Its outlet is the functional salivary orifice (SIO) located on the dista l extremity of the prementu m between the bas e of the glossa l tongue and the rudimentary hypopharynx (Hphy). There can be no doubt, therefore , that th e syring e i s a developmen t o f th e salivarium , which in its primitive for m i s a simple salivary pocke t at th e junctio n of the hypopharynx with the ora l surface o f the labiu m (Fig . 155 , Slv). I n Xylocopa the syringe is provided with two pairs of long muscles arising

302

PRINCIPLES OF INSECT MORPHOLOGY

in th e prementu m (Fig . 16 3 C, Is , 3s) , bu t i n Apis th e dorsa l pair (Is ) have thei r origi n o n th e latera l margin s o f the hypopharyngea l surfac e (Hphy) and thus correspond to the usual hypopharyngeal dilators of the salivariu m (Fig . 155 , Is) . The suckin g pump of the bee s is a larg e muscular sac lying entirel y within the head anterior to the brain (Fig. 163 B, Pmp). The morphology of th e orga n i s not entirel y clear , but , judgin g from it s musculature , i t includes withou t doub t th e pharyn x an d th e bucca l cavity an d perhap s the cibarium . It s dorsa l dilato r muscle s are separated int o two groups inserted anterior and posterior to the frontal ganglion (FrGng) and its connectives. Thos e o f the firs t grou p (dlbc) aris e o n the clypeus ; those of th e secon d group (dlphy) tak e thei r origi n on the irons . O n the floor of the pump, just within the mouth, is a broad sclerotic plate, from which a long arm (h) extend s posteriorly and dorsally on each side in the latera l wall o f th e orga n an d give s insertio n t o a pai r o f convergen t muscles, one arising on the clypeus, the other on the irons. I t i s perhaps possible that thes e bar s represen t th e ora l arm s o f the hypopharyngea l suspen soria o f mor e generalize d insects (Fig . 60 , y). A pai r o f larg e ventra l dilator muscles of the pum p arise on the transvers e bar o f the tentorium . The posterio r en d o f the pum p narrow s to a mor e slender tube , whic h passes betwee n the brai n an d the suboesophagea l ganglion, and enlarges again i n a smal l posterio r pharyn x (PPhy) lyin g in th e rea r par t o f th e head behind the brain. Th e posterior pharynx is provided with a pair of long slende r dorsal dilator muscle s (dlpphy), an d a pair o f short ventra l dilators arisin g on the tentorium . 5. TH E FEEDIN G MECHANIS M O F LEPIDOPTER A

In the Lepidopter a the structura l divergenc e betwee n the mout h parts of the larva and those of the adul t has been carried to a still greater degree than in the Hymenoptera. Th e mouth parts of a caterpillar show a genera l resemblanc e t o thos e o f th e larva e o f Hymenoptera , an d i n each grou p the salivar y gland s secret e a substanc e tha t become s silky when extrude d an d whic h ma y b e use d fo r constructin g a cocoon . The mout h part s o f a moth o r butterfly, on the othe r hand , hav e littl e to sugges t a common origin with the mout h parts of a wasp or bee, since in the majorit y of Lepidoptera the mandible s are rudimentary o r absen t and th e labiu m i s eliminate d fro m th e feedin g mechanism . I n al l bu t certain generalize d forms th e maxilla e remai n a s th e onl y appendage s involved in the apparatus o f ingestión, and they are greatly modified by a reduction o f the palpi , the los s of the laciniae , and th e elongatio n of th e galeae to form a tubular proboscis. Th e sucking pump of the Lepidoptera, as in th e Hymenoptera , i s formed largel y o f the precerebra l pharyngeal region o f the stomodaeum , but i t appear s t o includ e the cibarium . It s

??? ?????? ?? ????????? ? ?

dorsal dilato r muscle s tak e thei r origi n o n th e frontoclypea l plat e of the hea d wall . THE FEEDIN G AN D SPINNIN G ORGAN S O F A CATERPILLA R The mouth parts of a caterpillar ar e of the bitin g an d chewin g type of structure a s far a s the functio n o f feeding is concerned, but the y are used for variou s purpose s othe r tha n that o f takin g food . Th e mandibles , for example , serv e wit h man y specie s a s implement s fo r gnawin g an d tunneling, whil e th e maxillae , labium , an d hypopharyn x ar e alway s united i n a large under-lip comple x on which opens the duc t o f the silk forming labia l glands , an d it s activities , therefore , mostl y pertai n t o the function o f " spinning." The Mandibles.—-The caterpillar's mandibles are jaws of the ordinar y biting an d chewin g form (Fig . 16 4 B). Eac h i s hinged t o th e hea d b y posterior an d anterio r articulation s (a' , c ) o f the usua l type o f structure . The abducto r muscle s are relatively small , but th e great adductor s (Fig . 64 B , admd) occup y most o f th e latera l part s o f th e hea d cavit y an d appear t o determin e the siz e an d for m o f the latera l hemisphere s o f th e cranium. Som e species of caterpillars ar e provide d wit h a pai r o f large tubular mandibula r gland s reachin g ofte n fa r bac k int o th e thora x an d abdomen, th e duc t o f each extending down to th e inne r edg e of the bas e of the mandible close to the apódem e of the adducto r muscle. The Maxillolabial-hypopharyngea l Complex. —The maxillae , th e labium, an d th e hypopharyn x i n th e caterpillars , a s i n hymenopterou s larvae, are united to for m a large composite structure tha t project s like a thic k unde r li p behind th e mout h an d bear s th e spinnere t a t it s ti p (Fig. 16 4 C). Basall y th e orga n i s supported o n the hypostoma l lobe s (Hst, Hsf) of the postgenae, which are approximated medially between the nec k and the bas e of the labium . The wall s o f th e under-li p comple x (Fig . 16 4 C ) ma y b e largel y membranous, th e sclerotizatio n bein g usuall y broke n u p int o variou s small plates ; bu t b y observin g certai n landmark s th e component s of the orga n ca n b e prett y wel l defined . Tw o latera l lobes , representin g the maxillae , ar e more or less distinct fro m a median lob e formed o f th e labium and hypopharynx. A small plate (Cd) at the base of each lateral lobe is evidently the cardo, since it articulates (a") with the ????????? ????? ??? ????? ??? ?????????? ?? ??? ????????? ???????? muscles. O n th e mesa l borde r o f eac h latera l lob e i s th e lin e o f a strong interna l ridg e (Fig . 16 4 C, q) upo n whic h ar e inserte d th e usua l stipital muscles of the maxilla. Th e areas laterad o f the ridges, therefore, are the maxillary stipites (Sf). Each stipital area ends distally in a membranous lob e (Lo) , usuall y havin g smal l sclerite s i n it s wall s an d bearing termina l papilla e provide d wit h sens e organs . Thre e muscle s

304

PRINCIPLES OF INSECT MORPHOLOGY

are inserted o n the bas e of the lobe , two arisin g in the stipe s an d on e on the hypostoma , but th e homolog y of the lob e is difficult t o determin e in the ordinar y caterpilla r structure . I n th e micropterygi d Sabatinca, however, th e larva l maxilla , a s show n by Tillyar d (1922) , end s wit h a three-segmented palpu s an d a distinct lacinia an d galea (Fig . 16 7 A). The media n componen t o f th e under-li p comple x o f th e caterpilla r consists o f the labiu m an d th e hypopharynx . It s proxima l par t i s th e postmentum (Fig . 16 4 C, PraZ) , whic h may b e a n entirel y membranous area, thoug h i t frequentl y contain s a postmenta l sclen t e (Fig . 16 4

FIG. 164.—Mout h part s o f larval Lepidoptera. A , Lycophotia margaritosa, labrum , anterior view . B , Estigmene aerea, mandibles , ventra l view . C , same , labium , hypopharynx, an d maxilla e united an d suspende d fro m hypostoma l lobe s of cranium .

C, pmi). Th e dista l part , whic h form s a fre e media n lob e betwee n the terminal lobes of the maxillae (Lo), is the prementum (Prmt) with the hypopharyn x (Hphy) adnat e upo n it s anterio r surface . I t bear s dista lly th e spinnere t (Sr), a hollo w spin e havin g th e orific e o f the sil k duct a t it s extremity . Labia l palp i ar e absent , unles s the y ar e repre sented b y the pai r o f small papillae located a t th e side s of the spinneret . The mouth part s of larval Trichopter a ar e structurally identica l with those o f th e caterpillars . Th e maxilla e an d labiu m ar e unite d i n th e same manner, an d the hypopharyn x is fused wit h th e smal l prementum. The postmentum , however , is elongat e an d contain s a relativel y larg e plate include d betwee n th e elongat e postgena l area s o f th e cranium . In som e species the postmenta l plat e simulates a gula, but th e posterio r tentorial pits are always at th e nec k margin of the head . Al l the labia l muscles ar e inserte d o n the prementum , bu t th e media n retractor s ar e absent, a s the y ar e i n th e caterpillars . Th e maxillar y musculatur e is the sam e as that of a caterpillar . The Spinnin g Apparatus.—The materia l o f the sil k thread s spu n b y caterpillars i s secrete d b y th e labia l glands , whic h consis t o f a pai r of greatly elongat e tube s lyin g in th e bod y cavit y a t th e side s o f the ali mentary cana l (Fig . 196 , SkGl). Th e duct s o f thes e silk glands, afte r

THE ORGANS OF INGESTIÓN

305

receiving the duct s of a pair o f small accessory acinous glands, sometimes called the glands of Filippi, unite in a short median conduit that opens into the base of an organ known as the silk press (Fig. 165, A, B, E, Pr). The press lies in the media n lobe of the mout h parts formed o f the unite d hypopharynx an d prementu m (A , C). I t discharge s throug h a narro w terminal duc t tha t open s on the ti p o f the spinnere t (Sr) locate d o n th e distal surfac e o f th e hypopharyngeal-prementa l lobe . Th e dorsa l wal l of th e pres s i s deepl y invaginate d int o th e lume n o f the orga n (F ) an d contains a median sclerotic bar, or raphe (Rph), on which are inserted

FIG. 165.—Spinnin g apparatu s o f caterpillars . A , premento-hypopharyngea l lob e of a noctuid caterpillar, with silk press and spinneret , lateral view. B , same, ventral view. C, sam e of Malacosoma americana, lateral view . D , premento-hypopharyngea l lob e of a noctuid, showin g support o n maxillar y arm s (q), an d muscles , lateral view . E , sam e in cross-section, posterior view . F , diagrammati c cross-section of silk press.

two o r three pairs o f muscles (A , B, C , E, F , 17, 18) having their origin s on the dorsolatera l parts of the spinneret-bearin g lobe . Anothe r pair of muscles (19) arisin g ventrally i n the prementa l part of the lob e is inserted on the lateral walls of the pres s (E , F). Bot h sets of muscles apparentl y are dilators o f the pres s lumen, the antagonisti c forc e being the elasticit y of th e infolde d dorsa l wall of the organ . Morphologically the silk press of the caterpillar s i s a highly specialized development of the salivariu m o f more generalized insects (Fig . 155 , Slv). By th e complet e unio n o f th e hypopharyn x wit h th e prementu m th e press ha s becom e entirel y enclose d betwee n th e componen t element s of th e hypopharyngeal-prementa l lobe , and its outlet duc t represents th e persisting remnant of the salivary passage (sm) between the hypopharynx and th e labium . It s dorsa l muscle s ar e th e hypopharyngea l dilator s

306

PRINCIPLES OF INSECT MORPHOLOGY

of th e salivariu m (Is) , it s ventra l muscle s are thos e normall y arisin g i n the prementu m (2s , 3s). The spinneret-bearin g lob e o f th e caterpilla r i s supporte d laterall y at it s bas e o n two sclerotic bars (Fig . 16 5 D, E, q), whic h are th e dista l arms o f the mesa l ridges of the maxillar y stipites (Fig . 16 4 C, q ) articu lated b y their extremities (Fig . 165 D, E, r) with the side s of the premen tum. Upo n these fulcr a th e entir e spinnin g apparatus ca n be swung up and down , o r anteriorl y and - posteriorly, b y muscle s inserte d upo n it s base. Th e spinning muscles comprise a pair of ventral labial muscles (15} inserted o n the bas e of the prementum , and a pair of dorsal hypopharyn geal muscle s (16) inserted o n th e bas e o f th e hypopharyngea l surface , both pair s takin g thei r origi n o n th e tentorium . Th e fulcra l arm s (q ) give insertio n t o a pai r o f maxillar y adductor s (11). Th e numerou s

FIG. 166.—Anterio r part of the stomodaeu m o f a noctuid caterpillar . 20-23, dilator muscles o f buccal cavit y arisin g o n clypea l triangl e o f cranium ; 24-27, precerebral dorsa l dilators o f pharynx ; 28-30, postcerebra l dorsa l dilator s o f oesophagus ; 31-36, ventra l dilators o f pharynx an d oesophagus .

other movement s mad e b y th e caterpilla r i n th e ac t o f spinnin g ar e produced by th e elaborat e musculature of the bac k of the hea d an d th e anterior par t o f the body . The Hea d Stomodaeum. —The mout h o f th e caterpilla r open s int o a wid e stomodaea l chamber , covere d externall y b y broa d plaque s o f muscle fibers , lyin g anterio r t o th e nerv e rin g o f th e hea d (Fig . 166 , BuC, Phy). Th e chambe r i s evidentl y th e buccopharyngea l region of the stomodaeum , since its dorsa l dilator muscle s are separated int o tw o groups b y th e fronta l ganglio n and it s connectives , those o f one group (22j 23 ) arising o n the triangula r clypeus , while those of the othe r group (24-, 25 , 26, 27 ) aris e o n th e postclypea l area s o f th e head . A thir d anterior se t o f muscles (20, 21), arisin g als o o n th e clypeus , is inserte d just before the first transverse muscles (a) of the stomodaea l wall. Thes e muscles are clearly cibarial muscles. Th e par t o f the stomodaeu m lying

THE ORGANS OF INGESTIÓN

307

in the hea d behind the nerve ring is merely a wide cylindrical oesophagus (Oe) with strong circular muscles. The dilator muscles inserted upon it, however, sho w that this part o f the stomodaeu m in the caterpilla r corre sponds to the posterior pharynx inOrthoptera and Coleóptera (Fig. 156 D 192, 193, PPhy). THE FEEDIN G ORGAN S O F ADUL T LEPIDOPTER A With mos t o f th e Lepidopter a th e mout h part s underg o a radica l change i n structur e durin g th e metamorphosi s fro m th e larv a t o th e imago. Th e mandibles become rudimentary o r are entirel y suppressed ;

FIG. 167.—Mout h parts of Micropterygidae . A , Sabatinca barbárica, larval maxilla . B, Micropteryx ammanella, hea d o f adult . C , Micropteryx aruncella, maxill a o f adult . D, Mnemónica auricyanea, maxill a o f adult . E , same , labiu m o f adult . F , same , hea d of pupa . G , Sabatinca incongruella, mandible s o f pupa . (A , C , G from Tillyard, 1922 , 1923; B , D , E , ¥ from Busck an d Boving, 1914. )

the termina l part s o f the maxilla e are transforme d into lon g half tubes , which, together, form the characteristi c coiled proboscis of the imago (Fig. 168, Prb); the labiu m is reduced to little more than a flap, but it acquires a pai r o f larg e palp i (LbPlp) ; th e anterio r par t o f the stomodaeu m i s developed int o a n efficien t suckin g pum p (Fig . 16 9 F, Pmp). I n th e primitive famil y Micropterygida e th e adul t mout h part s hav e a mor e generalized structure , an d on e which clearly demonstrates th e evolutio n of th e suckin g apparatu s o f th e Lepidopter a fro m mout h part s o f th e usual biting type, and not from a mouth-part structur e of the larval type. The feeding and spinning mouth parts of the caterpillar, therefore , appea r to represen t a specialize d larval conditio n adaptiv e t o th e need s o f th e caterpillar. Generalized Lepidopterou s Mout h Parts.—Th e member s o f th e Micropterygidae ar e moth s i n ever y essentia l respect , bu t the y hav e mandibulate mout h part s i n th e larval, pupal , an d adul t stages . Th e mandibles o f the pup a an d imag o ar e typica l functiona l jaws (Fig . 16 7

308

PRINCIPLES OF

INSECT MORPHOLOGY

B, G) . Thos e o f the adul t ar e sai d b y Tillyar d (1923 ) t o wor k in con junction with brushes of the epipharyn x and hypopharyn x and a basket like structur e o n th e hypopharyn x "a s grinder s o f th e minut e polle n grains or other fine vegetable matter which forms the foo d o f the imago/ 7 The maxilla e have a typica l generalize d structure, eac h being composed of a basi s forme d o f a card o and stipe s (C , Cd, St) an d provide d wit h a long palpu s (Pip) an d tw o termina l lobe s ((?a , Lc) . Th e lacini a (Lc) , however, i s muc h smalle r tha n th e gale a (Go). Th e labiu m i s rudi mentary i n that its median part is reduced to a simple lobe, but i t bear s two large three-segmented palpi . In th e Eriocranidae , a relate d family , th e mout h part s sho w thei r origin fro m th e micropterygi d typ e o f structure, bu t the y tak e o n th e

FIG. 168.—Hea d o f peach-tree bore r moth , Synanthedon exitiosa.

peculiarities o f th e typica l lepidopterou s feedin g organs . Mandible s are present i n the adul t stage , thoug h the y ar e very smal l and probabl y functionless. I n th e pupa , however , the y ar e extraordinaril y larg e (Fig. 167 F, Md) and are so constructed that they open forcibly outward, thus enabling th e pup a t o use its jaws for liberating itsel f fro m it s toug h underground cocoo n and fo r diggin g upward t o th e surfac e o f the earth . The maxill a o f th e adul t (D ) ha s a larg e six-segmente d palpu s (Pip) and a lon g slender gale a (Go), bu t th e lacini a i s absent. Th e galea e of the tw o maxilla e ar e groove d o n thei r opposin g surfaces , whic h ar e joined t o for m a curve d probosci s capabl e o f being partl y coiled . Th e labium i s a simple lob e with lon g three-segmented palp i (E) . The Typica l Feedin g Mechanis m o f Moth s an d Butterflies.—Th e feeding mechanis m characteristi c o f the Lepidopter a i s a simpl e devic e for extractin g necta r fro m th e depth s o f flowe r corollas . I t consist s essentially of a long tube, th e probosci s (Fig . 16 8 A, B, Pr6), arising fro m the ora l regio n o f the head , wher e its lume n opens into th e mout h (Fig . 169 F, mth), and of a pumping organ (Pmp) formed of the anterior part

THE ORGANS OF INGESTIÓN

309

of th e stomodaeum . Thoug h nectar is the principa l foo d o f adult moth s and butterflies , the feedin g apparatu s serve s as well for imbibing expose d liquids, suc h a s wate r an d th e juice s of fruits. Man y species, however , take no food o f any kind, and in such species the mout h parts are usually reduced, in some they ar e rudimentary an d functionless, an d th e mout h pump is entirely absent, the stomodaeu m being reduced in the hea d t o a threadlike tube . The Labrum.—The labrum is never large; usually i t is a narrow transverse band a t th e lowe r edge of the larg e clypeal region of the fac e (Fig . 169 A, C , D , Lra) . O n its latera l extremitie s i t bear s a pai r o f small , hairy lobes, the pilifers (A, Pf), which are present likewise in the Micropterygidae (Fig . 16 7 B, Pf). The Mandibles. —Rudiments o f the mandible s occu r i n som e macro lepidopterous moth s a s smal l immovabl e lobe s projectin g fro m th e cranial walls at the sides of the labrum (Fig. 169 D, Md). The reduction of th e jaw s fro m th e larva l siz e take s plac e a t th e transformatio n fro m the caterpillar to the pupa, and again at the change from th e pupa to the moth. I n mos t specie s th e mandible s ar e entirel y obliterate d i n th e adult. The Proboscis. —The essentia l externa l par t o f the suckin g apparatu s is th e proboscis . Thi s orga n i s formed o f the greatl y elongat e termina l lobes o f the maxillae , which , as w e have seen , ar e probabl y th e galeae , the lacinia e bein g reduce d an d suppressed . Th e probosci s i s thus com posed o f tw o latera l pieces , whic h ar e hel d togethe r b y interlockin g grooves an d ridge s (Fig . 16 9 E). Th e oppose d walls are thickene d an d concave and enclose between them a canal (fc) through which the liquid food o r drink o f the insec t i s drawn up t o th e mout h b y th e stomodaea l pump (F , Pmp). Th e basal part of each maxilla usually show s a divisio n into a small cardo (B, Cd) and a larger stipes (St), the latter bearing a rudimentary palpu s (Pip) and the elongat e galea (Go). When the probosci s is not i n use it i s tightly coiled beneath the head , but i t ca n be completely extende d i n response t o a foo d stimulus . Th e mechanism o f extensio n an d coiling , however , i s no t wel l understood . The oute r wal l o f eac h hal f o f th e probosci s show s a closel y ringe d structure produce d b y a successio n o f scleroti c arc s alternatin g wit h narrow membranou s spaces. Thi s structure probabl y allow s the coilin g of th e tube . Withi n eac h hal f o f th e latte r ther e i s a serie s o f shor t muscle fibers arising near the middle of the outer wall (Fig. 169 G, mcls) and extendin g obliquely distad an d toward the inne r edg e of the concav e side o f th e organ , o n whic h the y hav e thei r insertions . Th e muscle s occupy the entir e length o f each half o f the proboscis , an d thei r arrange ment suggest s tha t the y serv e t o coi l th e proboscis . Unles s ther e i s some mechanica l principl e her e involve d that i s not ye t understood , w e

310

PRINCIPLES OF INSECT MORPHOLOGY

must assume , then, tha t th e probosci s Is extended b y bloo d pressure, in the same way that a toy paper "snake " is unrolled by inflating it, an d it must b e observe d tha t th e natura l uncoilin g o f th e lepidopterou s pro boscis, beginning at th e base and progressing toward the tip, ha s a striking resemblance to the unrolling of the inflated "snake." Th e mechanism for creatin g th e assume d blood pressure, however , is not evident . With species that do not feed in the adult stage the proboscis is usually short an d weak , an d i n som e form s th e entir e maxilla e ar e reduce d t o small lobes projecting a t th e side s of the mout h (Fig . 16 9 C, MX).

FIG. 169.—Mout h parts and suckin g apparatu s of adult Lepidoptera. A, Synanthedon exitiosa, labrum, epipharynx , an d pilifers . B , same , bas e o f maxilla. C , Malacosoma americana, showin g rudimentar y maxillae . D , Hyphantria cunea, hea d an d proboscis . E, cros s sectio n o f probosci s o f Dañáis archippus. (From Burgess, 1880. ) F , sectio n o f head o f sphin x moth , showin g suckin g pump , diagrammatic . G , diagra m o f par t o f proboscis an d its muscles .

The Labium. —In al l adul t Lepidopter a th e labiu m ha s th e simpl e form i t ha s i n th e Micropterygida e an d Eriocranida e (Fig . 16 7 E) , being at mos t a small lobe or flap, but ofte n i t is reduced to a mere membranous area behind the base of the probosci s supported posteriorl y o n a hypostomal ba r unitin g th e postgena l area s o f th e epicranium . Th e three-segmented labia l palpi , however , ar e usuall y wel l develope d an d covered wit h lon g hairlike scales , formin g thus two conspicuou s brushes projecting upward at th e side s of the proboscis . The Sucking Pump.—A suckin g orga n i s highl y develope d i n adul t Lepidoptera havin g functiona l mout h parts ; bu t wit h specie s i n which the proboscis is rudimentary or absent, th e pump is likewise rudimentary, the mout h openin g int o a smal l funne l leadin g int o th e simpl e slende r stomodaeum. Th e functionall y develope d pum p o f th e Lepidopter a includes th e buccopharyngea l regio n o f th e stomodaeum , a s i t doe s i n Hymenoptera, sinc e th e fronta l ganglio n lie s on its dorsa l wall , and th e

THE ORGANS OF INGESTIÓN

311

dorsal dilato r muscle s are inserte d befor e an d behind th e connective s of the ganglion; but i t is to be suspected that the anterior par t of the organ may be formed b y the cibarium . Th e sucking pump of the Lepidoptera , however, ha s bee n bu t littl e studied , an d n o definit e statement ca n b e made as to it s morpholog y until w e have mor e comparative informatio n on its structur e an d the relation o f its muscles to the hea d wall. In th e sphin x mot h th e suckin g pum p i s a larg e bulblik e structur e with strongly muscula r walls lying in the anterio r par t of the hea d (Fig . 169 F, Pmp). I t open s o n the bas e o f the probosci s throug h a narrow neck an d tapers posteriorly int o th e oesophageal tube. Th e firs t dilato r muscle (1) consist s o f a transvers e shee t o f fiber s arisin g o n th e lowe r edge o f th e clypea l region o f the hea d wal l and inserte d o n th e nec k of the pump . Abov e these muscle s is a media n mas s o f fiber s (2 ) inserte d on th e anterio r en d o f th e bulbou s par t o f th e pump . Th e principa l dilators, however , comprise two thick, paired bundle s of fibers (8) arising on the uppe r par t o f the facia l regio n of the hea d an d inserte d laterall y on th e dorsa l wal l of the pum p anterio r t o th e fronta l ganglio n an d it s connectives. Posterio r t o th e fronta l ganglio n ar e a pai r o f smalle r muscles (4 ) and a single median muscle (5), the insertion s o f which show that the rea r par t o f the suckin g organ at leas t i s the pharynx , a s it is in Hymenoptera. 6. TH E FEEDIN G MECHANIS M O F DÍPTER A

The differentiatio n between larval an d imagina l structure s i n insect s has reache d it s highes t degre e i n th e mout h part s o f th e Diptera , fo r here bot h adult s an d larva e hav e widel y diverge d fro m th e ancestra l norm, an d i n th e highe r familie s th e maggot s hav e outdon e th e flies. In fact , i n the Cyclorrhaph a i t appear s tha t th e larva e hav e developed not only a feeding mechanism but als o a functional head and mouth that have littl e relatio n t o th e cephali c structure s o f the imago . Th e tru e mouth part s o f th e fl y ar e entirel y suppresse d durin g th e whol e larva l period, the major part of the head is invaginated into the body, and a new set o f organ s i s develope d t o serv e th e purpose s o f th e maggot . Th e imaginal part s ar e restore d durin g th e pupa l transformatio n an d the n developed directl y int o th e specialize d for m characteristi c o f the adult . In th e mor e generalize d Orthorrhapha , however , the larv a retain s th e usual hea d structur e an d a feedin g mechanis m tha t clearl y demon strates the origi n of the Dipter a fro m insect s having typical biting mout h parts. MOUTH PART S O F DIPTEROU S LARVA E Only th e extrem e type s o f dipterou s larva l mout h part s wil l b e described here , on e representin g th e mos t generalize d form , occurrin g

312

PRINCIPLES OF

INSECT MORPHOLOGY

in the Orthorrhapha , th e other th e highly specialized structure develope d in the muscoid maggot. Th e intermediate stage s between these extreme s are stil l no t wel l understoo d an d offe r a n invitin g fiel d fo r furthe r exploration. The Orthorrhaphou s Type o f Larval Mout h Parts.—I n th e larva e of Tipulidae o r Tabanida e an d relate d familie s th e hea d i s a n elongat e oval capsul e wit h stron g scleroti c walls , bu t i t i s almos t completel y retracted int o th e anterio r par t o f the thora x (Fig . 17 0 A), wher e it i s enclosed i n a membranou s sheat h forme d b y a n inflectio n o f th e nec k membrane (Cv). Th e dorsa l wal l of the head , however , is muc h longer than the lateroventral walls , which taper anteriorl y an d are united below in a small, toothed, triangular hypostomal lobe (Hsf) projecting beneath the mouth (Mth) and the rudimentary labium (L6). The brain and the suboesophageal ganglio n ar e withdraw n fro m th e hea d an d li e i n th e middle o f the thoraci c region o f the body , bu t lon g nerve trunk s exten d forward fro m the m t o th e organ s o f th e hea d normall y innervate d b y these ganglia . The mandible s i n th e tipuli d larv a ar e strongl y musculate d jaw s of the generalize d bitin g typ e o f structur e (Fig . 17 0 B). I n th e tabani d larva th e mandible s ar e rudimentary , bu t i n variou s othe r familie s of the mor e generalize d flies, as i n th e Chironomida e an d Culicidae , th e larval mandible s ar e also jawlike in form . The maxilla e o f th e tipuli d larv a ar e smal l fla t lobe s (Fig . 17 0 C) , in which the usual parts of a maxilla are somewhat indefinitely separated . In the larva of TabanuSj however, the maxillae have a more generalizeda structure, eac h orga n (D ) comprisin g a basal lob e (Cd), whic h is appar ently the cardo, and a larger stipes (Sf) bearing a distinct galea (Go) and a lacinia (i/c) , but th e palpu s i s absent . The labiu m i s rudimentar y i n al l fl y larvae , an d th e hypostoma l lobe of the ventra l hea d wall is frequently mistaken fo r it. I n th e larv a of Típula the labium is a small median projection beneath the mouth (Fig. 170 A, 1/6), but it is concealed above the hypostoma (Hsf). United with the labium is a small hypopharyngeai lobe (Hphy), and between the latter an d th e labiu m open s the duc t o f the salivar y gland s (SID). The hea d stomodaeum of the tipuli d larv a is a straight tube (Fig . 170 A, Stom), slightl y widene d anteriorly , bu t showin g n o structura l differ entiation int o buccal , pharyngeal , an d oesophagea l regions . O n it s dorsal wal l ar e inserte d thre e group s o f dilato r muscles . Th e fiber s o f the firs t grou p (dlcb, dlbc) belon g to the cibariobucca l region ; those of th e second and third groups (dlphy) correspon d to the dilator s o f the anterio r pharyngeal region in other insects (Fig . 155 , Phy), sinc e they ar e inserted posterior t o th e connective s o f th e fronta l ganglio n (FrGny) an d ar e precerebral i n position , th e brai n bein g withdraw n int o th e thorax .

THE ORGANS OF INGESTIÓN

313

The Muscoi d Typ e o f Larva l Mout h Parts.—I n th e highe r cyclor rhaphous Dípter a th e usua l mouth part s ar e entirel y suppresse d in th e larval stage , an d th e onl y externa l feedin g organ s o f th e maggo t ar e a pai r o f strong mout h hook s movable in a vertica l plane . Moreover , the entir e facia l regio n o f the hea d posterio r t o th e clypeus , including the are a o f the irons an d tha t o f the imagina l antenna e an d compound eyes, is invaginated (no t merely retracted) into the thorax, and a circular fold o f the neck projects beyond the mouth to form a conical snout, which is the functiona l "head " of the maggot .

FIG. 170.—Hea d an d mout h part s o f larva l Díptera . A , Típula abdominalis, dia grammatic sectio n o f retracted head . B , same , righ t mandible , mesa l view . C , same , left maxilla, outer view. D, Tabanus punctifer, left maxilla, outer view. E, F, G, diagrams illustratin g stages in th e invaginatio n o f the hea d o f a muscoid maggot .

To understan d th e morpholog y of the extraordinar y hea d structur e of th e cyclorrhaphou s maggot, we must trac e its evolution from th e head structure o f a simple r type , possibl y fro m a n invaginate d hea d o f th e tipulid o r tabanid variety (Fig . 17 0 E). First , we must assum e that th e neck membrane (Ctf), between the points x and y above, and xr and yr below, has been extended in a fol d (F , Cv ) somewhat beyond the mout h (mtk) of the retracted head, while, at the same time, the areas of the dorsal wal l o f th e hea d o n whic h the antenna e an d compoun d eyes of the adul t ar e t o b e forme d hav e becom e invaginated posterio r t o th e clypeus (F , Clp) a s a pai r o f lateral pouche s (aop , onl y th e righ t pouch shown i n th e figure) , whic h contai n th e histoblasti c rudiment s o f th e antennae an d compoun d eyes . Next , apparently , th e clef t betwee n

314

PRINCIPLES OF INSECT MORPHOLOGY

the tw o antenno-ocula r pouche s has bee n extended forward throug h th e clypeal region, dividing the latter mesally into tw o lateral plate s covere d by the closel y appressed nec k membrane, formin g thu s tw o flat , double walled wing s on which the clypea l muscles (dlcb) tak e thei r origin , an d from the posterior ends of which the antenno-ocular pouches (aop) extend into the thorax . Finally , by a further extensio n o f the nec k fold , ther e is establishe d withi n th e latte r a preora l cavity , know n a s th e atrium (G, Atr). Th e functional mouth (mth) i s thus situated a t th e inne r en d of th e atrium , an d th e close d passages fro m th e antenno-ocula r pouche s over the wings of the clypeus are confluent a t the labrum (Lra ) in a dorsa l diverticulum o f th e atriu m jus t befor e th e mouth . Th e nec k fol d enclosing th e atriu m no w become s th e apparen t larva l head . Belo w the ora l apertur e i s a pai r o f ver y smal l ectoderma l pouche s sai d t o contain th e histoblast s o f th e imagina l labiu m (Lb), the labiu m bein g suppressed a s an externa l orga n in the larv a i n the sam e way as are th e thoracic appendages . Jus t withi n th e lip s o f th e atria l openin g ther e project, on e o n eac h side , th e tw o mout h hook s (mhk), th e substitut e jaws of the maggot . Along wit h th e numerou s transposition s tha t s o alte r th e cephali c structure o f the cyclorrhaphou s larva , th e ingestiv e apparatu s become s highly develope d an d specialized . I n th e tipuli d larv a (Fig . 17 0 A) i t is t o b e observe d tha t th e cibariu m (Cb) has a typicall y orthopteroi d position and structure, and that the cibarial and buccal dilators (dlcb, dlbc) for m a distinct grou p of muscles separated fro m the tru e pharyngea l dilators (dlphy) by the frontal ganglion connectives. In the cyclorrhaphous larv a (G ) th e cibariu m i s transforme d int o a larg e suckin g pump (Pmp), wit h its grea t mas s o f dilator muscle s (did)) arisin g o n th e invaginated clypeal surface of the head (Clp). The orifice of the pump (mth) is the functional mouth, but the true mouth is the opening from the pum p lume n int o th e stomodaeu m (Stom). Th e floor of the pum p is the base of the hypopharynx, and the salivary duct (SID) opens in the normal position betwee n the hypopharyn x an d the labia l rudiment (Lb). The lateral wall s of the pum p and th e wall s of the clypea l wings leading back t o th e antenno-ocula r pouche s (aop) become strongly sclerotized , forming th e conspicuou s scleroti c structur e lyin g in th e anterio r en d of the maggot, commonly known as the " pharyngeal skeleton" or "buccopharyngeal armature. " The mout h hook s of the cyclorrhaphou s larv a (Fig . 17 0 G, mhk) ar e often calle d " mandibles," bu t sinc e the y ar e solid cuticular structures, shed with each moult, arising from the lips of the atrial cavity (Atr), which i s evidently derive d fro m th e infolde d nec k membrane , i t i s no t clear ho w the larva l jaw s ca n have an y relatio n whateve r t o tru e man dibles. Furthermor e they lie in vertical planes and are moved by muscles

THE ORGANS OF INGESTIÓN

315

taking their origins on the lateral walls of the cibarial pump. Th e mouth hooks thus appea r to be secondary cuticular structures developed for the purposes o f the maggot . Afte r th e thir d moul t the y ar e no t renewed . The accompanying diagrams (Fig. 170 E, F, G) perhaps do not express accurately all the relation s i n the larva l hea d structures, sinc e there ar e still certain points that are obscure, but th e know n facts o f development and of comparative anatomy demonstrate that in some such way as that indicated the peculiar structures of the cyclorrhaphous maggot have been evolved fro m th e mor e usua l typ e o f head structure . Essentially , th e condition is one in which not onl y the appendage s but a large part of the head as well have been reduced to histoblastic rudiments and withdrawn into th e bod y durin g th e larva l stage , t o b e everte d late r durin g the pupal transformation i n order to complete their development into the imaginal parts . I n mos t insect s th e invagination s tha t contai n the imagina l histoblasts d o not appea r unti l th e last larval stage; in the higher Dipter a the y ar e forme d i n the embry o and thu s becom e a par t of the larva l structure. A much more primitive condition occurs in some of th e lowe r Diptera , a s i n Chironomus an d Psychoda, i n whic h th e antenno-ocular pouche s are formed onl y in the pupa l integument devel oped i n th e las t larva l instar , whil e th e othe r feature s o f th e cyclor rhaphous larva d o not appea r a t all . THE FEEDIN G MECHANIS M O F ADUL T DÍPTER A No adul t diptero n ha s mout h part s o f th e typica l bitin g typ e o f structure. Thoug h certai n flie s ar e sai d t o "bite, " th e effec t i s th e result o f a puncture an d no t o f a pinch. Th e majorit y o f flies are inca pable o f inflictin g an y kin d o f wound . Th e familia r "bitin g flies " belong to two groups; in one group the mandibles are the piercing organs, in th e othe r th e labiu m i s th e effectiv e instrument . Mandible s occu r only in a fe w of the mor e generalized families of Diptera, being presen t in th e female s o f Phlebotomus (Psychodidae) , Dixidae , Culicoides (Chi ronomidae), Culicidae , variou s specie s o f Simuliidae, and i n Tabanidae . Among mal e flies mandibles ar e sai d t o occli r onl y i n certai n specie s of Simulium. Flie s havin g a piercing labium include principally the tsetse fly, stabl e fly , an d hor n fl y o f the famil y Muscidae . Th e robbe r flies (Asilidae) als o shoul d b e include d amon g th e bitin g flies , thoug h the y confine thei r attack s t o othe r insects . Th e piercing organ of the robbe r flies, however , appear s t o b e th e hypopharynx , whic h i s long , shar p pointed, an d protractile . Th e onl y truly bitin g flies are certai n specie s of Dolichopodidae , in which the termina l lobes of the labiu m are strongl y sclerotized and jawlik e in form an d action . The bes t known of the mandibulat e piercin g flies are the femal e horse flies (Tabanidae), and female mosquitoes. Th e mouth parts of a horse fly

316

PRINCIPLES OF INSECT MORPHOLOGY

may b e studie d a s a n exampl e of the mor e generalize d condition o f thf c mouth part s a s i t occur s in adul t Diptera . T o understan d th e entir e feeding mechanism, however, it wil l be necessary to know also somethin g of th e structur e o f the clypea l region of the head . The Feedin g Organ s of a Horse Fly.—The mouth parts of the horse fly form a compact group of organs projecting downward from the peristoma l margin of the hea d (Fig . 17 1 A). I n th e femal e fly there are nine pieces included i n th e group , thre e o f whic h ar e media n an d unpaired , whil e the othe r si x represent thre e pair s of lateral organs. Th e mos t anterio r of the median organs is the broad, dagger-shaped labrum (Lm). Its lateral edges are overlapped in the usua l position b y a pair o f large two????????? ????? ???????? ????? ?????? ?? ??? ????????? ????????? ?? the labru m ar e tw o pair s o f long, slender, taperin g blades , th e anterio r ?? ????? ??? ??? ????????? ????? ??? ????????? ??? ????? ?? ??? ???????? (Mx). Posterior to the mandibles and between the maxillae is a second

???? ????????? ??? ????? ????? ?? ?? ????? ????? ???? ??????? ???????? ?? ???????? view. B, detail s of clypeus and labrum . C, labium and maxillae , posterio r view .

median piec e resemblin g the labru m bu t slenderer . Thi s i s th e hypo pharynx. Behin d an d partl y enclosin g al l thes e part s i n th e norma l position i s the larg e median labium (L6 ) endin g in two broad lobes . The Labrum and th e Clypeus.—The labru m o f the hors e fly, as note d above, i s shape d lik e th e blad e o f a dagger , bu t i t i s no t particularl y rigid, an d it s poin t i s blun t (Figs . 17 1 A, B , 17 2 A, Lm). Proximall y it i s attache d t o th e lowe r margi n o f the facia l are a o f the hea d b y a median membranou s are a an d tw o divergen t latera l arms . A shor t median apódeme projects dorsally into the head cavity fro m th e anterio r wall of the labru m an d give s insertion t o a fan-shaped muscle arising o n the clypea l plat e o f th e hea d (Fig . 17 1 B , dp). Thi s muscl e serve s apparently t o kee p the labru m i n clos e contac t wit h th e othe r piece s of the mout h parts . Th e presenc e o f a clypeolabra l muscl e i s a specia l feature o f the Dipter a an d constitute s a n exceptio n to th e genera l rul e that the labra l muscles take thei r origi n on the irons. Th e posterior, o r epipharyngeal, wal l o f th e labru m i s excavate d b y a media n channe l

??? ?????? ?? ????????? ???

continued proximall y int o th e smal l mouth apertur e locate d behin d th e base of the labru m (Fig . 17 2 A, mth). When th e mout h part s o f th e femal e hors e fl y ar e i n th e norma l position, th e labral groove is closed posteriorly by the broad, overlapping mandibles, an d there i s thus formed a tubular passag e leading up to th e mouth. Thi s condui t i s th e food meatus, which , i n femal e Tabanidae , as pointe d ou t b y Voge l (1921) , lie s thu s betwee n th e labru m an d th e mandibles. I n th e femal e mosquito , however, Vogel shows, the labru m itself form s th e foo d cana l since its concavity is closed posteriorly b y th e approximation o r overlapping of its incurve d lateral margins . In many o f the Dolichopodida e the epipharyngea l wall of the labru m bears a n armatur e o f spine s o r movabl e teeth , th e latte r bein g highl y developed i n th e genu s Melanderia; bu t since , i n general , th e posterio r surface o f the dipterou s labru m i s smoot h an d present s n o structur e of any kin d t o b e specificall y terme d a n epipharynx , th e write r see s n o reason fo r followin g th e usua l custom of calling the elongat e labral lob e of th e Dipter a a "labrum-epipharynx. " ??? ???? ?? ??? ???? ???? ?? ??????? ???? ????? ??? ?????? ?? ???? pended i s a median par t o f the clypeu s (Fig . 17 1 B, dp ) separate d fro m the latera l part s o f the clypea l area by a membranous fold o n each side. ??? ??????? ?????? ?? ??? ???? ??????? ????? ??? ?????? ?? ??? ???? ??????? anterior tentoria l pit s (atf at), from which the epistomal suture (es) is arched upwar d and crosse s the lowe r part o f the fac e beneat h th e base s of the antennae. Upon the median clypeal plate (dp), as we shall presently see, the dilato r muscle s of the suckin g pump take their origin . In th e Tipulida e th e clypea l area form s th e uppe r wal l of a snoutlik e projection of the hea d extending anterior to the eyes , and in some species bearing a stron g spine-lik e proces s nea r it s dista l end . I n th e highe r Diptera th e media n part of the clypeu s becomes an independent sclerite , but th e dilato r muscle s of the pum p retai n thei r attachment s upo n it . The Mandibles. —The mandible s o f th e hors e fly are long , flattened , sharp-pointed blades , their tip s reachin g t o th e apex o f th e labru m (Fig. 171 A, Md). Each is articulated by an expanded base (Fig. 172 B) to th e lowe r edge of the hea d a t th e side s of the mout h an d i s provided with antagonisti c muscle s that tak e thei r origi n on the hea d wall . Th e mandibles o f the fly are thus, a s in biting insects, capabl e of being moved in a transvers e plane , bu t the y hav e n o movement s o f protractio n o r retraction. Th e thrus t o f the piercin g mandibles i s made b y a forcefu l action o f the hea d an d bod y of the fly . The Maxillae.— Each maxill a consist s o f a basal part evidentl y com posed o f th e card o an d stipe s (Fig . 172 , C , Cd , St), o f a large , thick , two-segmented palpu s (Pip), an d a long , slender , taperin g blad e (Go). The maxillary bases underlap the proximal part of the labium (Fig. 171 C,

318

PRINCIPLES OF INSECT MORPHOLOGY

Pmf) an d ar e articulated b y their cardina l extremitie s t o the lowe r edges of th e postgena e (a") jus t belo w th e posterio r tentoria l pit s (pt, pt). The palpus (MxPlp) arise s by a narrow stalk fro m th e oute r edg e of th e stipes. Beyon d th e bas e o f the palpu s th e stipe s i s continue d int o th e long blade-lik e maxillar y lobe . Th e latte r (Fig . 17 2 C , Go) ma y b e regarded a s th e galea , sinc e i t appear s t o correspon d t o th e principa l lobe o f th e maxill a i n Hymenopter a an d Lepidoptera . Nea r it s bas e it give s of f a smal l stri p (a ) that connect s mesally wit h th e hea d wall . Patten an d Evan s (1929 ) regar d thi s smal l lob e in Haematopota a s th e lacinia, but it s origin from th e base of the larger lobe makes this interpretation see m doubtful. Th e maxillae are well developed in many orthor rhaphous flies that lac k mandibles.

FIG. 172.—Mout h part s o f Tabanus atratus, separated , latera l view . A , labru m and suckin g pum p (cibarium ) wit h dilato r muscle s arisin g o n clypeus . B , lef t mandible . C, lef t maxilla . D , labium , hypopharynx , an d salivary syringe.

The Hypopharynx an d th e Salivary Syringe. —The hypopharyn x i s a long, narrow, tapering style t (Fig . 17 2 D, Hphy) arisin g from th e ventra l wall of the head just behind the mouth aperture. Normally it lies in th e deep groov e of the anterio r fac e o f the labium . I t i s traversed b y th e outlet duc t of the salivar y glands (sra) , which opens by an aperture on its tip (Slo). Jus t proxima l t o th e bas e o f th e hypopharyn x th e duc t i s enlarged t o for m a syringe-lik e apparatu s (Syr), th e anterio r wal l of which i s inflecte d into th e lume n an d i s provide d wit h dilato r muscle s (dlsyr) arisin g on the posterio r surfac e o f the suckin g pump of the feedin g apparatus (A , Pmp). Sinc e th e foo d pum p o f Dipter a represent s th e cibarium o f orthopteroid insect s (Fig . 155 , Cb), its floor is formed b y th e adoral surfac e o f the hypopharynx . Th e dilato r muscle s of the salivar y syringe, therefore , ar e th e usua l hypopharyngeal muscle s (Is ) o f th e salivarium; th e ventral , o r labial , muscle s ar e absent . Jus t ho w the syring e an d th e salivar y channe l have becom e enclosed within th e hypopharynx cannot be explained. Th e salivary glands of the Tabanida e

THE ORGANS OF INGESTIÓN

319

are long , simple , tubula r organs ; thei r secretio n i s sai d t o contai n a powerful blood anticoagulin (se e Patten and Evans, 1929) . The Labium. —The labiu m o f the hors e fly is a large, thick, elongat e appendage (Fig . 17 2 D, Lb) endin g i n tw o broa d lobe s know n a s th e lobelia (La). I t i s suspended fro m th e posterio r par t o f the hea d b y a membranous basa l regio n (Fig . 17 1 C, Pmt) lyin g between the maxillar y stipites an d i s separated fro m th e forame n magnu m (For ) b y a narrow hypostomal bridg e (Hst) connectin g the posterio r angle s of the postgenae . The stalk of the fre e part of the orga n is the stipital region of the append age an d i s therefore the prementu m (Figs . 17 1 C, 17 2 D, Prmt). I t i s deeply evacate d longitudinall y o n it s anterio r surface . I n th e norma l position of the mouth parts, the labrum, the mandibles, the hypopharynx , and the maxilla e all lie within the cavit y o f the labium, the hypopharyn x immediately behin d th e mandibles , wit h th e maxilla e to eithe r sid e of it but posterio r t o it s broa d lateral margins . Th e relation s o f the severa l elements of the mouth parts to one another ar e best seen in a cross section (see Vogel, 1921) . The terminal lobes of the labium, or labella, are large soft pads capable of bein g spread outwar d fro m th e en d o f the stipita l stal k (Fig . 17 2 D) to form a broad disc, sometimes called the "ora l sucker." Th e posterior halves o f the labell a ar e united , bu t thei r anterio r part s ar e separate d by a deep median cleft. Th e under surface of each lobe is marked by th e lines o f numerous close-set, transvers e channel s in its membranou s wall, called pseudotracheae fro m thei r superficia l resemblanc e t o half-ope n tracheal tubes . Th e mesa l ends of the canals lead to the base of the clef t between th e labella r lobes , at whic h point normall y lie s the ape x of th e labrum. I n th e feedin g fly, the bloo d collected by the labella r channel s is here taken int o the foo d cana l between the labru m an d the mandibles . In th e genu s Melanderia o f th e Dolichopodida e th e labell a hav e a very unusua l developmen t (se e Aldrich, 1922 ; Snodgrass, 1922) . Eac h labellum ha s a strongl y sclerotize d movabl e lob e wit h th e free , shar p apical part turned inward. Th e two lobes give the appearanc e of a pai r of mandibles , an d they are provided wit h muscles arising in the premen tum s o attached o n the lobe s that th e latte r apparentl y ca n be opened and close d i n th e manne r o f a pai r o f jaws. Sinc e the Dolichopodida e are predacious, it i s highly probable that the labella r lobes of Melanderia serve to grasp and hold the living prey on which the insects feed . The labell a o f th e fl y labiu m hav e bee n generall y regarde d a s th e paraglossae, apparen t rudiment s o f the glossa e being sometime s presen t between them ; bu t Crampto n (1923 , 1925&) , fro m a comparativ e stud y of the labiu m in Diptera an d related insects , ha s given reasons for believing tha t th e labella r lobe s ar e th e labia l palpi . Palpi , however , ar e typically provide d with antagonistic muscles ; the lobe s of the fly labium

320

PRINCIPLES OF INSECT MORPHOLOGY

have usually eac h only on e muscle inserted directl y upo n it. Th e ter m "labella" is used b y som e writers i n a singular sense , bu t i t i s properly the plura l o f labellum (diminutiv e o f "lip"), which is the Lati n for m of the singular . The Sucking Pump.—The suckin g apparatus o f the hors e fly is a small chamber whic h extend s upward in th e lowe r part o f the hea d fro m th e functional mout h (Fig . 17 2 A, Pmp). Th e posterio r an d wid e latera l walls o f the orga n ar e strongl y sclerotize d an d fixed to th e uppe r en d of the labrum . Th e anterio r wall , o n the othe r hand , i s thin an d flexible and is ordinarily deepl y invaginated int o the lumen of the pump, but i t is provided with two large groups of muscle fibers taking their origi n on th e median clypeal plate of the head wall (Fig. 171 B, dp) an d is thus capable of exertin g a suckin g action o n the liqui d foo d ascendin g t o th e mout h through th e foo d cana l o f the mout h parts . Th e origi n o f th e dilato r muscle o f the pum p o n the media n plat e o f the clypeu s show s that th e sucking apparatus o f the Dipter a i s the cibariu m o f orthopteroid insects , together wit h it s dilato r muscle s an d th e clypea l plat e o n whic h thes e muscles take thei r origin . Th e functional mouth apertur e (mth) leadin g into the pump chamber, therefore, i s not the true mouth, the latter being the opening into the stomodaeum at the inner end of the pump . The Muscoi d Type s o f Mout h Parts.—I n th e highe r Dipter a th e external feedin g apparatu s o f the adul t fl y consist s o f a proboscis. Th e proboscis i s a composit e structur e forme d o f th e labrum , th e hypo pharynx, an d th e labium , al l supporte d o n a membranou s bas e tha t contains i n it s anterio r wal l th e media n clypea l plat e o n whic h aris e the dilato r muscle s o f th e suckin g pump . Th e probosci s support s a pair o f palpi, whic h are probabl y th e maxillar y palpi , an d i t terminate s in a pai r o f labellar lobes . Tw o functiona l type s o f structur e ar e dis tinguishable i n th e proboscis , dependin g o n th e natur e o f th e labella r lobes. I n mos t o f th e muscoi d flie s th e labell a ar e broad , sof t pad s resembling those o f the horse fly, and such species are incapable of biting, though som e are provided wit h smal l labellar teet h tha t enabl e them t o rasp th e foo d substances . Flie s havin g mout h part s o f thi s kin d ar e designated b y Metcal f an d Flin t (1928 ) a s the sponging type o f Diptera . With thes e flies the probosci s is usually flexible and extensibl e an d when not i n use is folded agains t th e lowe r part of the hea d or retracted int o a ventral cavit y o f th e hea d wal l surrounde d b y th e projectin g margin s of th e peristome . I n a few species of the famil y Muscidae the probosci s is rigid, an d th e labella r lobe s ar e small , cuttin g plates . Thes e specie s constitute th e so-calle d "biting " muscoid flies. The Nonpiercingj or Sponging, Type of Muscoid Mouth Parts.—The typical muscoid proboscis, as seen in the blo w fly or house fly (Fig. 17 3 A, Pr6), consist s o f three parts : first, a large basiproboscis, o r rostrum (Rst);

THE ORGANS OF INGESTIÓN

321

second, a mediproboscis, o r haustellum (Hstl) ; and , third, a distiproboscis, formed o f the labell a (La) , or lobes of the so-calle d "oral sucker/' The rostrum i s a broad inverte d con e having for the most part flexibl e membranous walls. O n the uppe r part o f its anterio r surface , however, there ar e usuall y tw o media n plate s (Fig . 174 , C , c , dp). Th e mor e ventral o f these plates (cZp) , having the for m o f an inverte d V , is a con stant feature of the rostru m in the muscoi d proboscis; the uppe r plate (c) is a weaker selerotization hingin g th e V-shape d plat e to th e lowe r edge of th e crania l capsule . O n th e lowe r par t o f th e anterio r fac e o f th e rostrum ar e two small lateral sclerite s (mxpl), whic h support th e pai r of long palpi (MxPlp). The cylindrical haustellum projects downward and somewhat forwar d from th e en d o f th e rostru m whe n th e probosci s i s protracted ; i n th e

FIG. 173.—Hea d an d probosci s o f th e hous e fly , Musca domestica. A , latera l vie w of hea d wit h probosci s extended . B , anterodista l vie w o f extende d proboscis , showin g ventral surfaces o f labella wit h apertur e (a ) leading int o food meatu s (/m ) betwee n labru m and hypopharynx.

retracted conditio n i t fold s upo n th e anterio r surfac e o f th e rostrum . The posterio r wal l of the haustellu m i s occupie d by a prominen t plat e known to students of Diptera as the thyroid or "mentum." The anterior surface i s covere d b y a long , tapering , strongl y sclerotize d fla p (Figs . 173 A, 174 C, Lm), which arises from the distal margin of the rostrum an d is partly overlappe d b y lateral fold s o f the haustellum . Thi s flap is th e labrum. B y lifting its distal en d (Fig. 17 3 B, Lm) ther e is exposed in th e anterior par t o f the haustellu m a deep , lengthwis e cavit y i n whic h lie s the blade-like hypopharynx (Hphy). Between the labrum and the hypopharynx is the foo d canal (fm) o f the proboscis, which leads up to th e functional mouth situated behind the base of the labrum as in the horse fly (Fig. 17 2 A, mth). The labella r lobe s (Fig . 173 , La ) terminatin g th e probosci s i n th e nonpiercing, o r sponging , type o f muscoid mouth part s ar e broa d pad s

322

PRINCIPLES OF INSECT MORPHOLOGY

similar t o thos e o f Tabanus. Whe n sprea d ou t fla t the y for m a n ova l disc (B , La ) crosse d b y th e pseudotrachea l channel s (6) , and enclosin g centrally a n opening (a) at th e posterio r en d of the anterio r clef t betwee n the componen t lobes . Thi s opening , which leads int o th e foo d cana l of the proboscis , i s known a s the "ora l aperture, " but i t mus t no t b e con fused wit h the functiona l mout h apertur e of the fly, which , as we hav e seen, lie s a t th e uppe r en d o f the foo d cana l betwee n th e base s o f th e labrum an d hypopharynx. Th e clef t betwee n the labella r lobe s anterio r to th e apertur e o f the foo d cana l is known as the prestomum. It s inne r walls i n som e flie s ar e arme d posteriorl y wit h severa l row s o f smal l prestomal teeth. The morpholog y o f the muscoi d probosci s i s difficul t t o understan d in all its details, and there are many features in the structure an d musculature o f this comple x feedin g apparatu s whic h show that i t i s a highl y specialized composit e organ . Th e part s o f the probosci s dista l t o th e rostrum ar e clearl y th e homologue s o f correspondin g element s i n th e mouth parts of the horse fly, including the premental region and termina l lobes o f th e labium , togethe r wit h th e labru m an d th e hypopharynx . The compositio n o f the rostrum , however , i s less eas y t o determine , bu t it apparentl y include s th e bas e o f the labiu m an d a part o f the clypea l region o f the head . The inverte d V-shape d plat e o f th e anterio r wal l o f th e rostru m (Fig. 174, C, D, dp) bears upon its lateral arms the origins of the dilator muscles o f th e cibaria l pum p (D , 3) . Ther e ca n b e littl e question , therefore, tha t thi s sclerit e represent s a t leas t th e media n par t o f th e clypeus i n th e hea d o f Tabanus (Fig . 17 1 B, dp). Th e latera l wall s of the pum p in the muscoi d flies, however, are attached b y wide apodema l inflections t o th e margin s o f th e V-shape d clypea l plate , an d fo r thi s reason anatomists hav e often regarde d the latter as a part of the suckin g mechanism. Th e pump, the clypea l plate, an d the connecting apodemes are describe d as a stirrup-shaped structur e which , in the specia l termin ology o f th e fl y head , i s calle d th e fulcrum, becaus e certai n muscle s attached upo n i t serv e t o fle x th e probosci s (se e Lowne , 1890-1895 ; Graham-Smith, 1930) . Fre w (1923 ) recognize s "thi s expose d par t of the fulcrum " a s th e clypeus . Peterso n (1916) , o n th e othe r hand , attempted t o explai n th e anterio r plat e o f the rostru m a s derive d fro m the tormae , which are sclerotic processes in the base of the epipharyngea l wall o f th e labrum . Th e attachmen t o f th e dilato r muscle s o f th e cibarial pum p on the arms of the V-shaped rostra l plate, however, clearl y demonstrates th e clypeal origin of this sclerite, and confirmatory evidence of it s homolog y with the media n clypea l region in Tabanus i s seen in the fact tha t a pair o f labral muscles (Fig . 17 4 D, 2 ) take their origi n o n it s dorsal part . Th e smalle r sclerit e abov e th e V-shape d clypea l plat e

THE ORGANS OF INGESTIÓN

323

(C, c) i s either a part of the clypeu s or a secondary sclerotization hinging the latter to the lowe r margin of the face . W e must, then , assum e that the rostru m o f the fly' s probosci s includes at leas t a part o f the clypeus , which ha s becom e detached fro m th e hea d wall s and shifte d ventrally . The origi n o f th e maxillar y palp i (Fig . 17 4 C , MxPlp) fro m smal l sclerites i n the ventra l par t o f the rostru m show s that th e las t ha s als o absorbed th e basa l part s o f the maxillae . Th e remainin g major par t of the rostru m i s apparentl y t o b e attribute d t o th e basa l par t o f th e labium.

FIG. 174.—Probosci s of a fruit fly and a blow fly . A , Rhagoletis pomonella, proboscis, anterior view . B , same , hea d an d proboscis , latera l view . C , Calliphora erythrocephala, proboscis, anterio r view . D , same , hea d an d proboscis , latera l view , showin g muscles . (Adapted from Graham-Smith, 1930. )

Within th e rostru m li e the suckin g pump, the salivar y duc t wit h it s syringe, and a pair o f long rodlike apodemes arising from th e basal angle s of th e labru m (Fig . 17 4 C, D , Ap). Th e suckin g pump of the blo w fly does not diffe r fro m tha t o f the hors e fly, except for the connectio n of it s lateral wall s with th e margin s o f the clypeu s an d i n detail s o f its form . The rostral apodeme s appear t o belon g to the labrum, bu t som e writers have regarded the m a s remnants o f the maxillae . Muscle s are inserte d upon the m whic h hav e n o apparen t homologue s in th e hea d o f bitin g insects. The musculatur e of the probosci s is somewhat comple x (Fig. 17 4 D) but i s not difficul t t o study . Mos t o f the muscles , however, appear t o be specia l adaptation s t o th e function s o f th e fl y proboscis , an d the y cannot b e satisfactoril y homologize d wit h th e muscle s o f th e mout h parts in biting insects ,

324

PRINCIPLES OF INSECT MORPHOLOGY

For a ful l accoun t o f the structura l detail s an d musculatur e o f th e head an d mout h part s o f muscoi d flie s th e reade r i s referre d t o th e elaborate pape r o n the probosci s of the blo w fly by G . S. Graham-Smit h (1930), an d t o tha t o n the hea d an d mout h part s o f the tsets e fl y b y Jobling (1933) , thoug h th e man y specia l term s use d b y thes e writers , taken largel y fro m Lown e (1890-1895) , wil l b e somewha t confusin g t o the studen t of general insect morphology. The blo w fly Calliphora erythrocephala, accordin g to the observation s of Graham-Smith , ha s several different method s of feeding, which involve a us e o f the labella r lobe s i n a s many correspondin g differen t positions . In th e nonfeedin g position , th e lobe s ar e flexed posteriorly agains t th e haustellum wit h thei r pseudotrachea l surface s i n apposition . Whe n the fl y feed s o n a film of liquid th e labell a ar e sprea d ou t fla t lik e th e leaves o f a book , bu t onl y th e part s o f thei r surface s covere d b y th e pseudotracheae ar e applie d t o th e foo d substance ; th e liqui d i s the n sucked u p throug h th e pseudotrachea l opening s ("interbifi d grooves") , and "al l particle s to o larg e t o pas s throug h the m ar e filtere d ou t an d rejected." Thi s wa y o f using th e labell a i s terme d b y Graham-Smit h the filterin g position. I n som e case s th e edge s o f the lobe s ar e turne d down, producin g a marginal ri m aroun d th e labella r disc , thu s givin g a second or cupping position. B y a separation o f th e labell a the prestoma l teeth may be partly exposed and used to some extent while the liquid foo d is stil l bein g filtere d throug h th e pseudotracheae . Thi s give s a n intermediate position leading t o th e next , o r scraping position, in whic h the labellar lobe s ar e turne d upwar d unti l th e prestoma l teet h ar e full y exposed for the purpos e o f rasping. I n thi s position th e pseudotrachea l surfaces ar e ou t o f action . Finally , ther e i s th e direct feeding position, produced by foldin g th e labell a upwar d and outwar d agains t th e side s of the haustellum , i n orde r tha t th e apertur e o f th e foo d cana l ma y b e applied directly to the food, thus allowing not only the liquid but particle s in the liqui d t o be freely ingested . In some of the nonpiercing Muscidae the prestomal teeth are large and strong, a s in Musca crassirostris, which attack s cattl e an d obtain s bloo d from the m b y scratching the ski n with its powerfu l labella r armature . The Piercing Type o f Muscoid Mouth Parts.—Muscoid flies having th e "biting'7 type of mouth parts are principally the stable flies, the horn flies, and the tsetse flies, that is, members of the gener a Stomoxys, Haematobia, and Glossina. Th e piercin g organ of these flies is the probosci s (Fig . 175 A, Prb), whic h consists o f the sam e parts as does that of the nonpiercin g muscoids. Th e haustellum , however , i s elongat e an d rigid , swolle n a t the base to accommodate the contained muscles, and tapering toward the extremity. Th e labellar lobe s (C, La) are small, flat, and densely horny , and th e prestoma l teet h ar e wel l developed . Th e labru m i s almos t

THE ORGANS OF INGESTIÓN

325

circular i n transvers e sectio n (B , Lm) an d i s firml y locke d withi n th e upturned edges of the labiu m (L6) , the tw o parts formin g thu s a tubula r food channel (/m), within which lies the slender hypopharynx (Hphy) traversed b y th e salivar y channe l (sm). Th e beaklik e probosci s i s forced into the flesh of the victim by a strong thrust of the head and body of th e fly , an d th e bloo d i s sucke d up directl y throug h th e foo d canal . The salivar y secretio n of Glossina is said to preven t clotting of the blood . For furthe r informatio n o n the structur e o f the mout h parts and th e feeding mechanis m o f the piercin g muscoid flies, the studen t shoul d con-

FIG. 175.—Hea d an d mout h parts o f a tsets e fly . A , Glossina palpalis, male , head and proboscis. B , G. fusca, cros s section of proboscis. (From Vogel, 1920.) C , G. palpalis, mouth parts an d sucking pump.

suit th e work s of Cragg (1912) , Hanse n (1903) , Jobling (1933), Minchin (1905), Patte n an d Evan s (1929) , Stuhlmann (1907) , and Voge l (1920). 7. TH E MOUT H PART S O F SIPHONAPTER A

The mout h part s o f the flea s appea r t o b e o f the dipterou s typ e of structure, bu t the y ar e mor e generalize d tha n thos e o f an y adul t fly . None of the usual pieces is lacking, and both the maxillae and the labiu m retain long , segmented palpi . Th e mandibles ar e said t o b e the cuttin g and piercin g organ s of the fleas, and th e preora l foo d cana l lie s betwee n them an d th e concav e unde r surfac e o f the labrum , a s i t doe s i n th e Tabanidae. Th e essential characters of the flea mouth parts, as described by Patte n an d Evan s (1929) , ar e a s follows : Th e labru m i s lon g an d slender bu t i s blunt a t th e apex ; its latera l edge s ar e rolle d downward between th e mandibles , formin g th e anterio r wal l o f th e foo d canal . The mandibles are long, sharp-pointed blade s armed distally with minut e

326

PRINCIPLES OF INSECT MORPHOLOGY

teeth. Th e maxillae are short , rathe r wid e plates, bearin g each a long, segmented palpus. Th e labium consists of a short median body, hollowed anteriorly, bearing distally a pair o f segmented palpi. Th e shor t hypo pharynx project s into th e proxima l en d o f the foo d cana l betwee n th e bases of the mandibles ; upon it open s the duc t of the salivar y glands, the secretion o f which is conveyed to th e woun d throug h a channel between the posterio r edge s of the mandibles . 8. TH E FEEDIN G MECHANIS M O F THYSANOPTER A

There ar e fe w insects s o isolated fro m othe r order s by som e peculiar feature o f their anatom y a s ar e th e Thysanopter a an d th e Hemipter a in the for m an d structure of their mout h parts. I t ha s ofte n seeme d to entomologists tha t th e Hemipter a i n particula r mus t b e a grou p bu t distantly relate d t o othe r insects , an d ye t suc h a n assumptio n i s dis -

FIG. 176.—Mout h part s an d suckin g apparatu s o f Thysanoptera . A , Heliothrips femoralia, section of head and beak, showing food meatus (fni) between labrum and hypopharynx, an d suckin g pum p (Pmp) wit h it s dilato r muscles . B , same , mandible . C , Frankliniella tritici, maxilla . D , Heliothrips haemorrhoidalis, maxill a an d muscles . (A, B , C from Peterson, 1915 ; D from Reyne, 1927. )

credited in all parts of their organizatio n except the mout h parts. I t i s true, however , that th e member s of these tw o suckin g orders posses s a piercing mechanism, at least, that has no counterpart i n any other group of insects , though some of its feature s are suggested in certain structure s found i n the Psocida e and Mallophaga. The mout h part s o f the thrips , whil e aberrant i n som e respects, ar e distinctly more generalized than ar e those of the Hemiptera , an d studie s of thei r developmen t giv e us a n insigh t int o th e natur e o f the unusua l modifications tha t hav e produce d the distinctiv e character s o f both th e Thysanoptera an d th e Hemiptera . Th e most importan t paper s o n th e mouth parts of the thrips are those of Peterson (1915 ) and Reyn e (1927). The curiousl y distorted hea d o f the Thysanoptera , whic h usually is produced forwar d fro m th e thorax wit h th e facia l are a turne d ventrall y (Fig. 17 6 A), bear s a short , thick , conica l bea k projectin g downward from th e posterio r par t o f th e unde r surface . Externall y th e bea k i s formed b y th e labru m in fron t (Lm), th e maxilla e on the sides , an d th e

??? ?????? ?? ????????????

labium behind (Lb). Within the beak is contained a single mandible, which is the lef t one , two piercing stylets associate d with th e base s of the maxillae, and the hypopharynx (Hphy). All the elements of the piercing mechanism and th e lowe r parts o f the hea d wall are subject to a n asym metry o f shape, an d the y ma y diffe r i n detail s o f form i n th e tw o thy sanopterous suborders , th e Terrebranti a an d Tubulifera , bu t thei r essential structur e i s the sam e i n bot h groups . The Labrum.—Th e labru m (Fig . 17 6 A, Lm) i s a broad , triangula r lobe, usually of irregular form, coverin g the anterio r surfac e o f the beak . Between its wid e base and th e lowe r edge of the facia l region of the hea d is an asymmetrica l triangula r sclerit e whic h is probably a basa l par t of the labrum , since , fo r reason s t o b e give n presently , th e write r woul d regard the clypeus as being contained in the large facial region of the hea d capsule (Clp). The Mandible.—Th e mandible developed on the lef t sid e of the hea d is contained in a mandibular pouch invaginated within the hea d fro m th e basal angl e between the contiguou s surfaces of the labru m and the lef t maxilla. Th e mandibl e i s a n elongat e piercin g orga n (Fig . 17 6 B) , consisting of a wider basal part (6) and a slender distal stylet (Stl). On the bas e ar e inserte d retracto r muscle s arisin g o n th e hea d wall , bu t protractor muscles are said to be absent. A functionless rudiment o f the right mandible , according to Peterson (1915) , i s present o n the righ t sid e of th e head . The Maxillae.—Th e maxilla e o f th e thrips , wit h thei r associate d stylets, ar e o f particular interes t becaus e of the ligh t the y hav e thrown on th e obscur e morpholog y o f th e maxilla e i n th e Hemiptera . Eac h maxilla consists o f an elongate , triangular plat e (Fig . 17 6 C, St) formin g the latera l wal l of the conica l beak. Nea r its dista l en d it bear s a shor t ?????? ????? ?? ??? ?? ???? ????????? ???? ??? ????? ???? ?? ??? ???? of th e maxillar y plat e ther e i s give n of f mesally (i n th e Terrebranti a ?? ?????? ? ????? ??? ?????? ????? ???????? ? ????? ??????? ????????? ????? (Stl) wit h a n enlarge d base, which is contained in a pouch of the ventra l head wall . I t i s shown b y Reyn e (1927 ) tha t th e maxillar y style t i s formed i n the embry o from th e bod y of the maxilla . Th e latter , a t th e end of the sixt h day o f development, is still a simple lobe, but soo n a clef t appears i n the maxillar y rudiment , whic h separates th e basa l par t fro m the rest , an d late r fro m thi s basa l par t th e style t i s forme d a s a n out growth. Retractor muscles from the head wall (D, rstl) become inserted ?? ??? ??? ????? ?????????? ??? ?????? ???? ??? ??????? ?????? ??? ???? tractor muscles (pstl) are developed in the tissue between the bar and the inner face of the plate. Th e latera l part of the maxilla bearing the palpus is clearly the stipes , whic h terminates i n a single lobe (mxl) representin g the fuse d gale a an d lacinia . Th e stylet , accordin g t o Reyne , i s o f th e

328

PRINCIPLES OF INSECT MORPHOLOGY

nature o f a larg e cuticula r spin e produce d fro m th e mesa l par t o f th e maxillary base , secondaril y spli t of f fro m th e regio n o f th e card o an d stipes. The structur e o f th e thrip s maxilla , a s w e shal l presentl y see , i s almost a n exac t duplicat e o f tha t o f th e hemipterou s maxill a (Fig . 181 B), i n whic h th e par t representativ e o f the style t i s usually draw n out into a long slender bristle (MxB), Furthermore, the " mouth forks" o f the Psocida e an d simila r rod s associate d wit h th e maxilla e i n some of the Mallophag a appear t o b e structures analogou s at leas t wit h the thysanopterou s maxillar y stylets . Al l thes e anomolou s organ s Hansen (1930 ) woul d deriv e fro m th e superlingua e an d no t fro m th e maxillae. Hansen , however , entirel y ignore s th e impor t o f Reyne' s studies o f the developmen t of the mout h parts i n the Thysanoptera , an d he give s n o weigh t t o Reyne' s assertio n tha t th e maxillar y stylet s ar e actually split of f from the maxillary rudiments during embryonic growth. The Labiu m and Hypopharynx.—Th e labium of the Thysanopter a i s a wid e triangular appendag e formin g th e posterio r surfac e o f the bea k (Fig. 17 6 A, Lb). Distall y i t bear s a pair o f short two-segmente d palp i and terminates i n one or two pairs of flaps which are evidently the glossa e and paraglossae . Th e hypopharyn x (Hphy) i s a shor t mediu m lob e arising fro m th e anterio r surfac e o f th e bas e o f th e labium . Betwee n it an d th e labru m i s th e foo d meatu s o f th e bea k (fm) leadin g t o th e mouth apertur e (mth) behin d th e bas e o f the labrum . Th e duc t o f th e labial gland s (SID) open s posterior t o th e bas e of the hypopharynx , an d the salivar y liqui d i s conveye d t o th e ti p o f th e bea k throug h a channel, th e salivar y meatu s (sm) , betwee n th e hypopharyn x an d th e labium. Th e salivarium into which the salivar y duc t opens has muscles inserted upo n it, formin g thu s a structure suggestive of the mor e highl y evolved salivary syring e of the Hemiptera . The Suckin g Pump.—Th e suckin g organ o f the thrip s (Fig . 17 6 A, Pmp) is very similar to that of the Hemiptera (Fig. 179, Pmp). It is a cibarial chambe r enclose d withi n th e head , an d it s externa l apertur e (mth) i s directl y continuou s wit h th e foo d meatu s (fm) o f th e beak . On its dorsa l wall are inserted long bundles of dilator muscl e fiber s (dlcb) that take their origin on the anterio r par t of the facia l region of the hea d capsule. Th e crania l suture s ar e obsolet e i n th e Thysanura , bu t i t i s evident tha t th e hea d are a o n whic h th e pum p muscle s aris e (Clp) corresponds t o th e enlarge d clypea l plat e i n th e hea d o f a cicad a (Fig . 177 B, Clp) or a psocid (A, Clp). 9. TH E FEEDIN G MECHANIS M O F HEMIPTER A (RHYNCHOTA )

The typica l hemipterou s feedin g mechanis m differ s fro m tha t o f th e Thysanoptera i n the followin g respects: (1) The beak is usually long and

THE ORGANS OF INGESTIÓN

329

slender an d consist s principall y o f th e labium , whic h lack s palp i an d terminal lobes , th e shor t labru m coverin g onl y it s basa l part ; (2 ) bot h mandibles are symmetrically developed , and their apica l parts are drawn out into long slender bristles , eac h movable by retractor an d protracto r muscles; (3 ) the latera l plate s o f the maxilla e ar e mostl y incorporate d into th e hea d capsul e wit h onl y thei r termina l part s free , maxillar y palpi ar e lacking , an d th e maxillar y stylet s ar e lon g slende r bristle s similar t o th e mandibula r bristles ; (4 ) th e salivar y syring e i s a well developed force pump for ejecting the saliva , an d its duct trans verses the short hypopharyn x t o it s tip ; (5 ) both th e foo d cana l an d th e salivar y canal o f the bea k li e between th e closel y apposed inne r surface s o f th e maxillary bristles . The Structur e o f the Head.—I n a typica l homopterou s insect , suc h as the cicada , th e hea d capsul e presents anteriorl y a prominen t conve x plate (Fig . 17 7 B, Clp). Thi s plat e clearly belongs to the clypea l region of th e head , sinc e th e dilato r muscle s o f the suckin g pum p (cibarium ) take thei r origin s upo n it . Th e plate i s bounded laterall y an d dorsall y by a deep groove (es) identified as the epistomal suture by the presence of th e pit s of the anterio r tentorial arms in its lateral parts (Figs . 177 A, 178, at). Th e epistoma l sutur e i s strongl y arche d upwar d o n the face , its transvers e dorsa l par t lyin g betwee n th e base s o f the antennae . A comparison of the cicad a head (Fig . 17 7 B) with the hea d of a procid (A) leaves little doubt of the homologies of the facia l plates in the two insects. The iron s o f the cicad a i s reduced to th e small , imperfectl y demarked, triangular area (Fr) on the top of the head, immediately above the large clypeal plate , bearin g th e media n ocellus . Betwee n th e larg e clypea l plate an d th e bas e o f the labru m (Lm) ther e i s in th e cicad a a smalle r anteclypeal plate (Aclp), but in some of the Homoptera the separation between the two clypeal areas is indistinct o r absent. Th e labrum in the cicada is a small, slender, tapering lobe (Lm) closely applied to the anterio r side o f the bas e o f the beak . I t i s often calle d the "epipharynx. " On eac h sid e o f th e hea d belo w th e compoun d ey e ar e tw o latera l plates (Fig . 178 , A , B ) separated b y a deep membranous groove (h), bes t seen in the sof t hea d o f a newly emerged imago (B) . Th e anterio r plat e (B, A), termed the lorum by homopterists, migh t appea r fro m its positio n to represent th e mandible , sinc e it s dorsa l extremit y lie s immediatel y beneath th e roo t o f the anterio r tentoria l ar m (at) and it s lowe r part is continuous with the lateral wall of the hypopharynx (Hphy), but an identity with the mandible i s not borne out by studies of the developmen t of thi s plate. Th e sclerite , however , is commonly called the mandibular plate, since the mandibula r bristl e i s articulated b y a leverlike arm wit h the posterior borde r of its upper part (Fig. 181 A, for), and the mandibula r protractor muscles (pmdb) arise on its lower part. The second lateral

330

PRINCIPLES OF

INSECT MORPHOLOGY

plate (Fig . 178 , #) , which is continuous dorsall y wit h the crania l wall, is known a s the maxillary plate. It s uppe r par t i s probably th e gena , bu t its lowe r extremit y (B , MX ) an d th e smal l taperin g appendicula r lob e

FIG. 177.—Types of head structure in Corrodentia and Hemiptera. A, head of a psocid. B , Magicicada septendecim. C , Deltocephalus simplex (Cicadellidae) . D , E , Lepyronia quadrangularis (Cercopidae) . F , Ocleus borealis (Fulgoridae) . G , Ceresa diceros (Membracidae) . H , Laternaria (Fulgoridae) . I , Corixa (Corixidae) . J , K , Euschistus variolarius (Pentatomidae) .

suspended fro m i t (mxl) hav e bee n show n from embryologica l evidence to be the basa l part of the maxilla fuse d wit h the lateral wall of the head . The maxillar y bristl e i s articulated b y a leve r wit h th e maxillar y plat e (Fig. 18 1 B), an d th e maxillar y protracto r muscle s (pmxb) aris e o n th e lower part o f the latter .

THE ORGANS OF INGESTIÓN

331

The posterior par t o f the craniu m is imperfect i n the cicada , an d th e labium i s suspende d fro m a larg e membranou s ventra l are a continuou s with the neck . It i s no t difficul t t o identif y th e crania l area s o r sclerite s o f othe r Homoptera wit h thos e o f th e cicad a i f th e criterio n o f muscl e attach ments i s consistentl y followed . Th e clypeu s (o r postclypeus ) i s i n al l cases th e plat e o n which arise th e dilato r muscle s of the suckin g pump. In Cicadellida e (Fig . 17 7 C), Cercopida e (E) , an d Membracida e (G) , it has a positio n simila r t o tha t i n th e cicada ; i n th e cercopid s its uppe r part is reflected beyon d the fastigia l angle of the craniu m and appear s a s a small sclerite in the dorsa l wall of the hea d (D , Clp). Th e anteclypeu s is not alway s distinctl y separate d fro m th e postclypea l are a (E) , an d i n some Fulgorida e th e clypeu s is undivided (F , Clp). Th e clypeu s of th e fulgorids i s relativel y smal l an d ha s a ventra l (F ) o r posteroventra l position o n the under side of the hea d (H) ; the vertex , on the othe r hand , is larg e an d ofte n elongat e o n th e facia l aspec t o f th e hea d (F , Vx), in th e lanternflie s (Laternaria) i t attain s a n extrem e siz e an d a highl y grotesque for m (H) . I n th e Psyllida e th e hea d i s markedl y opisthog nathous, th e clypeu s bein g ventral , an d th e bea k se t fa r bac k o n th e under sid e o f the head . Thi s conditio n i s still more exaggerated i n th e flattened " sternorhynchous" larva l an d adul t femal e Coccidae . O n the side s o f th e homopterou s hea d th e mandibula r an d th e maxillar y plates ar e i n mos t case s easil y recognize d (C , E , G , A , E ) o r ca n b e identified respectivel y a s th e area s o n whic h aris e th e mandibular'an d maxillary protracto r muscles . In th e Heteropter a th e hea d differ s i n severa l respect s fro m tha t of th e Homoptera , an d th e homologie s of its part s ar e mor e difficul t t o determine. Th e beak usually arises anteriorl y (Fig . 17 7 K), the clypea l area i s typicall y dorsa l (J , K , Clp), an d ther e i s a larg e ventra l are a of th e hea d behin d th e beak , walle d b y a scleroti c hypostoma l bridg e (usually calle d th e "gula" ) betwee n th e bas e o f th e labiu m an d th e foramen magnum . I n Corixida e and Notonectidae, however, the mout h parts ar e ventral an d the facia l region is directed forwar d (I) . Th e are a of the clypeu s is marked by the origin s of the dilato r muscle s of the suck ing pump (Fig . 184 A, did)) an d may exten d far back on the dorsa l surfac e of the hea d (Fig . 17 7 J, Clp), bu t it s upper or posterior part is not define d by a suture (I , J). O n the othe r hand , th e dista l part o f the clypeu s or anteclypeal region (J, Aclp), calle d the tylus by heteropterists, i s margined by dee p cleft s tha t separat e i t fro m latera l lobe s o f th e hea d (A, A ) known as the juga. Thes e paraclypeal lobes appear to be the mandibula r plates o f the Homoptera , sinc e th e mandibula r bristle s ar e articulate d to thei r latera l margin s an d th e mandibula r protracto r muscle s aris e upon them . I n Corixida e an d Notonectida e th e mandibula r muscle s

332

PRINCIPLES OF

INSECT MORPHOLOGY

arise o n th e inflecte d mesa l margin s o f th e lobes , whic h suppor t th e sucking pump . Th e maxillar y plate s generall y hav e th e usua l positio n on th e side s of the hea d (K , J5) , bu t i n Notonectida e the y ar e inflecte d and mostl y conceale d at th e bas e o f th e labium . Th e heteropterou s labrum is relatively long (K, Lm), in Notonectidae it is a large triangular flap. The Beak. —The typica l hemipterou s bea k i s forme d principall y of the slender , segmented, but usuall y rigid labium (Fig. 178 A, Lb), which, in th e cicada , hang s freel y fro m th e nec k membrane behind th e lowe r

FIG. 178.—Hea d an d bea k o f Magicicada septendecim. A , head o f fully mature d imago . B, sof t hea d o f imago emerging from nympha l skin , with parts separated.

extremities o f the maxillary plates. Th e basal part of the beak, however, includes th e shor t labru m (Lm) , an d th e latera l space s betwee n th e labrum an d the labiu m are closed by the termina l lobe s of the maxillar y plates (mxl). Withi n th e bea k ar e enclose d the mandibula r an d maxil lary bristles (B , MdB, MxB), whic h lie in a dee p groove of the anterio r surface o f the labiu m (Fig . 18 2 A). Th e ti p o f the hypopharyn x (Fig . 178 B, Hphy) projects into the proximal part of the beak between the bases of the bristles, which issue from pouches of the head wall invaginated between the side s of the hypopharynx and the inner walls of the maxillary plates (Fig . 18 0 A). The Hypopharynx. —The hypopharyn x o f th e cicad a (Fig . 17 8 B , Hphy) i s a median conical lobe of the ventral wall of the head between the lower end s of the mandibula r plates (A) , where in th e norma l condition it is entirely concealed by the approximation of the surrounding parts (A) . The anterio r surfac e o f the hypopharyn x is continued dorsally int o th e

THE ORGANS OF INGESTIÓN

333

posterior (ventral ) wal l of the chambe r of the suckin g pump (Fig . 17 9 A, B, Pmp). Withi n th e hypopharyn x i s locate d th e salivar y syring e (Syr), the terminal duct of which (B, sm) opens on the tip of the hypopharynx (Figs . 17 9 B, 18 0 B, SIO). Th e side s o f the hypopharyn x ar e prolonged upward a s two flat, strongly sclerotize d plates (Fig . 18 0 A, B, hpl) formin g th e inne r wall s o f th e bristl e pouche s (bp). Th e uppe r extremities o f these plate s ar e secured to th e posterio r transvers e ba r of the tentorium (Tnt), th e unio n being so close in the cicad a as to make i t appear tha t the plates ar e united wit h the tentorial bar .

FIG. 179.—Th e suckin g pump and salivar y syring e of Magicicada septendedm. A , section o f the hea d showing position of the sucking pump (cibarium ) wit h dilator muscles arising on the clypeus. B, section through the mouth region, showing food meatus (fm), suck-pump (Pmp), and salivary syringe (Syr).

The Mouth.—I n the sof t immatur e stag e o f the cicad a imag o newly emerged from th e nympha l ski n (Fig . 17 8 B), there i s seen to b e a wide, open, transverse cleft between the base of the anteclypeus (A dp) and the hypopharyn x (Hphy), whic h expose s th e chambe r o f th e suckin g pump (Pmp). In the fully matured insect the lips of this cleft are always tightly shu t b y th e contac t o f the anteclypeu s agains t th e lowe r parts of th e mandibula r plate s (A) , an d b y th e closur e of the epipharyngea l wall o f the anteclypeu s upo n th e anterio r surfac e o f the hypopharynx . But th e surfac e o f th e hypopharyn x tha t i s thus covere d b y th e epi pharynx contain s a media n groove , an d thi s groove , converte d int o a tube (Fig . 179 , fm) b y th e overlyin g epipharyngeal wall , remains a s th e only entranc e int o th e pum p chambe r an d become s thus th e functional mouth. Th e chamber of the suckin g pump of the Hemiptera, however, as we shal l presentl y see , represent s th e preora l cibariu m o f orthopteroi d insects (Fig . 155, Cb). Th e true mouth, therefore, is the posterior openin g of th e pump into the stomodaeu m (Fig . 17 9 B, Mth).

334

PRINCIPLES OF INSECT MORPHOLOGY

The Suckin g Pump.—Th e narrow , tubula r functiona l mouth o f th e Hemiptera, o r channel between the anterio r surfac e o f the hypopharyn x and th e appose d epipharyngea l surfac e o f the anteclypeu s (Fig . 17 9 A, B, fm), o n the on e hand, connect s wit h th e foo d cana l between the max illary bristles (B,/c ) and, on the other, leads into the cavity of the sucking pump (Pmp). Th e latte r i n th e cicad a i s a larg e ova l chambe r lyin g almost vertica l i n th e lowe r par t o f th e hea d (A) . It s posterio r an d lateral wall s ar e conve x an d strongl y sclerotized . Th e anterio r wal l i s flexible and i s deepl y invaginate d int o th e lume n o f the chamber . O n its midline are inserted the convergin g ends of two large groups of muscle fibers (A , B, dlcb), whic h have their origin s on the entir e inner surface of the postclypea l plat e o f the hea d wal l (A , Clp). Thes e muscle s ar e th e dilators of the pump . Thei r contractio n lift s the infolde d anterio r wal l of th e organ , thus creatin g a n upwar d suctio n throug h th e tubula r entrance t o th e chamber ; with th e relaxatio n o f the muscles , th e lifte d wall spring s bac k int o th e lume n b y th e forc e o f its ow n elasticity, it s lower en d descending first . B y this mechanism the foo d liqui d is drawn into th e pum p chambe r fro m th e foo d cana l o f the bea k (B , /c) an d i s expelled upward into the anterio r par t of the stomodaeum . The pum p chambe r o f the Hemipter a ver y evidentl y represent s th e preoral cibariu m o f mor e generalize d insect s (Fig . 155 , Cfe) , thoug h i t has usuall y bee n referre d t o th e bucca l cavit y o r to th e pharynx . Its floor is formed b y the proxima l par t of the anterio r surfac e o f the hypo pharynx, an d it s roo f i s the epipharyngea l wal l of the anteclypeus . I n the cicad a th e latera l wall s o f the pum p ar e deepl y clef t b y th e wid e opening between the hypopharyn x and the epipharyngeal surface, as seen in th e newl y emerged imago (Fig . 17 8 B), an d i t i s only in th e matur e condition tha t the pum p cavit y is concealed by the firm closure o f these opposing parts. Th e true mouth of the insec t is the inne r opening of the pump (Fig . 17 9 B, Mtti) int o th e stomodaeum . The stomodaeum of the cicad a extends upward from th e inne r mout h of th e pum p i n th e usua l fashio n an d enlarge s int o a smal l sa c (Fig . 179 A, B , Phy) restin g upo n th e transvers e ba r o f the tentoriu m (Fig . 180 A). Thi s sa c i s th e tru e pharynx , a s show n b y th e fac t tha t th e frontal ganglio n lies o n its anterio r end . Th e wall s of the pharyn x ar e muscular, and th e orga n i s provided with dilato r muscle s arising o n th e postocular regio n o f th e hea d an d o n th e tentorium . Followin g th e pharynx i s a long tubular oesophagu s (Fig . 179 , Oe). The prototyp e o f the hemipterou s suckin g pump is evidently presen t in th e Corrodenti a (Copeognatha) , where , a s show n b y Webe r (1933 , Fig. 56), the ingestive apparatus includes a pumplike mechanism provided with hug e dilato r muscle s arisin g o n th e larg e postclypea l plat e tha t forms mos t o f the facia l are a o f the head . Thoug h th e psoci d pum p i s

THE ORGANS OF INGESTIÓN

335

attributed b y Webe r to th e "pharynx, " i t i s clear fro m it s relation s t o surrounding parts, and by its clypeal musculature, that it belong s to th e cibarial regio n of the preora l cavity lyin g proximal to th e mola r surface s of th e close d mandibles an d thus corresponds to th e mout h pum p o f the Dytiscus larva, which Weber refers to the cibarium (Mundhóhle). The position o f the fronta l ganglion in the cicad a on the muscula r pharyngea l sac leaves n o doub t tha t th e suckin g pump o f the Hemipter a i s a pre pharyngeal structure . Th e wholl y nonmuscular walls of the pum p an d the origi n of the dilato r muscle s on the clypeu s attest that the pum p has been evolve d fro m th e preora l cibarium . Th e enlargemen t an d dorsa l or posterio r extensio n o f the clypeu s in Corrodentia , Thysanoptera , an d Hemiptera i s clearl y correlate d wit h th e grea t development , o f th e cibarium and its dilato r muscles .

FIG. 180.—Th e salivar y syringe an d associate d structures of Magicicada septendecim A, posterio r view lookin g into bac k o f head between maxillar y plates. B , diagrammatic cross sectio n through the maxillar y plate s (B) , bristl e pouche s (bp), hypopharynx , and salivary syringe.

The position of the suckin g pump within the head varies considerabl y in different group s of Hemiptera. I n the Heteroptera th e organ generally lies farthe r bac k tha n i n th e Homopter a an d ha s a mor e horizonta l position (Fig . 18 4 A). It s variou s type s o f structure an d man y detail s in th e mechanis m o f the suckin g apparatu s hav e bee n admirabl y por trayed b y Webe r (1928 , 1928a , 1929 , 1930 , 1933) . The Salivar y Syringe.—Th e salivar y syring e o f the Hemipter a i s a small, hollow , cup-shape d orga n (Fig . 17 9 B, Syr), whic h a t it s dista l end receive s th e commo n duc t o f th e salivar y gland s (SID), an d dis charges to the exterior through an outlet tube (sm) opening on the tip of the hypopharynx (Hphy). The wider inner end of the cup is deeply invaginated an d support s a short apódem e on which are inserted a pai r of larg e muscles (dlsyr) arisin g o n th e mesa l surface s o f the lon g latera l plates o f the hypopharyn x (Fig . 18 0 B). Th e mechanis m of the appa ratus i s very simple: the contractio n o f the muscle s lifts th e invaginate d

336

PRINCIPLES OF INSECT MORPHOLOGY

end wall of the cup, and the latter springs back by its own elasticity when the muscle s relax. Thu s th e salivar y liqui d i s drawn into th e chambe r of th e cu p through the salivar y duc t an d i s forcibly expelle d through th e outlet tube . I n som e of the Hemipter a a t least , a s described by Webe r (1930), th e entranc e an d exi t o f the pum p chambe r ar e provide d wit h valvular flaps to prevent th e backwar d flow of the liquid . Thoug h th e organ is commonly known as the "salivary " pump, or syringe, the secre tion o f the connecte d glands, whic h undoubtedly ar e th e homologue s of the labial glands of other insects, probably does not have in all Hemipter a a strictl y digestiv e function . Structurally th e salivar y syring e of the Hemipter a i s very simila r t o the correspondin g organ o f Diptera; in eac h cas e the pum p chambe r i s provided with hypopharyngeal muscles only, and the exi t duct traverse s the hypopharynx to open on the tip of the latter. Morphologicall y there can b e little doub t tha t th e syring e is a highly specialize d development of th e salivar y pocket , or salivarium , o f orthopteroid insect s (Fig . 155 , Slv). It s dilato r muscle s are the dorsa l hypopharyngeal muscles of th e salivarium (Is). Representative s o f the ventra l labia l muscle s (2s , 3s) are absen t i n bot h Dipter a an d Hemiptera . Ther e i s n o evidenc e t o show how the syring e and its exi t duc t hav e become enclosed within th e hypopharynx; but i f the duc t represent s th e salivar y meatu s o f generalized insect s (Fig . 155 , sm), i t i s perhap s possibl e tha t th e apparen t posterior wal l of the hypopharyn x in the Hemipter a i s a fold o f the labia l wall. O n the othe r hand , th e whole apparatus ma y b e simply infolde d within a close d groove of the hypopharyngea l wall. I t i s interesting t o observe i n thi s connectio n th e muc h mor e primitiv e structur e i n th e Thysanoptera (Fig . 17 6 A). The Mandibula r an d Maxillar y Bristles.—Th e lon g bristle-lik e stylets characteristic of most of the Hemipter a arise from th e wall s of th e bristle pouches , which, as already noted, are invaginations o f the ventra l wall o f the head between the inne r surface s o f the maxillar y plates an d the oute r surfaces of the hypopharyngeal plates (Fig . 180 B, bp). Emerg ing fro m th e pouche s the bristle s converg e along the side s o f the hypo pharynx (A , Hphy), and , a s the y ente r th e groov e of the labium , the y become adheren t t o on e another i n a compac t bundle , o r fascicl e (Fig . 182 A). Th e base s o f the bristle s ar e enlarged; those of the mandibula r pair li e anteriorly i n the pouches , those o f th e maxillar y pair posteriorly . Within th e labium , th e mandibula r bristle s ar e th e oute r pai r o f th e fascicle (MdB), th e maxillar y bristles th e inne r pair (MxB). The mandibula r bristle s o f the cicad a ar e slightl y thicke r tha n th e maxillary bristles . Th e enlarge d base o f each lies i n th e bristl e pouc h just behin d th e lowe r end o f the correspondin g mandibular plat e (Fig . 181 A) an d i s produced proximally into tw o lon g arms. On e arm (ra )

THE ORGANS OF INGESTIÓN

337

proceeds dorsall y i n th e inne r wal l of the pouc h (bp) and give s attach ment t o th e retracto r muscle s (rmdb) arisin g o n th e dorsa l wal l o f th e head. The other arm (Ivr) goes dorsally in the external membranous groove (Fig . 17 8 B, h) betwee n the mandibula r an d maxillar y plate s o n the sid e of the head , and its upper part bends forward t o articulat e wit h the dorsa l en d o f the mandibula r plat e (Fig . 18 1 A, g) . Thi s ar m sup ports for most of its length a wide, thin apodemal inflection (ap), on which ar e inserte d th e protracto r muscle s of th e bristl e (pmdb)j whic h have their origins on the inner face of the mandibular plate (A). The protractor ar m o f the mandibula r bristl e (Ivr) thu s function s as a lever , and its relation s t o th e mandibula r plat e an d the bas e o f the bristl e ar e

FIG. 181.—Moto r mechanis m o f th e mandibula r an d maxillar y bristle s o f Magicicada septendecim. A , bas e o f a mandibula r bristl e wit h retracto r (rmdb) an d protracto r (pmdb) muscles . B , bas e o f lef t maxillar y bristle , posterio r view , wit h retracto r an d protractor muscles .

very similar to those of the maxillar y lever (B , Ivr) t o the maxillary plat e and th e bas e o f the maxillar y bristle . Th e mandibula r leve r i s differ entiated fro m th e mandibula r plat e durin g the transformatio n fro m th e nymph to the imago. The maxillar y bristle s aris e fro m th e wall s of the bristl e pouche s a t a highe r leve l than d o the mandibula r bristles . Upo n the bas e o f each (Fig. 18 1 B , MxB) ar e inserte d th e retracto r muscle s (rmxb), whic h arise o n the dorsa l walls of the head , an d als o a large protractor muscl e (Ipmxb) havin g it s origi n ventrall y o n th e inne r fac e o f th e maxillar y plate (B). I n addition to these muscles there is a second set of protractor fibers (2pmxb) arisin g o n the maxillary plate and inserted o n the leverlik e sclerite (Ivr) tha t lie s in th e wal l of the bristl e pouc h and connect s th e base of the bristl e wit h the posterio r edg e of the maxillar y plate . The tw o set s o f bristles exten d ou t o f the bristl e pouche s along th e sides of the hypopharynx , where the maxillar y bristles slid e upon track -

338

PRINCIPLES OF INSECT MORPHOLOGY

like ridges o f the latera l hypopharyngeal plates. Beyon d the ti p o f th e hypopharynx th e bristle s tur n downwar d t o ente r th e groov e o f th e labium an d thos e fro m opposit e side s converge , th e maxillar y bristle s becoming her e interlocked , whil e th e mandibula r bristle s tak e thei r positions a t th e side s o f th e maxillar y bristles . I n th e groov e o f th e labium th e fou r bristle s ar e thus assemble d i n a slende r fascicle . Th e maxillary bristles for m the cor e of the fascicl e (Fig . 182 , MxB) wit h thei r inner faces closel y applied to eac h other an d usually held firml y togethe r by dovetailing grooves and ridges extending throughout their lengths (C) . In some species of Hemiptera th e mandibular bristles ar e similarly locked to th e maxillar y bristle s (C) . Betwee n th e maxillar y bristle s ther e ar e two minut e tubula r canal s forme d b y opposin g groove s o n th e inne r surfaces o f th e bristle s (B , C , /c, sc). Th e positio n o f thes e canal s i s

FIG. 182.—Section s o f th e bea k an d feedin g bristle s o f Hemiptera . A , Magicicada septendecim, labiu m an d bristles . B , Aphis rumiéis, labrum, labium , an d bristles . (From Davidson, 1925. ) C , Anasa tristis, bristle fascicle. (From D . G. Tower, 1914. ) D , Cimex lectularius, bristl e fascicle . (From Kemper, 1932. )

such that , wher e the bristle s diverg e t o ente r th e bristl e pouches , th e anterior canal (Fig. 179 B,/c) opens into the mouth channel (fm), an d th e posterior cana l (sc) receive s th e ti p o f th e hypopharynx , o n whic h i s located th e apertur e o f the salivar y meatu s (sm). Th e anterio r cana l is, therefore, the foo d cana l (/c) , and the posterio r on e the salivar y cana l (sc). I n th e dista l par t o f the labiu m th e bristl e fascicl e ma y becom e twisted, with a consequent chang e in the relativ e position s o f the canals . The Labium.—Th e labiu m o f the Hemiptera , whe n well developed , is a long, slender , rigi d orga n divide d int o thre e o r four parts , o r " segments" (Fig . 179 , Lb). It s anterio r surfac e i s deepl y concav e to for m the channe l of the beak containing th e mandibular an d maxillary bristle s (Fig. 18 2 A, Lb). Th e morpholog y o f th e hemipterou s labiu m i s no t understood. Th e crania l muscle s that mov e it ar e inserted o n the firs t or second segment , an d it woul d seem , therefore, tha t the principa l par t of the labiu m of the Hemipter a consist s o f the prelabium alone, the post labium being represented by a basal segment or by the ample membranous area at the base of the organ. Th e cranial muscles act as either retractor s or protractor s accordin g t o whethe r the y ar e inserte d directl y o n th e labial bas e o r o n a n apodema l ar m o f th e latter . Th e interio r o f th e

339

THE ORGANS OF INGESTIÓN

labium contain s an elaborate musculature, which has been fully described in Aphis fabae b y Webe r (1928a ) an d i n Cimex by Kempe r (1932) . The Piercin g Mechanism.—I n most o f the Hemiptera , an d probabl y in al l members o f the order , as has bee n shown by Webe r (1928 , 1930) , the mout h bristle s are not move d by simultaneou s contraction s o f thei r muscles. Th e mandibula r bristle s are the chie f piercin g organs. Whe n the insec t begin s a n insertio n o f it s bristl e bundl e (Fig . 183 , 1) , on e mandibular bristl e i s thrus t ou t a shor t distanc e i n advanc e o f th e other t o punctur e the foo d tissu e (2) , and then th e opposite mandibula r bristle i s protracte d unti l it s ti p meet s tha t o f th e firs t (3) . No w the tw o maxillar y bristle s ar e lowere d togethe r unti l thei r tip s li e between thos e o f th e tw o mandibula r bristle s (4) . A t a singl e thrus t a bristl e i s extrude d n o farthe r tha n th e maximu m distanc e th e shor t protractor muscl e can driv e i t wit h on e contraction. Thi s distanc e a t

1

2

3

4

5

6

7

8

FIG. 183.—Successiv e stage s i n th e insertio n o f th e feedin g bristle s o f Hemiptera . (Diagrams based o n figures from Weber, 1928. )

best is insignificant compared with the dept h to which the bristl e bundle can finally be sunke n into the foo d tissue . Repeate d thrusts , therefore , are necessary ( 5 to 8) . Bu t a repetition o f the insertio n proces s necessitates tha t th e protracte d bristle s b e i n som e wa y secure d i n th e ne w position i n orde r t o resis t th e backwar d pul l o f th e retracto r muscle s that restore s th e protractor s t o thei r functional lengths. I n some cases the bristle s ar e anchore d i n th e foo d tissu e b y barb s o n thei r tips ; i n others they ar e held in a clasp of the enclosin g labium. When the mouth bristles are not in use they d o not normally protrude from th e ti p o f the labium . Wit h mos t species , moreover , the bristle s are not lon g enough to be projected from th e labiu m except for the ver y short thrus t give n the m b y th e protracto r muscles , an d i n suc h case s the exposur e an d insertio n o f th e dista l par t o f the bristl e fascicl e ar e made possibl e b y a retractio n o r foldin g o f th e labiu m tha t doe s no t involve the bristles . In homopterou s form s havin g mout h bristle s o f usua l length , th e labium i s suspended from a membranou s are a o f the hea d an d i s ofte n flexible at it s bas e (Fig . 18 4 C). Th e exposur e of the bristle s i n suc h species is brought about b y a retraction o f the labiu m or by a backward

340

PRINCIPLES OF INSECT MORPHOLOGY

folding o f it s basa l segments , allowin g th e hea d t o b e lowere d a s th e bristles penetrat e th e foo d tissu e (D) . I n som e specie s o f aphid s a n individual i n the ac t o f inserting it s bristle s stand s high on its fron t leg s and plant s th e bea k verticall y agains t th e lea f (Fig . 185) ; as the bristle s sink int o th e lea f tissue , th e bod y i s lowere d anteriorly an d th e basa l part of the labium bends back like the elbo w of an arm, while the termina l part retain s it s gras p on the bristles ; finally, when the bristle s ar e in a t full length , th e insec t stand s almos t o n it s head . I n th e adult s o f Aleurodidae, accordin g t o Webe r (1928) , th e labiu m i s equippe d wit h protractor muscles ; this provisio n allow s these insect s t o mak e a quic k departure fro m a feedin g puncture , bu t th e aphids , whic h have n o pro tractor mechanism for the labium, often have much difficulty in extractin g the mout h bristles .

FIG. 184.—Variou s positions of the hemipterou s labiu m durin g feeding . A , Graphosoma italicum. (From Weber, 1930. ) B , Cimex. (From Kemper, 1932. ) C , D , Trialeurodes, befor e and afte r insertio n o f the bristles . (Adapted from Weber, 1928. )

With man y o f the Heteropter a the lon g rigid labium is firmly articulated t o th e hea d somewha t behin d th e exi t o f th e bristle s fro m th e latter, an d i n suc h specie s i t i s probable , a s show n b y Webe r (1928) , that a preliminary exposur e of the tip s of the bristle s i s effected merel y by th e forwar d swin g of the bea k fro m it s horizontal positio n o f repose. The furthe r exposur e and th e insertio n o f the bristle s ar e usually accompanied b y a n elbowlik e bend of the labiu m between the firs t an d second segments (Fig . 18 4 A), whil e the bas e o f the bristl e fascicl e (bf) i s held in th e groov e of the labru m (Lm). I n th e be d bug, however, according to Kempe r (1932) , th e labiu m bend s betwee n th e thir d an d fourt h segments (B ) an d i s furthe r shortene d b y a n invaginatio n o f it s bas e into the head , an d to a smaller degre e by a telescoping of its segments . It i s evident , now , that suc h simpl e device s as thos e jus t describe d for th e exsertio n o f the mout h bristle s ca n giv e effectiv e servic e onl y t o larger specie s o r to specie s that obtai n thei r foo d bu t a shor t distanc e below the surfac e o f the foo d tissue . Ver y small sucking insects o r those

THE ORGANS OF INGESTIÓN

341

that draw their food from relatively greate r depth s must have proportion ately lon g mout h bristles . Suc h species , therefore , ar e confronte d wit h the proble m o f storage fo r bristle s ofte n o f greate r lengt h tha n th e

FIG. 185.—Attitude s of an aphi s during feeding.

body, an d wit h tha t o f exserting th e bristle s fa r beyon d th e ti p o f th e labium. Hemiptera wit h bristle s muc h longe r tha n th e labiu m includ e th e larvae o f Psyllida e an d Aleurodidae , larva e an d adul t female s o f Coccidae, the Coptosmatidae , an d the Aradidae . Th e problem o f bristle storage has bee n solve d by thes e insect s in differen t ways . Wit h the

FIG. 186.—Structura l detail s an d variou s devices for the storag e of the mout h bristles of long-bristle d Hemiptera . A , Psylla mali larva , bristle s loope d outside of head. B , Pseudococcus adul t female , bristle s i n crumena . C , Tropidotylus fasciolatus, bristle s i n crumena. D , Aradus, bristle s coile d i n preora l cavity . E , Bozius respersus, bristle s looped i n bas e o f labium . F , Pseudococcus, labiu m an d bristl e clamp . G , Psylla mali, bristle clamp of labium. (A , B, F , G from Weber, 1928 ; D from Weber, 1930 ; C , E from China, 1931. )

larval psyllid s th e bristl e fascicl e whe n retracte d i s projecte d forwar d from th e bas e o f the labiu m i n a larg e fre e loo p beneath th e hea d (Fig . 186 A). I n th e Coccida e an d th e larva e o f Aleurodidae th e retracte d fascicle i s receive d int o a lon g interna l pouch , th e crumena, extendin g

342

PRINCIPLES OF INSECT MORPHOLOGY

backward fro m abov e th e bas e o f the labiu m int o th e thora x (B , Cru). In som e member s o f the heteropterou s famil y Coptosomatida e (Plata spididae), a s show n b y Chin a (1931) , th e retracte d bristl e fascicl e i s looped posteriorl y i n a larg e membranou s diverticulu m a t th e bas e of th e secon d labia l segmen t (Fig . 18 6 E), whil e in other s (C ) th e fas cicle i s receive d int o a lon g crumena l sa c (Cru) resembling tha t o f th e Coccidae, extendin g fro m th e hea d int o th e bas e o f the abdomen . Th e coptosomatids are mostl y fungu s feeders , and the lengt h of the mout h bristles, a s suggeste d b y China , i s probabl y a n adaptatio n fo r probin g lengthwise throug h th e mycelia l filaments . I n th e Aradidae , finally , which ar e als o fungus feeders , th e retracte d bristl e fascicl e i s coiled in a large chambe r o f the preora l cavit y anterio r t o the mout h of the suckin g pump (Fig . 18 6 D). The mean s b y whic h th e long-bristle d Hemiptera , particularl y th e minute Coccidae , ar e abl e t o protrud e their threadlik e mout h bristle s from th e hea d and to insert the m int o wood y tissues wa s for a long tim e an outstandin g entomologica l mystery . Som e writer s attempte d t o explain th e exsertio n o f th e bristle s a s brough t abou t b y a muscula r contraction o f the crumena , and others postulate d blood pressure agains t the sa c as the activ e force , bu t thes e theorie s coul d not appl y t o larva l Psyllidae with the bristles loope d outside the head, nor would they in any case accoun t fo r th e retractio n o f th e bristles . A consisten t an d con vincing explanatio n o f th e mechanis m o f protractio n an d retractio n of th e mout h bristle s in thes e species , however, has recentl y been give n by Webe r (1928 , 1930 , 1933) , an d th e followin g description s ar e base d on his observations . Three anatomica l fact s explain the principle by which the mechanis m of exsertion an d retraction accomplishe s its results. First , the protracto r and retracto r muscle s ar e abl e t o mov e th e bristle s bu t a ver y shor t distance wit h eac h contraction ; second , th e fou r bristle s ar e firml y interlocked in the fascicl e but slid e freel y upo n one another; third , ther e is some provision fo r holding the bristle s i n place, after eac h protractio n or retraction , tha t prevent s th e antagonisti c muscl e fro m undoin g th e work of th e other . Th e holding apparatus i n the Psyllidae , Aleurodidae , and Coccida e i s a clam p i n th e labium , consistin g o f a narrowe d an d strongly sclerotize d are a i n th e labia l groov e wit h muscle s t o regulat e its pressure on the bristle fascicl e (Fig . 186 , F, G) . I n othe r families the same effec t i s accomplished by barb s o n the end s of the bristles . The musculatur e o f th e mout h bristle s i s mechanicall y th e sam e in al l cases , an d th e alternatin g thrust s an d pull s ar e exerte d o n th e several bristle s o f the fascicl e i n th e manne r alread y describe d fo r th e Hemiptera i n genera l (Fig . 183) . Th e onl y differenc e i n th e long bristled form s i s that th e retracte d fascicl e i s thrown into a loo p or coil

THE ORGANS OF INGESTIÓN

343

somewhere betwee n it s bas e an d it s extremit y (Fig . 187) . Th e loop , however, make s n o differenc e i n th e movemen t o f the bristles , becaus e the latte r ar e securel y hel d togethe r b y interlockin g groove s and ridge s and slid e freel y o n on e another . Th e successiv e contraction s o f th e protractor muscles hav e n o effect o n the loo p ( 1 to 4), the bristle s bein g moved alik e a t bot h ends . But , afte r eac h thrust, whe n the fascicl e is held i n plac e b y th e labia l clamp , th e simultaneou s contractio n o f th e retractor muscle s takes u p a littl e o f the slac k i n th e loo p (5) . Henc e the bristle s penetrat e deepe r an d deepe r wit h th e succeedin g outwar d thrusts, whil e the serie s o f pulls o n their base s i s expende d agains t th e

FIG. 187.—Th e mechanis m of insertio n o f th e feedin g bristle s b y Hemipter a having long bristle s store d i n a loo p o r coi l whe n retracted . (From Weber, 1933. ) 1 , beak placed agains t th e plan t surface . 2 , 3, 4, firs t insertion of the mandibula r and maxillary bristles, a s i n Fig . 183 , b y contractio n of th e protracto r muscles . 5 , fascicl e o f bristle s held i n labia l clam p whil e loo p shortene d b y contractio n of th e retracto r muscles . 6 , bristles inserted ful l lengt h after successiv e repetitions of movements 2 to 5 .

loop, wit h th e resul t tha t th e latte r i s graduall y shortened , unti l i t i s obliterated whe n the bristles ar e exserted at ful l lengt h (6) . Th e looping of th e fascicl e durin g retractio n o f th e bristle s require s onl y a revers e action o f the labia l clamp . It stil l seem s almos t beyond belie f tha t th e delicat e bristle s o f such small insect s a s Coccidae can penetrate th e bar k o f trees; but sinc e it i s an observabl e fac t tha t the y d o so, the fea t evidentl y i s not impossible . It i s known , however , tha t th e salivar y secretio n o f som e Hemipter a has a solven t effec t upo n plan t tissue s an d thus facilitate s th e insertio n of th e bristles . Th e salivar y cana l o f th e beak , i t shoul d b e recalled , accompanies th e foo d channe l throughou t th e lengt h o f th e bristl e fascicle.

344

PRINCIPLES OF INSECT MORPHOLOGY 10. TH E FEEDIN G MECHANIS M O F ANOPLUR A

The feedin g equipmen t o f the tru e lic e is a highly specialized piercing and suckin g mechanism. Th e morpholog y o f the piercin g organs is no t definitely known , an d observation s o n the structur e o f the mout h part s given by various investigators do not agre e in all respects, though detail s have bee n minutel y describe d an d figured . Ou r presen t informatio n on the mout h part s of the Anoplur a is contained i n the wor k of Cholod kowsky (1904) , Enderlei n (1905 , 1905a) , Pavlowsk y (1906) , Harriso n (1914), Sikor a (1916) , Peacoc k (1918) , Florenc e (1921) , Voge l (1921a) , and Fernand o (1933) , whil e summarized account s ar e give n by Metcal f and Flin t (1928) , Patte n an d Evan s (1929) , an d Imm s (1934) , thoug h with variations i n detail accordin g to th e sourc e selected .

FIG. 188.—Th e piercin g an d suckin g apparatu s o f Anoplura . A , sectio n o f th e head showin g buccal and pharyngea l pumps (BuC, Phy) , an d subora l sac (*Sac ) containin g the piercin g stylets (Stl). (Diagram composed from Sikora, 1916 , an d others.) B , sectio n of th e labru m an d piercin g stylets o f Pediculus vestimenti. (From Vogel, 192la. )

The essentia l structur e o f th e piercin g an d suckin g apparatu s o f Pediculus appear s t o b e a s follows . Th e elongat e hea d terminate s anteriorly i n a small , protractile , snoutlik e tube , know n a s th e mouth cone, rostrum, proboscis, o r prestomum. Th e orga n appear s t o b e th e labrum (Fig . 188 , A , Lm). I t ha s a terminal apertur e continue d int o a median ventra l cleft , an d it s inne r wall s are arme d wit h smal l recurve d teeth (b), which , whe n everted , enabl e th e parasit e t o obtai n a hol d on th e ski n o f it s host . Othe r member s o f th e mout h part s ar e no t ordinarily visibl e externally . Th e ventra l channe l o f th e labru m leads int o a tubula r preora l cavity , th e "bucca l funnel " (PrC), i n th e anterior par t o f the head . Th e hea d capsul e is closed belo w by a lon g hypostomal wall, the distal extremity of which forms the "lower lip (hsi) of th e preora l cavity . Fro m th e posterio r en d o f th e preora l cavit y the mout h (Mth) open s dorsall y int o a two-chambere d suckin g pum p (BuC, Phy), whic h terminates i n a slender oesophagea l tube (Oe). Ven trally th e preora l cavit y i s extende d belo w th e mout h i n a lon g sa c (Sac) containin g a grou p o f slende r piercin g organ s (Stl). Th e firs t chamber o f the suckin g pump (BuC) i s perhaps th e bucca l cavity, sinc e it lie s anterior to th e fronta l ganglion (FrGng) ; th e secon d (Phy) is with-

THE ORGANS OF INGEST IÓN

345

out doub t th e tru e pharynx ; both evidentl y belon g to th e stomodaeu m since thei r wall s hav e a sheat h o f circula r muscl e fibers . A mor e careful stud y o f th e relation s o f th e dilato r muscle s t o th e hea d wal l and t o th e fronta l ganglio n connective s may b e expecte d to giv e mor e conclusive evidenc e a s t o th e identitie s o f the severa l part s o f the foo d passage. The piercin g organ s o f th e lous e (Fig . 18 8 A , Stl), accordin g t o Vogel (1921a) , consis t o f thre e superpose d stylet s (B , MX, Hphy, Lb). The stylet s aris e posteriorl y fro m th e wall s o f the containin g sa c (A) , and, i n th e retracte d condition , thei r dista l end s exten d t o th e bas e of the labru m i n th e preora l cavity , wher e the y ar e ensheathe d i n fold s of th e wall s of the cavity . Th e mos t dorsa l stylet appear s to b e forme d of tw o unite d appendage s (B , MX), th e dista l part s o f which have thei r free edge s rolle d upwar d t o for m a tubula r channe l (/c) , whic h i s th e food cana l servin g t o conduct th e ingeste d bloo d fro m it s sourc e t o the mouth. The intermediate stylet (Hphy) is a slender rod traversed by the salivar y duc t (A , SID), whic h opens on its extremity. Th e ventral stylet (B , Lb) i s a broade r appendag e with distinc t dorsa l an d ventra l walls (whic h hav e bee n mistaken fo r separate pieces) . Th e dorsa l wall is deepl y groove d by a channe l containing the media n stylet . Distall y the ventra l style t end s i n thre e sharp-pointed , serrat e lobes , whic h are the piercin g organ s o f the louse . Th e proxima l ends o f the dorsa l an d ventral stylets giv e off long apodemal arms (A) , one pair from th e former , a dorsa l an d a ventral pai r fro m th e latter , which are imbedded in fold s of th e wal l o f th e sa c an d giv e insertio n t o protracto r muscle s arisin g anteriorly o n th e sa c walls . Othe r muscles , arisin g o n th e hea d an d inserted o n the sac , serve for the retractio n o f the sa c and the stylets . The stylet s hav e bee n generall y assume d t o represen t i n som e way the mout h part s o f the louse . Investigator s ar e agree d tha t th e man dibles ar e absen t i n adul t Anoplur a o r ar e reduced t o a pai r o f smal l plates lying at the sides of the preoral cavity. Th e mandibulate elephan t louse (Haematomyzus), a s show n b y Ferri s (1931) , ha s non e o f th e special feature s o f th e suckin g lic e an d i s perhap s t o b e classe d wit h the Mallophaga. Cholodkowsk y (1904) claimed that both the mandible s and th e maxilla e o f the Anoplur a disappear durin g embryoni c develop ment, an d tha t th e piercin g organ s ar e secondar y structure s conceale d by the labium . Enderlei n an d Vogel , however, from anatomica l studie s of th e adult insect, have contended that the dorsal stylet (Fig. 188 B, MX ) represents th e unite d maxillae , tha t th e intermediat e style t (Hphy), traversed b y the salivary duct , is the hypopharynx, an d that the ventral stylet (Lb) i s the labium . Thi s interpretatio n appear s t o b e confirme d by the mor e recent study o f Fernando (1933 ) o n the embryoni c develop* ment o f the mout h parts of Pediculus humanus.

346

PRINCIPLES OF INSECT MORPHOLOGY

According to Fernando , th e usua l gnathal appendages appear on the head o f th e embry o of Pediculus, there being present at a n earl y embryonic stage paired rudiments of mandibles, maxillae, and the labium. Th e mandibles underg o no developmen t and finall y disappear . Th e maxil lary an d labia l rudiments , however , elongat e an d thos e o f eac h pai r unite, forming thus two median organs which become the dorsal stylet and the ventra l stylet . Th e stomodaeu m i s formed i n the usua l manner a s a media n invaginatio n betwee n th e antenna e an d th e mandibles , an d the labru m appear s anterio r t o th e mouth . Th e stylet s ar e now withdrawn int o a n invaginatio n o f the ventra l wal l of the hea d behin d th e mouth, an d th e lip s o f the pouc h gro w ou t t o for m th e enclosin g sac. The intermediat e style t i s the n forme d b y a n outgrowt h betwee n th e bases o f th e maxillar y an d labia l stylet s involvin g th e termina l par t of th e salivar y duc t an d evidentl y represent s th e hypopharynx . Th e labrum becomes the conica l snoutlike rostrum embracin g the tip s o f th e retracted stylets .

CHAPTER XII I THE ALIMENTAR Y CANA L The organ s o f alimentation i n metazoic animals hav e t o d o with th e intake o f ra w foo d materials , th e digestio n an d absorptio n o f nutrien t substances fro m thes e materials , th e ejectio n of the unuse d residue, an d the distributio n withi n th e bod y cavit y o f th e absorbe d product s o f digestion t o the cellula r tissues wher e they ar e utilized in the processe s of growth and metabolism . Th e organ s of ingestión, digestion , absorption , and egestio n ar e th e part s o f th e alimentar y cana l an d th e digestiv e glands tha t pou r thei r secretion s int o it . Th e mediu m o f distributio n is the blood . Feeding i s primaril y a matte r o f gettin g nutrien t material s fro m the environmen t through the integumen t o f the organism ; assimilation i s the utilizatio n o f the absorbe d materials b y the cell s of the bod y tissues . Most metazoi c animals in their feedin g habits diffe r fundamentall y fro m such protozoan s a s the amoeb a i n that the y d o not tak e soli d particle s of foo d matte r throug h th e bod y wall ; the requisit e nutrien t substance s are dissolve d i n liquids throw n of f from a part o f the bod y an d ar e the n absorbed int o th e latter . Th e primitive stomach , o r archenteron o f the gastrula, i s simpl y a foo d pocke t invaginate d o n on e sid e o f the body , the wal l o f whic h i s forme d o f specialize d digestiv e cells . Th e mor e complex alimentar y cana l o f th e highe r animals , therefore , mus t b e regarded a s merel y a mor e efficien t devic e for holdin g food material s i n proximity to a digestive and absorptive surface, to which have been added special mechanism s fo r ingestió n an d egestion . Th e lume n o f the foo d tract is a part o f th e environmen t enclose d within th e animal . 1. DEVELOPMEN T O F TH E ALIMENTAR Y CANA L

The embryoni c developmen t o f th e alimentar y canal , describe d in Chap . II , give s u s a misleadin g concep t o f th e tru e natur e o f th e digestive trac t o f arthropods , especiall y o f insects , fo r w e ar e induce d to thin k o f i t a s consistin g o f a n endoderma l stomac h forme d entirel y within the body, which only secondarily acquires openings to the exterio r through a n ectoderma l stomodaeu m an d a n ectoderma l proctodaeum . The ontogeneti c developmen t o f the digestiv e tube , however , i s clearl y an embryoni c adaptation t o th e condition s o f life i n th e eg g and i s no t to b e take n a s a literal repetitio n o f phylogenetic history. Th e mesen 347

348

PRINCIPLES OF INSECT MORPHOLOGY

teron i s the primar y stomac h an d ther e i s little probabilit y tha t i t wa s ever a close d sa c i n an y o f th e adul t ancestor s o f th e arthropods ; th e stomodaeum an d proctodaeu m ar e late r ingrowth s o f th e ectoder m a t the primitive oral and anal apertures of the mesenteron. Th e stomodaeal and proctodaeal openings into the stomach , therefore , i n a sense, are th e true mout h an d anu s o f the arthropod , whic h hav e bee n carrie d inter nally b y a n inwar d growt h o f th e circumora l an d circumana l part s of th e ectoderm . Accordin g t o Henso n (1931) , th e innermos t cell s of the stomodaeum and proctodaeum in lepidopterous larvae form interstitial rings of ectodermal cell s that retain th e powe r of mitotic divisio n and a t the tim e of metamorphosis regenerat e the epitheliu m o f the stomodaeu m and th e proctodaeum . Durin g th e larva l perio d thes e part s o f th e alimentary cana l grow by enlargement of the epitheliu m cell s but no t b y cell multiplication . Thoug h th e stomodaeu m an d proctodaeu m ar e primarily organ s of ingestión and egestion , they hav e come to serv e als o in various othe r capacities accessor y to th e functio n o f the stomach . The cell s of the mesentero n maintai n thei r earl y acquire d activitie s that particularly adapte d the m to the function s o f digestion and absorp tion. The y ar e continually subject to disintegrating processes , and some of them , a t least , retai n th e powe r of mitotic divisio n t o replac e thos e depleted b y digestiv e activities , o r to regenerat e th e entir e epitheliu m at th e tim e o f metamorphosis o r even at th e larva l moults . 2. GENERA L STRUCTURE O F TH E ALIMENTAR Y CANAL

Since the digestiv e tract i s but a n infolde d par t o f the bod y wall, it s own walls have the sam e essentia l structur e a s that o f the bod y integument. The y consis t o f a laye r o f cells , th e enteric epithelium, restin g upon a basement membrane turne d towar d th e somati c cavit y an d line d internally b y a cuticula r intima. Th e intim a i s best develope d in th e stomodaeum an d proctodaeum ; i n th e mesenteron , i f presen t a t all , i t has a very delicate texture an d is often disrupte d by the activitie s o f the epithelial cells . Al l parts o f the alimentar y cana l ar e usuall y investe d in a muscular sheath, o r muscularis, derived principally from the splanch nic laye r o f th e mesoderm . Othe r muscles , probabl y o f somatopleur e origin, exten d fro m th e bod y wal l t o th e alimentar y canal . Thes e extrinsic muscle s ar e know n a s th e suspensory o r dilator muscleSj th e second term probabl y bette r expressin g their function . In for m th e alimentar y cana l o f insects i s a tube , eithe r straight , or variousl y loope d upo n itsel f i f it s lengt h exceed s that o f th e body . In it s simples t developmen t th e tub e show s little differentiation beyond the primary division into stomodaeum (Fig. 189, Stom), mesenteron (Menf), and proctodaeum (Proc). Th e functiona l stomach , o r ventriculus, is th e mesenteron. Usuall y a circula r valve-lik e fol d separate s th e cavitie s

THE ALIMENTARY CANAL

349

of adjoining sections, that between the stomodaeum and mesenteron being known a s th e stomodaeal, o r cardiac, valve (SVlv), th e on e closin g th e entrance to the proctodaeum as the proctodaealj or pyloric, valve (PVlv). Few insects , however , hav e a n alimentar y cana l s o simpl e a s tha t just described . Generall y eac h o f th e primar y section s o f th e tube , particularly th e firs t an d th e third , ar e differentiate d int o severa l mor e or les s distinc t regions , an d diverticul a o f various form s gro w ou t fro m the wall s (Fig . 190) . Th e principa l outgrowth s o f th e alimentar y canal ar e th e Malpighia n tubule s (Mai), whic h ar e attache d t o th e

FIG. 189.—Th e alimentary canal of a collembolan , Tomocerus niger, showin g in simpl e form th e primar y components of the foo d trac t withou t secondary specializations. (From Folsom and Welles, 1906.) Ment, mesenteron; Proc, proctodaeum; PVlv, proctodaeal, or pyloric, valve; Stom, stomodaeum; SVlv, stomodaeal, or cardiac, valve.

anterior en d of the proctodaeum, but variou s diverticula occur also on the mesenteron, an d gland s ma y ope n int o th e stomodaeum . Th e ali mentary cana l i n al l it s part s i s subjec t t o man y variation s o f form i n different insects . Som e of its principa l type s o f structure ar e show n in Figs. 195 , 196 , 198 , 199 . Durin g metamorphosi s th e entir e digestiv e tract ofte n undergoe s muc h reconstructiv e alteratio n bot h i n externa l form an d i n it s histologica l structure , a s i s wel l illustrate d i n th e Lepi doptera (Fig . 197) , th e change s being adaptiv e t o th e differen t feedin g habits of the young and th e adul t o f the sam e species. 3. TH E STOMODAEU M

In it s simples t conditio n the stomodaeu m is little more than a n inle t to the stomach or a short condui t to the latter from th e mouth (Fig . 189 , Stom). I n mos t insects , however , th e stomodaeu m i s a lon g tub e of which the middl e part is enlarged to for m a storage chambe r for reserv e supplies o f food ; an d thi s functio n assume d b y th e middl e regio n wa s evidently th e precurso r o f a specializatio n o f the for e par t o f th e tub e into a n orga n o f ingestión, an d o f the posterio r par t int o a " stomach mouth" for regulating th e passag e o f food int o th e ventriculus , o r even in some cases for giving it a second chewing. Thu s th e stomodaeum , or primitive oesophagus , ha s becom e differentiate d int o thre e primar y regions, namely , th e pharynx (Fig . 190 , Phy), th e crop (Cr), an d th e proventriculus (Pvent). A n undifferentiated par t of the tub e may remain as a definit e oesophagus (Oe) betwee n the pharyn x an d th e crop , and, a s

350

PRINCIPLES OF

INSECT MORPHOLOGY

we sa w in th e las t chapter , th e initia l regio n jus t withi n th e mout h i s often distinguishabl e fro m th e pharyn x a s a buccal cavity (BuC). The primary functions of the stomodaeu m thus appear to be mechanical; bu t ther e i s little doub t tha t th e orga n i n insect s ha s secondaril y come t o b e als o a physiologica l adjunc t t o th e stomac h b y increasin g

FIG. 190.—Diagra m showin g th e usua l subdivisions and outgrowth s of the alimentary canal. AInt, anterio r intestine; An, anus ; BuC, buccal cavity ; Car, cardia; Cln, colon; Cr, crop ; GCa, gastric caecum ; II, ileum ; Mai, Malpighia n tubules; Ment, mesentero n (ventriculus); Mth, mouth ; Oe , oesophagus; Phy, pharynx ; Pint, posterior intestine (rectum); Proc, proctodaeum ; Pvent, pr o ventriculus; Py , pylorus ; Rect, rectu m (rect, rectu m proper; rsc, rectal sac); Stom, stomodaeum; Vent, ventriculus.

the spac e availabl e fo r digestiv e purposes , sinc e the foo d store d i n th e crop i s subject to th e actio n bot h o f the salivar y liqui d mixed with th e food durin g ingestión and o f gastric juices that flow forward int o the crop from th e ventriculus. Histology o f th e Stomodaeum.—Th e wall s o f th e stomodaeu m i n general hav e a simpl e structure. Th e epitheliu m (Fig . 19 1 A, Epth) i s

FIG. 191.—Section s o f th e stomodaeu m o f a grasshopper , Dissosteira Carolina. A , the crop . B , th e pr o ventriculus. BMb, basemen t membrane ; cmds, circula r muscles ; Epth, epithelium ; In, intima ; Imcls, longitudinal muscles; Lum, lumen; m, muscles in folds ; Tra, trachea.

usually flat, and the cell boundaries often indistinct. The intima (In) is relatively thick ; its surfac e fo r the mos t part i s sparsely covere d with short hair s o r spicules , thoug h i n th e pharyn x an d th e proventriculu s there ma y b e area s closel y bese t wit h lon g hair s o r spines . I n th e proventriculus o f som e insect s th e intim a i s dens e an d produce d int o

THE ALIMENTARYCANL

351

lobes an d teet h formin g a specia l armatur e (Fig . 194) . Bot h th e epi thelium an d th e intim a ar e throw n int o longitudina l fold s whic h i n most part s o f the stomodaeu m allo w for expansio n o f the lume n a s th e latter become s filled with food ; bu t a certai n numbe r o f the lobe s ar e often definit e structures , a s show n b y th e increase d thicknes s o f th e intima coverin g them an d o f the underlyin g epithelial cell s (Fig. 19 1 B). These definit e fold s usuall y occu r i n multiple s o f tw o o r three , ther e being commonly four, six, or eight major folds, with the same or a greate r number o f mino r intermediat e fold s betwee n them . Th e majo r fold s are particularly develope d in the pharyngea l an d proventricular regions . The muscula r sheat h i s a ver y importan t par t o f the stomodaeum . It consist s i n genera l o f a n oute r laye r o f circula r fiber s (Fig . 19 1 A, cmcls) an d o f an inne r laye r o f longitudinal fiber s (Imcls) ; bu t a detaile d study o f the stomodaea l musculari s show s that it s fiber s d o no t neces sarily adher e strictl y t o th e typica l arrangemen t i n al l part s o f th e stomodaeum. Th e circula r fiber s generall y ru n continuousl y aroun d the tub e withou t attachment s t o th e latter . Th e longitudinal muscles , on th e othe r hand , ar e sometime s inserte d o n th e intim a i n th e sam e manner a s th e somati c muscle s ar e attache d t o th e bod y wall , bu t i n other case s they to o appea r t o hav e n o connection s with th e intim a o r the epitheliu m an d aris e a s confluen t branche s o f the circula r muscles . The last condition is well shown in the cro p of a caterpillar (Fig . 166 , Cr) where the musculari s form s a veritable plexu s of branching and unitin g fibers constituting a sheath abou t th e inner walls of the tube , bu t havin g no intimat e connection s wit h th e latter . Wher e th e foldin g o f th e stomodaeal walls is pronounced, the longitudinal muscles tend t o become grouped i n the space s of the fold s (Fig . 19 1 B). The stomodaeu m i s generall y wel l provide d wit h dilato r muscle s (Figs. 166 , 193) . Thes e muscle s tak e thei r origin s o n th e wall s an d apodemes o f th e hea d an d o n th e wall s o f on e o r mor e o f the thoraci c segments. Thei r centra l end s usuall y penetrat e betwee n th e fiber s o f the musculari s t o b e inserte d eithe r o n th e stomodaea l epitheliu m o r on th e intima , bu t i n certai n case s some of their fiber s appea r t o unit e with those o f the muscularis . The Bucca l Cavity.—Th e tru e bucca l cavit y o f th e insec t i s th e oral par t o f the stomodaeu m (Figs . 155 , 190 , 192 , 193 , Bud) an d shoul d not be confused wit h the preoral cavity (Fig . 155, PrC), o r external space enclosed betwee n the mout h parts, whic h is often incorrectl y calle d th e "mouth cavity. " A s show n i n th e las t chapter , th e bucca l cavit y usually is not structurally differentiate d fro m th e pharynx (Fig . 192) , bu t it ma y b e define d a s th e initia l par t o f the stomodaeu m o n whic h ar e inserted th e secon d group of dilator muscle s taking thei r origin s o n th e clypeus, o r th e clypea l are a o f th e head , an d havin g their insertion s

352

PRINCIPLES OF INSECT MORPHOLOGY

anterior t o th e fronta l ganglio n an d it s connective s (Figs . 155 , dlbc, 193, 34). The Pharynx.—Th e pharyngeal par t o f the stomodaeu m follow s th e buccal cavit y (Fig . 190 , Phy), and , i f no t structurall y differentiate d from the latter, it is to be identified as that part of the stomodaeum whose dorsal dilato r muscle s tak e thei r origi n o n the fronta l an d dorsa l area s of th e hea d wal l an d ar e inserte d posterio r t o th e fronta l ganglio n an d its connective s (Fig . 155 , Phy). Th e pharyn x typicall y lie s befor e th e nerve connective s betwee n th e brai n an d th e suboesophagea l ganglion , but i n some insects ther e i s a second pharyngeal chambe r o f the stomo daeum behind th e connectives , the tw o parts being differentiated either

FIG. 192.—Longitudina l vertical sectio n o f the hea d par t of the stomodaeu m o f a cock roach, Blatta orientalis, showin g precerebra l anterio r pharyn x (APhy) an d postcerebra l posterior pharynx (PPhy). (From Eidmann, 1924, but relettered.)

by the contou r of the stomodaeal tube or by their musculatur e or internal structure (Fig . 192) . Th e precerebra l an d postcerebra l pharyngea l regions ar e distinguishe d a s the anterior pharynx (Figs . 192 , 193 , APhy) and posterior pharynx (PPhy) y respectively. When there is no posterior pharyngeal developmen t th e postcerebra l regio n o f th e stomodaeu m becomes a par t o f the oesophagus . I n genera l th e distributio n o f th e dilator muscle s serve s bette r t o identif y correspondin g morphologica l parts o f the cephali c stomodaeu m tha n doe s the structur e o f the part s themselves. Th e principal modifications of the pharynx and its musculature hav e bee n sufficientl y note d i n connectio n wit h th e feedin g mech anism describe d in the las t chapter . The Oesophagus. —The oesophagu s ha s n o definit e morphologica l status; i t i s merel y th e narro w par t o f the stomodaeu m followin g th e pharynx tha t i s not differentiate d for purpose s othe r tha n tha t o f foo d conduction. Typicall y th e oesophagus is a slender tube an d may exten d direct t o th e stomach , bu t mor e generally it i s limited posteriorl y b y a proventricular o r ingluvial section of the stomodaeum (Figs. 190,198, 199 ,

THE ALIMENTARY CANAL

353

Oe). Whe n the ingluvies, or crop, is a simple dilatation of the stomodaea l tube th e oesophagu s usually widen s gradually int o th e cro p (Fig . 190) , and th e latte r ma y exten d s o far forwar d a s practicall y t o exclud e th e oesophagus (Figs . 193 , 195) . The Crop , o r Ingluvies.—The cro p i s ordinaril y bu t a n enlargemen t of th e posterio r par t o f the oesophagu s (Figs . 190 , 195 , Cr) . I n som e insects, however , i t i s a latera l diverticulu m o f the oesophagu s havin g the for m eithe r o f a simple sac (Fig . 19 7 B, C, Cr) or, as in some Dipter a (Fig. 198) , o f a long, slender tube wit h a bladderlike swelling at th e en d (Cr). Th e intim a o f the cro p is usually thick , th e epitheliu m flat , an d the walls of the entir e organ , when not stretche d b y the foo d content , ar e thrown int o numerou s lengthwis e fold s an d transvers e wrinkle s tha t allow o f distention . That th e primar y functio n o f th e cro p i s on e o f storag e i s ampl y attested b y it s siz e an d structure . Mos t insect s fee d rapidl y whe n food i s available i n abundanc e and accomplis h more leisurely th e diges tive processes . Sanfor d (1918 ) foun d tha t cockroache s fed t o repletio n on a diet o f oil and suga r could go for nearly two months befor e th e cro p content wa s exhausted . I t seem s equall y clear , however , no t merel y that the cro p is an antechamber o f the ventriculus, or waiting room where the foo d i s held in anticipation o f its admissio n t o th e stomach , bu t tha t it i s i n itsel f th e sea t o f a certai n amoun t o f foo d digestion , sinc e i t receives digestiv e liquid s bot h fro m th e salivar y gland s an d fro m th e ventriculus. Analyse s hav e show n the presenc e o f numerous digestiv e enzymes in the stomodaeu m o f various insects, but al l enzymes reported from th e stomodaeu m occur also in the salivar y gland s or the ventriculu s or i n both , an d i t i s probabl e tha t thes e organ s ar e th e source s o f th e enzymes discovered in the crop . A few writers have believed, however, that certai n enzyme s may be formed i n the stomodaea l epitheliu m itself . Sanford (1918) , for example, claimed that the fat-splittin g enzym e lipase is a produc t o f th e cro p wall s i n th e cockroach , an d Swingl e (1925 ) thought i t likel y tha t mal t ase and inver t ase as wel l as lipas e occurring in th e cro p must b e produce d there. O n the othe r hand , bot h Abbot t (1926) an d Wiggleswort h (1928 ) asser t tha t lipas e canno t b e demon strated in the cro p walls of the cockroach , and Abbot t say s the presenc e of lipase in the cro p is the resul t o f regurgitation fro m th e stomach . Th e production o f enzymes in th e stomodaeu m o f any insec t has , therefore, not ye t bee n established . The questio n a s t o whethe r absorptio n take s plac e throug h th e walls o f the stomodaeu m i s one also that canno t b e regarde d a s settled. Petrunkevitch (1900 ) and Sanfor d (1918 ) hav e contende d no t onl y that absorption take s plac e i n th e crop , bu t tha t i n th e cockroac h the cro p is th e chie f sea t o f absorption . Thi s clai m the y bas e o n histologica l

354

PRINCIPLES OF INSECT MORPHOLOGY

studies o f th e cro p epitheliu m o f oil-fe d roaches , th e cell s o f whic h are foun d t o b e ful l o f oi l globules . Sehlute r (1912) , however , came to quite opposite conclusions from th e sam e methods of study carried out on variou s orthopteroi d specie s a s wel l a s o n odonat e larva e an d o n beetles. H e assert s definitel y tha t absorptio n doe s no t tak e plac e i n the crop , an d tha t i f fa t appear s i n th e ingluvia l cell s i t get s ther e i n some other way than by direct absorption from th e cro p lumen. Abbot t (1926), again, agrees with Petrunkevitch and Sanford that the crop of the roach i s an importan t orga n for the absorptio n o f fat, but h e say s tha t water and water-soluble substances are not absorbe d in it. The thicknes s o f the stomodaea l intima woul d appear to b e a n effec tive barrier t o mor e than a minimum of absorption takin g place in an y

FIG. 193.—Th e cephali c dilato r muscle s o f th e stomodaeux n o f a grasshopper, Dissosteira Carolina.

part o f the stomodaeum , and th e experiment s of Eidmann (1922 ) on th e relative permeabilit y o f th e intim a o f th e cro p an d intestin e i n Blatta orientalis giv e littl e suppor t t o th e ide a o f absorptio n i n th e former . Eidmann foun d tha t bot h alkalin e an d aci d substance s diffus e ver y slowly, only in the cours e of hours, through the intima o f the crop , which has a thicknes s o f 5 to 8 microns, though the y penetrat e th e relativel y thin intim a o f the intestin e i n 1 0 or 1 5 minutes. I n othe r Orthopter a the cro p intima i s often muc h thicker than that of the cockroach , and, in general, i t woul d appea r that , a s Sehlute r remarks , "a n orga n coul d scarcely be made less fitted for absorption." In th e Dipter a th e bladderlik e cro p (Fig . 198 , Cr) usuall y contain s a clea r liquid . Tha t o f Tabanus, accordin g to Crag g (1920) , does no t serve a s a foo d reservoir ; it s content s ar e apparentl y derive d fro m th e mesenteron and then agai n returned to the latter, a process that insure s a thorough mixing of the gastri c secretio n wit h the ingeste d blood . The Proventriculus. —This, th e termina l regio n of the stomodaeum , is ofte n structurall y th e mos t highl y specialize d part o f the alimentar y

THE ALIMENTARY CANAL

355

canal. I n it s simple r forms , however , a s see n i n th e larva e o f man y insects an d i n som e adults , i t i s merel y th e narrowe d posterio r en d of the stomodaeu m which is more or less invaginated into th e anterio r en d of th e mesentero n t o for m th e cardia c valve (Fig . 189 , SVlv). In adul t insect s tha t fee d o n soli d foo d th e proventricula r regio n usually becomes differentiated a s a definit e par t o f the alimentar y trac t between th e cro p an d th e ventriculus , an d it s inne r wall s develo p a mechanism, ofte n arme d with strong cuticular plates o r teeth (Fig . 194) , that ma y serv e severa l purposes . Th e armatur e o f the pr o ventriculus lies anterior to the funnel-shaped posterior part of the organ (SVlv) that enter s th e stomach , an d fo r this reaso n i t woul d appea r t o b e a secondary additio n to a more simple primitive structure; but sinc e some modification o f the mechanis m at leas t i s present i n most o f the chewing insects an d als o in the Hymenoptera , Eidman n (1924 ) suggests that the proventricular armature is a primitive equipment of the insect alimentar y canal, whic h has bee n lost i n most o f the suckin g orders. The proventricula r mechanis m consist s fundamentall y o f stron g longitudinal fold s o f th e wall s o f th e orga n projectin g into th e lumen . The fold s ar e usuall y continuation s o f th e les s pronounce d plication s of th e wall s o f th e crop , an d ther e ar e consequentl y four , six , o r eigh t major proventricula r fold s an d a varying number o f minor intermediat e ridges. A simpl e conditio n i s foun d i n th e Acrididae , wher e th e wall s of th e proventriculu s ar e produce d int o si x longitudina l elevation s (Fig. 19 1 B), eac h deepl y groove d anteriorl y an d taperin g posteriorl y to th e margi n o f th e shor t proventricula r valve . Th e surface s o f th e lobes are not strongl y sclerotized in the grasshopper , and they ar e armed only with a few small marginal teeth an d wit h area s o f minute granulations o n thei r dista l halves . A laye r o f stron g circula r muscle s run s continuously aroun d th e proventriculu s (cmds), bu t th e longitudina l fibers ar e aggregate d int o si x group s occupyin g the base s o f th e fold s (Imds). There appear to be also short inner transverse fibers in the crests of the folds (m) serving to compress the latter. By a contraction of th e circula r muscles the si x major fold s ar e evidently brought togethe r and effectivel y bloc k th e entranc e t o th e ventriculus . Th e channel s between thes e folds , however , ma y permi t th e egres s o f ventricula r liquids int o th e stomodaeum , an d th e brow n liqui d tha t grasshopper s sometimes ejec t fro m th e mout h probabl y escape s fro m th e stomac h in this manner. In th e Blattida e (Fig . 194 ) th e si x major fold s o f the proventricula r wall are densel y sclerotized anteriorly formin g a n armatur e o f six plate s (a), eac h o f whic h i s produce d centrall y int o a strong , shar p proces s with th e poin t turne d somewha t posteriorly . I n th e mor e taperin g posterior hal f o f th e proventriculu s behin d th e plate s th e fold s ar e

356

PRINCIPLES OF

INSECT MORPHOLOGY

again thickened , formin g her e a circl e o f si x soft , cushionlik e lobe s (& ) covered wit h hair s o r spine s directe d backward . Th e proventricula r region i s thu s divide d int o a proventriculus anterior arme d wit h th e plates, an d a proventriculus posterior containin g th e cushions . Beyon d the cushion s i s th e regio n o f th e stomodaea l valv e (SVlv\ whic h i s a long, narro w tubula r fol d i n th e cockroach , on the inne r wall s of which the proventricula r folds ar e continue d a s lo w ridges tha t tape r gradually t o th e en d o f th e valve . A mor e detailed accoun t o f the proventriculu s o f th e cockroach i s give n b y Sanfor d (1918 ) an d b y Eidmann (1925) . The general structure of the proventriculu s in the Gryllida e and Tettigoniidae i s the sam e as th e Blattidae , bu t th e scleroti c plate s ar e here longe r an d ar e broke n u p int o serie s of transverse ridge s ending in points that appea r as six rows of overlapping teeth directe d pos teriorly. Th e proventriculu s o f Gryllus i s described by DuPorte (1918), that of Grylloblatta b y Sayc e (1899) , an d tha t o f Stenopelmatus by Davi s (1927) . The suckin g insect s usuall y lac k a proventriculus, othe r tha n th e regio n o f th e cardiac valve , thoug h th e Siphonapter a ar e said t o hav e a proventricula r regio n arme d FIG. 194.—Sectio n o f th e with cuticula r teeth . Th e flat , circula r sa c crop, proventriculus, stomodaea l valve, an d cardia c en d o f th e that intervene s betwee n th e oesophagu s an d ventriculus of a cockroach, the stomac h i n muscoi d Diptera , whic h mos t Blatta orientalis, diagrammatic , a, 6 , proventricula r plate s an d writers refe r t o a s the " pro vent riculus" (Fig . pads; c , circula r muscles ; d, d , 198, Car) , i s th e anterio r part , o r cardia , of junction o f proventriculu s an d ventriculus; Z , longitudina l the ventriculu s (Vent), a s wil l late r b e muscles. shown. The functio n o f th e proventriculu s unquestionabl y differ s accordin g to th e structur e o f th e orga n an d th e natur e o f th e foo d materia l i n different insects . I n it s simple r forms , a s we have seen , i t act s merel y as a sphincter betwee n the cro p and the stomach t o regulate the passag e of foo d materia l int o th e latter . Wit h th e developmen t o f fold s an d sclerotic armatur e o n it s inne r walls , however , th e orga n acquire s a more diversifie d function . I n th e firs t place , th e fold s projectin g int o the lume n serv e t o hol d bac k th e foo d i n th e cro p withou t completel y closing th e ventricula r entrance . Digestiv e liquid s fro m th e stomac h may thus be permitted to flow forward into the crop through the channels

THE ALIMENTARY CANAL

357

between th e fold s an d brin g abou t a partia l digestio n o f the cro p foo d before th e latte r i s transmitted t o th e stomach . Thi s possibl e function of th e proventriculu s in Orthoptera and Coleópter a has been particularl y stressed b y Ramm e (1913) , wh o points ou t furthe r that th e movement s of th e proventricula r lobes , brough t abou t b y th e stron g muscle s sur rounding th e latter , mus t serv e t o mi x the digestiv e fluids thoroughl y into the foo d mass. Som e insects accomplish an extraintestinal digestio n of th e food , supposedl y b y gastri c juices ejecte d fro m th e mouth . The armatur e o f the proventriculu s often ha s th e for m o f convergent lamellae, an d thi s typ e o f structur e ha s suggeste d tha t th e apparatu s serves a s a straine r t o preven t large r piece s of hard indigestibl e matte r in the foo d fro m enterin g the stomach , such material being later dispose d of b y regurgitation . Th e onl y definite evidenc e of normal regurgitatio n by insects, however, pertains toDytiscus, which is said by Rungius (1911) ,

FIG. 195.—The alimentary canal, salivary glands, and Malpighia n tubules of a grasshopper, Dissosteira Carolina.

Ramme (1913), Blunck (1916), and others to disgorge indigestible parts of the animal s o n whic h i t feeds . Accordin g to Blunck , Dytiscus has n o salivary glands , and digestion takes place in the cro p by liquids from th e stomach. Th e proventriculus , h e says, grind s the foo d mas s and pushe s the large r fragments back into the crop , while at th e sam e time it allow s the liquefie d residu e t o filte r throug h int o th e stomach . A few hours after mealtim e th e beetl e suddenly eject s several times fro m it s mouth a turbid clou d o f material, which , as i t disperse s i n th e water , i s see n t o contain undigeste d remnant s o f th e food . Othe r insects , a s fa r a s observed, ordinaril y pas s al l undigeste d refus e throug h th e stomac h and intestine . Sanfor d (1918 ) observe d regurgitatio n b y overfe d cockroaches, but i t i s here evidently th e resul t o f too muc h feeding an d not a n exampl e of a normal physiological process. The movement s o f th e stomodaeu m o f Periplaneta fuliginosa hav e been studie d b y Yeage r (1931 ) wh o find s tha t peristalsi s take s plac e in th e cro p i n bot h a posterio r an d a n anterio r direction , an d tha t th e proventricular movement s ar e contractil e only . Th e activitie s o f th e proventriculus, h e says , appea r t o b e largel y controlle d b y th e firs t thoracic ganglio n o f the ventra l nerv e cord.

358

PRINCIPLES OF

INSECT MORPHOLOGY

The earlie r entomologist s commonl y regarde d th e proventriculu s as a gizzard ; judging from it s structur e i n Orthopter a an d Coleópter a they di d not hesitat e t o nam e i t th e "chewin g stomach " (Kaumagen). It wa s Plateau wh o first threw discredit o n this idea , an d later Ramm e (1913) claime d t o demonstrat e tha t th e proventriculu s i s i n n o cas e able t o brea k u p har d part s o f th e food . Muc h discussio n ha s sinc e ensued, an d experimenta l evidence has seeme d inconclusive. Recently , however, Eidman n (1924 ) ha s mad e observation s tha t appea r t o b e decisive. H e find s tha t i n cockroache s durin g moulting th e post cephalic par t o f the stomodaea l exuvia e remains intac t withi n th e cro p until th e armatur e of the ne w pro ventricular cutícul a is fully sclerotized ; after thi s th e ol d cuticul a i s broke n u p an d th e piece s discharge d b y way o f th e stomac h an d intestine . Furthermore , a n examinatio n o f

FIG. 196.—Th e alimentar y canal, sil k glands , dorsa l bloo d vessel , an d nerv e cor d o f a caterpillar.

the foo d conten t o f th e cro p an d ventriculus , mad e a t a certai n tim e subsequent t o feeding , show s that th e foo d particle s o f th e latte r ar e smaller tha n thos e i n th e crop . Fro m thes e observation s Eidman n concludes that the proventriculu s anterior o f the cockroac h is a chewing apparatus, an d tha t afte r trituratio n th e foo d i s returne d t o th e cro p where it undergoes a preliminary digestion by the enzymes of the salivar y secretion. Th e food is then passed into the stomach through the proven triculus posterior, whic h otherwise serves merely as a closing apparatus . Confirmatory evidenc e o f th e chewin g function o f th e proventriculu s is adde d b y Davi s (1927) , who inserte d smal l strand s o f wax int o th e proventriculus of live Stenopelmatus an d foun d th e wa x indented b y th e pro ventricular teeth . Finally, w e ma y observe , th e proventriculu s serve s i n som e case s as a stomach mouth (Magenmund), o r pump (Pumpmagen, Emery , 1888). This functio n i s particularl y eviden t i n th e aculeat e Hymenoptera . Here th e fou r thick , inne r lobe s o f th e orga n reac h forwar d int o th e crop (hone y stomach of bees), and the posterio r part extends as a funnel shaped tub e int o the ventriculus . Th e lobe s open and clos e like a four lipped mouth, and apparently i t i s by their activit y tha t the foo d i n the crop is transferred to the stomach .

THE ALIMENTARY CANAL

359

The Cardiac Valve. —The cardiac , or stomodaeal , valv e i s essentiall y a circula r fol d o f th e stomodaea l wal l projectin g int o th e ventriculu s from the posterior end of the stomodaeum (Fig. 194, SVlv). The valve is composed , therefore , o f two cellula r lamella e an d i s covere d o n eac h side b y th e stomodaea l intima . Th e basa l rin g o f th e oute r lamell a (d, d ) mark s th e morphologica l terminus o f the stomodaeum . I n for m the cardia c valv e i s generally cylindrical or funne l shaped , bu t i t i s no t always symmetricall y developed . Th e tw o lamella e ar e usuall y mor e

FIG. 197.—Transformation o f the alimentar y canal of a moth, Malacosoma americana, from the larv a (A ) throug h the pup a (B ) to th e imag o (C).

or less free fro m eac h other an d ma y includ e between them a n extension of th e stomodaea l muscles , but i n som e cases the tw o wall s are adnate . The functio n o f th e cardia c valv e i s generall y suppose d t o b e tha t of preventin g a retur n movemen t o f th e foo d fro m th e stomach , bu t the fol d doe s no t entirel y occlud e the stomac h entrance , sinc e i n som e insects digestiv e juice s flow forward fro m th e latte r into the crop . Th e projecting valvula r tub e conduct s th e foo d fro m th e proventriculu s well int o th e stomac h lume n an d partl y shut s of f a spac e aroun d i t i n the cardia c en d o f the stomach , int o which may ope n the gastri c caeca , and i n whic h may b e situate d specia l secretor y cell s of the ventricula r wall that for m th e peritrophi c membrane (Fig . 20 4 A, B). 4. TH E MESENTERO N

The middl e sectio n o f th e alimentar y cana l (Fig . 190 , Ment) i s th e stomach o f th e adul t insec t an d i s therefor e commonl y calle d th e ventriculus. Onl y th e epithelia l wal l o f th e ventriculu s i s forme d fro m

360

PRINCIPLES OF INSECT MORPHOLOGY

the endodermal mesenteron of the embryo (Fig. 13 D, Ment), but usually the entir e adul t orga n is termed th e mesenteron , o r mid-gut. In th e composit e definitiv e alimentar y cana l th e ventriculu s begin s morphologically a t th e bas e o f th e oute r fol d o f th e stomodaea l valv e (Fig. 194 , d), th e lin e bein g marke d b y th e terminatio n o f th e stom odaeal intima . Th e wall s o f th e ventriculu s ar e distinguishe d fro m those o f the stomodaeu m by the large r size and more spongy appearanc e of th e epithelia l cells , by th e absenc e of a permanent o r uniform intima , and by a reversal in the arrangement of the fibers in the muscular sheath ,

FIG. 198.—The alimentary canal an d salivar y glands of a fruit fly, Rhagoletis pomonella, showing th e diverticula r cro p (Cr) an d th e cardia c sa c (Car) o f th e ventriculus , charac teristic of many Diptera .

the principa l longitudina l muscle s o f th e ventriculu s (Fig . 201 , Imcl) being externa l t o th e circula r muscle s (cmcl). Th e ventriculu s end s posteriorly a shor t distanc e befor e th e base s o f the Malpighia n tubule s (Fig. 190, Mai), which, when present, define approximately the anterior end o f the proctodaeum . General For m o f th e Ventriculus. —The ventriculu s commonl y ha s the for m o f a tub e o r elongat e sa c o f approximatel y unifor m diamete r (Figs. 195 , 196 , Vent). Onl y occasionally doe s it sho w a differentiatio n into regions , thoug h i n som e insect s i t i s quit e distinctl y divide d int o two, three, o r four parts . The anterio r en d o f the ventriculu s surroundin g th e stomodaeal , or cardiac, valve is sometimes distinguished as the cardia (Figs. 190,194 Car). In th e muscoi d Dipter a th e cardi a become s a small , flattened , circula r sac containin g th e stomodaea l valve , separate d fro m th e res t o f th e ventriculus by a narrow constriction (Fig . 198 , Car). Nearl y all students of the alimentary canal of Diptera have called the cardia the " pro ventriculus/' bu t it s tru e natur e i s shown by th e fac t tha t th e stomodaea l valve is invaginated into its anterior en d (Fig. 20 4 B). I n the mosquit o (A) th e cardi a is less differentiated an d i s clearly the anterio r part o f th e ventriculus.

THE ALIMENTARY CANAL

361

In the horse fly Tabanus, as described by Cragg (1920), the ventriculus is differentiated int o a slender anterior tubular region and into a posterior dilated region , the tw o differin g bot h histologicall y an d functionall y as well a s i n form . Th e firs t par t Crag g call s the "cardia, " thoug h thi s term shoul d b e restricte d t o th e anterio r en d o f th e ventriculus ; th e second h e says i s functionally th e tru e stomach o f the hors e fly, since all the bloo d swallowed at th e tim e of feeding i s passed into it . The regional differentiation of the ventriculu s is carried to its highes t degree in the Hemiptera. I n the more generalized Homoptera the organ is usually divided into three quite distinct part s (Fig . 209 A). Th e first part ( 1 Vent) i s a sac lying within the filter chamber (FC) ; th e secon d is a large croplik e enlargement ( 2 Vent); an d th e thir d i s a long slender tub e (3 Vent), ofte n calle d the "ascendin g intestine" since it turns forwar d t o

FIG. 199.—Th e alimentary cana l o f a scarabaeid larva, Popülia japónica, wit h thre e set s of gastri c caec a (IGCa, 2GCa, SGCa).

reenter th e filte r chamber . I n th e Heteropter a th e ventriculu s i s commonly differentiate d int o fou r well-define d region s differin g i n lengt h and diamete r (Fig . 20 0 B), th e fourt h bein g provided i n man y families with numerou s caeca l diverticul a (GCa). Th e principa l modifications in the for m o f the hete r opt eran alimentar y cana l are shown by Glasgow (1914) i n a long series of figures . Caecal Diverticul a o f th e Ventriculus.—Blin d pouche s varyin g i n number an d i n lengt h ma y b e develope d on differen t part s o f the ven triculus. Mos t commonl y they occu r a t th e anterio r en d surrounding the stomodaeal valve. Ther e are usually from two to six of these anterior gastric caec a (Fig . 190 , GCa), thoug h th e numbe r ma y b e greater . I n form the y ar e generall y simpl e blun t o r taperin g processes , bu t i n th e Acrididae each is divided at its base into an anterior branch and a posterior branc h (Fig . 195 , GCa). Caeca l diverticul a sometime s occur , however, on other parts of the ventriculus , a s in the larva e of lamellicorn beetles, in which there may be three circles of them (Fig. 199, IGCa, 2GCa, 3GCa), tw o nea r th e anterio r en d o f th e stomach , th e othe r nea r th e posterior end . I n th e larv a o f th e fl y Ptychoptera contaminata, va n Gehuchten (1890 ) describe s a circl e of eigh t smal l diverticul a nea r th e

362

PRINCIPLES OF INSECT MORPHOLOGY

anterior en d o f th e ventriculu s (Fig . 20 0 A , GCa), an d a pai r o f long glandular pouche s (gl) arisin g fro m th e extrem e posterio r en d o f th e organ. I n man y Coleópter a a larg e par t o f the ventriculu s i s covered with small papilliform o r sometimes elongate diverticula, but thes e struc tures i n mos t case s ar e th e crypt s o f epithelia l regenerativ e cell s (Fig . 206 C, Cpt) rathe r tha n true caeca. A remarkabl e development o f caeca l appendages o n th e ventriculu s occurs in the Heteroptera , where in many families a group of diverticula,

FIG. 200.—Example s o f caeca l diverticul a o n variou s part s o f th e ventriculus , an d o f subdivision o f th e ventriculus . A , larv a o f Ptychoptera contaminata (Diptera ) wit h glandular diverticul a (gl) fro m posterio r en d o f ventriculus. (From Va n Gehuchten, 1890. ) B, C , Peliopelta abbreviate, and Blissus leucopterus (Heteroptera ) wit h fou r section s i n th e ventriculus, an d gastri c caeca arising fro m th e fourt h section. (From Glasgow, 1914. )

varying greatly in number, size, an d form , ar e give n off fro m th e fourt h section o f the stomach . A n extensiv e study o f the gastri c caec a of th e Heteroptera ha s bee n mad e b y Glasgo w (1914) , wh o give s numerous illustrations o f their variou s forms. I n genera l there ar e tw o type s of these organs : in on e type th e diverticul a are short , o f uniform size , an d arranged in two or four rows along most of the exten t of the fourt h section of th e stomac h (Fig . 200 B, GCa); i n the othe r type th e caec a are fewe r in numbe r but ar e lon g tubes o f varying length an d ofte n ver y unsym metrically groupe d (C). Accordin g to Glasgo w the gastri c caec a of th e Heteroptera, whereve r they occur , ar e invariabl y fille d wit h bacteria , and th e presenc e of the bacteri a i s hereditary, th e organism s appearing early i n the alimentar y cana l o f the developin g embryo. Glasgo w says that " these normal bacteria appear not only to inhibit the development of foreign bacteria but to exclude them altogether.'7 H e suggests, therefore,

THE ALIMENTARY CANAL

363

that th e functio n o f the caec a is merely t o provid e a saf e plac e for th e multiplication o f the norma l bacteria o f the alimentar y canal . Histology o f th e Ventriculus.—Th e epithelia l wall s o f th e stomac h are characteristicall y thicke r tha n those o f other part s of the alimentar y canal, bu t th e musculari s is usually more weakly developed than i n th e stomodaeal region . A n intima is not alway s present, a t leas t no t i n th e form o f a definit e cuticula r layer , an d whe n i t doe s occu r as suc h i t i s continually o r periodicall y she d int o th e lume n o f th e stomach . I n most insect s a thin peritrophic membrane surround s the foo d content s of the ventriculus . The Epithelium. —The appearanc e o f th e ventricula r epitheliu m (Fig. 201 , Epth) varie s greatl y according t o th e stat e of the digestive processes . Mos t o f it s cell s are columnar , wit h irregula r inne r ends more or less projecting into the stomach lumen . Th e cytoplas m appears granula r o r spongy ; th e nuclei are large and generall y occupy the middl e or distal parts of the cell bodies, where, in sections, the y FIG. 201.—Diagrammati c cros s sectio n form fairl y eve n row s o r line s of th e ventriculus . BMb, basemen t memfollowing th e inne r contou r o f th e brane; cmcl, circula r muscles ; Epth, epi epithelium. I n additio n t o thes e thelium; F, food material; IMcl, longitudinal muscles; Lum, lumen ; PMb, peritrophi c larger, spong y cells that form mos t membrane surroundin g th e foo d an d sepa of th e epithelia l wall , ther e ar e rated b y spac e (a ) fro m epithelium ; rg, usually to be seen other smaller cells regenerative cells ; sb , striate d border . (rg) o f a denser texture occurrin g either singly o r i n group s between th e bases o f th e large r cell s (Fig . 20 2 B ) o r aggregate d int o definit e clusters (C) , sometime s containe d i n pockets , o r crypts, o f th e epithelium (E) . Th e large r cells , havin g thei r inne r end s expose d o r projecting int o the stomac h lumen , are the digestive cells (B , dg), tha t is, cells that take an activ e par t i n th e processe s o f secretion o r absorp tion; th e smalle r basa l cell s ar e th e regenerative cells (rg), th e functio n of whic h i s t o propagat e cell s to replac e th e digestiv e cell s whe n th e latter are exhausted by secretory activities or shed at th e tim e of ecdysis. The digestive cells constitute th e functional epithelium of the stomach . Their central end s are often differentiate d as a weakly staining, margina l layer o f th e epithelium , which , i n sections , appear s t o b e crosse d b y numerous fin e line s perpendicula r t o th e surface . Thi s margina l zon e of th e stomac h epitheliu m i s know n a s th e striated border (Figs . 201 , 202 A , s&) . Th e natur e o f th e striate d borde r ha s bee n th e subjec t of much discussion . Earlie r investigator s believe d i t t o b e a coatin g of

364

PRINCIPLES OF

INSECT MORPHOLOGY

fine filament s coverin g th e inne r surfac e o f th e stomach , comparabl e with th e ciliat e linin g o f the mesentero n i n Annelida . I n th e insects , however, th e striate d zon e i s a continuou s laye r i n whic h th e darke r lines o f th e stria e alternat e wit h clea r line s o f a les s dens e materia l through which minute droplets of digestive liquids may b e extruded fro m the inner parts of the cells. Th e surface of the striated border is generally observed to be defined b y a delicate limiting membrane. In most insects the digestive cells are of uniform structure throughou t the ventriculus , excep t in that they ma y b e of different size s in differen t

FIG. 202.—Diagram s showin g variou s position s o f th e regenerativ e cell s (rg} o f th e ven tricular epitheliu m wit h relatio n t o th e digestiv e cell s (dg).

parts of the stomac h or may b e found i n various stages o f disintegration. In the larva e of Lepidoptera, however, there are two quite distinct type s of digestiv e cells . Thos e o f on e typ e hav e th e ordinar y columna r o r cylindrical form; thos e o f the other , characterized as calyciform or goblet cells, hav e eac h a larg e ampull a i n its mesa l par t openin g by a narrow neck throug h a smal l apertur e o n the inne r surface . Th e tw o type s of digestive cells of the caterpilla r have been studied particularl y in Gallería mellonella b y Yung-Ta i (1929) , wh o find s that the y ar e differentiated even i n th e embryo , an d tha t the y ar e generate d separatel y fro m th e replacement cells . Th e cavitie s o f th e goble t cell s ar e line d wit h a striated borde r lik e tha t o f th e columna r cells . Yung-Ta i conclude s that th e goble t cell s ar e exclusivel y secretor y i n function , whil e th e ordinary cylindrica l cell s may b e eithe r secretor y o r absorptive, thoug h the sam e individual cell s do not functio n i n both capacities . Th e goble t cells, he says, ar e not replace d after th e moult to the pupa . From it s ver y beginnin g th e endoder m o f insects appear s t o b e a n unstable tissue. A s we saw in Chap. II, th e formation of the mesenteron in th e embry o is apparentl y a regenerativ e proces s followin g a n earlie r

THE ALIMENTARY

CANAL

365

dissolution o f the primitiv e archenteron . Durin g postembryonic lif e of most insect s th e cell s o f the mesenteri c epitheliu m ar e continuall y sub ject t o variou s degree s o f disintegratio n a s a resul t o f thei r secretor y processes, o r for the purpos e of reconstructive growt h in the ventriculu s accompanying the moults . Th e replacemen t o f the epitheliu m i s eithe r gradual an d partia l o r rapi d an d complete , accordin g t o th e natur e of the disintegratio n processes. Th e ne w cells are formed fro m th e specia l regenerative cells , whic h tak e n o par t i n th e othe r activitie s o f th e ventriculus. Th e processe s o f regeneratio n ar e i n genera l th e sam e regardless o f th e degree , time , o r manne r o f cel l replacemen t o r th e reason for its occurrence. Both th e digestiv e an d th e regenerativ e cell s o f the ventriculu s ar e derived from th e primitiv e endoderm , the digestiv e cell s being so special-

FIG. 203.—Section s of the ventriculu s of a mosquit o larva and pup a showing regenerative cells . (From Samtleben, 1929. ) A , B , Culex pipiens, middle-age d larva. C , Aedes meigenanus, newl y formin g pupa . D , Culex pipiens, larv a just befor e pupation .

ized for the function s o f secretion and absorptio n that they hav e lost th e power o f reproduction, while the regenerativ e cell s maintain unimpaire d the propert y o f mitotic division . Th e regenerativ e cell s ar e usuall y of small siz e an d li e beneath th e other s agains t th e basemen t membran e (Fig. 20 2 B , rg). The y ar e show n i n a relativel y simpl e conditio n i n Gollembola of all stages and in some dipterous larvae (Fig. 203, rgr), where they occu r singl y o r in smal l group s scattered throughou t th e lengt h of the ventriculus . The regeneratio n cell s o f th e ventricula r epitheliu m generall y for m definite cel l groups, or nidi, sharpl y distinguishe d fro m th e surroundin g digestive cell s (Figs . 20 2 C , 20 5 B, 20 6 A, rg). Fro m thes e specialized regeneration center s ar e propagate d th e ne w cell s tha t replac e th e exhausted or discarded digestive cells. I n the Hymenoptera th e regeneration cell s are containe d i n ope n pockets o f the ventricula r epithelium , but i n other insects in which the regeneratio n cells are grouped in definite nidi, the pockets are generally closed by an overgrowth of the surrounding digestive cells (Figs. 20 2 C, 206 A), an d th e genera l contour of the inne r surface o f the epitheliu m give s no indication o f the positio n o f the nidi . The regeneratio n cells , however, may li e at th e bottom s o f deep folds o r

366

PRINCIPLES OF INSECT MORPHOLOGY

pockets o f th e epithelium . I n som e Coleópter a the y ar e containe d i n evaginations o f the stomac h wall , forming pouchlik e diverticul a known as the regenerative crypts (Figs. 202 E, 206 C, Cpf), which may be so numerous as to give the entire external surface of the ventriculus a villous structure. The Basement Membrane. —The epithelia l cell s of the ventriculu s rest upon a membrane (Fig. 201, BMb) whic h appears to be a tunica propria , or produc t o f the cel l bases , differin g i n n o respec t fro m th e basemen t membrane o f the bod y wall or fro m tha t o f the ectoderma l parts o f th e alimentary canal . Accordin g to Deegene r (1910 ) an d Rungiu s (1911 ; Korschelt, 1924) , however , th e ventricula r epitheliu m o f Dytiscus i s invested in a thick supporting layer (Stützlamelle) which is a nucleated connective tissue and is not to be identified with the tunica propria of the stomodaeum and proctodaeum. The Peritrophic Membrane. —The foo d conten t o f th e stomac h i n many insect s i s separate d fro m th e ventricula r epitheliu m b y a thi n membrane, which, though often i n more or less intimate contact with th e inner end s of the epithelia l cells, typically surround s the foo d mas s a s a cylindrical sheath fo r the mos t par t fre e fro m th e stomac h walls . Thi s food envelope is known as the peritrophic membrane (Fig. 201, PMb). It i s not presen t i n al l insects, bu t i t i s known to occu r in Collembola , Thysanura, Ephemerida , Odonata , Orthoptera, Neuroptera , Coleóptera , Hymenoptera, Diptera, an d larva l Lepidoptera , whil e it i s sai d t o b e absent in Hemiptera and adult Lepidoptera, as well as in certain members of th e order s in which it i s usually present . The peritrophic membrane is a product of the ventricula r epithelium , being forme d i n mos t case s fro m th e entir e surfac e o f th e ventriculus , but i n Dipter a i t appear s t o be produced by a band of specialized cells in the anterio r end of the cardi a encircling the base of the stomodaeal valve. In no case is it a continuation of the pro ventricular intima. Som e writers have assumed that the peritrophic membrane is a nonchitinous structure because i t i s produce d b y endoderma l cells ; bu t severa l investigators , including Weste r (1910) , Campbel l (1929) , and vo n Deh n (1933) , hav e found b y chemica l tests that th e peritrophi c membrane contains chitin , while Hovene r (1930 ) say s that it shows two characteristic propertie s of chitin, namely , double refraction and resistance to alkalies . Ther e is no reason for supposing that chitin should not be produced from endoderma l as well as from ectoderma l derivatives o f the blastoderm ; the peritrophi c membrane i s evidentl y t o b e regarde d a s a chitinou s intim a o f th e ventriculus. The componen t materia l o f th e peritrophi c membran e i s probabl y a secretio n produc t o f th e matri x cells . Folso m an d Welle s (1906 ) claimed tha t th e peritrophi c membrane of Collembola is a direc t trans -

THE ALIMENTARY CANAL

367

formation o f th e striate d borde r o f th e ventricula r epithelium , cas t off from tim e t o time , a s a ne w striated borde r i s forme d beneat h it , an d Ertogroul (1929 ) describe d th e peritrophi c membran e i n th e silkwor m as formed in the same manner. Accordin g to Yung-Tai (1929), however, the peritrophi c membran e o f the larv a o f Gallería mellonella consist s of successive delamination s o f a surfac e membran e o f the ventricula r epi thelium, and that of the larv a of Vanessa urticae is said by Henson (1931 ) to b e a secretio n produc t o f th e epithelia l cells . Vo n Deh n (1933 ) contends that th e peritrophi c membrane is in no case identical wit h the striated borde r o f the epithelium , since th e membran e i s chitinou s an d the striate d borde r i s cytoplasmic. Th e chitinou s material o f the peri trophic membrane , sh e says , appear s i n liqui d droplet s beneat h th e striated border , i s extrude d throug h th e interstice s o f th e latter , an d runs together ove r the cel l surfaces t o for m a continuous layer, whic h is then separate d fro m th e epitheliu m t o becom e a peritrophic membrane. In th e proces s of separation th e striate d border may b e more or less dis rupted an d fragment s may adher e to th e membrane, but no t as constituent parts of it. In th e larva e o f mos t aculeat e Hymenopter a th e peritrophi c mem branes form a sac closed posteriorly abou t the foo d mas s of the stomach , since th e mesentero n doe s no t ope n int o th e proctodaeu m unti l th e termination o f larval life . Th e sam e is said t o b e tru e o f the larva e of some Neuroptera. Th e food sac of a mature wasp larva (Vespa) appear s as a ba g filled with a blac k mass , th e ba g lyin g fre e i n th e ventriculu s except for an attachment t o the walls of the latter around the bas e of the stomodaeal valve. A t th e tim e o f defecation the sa c becomes detached and i s ejected entire wit h its content s int o th e inne r en d of the cocoon , which the larva has already spun about itself, and here the dejectament a dry t o a har d blac k mass . Th e peritrophi c membrane s o f th e was p larva ar e described by Renge l (1903), and those of ant larva e by Strindberg (1913) , a s bein g given of f successively from th e genera l surface of the ventricula r epithelium . I n th e larv a o f the honey bee Nelson (1924) describes the peritrophi c membrane as a thick homogeneous layer, appar ently o f gelatinous consistency , covering the inne r surface o f the epithe lium, but a t th e anterio r en d of the ventriculu s he says there i s a ring of specialized cell s from th e surfac e o f which streams o f secretion issue an d run cauda d to join with the principa l mass of the peritrophi c membrane. Since th e peritrophi c sa c o f hymenopterous larvae become s entirel y free fro m th e wall s of th e ventriculu s in the matur e larva, excep t for it s anterior attachment , i t i s clea r that a n examinatio n o f a larv a a t thi s stage woul d sugges t that th e membran e is a product onl y of the rin g of specialized cells, noted by Nelson in the honeybee, surrounding the stomodaeal valve . Th e statemen t b y Cuéno t (1896 ) tha t th e peritrophi c

368

PRINCIPLES OF INSECT MORPHOLOGY

membrane o f Orthopter a i s th e produc t o f secretion b y cell s occupying the anterio r en d of the mesentero n must, therefore , be taken wit h some reserve, especiall y sinc e Davi s (1927 ) find s th e foo d envelope s of Stenopelmatus t o b e mostl y a serie s o f delaminations fro m th e entir e surfac e of the stomach epithelium, though possibly augmented from the secretion of specia l anterio r cells . O n the othe r hand , ther e appear s t o b e reason to believ e tha t i n the Diptera , bot h larva e an d adults , th e peritrophi c membrane ma y tak e it s origi n entirel y fro m a ban d o f specialized cells confined t o th e anterio r en d of the ventriculus . In th e adul t hone y bee the foo d materia l of the ventriculus is usually enclosed i n a serie s o f peritrophic membranes , whic h are give n or í suc cessively fro m th e inne r surfac e o f th e epithelium . Durin g period s of secretory activit y the secretio n product s forme d i n the cell s accumulate beneath a surface film, or border membrane, and the whole mass eventu ally separate s fro m the cel l layer, whic h then form s a new border membrane. Mos t o f the discarde d substance s ar e dissolved, an d th e residu e becomes a peritrophic membrane . That th e peritrophi c membran e o f Diptera i s produced from cell s in the anterio r en d of the mesentero n was first suggested by van Gehuchten (1890); but va n Gehuchte n called the cardia c enlargement o f the mesen teron (Fig . 198, Car) th e " pro ventriculus/' and this terminology, adopted by man y subsequen t student s o f th e alimentar y cana l o f Diptera , ha s been a source of confusion to those who have not perceived that the orga n in questio n i s no t th e proventriculu s o f other insect s (Fig . 190 , Pvent) but i s the cardiac section of the mesenteron. Th e cardia is best developed as an antechamber o f the stomach in the muscoid Diptera, where it take s the for m o f a flattened , circula r sac (Fig . 198 , Car) with the stomodaea l valve invaginated int o its anterior en d (Fig . 204 B, SVlv). I n th e lower flies this regio n of the stomac h is less differentiated, but i t i s recognized by Imm s (1907 ) i n th e mosquit o larv a a s the cardi a (Fig . 20 4 A, Car). The anterio r par t of the wal l of the cardi a is formed b y a band of specialized epithelia l cell s (e ) surroundin g th e bas e o f th e stomodaea l valv e (SVlv), an d it i s these cell s apparently tha t secret e the substanc e which forms th e peritrophi c membrane (PMb). The formation of the peritrophic membrane in Diptera a s an apparen t secretion fro m a ring of specialized cells in the anterio r en d of the mesen teron ha s bee n describe d b y Hasema n (1910 ) an d Hovene r (1930 ) i n Psychoda alternata, an d b y Wiggleswort h (1930 ) i n Glossina. I n Psychoda, accordin g t o Haseman , th e glandula r membrane-formin g cell s occupy a circula r are a 6 to 1 2 cells in length just beyon d the bas e of the stomodaeal valve ; the inne r granular surface o f the cell s hardens to for m the delicat e peritrophi c membrane , " which is continually fed back into the mid - an d hind-intestin e t o envelo p th e foo d materials. " I n th e

THE ALIMENTARY CANAL

369

description o f the formatio n o f the peritrophi c membran e o f th e tsets e fly give n b y Wiggleswort h th e reader mus t understan d tha t th e ter m "proventriculus" refers t o the cardia , or anterior en d of the mesenteron . Figure 20 4 C , base d o n Wigglesworth' s drawing s o f Glossina, show s a section throug h on e side of the stomodaea l valve (SVlv) an d th e wal l of the surroundin g cardia (Car) . A s in most o f the highe r Dipter a th e lip s of th e valv e ar e reflecte d forward , an d in the circula r space thus enclosed at th e bas e o f th e valv e i s th e rin g o f larg e secretor y cell s (e) o f th e anterior en d o f the mesenteri c epithelium . Th e discharge d products of these cells condense to form a cylindrical peritrophic membrane (PMb) fed bac k into th e ventriculu s from aroun d the peripher y o f the reflecte d lips of the stomodaea l valve. I n th e mosquit o larva, as shown by Imms

FIG. 204.—Section s o f th e stomodaea l valve , cardia , an d peritrophi c membrane o f Diptera. A , Anopheles maculipennis larva . (From Imms, 1907. ) B , Calliphora erythrocephala, stag e almos t adult . (From Pérez, 1910. ) C , Glossina adult , on e sid e o f stomodaeal valve and wal l o f cardia. (Diagrammatic from Wigglesworth, 1929. )

(1907), th e peritrophi c membran e i s forme d i n a simila r manne r (Fig . 204 A) , bu t th e generativ e cell s here occup y most o f the lengt h o f th e cardia. Inasmuch a s th e peritrophi c membrane , when present, usuall y completely surrounds the food content of the stomach (Fig. 201), the products both o f epithelia l secretio n an d o f gastric digestio n mus t penetrat e th e membranous envelope , the firs t t o ac t upo n th e food , th e secon d to b e absorbed b y th e ventricula r cells . Th e spac e betwee n th e epitheliu m and the peritrophi c membrane (a) is generally filled with digestive liquid, granules, globule s of secretion products , discharge d epithelia l cells , an d presumably als o with foo d materia l i n solution tha t has passed outwar d through th e peritrophi c membrane. Th e permeability o f the peritrophi c membranes o f the hone y bee and th e blo w fly to various stains has been demonstrated b y von Dehn (1933) . A t present n o satisfactory explanation ca n b e offere d a s t o th e genera l functio n o f the membrane , which occurs also in other arthropod s tha n insects .

370

PRINCIPLES OF INSECT MORPHOLOGY

The Muscularis. —The muscula r sheat h o f th e ventriculu s i s les s strongly develope d tha n tha t o f the stomodaeum . Th e circula r fiber s (Fig. 201, cmd) generally constitute the principal layer, the longitudinal fibers (Imcl), lyin g external to th e circulars , being usually widely spaced , and sometime s group s o f longitudina l fiber s for m specia l lengthwis e muscle bands that look like cords stretched betwee n the tw o ends of th e stomach. Muscle s of the latte r type are particularly conspicuou s in th e caterpillars (Fig . 207 A, B, Vent). Whil e in most insect s all the longitu dinal muscles of the ventriculus lie external to the circula r muscles, a few lengthwise fiber s ar e sai d b y Renge l (1898 ) t o li e withi n th e circula r fibers in Hydrophilus, an d Whit e (1918 ) claim s that ther e i s likewise in the hone y bee an inne r laye r o f very fine longitudinal fibers between th e circular muscle s and th e basemen t membran e of the ventricula r epithe lium. A peritonea l coverin g o f loos e cellula r tissu e i s sai d b y som e writers t o surroun d th e musculari s i n certai n insects , bu t usuall y th e muscles of the alimentar y cana l have no very definite investiture . Activities o f th e Ventricula r Epithelium.—Th e activitie s o f th e epithelial cells of the ventriculu s may b e divided for descriptive purpose s into four classes , a s follows : (1 ) secretion an d absorption , (2 ) excretion , (3) degeneratio n an d regeneratio n o f th e digestiv e cell s accompanyin g or followin g secretion , an d (4 ) periodical delaminatio n an d replacemen t of the entir e epithelium, mostly accompanying the moults. Secretion an d Absorption. —The primar y function s o f the cell s o f th e ventricular epitheliu m ar e the productio n of liquids containing digestiv e enzymes, and the absorption and transmission to the blood of the products of digestion . Probabl y i n mos t insect s bot h thes e activitie s ar e prop erties o f the sam e cells , but va n Gehuchte n (1890 ) has claimed that th e two function s pertain t o tw o set s o f cells in the fl y Ptychoptera contaminóla, an d Yung-Ta i (1929 ) give s convincin g evidenc e tha t th e goble t cells i n th e larva l epitheliu m o f the mot h Galleria mellonella ar e exclu sively secretory , whil e th e columna r cell s ma y b e eithe r secretor y o r absorptive i n function , thoug h th e tw o activitie s ar e no t performe d by the sam e cells of this group. The discharg e o f th e secretio n products , i n it s simples t form , undoubtedly, i s accomplishe d b y th e direc t passag e o f th e elaborate d substances throug h th e striate d borde r o f th e secretin g cells , an d i t i s possible tha t th e secretio n discharg e i n al l case s take s plac e b y thi s method. Wit h mos t insects , however , ther e i s t o b e observe d i n th e ventriculus a conspicuou s process of budding from th e inne r end s o f th e epithelial cells . Th e extrude d globule s either disrup t an d scatte r thei r contents in the ventricular lumen or they become detached and are given off a s fre e bodie s whic h later disintegrate . Generall y i t ha s bee n sup posed tha t thes e activitie s o f th e ventricula r cells , whic h hav e bee n

THE ALIMENTARY CANAL

371

studied onl y a s physica l phenomen a i n histologica l preparations , ar e processes of holocrine secretion, but ther e is a recent tendency to regar d them as disintegration processes followin g exhaustiv e period s o f ordinar y secretion. I n an y cas e they ar e anatomically degenerativ e change s and will be described in this category . Excretion.—There i s littl e doub t tha t th e wall s o f th e ventriculu s play som e part i n excretion , in eithe r a n activ e o r a passiv e role . Th e epithelial cell s ar e ofte n observe d t o contai n larg e number s o f smal l crystalline bodies, which are found to be principally calcium salts, though some als o ar e sai d t o hav e the properties o f uric acid concretions. Suc h deposits, togethe r wit h bacteria l inclusions , are at leas t eliminate d wit h the sheddin g o f the epitheliu m at th e tim e o f ecdysis.

FIG. 205.—Examples of disintegration processe s (supposedly holocrine secretion) in th e stomach epithelium . A , Ptychoptera contaminata larva . (From Va n Gehuchten, 1890. ) B, Gomphus descriptus larva . (From Needham, 1897. ) C , Tabanus adult . (From Cragg, 1920.)

Degeneration and Regeneration of the Digestive Cells. —Throughout the activ e lif e o f most insect s ther e take s plac e in th e epitheliu m o f th e ventriculus a partia l o r complet e disintegratio n o f th e digestiv e cells , followed b y a replacement of the los t cells with new cells formed fro m th e regeneration cells . The simples t for m o f disintegration i n th e digestiv e cell s consists of the accumulatio n o f granula r materia l i n th e inne r end s o f th e cells , succeeded by a rupture o f the cel l wall and th e discharg e of the materia l into the ventricular lumen. Th e cell wall then closes, the striated border is reestablished, an d the cell continues its digestive functions. A second and more intensive form o f disintegration involve s a separation o f the inne r parts o f the cell s containing th e granule s and globules . The mesa l border o f the cel l in this case swells out i n the for m o f a bud , which become s constricte d a t it s bas e an d finall y separate d a s a fre e sphere from th e bod y of the cel l (Fig . 205 A). I n Collembola , according to Folsom an d Welles (1906), the bud is at first surrounded by a striated zone, whic h later i s lost ; bu t i n mos t othe r insect s th e striate d borde r disappears o n the evaginatin g bud . Th e liberate d spher e floats off into

372

PRINCIPLES OF INSECT MORPHOLOGY

the stomac h lume n and there undergoes a gradual dissolution whic h sets free it s contents . Thi s for m o f disintegratio n i s th e on e generall y observed i n adul t insects . Th e bud s var y fro m rounde d protuberance s (A, 6 ) to fingerlik e processe s o r appea r a s smal l globule s at th e end s of long slender stalks (C , b). I n mos t insect s th e bud s are formed prio r t o feeding, and , a s shown by Needha m (1897 ) in odonate larvae, they may in suc h case s increase enormousl y in siz e and number s in starve d indi viduals. I n th e hors e fly Tabanus, however, described by Crag g (1920), the bud s ar e extende d durin g the perio d o f feeding, an d afte r thei r dis charge the epithelial cells go back at onc e to the normal resting condition. The horse fly, Cragg says, feeds at intervals of two or three days. A thir d typ e o f cell disintegratio n i s similar t o th e last , excep t that the par t o f th e cel l give n of f contain s a nucleu s an d is , therefore , a n extruded cell. Th e cell, loaded with granular matter, degenerate s and is dissolved i n th e stomac h lumen . Th e liberatio n o f nucleate d cells , Needham says, is characteristic o f dragonfly larvae (Fig . 205 B); in other insects i t frequentl y accompanies the discharg e o f nonnucleated bodies, as in the horse fly (C, c) and in the honey bee. Thi s form of disintegration, as wel l a s the las t o r the tw o together , result s i n a rapi d an d extensiv e depletion o f th e digestiv e cell s o f th e epithelium , necessitatin g thei r replacement by cell s propagated fro m th e regenerativ e cells. All thes e form s o f cel l disintegratio n i n th e ventricula r epitheliu m have generall y bee n describe d a s method s fo r th e rapi d discharg e of secretion products . Onl y recentl y thi s interpretatio n i s challenge d b y Yung-Tai (1929) , who points ou t tha t secretion s are alway s in the for m of a diffusibl e liquid , an d tha t th e coars e granular content s o f the bud s and glpbule s give n of f from th e digestiv e cell s hav e al l th e aspect s of cytoplasmic degeneratio n products . H e therefor e contend s tha t th e processes ordinarily described as secretion discharge are really disintegra tion processe s following activ e period s of secretion o r absorption . Thi s view i s endorse d als o b y Henso n (1930) . Th e subject , however, mus t be studie d fro m a physiologica l standpoin t befor e conclusion s ca n b e justified. Periodic Delamination and Replacement of the Ventricular Epithelium. — Reconstructive processes , varying in degree, usually occur in the stomac h walls at th e tim e o f ecdysis, particularly a t th e moul t o f the larva to th e pupa i n holometabolous insects . I n som e insects th e entir e ventricula r epithelium i s shed an d renewe d at eac h moult, an d i n certai n beetle s a complete regeneratio n o f the stomac h wal l is sai d t o occu r periodicall y throughout adul t life . A replacemen t o f th e entir e ventricula r epitheliu m accompanyin g each ecdysi s ha s bee n describe d b y Folso m an d Welle s (1906 ) an d b y Boelitz (1933 ) i n Collembola, which moult throughou t life , an d a similar

THE ALIMENTARY CANAL

373

process accompanyin g th e larva l ecdyse s has bee n observe d i n Dermes tidae by Mobus z (1897 ) and Braun (1912) , in the moth Gallería melonella by Yung-Ta i (1929) , an d i n th e fl y Psychoda altérnala b y Hasema n (1910). Th e renewa l o f th e ventricula r epitheliu m i n Collembola , according t o Boelitz , i s preceded by a n evacuatio n o f the stomac h an d starts with mitoti c divisio n i n th e regenerativ e cells , the activit y of the latter beginning anteriorly an d proceeding posteriorly. A s the ne w cells multiply, th e ol d epithelium is separated fro m th e basemen t membrane ,

FIG. 206.—Regenerativ e cell s of the ventricula r epithelium . A , a nidu s o f regenerative cells of Stenopelmatus. (From Davis, 1927.) B , a crypt o f regenerative cell s of larva of Dytiscus marginalia. (From Rungius, 1911. ) C , tw o crypt s o f adul t Hydrophilus piceus. (From Rengel, 1898.) D, same during regeneration of new epithelium (2Epth), with ol d epitheliu m (lEpth) cas t off .

which remain s intact , an d i s pushed towar d th e lume n o f the stomach , finally to be thrown of f int o the latter , where it i s digested an d absorbe d by th e ne w epithelium . Folso m an d Welle s describe d th e rejecte d epithelium o f Collembol a a s forme d b y a longitudina l divisio n o f th e primary epithelium , th e oute r laye r remainin g a s th e nex t functiona l epithelium; thei r accoun t make s n o mentio n o f th e regenerativ e cell s later describe d by Boelitz. In mos t holometabolou s insect s ther e i s probabl y mor e o r les s o f a renovation o f the stomac h epitheliu m accompanyin g each moult o f th e larva; bu t i n th e majorit y o f case s observe d th e renovatio n doe s no t involve a complete loss of the ol d cell wall. Accordin g to Brau n (1912) , in specie s o f Lepidoptera, Coleópter a (excep t Dermestidae) , Hymenop -

374

PRINCIPLES OF

INSECT MORPHOLOGY

tera, an d Dípter a studie d b y him , activ e cel l divisio n an d epithelia l growth, i n som e cases accompanied by th e los s o f a fe w cells thrust ou t into th e stomac h lumen , take plac e during the period s of larval ecdysis . These activitie s o f the mesentero n cells, however , he says ar e primaril y for th e purpos e o f growth i n th e alimentar y cana l followin g th e moult , and only to a small degre e do they hav e a regenerative significance . At th e penultimat e moul t o f holometabolou s insects , tha t is , wit h the change from the larva to the pupa, it is well known that the ventricular epithelium is cast off and replaced by a new cell layer that takes o n more nearly th e for m o f the ventriculu s o f the adul t insect . Mos t investiga tors find that the pupal, or imaginal, epithelium is formed fro m th e sam e regenerative center s that produce the ne w cells of the larva l ventriculus . The statement by Mansour (1928) , therefore, that the imaginal stomac h of Rhynchophor a i s generate d fro m th e stomodaeu m an d no t fro m th e cells of the larva l mesenteron, i f true, woul d establish a most exceptional condition i n these beetles , sinc e it implies , a s Mansou r claims, that th e imaginal stomach i s of ectodermal origin. A replacemen t o f th e stomac h epitheliu m betwee n th e pupa l an d imaginal stage s ha s no t bee n generall y observed , bu t Deegene r (1904 ) says tha t th e epitheliu m i s renewe d a t th e pupa l moul t i n the beetl e CybisteTj an d Rus s (1907 ) describe s a partia l degeneratio n an d replace ment o f the pupa l epithelium i n Trichoptera . Finally, it appear s that a complete renewal of the stomach epitheliu m may occu r eve n i n th e imagina l insta r o f pterygot e insects . Rengel (1898), fo r example , claim s tha t a periodi c sheddin g an d regeneratio n of th e entir e ventricula r epitheliu m take place in members of the Hydrophilidae throughou t th e lifetim e o f th e adul t beetles . H e describe s both processe s i n detai l for Hydrophilus (Fig . 20 6 D). Thoug h h e say s nothing o f the physiologica l significance , i t i s t o b e suppose d tha t th e shedding o f th e ol d cel l laye r (lEpth) i s a preliminar y t o th e renewa l of th e epitheliu m (2Epth) followin g exhaustio n fro m secretor y activities . The ol d epitheliu m i s entirel y replace d b y a ne w cel l laye r (C , Epih) formed from the regenerative cells (rg) of the ventricular crypts (Cpt). 6. TH E PROCTODAEU M

The proctodaeu m is the posterio r ectoderma l par t o f -the alimentar y canal. I n its lesse r degrees of development it i s a simple tube (Fig . 189 , Proc) constitutin g merel y a conduit fro m th e stomac h t o th e anus ; but, as the stomodaeum , the proctodaeum also is generally differentiated into several mor e o r les s distinc t regions . Th e anterio r en d o f the procto daeum i s approximatel y marke d b y th e base s of the Malpighia n tubules (Fig. 190 , Mai), sinc e thes e tubule s ar e diverticul a o f th e stomodaea l walls; bu t th e tru e dividin g line between mesenteron and proctodaeum

THE ALIMENTARY CANAL

375

is usuall y somewha t anterio r t o th e base s o f the tubules , an d i n some insects i t lie s a considerable distance befor e them . Th e entrance to th e intestine fro m th e stomac h i s generall y mor e o r les s constricted , an d the openin g is guarded by a regulatory structure commonl y known as the pyloric valve (Fig . 189 , PVlv). Th e analog y wit h vertebrat e anatom y implied i n th e term , however , i s not exact , fo r the valvula r apparatu s in insect s i s usually, thoug h no t always , located behin d th e stomac h in the anterio r par t of the proctodaeum . The proctodaeu m i s furnishe d wit h extrinsi c muscle s tha t exten d to it s posterio r part s fro m th e wal l o f th e abdomen . Thes e muscles , often calle d th e "suspensory " muscle s o f th e proctodaeum , probabl y serve in part t o maintain th e position of the intestine, bu t the y evidentl y have als o a mor e activ e function . I n som e insect s the y ar e clearl y dilators o f th e proctodaeum , sinc e the y sprea d i n fan-shape d bundle s from thei r origin s to thei r insertion s o n the proctodaea l walls; in others, as in the caterpillar s (Fig . 20 7 A), where they tak e a more longitudinal course, it woul d appear that they pla y som e part i n evacuation. Subdivisions o f the Proctodaeum.—The regions into which the procto daeum i s usuall y differentiate d var y i n differen t insects , an d fo r thi s reason i t i s difficul t t o appl y a consisten t terminolog y t o them . Th e names by which they are commonly designated are borrowed from huma n anatomy, an d the y hav e n o excus e i n entomolog y other tha n tha t of nomenclatural convenience . The mos t genera l divisio n o f th e proctodaeu m i s int o a n anterior intestine (Fig . 190 , AInt) an d a posterior intestine (Pint), th e secon d being commonly termed the rectum (Red). The two parts are usually separated externall y b y a shar p constriction , and internall y b y a rectal valve. I n man y insects , however , ther e i s a shor t bu t distinc t sectio n of th e proctodaeum that intervenes between the ventriculus and the tru e intestinal tube, whic h contains the sphincter valve that regulates the exit from th e stomach . Thi s sectio n i s th e pylorus (Py). Th e Malpighia n tubules (Mai) ope n into the anterio r part of the proctodaeum , sometimes immediately behin d the ventriculus; but whe n there is present a distinc t pyloric region, they discharg e into the latter. The anterio r intestin e ma y b e a simpl e tube , varyin g i n lengt h i n different insects , but it is often subdivided into an anterior ileum (Fig. 190, II) an d a posterio r colon (Gin). Th e posterio r intestin e i s generall y dilated anteriorl y int o a rectal sa c (rsc) an d narrowe d posteriorl y i n a straight tubular part, or rectum proper (reel), tha t goes direct to the anu s (An). Frequently the anterior intestine opens into the posterior intestine on the sid e of the recta l sa c (Fig . 21 0 B), an d i n such cases the anterio r end of the latte r becomes a blind pouch, or rectal caecum. I n som e of the Heteroptera almos t th e entir e proctodaeu m consist s o f a larg e sa c

376

PRINCIPLES OF INSECT MORPHOLOGY

(Figs. 200 B, C , Beet, 219 A, r). I f this sac is the rectum , as it appear s to be , a shor t tubula r invaginatio n o f th e intestina l wal l behin d th e swollen base s o f th e Malpighia n tubule s i s perhap s a remnan t o f th e anterior intestine . Histology o f th e Proctodaeum.—Th e wall s o f th e proctodaeu m resemble in structure thos e o f the stomodaeum . Th e cell s of the epithe lium are flat or columnar, in most places showing little evidenc e of having a secretor y function , an d the y ar e covere d internall y wit h a distinc t cuticular intima . Th e muscl e layer o f the proctodaeu m i s les s regula r than that o f the othe r section s of the alimentar y cana l and is frequently absent o n som e o f th e intestina l regions . I n genera l th e musculari s includes interna l circula r fibers and externa l longitudina l fibers , resem bling thu s th e muscl e sheath o f the ventriculu s rather tha n tha t o f th e stomodaeum; but th e relativ e developmen t o f the tw o sets o f fibers often varies greatl y i n differen t part s o f the proctodaeum , an d ther e ma y b e additional muscle s eithe r outsid e o r insid e th e usua l layers . Specia l histological feature s o f th e proctodaeu m wil l b e describe d i n treatin g of th e severa l intestinal region s individually . The Pylorus.—Th e anterio r par t o f the proctodaeu m is ofte n differ entiated a s a well-defined regio n into which open the Malpighia n tubules (Fig. 207 A, B, Py). Sinc e the pyloric valve is usually situated here , this region i s termed th e pyloru s ("gatekeeper" ) o f the intestin e (Deegener , 1904; Rungius, 1911 , 1924 ; Weber, 1933) , though the ter m in vertebrat e anatomy applie s t o th e posterio r par t o f the stomach . Example s o f a well-differentiated pylori c regio n ar e t o b e foun d i n Coleópter a an d i n the larva e of Lepidoptera. I n som e insects, however, there is no pyloric valve othe r tha n a smal l epithelia l fol d betwee n the mesentero n and proctodaeum, an d i n suc h cases there i s consequently no differentiatio n in the externa l structur e o f the alimentar y tub e t o distinguis h a pyloric region fro m th e res t o f the intestine . In th e caterpillar s th e pyloru s constitute s a distinc t an d highl y specialized proctodaea l regio n (Figs . 196 , 20 7 A, Py) betwee n the ven triculus (Vent) an d th e enlarge d middle chamber (AInt) o f the intestine . The Malpighia n tubule s (Mai) ope n into it s posterio r part . Th e organ , when full y stretche d ou t (Fig . 20 7 A, Py), present s a narro w posterio r neck o r stal k surrounde d b y a strong , externa l sphincte r muscl e (sptr) just behin d th e base s o f the Malpighia n tubule s (Mai), an d a widened anterior par t continue d forwar d a s a calyxlik e expansio n continuou s with the posterior end of the ventriculus (Vent). The line between the mesenteron an d th e pyloru s i s marke d externall y b y a stron g b and of circular muscle s (A , g), an d internally b y a correspondin g fold (B , g) . The proctodaea l intim a u p t o thi s fol d i s covere d wit h smal l spicules . Midway i n th e wall s of the anterio r par t o f the pyloru s there is a second

THE ALIMENTARY CANAL

377

internal fol d (A , B, h) which varies in height according to the contractio n of th e organ . Th e entir e lengt h o f th e pyloru s i s closel y surrounde d by a serie s o f circula r muscle fibers , outside o f which there ar e widel y spaced, branchin g longitudina l muscle s (A , Imcl) tha t ar e fre e fro m th e pyloric walls except at their ends. Posteriorl y these muscles pass beneath the sphincte r (sptr). I n appearanc e the pylorus of the caterpilla r varie s much accordin g to th e stat e o f contraction o f the longitudina l muscles ;

FIG. 207.—Th e proctodaeum , pylorus , an d pylori c valve . A , proctodaeu m o f a noctuid caterpillar , showin g highl y develope d pyloru s (Py). B , interna l vie w o f pyloru s of sam e in a contracted condition . C , sectio n o f proctodaeal pylori c valve (PVlv) o f adult Phyllophaga gracilis. (From Fletcher, 1930. ) g, junction o f mesenteron an d proctodaeum ; h, fol d o f pyloric wall .

in the sam e species it ma y be stretched out , a s in Fig. 20 7 A, or again i t may b e contracted an d thrown into stron g circula r folds a s at B . The Pyloric Valve. —Two differen t type s o f valvula r structure s ar e associated wit h th e openin g fro m th e stomac h int o th e intestine . I n some insects a small, internal, circula r fold, o r ring of long cells, projects from th e posterio r margi n o f th e mesenteri c epithelium , formin g a ventricular valve; in others a n apparatus fo r closing the entranc e into th e intestine i s develope d i n th e pylori c regio n o f th e anterio r en d o f th e proctodaeum an d constitute s a proctodaeal valve. Th e latter , whe n present, i s clearl y the mor e efficien t occluso r mechanism an d i s the on e generally foun d a t thi s region . A proctodaeal pylori c valve i s typically develope d in the Coleóptera , where it consist s of one or two transverse fold s o r thick rings of epithelial cells in the pylori c region at th e anterio r en d of the intestin e (Fig . 207 C,

378

PRINCIPLES OF INSECT MORPHOLOGY

PVlv) immediatel y befor e the opening s of the Malpighia n tubules (Mai). Usually the valvula r ring s are cut by the longitudina l folds o f the pyloric wall into a series of opposing lobes. Th e sphincte r muscle of the pylori c region serve s t o approximat e th e lobes , an d shor t longitudina l muscles from th e posterio r en d o f th e ventriculu s inserte d i n th e lobe s o f th e valve apparently for m the opening mechanism. A detailed description of the pylori c valve of Dytiscus is given by Rungius (1911; Korschelt, 1924) . The pylori c valve of caterpillars i s situated i n the narro w neck of th e pyloric regio n o f th e proctodaeu m (Fig . 20 7 A , Py). Th e inne r wall s of th e latte r ar e throw n int o numerou s longitudinal folds , fou r o f which are particularl y larg e (B , PVlv) an d for m opposin g lobes that bloc k th e narrow entranc e int o th e succeedin g intestina l chamber . Th e larg e external sphincte r muscl e of the pyloru s (A , B, sptr) surround s the poste rior end s o f th e valv e fold s posterio r t o th e base s o f th e Malpighia n tubules (Mai) an d whe n contracte d evidentl y shut s th e lume n o f th e pylorus fro m that o f the intestine . Th e externa l longitudina l muscle s of the pyloru s (A , Imcl), inserte d beneat h th e sphincter , perhap s serv e t o relax the valve, but when they ar e contracted a deep transverse, interna l groove cuts acros s the valv e lobes, forming a circular channel into which the Malpighia n tubule s ope n (B) . Th e lengthwis e stretchin g o f th e pyloric region is perhaps accomplished by the muscles from the body wall inserted o n the posterio r part s of the intestine . The Malpighia n Tubules.—Excretor y diverticul a o f the alimentar y canal occur in Arachnida, Crustacea, Myriapoda , and Hexapoda. Thos e of Chilopod a an d Hexapoda , know n as the Malpighian tubules, are com monly observe d t o b e attached t o th e anterio r en d of the proctodaeum , and embryologist s generall y asser t tha t the y tak e thei r origi n a s out growths fro m th e proctodaeum . Tirell i (1929) , however, quite circum stantially describe s th e tubule s o f Cloeon dipterum a s openin g int o th e mesenteron, an d Henso n (1932 ) claim s tha t i n th e mot h Hepialus th e tubules aris e fro m th e mesenteron . Henso n believes , therefore , tha t the functiona l part s o f the tubule s ar e o f endoderma l derivatio n i n al l Lepidoptera, an d that only the terminal duct s opening into the intestin e are o f proctodaeal origin . I n som e caterpillars th e pylori c region of th e proctodaeum extends so far forwar d beyon d the base s o f the Malpighia n tubules a s to become virtually th e posterior en d of the stomac h (Fig . 207 A, Py) an d migh t b e mistake n fo r a par t o f th e mesenteron . I n th e Acrididae (Fig . 195 ) an d othe r Orthoptera , however , th e Malpighia n tubules ope n s o nearl y o n th e lin e betwee n th e mesentero n an d th e proctodaeum tha t i t migh t b e questione d whethe r thei r epitheliu m i s continuous wit h tha t o f the ventriculu s o r wit h tha t o f th e intestine . Histologically th e wall s of the tubule s mor e closely resemble the epithe lium o f the ventriculu s tha n that o f the proctodaeum . I n th e majorit y

THE ALIMENTARY CANAL

379

of insects , however , ther e i s n o doub t tha t th e tube s aris e fro m th e proctodaeum, and in many cases they open into the latter well bac k fro m its anterio r end . The Malpighia n organ s o f insects ar e typicall y long , slender , much convoluted tubules , frequentl y branche d o r arrange d i n clusters . I n the Protura , however , according to Berles e (1910) , the y ar e represented by si x small oval masses of cells projecting from th e anterio r en d o f th e proctodaeum, eac h orga n consistin g o f tw o larg e oute r cell s supporte d on a peduncl e of slender cell s extended fro m th e intestina l epithelium . In th e Japygida e th e organ s ar e smal l bu t tubular ; a s describe d b y Silvestri (1905 ) and Tillyar d (1930 ) they consis t o f six very short diver ticula fro m th e anterio r en d o f the proctodaeum . Generall y th e Mal pighian tubule s lie entirely fre e i n the bod y cavity , but i n som e insect s their posterior end s penetrate beneath the oute r tissues of the walls of the rectum. Thi s latte r conditio n is usual in the larva e o f Lepidoptera (se e Ishimori, 1924) , and it occur s in some Neuroptera and many Coleóptera. Marcus (1930 ) says that in only a small proportion of the Coleópter a do the Malpighia n tubule s en d freel y i n th e bod y cavity . Th e termina l parts of the tubule s ar e frequently united i n various ways. The numbe r o f Malpighia n tubule s i s highl y variabl e i n differen t insects, an d eve n withi n a singl e order . I n th e Myriapod a generall y there are two tubules or not more than four ; in the insects the number is usually greater , rangin g fro m 1 to 150 . A summary o f the numbe r of Malpighian tubule s know n t o occu r in th e variou s order s o f insects i s given by Wheele r (1893a) an d Venezian i (1905), and additiona l information o n the numbe r in Apterygota is given by Tillyar d (1930) . From th e foregoin g source s and other s i t appear s tha t th e numbe r of Malpighia n tubule s i n Apterygot a i s usuall y 6 , thoug h onl y 4 ar e reported i n som e Lepismatidae, an d th e numbe r may b e a s larg e a s 16 in Campodea o r 20 in Machilis. I n Ephemerid a th e tubule s ar e sai d t o vary from 40 to more than 100, those of Heptagenia being arranged in 8 groups , accordin g to Marshal l (1927) . Adul t Odonat a hav e 5 0 to 60 tubules, an d a similar numbe r is present i n Plecoptera . I n Orthopter a the number is generally large in adults, 20 to 100 or more, but th e tubule s are usuall y arrange d i n fro m 2 t o 1 2 groups. Th e Gryllida e presen t an interestin g exceptio n in that onl y one short primar y tub e i s present, but thi s on e branches into a cluster of 100 to 12 0 long secondary tubules. According t o Wheeler , only 6 tubules ar e presen t i n embryo s of Blatta, Xiphidiunij and Melanoplus. In the Hemiptera, the Aphididae lack Malpighian tubules , th e Coccida e have 2 , an d al l other s 4 , th e end s of which are sometimes united i n pairs . Among the holometabolou s orders the numbe r of Malpighian tubule s is likewis e variable . Ther e ar e 6 t o 8 i n mos t Neuroptera , Wheele r

380

PRINCIPLES OF INSECT MORPHOLOGY

reporting th e od d numbe r o f 7 i n a larva , probabl y Chauliodes. Mecoptera an d Trichopter a hav e 6 . I n Coleóptera , most pentamerous forms hav e 4 tubules , an d other group s have 6 , but ther e ar e exceptions, a s Hydrophüus wit h 6 an d Sitaris wit h 4 . Th e Lepidoptera , with rar e exceptions , hav e alway s 6 tubules i n both larva e and adults , but th e 3 o n eac h sid e branc h fro m a commo n stalk . I n Gallería melonella, however , ther e ar e numerou s irregularl y branche d tubule s from eac h stalk , an d i n certai n Tineida e th e 6 typica l tubule s o f th e larva ar e reduce d t o on e pai r i n th e adult . Amon g Hymenopter a there ar e 20 to 2 5 tubules i n adult Tenthredinidae , onl y 6 in some ants, but i n most forms a larger number is present, 12 to 150, generally arranged in 2 , 3 , o r 4 groups . I n larva l Hymenopter a th e numbe r o f tubules i s generall y 4 , bu t ther e appea r t o b e onl y 2 o r 3 i n chalcid larva e (Thomsen , 1927). Th e Diptera , bot h larva e an d adults , generally hav e bu t 4 tubules . I n som e familie s al l aris e separatel y from the intestine ; in other s the y are unite d in 2 group s of 2 each . Culex an d Psychoda hav e 5 tubules. It i s difficult t o form an opinion as to what may have been the primary number o f Malpighian tubules in insects, bu t fro m th e foregoin g review it is evident that it was not large, since, when the tubule s are numerous, they ar e usuall y arrange d i n a fe w groups . Wheele r conclude d tha t the primitiv e insect s probabl y ha d 6 excretor y diverticul a o f th e intestine, correspondin g to th e groove s between the usual 6 longitudinal folds o f the proctodaea l wall. A description of the histolog y and functio n o f the Malpighia n tubule s will be given in the followin g chapter . The Anterio r Intestine.—Th e par t o f th e intestin e betwee n th e bases o f the Malpighia n tubule s an d th e rectu m i s frequently a simpl e tube showin g no differentiation into parts; it ma y be short an d straight (Fig. 199 , AInt), dilate d int o a saclik e chambe r (Fig . 196 , 20 7 A), o r greatly lengthene d an d variousl y loope d an d coile d (Fig . 198 , 21 0 A) . Very commonly, however, the anterio r intestine is more or less distinctly divided int o a n anterio r ileum (Fig . 195 , II) an d a posterior colon (Cln), which may diffe r i n histological characters and are usually separated by a constriction o f the intestina l wall . Th e tw o part s ar e sometime s designated th e "smal l intestine " an d "larg e intestine, " bu t thei r relativ e dimensions are not alway s in keeping with these terms . The Posterior Intestine, or Rectum.—The opening from th e anterio r intestine int o th e posterio r intestin e i s often guarde d by a circula r fol d or group of lobes termed the rectal valve. I n som e cases a fold resembling the cardia c valv e i s forme d b y th e invaginatio n o f the posterio r en d of the anterio r intestin e int o the rectum ; in others th e wall s of the rectu m at th e mouth of the openin g are produced into opposing lobes forming a n

THE ALIMENTARY CANAL

381

occlusor mechanis m simila r t o th e pylori c valv e i n th e anterio r en d of the proctodaeum. The muscula r sheath o f the posterio r intestine, whe n fully developed, consists o f interna l circula r fiber s an d externa l longitudina l fibers , bu t the strengt h o f the musculari s varies much in different insects , and fibers of eithe r se t may b e lacking. Th e longitudina l fibers are ofte n collecte d into six external lengthwis e bands. Accordin g to Davi s (1927) , there i s in Stenopelmatus a laye r o f scattere d interna l longitudina l fiber s insid e the circula r muscles of the rectum. I n the region of the anu s the circular muscles ar e usually well developed, forming her e a group that function s as a n ana l sphincter . A peritoneal sheat h i s usually absent , bu t som e investigators report the presence of a connective tissue membrane surrounding the muscularis . An unusua l conditio n exist s i n th e larva e o f mos t Lepidoptera , in which the rectu m i s provided wit h cellula r membranes lying beneat h the muscularis , whic h ar e penetrate d b y th e Malpighia n tubules . A s described b y Ishimor i (1924 ) there ar e tw o o f these recta l membranes , the oute r on e composed of a single layer o f cells, the inne r o f two layers . The six Malpighian tubule s ente r beneat h th e oute r membran e nea r th e anterior en d o f the rectu m (Fig . 20 7 A, Mai) an d ru n posteriorl y i n th e space between the two membranes; then they penetrat e th e inner double membrane and turn forward in the space between this membrane and the epithelium o f the recta l wall . Her e th e tubule s ar e eithe r dispose d i n various convolution s an d loop s befor e the y end , o r the y brea k u p int o a plexu s o f reunitin g branches . Ishimor i distinguishe s fiv e type s o f patterns forme d b y th e termina l part s o f th e tubules , mostl y charac teristic o f group s of families. I n th e Hepialida e alon e the tubule s en d in the bod y cavity, an d the rectu m lacks the membranous sheaths . The Rectal Organs. —In mos t insect s th e epitheliu m an d intim a o f the recta l sa c form structure s commonl y known as the "recta l glands." The organ s in question take on two principal forms: In on e they appea r as oval , o r elongate , padlik e thickening s o f th e intestina l wall ; i n th e other the y ar e conical processes projecting fro m th e wal l into th e lumen. Hence, t o b e noncommittal i n th e matte r o f function, w e may describ e the organ s as rectal pads an d rectal papillae. The recta l pad s ar e area s o f th e wal l o f th e rectu m o n whic h th e epithelium i s compose d o f high , columna r cell s formin g elongat e ova l bodies elevate d o n the sid e toward the lume n (Fig. 20 8 A). Th e intim a is usually thi n ove r th e surfac e of each pa d bu t form s a thickene d ri m around its margin. Typicall y th e recta l pad s are six in number, equally spaced aroun d th e anterio r par t o f the recta l sac , wit h thei r lon g axes longitudinal. I n som e cases , however , ther e ar e bu t three , a s i n th e larvae of Odonata, while in others there may be a much greater number,

382

PRINCIPLES OF INSECT MORPHOLOGY

as i n Mecoptera , Trichoptera , an d Lepidoptera . Th e organ s ar e sai d to b e absen t i n Ephemerida, Hemiptera , Coleóptera , an d i n most holo metabolous larvae ; bu t the y ar e know n t o exis t i n representative s o f Plecoptera, Odonata , Orthoptera, Neuroptera , Trichoptera, Lepidoptera , Mecoptera, and Hymenoptera , thoug h i n som e members of these order s they ar e but littl e developed or are absent . It i s shown by Tonkov (1923 , 1925) that two types o f structure ma y be distinguishe d i n th e recta l pad s o f differen t insects . Th e firs t i s a simple type , occurrin g i n Plecoptera , Odonata , an d Orthoptera , i n which eac h orga n consist s o f a singl e laye r o f cell s (Fig . 20 8 A) . I n

FIG. 208.—Example s o f differen t type s o f recta l organ s ("recta l glands") . A , section o f simpl e padlik e orga n o f Eremobia. (From Faussek, 1887. ) B , hollo w padlik e organ of Apis mellifica. (From Trappmann, 1923.) C, papilla-like organ of the mydas fly. (From Jahn, 1930. )

the secon d type ther e ar e tw o layers o f cells, and organ s of this variet y may b e eithe r compact , a s i n th e Mecoptera , Lepidoptera , an d som e Hymenoptera, o r hollow , a s i n Neuropter a an d certai n Hymenoptera , owing to th e presenc e of a lumen between the tw o cell layers (B) . Th e origin o f th e two-laye r structur e o f th e recta l organ s ha s bee n studie d in Vespa vulgaris by Evenius (1933), who says that the inner layer represents th e norma l wal l o f th e rectum , bu t tha t th e oute r laye r i s derived fro m cell s lying originall y in th e bod y cavit y aroun d the proc todaeum, whic h graduall y approac h th e rectu m an d becom e stratifie d upon it t o for m th e oute r walls of the recta l organs. Rectal organ s o f th e papillifor m typ e occu r i n th e Dipter a an d Siphonaptera. Th e recta l papilla e ar e hollow , conica l invagination s of th e intestina l wal l (Fig . 20 8 C) . Th e positio n o f eac h i s marke d externally by a pit, whic h is penetrated b y ramifyin g trachea l branche s (Tra). According to Engel (1924), the usual number of rectal papillae in th e Dipter a i s fou r o r six , th e large r numbe r bein g confine d t o th e Orthorrhapha; in Culex pipiens and i n Atherix ibis there ar e fou r i n th e

THE ALIMENTARY CANAL

383

male an d si x in th e female . Th e typica l arrangemen t o f the papilla e i s in a circle around the anterio r en d of the rectum , but i n some species the position i s irregular, an d all the organ s may fall into a single longitudina l row. I n the Myda s fly Mydas clavatus, Jahn (1930 ) reports th e presenc e of abou t 3 3 rectal papilla e dispose d approximatel y i n three longitudina l rows. The functio n o f th e recta l pad s o r papilla e i s no t definitel y known in an y case . Th e suppose d glandula r natur e o f th e organ s ha s neve r been demonstrated , an d som e writer s hav e suggeste d tha t th e organ s serve fo r breakin g u p th e peritrophi c membrane , other s tha t the y eliminate carbo n dioxid e fro m th e blood , an d stil l other s that the y absorb th e foo d residu e fro m th e rectum . Wiggleswort h (1932 ) ha s advanced th e theor y tha t th e organ s reabsor b wate r fro m th e faeca l matter i n the rectum and thus play an important par t in water conserva tion, bu t th e structur e o f th e organ s doe s no t i n al l case s appea r t o be particularly adapte d t o thi s role , sinc e the coverin g cuticula i s ofte n more dens e o n th e surfac e o f th e pad s tha n elsewher e i n th e rectu m (Fig. 208 B). Various other type s of rectal organ s occur frequently in larval insects , some havin g th e for m o f simpl e eversibl e lobes , an d other s a mor e complicated structure , suc h as the well-know n rectal gill s of anisoptera n dragonfly larvae . Anal Glands. —In som e insects , especiall y i n th e Coleóptera , ther e are ectoderma l gland s openin g i n suc h clos e proximity t o th e posterio r end o f the rectu m tha t the y ar e know n a s anal glands. Thes e gland s serve principall y a s organ s o f defense b y dischargin g substance s havin g strong an d repulsiv e odors , o r eve n explosiv e properties, a s i n th e cas e of th e bombardie r beetle . 6. TH E FILTE R CHAMBE R

In mos t o f the Homopter a a n unusua l modification o f the alimentar y canal produce s a n orga n know n a s th e filte r chamber, i n whic h tw o ordinarily distan t part s o f th e digestiv e tub e ar e closel y applie d t o each other an d bound together by a connective tissue sheath (Fig . 209 A, FC). Th e part s involve d i n th e filte r chambe r ar e usuall y th e tw o extremities o f the mesentero n an d th e anterio r en d o f the proctodaeum . The orga n thu s forme d i s suppose d t o b e a devic e fo r allowin g som e of the exces s water and soluble carbohydrates o f the foo d to be eliminated by diffusin g directl y fro m th e anterio r par t o f th e stomac h int o th e intestine, whil e the protein and fatty materials ar e retained t o be digested and absorbe d i n the stomach . The ventriculu s o f a typica l homopterou s alimentar y cana l consist s of thre e parts . Th e firs t (Fig . 20 9 A, IVent) i s an anterio r enlargemen t

384

PRINCIPLES OF INSECT MORPHOLO GY

lying immediately posterior to the stomodaeal valve (SVlv) and is enclosed in the filter chamber; the second is a large, croplike sac (2Venf) serving as a storage reservoir; the third is a long, tubular section (SVent), the functiona l stomac h o f the insect , whic h turn s anteriorl y t o reente r the filte r chambe r (FC) an d open s into th e proctodaea l intestin e (AInt) at th e poin t wher e the Malpighia n tubules (Mai) ar e give n off from th e latter withi n th e chamber . Mos t writer s apparentl y hav e no t under stood th e morphologica l relationships o f the variou s parts i n the compli cated homopterou s alimentar y canal , sinc e th e tw o ventricular sac s ar e usually regarde d a s part s o f th e crop . Ceci l (1930) , however , i n a

II

'

~

FIG. 209.—The filte r chambe r o f Homoptera. A , diagra m o f a simpl e typ e of filte r chamber in which the two extremities o f the ventriculus and the anterior end of the intestine are bound togethe r i n a common sheath. B , the ventriculus convolute d i n the filter cham ber an d th e intestin e issuin g fro m it s posterio r end . C , th e filte r chambe r o f Lecanium, diagrammatic. (From Weber, 1930.)

paper o n Philaenus ^ state s tha t th e " mid-intestine o r stomac h i s tha t part o f th e alimentar y cana l posterio r t o th e oesophagea l valv e an d anterior t o th e Malpighia n tubules/ ' Th e histolog y o f th e digestiv e tube, a s show n by Ceci l i n Philaenus an d b y Hickernel l (1920 ) i n th e cicada, leave s littl e doub t tha t th e ventriculu s begin s an d end s i n th e filter chamber , thoug h Hickernel l call s th e anterio r par t i n th e filte r chamber the "anterio r crop," and the second stomach sac the "posterio r crop." In most of the Homoptera the intestine, instead of issuing from the anterior en d of the filter chamber, as shown at A of the diagra m (Fig. 209), makes a loop within th e chambe r an d emerge s at th e posterio r en d (B). By this complicatio n th e chambe r evidentl y becomes more efficient a s a filter, an d it s effectivenes s i s usually agai n increase d b y a zigzag course assumed by the ventricular and intestinal tubes within the chamber,

THE ALIMENTARY CANAL

38

5

and by their enclosure in deep folds of the epitheliu m of the first ventricular sa c (Figs . 20 9 B, 21 0 D) . A mor e primitiv e typ e o f filte r chambe r i s describe d b y Knowlto n (1925) in the aphi d Longistima caryae. Th e digestive tract of this insect is for th e mos t par t simple , but , Knowlto n says , "fo r a shor t distanc e the tube is complicated by the anterio r en d of the mid-intestine doublin g back an d formin g a loop through the muscula r wall of the posterio r en d of th e mid-intestin e an d th e anterio r en d o f th e hind-intestine " (Fig . 210 A). A n eve n simpler for m o f the orga n occur s in som e of the Coc cidae, a s i n Lecanium (Fig . 20 9 C); wher e the anterio r en d o f the ven triculus (Venf) forms a loop imbedded in the anterior end of the short, wide proctodaeu m (Proc). Her e th e Malpighia n tubule s aris e outsid e the regio n o f th e filte r chamber . I f thi s structur e i n th e Coccida e is the prototyp e o f th e filte r chambe r i n othe r Homoptera , i t suggest s that th e muscula r an d peritonea l covering s o f th e orga n ar e derive d from th e proctodaeu m rather tha n fro m th e mesenteron. An extrem e typ e o f modificatio n i n th e alimentar y cana l occur s in the diaspin e Coccidae , in which the middl e part o f the tub e i s all bu t obliterated. Th e oesophagu s ends i n a stomac h sac , which , according to Berles e (1909) , ha s n o connectio n with th e intestin e excep t fo r tw o ligaments. Child s (1914) , however, believes that there is a very delicate membranous union between the stomac h and the intestin e i n Epidiaspis piricoldj whic h i s s o easil y destroye d i n specimen s prepare d fo r his tological study that it is usually lost. I n any case it seems probable that most o f the foo d materia l passe s throug h th e wall s of the stomac h int o the blood , and that superfluou s a s well as waste substances ar e excreted from th e bloo d throug h th e wall s o f th e intestin e an d th e tw o hug e Malpighian vessels . The usua l structur e o f the filte r chambe r i s well shown i n it s mor e simple for m o f developmen t i n th e Cicadidae . Th e alimentar y cana l of th e cicad a (Fig . 21 0 B) i s extremely long relative t o th e lengt h of the body; for most o f its exten t i t i s a slender tube throw n into many transverse loop s agains t th e dorsa l wall of the abdome n abov e th e grea t ai r chamber tha t usurp s a majo r par t o f th e abdomina l cavity . Th e oesophagus (Oe) open s through a stomodaeal valve into a large S-shape d stomach region composed of two elongat e sacs (FC, 2Venf), th e anterio r of whic h is the filte r chambe r and contain s th e firs t sectio n o f the ven triculus (Fig . 209 A, IVent). Fro m th e secon d stomach sa c the tubula r third sectio n o f the ventriculu s (Fig . 21 0 B, 3Veni) goe s first posteriorly and the n turn s forwar d t o reente r th e filte r chamber , withi n whic h i t continues anteriorl y i n man y loop s buried i n th e epithelia l wal l o f th e first part o f the ventriculus , until i t end s a t it s junctio n with th e proc todaeum. Th e latter emerges from th e anterio r end of the filter chamber

386

PRINCIPLES OF INSECT MORPHOLOGY

as a narro w intestina l tub e (AInt), whic h turn s posteriorl y an d afte r many crosswise loops finds its way to the rectum (Reef). The four Malpighian tubule s (MaT), arisin g fro m tw o shor t basa l trunk s withi n the filte r chamber , g o posteriorl y an d emerg e fro m th e posterio r en d of th e chamber . A n interestin g accoun t o f th e histolog y o f th e ali mentary cana l of Magicicada septendecim i s give n by Hickernel l (1920) , who shows that the third tubular section of the mesenteron (ZVent) is the part of the stomac h in which the secretor y processe s are most active .

FIG. 210.—The alimentary cana l an d filte r chambe r o f Homoptera. A , Longistigma caryae, th e tw o extremitie s o f th e ventriculu s an d th e uppe r en d o f th e proctodaeum . (From Knowlton, 1925. ) B , Magicicada septendecim, entir e alimentar y canal . C , Tricentrus albomaculatus, entir e alimentar y canal . D , same , sectio n o f filte r chamber . E , same, section behin d th e filter chamber through tw o parts of ventriculus, anterio r intestine , and Malpighia n tubules . (C , D, E from Kershaw, 1913. )

The croplik e second part (2Venf) serve s a s a foo d reservoi r an d i s ofte n found distende d wit h liquid. In th e more typical structur e o f the filter chamber, as already noted , the anterio r en d o f the intestin e form s a loo p within th e chambe r an d emerges from th e posterio r en d of the latte r (Fig . 20 9 B). Thi s typ e of structure i s shown in a simple form i n the membraci d Tricentrus albomaculatus (Fig . 210 C) describe d by Kersha w (1913) . A cross section o f a filter chambe r o f thi s for m take n belo w the origi n o f th e Malpighia n tubules (D ) cuts throug h th e firs t ventricula r chambe r (IVenf) a t it s junction with the large second ventricular sac (2Venf), through the posterior en d o f th e ascendin g thir d par t o f th e ventriculu s (SVeni), and the descending intestinal tube (AInt), as well as through the Mai-

THE ALIMENTARY CANAL 38

7

pighian tubule s (MaZ) , th e tw o basa l stalk s o f whic h i n Tricentrus d o not divid e unti l afte r the y emerg e from th e filte r chambe r (C , E, Mai). GLOSSARY O F TERM S APPLIE D T O TH E ALIMENTAR Y CANA L Alimentary Canal.—The food tub e traversing th e body ; in insects consistin g of an endodermal mesenteron, a n ectoderma l stomodaeum, an d a n ectoderma l proctodaeum. (Darmkanal.) Anal Glands.—Ectoderma l glands opening close to th e anus . Anterior Intestine (AInt). —The par t o f the proctodaeu m between the ventriculus and the rectum , o r between the pyloru s and th e rectu m when a proctodaeal pylorie region is distinct. (Small intestine, Dunndarm.) Anterior Pharynx (APhy). —The precerebral part of the pharyn x in insects having also a pharyngeal section o f the stomodaeu m behind th e brain . Anus (An). —The posterio r openin g of the alimentar y canal . Buccal Cavit y (BuC). —The firs t par t o f the stomodaeum , lying jus t withi n th e mouth; it s dilato r muscle s arisin g o n th e clypeus , an d inserte d befor e th e fronta l ganglion and its connectives . Cardia (Car). —The anterio r par t o f the ventriculus ; i n many Dipter a takin g th e form o f a small spherical sac, ofte n mistake n fo r a pro ventriculus. Cardiac Valve (SVlv). —See stomodaeal valve. Colon (Clri).— The posterio r part of the anterio r intestine, between the ileu m an d the rectum . (Large intestine, Dickdarm.) Crop, or Ingluvies (Cr). —An enlargemen t o f the oesophagea l region of the stomo daeum. (Kropf, jabot.) Digestive Cell s (dg). —The secretor y an d absorptiv e cell s o f th e ventricula r epithelium a s distinguished fro m th e regenerativ e cells . Dilator Muscles.—Muscle s extending from the body wall to the alimentary canal ; called als o suspensory muscles. Filter Chamber (FC). —A par t of the alimentar y cana l in Homoptera in which th e two ends of the ventriculu s an d the beginning of the intestin e ar e bound together in a membranous an d muscular sheath . Ileum (II). —The anterio r par t o f the anterio r intestine , betwee n th e ventriculu s or pylorus an d th e colon . (Small intestine, Dunndarm.) Ingluvies.—See crop. Intestine (Int). —The proctodaeum , o r th e par t o f the proctodaeu m beyon d th e pylorus. (Th e term applie d als o to the entir e alimentar y canal. ) Mesenteron (Ment). —The endoderma l stomach , o r ventriculus, o f th e insect . (Mid-gut, midintestine, Mitteldarm, Chylusdarm, Magen.) Mouth (Mth). —The anterio r openin g o f th e stomodaeum ; primaril y locate d i n the ventral wall of the head , but i n most sucking insects retracted int o the head wit h the transformatio n o f the cibariu m into a sucking pump, in which case the functional mouth (mth) i s the entranc e t o the pum p chamber. Nidus.—A grou p o f regenerative cell s in th e ventricula r .epithelium. (Regenerationsherde.) Oesophagus (Oe). —A tubula r part o f the stomodaeu m between the pharyn x an d crop, or sometimes extending to the stomach. (Speiserohre.) Peritrophic Membrane (PMb). —A cylindrica l membranous envelope surrounding the foo d i n th e ventriculus , an d sometime s extending into th e proctodaeum ; gener ated fro m th e ventricula r epithelium , eithe r fro m al l o r a par t o f the lengt h o f th e latter or from a ring of specialized cell s at it s anterio r end .

388

PRINCIPLES O F INSECT MORPHOLOGY

Pharynx (Phy). —The par t o f th e stomodaeu m betwee n th e mout h o r bucca l cavity an d th e oesophagus , the dorsa l dilator muscle s of which arise on the f rons and the dorsa l part o f the craniu m and ar e inserted posterio r to th e fronta l ganglion and its connectives ; usually not extendin g beyond the nerv e ring of the head , but i n some insects there i s a posterior pharynx behind the brain . Posterior Intestine (Pint), o r Rectum (Rect). —The termina l section of the procto daeum commonl y termed th e "rectum/ ' bu t usuall y divide d int o a n anterio r rectal sac (rsc)} and a posterior rectum proper (rect). (Mastdarm.) Posterior Pharynx (PPhy) .—A pharyngea l chamber of the stomodaeum behind th e brain, presen t i n Orthoptera , Coleoptera , an d som e other insects . Proctodaeum (Proc). —The posterio r ectoderma l par t o f th e alimentar y canal . (Hind gut, hind intestine, Hinterdarm, Enddarm.) Proventriculus (Pvent). —A specialize d par t o f th e stomodaeu m immediatel y anterior t o th e ventriculus . (Gizzard, gesier, chewing stomach, Kaumagen.) Pyloric Valve (PVlv). —A valvula r fold usuall y situated i n the pyloric region of the proctodaeum, but sometime s formed b y th e posterio r end of the ventriculus. Pylorus (Py). —An anterio r part of the proctodaeum usually containing the pyloric valve, sometime s distinctly differentiate d fro m th e tru e intestinal regio n and forming anatomically th e rea r en d of the stomach . Rectal "Glands."—Padlike or papilliform structures on the inner wall of the recta l sac (no t demonstrate d t o be glands). Rectal Sa c (rsc). —The enlarge d anterior par t o f the rectum , sometimes produced into a large rectal caecum. Rectal Valve.— A circula r o r lobat e fol d o f th e proctodaea l wal l betwee n th e anterior intestin e and the rectum . Rectum (Rect). —The posterio r intestine, includin g the rectal sac and the posterior , narrow rectum proper openin g at th e anus . Regenerative Cell s (rg). —The cell s tha t generat e th e replacemen t cell s o f th e ventricular epithelium . Regenerative Cryp t (Cpt). —A pocket o f the ventricula r epitheliu m containin g a group (nidus ) o f regenerative cells . Stomach.—The ventriculus, o r mesenteron. Stomach Mouth.— A ter m sometime s give n t o th e proventricula r apparatu s forming a mouthlike entranc e to th e ventriculus . (Magenmund, Pumpmagen.) Stomodaeal Valv e (SVlv). —The cylindrica l or funnel-shape d invagination o f th e posterior en d o f the stomodaeu m into th e cardia c part o f the ventriculus . (Cardiac valve.) Stomodaeum (Stom). —The anterio r ectoderma l par t o f th e alimentar y canal . (Fore-gut, fore intestine, Vorderdarm.) Striated Borde r (sb). —The inne r cytoplasmi c laye r o f th e ventricula r epithelia l cells wit h fin e line s perpendicula r t o th e surface . (Stdbchensaum, Hdrchensaum, plateau strie, bordure en brosse.) Suspensory Muscles.—Se e dilator muscles. Ventriculus (Vent). —The endoderma l stomach of the insect . (Se e mesenteron.)

CHAPTER XI V

THE ORGAN S O F DISTRIBUTION , CONSERVATION, AN D ELIMINATION The organs and tissues to be described in this chapter have no morphological relation t o on e another, bu t the y hav e this in common that the y are al l closel y associate d wit h th e blood , o r circulatin g mediu m o f th e body cavity . The product s o f digestion, o n bein g passe d throug h th e wall s o f th e alimentary canal , ar e discharge d directl y int o th e bloo d an d mus t b e carried b y th e latte r t o th e bod y tissues . Th e mechanis m o f food dis tribution in insects includes the blood as the carrier , and the organ s that keep th e bloo d i n circulation , namely , th e heart , th e diaphragms , an d other pulsating structures . Conservatio n and elaboration o f reserve food materials are function s of the fat body . The eliminatio n of wast e products i s accomplished by various organs, including the bod y wall, th e alimentary canal , th e trachea l tubes , an d specia l organ s o f excretion . 1. TH E BLOO D

The bod y liqui d o f insects, know n as the blood, i n commo n with that of mos t othe r animals , consist s o f a flui d part , th e blood plasma, o r haemolymphj and of free floating cells, the blood corpuscles, or haemocytes. During th e developmen t o f the arthropo d variou s free space s appea r in th e bod y o f the embry o betwee n th e principa l ger m layers . Thes e spaces, whic h ar e remnant s o f th e primitiv e blastocoele , becom e filled with a clear liquid and constitute th e haemocoele, or primary blood cavity of th e animal . Th e principa l haemocoel e cavities o f the insec t embry o are a ventra l epineural sinus (Fig . 1 9 B, EpnS) lyin g belo w th e yolk , and a dorsal cardiac sinus (Fig. 21, CdS) extending along the midline of the bac k abov e the yolk . Mos t o f the cardia c sinus eventuall y becomes enclosed i n a mesoderma l tube , whic h is the dorsa l bloo d vessel . Th e epineural sinu s remain s open ; it expand s laterall y an d unite s wit h th e cavities o f th e rudimentar y coelomi c sac s (Fig . 1 9 B , Coel), o r wit h the coelomi c spaces between the mesoder m layers (Fig . 21) , to becom e a part o f th e definitiv e bod y cavit y (BC). Th e clea r lymphlik e liqui d of th e embryoni c haemocoele thus come s to fill all the space s of the bod y and appendage s no t occupie d b y cellula r tissue s and , togethe r wit h 389

390

PRINCIPLES O F INSECT MORPHOLOGY

other substance s late r adde d t o it , form s th e plasmati c haemolymp h of th e definitiv e bod y cavity . Almost fro m th e beginnin g the embryoni c haemolymph contain s fre e floating cell s (Fig . 21 , BCls), whic h becom e th e definitiv e bloo d cells , or haemocytes . Th e bloo d cell s o f th e embry o ar e describe d a s bein g formed by proliferation from the ventral part of the mesoderm (Fig. 19 B, BCls); bu t th e phagocyti c an d digestiv e propertie s whic h the y late r develop sugges t a n affinit y wit h th e endoderm . I t ma y b e possible , therefore, tha t th e bloo d cell s ar e reall y derive d fro m th e intermediat e strand o f th e endoderm , an d tha t the y ar e geneticall y relate d t o th e vitellophags an d th e cell s o f the mesenteron . Th e bloo d cell s multiply by mitoti c an d amitoti c division , an d i t i s probable that thei r number s are kep t u p durin g postembryoni c stage s entirel y b y division , thoug h various writer s have claime d that ne w haemocytes ar e produce d i n th e adult fro m cel l masse s lyin g nea r th e heart , fro m th e fa t tissue , fro m the epidermis , or from unknow n sources. The Haemolymph.—Th e plasma o f insect bloo d is a somewhat visci d liquid. Usuall y i t i s transparent , but , owin g t o containe d pigments , it i s commonly tinted wit h amber, yellow, brown, or green, or, especially in larval insects, more strongly and often brightly colored with these same hues and sometime s with orang e and red. Th e blood colo r is character istic of species or of different stage s of the sam e species, but no t o f families or orders. I t ha s no correlation with th e natur e o r color o f the food , al l shades o f blood tint s bein g foun d i n bot h herbivorou s an d carnivorou s insects. Ofte n th e bloo d is darker or more brightly colore d in the larva e of holometabolous insects than in the adults , i n which pale tints are more generally prevalent . I n lepidopterou s larva e an d pupae , accordin g t o Geyer (1913) , there is in many species a difference o f blood color between the sexes . A s a rul e th e bloo d o f female caterpillar s i s green, an d tha t of mal e caterpillar s pal e yello w o r colorless . Th e gree n colo r o f th e female blood , Geye r says, is due to chlorophyl l dissolve d i n the plasma , that of the mal e to xanthophyll . Composition o f the Haemolymph.—Numerous substance s ar e containe d in the blood plasma; some are component parts of it, but mos t of them ar e associated bodie s or the product s of digestion, oxidation , and metabolism . Cuenot (1891 ) distinguishe d i n th e bloo d o f the larv a of - Saturnia pyri four principal groups of ingredients. First , an albuminoid, haemoxanthin, primarily yello w bu t becomin g blackis h brow n b y th e absorptio n o f oxygen from the air; second, fibrin, which he says is very abundant; third, lutein, a yello w substance extracte d b y alcohol ; and , fourth , uranidin, normally dissolve d i n th e plasma , bu t precipitate d i n extracte d bloo d as blackish-green granules. Al l these substances, however, Cuenot says , are no t presen t i n th e bloo d o f al l insects . Th e bloo d albumin s h e

THE ORGANS O F DISTRIBUTION

391

believed t o b e nutritiv e material s derive d fro m th e food ; th e lutein , which is particularly abundant i n caterpillars, he attributed t o the lutein s of plant s (chlorophyl l and xanthophyll) . A summary of later studie s on the compositio n of insect blood plasma given b y Muttkowsk i (1923 ) enumerate s a lon g lis t o f substance s o f several physiologica l groups . Th e haemolymph , Muttkowsk i says , contains serum , gelatin , fibrinogen , an d variou s substance s i n solution , including water , gases , salts , foo d materials , pigments , respirator y proteins, wast e products , enzymes , an d specia l substances . Wate r constitutes full y three-fourth s of the plasma . Th e gase s oxygen , nitrogen, and carbon dioxide are always present, the last being a waste product. In ashe d blood, Muttkowsk i report s th e findin g o f iron, copper , sodium, potassium, calcium , magnesium, and probably ammonia , present mostl y in the for m o f chlorides, sulphates, nitrates , phosphates, an d carbonates . During feeding , nitrite s ar e present i n abundance , but onl y nitrates ar e found i n th e bloo d o f starve d specimens . Th e organi c conten t o f th e haemolymph include s albumin , globulin , fibrinogen , haemoxanthin , gelatin, nucleoprotein , an d durin g feedin g variou s hydrolyze d proteins . Fat globule s are always present bu t ar e said to be more abundant durin g feeding. Othe r groups of substances in the bloo d include waste products, pigments of various kinds, enzymes, and certai n specia l substances, such as cantharidi n characteristi c o f th e bloo d o f bliste r beetle s (Meloe). Little i s know n specificall y o f th e bloo d enzyme s o f insects , bu t th e presence o f enzyme s is indicate d b y th e histolysi s o f tissu e fragment s in th e bloo d suc h a s take s plac e particularl y durin g th e pupa l meta morphosis. Analyse s o f th e ga s conten t o f th e bloo d o f Dytiscus an d Hydrophilus given by Barratt and Arnold (1911) show the presence of carbon dioxid e an d nitrogen , bu t n o oxygen . Thes e writer s contend , therefore, tha t th e insec t bloo d serve s a s a nutritiv e mediu m bu t no t as an oxyge n carrier. Hydrogen-ion Concentration in th e Blood.—The blood of insects usually has a n alkalin e reaction . Studie s mad e o n th e hydrogen-io n conten t show a considerable range of pH values , but i t i s likely that som e of th e differences recorde d ar e th e resul t o f different method s o f measurement . Thus Glase r (1925) , fro m test s mad e o n severa l specie s o f Orthoptera , Diptera, an d Lepidoptera , foun d a variatio n i n th e bloo d p H fro m 6. 4 to 8.0 , whil e Bodin e (1926) , studyin g th e bloo d o f variou s specie s of Acrididae, reports a range of pH values from 6. 4 to 7.05. Bot h investiga tors agree that there is no correlation between the hydrogen-ion concentration o f th e bloo d an d th e ag e o r se x o f th e insect . Glase r foun d n o constant p H difference s betwee n larval , pupal , an d imagina l stage s of Bombyx morij Malacosoma americana, and Musca domestica; an d Bodin e observes that fluctuation s o f pH value s ar e no t produce d by difference s

392 PRINCIPLES

O F INSECT MORPHOLOGY

of food . Makin g du e allowanc e fo r error s o f technique , i t i s eviden t that th e hydrogen-io n concentratio n o f insec t bloo d range s clos e t o neutrality, bu t tha t i t varie s normall y withi n rathe r wid e limit s a s compared wit h that o f human blood . Respiratory Proteins o f th e Blood. —The principa l oxygen-fixin g pro teins foun d i n th e bloo d o f animals ar e haemoglobi n an d haemocyanin . The firs t i s a n iro n compoun d havin g a bright-re d color , which make s its presenc e easil y detected . Th e secon d i s a colorles s compoun d of copper, which becomes blue on oxidation; but becaus e it generall y occurs in very small quantities in the blood, it usually does not make its presence visibly apparent . The re d respirator y protei n haemoglobin , s o characteristi c o f th e blood of vertebrates, i s present als o in some invertebrate animals , including man y Annelida , certai n Mollusca , an d som e o f th e Arthropoda . Among insects i t is notably presen t i n the " bloodworms," which are the larvae o f certai n specie s o f th e dipterou s genu s Chironomus. Thes e larvae ar e mostly aquati c an d live either i n tubes attache d t o the surfac e of submerge d object s o r i n burrow s i n sof t mu d a t th e botto m o f th e water. The y ar e conspicuou s b y reaso n o f thei r bright-re d color . A Chironomus larva , accordin g t o Leitc h (1916) , i s by weigh t 5 0 per cen t blood, an d th e bloo d ha s a n oxyge n capacit y o f 0.0 6 cubi c centimete r per larva . Th e haemoglobi n o f the bloodworms , Leitch shows , is not a storer o f oxygen , a s ha d bee n supposed , bu t i s a mediu m fo r oxyge n transportation. "Ther e is, " sh e says , " a constan t bindin g o f oxygen at the surface of the body and the constant givin g up of it on the interior," resulting in " a continuous mixing and interchange of oxidized and reduced blood kep t i n motio n b y th e beatin g o f the heart. " Th e haemoglobi n is carrie d i n solutio n b y th e blood , an d it s quantity , Leitch observes , is just that amount whic h will enable the larva to utilize the small quantit y of oxyge n i n it s mediu m withi n th e confinemen t o f its tub e o r burrow . The Chironomus larvae hav e but a rudimentary trachea l syste m an d th e spiracles ar e closed . Thei r bodie s ar e provide d wit h externa l blood containing filament s know n a s " blood gills, " an d ye t i t i s claime d b y Fox (1921 ) tha t thes e filaments have n o respiratory function , th e inter change of gases taking plac e through th e genera l body integument . Copper i s known to b e present i n considerabl e quantity i n the bloo d of many invertebrates. I n some of the decapo d crustaceans, in scorpions, and i n th e horsesho e crab , a n organi c coppe r compound , haemocyanin, is held i n solution in the blood and is said to act as a respiratory oxygen fixing medium. The presenc e o f haemocyani n i n th e bloo d o f insect s has ; no t bee n definitely proved ; bu t i t ha s bee n show n by Muttkowsk i (1921 , 1921a) , from test s made o n the as h o f entire incinerate d insects , tha t coppe r i s

THE ORGANS O F DISTRIBUTION

393

of genera l occurrence in insects of all the principa l orders and is present in nymphs, larvae, an d adults . Th e amoun t o f copper in insects he says is entirely comparabl e wit h tha t foun d i n crayfis h blood . Muttkowsk i infers, therefore , that th e rol e of copper in insect s i s the sam e as that of copper i n th e decapods , " namely, tha t i t serve s a s th e nucleu s o f a respiratory protein—haemocyanin. " I t ha s no t bee n found , however , that oxyge n i s presen t i n th e bloo d o f insect s lackin g haemoglobi n i n amounts greate r tha n tha t whic h woul d b e normall y dissolve d i n th e plasma; though the quantit y thu s held in solution appears to be sufficien t for th e need s of many internal parasiti c larvae . The presenc e of copper in insects has bee n verified b y Melvi n (1931) , who finds that, while measurable quantities o f copper are present in newly hatched nymph s an d in larvae, pupae , an d adults, there is found a larger percentage o f i t i n certai n product s give n of f b y insects , suc h a s cas t skins, empty egg cases, and cocoon linings, than in the insect s themselves. The Haemocytes.—Th e cellula r element s o f insec t bloo d occu r i n such variet y o f forms tha t i t i s impossible to giv e a satisfactory genera l description o f them . Th e fac t tha t the y frequentl y tak e amoeboi d shapes show s that the y ar e cell s o f a primitiv e nature ; and , a s fa r a s known, al l th e tru e bloo d corpuscle s are descendant s b y divisio n fro m the embryoni c blood cells formed i n the embry o (Fig . 1 9 B, BCls). Attempts hav e bee n mad e b y severa l writer s t o classif y th e bloo d cells o f insects . Holland e (1911 ) differentiate s fou r group s o f them , distinguished a s proleucocytes, phagocytes, granular leucocytes, and oenocytoids, whic h ar e presen t i n insect s generally , excep t tha t th e las t ar e absent i n Orthoptera . Th e proleucocyte s ar e youn g leucocytes , th e cytoplasm of which is basophile, and which reproduce actively by mitotic division. Fro m the m ar e derive d th e othe r mor e specialize d form s of haemocytes. Th e phagocyte s ar e distinguishe d b y thei r large r size , their hyalin e cytoplasm , an d thei r phagocyti c activities . Th e granula r leucocytes ar e characterize d by a granula r structur e o f their cytoplasm , and the y als o ar e sometime s phagocytic . Th e oenocytoids , s o name d from thei r resemblanc e t o oenocytes , ar e larg e cell s wit h rounde d o r spherical forms , incapabl e o f phagocytosis , havin g a homogeneou s cytoplasm strongl y acidophile . In additio n t o thes e mor e commo n form s o f haemocytes , Holland e (1909, 1911 ) describe s in certain Coleoptera and in the larva e of Lepidoptera othe r bloo d cells having ova l o r spherica l form s i n whic h the cyto plasm is filled with spherules , sometime s colorless , and sometime s tinted with yellow . Thes e bodie s h e call s cellules a spherules. The y ar e particularly abundan t i n Coccinellidae, Chrysomelidae, and Cantharida e and giv e a n opacit y t o th e bloo d characteristi c o f thes e beetles . Th e spherules, Holland e says , ar e elaborate d withi n th e cel l cytoplasm , an d

394

PRINCIPLES OF INSECT MORPHOLOGY

he believe s the y ar e enzyme-containin g bodies . Eventuall y the y ar e given of f into th e bloo d b y th e ruptur e o r disintegratio n o f th e cell . The spherul e cell s o f caterpillar s diffe r i n man y way s fro m thos e o f Coleoptera, bu t the y appea r t o b e cells of the sam e type. Stil l anothe r form o f haemocyt e Holland e (1911 ) distinguishe s a s adipoleucocytes,

FIG. 211.—Example s o f haemocyte s o f variou s insects . (From Muttkowski, 1924. ) 1, Dytiscus larva. 2 , 3 , 4 , 7 , 9 , Leptinotarsa. 5 , Aeschna. 6 , Dytiscus. 8 , Pieris rapae. 10, Enallagma. 11 , Dytiscus imago. 12 , Hydrophilus.

these bein g blood cells in which the cytoplas m ha s become charged wit h small oil y fat droplets . In a mor e recen t stud y o f th e bloo d cell s o f insects , Muttkowsk i (1924) recognizes two principal type s of cells distinguished b y difference s in size , form , an d stainin g reactions . Th e cell s o f on e kin d h e call s chromophile leucocytes, becaus e o f th e stron g affinit y o f thei r cytoplas m for anilin e dyes; those of the other , whic h are larger and colo r but faintl y in anilin e stains , h e call s amebocytes. Th e chromophil e leucocyte s Muttkowski describe s a s rounde d an d slightl y flattene d bodies , whic h take on various forms (Fig. 211, 7 to 12) . The y may extend pseudopodi a

THE ORGANS O F DISTRIBUTION

395

in varyin g number s (7 , 8) , bu t th e processe s ar e alway s short . Som e of thes e leucocyte s ar e phagocytic , th e phagocyti c form s bein g particu larly abundan t durin g metamorphosis . Th e chromophil e leucocyte s are the mor e primitive bloo d cells, their nucle i are large, and they divid e by mitosis . Th e simpl e form s o f these cell s are evidentl y th e proleuco cytes o f Hollande . The amoebocytes (Fig . 211 , 1 to 6), as described by Muttkowski, are generally large r tha n th e chromophil e leucocytes an d ar e mor e variabl e in size and shape. Whe n active they resemble free amoebae (1), but the y take o n variou s forms whe n contracted o r floatin g (5 , 6). Th e nucleu s is generall y ova l bu t i s frequentl y irregula r (2 , 4 ) an d i s surrounde d by a clear perinuclear area (1 , ps). Th e cytoplas m i s differentiated int o a thin film of ectoplasm and a granular endoplasm , the latte r containin g numerous smal l vacuole s (v), occasionall y cytoplasmi c bodie s (c6) , an d generally a few small fat globule s (/). Th e pseudopodi a (p ) are usuall y slender bu t var y greatl y i n length an d number s (4) ; during coagulation of th e bloo d the y becom e enormously long , an d i n clottin g the y for m a pseudopodial meshwork. For a mor e complet e revie w o f th e stud y o f bloo d corpuscle s i n insects th e studen t i s referre d t o th e wor k o f Paillo t (1933) . I n th e blood o f caterpillar s Paillo t distinguishe s fou r principa l type s o f cells , which h e call s micronucleocytes, macronucleocyteSj micronucleocytes with spherules (tha t is , having th e cytoplas m filled with refringen t granules), and oenocytoids. Al l form s ar e capabl e o f mitoti c division , eac h cel l producing daughter cell s of its own kind. Onl y the micronucleocytes are phagocytic. Bloo d cell s simila r t o thos e o f caterpillars , Paillo t says , are foun d i n the larv a o f Neurotoma (Hymenoptera) , bu t i n coleopterou s larvae (Hydrophilus an d Elateridae ) th e haemocyte s ar e quit e differen t and appea r principall y i n two forms . The Blood during Metamorphosis.—During the perio d of transformation fro m th e larv a t o th e imag o i n holometabolou s insects , th e bloo d becomes so charged with the debris of disintegrating tissue s that the pup a appears t o b e fille d wit h a thick , cream y pulp . A t th e beginnin g of metamorphosis th e cell s o f th e fa t bod y detac h themselve s fro m on e another an d floa t fre e i n th e bod y liqui d (Fig . 218 , FtCl). Ver y soo n they underg o a dissolutio n (FtCl'', FtCl") b y whic h their contents , con sisting no w principall y o f fa t globule s (/ ) an d grea t number s o f smal l proteid granule s (a) , ar e se t fre e i n th e blood . Amon g thi s mas s o f liberated material there may still be seen a few normal haemocytes (BCl). With man y insects, durin g the tim e o f metamorphosis, th e prevailin g type o f bloo d cell s ar e th e phagocytes , whic h ar e ofte n distende d wit h particles o f degeneratin g tissues , especiall y muscl e fragments . I n th e pupa o f Diptera spherica l bodies filled with muscle fragments (sarcolytes)

396 PRINCIPLES

O F INSECT MORPHOLOGY

are particularl y abundan t an d ar e know n a s spherules o f granules, or Kornchenkugeln. Mos t investigator s hav e regarde d thes e spherule s a s blood phagocyte s gorge d wit h ingeste d sarcolytes . I t canno t alway s be demonstrated, however , that such bodies are of a cellular nature, an d the write r (1924 ) has foun d tha t sarcolyt e spherules , occurrin g in grea t abundance i n th e larv a o f th e appl e frui t fl y Rhagoletis pomonella ar e almost certainl y mere masses of sarcolytes given off directly from muscle s undergoing histolysis . Thes e sarcolyt e spherule s eventuall y brea k u p and ad d thei r content s t o th e accumulatio n o f material alread y i n th e blood. Include d in the latter, besides the bodies already mentioned, ar e also numerous small nucleated masses of protoplasm (caryolytes) probably derived fro m th e muscles , and grea t number s of minute grains and othe r unidentifiable fragment s o f disintegratin g tissues . Towar d th e en d o f the pupa l perio d these temporary inclusion s of the bloo d begin to disap pear, probabl y goin g into solutio n b y complet e histolysis , and , shortl y after th e transformatio n to the adult , th e bloo d again becomes clear and regains it s normal condition. Clotting o f th e Blood.—Wit h mos t insect s whe n th e bloo d issue s from a smal l woun d i t thicken s an d form s a viscou s plu g closin g th e aperture i n th e integument . Thi s " clotting" o f the insec t bloo d ha s been studie d b y Muttkowsk i (1924a) , b y Yeager , Shull , an d Farra r (1932), an d b y Yeage r an d Knigh t (1933) . Th e las t writers , afte r investigating th e bloo d o f 47 species, found tha t insect s ca n b e divide d into thre e group s accordin g to th e clottin g propertie s o f the blood , a s follows: (1 ) species in whic h clotting doe s not tak e place , (2 ) species in which clotting i s produced mainly by an agglutination o f the haemocytes , and (3 ) species in which clotting is principally a coagulation of the plasma . The groupin g o f specie s according to bloo d clottin g ha s n o relatio n t o taxonomic classification . Insect s havin g n o clottin g o f the bloo d occur among Homoptera , Coleoptera , Lepidoptera , an d Hymenoptera . Th e honey bee larva furnishe s a typical exampl e in this class, the nonclottin g properties of its blood having been shown by Bishop, Briggs, and Ronsoni (1925). Insect s formin g a cellula r coagulu m includ e Orthoptera , Homoptera, Coleoptera , Lepidoptera , Hymenoptera , an d Dip t era; those in which the blood clot is principally a plasmatic coagulu m occur in Heteroptera, Orthoptera , Coleoptera , an d Lepidoptera . Where clottin g consist s essentiall y o f a coagulu m forme d b y th e blood cells , as in the cockroach , described by Yeager , Shull, an d Farrar , the haemocyte s thro w ou t fin e threadlik e pseudopodi a an d becom e agglutinated int o clumps , which spread ou t an d appea r t o disintegrate . The plasm a undergoe s littl e chang e visibl e unde r th e microscop e an d exhibits no fibrin formation, but there appears in it a granular precipitate. When th e haemocyte s tak e n o part i n th e clotting , a fibrous coagulum

THE ORGANS O F DISTRIBUTION

397

is forme d i n th e plasma . A n intermediat e typ e o f clottin g involvin g both the plasma an d the haemocytes is described by Muttkowski (1924a). In thi s cas e th e gelati n o f th e bloo d coagulate s an d th e haemocyte s become agglutinated, whil e thos e o f th e amoebifor m typ e thro w ou t pseudopodia an d for m a meshwor k whic h contract s tightl y an d draw s the lip s o f the woun d together . I n addition , however , a fibrou s ne t i s formed amon g th e haemocytes , which , b y contraction , bring s abou t a still bette r closur e o f the woun d and a complet e stoppag e o f the bloo d flow. Muttkowsk i wa s abl e t o demonstrat e th e effectivenes s o f th e haemolynaph alon e t o produc e a clo t b y killin g th e haemocyte s wit h cyanide fumes . 2. TH E ORGAN S O F CIRCULATION

The primar y bloo d cavity o f an anima l i s the embryoni c haemocoele, or body space between the ectoder m and endoderm, which is the remnan t of th e earlie r blastocoel e lef t afte r th e invasio n o f th e latte r b y th e gastric endoderm . Th e haemocoel e i n triploblasti c animal s i s the n again invade d b y th e mesoder m an d i n most case s is finally reduced t o tracts enclose d in mesodermal walls, which constitute th e bloo d vessels . The cardia c sinu s o f th e youn g insec t embry o i s th e dorsa l par t of the haemocoele lying above the yolk or the alimentar y cana l between the upper ends of the latera l sheet s of mesoderm (Fig . 21, CdS). A s development proceeds , th e mesoder m layers approac h eac h other fro m opposit e sides i n th e cardia c sinus ; thei r opposin g surface s becom e hollowe d lengthwise, and, whe n the tw o layers meet, the lip s of their furrows unit e to for m a median dorsa l tube. Thi s mesodermal tube i s the dorsal blood vessel (Figs . 22 , 212, 214 , DV). It s lume n is merely a restricted par t of the cardia c sinu s an d i s filled with bloo d liqui d containin g bloo d cell s (Fig. 213 A, BCls). Th e dorsa l vessel is the onl y part of the haemocoel e of insects that is closed by definite walls; elsewhere the bloo d is contained in th e bod y cavit y becaus e o f the embryoni c unio n o f the ventra l par t of th e haemocoel e wit h th e cavitie s o f th e coelomi c sacs . I n som e of th e othe r arthropod^ , however, a s in the large r Crustace a an d Arach nida, there may be present an elaborate system of blood vessels branching from th e dorsa l organ. The part s o f the dorsa l mesoder m no t include d i n th e cardia c rudi ments exten d latera d i n th e uppe r par t o f the bod y fro m th e wall s of the dorsa l vessel (Fig . 22 , DDph) an d ar e drawn ou t int o thi n sheet s of cells. Thes e cell s give rise to th e transvers e dorsa l muscle s of the bod y and to connective tissue membranes more or less uniting the muscle fibers. The tw o wing s thu s forme d extendin g latera d fro m th e dorsa l vesse l constitute th e dorsal diaphragm (Figs . 212 , 213 A, DDph). Th e muscles become attached to the bod y wall along the latera l part s of the dorsum ;

398

PRINCIPLES OF

INSECT MORPHOLOGY

the membrane s become continuous with th e delicat e peritoneal covering of th e somati c muscles. Th e dorsa l diaphrag m shut s of f a spac e in th e upper par t o f the bod y cavit y containin g th e dorsa l bloo d vessel , which is know n a s th e pericardial cavity , o r dorsal sinus (Fig . 212 , DS). I n some insects the ventral transvers e bod y muscles, stretched betwee n th e lateral margin s of the sterna l regio n of the bod y wall, form a continuou s series o f fin e fiber s ofte n hel d i n a delicat e membrane . Thi s struc ture, whe n present , constitute s a ventral diaphragm (Fig . 212 , VDph). Beneath th e ventra l diaphragm i s a fre e spac e enclosing the nerv e cor d (VNC) an d th e ventra l longitudina l muscles ; it represent s a part o f the

FIG. 212.—Diagrammati c cros s sectio n showin g subdivision s o f th e bod y cavity . AlCnl, alimentar y canal ; BCl, blood cell ; DDph, dorsa l diaphragm ; DS , dorsa l sinus ; DV , dorsal bloo d vessel ; Ft , fa t tissue ; Oen, oenocytes ; PCls, pericardia l nephrocytes ; PvS, perivisceral sinus; VDph, ventra l diaphragm; VNC, ventra l nerve cord ; VS , ventra l sinus.

embryonic epineura l sinus and i s known as the ventral sinus (VS) i n th e adult anatomy . Th e bod y cavit y betwee n th e dorsa l diaphrag m an d the ventral diaphragm, when the latter is present, containing the principal visceral organ s is the perivisceral sinus (Fig . 212 , PvS). The dorsa l vesse l i s regularl y pulsatil e an d i s th e principa l orga n by whic h th e bloo d i s kep t i n motion . Th e dorsa l an d ventra l dia phragms, however, are rhythmically contractile and function as important adjuncts t o th e dorsa l vesse l i n th e circulatio n o f the blood . Finally , there are present in many insects other pulsating organs located in various parts o f th e bod y an d i n th e appendages , whic h apparentl y serv e t o drive the bloo d into th e extremities . The Dorsa l Vessel.—Th e dorsa l bloo d vesse l whe n full y develope d extends from the posterior end of the abdomen into the head. Th e organ is differentiated int o an anterior part known as the aorta, and a posterior

THE ORGANS OF DISTRIBUTION

399

part called the heart, but, while the two sections are anatomically different , their distinguishin g feature s ar e difficul t t o define . Th e hear t i s i n general th e pulsatin g par t o f the tube , thoug h th e aort a frequentl y i s provided wit h pulsatin g vesicula r diverticula , and , accordin g t o Wet tinger (1927) , th e dorsa l vesse l o f th e larv a o f Tipula selene pulsate s throughout it s length . Th e cardia c sectio n i s usuall y confine d t o th e abdomen, bu t i t ma y exten d int o th e thorax . Th e hear t i s character -

FIG. 213.—Structure of the dorsal blood vessel and associated tissues as seen in sections . A, hear t o f proctotrypid larva , Phaenoserphus viator. (From Eastham, 1929. ) B , middl e part o f dorsa l vesse l o f a sarcophagi d larva , Miltogramma punctatum. (From W . R . Thompson, 1921. ) C , posterio r par t o f same . D , tangentia l sectio n o f hear t wal l of Aeschna larva. (From Zawarzin, 1911. ) E , hear t o f adult Apis mellifica an d neighboring parts o f dorsal sinus .

istically chambered , tha t is , slightly dilate d i n eac h bod y segment , bu t the pulsating vesicles of the aorta als o are segmental dilatations tha t may be regarded as of the sam e nature a s the cardia c chambers. Th e heart i s provided wit h opening s (ostia ) fo r th e admissio n o f the blood , an d th e aorta likewis e may have aperture s fo r the sam e purpose . The Heart. —The cardia c par t o f the dorsa l vesse l i s usually marke d by th e presenc e of more or less distinct symmetrical segmenta l swelling s of the tube (Fig . 215 A, Ht). Thes e dilatations are known as the chambers of th e heart . Typically , eac h hear t chambe r ha s a pai r o f vertica l o r oblique slitlik e openings , the ostia (Ost), i n it s latera l walls , on e ostiu m on eac h side , place d generall y behin d th e middl e o f th e chamber , an d

400 PRINCIPLES

O F INSECT MORPHOLOGY

sometimes clos e t o th e posterio r end . I n som e insect s th e cardia c chambers ar e separated b y well-marked constrictions; i n others the y are apparent onl y as slightly widene d parts o f the vessel . I n genera l ther e are n o interna l valve s betwee n th e hear t chamber s excep t th e ostia l flaps, to be described presently, but Wettinger (1927 ) reports the presence of well-develope d valvular structure s i n th e hear t wall s o f th e larv a of Tipula selene i n additio n t o th e ostia l flaps , an d Popovici-Baznosan u (1905) describe s a pai r o f valve s i n th e hear t o f certai n chironomi d larvae lyin g jus t befor e the las t cardia c chamber , whic h evidentl y prevent th e backwar d flow of the bloo d into th e latter . I n som e othe r chironomid larva e th e sam e write r find s swelling s o f th e hear t wal l between th e consecutiv e pairs o f ostia, bu t thes e structure s h e say s ar e protruding hear t cell s and not valves . Valve-lik e folds o f the hear t wall in some cases may b e the lip s o f closed and otherwis e functionless ostia . The chambere d par t o f th e dorsa l vesse l i s usuall y limite d t o th e abdominal regio n o f th e bod y an d commonl y begin s i n th e secon d abdominal segmen t (Fig . 21 5 B). I n Blattidae , however , according t o Brocher (1922) , an d i n Japyx (Fig . 214) , a s show n b y Grass i (1887) , there is a typical cardiac chamber not only in the first abdominal segment but als o in the third and the second segments of the thorax, each provided with a pair o f lateral ostia . I n th e larva e o f Ephemerida, accordin g t o Popovici-Baznosanu (1905) , a pair o f ostia i s present i n the metathorax . Ampulla-like enlargement s o f th e aort a occu r i n th e tw o wing-bearing segments an d i n th e firs t abdomina l segmen t o f various insects , some times provide d wit h ostia l openings , but thes e aorti c swelling s or diver ticula d o not hav e the usua l structure o f the cardia c chambers . The numbe r of chambers in the abdomina l section of the dorsa l vesse l may coincide with the number of segments occupied here by the tube, the maximum bein g nine. Bu t generall y there i s not a distinc t chambe r in the first segment, and the numbe r of chambers may be variously reduce d to a minimum of one, as in Mallophaga an d Anoplura, though th e singl e large termina l swellin g of the hear t i n thes e insect s i s probably a com pound chamber , sinc e i t contain s tw o o r thre e pair s o f osti a (Fulmek , 1906). Th e heart o f larval Odonat a has a single large posterior chambe r (Zawarzin, 1911 ; Brocher , 1917a ) provide d wit h tw o pair s o f osti a i n Aeschnidae an d on e pai r i n Agrionidae , an d havin g tw o pair s o f alar y muscles. I n th e hemiptero n Nezara th e heart , a s describe d b y Malou f (1933), consist s likewis e o f a singl e larg e swellin g o f th e dorsa l vessel , provided wit h three pair s o f ostia. Though reductio n i n th e numbe r o f chamber s i n th e hear t usuall y results fro m a suppression of the cardia c swellings in the anterio r abdomi nal segments, the heart may be shortened also at the posterior end. Th e last chambe r of the heart commonly ends either abruptly o r in a narrowed

THE ORGANS OF DISTRIBUTION

401

tapering process that may extend posteriorly through the tenth abdomina l segment. Th e en d o f th e vesse l i n eithe r cas e i s generall y closed ; i n certain dipterou s larva e (Tipula, Chironomus) th e hear t i s said t o hav e a dorsa l termina l opening , bu t i t i s possibl e tha t thi s media n ostiu m represents th e las t pair o f ostia unite d dorsall y i n a com mon aperture . In the larva e of Ephemerid a the hear t terminates i n three slende r branche s whic h open int o th e bases o f the cerc i and th e media n cauda l filament . The typica l cardia c osti a hav e usuall y th e for m o f vertical o r obliqu e slit s i n th e side s o f th e segmenta l chambers o f th e heart . Frequentl y th e opening s li e i n deep latera l inflection s of the hear t walls , and these ostia l pouches, projectin g inward and forward withi n the lumen of th e heart , form a series of paired valve-like flaps having the true osti a at thei r fre e inner ends. Th e flaps virtually mark the division o f the cardiac tube into chambers. Th e ostial valve s evidentl y ar e s o constructed a s to admit th e blood int o th e hear t whe n th e wav e o f diastole run s forward throug h th e tube , an d t o preven t it s escap e during systole; the y possibl y serv e als o t o obstruc t a rearwar d flow of bloo d within th e vessel ; an d ye t th e hear t o f certain insect s i s known a t time s t o revers e th e direction of its beat , an d t o caus e a backwar d flow of blood when the systolic wave s run caudad . The wall s of the hear t consis t o f muscle tissu e imme diately derive d fro m th e cardioblasts , o r heart-formin g 214. — cells of the embryoni c mesoderm, which are converted int o D oFIG. r s a l bloo d semicircular o r circular striated muscle fibers that compose vessel o f Japyx. (From Grassi, the hear t tub e (Fig . 213). In larval insects the striation s 1887.) of the cardia c fibers are sometimes indistinct o r not evident. The ostia l valve s ar e als o muscular, an d therefor e probabl y contractile . The linin g o f the hear t i s simply th e norma l muscl e surface , consistin g usually o f a thi n laye r o f hyaline sarcoplasm beneat h a delicate limiting membrane. Outsid e the heart there may be a sheath o f adventitious con nective tissue . Th e vesse l i s usuall y suspende d fro m th e dorsa l wall of the abdome n b y fine radiating strand s attache d t o th e epidermi s (Fig. 212), whic h appea r t o b e filamentou s processe s o f the hear t wall s (Fig. 213 A). The mechanis m o f the hear t i s no t full y understood . Th e contrac tions o f th e orga n ar e produce d undoubtedl y b y th e muscle s o f th e heart walls . Diastole , i t i s ofte n assumed , i s effecte d b y th e muscle s of th e diaphragm , bu t thes e muscle s are attache d t o th e ventra l wal l of the heart, an d it has been observed in some cases that the heart continue s

402

PRINCIPLES OF INSECT MORPHOLOGY

to bea t whe n the diaphrag m muscle s are cut. Sinc e the systoli c wave s run through th e tube , i t woul d seem that th e successiv e chambers may be expanded by the blood forced into them. Whil e ordinarily the succession o f contraction s i n th e dorsa l vesse l i s i n a forwar d direction , a temporary reversal of the heart bea t has been observed in various insects. This appear s t o b e a norma l proces s an d recentl y ha s bee n carefull y studied b y Geroul d (1929 , an d othe r papers) . A periodi c reversa l o f

FIG. 215.—Th e dorsal bloo d vesse l an d th e dorsa l diaphragm . A , diagram o f aorta and thre e chamber s of the hear t wit h correspondin g par t o f the dorsa l diaphragm , dorsa l view, arrow s suggestin g th e cours e o f bloo d circulation . B , dorsa l vesse l an d dorsa l diaphragm o f Dissosteira Carolina fro m metathora x t o fift h abdomina l segment , ventra l view.

the hear t beat , Geroul d finds, occurs particularl y i n Lepidoptera a t th e end of the larval period and may continue through the pupal and imaginal periods. I n th e silkworm , phases of antiperistalsis begi n about 4 8 hours before pupation , alternatin g wit h phase s o f forward beating . Pupatio n is accompanied by a vigorous backward pulsation, an d durin g th e pupal and imagina l periods the hear t beat s regularl y with alternatin g forwar d and backwar d series o f pulsations, th e tim e o f each phase being greatly shortened i n th e adult . A reverse d flo w o f th e bloo d durin g anti peristalsis, Geroul d says, ca n be shown by injection s of india ink . The dorsa l vesse l i s innervate d bot h fro m th e occipita l ganglio n of th e stomodaea l nervous system an d fro m gangli a of the ventra l nerv e cord. The Aorta. —The aort a i s th e slende r par t o f the dorsa l vesse l con tinued forward fro m the first chamber of the heart into the head (Fig . 215 A, Ao). Usuall y it begin s i n th e firs t segmen t o f the abdomen ; but i f the cardia c region of the vesse l extends into th e firs t abdomina l segment

THE ORGANS O F DISTRIBUTION

403

or int o th e thorax , th e aort a i s correspondingl y shortene d (Fig . 214) . The aort a i s not alway s a simple tube; in the honey bee, as it enter s th e thorax, i t i s throw n int o a serie s o f short loop s closel y boun d togethe r in a sheath of connective tissue; in the Lepidoptera it makes a large bend upward i n th e mesothora x (Fig . 216) . Dorsa l diverticul a o f the aort a occur i n Odonata , Orthoptera , Coleoptera , an d Lepidoptera . Withi n the head the aorta opens into the head cavity behind or beneath the brain, and it s latera l an d ventra l walls , continue d befor e th e brain , en d i n delicate attachment s t o variou s tissues o f the head . The ampullalik e swellings or diverticula o f the aort a are o f particular interest sinc e by thei r structur e the y sugges t tha t the y are modifie d segmental chamber s o f th e dorsa l vesse l anterio r t o th e tru e cardia c region. I n the acridi d Dissosteira the aort a begins in the first abdominal segment (Fig . 215 B), but i t has here a large dorsal dilatation (no t visibl e from below) , whic h lie s i n th e posterio r par t o f th e concavit y o f th e strongly elevated tergal plate o f this segment. Again , in the metathorax and th e mesothora x th e aort a give s of f dorsal diverticul a formin g tw o sacs lodged in the scutella r cavitie s o f the metatergu m an d mesotergum , respectively. A weakly developed extension o f the diaphrag m i s present in eac h o f thes e segment s attache d t o th e dorsa l vesse l i n th e usua l manner. Th e aorti c sac s o f Dissosteira ar e covere d b y thic k spong y masses of compactly aggregated cells closely attached t o th e wall s of th e sacs, an d throughou t it s lengt h th e aort a i s accompanied by strand s of similar cells , which are probabl y pericardial nephrocyte s (se e page 416). The larva e o f Odonata , a s describe d b y Broche r (1917a) , hav e a dorsal tubula r diverticulu m o f the aort a i n th e mesothora x an d i n th e metathorax. I n each segment a delicate membrane containing transverse muscle fiber s i s stretche d abov e th e diverticulu m betwee n th e base s of th e wings . Eac h membran e i s perforate d b y tw o aperture s havin g valvular lip s openin g int o th e aorti c diverticulu m beneat h it . Thes e membranes, Broche r claims , ar e pulsatin g organ s tha t dra w th e bloo d into the dorsa l sinuse s above them an d discharge it int o the aorta . In Dytiscus likewis e th e aort a give s of f dorsa l diverticul a i n th e mesothorax an d th e metathorax , an d eac h diverticulu m i s covere d b y a shee t o f muscle tissu e attache d laterall y o n th e wal l o f th e tergum , above whic h i s a blood-fille d sinus . Th e aorti c diverticul a hav e th e form o f stalked ampulla e widened above into flattene d chambers . Th e first i s lodged in the cavit y o f the mesoterga l scutellum , th e secon d lies beneath the media n tongu e o f the metascutellum . Th e aorti c ampulla e and associate d structure s o f Dytiscus ar e describe d b y Korschel t (1924 ) from th e wor k o f Oberl e an d Kuhl . Thes e author s believ e tha t th e muscular membranes serve to compress the ampullae, and that the latter thus act as accessory pulsating organs to drive the blood forward throug h

404

PRINCIPLES OF

INSECT MORPHOLOGY

the aorta . Broche r (1916) , however, claims that there i s a pair o f openings int o eac h aorti c ampull a fro m th e sinu s abov e it , an d h e explain s the structure s as organ s for creatin g a circulatio n of the bloo d throug h the wings, the blood returning to the sinus being then sen t int o the aorta . A pulsatin g orga n connecte d wit h th e aort a occur s also , accordin g to Broche r (1919) , i n th e mesothora x o f Lepidoptera . A muscula r membrane i s stretche d acros s th e anterio r hal f o f th e cavit y o f th e scutellum (Fig . 216 , p) abov e a trachea l ai r sa c lodge d i n th e media n scutellar lob e an d enclose s a smal l sinus (i) in the dorsal part of the thorax. Th e aort a enterin g th e thorax dip s ventrall y beneat h th e second phragma (2Pk) and then makes a larg e loo p do r sally (j , Z ) in the mesothorax. Fro m the dor sal part o f the loo p a diverticulu m (k) goe s straigh t upwar d t o th e back an d terminates i n a smal l bilobed vesicl e in th e anterio r en d FIG. 216.—Vertica l sectio n o f thora x of th e sinu s abov e th e pulsatin g and bas e o f abdome n o f Sphinx convolvuli, showing aorti c diverticulu m (k ) an d pul - membrane. Th e posterio r wal l of sating membran e (p ) i n mesothorax . the vesicl e i s perforate d b y tw o (From Brocher, 1919. ) small osti a (Ost), eac h havin g it s ventral li p prolonged into th e lume n of the vesicl e in th e for m o f a valvular flap . Broche r gives experimental evidence showing that th e pul sations o f the membran e (p ) dra w the bloo d int o th e sinu s (i) abov e it from th e thora x an d fro m th e wing s and discharg e it through th e osti a into th e aorti c vesicle , whence it i s carried forwar d i n the aorta,t o th e head. The Dorsa l Diaphragm. —The dorsa l diaphragm , whe n typicall y developed, consists of two delicate connective tissue membranes enclosing between the m the dorsa l transvers e bod y muscles , whic h are inserte d medially o n th e ventra l wal l of the heart . Th e degre e of developmen t in both the membranou s an d th e muscula r element s o f the diaphragm , however, varies much in different insects . Usuall y the dorsal diaphragm is well developed only in the abdomen , though i t ma y exten d i n a much reduced conditio n into th e thorax . The diaphragm muscle s are commonly known as the "win g muscles " of th e hear t (alar y muscles ) becaus e typically the y occu r in fan-shaped groups of fibers spreadin g from thei r points o f origin on the terga l plate s to their insertions on the heart (Fig. 215 A, df). In some insects, however, the diaphragm fibers are all approximately transversely paralle l an d arise serially alon g the laterodorsa l part s o f the bod y wal l (B) . Th e media n

THE ORGANS O F DISTRIBUTION

405

ends o f the muscle s terminate i n fine branching, tendonlike fibrils either attached t o th e lateroventra l part s of the hear t wal l or continuous across the ventra l wal l wit h fibril s fro m th e opposit e side . I n genera l th e diaphragm muscle s ar e presen t onl y i n th e bod y segment s containin g a chamber of the hear t (Fig . 215 B, //, III, IV ) an d are therefore usuall y limited t o the abdomen . I n th e Blattidae , however , in which the hear t and dorsal diaphragm are continued into the metathorax an d mesothorax, there are , accordin g to Broche r (1922) , group s o f muscle fiber s i n eac h of thes e thoraci c segments . The diaphrag m membrane s i n some cases ar e almos t entirel y absen t or for m bu t a scan t bindin g betwee n th e muscl e fibers , consistin g o f a weblike tissue ful l o f large and smal l fenestrae. O n the othe r hand , th e membranes ma y for m a continuou s an d fairl y toug h septu m entirely separating the dorsa l sinus from th e periviscera l space of the body cavity. In Dissosteira (Fig . 21 5 B) th e membran e o f the diaphrag m behin d th e first abdomina l segmen t appear s t o leav e n o opening s int o th e dorsa l sinus, and , latera d o f th e diaphrag m muscles , i t i s continuou s wit h a connective tissu e coverin g ove r th e muscle s o f th e bod y wall . Whe n the diaphrag m muscle s ar e arrange d i n groups , th e latera l edge s o f th e diaphragm usually for m fre e fold s betwee n the point s wher e the muscle s are attached on the terga l wall s (Fig. 215 A), thus leaving openings fro m the perivisceral sinus into the dorsal sinus. Th e membranous diaphragm usually extend s anterio r t o th e firs t hear t muscle s in th e abdome n an d may be continue d a varyin g distanc e int o the thora x as membranou s fringes alon g the side s o f the aorta . The Dorsa l Sinus.—Th e dorsal , o r pericardial , sinu s i s coinciden t in extent wit h th e dorsa l diaphragm , and , accordin g to th e developmen t of th e diaphragm , it i s more or less shu t of f by th e latte r fro m th e peri visceral sinus . Th e dorsa l sinu s contains , beside s th e dorsa l vessel , some o f th e media n longitudina l muscle s o f th e bod y wall , th e dorsa l longitudinal trachea l trunk s whe n th e latte r ar e present , masse s o f fa t cells, an d usuall y sheet s o r masses o f special " pericardial cells " restin g on th e diaphrag m alon g eac h sid e o f th e hear t (Fig . 212 , PCls). Th e pericardial cell s are in most cases nephrocytes (se e page 416). Segmenta l tracheal trunk s ente r th e dorsa l sinu s laterally , eithe r abov e th e latera l margins of the diaphrag m or between the group s of muscle fibers (Fig. 215 B), an d unit e her e wit h th e dorsa l longitudina l trachea l trunk s i f th e latter ar e present ; otherwis e th e transvers e trunk s fro m opposit e side s may becom e continuou s i n dorsa l commissure s abov e th e heart . Th e dorsal diaphrag m i s sometime s penetrate d b y part s o f the Malpighia n tubules, whic h make convoluted loop s within the sinus . The Ventra l Diaphragm.— A ventra l diaphrag m i s no t a constan t feature o f insect anatomy . Whe n wel l developed , a s i n Acridida e an d

406 PRINCIPLES

O F INSECT MORPHOLOGY

Hymenoptera, the ventra l diaphragm forms a continuous ventral sheet of tissue composed mostly of the ventral transverse muscles of the abdomen . In Dissosteira th e ventra l diaphragm extend s throug h th e lengt h o f th e body from the head into the seventh abdominal segment. I n the anterio r part o f the thora x i t i s a ver y delicat e membran e without muscle s and appears t o b e attache d laterall y t o th e salivar y gland s an d sheet s of fat tissue . Betwee n th e spreadin g base s o f th e sterna l apophyse s i n the metathorax , however , there appears in the diaphrag m a serie s of fine transverse muscl e fibers attached laterall y o n the apophyses . Th e fibers continue throughout the length of the abdominal part of the diaphragm as its principal tissue. I n the abdomen the muscles have their attachment s on the stern a a t th e base s o f the anterio r an d latera l sterna l apodemes . The anterio r an d posterio r fiber s i n eac h segmen t sprea d somewha t forward an d rearward to bridge the space s intervening betwee n the con secutive segments . B y thi s arrangemen t ther e i s lef t a serie s o f intersegmental notches along the latera l margin s of the diaphrag m where the latter has no connection with the body wall, and the openings thus formed appear t o b e th e onl y connectio n betwee n th e ventra l sinu s an d th e perivisceral cavit y o f the abdomen , except for th e wid e spac e below th e free posterio r margin of the diaphragm . In som e insect s th e ventra l transvers e muscle s consis t o f compac t bundles o f fibers in eac h segment, an d i n suc h cases there i s no ventra l diaphragm. Other Pulsatin g Organs. —Muscular pulsatin g membrane s simila r to thos e associate d wit h th e aorti c vesicle s o r ampulla e ar e presen t i n the dorsa l part of the thora x in some insects where there are no modifications o f th e aort a an d n o opening s int o th e latter . Jane t (1906 ) first called attentio n t o a pulsatin g orga n o f thi s kin d i n th e cavit y o f th e mesoscutellum o f ants , an d Broche r (1919 ) say s tha t a ver y delicat e pulsating membran e i s presen t i n th e bac k o f th e metathora x i n th e sphinx moth , thoug h th e aort a make s n o connection s wit h i t i n thi s segment (Fig . 216). Freudenstei n (1928 ) describes pulsating membranes in both th e mesothora x and the metathorax o f the hone y bee, that of the mesothorax containin g muscl e fiber s an d havin g perforation s t o allo w the bloo d t o escap e fro m th e sinu s abov e i t int o th e thoraci c cavity . A simila r pulsatin g membrane , accordin g t o Broche r (1921) , occur s in th e mesothora x o f Vespa an d als o in Tabanidae . Bot h Freudenstei n and Broche r regard these structures as organs for maintaining a circulation o f blood through the wings , the returnin g channel s fro m th e latte r being the posterio r part s o f the basa l win g membranes. Small pulsating membranes have been observed in various other parts of th e bod y i n a fe w insects. I n th e hea d a pulsatin g vesicl e situate d between the bases of the antennae, which apparently drive s the blood into

THE ORGANS O F DISTRIBUTION 40

7

these appendages , i s describe d in Blattida e b y Pawlow a (1895 ) an d b y Brocher (1922) , and a similar structur e ha s been found i n the hone y bee by Janet (1911 ) an d Freudenstein (1928) . Pulsatin g membrane s i n the legs o f certai n Hemipter a ar e describe d b y Broche r (1909 ) an d other s (see Weber , 1930) . Course o f th e Bloo d i n Circulation.—Th e hear t ordinaril y pulsate s in a forward direction ; but, a s we have seen, it may reverse the direction of it s beat , an d i n some insects it s actio n appear s t o alternat e regularl y between phase s o f anterio r an d posterio r pulsation . Th e bloo d i n th e dorsal sinus is drawn into the heart through the ostia with the expansions of th e hear t chambers , and , whe n the systoli c wave s run forward , i t i s expelled int o th e hea d fro m th e ope n anterio r en d o f the aorta . Her e it percolate s laterall y an d posteriorly (Fig . 21 5 A). I f pulsatin g organ s are present in the antenna l region, as in the cockroach and the honey bee, an active circulation takes place through the antennae. I n the thorax the blood collect s in the ventra l sinus , fro m whic h it circulate s through th e legs. Fro m the dorsa l part o f the thora x i n the ala r segment s it enter s the anterio r part s o f th e win g base s an d return s b y wa y o f channel s in th e posterio r parts , th e centripeta l strea m bein g expedite d b y th e pulsating membranes of the thorax , where such membranes are present , and is then discharged into the aorta, if there are openings into the latter, or otherwis e sen t bac k int o th e thoraci c cavity . Fro m th e thora x a large part of the bloo d must b e drawn into the ventra l sinu s of the abdo men, especially in insects in which the ventral diaphragm is well developed and activel y pulsate s i n a posterio r direction . Leavin g th e ventra l sinus b y th e opening s along th e side s an d a t th e posterio r en d o f th e ventral diaphragm , th e bloo d goe s upwar d throug h th e periviscera l spaces and is drawn into the dorsa l sinus by the movements of the dorsa l diaphragm, which pulsates rhythmicall y i n a n anterio r direction . Her e it reenters the dorsal vessel through the osti a and is again driven forward into the hea d by the beatin g o f the heart . For a more detailed account of the cours e of the bloo d in circulatio n the studen t shoul d consul t th e variou s paper s b y Broche r (1909-1922) . The subjec t of blood circulation in th e wing s of insects i s reviewed in a recent pape r b y Yeage r an d Hendrickso n (1934 ) i n connectio n with a special study of the circulation in the tegmina and wings of the cockroach. 3. TH E FA T BOD Y

The majorit y o f insect s fee d mos t intensel y durin g th e immatur e stages of their lives. Man y insects take but littl e food, and some none at all, durin g th e imagina l period , an d ye t i t i s usuall y i n th e earl y par t of the adult stage that the reproductive elements are brought to maturity . The physiological economy of the insect, therefore, must include a system

408

PRINCIPLES OF INSECT MORPHOLOGY

of foo d conservatio n t o guarante e a consummatio n o f the reproductiv e function, o r t o compensat e fo r th e inequalit y o f ingestion betwee n th e immature an d adul t stages . I n th e Holometabola , moreover , th e constructive metabolis m that takes place in the pupa is partly dependent on food reserve s stored i n the bod y durin g the larva l period . The principa l tissu e tha t serve s fo r th e deposi t o f nutritiv e an d energy-forming substances is that which constitutes th e so-called fat body.

FIG. 217.—Fa t tissue , oenocytes , an d nephrocytes . A , grou p o f unstained fa t cell s of youn g mal e mot h o f Malacosoma americana. B , par t o f larval fa t body , wit h nuclea r stain, o f Hyphantria cunea. C , larva l fa t cell s o f Malacosoma treate d wit h osmi c acid . D, fa t cel l o f tenthredini d pupa , Macrocentrus ancylivora. (From Schmieder, 1928. ) E , oenocytes of dipterous larva, Cryptochaetum iceryae. (From Thorpe, 1930. ) F , nephrocyt e of Pediculus humanus. (From Nuttall an d Keilin, 1921. )

a derivativ e o f the mesoderm , forme d i n th e embryo . I n normall y fed insects thi s adipos e tissu e consist s o f loosel y aggregate d o r compac t masses o f cell s (Fig . 212 , Ft), usuall y opaquel y white , bu t sometime s brightly colored , irregularl y distribute d i n th e periviscera l space s o f the abdome n an d thorax, i n the dorsa l an d ventral sinuses , i n the head , and i n the appendages . Th e fat bod y ca n scarcely be termed a n orga n since i t ha s no definite organization , an d its cell s apparently hav e littl e interdependence o r interrelation . Th e fa t cells , however , ar e usuall y closely adherent, except at the time of metamorphosis, an d are so densely packed tha t they assum e polygona l shape s (Fig . 21 7 A). Th e externa l surfaces o f th e cel l masse s ar e ofte n smoot h ari d regular , an d i n suc h cases the y appea r t o b e covered by a delicat e membranou s sheat h (B) . Numerous finel y branchin g trachea e endin g i n cluster s o f tracheole s penetrate t o al l parts of the fa t body .

THE ORGANS O F DISTRIBUTION

409

A. differentiation occur s in th e cell s o f th e fa t bod y o f mos t insect s that result s i n th e appearanc e o f tw o type s o f cells , th e difference s becoming more marked with the ag e of the individual . Th e cell s of one type, whic h always constitute th e bulk of the fa t body , are characterized by a strong vacuolization of their cytoplas m with globules of oil-like fat ; those of the other type have little fat, but thei r cytoplas m contains many small, refringent granules, which are found to be uric acid products. Th e fat-containing cells , becaus e o f thei r functiona l relatio n t o nutrition , are trophocytes; th e other s ar e know n as urate cells. Th e tw o type s of cells, however , are no t distinc t i n thei r origin , bot h bein g derive d fro m the primitiv e undifferentiate d cell s of the youn g fat tissue . The Trophocyte s of the Fa t Body.—Durin g postembryonic periods of feeding, th e trophi c cell s of the adipos e tissue increase in size until the y become usually the larges t cell s in the body , excep t for the nephrocyte s and th e oenocytes . Fro m a n earl y period , wit h som e insect s i n th e embryo, these fat-body trophocyte s store up fatty material s within thei r cytoplasm in the for m o f oil-like globules (Fig. 217 C, /). I n the matur e cells the fa t globule s become so large that they ma y occup y most o f the perinuclear part s o f th e cell s (A) , an d th e nucle i ar e ofte n distorte d in shape by their pressure . Th e fat o f insects occurs mostly i n the for m of a n oily liquid. Fo r a discussion of the natur e of insect fat th e studen t is referred t o th e wor k of Timon-David (1930) . Though th e fat-formin g an d fat-storin g activitie s o f th e adipos e trophocytes giv e the latte r their characteristi c appearanc e of "fat cells, " the sam e cells have also other important functions . I n som e insects th e fat cell s store u p glycoge n in their cytoplasm ; according to d e Boissezon (1932) glycoge n is to b e foun d i n th e mosquit o only in th e trophocyte s of th e fa t body . I n th e Holometabola , a s th e tim e o f metamorphosi s approaches, th e fa t cell s elaborate larg e quantities o f albuminoid bodies in the for m o f small granules held in the cytoplas m between the globule s of fa t (Fig . 21 7 D, a , fr) . Thes e protei d inclusion s ar e forme d i n th e fat cell s apparently b y the actio n o f nuclear enzymes on nutritive mate rials take n i n from th e blood . During th e perio d o f metamorphosis i n holometabolou s insects , th e fat bod y disintegrates, an d it s cells , now released from pressure , become large spheres floating fre e i n the bloo d (Fig . 218, FtCl). Soon , however, most o f the cell s themselves begin a process of gradual dissolution (FtCl', FtCl")', thei r nutritiv e content s ar e thu s disperse d i n the bloo d (a , /) , where they become available to the growing tissues o f the pupa. Th e fat body o f th e adul t i s regenerate d toward th e en d o f th e pupa l perio d apparently fro m larva l fa t cell s that d o no t suffe r complet e histolysis, and thes e pupa l cell s may g o over into th e imagina l stage full y charge d with fatty inclusion s for the us e of the adul t (Fig . 217 A).

410

PRINCIPLES OF

INSECT MORPHOLOGY

The Urat e Cells.—I t wa s formerl y hel d tha t th e urat e cell s o f th e fat body are ductless excretory organs, which extract waste products from the bloo d an d retai n the m i n thei r cytoplasm . Berles e suggested tha t they ac t a s substitutes fo r the Malpighia n tubule s durin g the perio d of metamorphosis when the excretory tubules are undergoing reconstruction. Hollande (1914) , however , i n a stud y o f th e urat e cell s o f Vanessa, claims tha t th e urat e granule s ar e produce d within th e cell s o f the fa t body itself, an d that they are waste products of the final transformations of th e albuminoi d inclusion s o f th e activ e trophocytes . Certai n cell s of th e fa t body , absorbing these product s fro m contiguou s cells , become thus differentiate d a s the urat e cells . Holland e assert s tha t th e urat e granules ar e firs t forme d aroun d th e nucle i o f the trophocyte s an d ar e

FIG. 218.—Disintegratin g fa t cell s i n th e pupa l bloo d o f Malacosoma americana. a7 albuminoid granules ; /, fa t globules .

later disperse d in the cytoplasm . Th e granules remain in the urate cell s through the pupa l stag e and for a long time durin g the lif e o f the imago . In th e mosquito , accordin g to de Boissezon (1930a), crystallin e deposit s are neve r seen in cell s of the fa t bod y during the firs t stag e o f the larv a but appea r afte r th e firs t moul t an d increas e with th e ag e of the larva . During metamorphosis , h e says , the y becom e stil l more abundant , especially in the periphera l cells, and in the adul t stag e they are presen t in greatest number s in'hibernating females . The histochemica l studie s o f d e Boissezo n (19306 ) o n th e urinar y cells o f Culex indicat e tha t th e crystallin e deposit s ar e probabl y purin e bodies. The y ar e alway s situate d i n th e cytoplas m nea r th e nucleus , and microscopic examination shows that they are formed by the nucleolus. It is probable, therefore, that they are disintegration products of nucleinic acid, which, by further reduction , forms purine bases. I n s o far as these products ar e retained in the cells , de Boissezon points out, the urat e cells may b e said t o b e "kidneys of accumulation." 4. TH E OENOCYTE S

The oenocytes are large cells found in the bod y cavity o f most insects , either arrange d i n group s (Fig . 21 7 E) o r scattered individuall y among

THE ORGANS O F DISTRIBUTION 41

1

the tissue s o f the abdomen , wher e they ar e usuall y associate d wit h th e fat cells . The y occu r apparentl y i n al l th e pterygot e order s bu t ar e said t o b e absen t i n th e Apterygota . Investigator s agre e tha t th e oenocytes originat e i n th e embry o from th e ectoder m alon g the side s of the abdomen , arisin g her e fro m point s jus t posterio r t o th e spiracle s i n the firs t eigh t segments . Accordin g to Wheele r (1892) , th e oenocyt e cells remain imbedde d in the epidermi s at thei r point s o f origin in Ephe merida, Odonata , Plecoptera , an d Isoptera , bu t i n Corrodenti a an d Thysanoptera, thoug h stil l attache d t o th e epidermis , the y for m seg mental cluster s of cells projecting into the bod y cavity. I n Trichopter a and Lepidoptera the oenocyte s are fre e fro m th e bod y wall and occu r in groups connecte d wit h trachea e i n th e neighborhoo d o f th e spiracles , the cell s bein g hel d togethe r b y finel y branchin g trachea l tubes . I n the Coleopter a the y for m loos e clusters o r bands o f cells along the side s of the abdome n lying against th e viscera l organs. I n most adult Hemip tera an d Hymenopter a th e oenocyte s ar e generall y distribute d a s fre e cells amon g the cell s of the fa t tissue , bu t i n some larval Hymenopter a they ar e sai d t o b e arrange d i n paire d segmenta l group s lying mostl y in contac t wit h th e epidermi s o f th e firs t eigh t abdomina l segment s (Eastham, 1929) , and in the hemiptero n Rhodnius, according to Wigglesworth (19336) , they ar e imbedde d i n th e epidermis . Oenocyte s are not known t o occu r in adul t Diptera , thoug h the y ar e present i n dipterou s larvae as metameric cel l clusters attache d t o the bod y wall (Fig . 217 E). Though much attention has been given to th e histolog y o f the oeno cytes an d thei r growt h an d change s fro m th e larv a t o th e imago , n o consensus of opinion has yet bee n reached concerning the functio n o f th e cells. Som e o f th e earlie r writer s regarde d th e oenocyte s a s excretor y organs tha t retai n wast e product s i n thei r cytoplasm , whil e other s have favored tn e view that they ar e glands of internal secretion. Glase r (1912), studyin g th e oenocyte s of the larv a o f the leopar d moth Zeuzera pyrina, concluded that the cell s are the sea t of production o f oxydases in insects. Holland e (1914a) , however , claims that th e oenocyte s ar e i n a sense complementar y t o th e fa t cell s in that the y for m an d conserve in their cytoplas m deposit s o f wax . Th e wax , h e says , disappear s fro m the cell s during metamorphosis, an d also after fastin g or after eg g laying, just a s the fa t disappear s fro m th e fat cells . Hollande , therefore, would rename th e oenocyte s cerodecytes. Th e ter m " oenoeyte" refer s t o th e usual pal e ambe r colo r o f th e cells , bu t th e latte r ar e i n som e insect s brown, yellow, green, or red, an d sometime s colorless. 5. TH E CORPOR A ALLAT A

The corpor a allat a ar e small , compact , cellula r bodies , generall y oval, elliptical, o r spherical in form , lyin g at th e side s of the stomodaeu m

412 PRINCIPLES

O F INSECT MORPHOLOGY

behind th e brai n in the bac k of the hea d o r in the neck , and sometime s in the prothorax , where they ar e closel y associated wit h the stomodaea l nervous system (Fig . 249, CA). The y ar e said to be present in all orders of insects . Formerl y th e bodie s wer e assume d t o b e nerv e ganglia ; they wer e describe d b y Hofe r (1887 ) a s " paired posterio r viscera l ganglia." Heymon s (1895 ) late r discovere d that the y tak e thei r origi n from th e ectoder m o f th e latera l wall s o f th e head , an d he , therefore , termed the m " ganglia allata" in reference t o their transpose d definitiv e position. Mor e recen t investigation s b y Heymon s (1899a) , Naber t (1913), It o (1918) , an d other s hav e shown , however , tha t th e organ s do no t hav e th e structur e o f nerv e tissu e an d cannot , therefore , b e regarded a s ganglia . Thei r origi n fro m th e ectoder m suggest s a n ana tomical analog y at leas t wit h the oenocytes . The origi n of the corpor a allata fro m th e ectoder m has been observed by Heymon s (1895) , Carrier e an d Burge r (1897) , Strindber g (1913) , Nelson (1915) , an d It o (1918) . Th e firs t thre e writer s asser t tha t th e bodies ar e proliferate d fro m th e wal l o f th e firs t maxillar y segment . Nelson, however , describes the bodie s i n th e hone y be e a s arisin g fro m invaginations betwee n the base s o f the rudiment s o f the mandible s an d first maxillae, associate d wit h th e invagination s tha t for m th e adducto r apodemes o f the mandibles ; according to Ito , thos e o f the silkwor m are formed a s cellula r ingrowth s fro m th e mesa l surface s o f th e adducto r apodemal invaginations . Subsequentl y th e rudiment s o f th e corpor a allata become solid masses o f cells; they los e their connection s with th e ectoderm, migrate mesall y an d dorsally , an d finally become attached t o the ventrolatera l angle s o f th e coelomi c sacs o f th e antenna l segment. Those of Forficula are said by Heymons to unite with each other in a single media n body . I n thei r definitiv e position th e corpor a allat a ar e innervated fro m th e occipita l ganglio n (o r ganglia ) o f th e stomodaea l nervous system , an d eac h i s investe d i n a sheat h continuou s wit h th e neurilemma o f the nerve ; the latter , accordin g to Nabert , goe s in som e cases to the surface of the corpus allatum, and in others branches within it . The comparativ e structur e o f the corpor a allat a ha s bee n minutel y described b y Naber t (1913) . I n som e insects , Naber t says , th e com ponent cell s of the bodie s ar e distinct , bu t generall y each body consist s of a more o r les s uniform , reticulated , multinucleat e granula r mass , coloring strongly in stains, containing numerous vacuoles, with the nuclei rich i n chromatin . I n Phasmida e ther e i s a centra l cor e o f secretio n products. Durin g metamorphosi s o f th e silkworm , accordin g t o It o (1918), "th e corpor a allata become conspicuously enlarged and altered in shape. Th e structur e o f the cell s undergoes a gradua l change , and th e nuclei becom e greatl y ramifie d an d contai n a grea t man y chromati n granules." I n th e imago , It o says , th e bodie s increas e t o fou r o r five

THE ORGANS O F DISTRIBUTION

413

times th e larva l size ; th e nucle i ar e irregula r i n shap e an d underg o chromatolysis; th e cytoplas m i s alveolate d wit h vacuole s containin g granular secretio n products . Bot h Naber t an d It o conclud e tha t th e corpora allat a ar e gland s o f interna l secretion ; Wiggleswort h (1934 , 1934a) ha s produce d evidenc e suggestin g tha t the y ar e th e sourc e of hormone s that induc e moulting. 6. TH E ORGAN S O F ELIMINATIO N

The ter m eliminationj i n a n unrestricte d sense , refers t o th e physio logical discharg e o f an y useles s o r wast e substance s fro m th e bod y tissues; b y a mor e limite d definition , excretion i s th e eliminatio n o f waste product s o f metabolism . Man y substance s discharge d throug h the excretor y tissue s o r organs , an d therefor e classe d a s excreta , ar e simply materials , suc h a s salts , unavoidably take n int o th e bod y i n excess o f the nee d for such matter. O n the othe r hand , wast e products of metabolis m tha t ar e trul y excretor y substance s ar e no t necessaril y removed fro m th e body ; the y ma y b e merel y separate d fro m certai n tissues wher e their presenc e woul d b e detrimenta l an d store d i n other s where the y becom e harmless . Finally , excretor y substances , whethe r stored o r eliminated , ma y serv e som e usefu l purpos e i n th e econom y of th e animal . Unuse d or indigestible parts of the foo d material ejecte d from the alimentary cana l are not excreta, though much excretory matter may be voided with the foo d refus e i n the faeces . Eliminated substance s ar e solid , liquid , o r gaseou s i n form . Th e principal ga s expelle d a s a wast e i s carbo n dioxide , an d th e principa l liquid i s water. Soli d excret a ar e give n off mostly i n crystallin e masse s or i n aqueou s solution . The y includ e nitrogenou s an d nonnitrogenou s organic compounds and inorganic salts. Almost an y epithelia l tissu e o f th e bod y ma y assum e a n excretor y function, bu t specia l organ s o f excretio n ar e develope d generall y fro m the ectoder m o r th e mesoder m o r fro m bot h thes e ger m layers . Mos t of the arthropods have no excretory organs corresponding morphologically to the nephridi a o f Annelida and Onychophora , their excretor y functions being accomplishe d by th e integumen t an d th e wall s o f the alimentar y canal. Exception s ar e foun d perhap s i n th e hea d gland s o f Crustacea, which ar e generall y regarded a s modified nephridia l organs . The organ s o r tissue s o f insect s tha t serv e t o eliminat e substance s that cannot b e physiologically utilized in the body include (1) the general body integument ; (2 ) certai n specialize d parts o f the integument , suc h as the surface s o f gills and o f integumentary glands ; (3) the capillar y en d tubes o f th e trachea l system ; (4 ) th e wall s o f th e alimentar y canal ; (5) specifi c excretor y tube s openin g int o th e proctodaeum , know n a s the Malpighian tubules. I n addition , ther e ar e the masse s o f pericardial

414 PRINCIPLES

O F INSECT MORPHOLOGY

cells, an d group s o f simila r cell s i n othe r part s o f the body , generall y called "nephrocytes, " whic h hav e bee n suppose d t o eliminat e foreig n and waste substances from the blood and to store them in their cytoplasm , but whic h perhap s functio n i n a manne r preliminar y t o excretio n b y changing colloida l substance s t o crystalloid s tha t ca n b e eliminate d through th e Malpighia n tubule s an d excretin g tissue s i n othe r part s of th e body . Though we define excretory substances as the en d products of metabolism an d exces s matte r tha t canno t b e pu t t o an y physiologica l use , this is not t o sa y that suc h substance s ma y not b e made to serv e some practical purpose i n th e biologica l econom y of the animal . Numerou s instances migh t b e cite d i n whic h insect s mak e us e o f thei r excrete d matter. Excretor y substance s discharge d fro m gland s ofte n hav e offensive odor s whic h becom e a par t o f th e insect' s means o f defense. The products of the Malpighian tubules may be employed in the construc tion o f larva l case s o r injecte d int o th e fabri c o f th e cocoon . Othe r excretory substance s hel d withi n th e bod y ma y be deposited withi n th e integument, wher e they serve as pigments giving surface colo r markings, and i t i s possible eve n that th e nitrogenou s componen t o f the cuticula , a mos t importan t par t o f the physica l organizatio n o f insects, i s t o b e regarded a s an excretory product eliminate d wit h th e moults . The Integumen t a s a n Excretor y Tissue.—Th e bod y wal l o f man y animals serve s a s a n excretor y orga n i n th e eliminatio n o f waste sub stances fro m th e body , includin g inorganic salt s an d nitrogenou s com pounds. Th e arthropo d integumen t has , a s its oute r layer , a cuticul a in whic h th e nitrogenou s substanc e chiti n i s a prevailin g i f no t th e predominant element , an d i n whic h ther e usuall y occu r calciu m salt s or othe r incrustation s i n varying amounts . Th e cuticul a is periodically cast of f and renewed . I n thi s wa y the arthropo d lose s with each moult a larg e amoun t o f nitroge n an d whateve r calciu m o r othe r substance s may be contained in the exuviae. It ha s bee n suggested , therefore, tha t moultin g i n the arthropod s i s primarily a proces s o f excretion , particularl y o f nitroge n excretion . Chitin constitute s probabl y 30 to 4 0 per cent o f the insec t cuticula , but , as Uvaro v (1928 ) point s out , ther e ar e othe r substance s associate d with i t whic h are als o nitrogenous. Thes e substances includ e pigments, such a s thos e i n th e scale s o f Lepidoptera, which , Uvaro v says , "ar e clearly th e fina l product s o f metabolism transferre d t o certai n part s of the integument , an d deposited there instead o f being excreted." I n thi s connection it is interesting to recall the suggestio n of N. Holmgren (1902) that the cuticula r covering of the integument has originated a s a hardening of secretions thrown off from th e epidermi s between filaments'o n th e outer surfac e o f the latter .

THE ORGANS O F DISTRIBUTION

415

The product s o f mos t o f th e integumentar y gland s o f insect s serv e some specifi c purpos e i n th e biolog y o f eac h species , but , a s alread y mentioned, i t i s probabl e tha t som e o f the m ar e primaril y excretor y substances. Respiration i n insects, n o matter what mechanica l device s are devel oped to facilitate it, always takes place through some part of the ectoderm , and, i n thi s way , th e ectoder m serve s a s a n excretor y tissu e fo r th e elimination o f carbo n dioxid e an d water . Insects , suc h a s mos t o f th e Collembola, lackin g trachea e o r certai n larva e havin g close d o r rudi mentary trachea l system s respir e directl y throug h th e integument , and i t i s know n tha t man y insect s havin g well-develope d trachea e discharge at least a part of their carbon dioxide directly through the body wall. Bu t th e trachea l tube s themselve s ar e merel y invagination s of the bod y wall , and th e ultimat e tracheole s ar e develope d in cell s o f th e ectodermal trachea l epithelium . Bot h externa l gill s and interna l recta l gills ar e als o part s o f th e ectoder m speciall y modifie d fo r respirator y purposes. Excretion through the Walls of the Alimentary Canal.—Many investigators hav e observe d the accumulatio n of crystalline bodie s i n the wall s of th e mesentero n o f various insects . Mos t o f these bodie s ar e salt s of calcium, bu t som e also ar e sai d t o reac t t o test s for uric aci d salts . I n either cas e th e bodie s ar e probabl y excretory product s o r exces s sub stances that cannot be utilized. Th e insect, making little use of calcium in it s bod y structures , mus t necessaril y eliminat e mos t o f th e calciu m absorbed fro m it s food , an d analyse s o f the faece s alway s show a hig h percentage o f calcium in th e latter . I t seem s probabl e tha t excretor y matter ma y b e eliminate d als o directly throug h th e wall s of the procto daeum, bu t mos t o f th e intestina l excretio n take s plac e throug h th e Malpighian tubules . Th e functio n o f th e recta l glands , a s w e hav e seen (pag e 383), ha s not bee n exactly determined . The Nephrocytes.—I n nearl y al l th e Arthropod a an d i n th e Ony chophora, variousl y distribute d throughou t th e bod y an d eve n i n th e appendages, there ar e groups of special cells having the commo n property of absorbin g ammonia carmine injected into th e blood , and o f retaining a precipitate o f carmin e i n thei r cytoplasm . Becaus e o f thei r actio n o n carmine, these cells have been supposed to have a similar actio n o n other substances naturall y presen t i n th e blood , suc h a s wast e product s o f metabolism o r other injuriou s bodies, and for this reaso n they hav e been termed "storag e kidneys " (reins d'accumulation), o r nephrocytes. Th e structure an d distributio n o f the carmine-absorbin g cell s in the Arthrop oda hav e bee n elaboratel y studie d b y Kowalevsk y (1892) , Metalnikof f (1896), and Bruntz (1904) .

416 PRINCIPLES

O F INSECT MORPHOLOGY

The carmine-absorbin g cell s of insects occu r particularly i n th e peri cardial sinus , where they form masses or long strands o f cells on each side of the heart , known as the pericardial cells (Figs . 212,213,PCfe). Cell s of this kind , however , ar e ofte n t o b e foun d i n othe r part s o f the body , generally a s single cells or small irregula r cel l groups, though sometime s in cluster s o f definite shapes , whic h in certai n case s have been taken for glands. Th e genera l distribution i n th e bod y o f insects o f cells havin g the propertie s o f the pericardia l cells has been described by Keilin (1917 , 1924) and by Nuttall an d Keilin (1921). The pericardia l cell s o f insect s ar e generall y larg e an d ar e ofte n binucleate (Fig . 217 F). The y are always found to have an acid reaction . They ar e o f mesodermal origi n an d ar e derived , accordin g t o Heymon s (1895), from th e same parts o f the mesodermal layers that form the heart and th e dorsa l diaphragm . Holland e (1922 ) say s tha t th e cell s o f th e larva generall y persist durin g metamorphosis and become the pericardia l cells of the imago. Functionally , Holland e finds that the pericardial cell s have preponderantl y th e powe r of absorbin g colloids , suc h a s albumin s and globulins , o r thei r derivatives , an d h e claim s that their well-known property o f absorbin g certai n pigment s result s fro m thei r affinit y fo r colloids i n general . Fo r th e mos t par t th e cell s tak e u p onl y colorin g matter o f a colloidal nature. Rarel y the y contai n crystalloids . A t th e approach o f the pupa l metamorphosis, Hollande observes, the cytoplas m of th e pericardia l cell s i n som e insects , particularl y i n Coleoptera , Trichoptera, an d Lepidoptera , become s charged wit h albuminoi d inclu sions, but thes e inclusions usually disappear b y the end of the transforma tion period . The studie s o f Hollande (1922 ) o n th e physiolog y o f the pericardia l cells o f insects sustain s th e vie w that thes e cell s play a n important rol e in excretion , but the y discredi t the ide a tha t the cell s hav e a storag e function. Holland e claims that, when the cells absorb ammonia carmine, they precipitate th e carmine, and that the latter remains in the cytoplas m because i t i s bu t littl e solubl e i n th e cel l juices . Normally , h e finds , the pericardia l cell s are agents fo r breaking down complex colloids, which are transformed by ferments produced in the cell s into crystalloids. Th e latter are then give n off into the blood , from whic h they ar e removed by the Malpighian tubules. Thi s view is in accord with that earlier expressed by Cueno t (1896) . Holland e summarize s his findings on the functio n of the pericardial cell s as follows: Each cel l is a ductless glandular body with a merocrin e typ e o f secretion , possessin g th e propert y o f neutralizin g alkaline substance s presen t i n exces s i n th e blood , an d als o tha t o f absorbing certai n colloi d substances; by means of its diastases , functioning in a n aci d mediu m produced by th e cel l itself , it split s the comple x colloidal molecules , transformin g the m int o simple r crystalloi d com -

THE ORGANS OF DISTRIBUTION

417

pounds tha t ar e rejecte d int o th e blood , fro m whic h the y ar e finall y eliminated b y the Malpighia n tubules . The pericardial cells , therefore, Holland e point s out, ma y b e likene d in some respects to th e live r o f a vertebrate anima l an d migh t b e called hepatic cells mor e appropriatel y tha n "renal " cells , o r "nephrocytes, " though the y diffe r physiologicall y fro m a vertebrat e live r i n tha t n o

FIG. 219.—Structur e o f th e Malpighia n tubule s o f a hemipteron , Rhodnius prolixus. (From Wigglesworth, 193la. ) A , origi n o f Malpighian tubule s (mt) fro m intestine , wit h one tubul e show n i n full . B , par t o f tubul e a t junctio n o f dista l (us} an d proxima l (Zs) parts show n i n optica l section . C , basa l ampull a (am) o f a tubul e wit h elongat e cel l processes (amp) projecting into intestine. D, structural details of proximal part of tubule.

glycogen functio n ha s bee n detecte d i n connectio n wit h them . I n an y case, however , the pericardia l cell s an d presumabl y als o the simila r cells in other parts of the body play an important par t in the physiology of the insect. The Malpighia n Tubules. —The specifi c excretor y organ s o f insect s that correspond functionall y to the nephridi a of Annelida and Onychophora an d with th e kidney s o f Vertebrata ar e th e Malpighia n tubules , th e general morphology of which was discussed i n the las t chapter . Histologically th e wall s o f the Malpighia n tubule s resembl e i n som e respects th e wal l of the mesenteron rather than tha t of the proctodaeum , with whic h the y ar e anatomicall y continuous . Eac h vesse l i s a long ,

418 PRINCIPLES

O F INSECT MORPHOLOGY

narrow epithelia l tub e compose d o f larg e cell s limite d externall y b y a basement membrane (Fig. 219 B, D). Th e inner ends of the cell s usually project int o th e lume n of the tub e an d ofte n almos t occlud e the passage . The cytoplas m o f the cell s sometimes show s a fine striation i n it s oute r part perpendicula r t o th e basemen t membrane . Generall y ther e i s no evidence o f a cuticular linin g t o th e Malpighia n tubule s suc h as that of the proctodaeum , bu t th e cell s o f th e epitheliu m ma y hav e a distinc t " striated border " closel y resemblin g th e striate d borde r o f the mesen teron. Fin e cilialik e strand s hav e bee n describe d b y som e writer s a s projecting fro m th e inne r end s o f th e cell s i n certai n insects . A thic k coating o f such processes is said b y Noel and Tahi r (1929 ) to b e presen t in the tubules of Bombyx mori, almost filling the lumina, but th e filaments appear t o b e simply fine cytoplasmic prolongations o f the cells . Each Malpighia n tubul e i n some insects i s differentiated int o tw o or three parts . Ver y frequentl y ther e i s a vesicula r enlargement , o r ampulla, a t th e bas e of each tube (Fig . 219 A, am), but i n some cases also the dista l part of the tube is distinctly differentiate d int o two sections by histological difference s an d b y difference s i n th e content s o f the lumen . In the reduviid Rhodnius prolixus, as described by Wigglesworth (19316) , each tubul e abruptl y change s i n it s structur e an d content s a t a poin t between it s proxima l thir d an d dista l two-third s (Fig . 21 9 B). Th e ampulla at the base of each tubule has the cell s of its outer wall prolonged into slender processes that proj ect down into the lumen of the proctodaeu m (C, amp). The oute r surface s o f th e Malpighia n tubule s ar e richl y supplie d with fine branching tracheae, an d some writers have described an investing tuni c o f connectiv e tissue , whic h i s sai d i n som e case s t o contai n elastic strands an d even true muscle fibers . Thus , according to Leger and Duboscq (1899) , in the tunic of the Malpighia n tubules of Gryllidae there are smal l superficia l bundle s of elastic fibers , an d deepe r striated muscle fibers forming a spiral about the entire length of each tubule. I n certai n Tettigoniidae th e sam e writer s describ e a singl e lon g muscle takin g a spiral or almost axia l course along each tubule. Trappman n (1923 ) likewise reports the presence of muscle fibers in the Malpighia n tubules of the honey bee , eac h tubul e havin g si x or eigh t fibrilla e formin g fla t band s arranged i n wid e spiral s almos t paralle l wit h th e axi s o f th e tubule . Veneziani (1905 ) finds elastic fibers in the tubule s o f the larv a o f Cerambyx, an d tru e muscle s i n th e larva l tube s o f Melolontha. I n Dytiscus, Rungius (1911 ; Korschelt , 1924 ) says ther e ar e fine fibrillae in the tuni c of th e Malpighia n tubules, which are evidentl y contractile elements , bu t that no true muscle fibers having a cross striation ar e present. Al l these investigators asser t tha t th e tubule s o f freshl y narcotize d specimen s

THE ORGANS O F DISTRIBUTION

419

exhibit wavin g movements and als o pulsations o f their walls which expel the product s of excretion. Accordin g to Eastham (1925) , only the basa l stalks of the Malpighia n tubules of DrosophilaandCalliphoraare provide d with muscle fibers, but th e fibers here are arranged circularl y and longitudinally an d ar e continuou s with th e muscula r sheath o f the intestine . Peristalsis, he says, occurs in the musculated parts of the tubules, an d th e entire tube s exhibi t a waving movement a s a result o f the contractio n of the muscle s i n thei r basa l stalks . Wiggleswort h (19316 ) report s th e presence o f a fe w muscle fiber s o n th e ampulla e an d lowe r end s o f th e Malpighian tubules o f Rhodnius that ru n ou t from the wall of the mesen teron, but otherwis e he says the tubules in this insect have no muscles. The discharg e of excretory products from th e wall s of the Malpighia n tubules has been described by various writers as taking place by the usual method o f liqui d diffusio n throug h th e cel l walls , b y th e expulsio n of small masse s through th e striate d border , an d b y th e extrusio n o f cytoplasmic globules charged with excretory matter, which are constricted off and se t fre e i n th e lumen . Ther e i s reaso n t o believ e tha t th e firs t i s the normal method of excretion, and that the formation of globules on the ends o f the cell s i s a n artifac t resultin g fro m th e techniqu e o f fixation . Substances reporte d t o b e eliminate d b y th e Malpighia n tubule s of insects includ e urea, uric acid, urates o f sodium, of calcium and ammonia , leucin, an d variou s salt s o f calciu m an d potassium , suc h a s oxylates , carbonates, an d phosphates . Guanin , th e characteristi c excretor y product o f Arachnida , i s unknow n i n insects . Mos t o f th e nitroge n excreted i s i n th e for m o f uri c acid . Calciu m salt s ofte n constitut e a large part o f the Malpighia n output , bu t the y are not t o be regarded as true excretor y products . I n analyse s o f excretor y matte r fro m th e rectum, especially in carnivorous species, as pointed ou t by Wigglesworth (1931a), ther e mus t b e distinguishe d substance s derive d directl y fro m the foo d i n the stomach , an d substances that are products o f metabolism excreted through th e Malpighia n tubules . In additio n t o thei r excretor y function , th e Malpighia n tubule s of certain insects ar e known to produce just before th e tim e of pupation a substance whic h is spun out o f the anu s in the for m o f silk threads, with which th e cocoo n i s woven . Thi s silk-formin g functio n o f th e larva l Malpighian tubule s ha s bee n describe d i n th e neuropterou s specie s Mermeleon formicarius an d Chrysopa perla (Lozinski , 1911) , i n th e cur culionid Phytonomus arator (Lebedew, 1914), and in the chalci d Euplectrus bicolor (Thomsen , 1927) . I t wa s claimed by von Gorka (1914 ) tha t th e Malpighian tubule s produc e digestiv e enzymes , bu t a stud y o f th e enzymes of cockroaches and caterpillar s mad e by Dirk s (1922 ) shows an entire absenc e of ferments in the Malpighia n tubules.

420 PRINCIPLES

O F INSECT MORPHOLOGY

GLOSSARY O F TERM S USE D I N THI S CHAPTE R Alary Muscles.—Th e dorsa l transvers e muscle s o f th e body , attache d mediall y on th e heart , usuall y arrange d i n fan-shape d groups of fibers . (Wing muscles o f the heart.) Aorta (Ao). —The anterio r nonchambered part o f the dorsa l blood vessel. Blood.—The body-cavity liqui d and its contents . Blood Cells (BCls).—The cellula r elements of the blood . (Haemocytes, leucocytes.) Cardiac Sinus.—Th e dorsa l par t o f the embryoni c haemocoele, a par t o f which becomes the lume n of the dorsa l blood vessel. Cardioblasts (Cdbl). —The cell s o f th e uppe r edge s o f th e embryoni c mesoderm that for m th e dorsa l blood vessel. Cerodecytes.—See oenocytes. Corpora allat a (CA). —A pai r o f small cellular bodies o f ectodermal origin associ ated wit h the stomodaea l ganglia behind the brain . (Ganglia allata, corpora incerta.) Diaphragm (Dph). —One o f th e horizonta l partition s o f th e bod y cavity . (Se e dorsal diaphragm, an d ventral diaphragm.) Dorsal Diaphrag m (DDph). —The membranou s an d muscula r sheet s o f tissu e extending fro m th e dorsa l bloo d vesse l t o th e laterodorsa l part s o f the bod y wall , separating th e dorsa l sinus fro m th e periviscera l sinus . Dorsal Sinu s (DS). —The spac e o f th e definitiv e bod y cavit y abov e th e dorsa l diaphragm an d the heart . (Pericardial sinus.) Dorsal Vessel CDF).—Th e dorsal blood vessel, consistin g of the pulsatil e heart and the nonpulsatin g aorta. Epineural Sinus.—Th e ventra l par t o f th e embryoni c haemocoel e between th e yolk an d the ventra l nerv e cord. Fat Body.—Th e masses of fat-containing cell s usually distribute d throughou t th e body cavity . Fat Cell.—On e of the componen t cells of the fa t body . Haemocoele.—The bloo d space s o f th e embryo , o r remnant s o f th e blastocoel e after invasio n o f the latte r by th e mesoderm. Haemocytes (BCls). —The cell s of the blood . (Leucocytes.) Haemolymph.—The blood plasma, o r liquid part o f the blood . Heart (Ht). —The chambere d part o f the dorsa l blood vessel. Heart Chamber.—On e of the segmenta l swelling s of the heart . Hepatic Cells.—Se e nephrocytes. Leucocytes.—See haemocytes. Malpighian Tubules (Mai). —The excretor y tubules openin g into th e anterio r en d of th e proctodaeum . Nephrocytes.—Special cell s scattered i n group s in various part s o f the body , bu t especially abundan t i n th e dorsa l sinus , wher e they ar e know n a s pericardial cells; probably hav e som e functio n i n connectio n wit h excretio n an d hav e bee n terme d "hepatic cells." Oenocytes.—Large ectoderma l cell s occurrin g i n th e abdome n o f mos t insects , sometimes withi n th e epidermis , bu t generall y fre e i n latera l segmenta l groups , o r scattered through the fat body. (Cerodecytes.) Ostial Valves.—Valve-like pouches of the hear t wall containing th e osti a a t thei r inner ends . Ostium (Ost). —One o f the paire d an d usually lateral openings (ostia) of the heart . Pericardial Cell s (PCls). —Strands o f specia l cell s alon g th e side s o f th e heart , probably in most case s nephrocytes. Pericardial Sinus (DS).—See dorsal sinus.

THE ORGANS O F DISTRIBUTION

421

Perivisceral Sinu s (PvS). —The principa l par t o f the bod y cavity , betwee n th e dorsal diaphrag m an d the ventra l diaphrag m if the latte r is present'. Phagocytes.—Haemocytes having active ingestive an d digestiv e properties . Pulsating Membranes.—Smal l muscula r membranes foun d i n th e thorax , head , and appendage s o f variou s insects , th e rhythmi c contraction s o f whic h probabl y contribute t o th e circulatio n o f the blood . Trophocytes.—Cells that elaborat e nutritiv e materials ; th e cell s o f the fa t bod y having a trophic functio n i n distinction fro m th e urat e cells. Urate Cells.—Cell s of the fa t bod y that becom e charged with urat e crystals . Valves o f th e Heart.—Interna l valve-lik e lobe s o f th e hear t wall s betwee n th e chambers, said t o be present i n certain dipterou s larvae . Ventral Diaphragm (VDph). —A membranou s and muscular sheet presen t i n some insects stretched betwee n the lateral edges of the abdominal sterna, sometimes extending into th e thorax, separating the ventral sinu s from th e per i visceral sinus . Ventral Sinu s (VS). —The spac e o f the definitiv e body cavit y belo w the ventra l diaphragm, containin g the nerv e cord. Wing Muscles o f the Heart.—Se e alary muscles.

CHAPTER X V THE RESPIRATOR Y SYSTE M Respiration include s chemical and physica l processes. Th e chemical phase o f respiratio n i s th e oxidatio n accompanimen t o f metabolis m i n the bod y tissues , whic h results i n th e formatio n o f carbon dioxid e an d water. Physica l respiratio n ha s t o d o wit h th e transportatio n o f oxygen to th e tissue s an d the remova l o f carbon dioxide. I n th e many celled animals, in which the vital tissues are surrounded by an integument, the consumptio n o f oxyge n an d th e productio n o f carbo n dioxid e ar e mostly interna l processes . Th e respirator y gases , therefore , mus t penetrate th e body wall and must be transported withi n the body to and from th e cell s o f al l th e tissues . Henc e respiratio n i n th e Metazo a includes external respiration, ga s transportation, an d internal respiration. Internal respiratio n i s essentiall y th e proces s o f oxidation ; i t therefore belongs to the realm of physiology. Externa l respiration an d the internal transportation o f the respiratory gase s are physical processes, and a study of th e anatomica l structure s concerne d with thes e processe s is properl y a part of the subject of morphology. Th e special mechanical devices that may b e develope d t o facilitat e th e interchang e o f gase s betwee n th e environment and the blood or the cellula r tissues o f the anima l constitut e the respiratory system. Physical respiration , i n it s simples t form , i s probabl y nothin g mor e than ordinar y ga s diffusio n throug h permeabl e membranes , an d th e dispersal of the respiratory gase s within the body by diffusion i n the blood . In it s highe r development , the physica l phase o f respiration i s expedited by the developmen t o f special organ s for bringing ai r into the body , an d by the presenc e of chemical substances i n the bloo d tha t ac t a s carrier s of the respiratory gases . In som e o f th e smaller , soft-skinne d insects , externa l respiratio n is accomplishe d by ga s diffusio n throug h th e genera l bod y integument , and the transportation o f the respiratory gase s depends upon the diffusio n of th e latter in the blood. I n al l insects, externa l respiration take s plac e through som e par t o f the ectoderm , bu t generall y a t place s wher e th e ectoderm, b y reaso n o f th e delicac y o f it s externa l cuticula r layer , i s specially adapte d t o th e transmissio n o f gases . Suc h respirator y area s of th e ectoder m occur both in the integumen t an d in the proctodaea l section o f th e alimentar y canal . I n som e case s thes e area s ma y b e fla t 422

THE RESPIRATORY SYSTEM 42

3

surfaces wher e the cuticul a i s sufficientl y thi n t o allo w of ga s diffusion . Usually, however , they tak e th e for m o f evaginations , know n a s blood gillsj o r o f invaginations, terme d tracheae, including the termina l traehe oles. Th e predominan t mod e o f externa l respiratio n i n insect s i s b y means of tracheae. Gas transportation withi n th e bod y cavit y i s reduced to a minimum in insect s provide d wit h a well-develope d trachea l system , sinc e th e tracheal invaginations branch so profusely and penetrate so thoroughly t o the tissue s tha t oxyge n must b e carrie d directl y t o almos t ever y cel l of the body . An d yet i t i s probable that som e of it escape s into th e blood to supply the needs of the blood tissues, and, as shown in the last chapter, it is not certai n that there may not be present in the insect blood a small amount o f a n oxygen-carryin g protein . Carbo n dioxide , o n th e othe r hand, being given off from al l exposed cell surfaces, evidently , canno t b e taken u p entirel y b y th e tracheae ; investigation s hav e show n tha t a t least a part o f the carbo n dioxide produced in tracheate d insect s is diffused int o th e bloo d and eliminate d through the integument . 1. TH E INTEGUMEN T A S A RESPIRATOR Y ORGAN

Insects tha t d o no t hav e tracheae , o r tha t hav e a n imperfec t o r a secondarily close d trachea l syste m an d ar e no t provide d wit h othe r devices fo r respiration , mus t effec t th e oute r exchang e of gases directly through th e integument . Th e bes t know n examples of insects lacking tracheae ar e th e Collembola , the majorit y o f whic h have n o breathin g mechanism of any kind , and certai n species of aquatic Chironomidae, in which tracheae , i f present , ar e ver y imperfectl y developed . Man y parasitic insec t larvae , livin g entirely submerged in the liquid s or tissue s of th e host , als o mus t respir e throug h th e sof t bod y wall , though the y may be provided with a well-developed tracheal system . Seura t (1899 ) observes tha t som e interna l parasiti c hymenopterou s larva e hav e a system o f finel y branchin g trachea l tube s coverin g the inne r surfac e of the bod y .wall, into which air is absorbed from that dissolved in the blood of th e host . Certai n parasiti c larva e ar e provide d wit h filamentou s processes of the bod y wall that appea r t o b e gills. Though wit h th e majorit y o f free-livin g insects havin g a normall y open tracheal system inspiration takes place largely through the tracheae , there i s evidence to sugges t that th e expiratio n o f carbon dioxid e takes place a t leas t i n par t b y wa y o f the integument . Thu s Krog h (1913) , finding that the carbo n dioxide deficit i n the trachea e o f the hin d leg of a grasshopper after expiratio n is always considerably lower than the oxygen deficit, conclude d that a large part o f the carbo n dioxide formed i n th e tissues of the leg must be carried away by other means than the tracheae . The same idea has been expressed by othe r investigator s an d appear s t o

424 PRINCIPLES

O F INSECT MORPHOLOGY

be demonstrate d experimentall y for certai n insects . Vo n Buddenbrock and vo n Roh r (1923) , for example , claim that i n Dixippus morosus onefourth o f the carbo n dioxide produced is given off through the bod y wall, and Demol l (1927 ) reports findin g n o carbo n dioxid e in th e trachea e of Melolontha. O n the othe r hand , Wred e (1926 ) foun d littl e evidenc e of carbon dioxid e expiration throug h th e ski n o f ordinary caterpillars . I n aquatic caterpillars, however , such a s certain specie s of Bellura, Nymphula, and Pyrausta tha t lac k gills , respiratio n unde r water , Welc h (1922) has pointed out , mus t tak e place through th e genera l body integument . It ha s no t bee n show n that an y particula r par t o f the integument , when gill s ar e absent , serve s fo r th e eliminatio n o f carbo n dioxide , bu t we may suppose that diffusion woul d be most likely to take place through the less dense areas, suc h as the conjunctiv e membranes and othe r part s where sclerotization is weak or absent. I n the Acridida e there are fenes tralike, unpigmented areas of the bod y wall along the midlin e of the bac k above the hear t chamber s in the abdomen , and above the aorti c pouches in th e thorax , whic h suggest tha t the y ma y b e permeabl e t o gase s an d perhaps serv e fo r th e eliminatio n o f carbo n dioxide . I n soft-skinne d aquatic dipterou s larvae carbo n dioxide is usually foun d t o b e give n off over th e entir e bod y surface . 2. BLOO D GILL S

Various aquati c larva l insect s ar e provide d wit h thin-walled , hollo w diverticula of the integument or of the proctodaeum, which, in the absence of definit e knowledg e concernin g thei r function , ar e usuall y terme d "blood gills. " Externa l processe s of this kind, variousl y distribute d o n the body , occu r o n th e aquati c larv a o f th e beetl e Hygrobia (Pelobius) tarda, on the aquati c larvae o f certain specie s of Chironomus, and o n th e aquatic caterpilla r o f Cataclysta fulicalis. Th e taperin g flesh y processe s arising nea r th e anu s o f many tipulid larvae ar e sometimes cite d als o a s examples of blood gills, and i t i s said tha t stream s o f blood may b e seen to circulat e through the m (Brown , 1910; Gerbig, 1913), but eac h of these processes is penetrated b y a trachea, and Gerbi g suggests, therefore, that the organ s serv e i n a doubl e capacit y o f blood gill s an d trachea l gills . The " gills" of the Hygrobia larva , however , which consist o f cluster s of delicate filament s arisin g behin d th e base s o f each pai r o f thoracic legs , contain no tracheae; and the same is true of the grou p of slender processes arising a t th e posterio r en d o f the abdome n of Chironomus larvae, an d of the filamentou s appendage s distribute d ove r th e bod y o f th e larv a o f Cataclysta fulicalis. Thes e organs , therefore , ma y b e suppose d t o b e blood gills . Th e characteristi c tai l o f certai n parasiti c hymenopterou s larvae ha s bee n suppose d t o b e a gill . A respirator y functio n ha s no t been demonstrate d i n connectio n wit h thi s organ , bu t Wiggleswort h

THE RESPIRATORY SYSTEM 42

5

(1931) point s ou t that , whe n i t ha s th e for m o f a fluid-filled vesicle, it s surface offer s a respiratory possibility . Proctodaeal evagination s occu r in som e trichopterous larvae , an d i n the larva e o f certain Simuliidae , which are protractile fro m th e anu s an d are generall y regarded as organs for aquati c respiration . Th e structur e of th e slende r proctodaea l evagination s o f a trichopterou s larv a i s described by Branch (1922), who finds that the organs are hollow tubular diverticula produced from th e posterio r ends of the si x folds in the wal l of the prerecta l part o f the intestine . The y ar e capabl e of being extended from th e anus , apparentl y b y pressur e exerte d b y th e abdomina l walls , and eac h i s retractil e b y a branche d muscl e inserte d withi n it , whic h arises o n th e bod y wall . Th e simila r processe s o f Simulium larvae, a s described b y Headle e (1906) , consis t o f three simpl e o r branched , soft , translucent filament s protractil e throug h th e anu s fro m th e ventra l wall of the rectum , into which they may be retracted b y a pair of muscles arising o n the dorsu m of the abdome n and inserted b y branches on their bases. Sinc e eac h o f thes e processe s contain s masse s o f fin e trachea l tubes filled with air , Headle e suggest s tha t the organ s perhaps functio n both as blood gills and a s tracheal gills. Most of the suppose d "blood gills" of insects have not bee n subjected to physiologica l test s fo r a respirator y function . Experiment s o n Chironomus larva e mad e b y Fo x (1921) , usin g th e infusoria n Bodo sulcatus a s a n indicator , whic h is positively chemotacti c t o certai n con centrations o f dissolved oxygen, appear to show that oxygen consumption takes place through the genera l body wall and not throug h the so-calle d gill filaments . Simila r result s wer e obtaine d fro m microspectroscopi c tests. Carbo n dioxide , accordin g to Fox , i s give n of f likewise through the genera l integumen t o f th e Chironomus larva . Th e structur e an d function o f the gill-lik e anal lobe s of mosquito larva e hav e been studie d by Wiggleswort h (1933, 1933a) , who concludes that the organ s serve for the absorptio n o f water , sinc e respirator y test s sho w tha t oxyge n i s absorbed o n al l parts o f the body , though mos t activel y a t th e base s of the ana l lobes , an d tha t carbo n dioxid e i s give n of f equall y fro m th e entire body surface . 3. TH E TRACHEA L SYSTE M

Invaginations o f th e integumen t fo r respirator y purpose s occu r i n the Onychophor a and i n som e terrestrial form s a t leas t o f all the majo r groups o f Arthropoda . Th e organ s probably , a s claime d b y Rippe r (1931), ar e forme d independentl y i n most o f the severa l groups in which they ar e developed . Som e of the Arachnida , a fe w isopod crustaceans , most o f th e Chilopoda , an d th e majorit y o f insect s hav e branche d tracheae. Mos t of the Diplopod a are provided with segmental clusters of

426

PRINCIPLES OF INSECT MORPHOLOGY

unbranched respirator y tubules , an d on e famil y o f th e Protur a ha s unbranched trachea e arisin g onl y o n the thorax . Th e Arachnid a hav e tracheae an d respirator y pouche s know n a s "lun g books/ ' s o name d because thei r wall s ar e produce d int o paralle l lamellat e folds . I n th e Onychophora th e respirator y invagination s tak e th e for m o f group s of small tubules irregularly scattered ove r the inner surface of the body wall. Development o f the Tracheae. —The respirator y tube s o f insects ar e formed i n the embry o as simple invaginations o f the ectoder m alon g th e sides o f th e bod y (Fig . 220) . Th e externa l orifice s o f th e depression s become th e spiracles (Sp) ; th e interna l tubula r part s ar e th e rudiment s of th e trachea e (Tra). Th e trachea l pit s exten d inward and branc h into

FIG. 220.—Diagra m showin g the relation s of th e layer s of a trachea l invagination t o the layer s of th e bod y wall. BMb y basemen t membrane; BW, bod y wall; Ct , cuticula ; Ecd, epidermis ; Epth, trachea l epithelium ; In, intim a (trachea l cuticula) ; Sp , spiracle ; Tra, trachea.

ramifying tube s that eventually exten d to al l parts of the body in insects having a fully develope d tracheal system . Judgin g fro m th e positio n of the abdomina l spiracles in adult insects , it would seem that the primitiv e position o f the trachea l invaginations i s in the latera l part s o f the dorsum of the body segments just above the limb bases. I n embryos the spiracular rudiments commonl y are thus locate d (Fig . 221). Number of Spiracles.—It i s possible that th e primitiv e insect s had a pair o f trachea l invagination s i n eac h o f th e 1 7 somites o f th e gnathal , thoracic, an d abdomina l regions of the body ; bu t ther e i s no suggestion of trachea e ever having been formed i n the protocephalon . Direc t proo f of th e existenc e of tracheal invaginations, however , has bee n found onl y on 1 4 segments, whic h are th e secon d maxillary segment, the 3 thoracic segments, an d th e firs t 1 0 abdominal segments. Tracheal invagination s o f th e secon d maxillar y segmen t ar e sai d by Nelson (1915) to be formed in the embryo of the honey bee, where they appear o n the anterio r par t o f the segmen t above the base s o f the labia l

THE RESPIRAT ORY SYSTEM

427

rudiments. Thes e trachea l pit s o f th e secon d maxillar y segment , Nelson says , giv e ris e t o th e trachea l syste m o f the hea d bu t ar e soo n closed an d leav e no trace o f their existenc e in the hea d o f the adul t bee . In som e of the Sminthurida e (Collembola ) a pai r o f spiracles i s situated on th e side s o f th e nec k clos e behin d th e hea d (Fig . 22 2 B). Davie s (1927) believe s tha t thes e apparen t cervica l spiracle s belon g t o th e prothorax becaus e o f their positio n relativ e t o th e muscl e attachment s on th e posterio r margi n o f the hea d capsule . Whe n w e consider , however, tha t th e submargina l ridg e o f th e hea d o n whic h thes e muscle s are inserte d mark s th e intersegmenta l lin e betwee n th e tw o maxillar y segments (Fig . 5 4 A, pos), an d tha t th e membranou s neck is derived i n part fro m th e segmen t o f the labium , i t become s evident tha t th e nec k spiracles of Sminthurus lie in the regio n of the secon d maxillary segment .

FIG. 221.—Diagra m o f th e tracheatio n an d positio n o f th e spiracle s i n th e embry o of Dixippus morosus. (From Lehmann, 1925.)

These spiracles , therefore , ma y b e persistin g example s o f th e secon d maxillary spiracles, known otherwise only as temporary tracheal opening s in the embry o of the hone y bee . Prothoracic spiracle s ar e presen t i n th e embryoni c stag e o f som e insects, bu t thes e spiracle s ar e completel y obliterate d befor e hatching . The embryonic prothoracic spiracle s have been described by Cholodkow sky (1891 ) i n BlaHcUa, an d b y Wheele r (1889 ) i n Leptinotarsa. The thora x o f postembryoni c stage s o f al l insects , excep t Diplura , never has more than two pairs of spiracles, and these two pairs are formed in the embryo on the anterior part of the mesothorax and the metathorax , respectively (Fig . 221, /Sp 2, Sps). I n many insects, however, the thoraci c spiracles migrat e forwar d durin g development an d com e thus to hav e a definitive positio n i n the secondar y intersegmental membrane s or in th e posterior part s o f the segment s preceding . Th e mesothoraci c spiracle s particularly ar e subject to this anterior migration and hence often occur in larval o r adul t insect s o n th e side s o f th e prothorax , fo r whic h reaso n they ar e frequentl y calle d th e "prothoracic " spiracles . Th e anterio r

428

PRINCIPLES OF INSECT MORPHOLOGY

(mesothoracic) spiracle s ar e usuall y th e large r o f the thoraci c spiracles , those o f th e metathora x bein g generall y small , an d sometime s rudimentary, a s i n certai n larvae . I n adul t Pterygot a th e thoraci c spiracles have a "pleural" position, but it is probable that their definitiv e location between the pleura l sclerites is a result o f the secondar y upward extension o f th e subcoxa l plate s o f th e le g base s o n eac h sid e o f th e spiracles. The Diplur a diffe r fro m othe r insects in that some species have thre e and other s fou r pair s o f spiracles on th e thorax . Th e greate r numbe r

FIG. 222.—Unusual position of spiracles in certain apterygot e insects . A , Heterojapyx gallardi, wit h tw o mesothoraci c an d tw o metathoraci c spiracle s o n eac h side . B , Sminthurus viridis, with a spiracl e o n each sid e of the neck . (From Davies, 1927. )

occurs i n certai n specie s of Japygida e (Fig . 22 2 A) . Th e firs t spiracl e (Sp%) lie s in the extrem e posterior part of the prothorax ; the secon d (Sp%') is situated on the sid e of the mesothora x above and behind the le g base; the thir d (Sps) i s in th e anterio r par t o f the metathora x befor e th e le g f base; an d th e fourt h (Sp) corresponds in position to the mesothoracic 3 spiracle (Spz'). The first spiracle of the series (Sp*) is very evidently the usual mesothoracic spiracle of other insects displace d into the prothorax ; the third (/Sp 3), likewise, would appear to represent the usual metathoracic spiracle. Th e tw o posterio r spiracle s (SpS, Spz'), therefore , ar e no t represented i n Thysanur a an d Pterygota , bu t i t i s mos t interestin g to observ e tha t thei r position s o n th e segment s correspon d exactl y t o the usua l position o f the spiracle s in Chilopod a (Fig . 5 2 A). Moreover , these posterior thoraci c spiracles of Diplura fall i n line with the serie s of abdominal spiracles (Isp). Hence we might conclude that the anterior thoracic spiracles of Diplura represen t th e tw o thoracic spiracles present

THE RESPIRATORY SYST EM

429

in othe r insects , an d tha t th e posterio r spiracles , belongin g morpholog ically to the sam e series as the abdomina l spiracles, have been eliminated from th e thorax o f other insects. If , then, th e anterio r thoraci c spiracles have ha d a n independen t origin , w e hav e her e a n explanatio n o f th e curious fac t tha t th e structur e o f these spiracle s i n pterygot e insect s i s almost alway s differen t fro m tha t o f the abdomina l spiracles . I n Camported, according to Grass i (1886) , the anterio r metathoraci c spiracles ar e absent, bu t th e sam e spiracle s ar e frequentl y rudimentar y o r absen t i n Pterygota. On th e abdome n ther e ar e usuall y eigh t pair s o f spiracles , an d this is th e maximu m numbe r o f abdomina l spiracle s i n postembryoni c stages of all insects, but th e numbe r may be variously reduced . Cholod kowsky (1891 ) report s th e existenc e o f a pai r o f tracheal invagination s on eac h o f the firs t nin e abdomina l segment s i n th e embry o o f Blattella (Phyllodromia); an d Heymon s (1897 ) find s i n th e Lepisma embryo , i n addition t o nin e distinc t pair s o f abdomina l spiracles , masse s o f ectodermal cell s o n th e tent h segmen t a t point s correspondin g t o th e spiracular invaginations o n the precedin g segments, which he takes to be rudiments o f a tenth pai r o f abdominal spiracles . Th e firs t abdomina l spiracles ar e ofte n situate d clos e t o th e thorax , bu t thei r abdomina l relation i s show n b y th e fac t tha t the y alway s li e posterio r t o a lin e through th e bas e o f the thir d phragma , whic h i s an intersegmenta l fol d between the metathorax and the first abdominal segment. Organization o f th e Trachea l System.—A s th e primar y trachea l invaginations gro w int o th e bod y o f th e insect , the y divid e a shor t distance fro m thei r origin s int o majo r an d mino r branches , an d th e latter eventuall y ramif y t o al l th e tissues . I n insect s havin g a well developed trachea l syste m som e o f th e branche s fro m consecutiv e an d opposite spiracle s unite t o for m longitudina l trunks an d transvers e com missures. I n general, the mature tracheal system attains an organization having a pretty definit e fundamenta l pattern. General Plan o f the Body Tracheation. —It i s probable that in a primi tive stag e eac h somit e o f the bod y wa s independentl y tracheate d fro m its ow n pai r o f spiracles , an d tha t th e connectio n o f th e segmenta l systems b y longitudina l trunk s i s a secondar y conditio n evolve d t o give mor e efficien t aeration . I n genera l i t i s foun d tha t i n eac h hal f of eac h segmen t ther e ar e thre e principa l trachea e give n of f from th e longitudinal trun k (Fig . 22 3 B, LTra) i n th e neighborhoo d o f the con nection o f th e latte r wit h th e spiracle . Henc e w e may suppos e tha t primarily a short spiracular trachea (Fig. 223 A, a) extended inward fro m the spiracl e an d gav e of f three mai n branches . O f the latter , on e i s a dorsal trachea (6 ) going t o th e dorsa l musculatur e o f the bod y wal l an d to th e dorsa l bloo d vessel ; another i s a ventral trachea (c) supplyin g th e

430

PRINCIPLES OF INSECT MORPHOLOGY

ventral musculatur e an d th e ventra l nerv e cord , an d sendin g a branc h into th e le g in th e leg-bearin g segments ; th e thir d i s a media n visceral trachea (d ) havin g it s principa l ramification s o n th e wall s o f th e ali mentary canal , wit h branche s t o th e fa t bod y and , in th e appropriat e segments, t o the gonad s and the genita l ducts . The plurisegmenta l longitudina l trunk s ar e forme d b y th e unio n of anterior an d posterio r branche s fro m th e spiracula r trachea e o f con-

FIG. 223.—Diagrammati c cros s section o f the abdome n showing the principa l trachea e and trachea l trunks . a , spiracula r trachea ; b , c , d , dorsal , ventral , an d viscera l seg mental tracheae ; DCom, dorsa l commissure ; DDph, dorsa l diaphragm ; DTra, dorsa l plurisegmental trachea l trunk ; DV , dorsa l bloo d vessel ; LTra, latera l plurisegmenta l tracheal trunk ; Sp , spiracle ; VCom, ventra l commissure ; VDph, ventra l diaphragm ; VsTra, viscera l plurisegmenta l trachea l trunk ; VTra, ventra l plurisegmenta l trachea l trunk.

secutive segments . Th e lengthwise trunks most generall y presen t ar e a pair o f lateral longitudinal trunks (Fig . 223 B , LTra), on e o n eac h sid e of th e body , connectin g all the spiracula r tracheae fro m th e firs t thoraci c spiracle t o th e las t abdomina l spiracle . Bu t ther e i s ofte n presen t also a pair o f dorsal longitudinal trunks (C , DTra), connectin g the dorsa l tracheae o f successiv e segments , an d sometime s a pai r o f ventral longitudinal trunks (E, VTra) uniting the ventral tracheae. In some insects, finally, there are visceral longitudinal trunks on the side s of the alimentar y canal (E , VsTra). By anastomosi s o f the dorsa l o r the ventra l trachea e i n each segmen t there ar e frequentl y forme d commissural trunks continuou s fro m on e side o f th e bod y t o th e other . Thu s ther e ma y b e presen t a dorsal tracheal commissure (D , DCom) crossin g abov e th e dorsa l bloo d vessel , or a ventral tracheal commissure below the ventra l nerv e cor d (EFCom).

THE RESPIRATOR Y SYSTEM

431

The latera l longitudina l trunk s ar e usuall y th e larges t trachea e i n the insect. Becaus e of their size and their immediate connection with th e spiracles, these trunk s becom e generally the chie f avenue s o f air circula tion i n th e body . I n dipterou s larva e an d pupae , however , i n whic h the latera l spiracle s ar e suppresse d an d functionall y replace d b y dorsa l spiracles connected with the end s of dorsal trunks, i t i s the dorsa l trunks that become the major respiratory passages (Figs. 223 D, 228, 229, DTra). The lateral trunk s (LTra) i n suc h case s ar e reduce d an d appea r a s a series o f smal l connective s betwee n th e transvers e tracheae . Ventra l longitudinal trunk s ar e no t ofte n developed , bu t the y ar e presen t i n the abdome n o f som e Orthoptera . I n th e Odonat a a viscera l trun k arises fro m eac h of the dorsa l trunks i n the anterio r en d of the abdomen , crosses abov e th e cro p t o th e opposit e sid e o f th e body , wher e f t goe s posteriorly alon g th e latera l wal l o f the mesenteron , an d finall y unite s with the latera l trun k o f the sam e side in the eight h abdominal segmen t (see Tillyard, 1917) . Kennedy (1922a ) and Steiner (1929 ) have attempted, from a study of the Zygoptera, to deduc e a more detailed concep t than that given abov e of th e primitiv e tracheatio n springin g fro m eac h primar y trachea l invagination, fro m whic h migh t b e evolve d th e basi c patter n o f th e tracheal syste m i n eac h o f the insec t orders . Kenned y observes , how ever, that the " readiness o f the tracheal syste m to develop new branches has been one of the things which has made homologization of the branches seem a hopeless task." W e may ad d that th e sam e condition stil l pre vails t o suc h a n exten t tha t i t woul d b e useles s t o presen t her e an y attempt a t a comparativ e stud y o f th e trachea l system . A revie w of the fact s know n concerning the fundamenta l plan o f the trachea l syste m in the principa l order s of insects is given by Lehmann (1925) . Tracheation o f th e Head.—The tracheatio n proceedin g from th e firs t thoracic spiracles is necessarily different fro m tha t o f the othe r spiracles , because fro m thes e primaril y mesothoraci c spiracle s originate s th e tracheal suppl y not onl y of the prothora x an d mesothorax but als o of the head. The trachea e o f th e hea d usuall y aris e fro m tw o principa l pair s of head trunk s give n of f fro m th e firs t spiracles . On e o f thes e o n eac h side i s a dorsal head trunk y th e othe r a ventral head trunk. Th e actua l number o f trachea e enterin g th e bac k o f th e hea d fro m th e thorax , however, may be increased by an immediate branching of the two primary trunks, an d i t i s no t clea r tha t th e principa l trunk s themselve s ar e in all cases strictly homologous branches. A relatively simpl e condition o f the hea d tracheatio n i s described b y Lehmann (1925 ) in Dixippus morosus. Th e dorsal head trunk (Fig . 221 , g) send s branche s t o th e antennae , th e compoun d eyes , th e mandibles ,

432 PRINCIPLES

O F INSECT MORPHOLOGY

the brain , an d th e adducto r muscle s o f th e mandibles . Th e ventra l trunk (ti) branche s t o th e firs t an d secon d maxilla e an d t o th e man dibular adductor s an d form s a n anastomosi s wit h the dorsa l trunk . A similar distributio n o f th e hea d tracheae , Lehman n says , occur s i n Machilis an d i n Ephemerida . I n Lepisma, accordin g to Sul c (1927) , a short cephali c trunk spring s fro m th e dorsa l branc h o f the firs t (meso thoracic) spiracl e an d soo n divide s int o a trachea cephalica dorsalis an d a trachea cephalica ventralis. Th e firs t branche s t o th e prothoraci c tergum, th e dorsu m o f the head , th e brain , th e opti c lobes , th e uppe r parts o f the eyes , an d th e antennae . Th e secon d branches to th e pro -

FIG. 224.—Tracheatio n o f th e hea d an d thora x o f a noctui d caterpillar , inner vie w o f right side.

sternum an d neighborin g organs , th e posterio r par t o f th e head , th e salivary glands , the inner region of the eyes , and the gnathal appendages. From thi s i t woul d appear tha t i n genera l the procephali c part o f th e head i s tracheated fro m th e dorsa l hea d trunk , an d th e gnatha l regio n from th e ventra l trunk , thoug h Lehman n find s tha t th e mandible s receive thei r trachea e fro m th e dorsa l trunk . Th e ventra l trunk , Lehmann says, is the first head trachea formed in the embryo of Dixippus, the dorsa l trunk bein g an outgrowth from it . Studies o n the hea d tracheation o f other insects show a considerable diversity i n th e distributio n o f the branche s from th e principa l trunks . Thus, accordin g to Al t (1912 ; Korschelt , 1924) , the ventra l head trunk s of Dytiscus suppl y almos t al l th e tracheatio n o f th e hea d muscle s an d give branche s to al l the appendages , including the antenna e an d man dibles, a s well as to th e firs t an d secon d maxillae.

THE RESPIRATOR Y SYSTEM

433

In th e caterpilla r (Figs . 224 , 225) thre e larg e trachea l trunk s ente r the head from each anterior spiracle, one (/) being dorsal, another (h) ventral, an d th e thir d (g) having a middl e positio n betwee n th e othe r two. Th e dorsa l an d middl e trunks, however , branch fro m a common base o n each side. Th e dorsa l trunk s (/ ) g o upward and unit e t o for m a commissur e (DCom) i n th e posterio r dorsa l par t o f the head . Eac h gives of f posteriorly a n anterio r ar m o f the dorsa l X-shaped commissur e of the prothorax (q) and anteriorly several small branches to the mandibular muscles . Fro m th e ape x of the hea d commissur e a pai r o f long tracheae diverg e anteriorl y alon g th e arm s o f th e V-shape d epistoma l ridge o f the facia l wal l of the cranium , giving of f branche s to th e dorsa l muscles o f the pharynx , an d finall y endin g in th e la b rum. Th e middl e trunks (g ) enter th e side s of the hea d an d brea k u p int o larg e branches distributed t o the lateral part s o f the mandibula r muscles. Th e ventra l trunks (Ji) divid e eac h into tw o mai n branche s a s the y ente r th e head . One branc h turn s upwar d int o th e ventra l part s o f th e mandibula r adductors; th e othe r proceed s forwar d beneat h th e oesophagus , giving off firs t a dorsa l trachea , o f which a branc h penetrates th e brain , while the res t o f the mai n branc h i s distribute d t o th e oesophagus , pharynx , muscles o f th e maxill a an d labium , an d th e ventra l part s o f th e hea d generally. The hea d o f th e hone y be e larva , a s describe d b y Nelso n (1924) , is supplie d wit h thre e pair s o f trachea e fro m a transvers e commissur e uniting th e firs t spiracles . O f these th e mesa l pai r goe s to th e brain . The secon d pair give s off branches in th e uppe r part o f the hea d t o th e aorta an d th e brain , bu t th e mai n trunk s g o ventrally an d ramif y t o the maxillae , th e mandibles , th e antenna l rudiments , an d th e labrum . The lateral thir d pair goe s to the salivar y glands. I n th e embry o of the bee, Nelso n (1915 ) says , th e invagination s fro m th e temporar y secon d maxillary spiracle s giv e of f eac h fou r primar y trachea l branches , on e going posteriorly , on e dorsally , an d th e othe r tw o anteriorly . Th e posterior branche s connec t with th e trachea l syste m o f the thorax , th e dorsal branche s from opposit e side s unite t o for m th e anterio r commis sure, an d th e anterio r branche s on eac h sid e becom e the tw o principa l pairs o f head tracheae . Tracheation o f th e Thorax. —The trachea l syste m o f th e thora x i s often comple x an d differ s muc h i n differen t insects . I n it s simple r forms, however , i t depart s littl e fro m th e mor e generalize d pla n o f segmental tracheation i n the abdomen , except for the suppl y o f tracheae to bot h th e prothora x an d the mesothora x from th e mesothoraci c spiracles, an d i n th e frequen t reductio n o r obliteratio n o f th e metathoraci c spiracles.

434

PRINCIPLES OF

INSECT MORPHOLOGY

A goo d exampl e of generalize d tracheation i n th e thora x i s see n i n a caterpilla r (Fig . 225). Th e large lateral trunk s (LTrd) ar e continuou s from th e abdomen to the mesothoracic spiracles (Sp%), whic h are situated on the sides of the dorsu m of the prothorax . Th e metathoracic spiracles (Spa) ar e rudimentary, but th e sit e o f each is connected with the latera l trunk b y a smal l trachea l strand . Th e principa l somati c an d viscera l

FIG. 225.—Tracheatio n of the thora x an d firs t abdomina l segment o f a noctuid caterpillar , dorsal view .

branches are given off from th e latera l trun k i n the neighborhoo d of th e spiracles, and in each segment there is a well-developed ventral commis sure (VCom) crossin g the anterio r par t o f the sterna l region. The tracheatio n o f the thoraci c legs of the caterpilla r is of particular interest becaus e in some respects it illustrate s th e leg tracheation typica l of most insects. Eac h leg has two tracheae, one lateral, the other median. In th e prothora x th e latera l trache a o f the le g (i) i s derive d fro m th e ventral commissure; the median one (j) comes directly from the branches of th e first , o r mesothoracic , spiracle . I n th e mesothora x an d th e metathorax, th e latera l le g trachea (ra ) i s a branch from a tracheal loo p (kj I) forme d apparentl y b y th e unio n o f trachea e fro m th e spiracle s

THE RESPIRATORY SYSTEM 43

5

preceding and following, that is, from th e mesothoracic and metathoracic spiracles i n th e cas e o f the mesothoraci c legs (L 2), an d fro m th e meta thoracic an d firs t abdomina l spiracle s i n th e cas e o f th e metathoraci c legs (Z/s) . Eac h latera l le g trachea of the wing-bearin g segments, there fore, i n it s entiret y ha s th e for m o f a Y , the arm s of which (& , Z ) proceed from successiv e spiracles, while the stem (ra ) enters the leg. Th e median leg trache a (ri) i n thes e segment s spring s fro m th e latera l trun k i n th e neighborhood o f the spiracl e following . Tracheation o f th e Wings. —In th e caterpilla r i t i s t o b e see n tha t each internal wing rudiment (Figs . 224, 225, TF 2, TF 3) i s penetrated fro m opposite ends by two tracheae (o , p) give n off from th e arm s of the latera l Y-shaped le g trache a (& , I) o f th e sam e segment. Th e tw o basa l wing tracheae appea r t o becom e continuou s throug h th e win g rudimen t i n older larvae . I t ha s been shown by Chapma n (1918) , fro m a compara tive stud y o f the basa l connection s of the win g tracheae , that th e win g tracheation her e exemplifie d i n th e caterpilla r represent s th e primar y tracheation of the win g in all insects. Ther e are, of course, many devia tions from th e typica l conditio n an d many developments alon g differen t lines o f specialization; but al l such modifications, Chapman shows, ma y be derive d fro m th e fundamenta l simpl e pla n i n whic h tw o trachea e proceed fro m th e convergen t arm s o f the latera l le g trachea an d ente r the win g base. Accordin g to Kennedy (1922a) , th e anterio r branc h i s the origina l wing trachea. Ther e i s probabl y n o morphologica l significance in the origi n of the win g tracheae from th e Y-shape d leg tracheae, since thi s particula r tracheatio n o f th e leg s occur s onl y i n th e wing bearing segments . Th e spiracle s an d th e wing s belon g to th e latera l areas o f the dorsum , while the leg s arise fro m th e pleura l areas . The win g rudiments of holometabolous larvae are a t firs t penetrate d by bundles of tracheoles given off from the basa l trachea, bu t th e trache oles ar e late r replace d by th e definitiv e tracheae , whic h enter th e vein s of the matur e wings and persist throughou t the lif e of the insect to aerate the livin g tissue s withi n th e veins . Accordin g t o Comstoc k (1918) , the anterio r basal trachea of each wing gives off the trachea e of the costal , subcostal, radial, and medial veins; the posterio r one gives off the cubita l and ana l tracheae . Whe n th e connectiv e throug h th e win g base , o r transverse basal trachea, i s developed , however , "th e medial trachea/ ' Comstock says , " tends t o migrat e alon g th e transvers e basa l trache a toward th e cubito-ana l grou p o f tracheae " an d thu s become s mor e closely associate d wit h the posterio r wing tracheae in the wing s of more specialized insects . Tracheation o f th e Abdomen.—With insect s havin g a full y develope d tracheal system , th e latera l trachea l trunk s exten d posteriorl y t o th e last pai r o f spiracles , whic h are usuall y thos e o f th e eight h abdomina l

436

PRINCIPLES OF INSECT MORPHOLOGY

segment. I n the caterpilla r (Fig . 226) the principa l abdominal branches are give n of f from th e longitudina l trunk s i n th e neighborhoo d o f th e

FIG. 226.—Tracheation of the posterio r abdomina l segment s o f a noctuid caterpillar, dorsa l view.

spiracles. Transvers e ventra l commissure s (VCom) unitin g th e latera l trunks occur in each of the first seven segments, and in the eight h segment there i s a dorsal commissur e (DCom). Th e gangli a of the ventra l nerv e

FIG. 227.—Tracheatio n of the ventra l regio n and the righ t hal f of the thir d abdomina l segment o f a noctuid caterpillar .

cord, excep t th e last , ar e tracheate d b y branche s springin g fro m th e ventral commissure s of their prope r segments, regardless of the position s of th e gangli a (Figs . 226 , VII's, 227 , s) . Th e composit e last ganglion ,

??? ??????????? ?????????

lying i n th e sixt h segment , fo r whic h ther e i s no ventra l commissure , receives its trachea e (Fig . 226 , VII Is) directly fro m the latera l trunk s opposite th e eight h spiracles . Th e abdomina l leg s o f th e caterpillar , except the las t pair , als o are tracheate d fro m the ventra l commissure s ????? ???? ??? ??? ??? ???????? ?? ??? ???????? ?????????? ?????????? ?? the latera l trachea e o f the prothoraci c legs (Fig . 225 , i), whic h have n o counterparts i n the mesothora x and metathorax. Th e visceral trachea e of th e abdome n (Fig . 227, d), going principally to th e wall s of the alimentar y canal , arise fro m the latera l trunk s nea r th e spiracles . Th e ????? ???? ?? ?????????? ?? ???????? ???????? of th e dorsa l tracheae (6). Modifications of the Trachea l System.—Al l insects do not have a fully developed equipment of respirator y tubes . I n variou s holometabolous larvae , especiall y aquati c an d parasiti c species, th e trachea l syste m i s mor e o r les s reduced o r rudimentary , an d the spiracles may be closed . Thi s conditio n i n suc h insect s i s clearly th e resul t o f a secondar y degeneratio n of th e tracheae . I n Machilis, group s o f seg mental trachea e aris e fro m th e spiracles , bu t there ar e n o longitudina l trunks . Campodea and Protur a (Eosentomidae ) hav e thoraci c tracheae only , whic h i n th e secon d grou p ar e said t o b e unbranched . Mos t o f th e Collem FIG. 228.— A mosquit o bola hav e n o tracheae at all , and i n tracheate d larva, dorsa l view , showin g forms, suc h a s Sminthurus (Fig . 22 2 B) , th e dorsal trachea l trunks opening posterio r spiracle s tracheae aris e fro m a singl e pai r o f spiracle s through only (P/Sp) , and lateral trunks located a t th e bac k o f the head . I t ma y b e along th e lin e o f th e close d questioned, however , eve n i n th e Aptery - adult spiracles . gota, whethe r th e imperfec t stat e o r absenc e o f th e trachea l syste m represents a primitiv e conditio n o r i s th e resul t o f a degeneratio n o f the trachea e in form s whos e small siz e makes cutaneou s respiration suf ficient. Th e presence of a fully develope d tracheal system wit h complet e longitudinal trunks in Japygidae suggest s that the insect trachea l syste m in it s usua l for m i s a n inheritanc e fro m commo n ancestor s olde r tha n modern Apterygota . An interestin g exampl e o f specializatio n i n th e trachea l syste m i s seen in the larva e an d pupa e of most Diptera , i n which the dorsa l longi tudinal trunk s becom e th e principa l respirator y passage s (Figs . 228 , 229, Dtra). Th e dorsal trunks of all dipterous larvae open to the exterio r ??????? ????????? ???????? ?? ??? ??? ?? ?? ???? ???? ?? ??? ???? ?????

438

PRINCIPLES OF

INSECT MORPHOLOGY

PSp). Thes e dorsal spiracles appear to be secondary respiratory orifices , since i n som e case s ther e ar e presen t als o th e usua l lateral spira cles, though the latter are closed and remain rudimentary during the larval and pupa l stage s an d ar e no t functionall y restored unti l th e imagina l stage. Al l dipterou s larva e hav e a pai r o f posterio r dorsa l spiracles , and some have in addition a n anterior pair on the prothorax. Th e pupae have onl y th e anterio r dorsa l spiracles . I n a tipuli d larv a th e latera l and dorsa l trachea l trunk s ar e equall y developed ; bu t i n mos t othe r dipterous larvae, as is well shown in a mosquito larva or a muscoid maggot (Figs. 228 , 229) , th e dorsa l trunk s (DTra) ar e proportionatel y greatl y enlarged, while the lateral trunks (LTra) are reduced to inconspicuous connectives betwee n the root s o f the transvers e trachea e alon g the lin e of th e close d and rudimentar y lateral spiracles .

FIG. 229.—Larva o f a muscoid fly with anterior and posterio r dorsal spiracles (ASp, PSp) at end s of dorsal tracheal trunk; lateral spiracles absent.

According t o th e distributio n o f th e functiona l spiracles , severa l types o f respiratory condition s may b e distinguished . Th e holopneustic type is the generalize d one in which the insec t is provided with the usua l bilateral serie s o f 1 0 pairs o f ope n spiracles ; i f ther e ar e onl y anterio r and posterior functiona l spiracles , the insec t is said t o b e amphipneustic; if it has anterior spiracle s only, it is propneustic; if it breathes by posterio r spiracles only , i t i s metapneustic; i f al l th e spiracle s ar e closed , i t i s apneustic. Thes e term s an d other s referrin g t o th e distributio n o f th e functional spiracle s ar e mor e precisel y denne d i n th e Glossar y a t th e end o f this chapter . Structure o f the Spiracles.—I n thei r simples t for m th e spiracle s ar e merely the openings from the integument int o the tracheae (Fig . 230 A, t) representing th e primitiv e aperture s o f th e trachea l invagination s (Fig. 220 , Sp). Suc h spiracle s hav e n o provisio n fo r -regulating th e size of the trachea l aperture. Spiracle s of this kind occur in some of th e Apterygota, an d i t i s probable that some of the spiracle s of lower Pterygota, suc h as the thoraci c spiracles of Plecoptera, ar e o f the sam e type of structure. In general , however , th e primar y trachea l aperture s ar e mor e o r less sunke n int o secondar y depression s o f the integumen t (Fig . 23 0 B) . The externa l par t o f eac h spiracl e thu s become s a pitlik e o r tubula r

??? ??????????? ????????? ???????? ????? ?? ??? ?????????? ?????? ?????? ??? ?????? ?????? ?? the on e hand , t o th e exterio r b y th e secondar y atrial orifice (a ) and , ?? ??? ?????? ???? ??? ??????? ?? ??? ????????? ???????? ??????? ???? ??? walls o f the atriu m ar e ofte n rugos e and ma y b e strengthened b y trans verse circula r ridges , bu t suc h structure s ar e no t tru e taenidia , whic h pertain t o the wall s of the trachea e only. Th e atrial walls are also commonly clothe d wit h hair s o r othe r cuticula r processes , suc h a s occu r on the externa l bod y wall. I n som e cases the atriu m i s subdivided int o an outer and an inner chamber, which differ in diameter or in the structur e of their walls, but i n general the atriu m is quite distinct fro m the spiracu lar trachea . Th e lip s o f the atria l orific e ma y b e flush with th e surfac e of th e integument , raised in a marginal flange or short tube , o r produced into a pair o f valve-like plates, whic h ar e sometimes movable by specia l

FIG. 230.—Structur e o f spiracles, an d tw o principa l types of spiracular closing apparatus. A , simple spiracl e without an atrium. B , a n atriate spiracle. C , atriate spiracle with li p typ e of closing apparatus. D , atriat e spiracl e with closing valv e a t inne r en d of atrium. a , atrial orifice; Atr, atrium; b, filter apparatus; BW, bod y wall ; c, d, anterior and posterio r lips of atrium; Ptr, peritreme; t, tracheal orifice; Tra, trachea; Vlv, valve.

muscles. Th e openin g is often containe d in a small sclerotic plate o f the body wal l formin g a distinc t spiracula r sclerite , o r peritreme (B , Ptr). Atriate spiracles are usually provided with a mechanism for regulating the passage of air to and from the spiracular trachea. Thi s mechanism is generally called the closing apparatus, thoug h it serve s both t o ope n and to clos e th e spiracle . Th e structur e o f th e closin g apparatu s differ s much i n differen t insects , an d i t i s ofte n quit e differen t betwee n th e thoracic and the abdomina l spiracles of the sam e species. Tw o principa l types o f occluso r mechanism , however , ma y b e distinguished , wit h numerous modification s under each . Th e firs t typ e i s a devic e o f on e kind o r anothe r fo r closin g th e oute r lip s o f th e atriu m (Fig . 23 0 C) . The second is a mechanism for regulating the size of the tracheal apertur e at th e inne r en d of the atriu m (D) . Accessory structures ar e often presen t i n the oute r part of the atriu m in spiracle s o f th e secon d type , whic h simpl y guar d th e atria l orific e (Fig. 23 0 D, &) . Suc h structures commonl y hav e th e for m o f opposin g rows o f tapering processe s of the atria l wal l thickly clothe d wit h inter lacing hairs, th e whol e mass of which forms a filter apparatus tha t freel y

440

PRINCIPLES OF INSECT MORPHOLOGY

permits th e passag e o f air , bu t whic h prevents th e entranc e o f foreig n particles o r water into the atriu m (Fig . 233, 6). A closin g apparatus o f the spiracle s i s absen t i n som e of th e highe r insects, but th e lack of the mechanism in such cases is probably a secondary condition . Th e thoraci c spiracle s ar e mor e variabl e i n structur e than ar e th e abdomina l spiracles, an d i n genera l the li p typ e o f closing apparatus i s characteristi c o f them; th e abdomina l spiracle s more con-

FIG. 231.—Thoracic spiracles of a grasshopper, Dissosteira Carolina; examples of spiracles of th e lip-closin g type . A , first spiracle, oute r view . B , same , inne r view , with closin g and openin g muscles (79, 80). C , second spiracle , outer view. D , same , inne r view , with closing muscl e (111).

sistently have th e inne r type o f closing mechanism. Unusua l modifica tions o f th e spiracula r structur e occu r i n certai n holometabolou s larvae, a s i n th e so-calle d "biforous " spiracle s o f coleopterou s larva e and th e dorsa l spiracles of dipterous larvae . I t wil l be possible to giv e here onl y a brie f descriptio n o f th e principa l varietie s o f spiracula r structure, illustrate d b y a few typical examples. Spiracles with an External Closing Apparatus.—The typ e of spiracula r structure i n whic h the closin g apparatu s i s forme d b y th e lip s o f th e atrial apertur e i s wel l illustrate d i n th e thoraci c spiracle s o f Acrididae. In Dissosteira Carolina th e first , o r mesothoracic , spiracle , lyin g i n th e membrane betwee n th e prothora x an d th e mesothorax , i s a n obliquel y vertical sli t i n th e peritrema l sclerit e (Fig . 23 1 A) wit h strongl y pro truding anterio r an d posterior lips . Th e anterio r li p (c ) i s a rigid eleva tion of the anterio r edg e of the atria l aperture; its inne r face, however , is soft an d deepl y groove d paralle l wit h th e oute r margin . Th e posterio r lip (d ) is a weaker and freel y movabl e flap, but i t ha s a sharp , strongl y

THE RESPIRATORY SYSTEM 44

1

sclerotized margina l ban d (n), which , whe n th e spiracl e i s closed , fit s into th e groov e o f the anterio r lip . Th e atriu m o f this spiracl e i s th e shallow cavity between the lips. Fro m it there are given off two tracheae, a large dorsal one (B, dTra) and a smaller ventral one (vTrd). In the septum betwee n the tw o tracheal opening s is a strong ba r (o ) projecting anteriorly an d ventrall y fro m th e posterio r lip . Upo n th e fre e en d of this process is inserted a short muscl e (79), which has its origi n ventrall y on th e peritreme . A second muscl e (80) arises clos e to th e firs t an d i s inserted o n the bas e of the posterio r lip of the spiracle . Th e first muscle (79) i s th e occluso r o f th e spiracle ; th e secon d (80) i s it s antagonist . The differen t actio n o f th e tw o muscle s result s fro m th e oppositio n of their point s o f insertion relative t o the lon g axis of the posterio r movabl e

UP.

The second , o r metathoracic , spiracl e o f Dissosteira (Fig. 23 1 C, D ) is eve n mor e simpl e tha n th e first . Externall y i t present s tw o thick , elongate, oval , valve-lik e lip s (C , c , d ) separate d b y a vertica l cleft . Both lip s o f this spiracle are movable, though they ar e unite d ventrall y in a smal l scleroti c lob e (q). O n th e inne r surfac e o f th e spiracl e (D ) it is seen that a small muscle (111) i s inserted on this lobe, which takes it s origin o n a proces s (s ) o f th e margi n o f th e mesocoxa l cavity . Thi s muscle i s th e occluso r o f th e spiracle . It s contractio n revolve s th e spiracular lip s toward eac h other and close s the apertur e betwee n them . The lips ope n automaticall y b y the elasticit y of their basal connections . Spiracles havin g th e li p typ e o f closin g mechanis m ar e o f common occurrence o n th e thora x i n mos t group s o f insects. I n th e Blattida e the thoraci c spiracles diffe r fro m thos e o f Acrididae, but eac h is a simple structure close d b y th e externa l atria l lips . Th e metathoraci c spiracl e of a n adul t Dytiscus, as described by Al t (1909 ; Korschelt, 1924 ) is very similar t o tha t o f Dissosteira. Th e thoraci c spiracle s o f Hemipter a (see Mammen , 1912 ) ar e o f the lip-closin g type . I n th e hone y bee th e large firs t thoraci c spiracl e i s close d b y a n operculum , whic h i s th e large, flattened , lidlik e anterior li p of the atrium . Spiracles with an Internal Closing Apparatus. —Spiracles i n which th e closing apparatu s lie s a t th e inne r en d o f the atriu m an d regulate s th e tracheal opening comprise the majority of insect spiracles, but thi s type of spiracular structur e i s particularly characteristi c o f abdominal spiracles . With spiracle s o f thi s kind, th e atriu m i s usuall y a well-define d open cavity o f the integumen t (Fig . 230 D, Atr) an d may be long and tubular. The entrance , however , is ofte n guarde d b y a filte r apparatus , usuall y in th e for m o f tw o row s o f matte d brushe s projectin g fro m opposit e walls o f th e atriu m (b). Th e lip s o f th e atria l apertur e hav e variou s forms, bu t the y ar e neve r movabl e an d the y tak e n o activ e par t i n the closin g o f th e spiracle . Th e siz e o f th e atria l orific e varie s muc h

442

PRINCIPLES OF INSECT MORPHOLOGY

regardless o f th e siz e o f th e atrium ; sometime s i t i s contracte d t o a small por e openin g into a relatively larg e atrial chamber , an d in specia l cases it i s closed. Two commo n subtypes o f structure ar e foun d amon g spiracles o f th e inner closin g type. I n on e subtype th e occluso r mechanism is a simpl e pinchcock apparatu s (Fig . 23 2 A), consistin g o f two scleroti c bars (e , /) in opposit e wall s of the atriu m jus t befor e th e mout h o f the trache a (i), with a muscl e (osp) stretched betwee n thei r projectin g ends . Th e contraction of the muscle brings the bars together and thus closes the trachea l entrance (B) . Usuall y a secon d muscl e arisin g o n th e bod y wal l i s

FIG. 232.—Example s o f spiracle s wit h a n inne r closin g apparatus , diagrammatic . A, B , th e pinchcock , o r double-valve , typ e o f closing apparatus , ope n an d closed . C , a n abdominal spiracl e o f Blatta. D , sam e o f Dissosteira. E-H , varietie s of the single-valv e type of closing apparatus. dlsp, dilato r muscle; e, anterior bar;/ , posterior bar ; g, manubrium o f anterior bar ; h, bow; i, j, dorsal an d ventral muscle processes; k , closing valve ; I, lever o f valve; osp, occluso r muscle .

inserted o n th e en d o f the anterio r ba r opposit e th e attachmen t o f th e occlusor muscl e and act s a s a dilato r o f th e spiracl e (C, D, dlsp). Th e abdominal spiracle s o f Blattida e hav e a closin g apparatu s o f thi s kin d (C), bu t th e fre e en d o f the anterio r ba r i s prolonged a s a manubriu m (g) t o giv e stronge r effec t t o th e muscles . Th e dilato r muscl e (dlsp) in Blattida e arise s anteriorl y o n the deflecte d latera l lob e of the tergu m that contain s the spiracle . I n th e Acridida e (D) the closin g mechanism of th e abdomina l spiracle s i s a modification of the blatti d type , i n which the posterio r ba r i s absent , an d th e anterio r ba r i s -represented by th e entire anterio r wal l o f th e atriu m (e), whic h is movabl e an d produce d ventrally i n th e manubriu m (g). Th e occluso r muscl e arise s o n th e tergal wal l immediately behin d th e spiracle , an d the lon g dilator muscl e (dlsp) arises on the lateral edge of the sternum. In th e secon d subtyp e o f occluso r apparatu s i n spiracle s o f th e inner closin g typ e th e effectiv e orga n i s a valve . Th e valv e consist s

THE RESPIRATORY SYSTEM 44

3

of a fold o f the inne r end of one wall of the atriu m (Fig . 23 0 D, Vlv), an d of a mechanism for inflectin g th e fol d ove r the trachea l mout h (Fig . 232 E). A n occluso r apparatus o f this kin d i s th e commo n form o f closing apparatus i n th e abdomina l spiracle s o f holometabolou s insects . Th e essential element s o f th e closin g structure (Fig . 23 2 E ) include , first , a crescenti c o r semicircula r elasti c ba r (K), th e so-calle d closing bo w (Verschlussbugel of Landois and Thelin, 1867), the ends of which (if j) are produce d outsid e th e atria l wall s a s tw o thic k conica l processe s (Verschlusskegelri)} second , a soft , conve x fol d (&) , th e closing band (Verschlussband)j projecting into the atrial lumen from the wall opposite the bow ; and , third , a closin g muscl e (osp) stretche d lik e a bowstrin g between th e end s o f th e bow . Th e closin g band, o r valve , i s usuall y on the posterio r wal l of the atriu m an d i s located just befor e th e mout h of th e trache a (Fig . 23 0 D) . Th e contractio n o f th e occluso r muscle (Fig. 232 E, osp) pull s on the tw o ends of the bo w and forces the valve (k ) inward until it entirely closes the tracheal orifice (F , t). I n some spiracles the close d valv e overlap s externall y th e bo w o n th e opposit e margi n of the apertur e (Fig . 230 D). Th e opening of the spiracl e may b e caused entirely by the elasticit y o f the bow, but usuall y a dilator muscle , arising ventrally o n the bod y wall , is inserted o n the lowe r process o f the bo w opposite th e attachmen t o f the occluso r muscle. A simpl e closin g apparatus o f the for m jus t describe d occurs in th e abdominal spiracle s of many insects, bu t numerou s departures fro m th e typical structur e ar e foun d i n th e holometabolou s orders . A common modification result s fro m th e suppressio n o f th e ventra l proces s o f th e bow (Fig . 23 2 G) , an d th e developmen t o f a poin t o f flexur e betwee n the dorsa l proces s (Z ) and th e uppe r en d o f th e bow . Th e pul l o f th e closing muscl e on th e dorsa l proces s the n bring s th e bas e o f the latte r and th e closin g band (k ) agains t th e inne r edg e o f the bo w to clos e th e tracheal aperture . Thi s typ e o f structure, foun d wit h variation s i n th e Coleoptera, migh t als o be suppose d t o b e a derivativ e o f the pinchcoc k type o f mechanism (A, B, C), but it s effectiv e elemen t is a valve, a s in a typical valvula r spiracle (E). Th e valve mechanism is highly developed in Lepidoptera b y th e extensio n of the dorsa l muscle process into a long lever (H) . A dorsa l dilato r muscl e o r stran d o f elasti c tissu e (dlsp), arising o n the bod y wal l an d inserte d o n the lever , i s present i n cater pillars, i n additio n t o a ventra l muscle , whic h i s her e inserte d o n th e atrial wall. The structura l details an d actio n o f the closin g apparatus of a cater pillar spiracle are shown in Fig. 233, representing the first thoracic spiracle, in whic h the structur e i s th e sam e a s tha t o f the abdomina l spiracles , except that the positio n o f tne parts is reversed, th e leve r an d the valv e being anterior in the thoracic spiracles and the bow posterior. Th e lever

444 PRINCIPLES

O F INSECT MORPHOLOGY

(1) i s supporte d o n a loope d ba r i n th e membranou s valv e (B , Vlv), and th e latter , whe n closed , i s receive d int o a dee p concavit y o f th e posterior atria l wall. Dista l t o the valve the atrial apertur e is protecte d by a filte r apparatu s compose d of two opposin g mats o f thick, brushlik e processes (6 ) projectin g fro m th e anterio r an d posterio r wall s o f th e atrium. Biforous Spiracles. —In th e larva e o f variou s familie s o f Coleopter a there occur spiracles of a type known as "biforous." Th e term originall y implied tha t eac h spiracl e ha d tw o externa l openings , bu t i t i s no w commonly extende d t o othe r spiracle s similar i n appearance, but havin g only on e opening or probably, i n some cases, none at all .

FIG. 233.—Horizontal section of left firs t spiracl e of a noctuid caterpillar , showing ventral view of dorsal half. A , atrial valve open. B , atria l valve closed.

Good example s o f tru e biforou s spiracle s ar e foun d i n th e larva e of Elateridae . Th e functiona l openings of these spiracle s are secondar y formations, since the primary atrial orifice is closed except during ecdyses. A larval spiracle of Alaus oculatus presents externall y an ovate peritremal area (Fig . 23 4 A ) havin g anteriorl y a dar k scleroti c thickenin g (m) and posteriorl y tw o elongat e convergen t plate s (n). Th e thickenin g marks th e sit e o f th e close d atria l orific e (a) . Th e convergen t plate s have each a clear median area traversed b y an axial line (a 7). Internally , the spiracl e consist s o f a close d atria l chambe r (E , Atr), whic h give s off posteriorly a wide membranous pouch (p), the external wall of which consists o f a thi n doubl y conve x membrane strengthene d b y branchin g and inter joining trabeculae (B , E, g) . Externa l t o th e pouch , an d pro jecting beyon d i t posteriorly , ar e tw o shallo w cuticle-line d chamber s (r) beneat h th e convergen t externa l plate s o f th e spiracl e (A , n) . A manipulation o f th e spiracle s o f Alaus oculatus demonstrate s beyon d question tha t thes e chamber s ca n b e widel y opene d alon g th e axia l lines o f thei r oute r wall s an d give s ever y reaso n t o believ e tha t th e openings ar e natura l clefts , thoug h i n th e usua l conditio n thei r lip s ar e closely appresse d (D , a'). Robert s (1921 ) ha s show n that i n section s the spiracle s of Agriotes are clef t alon g the media n line s of the posterio r

THE RESPIRATORY SYSTEM

445

chambers. I t ha s ofte n bee n claimed , however, that th e chamber s ar e closed cavities , an d that observe d opening s are artifacts . The spiracle s o f Alaus oculatus thu s appea r t o consis t o f a close d atrial chambe r (Fig . 23 4 C, Atr) provide d with a broad posterio r diver ticulum (p), an d o f tw o open , secondar y posterio r chamber s (r) , th e inner wall s of which are adnat e wit h th e oute r wal l of the atria l pouch, forming a thin , doubl y arched , trabeculate d septu m (q) betwee n th e posterior chamber s an d th e pouc h of the atria l chamber . Ai r enterin g

FIG. 234.—Type s o f "biforus " spiracle s o f coleopterou s larvae. A , Alaus oculatus, functional spiracle . B , sam e spiracl e durin g moulting . C , sectiona l diagra m o f Alaus type o f spiracle . D , on e o f th e opening s (a') o f posterior chambe r of Alaus spiracle . E , inner structur e o f Alaus spiracl e an d en d o f trachea. F , Cactophagus validus, abdominal spiracle. G , same , inne r structure . a , primar y atria l orifice ; a' , orific e o f secondar y atrial chambe r (r); Atr, atrium ; c , d, anterio r an d posterio r atria l lips ; h, bow o f closing apparatus; j, muscl e process ; ra , cuticula r thickening ; n, oute r wal l o f secondar y atria l chamber (r) ; o, peritreme; p, atrial pouch; q, inner wall of secondary chamber; r, secondary atrial chamber ; Tra, trachea.

the posterio r chamber s evidentl y mus t diffus e throug h th e trabeculate d septum i n orde r t o ente r th e atriu m an d th e associate d tracheae . A t each ecdysi s th e primar y atria l apertur e i s opened (B , a) t o permi t th e withdrawal o f the trachea l exuviae , an d th e entir e spiracula r structur e is renewed. Other so-calle d "biforous " spiracle s o f coleopterou s larva e hav e a more simpl e structure . I n man y o f these th e primar y atria l apertur e is widel y ope n (Fig . 23 4 F, a) , bu t fro m on e en d o f th e atriu m ther e project tw o pouches (p) beneath the bod y wall and entirel y fre e fro m th e latter (G , p). Wit h suc h spiracle s ther e i s n o questio n o f separat e external openings into the pouches. O n the other hand, there are doubly pouched spiracle s havin g th e atria l orific e practicall y closed , a s i n

446 PRINCIPLES

O F INSECT MORPHOLOGY

Histeridae, in which the pouche s are not separate d fro m th e integument and hav e very thin oute r walls. I n suc h cases the exterio r wall of each pouch much resembles that of the posterio r chamber s of Alaus and ma y even b e marked b y a faint median line . Muc h discussion has centere d around the question a s to whether these pouches are open to the exterior or ar e closed . Steink e (1919 ) seem s to conced e that the y may b e ope n in som e cases , bu t h e rightly say s i t i s a ver y difficul t thin g t o prove . If the y ar e closed , a s the y appea r t o be , th e exchang e of respirator y gases mus t tak e plac e b y diffusio n throug h thei r ver y delicat e oute r walls. In th e larva e o f Donacia the dorsall y situated spiracles o f the eight h abdominal segment have each a pair of slender tubular pouches extended into a lon g fre e spine-lik e proces s o f th e peritreme . Th e respirator y spines o f thes e posterio r spiracle s ar e use d t o penetrat e th e vascula r tissues o f water plant s fo r obtainin g air . Accordin g to Bovin g (1910) , the spine s ar e inspirator y i n functio n an d ar e imperforate ; expiratio n takes plac e throug h th e ope n latera l spiracle s an d throug h th e atria l apertures o f th e posterio r spine-bearin g spiracles . I t i s eviden t that there i s neede d a thoroug h comparativ e stud y o f biforou s spiracle s i n Coleoptera. Fro m th e foregoin g discussio n i t appear s tha t ther e ma y be tw o type s o f closed spiracles here included , on e in whic h the atriu m gives off a pair of imperf orate diverticula, int o which air diffuse s throug h the externa l integument , th e othe r (Elateridae ) i n whic h the diffusio n surfaces ar e conceale d in secondary open invaginations of the bod y wall. Dorsal Spiracles o f Dipterous Larvae. —Spiracles o f a uniqu e typ e of structur e occu r in the larvae and pupa e of Diptera directl y connected with th e end s o f th e dorsa l trachea l trunks . Th e position , structure , development, trachea l connections , an d th e temporar y natur e o f thes e dorsal spiracle s sugges t tha t the y ar e secondar y respirator y structure s having no relation to the latera l spiracles , which are closed or suppressed during immature stages an d functionally restored in the adult . The anterio r larva l o r pupa l spiracle s tak e th e for m o f perforate d lobes o r tube s (Fig . 229 , ASp) o r o f trumpetlike horn s arising fro m th e posterior par t o f th e prothorax . Th e posterio r larva l spiracle s (PSp) are usually contained in a pair of prominent plates situated o n the eight h segment o r the composit e terminal segmen t of the abdomen , where they are generally exposed, though they may be concealed in a shallow cavity or elevate d o n a respirator y tub e (Fig . 228) . Th e posterio r spiracle s typically have one, two, or three openings. I n tipulid larvae the posterior spiracular plate s wer e formerl y suppose d t o consis t o f a mes h o f fin e rods branching from a central disc, admitting air through the interstices , but Gerbi g (1913 ) ha s show n tha t th e periphera l area o f the spiracl e is imperforate, an d tha t th e functiona l openin g is a media n sli t i n th e

THE RESPIRATORY SYSTEM 44

7

central dis c obscure d by its overlappin g lips. I n first-insta r larva e th e spiracular aperture s ar e plainly open . The external part of each prothoracic dorsal spiracle of cyclorrhaphous larvae ha s th e for m o f a smal l lobe , usuall y branche d o r digitate , wit h numerous pore s communicatin g wit h th e atriu m (Fig . 23 5 A) . Th e posterior spiracle s presen t eac h tw o opening s i n th e firs t insta r o f th e larva an d usually three i n the secon d and third instar s (B) . Th e spira cular apertures ope n into a large atrial chamber (Atr) connecte d with th e end o f th e correspondin g dorsa l trachea l trunk . A t th e firs t an d th e second moul t th e entir e spiracula r structur e i s forme d ane w an d take s on a differen t for m characteristi c o f the ensuin g instar . Investigator s do not agre e as to whether the new atrial chamber is an outgrowth of the one preceding or an ingrowth from th e integument , but i n either cas e the

FIG. 235.—Spiracle s o f a trypeti d fl y pup a an d maggot , Rhagoletis pomondla. A , anterior spiracl e of pupa. B , posterio r spiracles of a thir d insta r larva . a , spiracula r openings; u, outer scar of preceding spiracle; 0, remnant of preceding spiracle; w, inner scar.

old chambe r serve s fo r the discharg e o f the trachea l intim a an d i s then closed, while the ne w formation becomes the functiona l breathing orific e for th e succeedin g instar. Th e sit e o f the earlie r spiracl e i s marked b y a sca r o n the surfac e o f the integumen t (A , u), whic h remains connected with the bas e of the ne w atrial cavity (w ) by a strand of cuticular tissu e (v). A t the third moult of the larva the dorsal spiracles are not renewed. The latera l imagina l spiracle s o f thes e flie s appea r firs t o n th e fourt h instar o f the larv a (forme d withi n th e puparium ) jus t befor e the trans formation t o th e pupa . Structure o f the Tracheae.—Sinc e th e trachea l tube s ar e invagina tions o f th e bod y wall , thei r ow n wall s contai n th e sam e structura l layers a s doe s th e bod y wall , onl y i n revers e orde r (Fig . 220) . Th e matrix layer of a trachea i s an epithelium of flat polygonal cells (Fig. 236, Epth) continuou s with th e epidermi s aroun d th e spiracle . O n the out side is a basement membrane, and on the inside a strong cuticular intim a (In). Th e characteristic featur e of an insect trachea i s its closel y ringed appearance resultin g fro m th e presenc e o f fold s o r thickening s o f th e intima i n th e for m o f minute circular or spira l ridges, the taenidia (tn\

448

PRINCIPLES OF INSECT MORPHOLOGY

which projec t o n th e inne r surface . I n som e insects th e inne r wall s of the trachea e ar e covere d b y shor t spicule s o r clothe d wit h simpl e o r branched hair s arisin g fro m th e taenidia l ridges . The taenidi a ar e generall y not continuou s throug h an y considerabl e length o f th e trache a bu t for m a successio n o f ridges , eac h o f whic h makes a few turns around the tracheal wall and then terminates. Whe n a trachea i s broken, the torn edge usually pull s out in a long spiral ban d (Fig. 23 6 A), which , i t wil l b e observe d i n mos t cases , i s no t a singl e taenidium bu t a stri p o f the trachea l wal l containin g severa l taenidia . The taenidia o f the larg e dorsal trunks o f some dipterous larvae, however, appear to be simple, uninterrupted rings, since a single taenidial "thread " may be easily removed from the broken end of a trunk. Whil e in general the taenidi a ar e continuou s aroun d th e wall s of a trache a an d serv e t o

FIG. 236.—Structur e of a tracheal tube, and examples of tracheal air sacs. Epth, epithelium; In, intima; in, taenidium in spiral band of cuticular intima artificiall y separated.

keep th e tub e open , thos e o f the dorsa l trunk s o f a tabani d larva , an d presumably in other dipterou s larvae , ar e all jointed, o r broken by points of flexibility , i n a definite lin e along each side of the trachea . A trache a having thi s structure , whe n devoi d o f air , collapse s t o a fla t band . Dunavan (1929 ) ha s observe d tha t a collapsin g an d als o a shortenin g take plac e i n th e dorsa l tracheal trunk s o f a living Eristalis larva durin g respiration. I n insect s havin g a mechanical respiration, Krog h (1920a ) distinguishes respiration tracheae (tha t is , ventilation tracheae), whic h ar e oval i n cros s sectio n an d easil y compressible , fro m diffusion tracheae , which are rigid an d cylindrical . The Trachea l Air Sacs.—The tracheal tube s ar e seldom of a unifor m or an evenly tapering diameter ; generally they are widened in some places and narrowe d a t others . I f a widene d part o f a trache a form s a con spicuous enlargemen t in the cours e of the tube , th e dilatatio n i s called a tracheal air sac (Fig. 236 B, C). Ai r sacs are present i n certain member s of mos t o f the pterygot e order s and reac h their greatest developmen t i n some o f th e cyclorrhaphou s Diptera , an d i n th e Apida e amon g th e Hymenoptera, bu t the y ar e absent in Apterygota an d in holometabolous larvae. The y var y greatl y i n siz e fro m minut e vesicle s t o larg e bag s

THE RESPIRATORY SYSTEM

449

and ma y b e widel y distribute d i n th e body, i n th e head , an d i n th e appendages. I n the honeybee the lateral trachea l trunks of the abdomen are transformed into voluminou s air sacs, an d smalle r sacs occur abund antly throughou t th e res t o f the bod y an d i n the legs . Th e cicada s ar e remarkable fo r the grea t ai r spac e that occupie s most o f the abdomina l cavity (Fig . 237) . Thi s air-filled sac has been claimed to be a diverticulum of the alimentary canal , but it can readily be demonstrated i n various cicada specie s tha t th e abdomina l ai r chambe r open s directl y t o th e exterior throug h th e firs t abdomina l spiracles , an d tha t trachea l tube s issue from it s walls . The ai r sacs respond i n a greater degre e than d o the trachea l trunk s to increase d an d decrease d pressur e i n th e bod y resultin g fro m th e movements of respiration and thus give a more efficient ventilatio n to the

FIG. 237.—Longitudina l sectio n o f Magicicada septendecim, showin g th e grea t ai r chamber occupyin g mos t o f th e abdome n an d openin g t o th e exterio r throug h th e firs t abdominal spiracle s (ISp).

tracheal syste m durin g breathing . Th e ai r sac s i n som e insect s ar e particularly responsiv e t o th e respirator y movement s becaus e thei r walls lac k th e taenidia l ridge s characteristi c o f th e tracheae . I t i s generally assume d tha t th e intim a o f the ai r sac s a s well as that o f th e tracheae i s a chitinou s membrane . Test s mad e b y va n Wisseling h (1898) o n th e ai r sac s an d trachea e o f the hous e fly , an d b y Campbel l (1929) o n the ai r sacs and attached trachea e of both the house fly and th e honey be e faile d t o sho w the presenc e of chitin; but Koc h (1932 ) claims that wit h mor e delicate method s o f technique th e presenc e of chitin ca n be demonstrated i n the trachea l intim a o f both thes e insects . The Tracheoles. —The fina l lin k betwee n th e en d branche s o f th e tracheae an d th e cell s o f the bod y tissue s i s formed by minut e tubule s called tracheoles. Th e tracheole s ar e sai d t o diffe r fro m th e trachea e in tha t the y ar e containe d withi n singl e cells . The y ar e cuticula r canals, generall y less than a micron in diameter, lacking taenidial ridges , formed i n elongate and usuall y branched cells of the trachea l epithelium.

450 PRINCIPLES

O F INSECT MORPHOLOGY

When th e tracheole s ar e firs t develope d the y hav e n o opening int o th e lumen of the trachea , bu t wit h the remova l of the trachea ! intima a t th e succeeding moult th e lumin a o f the tracheole s becomes continuous with the cavit y o f the trachea . I n othe r words , a tracheole, apparently, i s a tubular outgrowt h of the newl y forming tracheal intima forme d withi n a single cell of the trachea l epithelium. A tracheole, therefore, is probably not a truly intracellula r structur e but resemble s the duc t of a unicellular gland, which penetrates th e cel l body as an invagination of the cel l wall. During the formativ e stage th e tracheol e becomes coiled within it s cell , but a s th e cel l elongate s th e tracheol e straighten s out , unti l finall y i t extends a long distance fro m it s poin t o f origin, an d th e attenuate d cell matrix aroun d th e tub e become s scarcel y perceptible . The tracheole s ar e usuall y give n of f in cluster s fro m th e tracheae . In som e cases they appea r t o b e simple tubules, bu t generall y they ar e dichotomously branched . Th e termination s o f the tracheole s hav e no t been studied i n many insects, but their final branches have been found t o anastomose i n a fin e capillar y networ k ove r th e tissu e cells , i n which there may b e united group s of tracheoles fro m severa l differen t trachea l sources. Vo n Wistinghausen (1890) has described the tracheole capillary net o f th e sil k gland s o f caterpillars , an d E . Holmgre n (1896 ) find s a similar networ k of anastomosing tracheole s no t onl y o n th e sil k gland s but als o o n the fa t cells , th e Malpighia n tubules , an d th e wall s o f th e mesenteron in caterpillars. Accordin g to Holmgren (1896a), the canaliculi o f the tracheol e net ar e formed in a different se t o f cells from thos e in which the primar y tracheoles ar e generated . The tracheole endings usually lie on the surface s of the cells , but the y are said in some cases to di p beneath the cel l surface an d thus appea r t o lie within the body of the cell . I t i s probable, however, that the trache oles d o no t ordinaril y penetrat e th e cel l cytoplasm . Wher e the y ar e seen t o li e withi n th e circumferenc e o f a n epithelia l cell , Holmgre n says, they are contained in a pouch or sheath o f the basement membrane. The tracheol e ne t o f mos t muscle s i s als o superficial . I n th e win g muscles, however , accordin g t o Athanasi u an d Dragoi u (1913 , 1915) , the trachea e branc h profusel y betwee n th e fibers , an d th e tracheole s penetrate int o th e fibers , wher e the y ramif y abundantl y amon g th e sarcostyles an d anastomos e t o for m a n intricat e networ k abou t th e latter. Tracheal Gills.—Organ s know n a s tracheal gills ar e hollow , thin walled evaginations o f the integumen t o r of the intestina l wall containing finely branched tracheae and usually an abundance of tracheoles. The y are presen t o n man y aquati c larva e an d o n som e pupae , an d i n a few cases they ar e retained i n the adul t stage . I n form , trachea l gill s vary from fine filaments to broa d plate s o r dilated sacs . The y ma y b e situ -

THE RESPIRATORY SYSTEM 45

1

ated o n any externa l part o f the body , includin g the head , th e thorax , and the abdomen , or in the recta l part of the proctodaeum , but the y are usually confine d t o th e exterio r o f the abdomen . Typical filamentou s trachea l gill s occu r o n the larva e o f Plecoptera , some Ephemerida , mos t Trichoptera , th e neuroptero n Corydalus cornutus, o n severa l specie s o f aquati c lepidopterou s larva e o f th e genu s Nymphuldj and on the pupa of the dipterous genus Simulium. The slender taperin g appendicula r processe s born e o n th e side s o f th e abdominal segment s o f siali d larva e an d o f gyrini d an d certai n othe r coleopterous larva e ar e usuall y regarde d als o a s havin g a respirator y function becaus e eac h i s penetrate d b y a trachea l branc h fro m th e lateral trachea l trunk . Familia r example s o f plate-lik e trachea l gill s are thos e that occu r along the side s of the abdome n of many ephemerid larvae and a t th e en d o f the abdome n of larvae of zygopterous Odonata. An interestin g descriptio n o f th e filamentou s gill s o f lepidopterou s larvae is given by Welch (1922), who finds that each gill filament contains a tracheal branch from the main lateral trunk of the tracheal system, and that th e inne r surfac e o f the gil l i s covere d by innumerabl e tracheoles lying parallel with one another. Nearl y five hundred gill filaments may be presen t o n a singl e individua l o f Nymphula obscuralis. Th e thre e terminal gill s o f zygopterou s larva e ar e born e b y th e epiproc t an d th e paraprocts. Usuall y the y hav e th e for m o f elongat e plates , bu t i n certain specie s the y ar e vesicular . A n accoun t o f thei r variou s form s and thei r structure i s given by Tillyard i n his Biology of the Dragonflie s (1917). Mos t highl y develope d o f al l trachea l gil l structure s ar e th e rectal gill s o f th e larva e o f anisopterou s Odonata . Thes e gill s consis t of si x set s o f invagination s o f variou s shapes projectin g in longitudina l rows fro m th e inne r wal l o f th e anterio r par t o f th e rectum , togethe r forming th e so-calle d "branchia l basket. " Th e respirator y lobe s ar e richly tracheated fro m the dorsa l and viscera l longitudinal trunks of the tracheal system. A detailed account of the for m an d structur e o f these organs is given by Tillyard i n the wor k above cited. Though parasitic larva e generall y have no special respiratory equip ment for breathing the oxyge n dissolved in the bloo d of the host , gill-like structures have been observed in a few cases. Thorp e (1930 ) has shown that a pai r o f long , well-tracheate d termina l lobe s o f th e parasiti c fl y larva Cryptochaetum iceryae tak e u p oxyge n more actively tha n doe s th e general integument o f the insect . Certai n parasiti c chalci d larvae hav e groups o f branche d trachealik e filament s arisin g externall y fro m th e body wal l i n th e neighborhoo d of th e anterio r an d posterio r spiracles , which hav e bee n regarde d a s gil l structures . Accordin g to Clause n (1932), however, these filaments are tracheae, but the y are given off from trunks of the host tracheal syste m that have become fused wit h the body

452 PRINCIPLES

O F INSECT MORPHOLOGY

wall of the parasit e and thus serve for the respiratio n o f the latter . Th e connecting trunks, Clause n points out, are usually broken in dissections, and hence the branching tracheae penetrating th e hos t tissue s appea r t o be outgrowths from th e integumen t of the larva . 4. GENERA L MECHANIS M OF TRACHEA L RESPIRATION

Respiration throug h tracheae branching to al l parts of the bod y may be accomplishe d entirely b y th e diffusio n o f gases withi n th e tracheae ; but probabl y the majorit y of adult insect s produce a partial ventilatio n of the trachea l system by means of movements of the bod y wall. I n th e second case , tracheal breathing ha s many features in common with lung breathing an d involve s the presenc e of a mechanis m fo r producin g an d controlling the respirator y movements. Respiration b y Gas Diffusio n i n the Tracheae.—Sinc e many insects , especially larva l forms , d o no t mak e an y perceptibl e breathin g move ments, i t i s evident tha t respiration i n such cases must b e accomplished largely o r entirely b y th e diffusio n o f gases through the tracheae . I t i s possible, however , that wit h some larvae th e movement s of the bod y or particularly th e successiv e contractio n o f th e latera l bod y muscle s overlying th e longitudina l trachea l trunk s ma y caus e an irregula r pas sage o f ai r throug h thes e trunks . Likewis e it ma y b e suppose d tha t in th e larva e o f Diptera th e pulsation s o f the hear t migh t effec t a compression o f the larg e dorsal trunk s lyin g to eac h side o f the heart . A s already mentioned , i t ha s bee n observe d b y Dunava n (1929 ) that th e dorsal trunk s o f a n Eristalis larv a bot h shorte n an d collaps e durin g respiration, thoug h Dunava n wa s no t abl e t o discove r th e mean s b y which th e activit y o f th e trachea e i s produced . I n general , however, there ca n b e n o doub t tha t diffusio n account s for th e majo r par t of gas transfer throug h the trachea e of insects that make no specific respirator y movements, and , eve n i n insect s tha t activel y breathe , i t i s onl y th e larger tracheae that are ventilated; the peripheral respiration is always by means o f ga s diffusion . The par t playe d b y diffusio n i n th e respiratio n o f insects ha s bee n conclusively show n by Krog h (1920 , 1920a) . Usin g tenebrionid larva e and th e larv a o f Cossus as subjects having an ope n tracheal system, an d aeschnid larva e a s example s o f aquati c insect s wit h a close d trachea l system an d breathin g b y mean s o f gills , Krog h demonstrate d experi mentally the interchange of gases in the tracheae by means of gas diffusio n entirely. Thoug h h e point s ou t tha t respiratio n b y diffusio n i s practicable only for small animals, since the rate of diffusion varie s directly with the diamete r o f the trachea e an d inversel y to thei r mea n length , ther e are many insects much smaller than th e form s o n which he worked that are activ e breathers . Th e productio n o f respirator y movement s i s

THE RESPIRATORY SYSTEM

453

dependent o n the bod y structure, an d i t woul d be quite impossible for a soft-skinned larva to make rhythmic respiratory movements such as those made by adul t insects . A Dytiscus larva, constructe d o n the pla n o f an adult insect , however , breathes b y activ e pulsation s o f the abdomen , a s Krogh has shown in a later paper . W e may conclude, therefore, that i t is practica l fo r al l insect s t o breath e b y ga s diffusion , bu t tha t activ e respiration give s a more efficient ga s exchange and i s practiced by man y insects structurall y capabl e of making rhythmic expansions and contrac tions o f the abdomen .

FIG. 238.—Diagrams of respiratory mechanisms . A , B, C , three types of abdomina l expiratory movement s mad e b y insects . (From Plateau, 1884) . D , sectio n o f segmen t with compressor muscles only. E , section of segment with compressor and dilator muscles. F, lengthwis e sectio n showin g protractor (de, ve) an d contracto r (di, vi) muscles .

Respiration b y Trachea l Ventilation.—Th e mechanica l element s necessary fo r th e productio n o f breathing movement s b y th e bod y ar e present i n th e fundamenta l relation s betwee n th e segmenta l plate s o f the bod y wal l an d th e somati c muscles . Th e possibilit y o f breathing , therefore, i s possesse d b y al l insect s wit h scleroti c plate s i n th e bod y wall, and , afte r th e acquisitio n o f tracheae , mechanica l respiratio n needed onl y th e developmen t o f control center s i n th e nervou s system . The Respiratory Movements. —The movement s o f respiratio n affec t principally the abdomen. The y are produced by the somatic muscles and by the elasticity of the body wall. Expiratio n result s from a dor so ventral compression o f the abdome n or , i n som e cases, als o fro m a longitudina l contraction o f the abdomen. Th e effectors i n the first case are the lateral tergosternal muscle s (Fig . 23 8 D, cpr); i n th e secon d they ar e the inter segmental longitudina l muscle s (F , di, vi). Bot h movement s may occur together i n th e sam e insect . Expiratio n ma y b e accomplishe d entirel y

454 PRINCIPLES

O F INSECT MORPHOLOGY

by the elasticit y o f the bod y wall; but, i n insects that breath e strongly , some o f th e vertica l an d longitudina l muscle s ar e generall y converte d into dilators and protractors of the abdomen by a change in their mechanical relation s t o th e plate s o n whic h the y ar e attached . Thus , wit h insects in which the abdomina l terga overla p the edge s of the sterna (E) , some o f the externa l lateral muscle s become dilators (dlr) i f their terga l attachments ar e ventral t o thei r sterna l attachments , an d in such case s the effectivenes s o f the dilator s i s usually increased by th e dorsa l exten sion of their sternal attachments on lateral apodemal arms of the sternu m (lAp). Similarly , a protracto r apparatu s i s forme d b y a transpositio n of th e anterio r end s o f the externa l dorsa l an d ventra l muscle s to th e posterior margin s o f th e terg a an d sterna , respectivel y (F , de, ve), s o that thes e muscle s becom e antagonisti c t o th e interna l longitudinal s (diy vi). The effectiveness of the protractors likewise may be increased by th e forwar d extensio n o f their point s o f insertion o n anterio r terga l and sternal apodemes. A comprehensiv e study o f th e breathin g movement s o f insect s ha s not bee n mad e i n recen t years , an d ou r bes t sourc e of information on the subjec t i s stil l th e wor k o f Platea u (1884) . B y mean s o f lanter n projections o f the shadow s of living insects, Platea u mad e observations on th e respirator y movement s o f insect s representin g th e principa l orders. Fro m his results he distinguished three principal types of respiratory mechanism, based on the structur e of the abdome n and the manner of breathing . I n th e firs t type (Fig . 238 A), th e stern a ar e usuall y firm and strongly convex and move but littl e i n respiration; the terga, on the other hand, are mobile and noticeably rise and fall wit h each inspiratio n and expiration. Insect s that breathe in this manner include Heteroptera and Coleoptera . I n th e second type (B) , the terg a are large and overlap the stern a laterally , usuall y concealin g the membranou s lateral zone s of th e segments . Bot h th e terg a an d th e stern a approac h and separat e in this type of structure, bu t th e movement s o f the stern a ar e the mor e pronounced. Her e Platea u include s th e Odonata , Acrididae , aculeat e Hymenoptera, and Diptera. H e observes, however, that in Phryganiidae and Hymenopter a th e dorsoventra l movement s o f th e abdome n ar e accompanied by more pronounced movements in a longitudinal direction. The third type of respiratory mechanism (C) is found in insects having the terg a an d stern a separate d o n th e side s o f the abdome n by ampl e membranous areas . Durin g breathin g th e terg a an d stern a approac h and separate, while the lateral membranes correspondingly bulge outward or ar e draw n inward . Insect s havin g thi s typ e o f structur e includ e Tettigoniidae, Neuroptera, Trichoptera , an d Lepidoptera . The rat e an d amplitud e of the breathin g movement s are character istically differen t i n differen t insect s an d var y als o i n eac h individua l

THE RESPIRATORY

SYSTEM

455

according to the strength of external stimuli and according to the activit y of th e insect . Le e (1925) , fo r example , record s th e averag e rat e of breathing fo r females o f Melanoplus femur-rubrum a s being 5.8 a minut e at 49°F. , an d increasin g t o 26. 6 a t 80° . Berbe r an d Slife r (1928) , however, find much variation i n the breathin g o f quiescent grasshopper s when observation s ar e continue d fo r a considerabl e length o f time, th e variations affectin g no t onl y the rat e of breathing bu t als o the dept h of the abdomina l pulsations . Thu s the y repor t fo r a mal e o f Melanoplus femur-rubrum, observe d fo r a n hour , a fluctuatio n fro m 21. 5 t o 67. 5 seconds i n th e tim e occupie d b y 1 0 respirator y movements . Th e Tettigoniidae appea r t o b e mor e activ e breather s tha n th e Acrididae , and durin g stridulatio n th e breathin g o f th e male s i s especiall y pro nounced. Th e Phasmidae , o n the othe r hand , ar e ver y slo w breathers . According to Stah n (1928) , the Europea n walkingstick Dixippus morosus when a t res t make s onl y 1. 4 to 2. 3 expiration s a minute , thoug h al l stimulating influence s caus e a n increas e i n th e respirator y rate . Th e breathing movement s o f Dixippus ar e sai d t o affec t bot h th e abdome n and the thorax . Course o f th e Ai r i n th e Tracheae. —During recen t year s ther e ha s been muc h discussio n o n th e questio n o f a differentia l functio n o f th e spiracles a s inspirator y an d expirator y orifices , an d o n that of the direc tion of the ai r current s in the longitudina l tracheal trunks . Experiments mad e b y Le e (1925 ) o n th e respiratio n o f grasshopper s (Acrididae) an d observatio n tha t th e thoraci c an d firs t tw o abdomina l spiracles ope n durin g th e expansio n o f th e abdomen an d clos e durin g contraction, whil e th e las t si x abdominal spiracle s open an d clos e wit h the revers e movements , seeme d t o sho w tha t i n norma l breathin g b y grasshoppers inhalatio n take s plac e throug h th e anterio r spiracles , an d exhalation throug h th e posterio r spiracles . Lee' s result s wer e dispute d by MacKa y (1927) ; and M e Arthur (1929) , after makin g similar experi ments on several species of Acrididae, arrived at the following conclusions: The firs t fou r spiracle s o f th e grasshoppe r ar e usuall y inspirator y an d the las t si x expiratory, bu t th e actio n o f the spiracle s i s variable unde r both norma l and abnorma l conditions ; the mechanis m of the spiracula r valves i s capabl e o f reversin g th e time s o f openin g an d closin g o f th e spiracles relativ e t o th e respirator y movement s o f th e abdomen ; th e direction o f air current s throug h th e trachea e ca n thu s b e reversed , o r the air can be forced into any one of several possible paths by the interna l control o f the spiracula r valves . Subsequent investigation s hav e confirme d i n genera l Lee' s origina l claim that the ai r stream goes posteriorly through the bod y of Acrididae. With a more efficient apparatu s tha n that used by the preceding writers, McGovran (1931) , experimentin g on Chortophaga viridifasciatusy reports

456 PRINCIPLES

O F INSECT MORPHOLOGY

that th e respirator y movement s produc e a pulsator y movemen t o f ai r through th e trachea l trunks, an d that inspiratio n i s principally int o th e thorax, whil e expiratio n i s principall y b y wa y o f th e abdomen . A n adult female , a t 28°C. , passe d a n averag e o f 0.22 2 cubi c centimete r of ai r throug h th e bod y pe r minut e pe r gra m bod y weight . Finally , the wor k of Fraenkel (1932 ) gives essentially th e sam e results o n Orthoptera. Th e thoraci c an d firs t tw o pair s o f abdominal spiracles, Fraenke l says, ope n durin g inspiratio n an d clos e durin g expiration , whil e th e other si x pair s o f abdomina l spiracle s sho w a revers e actio n relativ e to th e respirator y movement s o f th e body . Furthermore , Fraenke l demonstrated experimentall y in Schistocerca gregaria a movement of th e respiratory ai r posteriorly i n the tracheae . Quantitativ e measurement s showed fro m 5 t o 2 0 cubic millimeters transported pe r second , or fro m 7.5 to 24. 4 cubic millimeters wit h each expiratory movement. The wor k o f von Buddenbroc k and vo n Roh r (1923 ) o n the respira tion o f Dixippus morosus le d these investigator s t o th e conclusio n that the trachea l ai r strea m goe s forward i n th e walkingstick , th e thoraci c spiracles bein g expirator y an d th e abdomina l spiracle s inspiratory , except tha t a smal l quantit y o f air ma y sometime s issue fro m th e nex t to th e last pair o f abdominal spiracles. Stah n (1928 ) obtained th e sam e results i n experiments on Dixippus, bu t h e observes that th e expirator y stream appear s ofte n t o b e interrupted b y expiratio n throug h th e abdo men durin g passiv e breathing . Accordin g t o D u Buisso n (1926) , the actio n o f th e spiracula r valve s i n Dixippus i s variable . Durin g ordinary breathing , h e claims , inspiratio n take s plac e throug h al l th e spiracles, bu t th e revers e ma y occur , or , again , th e movement s ar e disordered an d hav e no rhythm. I n earlie r studies o n Stenobothrus an d Locusta, D u Buisso n (1924 , 1924a ) claime d tha t i n thes e insect s als o inspiration usuall y takes place through all the spiracles , but tha t expira tion i s ordinarily by wa y of the thoraci c spiracles only. Unde r unusual conditions, however, he says, Stenobothrus may keep the thoracic spiracle s continuously closed ; and expiration then takes place through the abdom inal apertures . That insect s hav e n o definitel y fixe d directio n o f breathin g i s als o the conclusio n o f Demol l (1927) , deduce d fro m experiment s o n Melolontha. B y subjectin g eithe r th e thora x o r th e abdome n o f a n intac t beetle t o nascen t chlorine , he foun d tha t th e insec t wa s quickly killed . If th e wing s were cu t off , however, and th e thora x protecte d fro m th e gas whil e th e abdome n wa s expose d t o it , th e insec t wa s unaffected , since th e ope n tracheae o f the win g stumps, togethe r wit h the spiracle s of th e thorax , afforde d a sufficien t mean s of respiration an d allowe d th e insect t o kee p the abdomina l spiracles closed.

THE RESPIRATORY SYSTEM 45

7

From th e diversit y o f the result s obtaine d b y differen t investigator s we may conclude that there is no law governing inhalation and exhalation through specia l sets o f spiracles applicabl e alik e to al l insects, an d tha t the directio n o f the respirator y current s may alternat e eve n in the sam e individual; bu t i t appear s tha t respiratio n ha s a usua l thoug h no t a fixed course in each species, which presumably is characteristic als o of the family, an d probabl y of the orde r in most cases . The Function of the Air Sacs. —The greater diameter o f the ai r sacs as compared with that of the trachea e makes the wall s of the sac s relativel y weaker, and the air sacs are, therefore, more responsive than the tracheae to variations of pressure in the surrounding blood or other tissues, create d by th e alternatin g respirator y movement s o f the bod y wall . Particu larly is this true of air sacs, such as those of the honey bee, which have no taenidia i n thei r walls . I n thei r respons e t o pressur e change s th e ai r sacs resembl e lungs ; bu t inasmuc h a s periphera l trachea e ar e give n off from them , thei r actio n i s more accurately stated b y Bett s (1923) , who says, "th e function o f the ai r sac s is that o f the ba g of a bellows/' or, as Demoll (1927 ) put s it , the y guarante e a n intensiv e ventilatio n o f th e tracheae durin g breathing. B y a device for making direct observation s on th e actio n o f th e trachea l sac s unde r varyin g pressures , Demol l demonstrated tha t th e sac s ar e compresse d wit h increasin g pressur e around them , a par t o f thei r ai r conten t bein g thu s drive n int o th e pressure-resisting trachea l tubes , an d that , wit h decreasin g pressure , they ar e inflated. Ther e is no direct evidenc e that the ai r sacs functio n as storage chamber s for air. I n specia l cases they serv e to giv e atmos pheric pressur e agains t th e inne r surface s o f tympana l organ s or , i n certain aquatic species, to maintain buoyancy in the water. The Respiratory Stimuli. —Most studie s o n th e respirator y stimul i of insect s appea r t o b e base d o n the assumptio n tha t carbo n dioxide is not carried by the blood, and that, therefore, th e respiratory movement s must be regulated by the relativ e amounts of carbon dioxide and oxygen in th e trachea l air , an d experiment s hav e shown , i n fact , tha t suc h i s the case . Temperatur e als o influence s th e rat e o f breathing , bu t it s primary effec t i s presumabl y o n th e processe s o f metabolism . Th e mechanism of breathing response to increased or decreased activity on the part o f th e insec t ha s receive d littl e attentio n experimentally ; bu t inasmuch as it has been shown that a part of the carbon dioxide produced by metabolis m ma y b e eliminate d b y othe r mean s tha n th e tracheae , it is probable that most of it is thrown off from th e tissue s into the blood . If so , it then becomes possible that the ordinary respiratory regulation in insects, a s i n vertebrates , i s brough t about b y fluctuation s o f th e hydrogen-ion concentratio n o f th e circulatin g medium . Th e stimulu s for th e fundamenta l rhythmi c movement s o f respiration , Fraenke l

458 PRINCIPLES

O F INSECT MORPHOLOGY

(1932a) concludes , arise s withi n th e controllin g nerv e center s an d ha s no peripheral source. The firs t attempt s a t determinin g th e regulator y valu e o f gases o n the respirator y movement s o f insect s ar e thos e o f Baba k an d Foustk a (1907). Fro m experiments on the breathin g reactions of libellulid larvae to alteration s i n th e carbo n dioxid e an d oxyge n pressure o f th e wate r medium, thes e investigator s conclude d that th e rat e an d amplitud e of breathing are dependent o n the oxyge n supply tha t reache s the nervous system throug h th e tracheae , bu t tha t carbo n dioxid e or carboni c aci d can scarcely be a regulatory stimulu s fo r respiration, since it i s effectiv e only i n excessiv e amounts . Stah n (1928) , however , claim s tha t th e experimental methods of Babak an d Foustk a were not reliabl e for determining the effect s o f small quantities of carbon dioxide. Experimentin g with Dixippus morosus, Stahn foun d tha t smal l increase s in th e carbo n dioxide content of the inspired air are reflected in the rate of the breathin g movements, an d tha t th e effect s o f a n exces s o f carbo n dioxid e ar e remarkably paralle l wit h th e effect s o f deficienc y o f oxygen . I n brief , Stahn conclude s that the primary stimulating agen t for increased breathing activity is carbon dioxide in small excess over the amoun t in ordinary air, the lower threshold being 0.2 per cent of carbon dioxide in the inspired air, and the effective maximum about 0.3 to 3 per cent. A slight decrease in th e oxyge n content , however , ha s th e sam e effec t a s a n increas e of carbo n dioxide , th e maximu m effectivenes s o f oxyge n a s a contro l stimulus bein g from 2 0 to 1 5 per cent . A strong an d apparentl y toxi c acceleration o f breathin g occur s whe n th e oxyge n conten t fall s belo w 8 per cent, or when the carbo n dioxide content exceeds 12 or 15 per cent. The respirator y effect s o f temperature hav e been studied b y Wallin g (1906), wh o found tha t norma l grasshoppers (Acrididae ) making on th e average 4 0 contractions o f the abdome n a minute a t 14°C . increase th e rate of breathing to 110 contractions a minute as the temperature, durin g a period of 4 hours, is increased to 54°C.; at stil l higher temperatures th e rate declines , an d respiratio n cease s a t 59°C . Lowere d temperatur e has a n opposit e effect . A t 5°C . grasshopper s breath e faintly , i f a t all , from five to si x times a minute, though breathing by normal individual s may no t ceas e unti l th e temperatur e fall s t o 0°C . I t wil l b e notice d that th e breathin g rat e o f grasshoppers a t ordinar y temperature s give n by Wallin g is considerably higher than the figure s o f Lee (1925 ) quote d above. Nothing is known definitely as to how the varying carbon dioxide and oxygen pressure in the trachea l ai r makes itself effectiv e a s a respirator y stimulus. I t ha s been supposed that ther e ma y be sensory nerves connecting th e trachea e wit h th e respirator y nerv e centers ; but a sensor y innervation o f the trachea e ha s no t bee n observed , and Stah n suggest s

THE RESPIRATORY SYSTEM 45

9

that the respiratory center s may be stimulated directl y b y the condition of the air that diffuses fro m the tracheoles into the nerve ganglia. The Respiratory Nerve Centers. —There is no specific respiratory cente r in th e nervou s syste m o f insect s fo r th e productio n an d regulatio n of the breathin g movements . Eac h ganglio n o f the ventra l nerv e cor d of the abdome n contain s a n independen t respirator y cente r controllin g the movements o f its segment , bu t i t appear s that the thoraci c gangli a als o play a par t i n th e productio n o r contro l o f the respirator y movements . Experimental results in some cases are possibly somewhat confused by th e fact that the ganglion proper to a segment may lie in some other segment, and that the thora x ofte n contain s on e or more of the abdomina l ganglia. Though Matul a (1911 ) claime d that in Aeschna larvae the activity o f the ventral gangli a i s unde r th e contro l o f a cerebra l breathin g center , hi s conclusion was disproved b y Wallengre n (1913) , wh o showed that head less larvae ar e still sensitive t o the oxyge n tension o f the water . O n th e other hand, Wallengren found that dragonfly larvae having the prothorax removed giv e n o respons e t o externa l respirator y stimuli , fro m whic h observation h e concluded that th e prothoraci c ganglio n plays a n impor tant rol e in the respirator y regulation . A t an earlie r dat e H . Z . Ewing (1904) ha d show n tha t i n grasshopper s eac h ganglio n o f th e ventra l nerve cor d contains a respiratory cente r and wil l activat e th e breathin g movements o f its segmen t whe n the latte r i s remove d fro m th e res t of the body . The mor e recent work of Stahn (1928 ) o n the respiratio n o f Dixippus morosus an d Aeschna larva e an d o f Fraenke l (1932a ) o n Schistocerca confirm th e vie w tha t th e hea d contain s n o respirator y nerv e center ; but Stahn concludes that there must be distinguished i n the body ganglia primary an d secondary respiratory centers . Th e first lie in the abdomi nal, th e metathoracic , an d th e mesothoraci c ganglia , an d possibl y als o in th e prothoraci c ganglion ; the secon d is containe d i n th e prothoraci c ganglion. Th e primar y center s o f Dixippus, a s show n i n insect s wit h both hea d an d prothora x removed , ar e responsive onl y to larg e dosages of carbo n dioxid e (1 2 to 1 5 per cent ) i n th e inspire d ai r o r t o larg e decreases in the oxygen content (1 0 to 8 per cent or less). Th e secondary center o f th e prothoraci c ganglion , o n th e other hand , i s a cente r fo r finer adjustments, sinc e insects fro m whic h the prothora x ha s no t bee n removed ar e responsive t o muc h smaller increase s o f carbon dioxid e (u p to 12 or 15 per cent), and to much smaller decreases in the oxygen content of th e inspire d ai r (dow n to 1 0 or 8 per cent). I n Aeschna larvae, Stah n says, th e respirator y contro l is almost entirel y taken ove r by the secondary center s of the prothorax . Mechanism o f Respiration i n th e Tracheoles.—I n livin g insect s th e tracheoles ar e filled to a varyin g exten t fro m thei r dista l end s wit h a

460

PRINCIPLES OF INSECT MORPHOLOGY

liquid. Th e compositio n o f this liqui d i s unknown , but i t i s o f such a nature tha t i t ca n be absorbed throug h th e wall s of the tracheoles . I t has been shown by Wigglesworth (1930a, 1931) i n mosquito larva e and in certain othe r insect s that the amoun t o f liquid i n the tracheol e branches distributed t o a muscle is inversely affecte d b y the activit y o f the muscle (Fig. 239) . Fro m variou s experiments Wigglesworth concludes that th e absorption o f the liqui d from th e tracheole s is a direct result o f increased pressure resultin g fro m th e formatio n o f metabolite s surroundin g th e ends o f th e tracheoles , t o whic h th e tracheol e wall s ar e impermeable .

FIG. 239.—-Diagram s o f movemen t o f liqui d i n th e tracheoles . (From Wigglesworth, 1930a.) 1, trachea ; 2 , tracheol e cell ; 3 , part s o f tracheole s containin g air ; 4, part s containing liquid; 5 , muscle .

Following metabolic activit y in the muscles , o r supposedly i n an y othe r tissue, therefore , th e liqui d i s absorbe d fro m th e tracheoles , an d ai r extends toward their extremities, wher e it come s into closer proximity t o the cell s requiring oxidation (B) . By severa l experiment s Wiggleswort h sough t t o demonstrat e hi s theory. I n th e firs t place , it wa s found tha t asphyxiatio n o f mosquito larvae cause s a t firs t a violen t muscula r reaction , whic h is followed b y penetration o f ai r fro m th e trachea e int o th e tracheole s goin g t o th e muscles. Lacti c acid, Wigglesworth showed, is produced by the mosquito larva durin g asphyxiation. I n a second set of experiments the liquid was absorbed fro m th e tracheole s followin g infiltration s int o th e bod y of 1 0 and 5 per cen t solution s o f sodium chloride, and o f lactic aci d an d

THE RESPIRATORY SYST EM

461

potassium lactat e at differen t strengths , th e las t bein g effective dow n t o 2 per cent . Again , it wa s found tha t th e sam e effec t wa s produced by allowing the body liquid from one larva to diffuse int o a second. Finally , testing th e effec t o f gases , Wiggleswort h showe d tha t carbo n dioxid e and hydroge n caus e muscula r contractio n followe d b y extensio n o f ai r in the tracheoles, until the insect is narcotized. Treatmen t with oxygen then restore s activity , wit h a consequent further penetration o f the air , followed agai n b y a ris e o f the liqui d i n th e tubules . Poisonou s gase s have th e sam e effec t a s nonpoisonous gases, but "th e exten t t o which air move s dow n th e tracheole s depend s upo n th e degre e o f muscula r activity whic h precedes the deat h o f the insect. " Soo n after deat h th e liquid rises in the tracheoles . Fro m these experiments it seem s clear, as Wigglesworth contends , tha t th e absorptio n o f the oxygenate d liquid , and conversely the penetration of air into the tracheoles going to muscles, is caused by the metaboli c activity i n the muscles. The respiratory effect o f the movemen t of the liqui d in the tracheole s is that the ai r of the trachea e is quickly brought into closer relation wit h the cell s of a tissue as metabolism in the latte r creates a need for it (Fig . 239 B) . Presumably , unde r norma l conditions , oxyge n i s continuall y dissolved i n th e tracheol e liquid an d wit h th e latte r i s take n int o th e cells. I t i s possible that th e entir e oxyge n supply o f the tissue s enter s the latte r b y wa y o f the tracheoles ; bu t i t doe s not see m possible that all the carbo n dioxide produced can be absorbed into th e tracheoles , no matter ho w intimately the latter may cover the cell surfaces or penetrate the cel l bodies. GLOSSARY O F TERM S APPLIE D T O TH E RESPIRATOR Y SYSTE M Air Sac.—An enlargement of a tracheal tube, usually without taenidi a i n its walls. Amphipneustic.—With only the first pair and one or two pairs of posterior spiracle s open. Apneustic.—Without specifi c externa l breathin g organs , eithe r spiracle s o r gills ; the trachea l syste m usuall y absent o r rudimentary. Atrial Orifice.—Th e externa l openin g of the spiracula r atrium . (Porta atrii.) Atrium (Atr}. —The spiracula r chambe r forme d b y a secondar y invaginatio n o f the body wall external to the primar y tracheal orifice . Biforous Spiracles.—Spiracle s o f coleopterou s larva e havin g tw o pouche s of th e atrium originall y supposed to open separately t o the exterior . Blood Gills.—Hollow, nontracheated, usuall y filamentous respiratory evagination s of th e bod y wall or the proctodaeum . Branchia.—A gill, either a tracheal o r a blood gill . Branchiopneustic.—The spiracles functionally supplanted by gills . Closing Apparatu s o f a Spiracle.—Th e closin g mechanis m an d opening , formed eithe r o f the lip s o f the atriu m o r by a valve at th e inne r end of the atrium. (Verschlussapparat.)) Closing Band.—Th e movabl e valvular fol d o f th e inne r closin g mechanis m of a spiracle. ( Verschlussband.)

462 PRINCIPLES

O F INSECT MORPHOLOGY

Closing Bow.—The rigid but elasti c lip of the inne r closing mechanism of a spiracle opposite the valve . (Verschlussbiigel.) Diffusion Tracheae.—Cylindrica l trachea e havin g noncollapsibl e wall s (se e ventilation tracheae). Dilator Muscl e of a Spiracle (dlsp). —A muscl e serving to open either the externa l or the interna l closing apparatus o f the spiracula r atrium . Dorsal Trachea.—Th e dorsal segmental trachea originatin g at a spiracle. Dorsal Tracheal Trun k (DTra). —A longitudina l dorsa l trunk uniting the serie s of dorsal tracheae. External Respiration.—Th e process of transferring th e respirator y gase s through the body wall; taking place in insects through thin areas of the ectoderm, either at tji e body surface o r in the wall s of evaginations (gills ) or invaginations (tracheae). Filter Apparatus.—Finel y branchin g processe s o f th e atria l wal l o f som e spira cles, formin g ofte n two thic k but air-perviou s mat s jus t withi n the atria l orifice . ( Reusenapparat.) Gills.—Respiratory evagination s o f the bod y wall or th e proctodaeu m (se e blood gills, an d tracheal gills). Hemipneustic, o r Hypopneustic.—Wit h som e o f th e spiracle s functionall y sup pressed. Holopneustic.—Having the usua l (generall y 10) pair s of open spiracles. Hyperpneustic.—With supernumerary spiracles, as in the thora x of some Diplura. Internal Respiration.—The process of oxidation accompanyin g metabolism in th e cells of the bod y tissues . Lateral Trachea l Trun k (LTra). —The usua l longitudina l tracheal trun k o n each side of the bod y closely connected with the lateral spiracles. Metapneustic.—With only the las t pair o f spiracles open. Occlusor Muscl e o f a Spiracle (osp). —A muscl e serving to close either an outer or an inner closin g apparatus o f the spiracula r atrium. Peripneustic.—With none or only a few spiracles closed in each lateral series. Peritreme (Ptr). —A sclerit e of the bod y wall containing the spiracula r opening. Propneustic.—With only the firs t pai r of spiracles open. Respiration.—The entire series of physical and chemica l processes accomplishin g oxidation and th e remova l of carbon dioxide. Respiratory System.—Th e anatomica l adaptation s o f th e anima l tha t facilitat e external respiration . Spiracle (Sp). —A primar y trachea l orifice , o r th e secondar y atria l orific e an d structures (peritreme , atrium , closin g apparatus) usuall y associate d wit h th e latter . (Stigma.) Spiracular Trachea.—Th e short, usually unbranched trachea arisin g directly fro m the spiracle . Taenidia (tn). —The circula r or spiral thickenings of the inne r cuticular walls of the tracheae. Tracheae (Tra). —The breathin g tube s forme d a s multicellula r invagination s o f the ectoderm . Tracheal Commissures.—Transvers e trachea l trunk s continuou s fro m on e sid e of th e bod y to the other . Tracheal Gills.—Gills containing tracheae and tracheoles. Tracheal Orifice.—Th e primar y opening at th e poin t o f formation o f a trachea, whether exposed externally or concealed in a secondary atrial depression of the bod y wall. Tracheal System.—Th e part o f the respirator y syste m compose d of the trachea e and tracheoles.

THE RESPIRATORY SYSTEM 46

3

Tracheoles.—The minute end tube s o f the trachea l system , forme d withi n single cells o f the trachea l epithelium , an d usuall y branche d i n digitat e extension s o f th e matrix cells. Ventilation Tracheae.—Trachea e wit h collapsibl e walls , respondin g t o varyin g surrounding pressure . Ventral Trachea.—The ventral segmental trachea originatin g at a spiracle . Ventral Trachea l Trun k (VTra). —A longitudina l ventra l trachea l trun k unitin g the serie s of ventral tracheae . Visceral Trachea.—Th e media n segmenta l trache a originatin g a t a spiracle , branching to the alimentar y canal , the fat tissue, an d the reproductive organs. Visceral Trachea l Trun k (VsTra). —A longitudina l tracheal trun k closel y associ ated wit h the wall s of the alimentar y canal .

CHAPTER XV I THE NERVOU S SYSTEM An anima l i s a highl y organize d mas s o f matte r charge d wit h th e potentiality o f chemica l an d physica l activity , bu t it s laten t energ y tends alway s to remain in a state of equilibrium with surrounding conditions. Ther e must be , therefore, some stimulus for the releas e of energy to activat e th e moto r tissues , an d stimul i mus t b e i n al l case s change s in th e force s oppose d t o th e energ y o f livin g matter . I n th e lowe r animals i t seem s probabl e that effectiv e stimul i consis t onl y o f change s in th e impingin g energ y o f the environmen t o r o f physical o r chemica l changes within the bod y of the animal . Th e reaso n for animal activit y is th e necessit y o f th e animal' s makin g advantageou s adjustment s t o changes i n it s immediat e surroundings. I f a n individua l i s t o liv e successfully, therefore , it must b e provided with a responsive mechanis m by whic h its behavio r wil l be brought int o harmon y wit h change s in it s environment. An anima l is formed o f chemical compounds of many differen t kinds , but mos t o f it s componen t substance s ma y b e classe d i n tw o groups . Those o f one group are stable compounds , the alteratio n o f which results only in damage to the organism. Thes e substances form the integument , the skeleton , th e connectiv e tissue , an d th e supportin g framewor k o f muscles, glands , an d cells . Substance s o f th e othe r grou p ar e labil e compounds, highl y unstabl e i n thei r molecula r structure , an d som e of them liabl e t o sudde n disruptio n o n sufficien t increas e o f stimulus. I t is these substance s tha t caus e the activitie s o f the animal . The y occu r principally i n th e secretin g cell s of glandula r tissues , i n th e contractil e tissue o f muscles , an d i n th e receptiv e an d conductiv e part s o f nerv e tissue. Since th e stimul i fo r actio n i n th e labil e tissue s o f the anima l com e primarily fro m th e environment , ther e mus t b e som e provisio n fo r transmitting thei r effec t fro m the stabl e peripher y of the anima l to the internal tissue s i n whic h is stored th e laten t energ y that make s actio n possible. Moreover , since the norma l activities o f a livin g creatur e ar e advantageous t o th e organism , ther e mus t b e als o som e provisio n fo r controlling an d directin g th e result s o f th e liberate d energy . Bot h of these requirements are furnished b y the nervous system, though directiv e 464

THE NERVOUS SYSTEM 46

5

movement i s partl y th e resul t o f th e mechanica l constructio n o f th e motor mechanism. The environmenta l stimuli include electromagnetic changes, chemical changes, changes in the rat e or kind of molecular motion, an d change s in the degre e o f pressur e exerte d b y materia l masse s touchin g upo n th e animal's body . Th e externa l stimuli , however , ar e no t transmitte d i n kind t o th e interio r o f the body ; al l forms o f energy in nature probabl y have on e effec t o n the periphera l receptiv e cell s of the nervou s system , which i s a n alteratio n i n th e rat e o f metaboli c activit y i n th e latter . This induced change in the rate of metabolism is then propagated throug h the nerve fibers to the tissues a t their inne r terminals. Wha t th e animal "feels" o r "does " i n respons e depend s upo n th e organizatio n o f th e central nervous system and the moto r mechanism . The concep t o f environmen t mus t b e broadl y understood . No t only does the anima l a s a whole have its environment , bu t eac h interna l tissue an d ever y individua l cel l o f th e bod y ha s it s ow n environmen t created b y th e condition s immediatel y surroundin g it. Stimuli , there fore, aris e both i n the externa l environmen t an d in the interna l environ ment. Th e distributio n o f internal stimul i ma y b e accomplished eithe r by a n inne r elaboratio n o f the nervou s syste m o r b y th e productio n of substances, know n as hormones, whic h ar e broadcas t i n th e circulatin g medium, an d which initiate activities i n certain tissue s o r organs remote from thei r source . Littl e is known of the presenc e or effec t o f hormones in insects . Th e insec t nervou s system , o n th e othe r hand , i s highl y developed, and it s structur e is now well understood in many respects . 1. GENERAL STRUCTURE , ORGANIZATION , AN D FUNCTIO N O F TH E NERVOUS SYSTE M

Anything tha t produce s a chang e i n th e metaboli c activit y o f th e labile constituent s o f livin g matte r i s calle d a stimulus. Th e qualit y of livin g matter that makes it responsiv e to stimul i is termed sensitivity. The effec t o f the stimulu s o n the tissue , however , does not en d with th e first impact; it is transmitted from molecule to molecule, and this property of progressiv e reactio n t o stimul i i s know n a s conductivity. Sinc e i t appears tha t reactio n t o a stimulu s i n al l cases involve s a destructiv e chemical action i n the labil e constituent s o f the tissu e involved , a period of recovery i s necessary for the restoratio n o f the discompose d substances to thei r origina l form . Fatigue arise s whe n stimul i follo w i n to o rapi d succession to allo w of complete recovery. The propertie s o f sensitivity an d conductivit y ar e presumabl y com mon to all protoplasm i n some degree. The y are highly developed in the protoplasm o f nerv e tissue ; the y constitute , i n fact , th e fundamenta l qualities o f nerve tissue upon which the function s of the nervou s system

466

PRINCIPLES OF

INSECT MORPHOLOGY

depend. Sinc e bot h ar e automati c processes , however , the y mus t b e controlled. Th e end s of nerves exposed to stimuli, therefore, are usually guarded b y specia l receptiv e en d organs , th e structur e o f which exert s a selectiv e limitatio n o n the effectiv e stimuli . Conductivit y i s effectiv e also onl y withi n certai n limits . Whe n th e stimulu s receive d fro m a n end orga n reache s a certai n intensity , reactio n i n the nerv e take s plac e at maximu m strength. On the othe r hand , whe n the stimulu s becomes too strong, th e reactio n again may cease. Development an d Organizatio n o f the Nervou s System.—It is a most interesting fact that the entire nervous system, a s we learned in Chap. II ,

FIG. 240.—Evolution o f motor and sensor y neurones and th e nerv e synapse, diagrammatic. A , a primitiv e sens e cell (SCI') i n th e ectoder m transmittin g stimul i directly to a muscl e (Mel). B , th e primitiv e sense cel l separate d fro m th e bod y wal l become s a motor cel l (MCI) an d mus t no w b e stimulate d fro m a secon d sensor y cell (SCI") i n th e ectoderm, thu s differentiatin g th e moto r neurone fro m th e sensor y neurone, whic h communicate through a synapse (Syri). C, a third, or association, neurone (ACl) interposed in tne synapse, allows more than one motor neurone (D) to be stimulated through a single sensory neurone, an d facilitates long distance transmission (E) .

is derived fro m th e ectoder m (Fig . 17) . W e can imagine that the primi tive longitudina l nerv e cord s o f annelid s an d arthropod s wer e highl y sensitive ventrolatera l tract s o f th e bod y wal l fro m whic h nerves wer e given off to the interna l organ s (Figs. 1 5 A, 240 A). I n this case stimula tion wa s direct fro m th e exterio r t o the interior . Differentiation of Motor and Sensory Nerves.—With progressive development, the primitive sensory cells of the ectoderm became detached internally fro m th e integumentar y cell s an d finall y cam e t o li e withi n the bod y cavit y wher e the y for m fre e strand s o f nerv e tissue . Th e primary nerve cells (Fig. 240 A, SClf)y thus cut off from direct contact with externa l stimul i (B , /SCZ') , mus t no w b e stimulate d indirectl y through a secon d se t o f sensory cell s (SCI"), which , on th e on e hand ,

??? ??????? ?????? ??? retain connection s wit h th e exterio r and , o n th e other , establis h com munication b y mean s o f nerv e fiber s wit h th e cell s o f th e firs t order . Thus a mor e highl y organize d nervous syste m i s evolved , consistin g of afferent, sensory nerves'(SNv) proceedin g inward from sensor y cell s on th e ???? ???? ?????? ?? ??? ?????? ????? ?????? ?? ??? ????? ???? ??? ?? ???????? ????????? ????? ?????? ?????? ????? ???? ??? ?????? ?? ??? ??????? ??????? of the body. Th e localized parts of the nervou s tracts where the sensor y ??? ????? ?????? ??????????? ?? ??? ??????? ??????? ??????? ??? incoming sensor y nerve s an d th e outgoin g moto r nerve s constitut e th e peripheral nervous system. ??? ?????????? ????? ????? ?? ?????? ? ???????????? ?? ? ????? cell, or neurocyte (Fig. 241, NCI). The neurocyte and all its branches

???? ???????????? ?? ? ???????? ?? ????????? ?? ??? ???? ????? ???? ????? ?? neurite; b, c, terminal arborizations; Col, collateral branch of the axon ; NCI, th e cel l body, or neurocyte.

constitute a neurone. Usuall y there is one principal branch, th e neurite, or axon (Axn), whic h is the nerv e fiber of the neurone . A lateral branch of th e axon , generally given off near th e neurocyte , is termed a collateral (Col). Bot h th e axo n and th e collatera l en d i n terminal arborizations of fine branching fibril s (6 , c). Simila r branchin g fibrils springing directl y from th e neurocyt e ar e distinguishe d a s dendrons, o r dendrites (a). I f a nerve cel l ha s bu t on e mai n nerv e proceedin g fro m it , i t i s sai d t o be unipolar; if two, it is bipolar; and if it has more than two nerve processes, it i s multipolar . Th e moto r nerv e cell s ar e typicall y unipola r (Fig . 242 A). Sensor y cells are either bipolar (B) , or multipolar (C) , according as the y hav e one distal process (d ) or several . Nerve Trunks. —The ordinar y nerves o f the bod y ar e usuall y bundle s of nerv e fibers . Som e nerve trunks contai n onl y motor fibers, and som e contain only sensory fibers, but i n most case s the tw o kinds o f fibers are contained i n the same bundle. Th e main branches of a nerve are smaller bundles o f fiber s separate d of f from th e mai n trunk , bu t th e termina l branches consis t o f singl e fibers . A nerv e trun k an d it s branches , including th e termina l fibers , ar e surrounde d b y a nucleate d sheath , termed th e neurilemma.

468

PRINCIPLES OF INSECT MORPHOLOGY

Ganglia.^An aggregatio n o f neurocyte s constitute s a ganglion ; bu t a ganglio n usually contain s als o th e collateral s o f the moto r nerves , th e terminal arborizations o f the sensor y axons , and generall y another grou p of cell s the branche s of which make connections between the sensor y an d motor neurones . Th e ganglion cells are mostly situate d at the peripher y of th e ganglio n (Fig . 25 9 A, GngCls). Th e centra l par t o f the ganglio n is occupied by a dense mass o f nerve fibrils, which constitutes th e neuropile (Npl), o r medullary tissue. Cluster s o f terminal fibril s forming small bodies i n th e neuropil e ar e terme d glomeruli. I n additio n t o th e tru e nerve cells , a ganglio n usuall y contain s als o othe r cell s o f ectoderma l origin whic h for m a supportin g tissue . Thes e cells , distinguishe d a s glia cells , hav e irregula r shape s an d ar e generall y profusel y branche d

FIG. 242.—Types o f neurones , diagrammatic . A , moto r neuron e wit h muscl e terminals. B , sensor y neuron e o f Typ e I , wit h unbranche d termina l proces s (d) goin g to a sens e organ . C , sensor y neuron e o f Typ e II , wit h branche d dista l processe s dis tributed o n various tissues.

among the elements of the true nerve tissue. Th e ganglion is surrounded by a nucleate d neurilemm a (Nlm) continuou s wit h tha t o f th e nerve s that issu e fro m it . The Synapse. —The centra l mechanis m b y whic h a moto r neuron e receives a n impuls e fro m a sensor y neuron e i s no t a direc t connectio n between the two systems. Th e motor and sensory neurones are separate in thei r origin , an d the y neve r unite . Communicatio n i s establishe d between the termina l arborizations of the sensor y axon and arborization s of a collatera l branc h o f the moto r axon , eithe r b y a direc t associatio n of th e fibril s (Fig . 24 0 B ) o r throug h a n intermediate , o r association, neurone (C , D , ACl). Th e interlockin g o f the termina l fibril s fro m th e communicating neurones is called a synapse (Syn). The Association, or Internuncial, Neurones. —Ordinarily, i n th e mor e highly organize d animals, the nerve synapses are not establishe d directl y between sensory neurones and motor neurones but, a s just noted, by way of a thir d elemen t i n th e nervou s syste m know n a s a n association, or internuncial, neurone (Fig . 24 0 C, D, E , ACl). Th e axo n and collatera l of a n associatio n neuron e make a two-way connection between neurones

THE NERVOU S SYSTEM

469

of th e sensor y an d th e moto r systems . Th e associatio n neurone s ma y thus giv e communicatio n betwee n neighborin g (C , D ) o r fa r distan t neurones (E) . Moreover , th e introductio n o f th e associatio n neuron e into the synapti c mechanism makes possible also a multiple internuncia l system, inasmuc h a s arborization s o f severa l o r man y differen t nerve s may be intermingled (D) ; and, finally, there may be a series of association neurones interpose d betwee n th e receptiv e neuron e an d th e moto r mechanism. Thu s the stimulu s from a single receptor may have a widespread effec t o n th e moto r system . Th e associatio n neurone s ar e con tained entirel y withi n th e gangli a an d th e ganglioni c commissures and connectives, bu t som e o f the m exten d lon g distance s i n th e centra l nervous system . The neurocytes of the associatio n neurone s of insects are mostly large, plasmatic cell s wit h larg e nucle i containin g bu t a smal l amoun t o f chromatin. Amon g them , however , ther e ar e certai n cells , occurrin g principally in the brain, which are sharply differentiated fro m th e general type o f associatio n cell s b y thei r smal l siz e an d roun d nucle i ric h i n chromatin. Associatio n cell s o f thi s typ e ar e distinguishe d a s globuli cells. Th e axon s o f globul i cell s ar e confine d withi n definitel y limite d regions. The Centra l Nervou s System.—Th e centra l station s o f the nervou s system ar e functionall y th e part s o f th e nervou s tract s containin g th e synapses between the sensory nerves and the motor nerves. I n the higher animals, a s we have seen , they ar e located i n the ganglia , which contain also the cel l bodies of the motor and association neurones. Anatomicall y the nerv e center s o f insect s ma y b e classe d a s somati c an d visceral . The somati c centra l system include s the comple x nervous mass called the brain (Fig . 246 , Br), lyin g i n th e hea d abov e th e stomodaeum , an d a chai n o f connected ventral segmenta l ganglia formin g th e ventral nerve cord (VNC), lyin g beneat h th e alimentar y canal , extendin g fro m th e mouth t o th e posterio r par t o f the body . Thi s i s the principa l centra l system o f al l arthropod s an d i s generall y terme d th e central nervous system. Th e viscera l gangli a ar e locate d o n th e dorsa l wal l o f th e stomodaeum and , togethe r wit h thei r periphera l nerves , constitut e th e stomodaeal nervous system, ofte n calle d th e "stomatogastric, " o r sym pathetic," system. Th e distributio n o f the nerve s fro m th e tw o centra l systems, however , does not correspon d entirel y wit h th e locatio n o f th e ganglia; som e o f th e nerve s o f the alimentar y cana l hav e thei r center s in the gangli a o f the ventra l nerve cord, and some of the somati c muscles may be innervated fro m th e stomodaea l ganglia . The Periphera l Nervou s System.—Th e periphera l nervou s syste m consists o f the axon s of the moto r neurones , the cel l bodies of which ar e contained i n th e centra l ganglia , an d o f th e axons , cel l bodies , an d

?? ?????????? ?? ?????? ?????????? terminal processes of the sensory neurones. Th e motor axons go outward to muscle s and glands. Th e sensory axons proceed inward from periph erally locate d neurocytes , whic h ar e alway s bipola r o r multipola r cells that maintai n a connection b y mean s of their dista l processes wit h surfaces fro m whic h sensory stimul i ar e received. In th e Arthropod a ther e ar e tw o distinct set s o f sensory neurocytes , distinguished as sensory cells of Type I, and sensory cells of Type II. Those o f the firs t typ e (Fig . 24 2 B, SCl-I) ar e alway s bipolar cell s lying either withi n o r jus t beneat h th e epidermi s o f th e bod y wal l (BW) o r the epitheliu m of the ectoderma l parts of the alimentar y canal , and thei r distal processes (d) are usually connected with specific ectodermal sense ?????? ????? ??? ????? ?? ??? ?????? ???? ??? ??????? ??? ?????? ??????? or multipolar. The y lie on the inne r surface o f the bod y wall and o n the wall o f th e alimentar y canal , an d thei r dista l processe s (d ) g o t o th e epidermis, th e connectiv e tissue , th e somati c an d splanchni c muscles , and the alimentar y epithelium . The Sens e Organs.— A sens e orga n i s a structur e designe d fo r th e reception o f a specifi c kin d o f stimulu s an d fo r th e transmissio n o f th e effect o f th e stimulu s t o a sensor y nerve . Sens e organ s ar e locate d either externall y o r internall y an d accordingl y ar e terme d exteroceptors or interoceptors. Th e interoceptors , however , includ e organ s tha t ar e morphologically externa l sinc e som e o f the m ma y b e situate d i n th e alimentary canal . Tru e interna l sens e organ s tha t li e within th e bod y cavity an d respon d t o interna l condition s o f th e organis m ar e distin guished, therefore , a s proprioceptors. N o specifi c proprioceptors ar e known to occu r in insects, the dista l processes of sensory cells of Type I I having always free endings on the tissues they innervate. Exteroceptors , on the othe r hand, are abundant o n many parts of the bod y and append ages, an d i n th e ectoderma l section s o f th e alimentar y canal . Th e sense organ s ar e usuall y comple x structures (Fig . 266 ) i n whic h ther e are associated with a sensory cell of Type I (SCI), or a group of such cells, on e o r mor e accessor y ectoderma l cells , an d usuall y a specia l modification o r developmen t o f th e coverin g cuticula . Th e specifi c structure o f th e externa l par t o f a sens e orga n i s presumabl y i n eac h case suc h a s to exclud e all but a certain kin d o f stimulus. Animal Behavior.—Whatever an anima l doe s in a state of nature, o r the way it act s under experimental conditions, zoologist s call its behavior. There appea r t o b e tw o type s o f behavior. I n one , the action s o f th e animal ar e direc t response s t o stimul i receive d fro m th e outsid e o r t o the stimul i o f physiologica l condition s o r hormone s withi n it ; i n th e other, consciousnes s seems to be an activating forc e not directl y dependent o n externa l conditions . Whethe r consciou s contro l o f nervou s activity is real or only apparent nee d not b e discussed in a work on mor-

THE NERVOUS SYSTE M

471

phology; bu t mechanisti c actio n evidentl y depend s o n anatomica l structure. The Mechanistic Theory o f Behavior. —Since th e activitie s o f th e motor mechanis m o f a n anima l depen d entirel y upo n stimul i receive d from a nerve, it is clear that the animal cannot make any action for which there i s n o nervou s mechanism . Th e immediat e sourc e of al l activity , therefore, i s at th e receptiv e ends of the moto r nerves. Th e problem to be solved, then, is how the moto r nerves are stimulated i n such a way as to produc e coordinated and apparentl y purposefu l action . The answer is that, if there is no internal sourc e of motor stimulation, the stimulatio n must b e received from th e sensory nerves, and that it is the organization of th e sensor y an d associatio n nerv e tracts tha t determine s th e specifi c activity of the motor tracts. Her e it must be understood that inhibition as wel l a s incremen t o f activit y play s a n importan t part . Henc e th e reactions of an animal to an external stimulus or set of stimuli may depend entirely on the organization of its nervous and motor mechanism. Simpl e reactions o f this kin d ar e reflexes; mor e complicate d performances that result i n specifi c orientatio n t o externa l condition s ar e calle d tropisms. Some instincts ar e mer e tropisms , bu t i n man y case s the y involv e sequences o f reaction s an d stimul i tha t mus t follo w i n regula r order . The operatio n o f a n instinc t ma y thu s tak e o n a mysteriou s aspect , but sequenc e o f actio n i s no w highl y develope d b y inventor s o f electrical an d othe r mechanica l apparatus , an d i t i s entirel y con ceivable that th e resul t o f on e physiologica l reactio n ma y giv e th e stimulus tha t allow s th e nex t t o b e operative . Whil e it i s not known what par t physiologica l condition s an d hormone s may play in initiating instinctive act s i n insects , i t woul d see m that suc h influences als o must be present. If a n anima l exhibit s a n abilit y t o "learn, " that is , to giv e a certai n reaction t o a give n set o f stimuli mor e readily afte r a number of repetitions, it is supposed that some primary resistance in the synapse has been broken down , an d tha t th e conductio n o f th e stimulu s ove r th e sam e tract i n th e centra l syste m become s smoothe r an d finall y automatic . "Learning" is usually connected with the associatio n of an unaccustomed stimulus wit h a commo n stimulus t o whic h there i s an establishe d reac tion, a s i n th e associatio n o f a color , a sound , o r som e mechanical contrivance wit h th e norma l foo d o f a n animal . Th e newl y acquire d habit o f reactin g t o th e secondar y o r circumstantia l stimulu s i s calle d associative memory , an d it s correspondin g actio n i n th e nervou s syste m is termed a conditioned reflex. I t seem s certain tha t suc h reflexe s mus t be conditione d throug h th e associatio n neurone s of the centra l nervou s mechanism, fo r i t i s the interventio n o f these neurone s between several sensory tracts , o n the on e hand, and the motor tracts, o n the other , that

472 PRINCIPLES

O F INSECT MORPHOLOGY

gives th e possibilit y o f combinin g two o r mor e sensor y impulse s i n a common motor reaction. Consciousness.—The propert y o f awarenes s is , o f course , positivel y known to exist only in ourselves. I n its simplest manifestation consciousness i s a translation o f the form s o f energy existing in th e environmen t into psychic equivalents . Red , fo r example , i s a for m o f consciousness corresponding t o electromagneti c "waves " o f a certai n length ; soun d is anothe r for m o f consciousnes s produced b y molecula r vibration , an d so on for everything else that is consciously perceived. Whethe r insects possess consciousnes s o r not i s a subject not worth discussing , sinc e th e fact canno t b e known. Th e reactions o f insects to stimul i tha t generate specific form s o f consciousness in us may be, an d i n most cases probabl y are, entirel y automati c an d unaccompanie d b y an y psychi c equivalen t of th e stimulus . However , som e insects ar e capabl e of developing conditioned reflexes , an d the conditione d reflex would seem to be the closes t physiological approac h t o reason . Th e studen t o f insec t behavior , however, must b e content t o record the observed reactions of his subjects to stimuli , withou t attributin g t o the m th e sensation s arouse d i n himself b y the sam e stimuli. 2. TH E CENTRA L NERVOUS SYSTE M

In th e evolutio n o f th e annelid-arthropo d nervou s system i t woul d appear tha t the firs t centralize d group of nerve cells had it s origi n in th e ectoderm a t th e anterio r pol e of the body , forming her e a small ganglion associated wit h a sensor y apical plate. Late r ther e appeare d severa l paired group s of sensory cell s at th e base s of tentacular o r other sensory organs behin d th e apica l plate . The n thes e variou s primar y nerv e centers united i n a single ganglionic mass, which is the so-calle d archicerebrum o f th e anneli d worm s (Fig . 1 6 A, Arc). Thi s primitiv e "brain " lies abov e th e anterio r en d o f th e alimentar y tract , where , in som e of the Annelida , i t i s still no t detache d fro m th e ectoder m (Fig . 24 3 A). Finally, tw o lateroventra l nerv e strands , consistin g o f nerv e cell s an d fibers, ar e develope d fro m th e ectoder m alon g th e entir e lengt h o f th e body (VC) an d ar e connected anteriorly wit h th e archicerebrum . Thu s is establishe d th e definitiv e nervou s system , consistin g i n it s simples t form o f a suprastomodaea l brain , whic h originate s i n th e prostomium , though subsequentl y i t ma y b e displace d posteriorly , an d o f paire d ventral nerve strands formed i n the postora l part o f the trunk . Following segmentatio n o f th e body , th e neurocyte s o f th e nerv e strands becom e aggregate d i n th e segments , producin g segmentall y arranged, ganglia. I n this stage (Fig . 16 B), therefore, the central nervous system consist s o f an anterio r media n brai n (archicerebrum ) located i n the hea d abov e th e alimentar y tract , an d o f tw o long , ganglionate d

??? ??????? ????????? nerve cord s situate d laterall y i n th e ventra l par t o f the body, i n which the gangli a (Gng) ar e unite d lengthwis e B y interganglioni c connectives of nerv e fiber s (Con), an d crosswis e by transvers e fibrou s commissures (Com). Finally , in most o f the Annelid a and al l Arthropoda, th e latera l cords hav e approache d eac h othe r (C), and th e pai r of ganglia in each segment have united to form a compound median ganglion, though the connectives i n mos t case s remai n double. Various stages i n th e evolutio n of the ventral nervous system, fro m a conditio n i n whic h th e latera l strands are widely separated t o one in whic h the y ar e unite d i n a FIG. 243.—Example s o f brai n structur e median cord , are wel l shown in th e ?? ????????? ?? ???????? ??????? ?? ????? o f Eunice punctata, showin g con Annelida. In some primitive forms , tomium tinuity o f brain an d epidermis . B , brai n of moreover, th e nerv e tissu e i s no t Eunice punctata, dorsal view. C , transverse o f brai n o f Podarke obscura. D , entirely detache d fro m th e ecto - section same o f Nereis pelagica, showin g positio n of derm. I n th e Onychophor a th e ??????? ???????????? ??? ? ???? ??????? 1925; C from Hanstrom, 1927 ; D from micronerve strands are widely separated, photograph b y Hanstrom, 1928.) lying above the bases of the legs, and they sho w scarcely mor e than a beginnin g of ganglionic differentiation. GENERAL STRUCTUR E O F TH E CENTRA L NERVOU S SYSTE M O F INSECTS The centra l nervou s syste m o f insect s consist s o f a mas s o f nerv e tissue, calle d the brain, lying above the anterio r en d of the stomodaeum , and o f a ventral nerve cord, compose d o f media n segmenta l gangli a an d paired connectives , lyin g beneath th e alimentar y canal . Th e two parts are joined by connective s embracing the stomodaeum . The brai n i s a composite structure, bu t ther e is a difference o f opinion as t o ho w many primar y segmenta l gangli a ente r int o it s composition . A distinc t prostomia l ganglio n i s no t eviden t i n th e ontogen y o f th e arthropod nervou s system , bu t th e interna l structur e o f the adul t brai n demonstrates, a s wil l late r b e shown , that a larg e par t o f the cerebra l mass, fro m whic h the opti c lobes of the compoun d eyes take their origin , is identical wit h a corresponding part o f the brai n i n the anneli d worms. This first part of the definitiv e arthropod brain is known as the protocere???? ????? ??? ?? ????? ?? ??? ????????? ?????????? ??? ??????????? appendages (Fig . 70 , Prnf), th e nerv e center s o f thes e appendage s la y immediately behin d th e opti c center s an d ar e include d i n th e proto cerebrum. Th e nerv e center s o f the firs t antenna e constitut e a distinc t

474 PRINCIPLES

O F INSECT MORPHOLOGY

second sectio n o f the brai n calle d the deutocerebrum (Fig . 245 C , 2Br). In al l insect s an d i n mos t crustacean s th e nerv e center s o f th e secon d antennae form a third part of the brain, or tritocerebrum (3Br). The tritocerebral lobes , however , ar e unite d b y a commissur e (Com/ ) tha t always lies beneath th e stomodaeum , and i n some Crustacea th e secon d antennal gangli a themselve s ar e no t containe d i n th e suprastomodaea l brain. Th e gangli a o f th e secon d antennal segment , therefore , ar e without questio n th e firs t gangli a o f th e primitiv e ventra l nerv e cor d (Fig. 24 4 A, B , 2AntGng), an d th e primitiv e arthropo d brai n include d only the gangli a contained in the protocerebru m and deutocerebrum.

FIG. 244.—Diagram s illustratin g two theorie s o f th e segmentatio n of th e arthropo d head an d th e compositio n o f th e brain . A , th e primitiv e hea d regio n a s commonl y supposed to include the prostomiu m (Prsf) an d si x somites (I-VI); th e brai n formed o f the prostomial archicerebru m (Arc) an d preantenna l an d firs t antenna l ganglia , wit h th e second antenna l gangli a added later . B , th e hea d region supposed to includ e th e pros tomium an d onl y four postora l somites ; the brai n forme d fro m th e archicerebru m differentiated int o protocerebru m (IBr) an d deutocerebru m (2Br), wit h th e firs t postoral , o r second antennal , ganglia finally added to for m th e tritocerebrum.

If th e preantenna l an d firs t antenna l appendage s represen t tru e somites of the body , the n th e suprastomodaea l brai n mas s include s th e highly develope d archicerebru m (Fig . 244 A , Arc) of th e prostomium , rudimentary preantennal ganglia (PrntGng) of the first somite (7), and antennal ganglia (lAntGng) of the second somite (//). It seems somewhat incongruou s tha t tw o pair s o f segmental gangli a shoul d li e above the stomodaeu m an d have no substomodaeal connectives. A more simpl e concep t o f th e brai n structure , an d incidentall y o f the procephali c segmentation, i s presented b y N . Holmgre n (1916 ) an d by Hanstro m (1927 , 1928a , 1930) , accordin g to whic h the entir e supra stomodaeal part of the brain is derived from the prostomial archicerebrum (Fig. 24 4 B, Arc) , whil e the secon d antennal center s ar e assume d t o b e the ganglia (2AntGng) of the first true somite (I). As a corollary to this view the preantenna e an d first antennae becom e appendicular structure s analogous to the prostomial tentacles of the Annelida , and the protocere bral and deutocerebral divisions of the brain (IBr, 2Br) ar e specializations of th e ocula r an d antenna l center s o f the archicerebrum . Th e theor y disregards th e evidenc e of metameris m i n th e postocula r regio n o f th e procephalon base d o n th e presenc e o f paire d cavitie s i n th e mesoderm ,

THE NERVOUS SYSTEM

475

which ar e usuall y regarde d a s coelomi c sac s o f preantenna l an d firs t antennal somite s (Wiesmann , 1926) . Accordin g t o th e othe r inter pretation, these apparent somites ar e "secondary segments." The foregoing theory, especially as elaborated by Hanstrom, has much to commen d it. Th e anneli d brai n i s often highl y evolve d and ma y b e differentiated int o severa l distinc t part s (Fig . 24 3 B , /& , mb, lib) cor responding t o th e center s of its principal sensory nerves; it contains welldeveloped corpor a pedunculat a (C , D , Cpd). Th e primar y arthropo d brain, therefore , ma y b e suppose d likewis e t o hav e bee n secondaril y differentiated int o protocerebra l an d deutocerebra l region s a s a resul t of th e specializatio n of the ocula r and antenna l centers . Th e protocere brum contain s corpor a pedunculata identica l i n structur e wit h thos e of

FIG. 245.—Diagram s illustrating the theoretica l evolution of the insec t brain from th e archicerebrum an d th e firs t postora l ganglia , an d th e formatio n o f th e suboesophagea l ganglion fro m th e gangli a of the gnatha l segments. Ant, antenna; Arc, archicerebrum; Br, definitive brain; IBr, protocerebrum; 2Br, deutocerebrum; SBr, tritocerebrum; CoeCon', CoeCon", primitiv e an d definitiv e circumoesophagea l connectives ; Com/ , commissur e of ganglia of first somite ; FrGng, fronta l ganglion ; Gnc, gnathocephalon ; GngI, ganglion of first somite ; OpL, optic lobe; Pnt, postantenna (secon d antenna) ; Pro, procephalon; Prst, prostomium; SoeGng, suboesophageal ganglion ; Stom, stomodaeuni.

the annelids . Th e procephali c par t o f th e arthropo d hea d (Fig . 24 5 A, Prc) is formed i n th e embry o of the externall y unsegmente d cephalic lobes, whic h evidently represen t principall y th e prostomiu m (Prst), bu t it usuall y include s als o th e reduce d tritocerebra l segmen t (7) . Th e tritocerebral, o r secon d antennal , nerv e centers , however , ar e actuall y the firs t gangli a o f th e ventra l nerv e cor d (A , GngI), thoug h i n mos t arthropods they are secondarily added to the primary cerebrum (C, SBr). They innervat e th e secon d antennae , th e regio n o f the mouth , an d th e preoral par t o f the prostomiu m and are united b y connective s with bot h the brai n an d th e stomodaea l nervou s syste m (Fig . 24 4 B, 2AntGng). With the addition o f the tritocerebral ganglia to the arthropo d brain , th e primitive brai n connective s (Fig . 24 5 A , CoeCon') ar e shortene d (B ) and finall y suppresse d (C) . Th e definitiv e connectives (C, CoeCon") ar e those betwee n th e tritocerebra l gangli a an d th e firs t gangli a o f th e gnathal region.

476

PRINCIPLES OF INSECT MORPHOLOGY

The nex t thre e gangli a o f th e ventra l nerv e cor d ar e thos e o f th e segments tha t becom e the gnatha l regio n o f the insec t hea d (Fig . 24 5 A, Gnc). Thes e ganglia are always united with one another in the matur e insect (C ) to for m a second composite nerve mass of the head , known a s

FIG. 246.—Ventra l nervous syste m an d brain o f a caterpillar , Malacosoma americana.

FIG. 247.—Ventra l nervou s syste m and brai n o f a grasshopper , Dissosteira Carolina.

the suboesophageal ganglion (SoeGng) becaus e i t lie s i n th e ventra l par t of th e hea d beneat h th e stomodaeu m (Fig . 249) . Th e principa l nerve s of thi s ganglio n ar e thos e o f th e mandibular , maxillary , an d labia l appendages. The thoraci c regio n o f th e insec t bod y contain s thre e primitiv e median gangli a correspondin g t o th e thre e thoraci c segments . Usuall y these ganglia remain distinct (Fig . 246) , but frequentl y th e mesothoracic and metathoraci c gangli a ar e united , an d th e definitiv e ganglio n of th e metathorax may includ e on e or more primitive abdomina l ganglia (Fig . 247, Gng3 +/ + // + 777) .

THE NERVOUS SYSTEM 47

7

In th e insec t abdome n ther e ar e a t mos t eigh t definitiv e segmenta l ganglia, correspondin g t o th e firs t eigh t abdomina l somite s (Fig . 246) ; but th e las t i s alway s a composit e ganglion , sinc e i t innervate s th e eighth an d succeedin g segments . Th e nerv e cor d o f th e abdomen , however, is often variousl y shortene d b y th e unio n of two o r more of th e posterior ganglia, and the ganglia are subject to a displacement anteriorly, so that a ganglion belonging to some particular segmen t may actuall y li e in a more anterior segmen t (Fig . 247 , GnglV, GngV). Th e nerve s fro m each ganglion , however , consistentl y g o t o th e segmen t i n whic h th e ganglion ha d it s origin . Hence , morphologically , a ganglio n shoul d b e numbered accordin g to th e segmen t i t innervates , an d th e distributio n of th e nerve s from a composit e ganglion is usually th e bes t index * of th e composition of such a ganglion. All th e gangli a o f the ventra l nerv e cor d hav e a tendenc y t o unit e with each other in various combinations in different insects . A n extreme condensation i s attaine d i n th e larva e o f cyclorrhaphou s Diptera , i n which th e entir e ventra l nerv e cord , includin g th e suboesophagea l ganglion, is consolidated into an elongate mass of nerve tissue, from which the entir e body is innervated. THE BRAI N AN D IT S NERVE S The insec t brai n i s principall y a cente r o f associatio n betwee n th e major sens e organ s locate d o n th e hea d an d th e moto r neurone s of th e gnathal, thoracic , an d abdomina l regions of the body . Mos t o f its bul k consists of a mass of neuropile tissue; but withi n this mass are contained , on th e on e hand , th e root s o f the nerve s fro m th e compoun d eyes , th e ocelli, an d th e variou s sens e organ s o f th e antenna e an d th e preora l cavity, and , on the other , th e anterio r terminal s o f nerve tracts from th e suboesophageal ganglio n an d th e gangli a o f th e ventra l nerv e cor d i n the thora x an d abdomen . Th e brain , therefore , i s necessar y fo r th e initiation o f al l activitie s tha t ar e normall y stimulate d throug h th e cephalic sense organs. I t take s n o part in the regulatio n o f such activities, whic h are directly controlle d from th e center s of the suboesophagea l and body ganglia. Henc e a decapitated insec t ma y be said t o b e incap able of "voluntary" action; but it s vital functions continue in operatio n as lon g a s it s bod y tissue s remai n alive , and , i f artificiall y stimulate d through somati c receptors , man y o f the moto r mechanism s ca n b e se t into normal activity. Th e insect brain contains,but few motor neurones . General Structur e o f the Brain.—I n externa l for m th e brai n varie s much i n differen t insects , bu t i t alway s show s a differentiatio n int o three successive parts, whic h are distinct a t leas t i n its internal organization an d ar e usually more or less apparent i n the externa l contou r of th e

478

PRINCIPLES OF INSECT MORPHOLOGY

adult brai n a s three pair s o f lateral swellings , or lobes (Fig . 248) . Th e first and largest part is the forebrain, or protocerebrum (IBr), the second is the midbrain , o r deutocerebrum (2Br), th e thir d i s th e hindbrain , o r tritocerebrum (3£r) . Th e latera l lobe s o f th e protocerebru m an d th e deutocerebrum ar e unite d wit h eac h othe r b y interna l commissura l tracts (Fig . 251 , ICom , 2Cora) ; th e tritocerebra l lobe s ar e connecte d generally by a free nerve trunk, the suboesophageal commissure (Figs. 249, 250, 251 , 3Com) , tha t passe s belo w th e stomodaeum . Fro m th e trito cerebral lobe s ther e procee d posteriorl y an d ventrall y th e circumoesophageal connectives (CoeCori) t o th e suboesophagea l ganglio n (Figs . 248, 249 , SoeOng) i n the lowe r part of th e head . Th e lobe s o f th e forebrain bear laterall y th e larg e optic lobes (OpL), whic h contain th e complex visua l center s of the compound eye s (Figs . 256 , 257). Th e optic lobe s ar e generall y narrowe d at thei r bases , an d i n som e insects they are greatly elongate (Fig. 252, OpL). Th e facia l ocell i ar e con nected wit h th e anterio r o r dorsal aspect o f th e protocerebru m b y long slende r stalks , th e ocellar pedicels (Fig . 249 , O P del), a t th e end s of whic h ar e conica l enlargement s containing th e center s of the shor t ocellar nerves . Th e tru e nerv e trunks of the brain arise principally from th e deutocerebru m an d tritocerebrum. FIG. 248.—Hea d nervou s syste m o f The substanc e of the brai n conDissosteira Carolina, anterio r view . (Fo r lettering se e Fig. 249.) sists largel y o f a neuropil e mass of intricately entangle d arborization s of associatio n neurones , th e cel l bodies o f which are locate d fo r th e mos t par t i n th e cortica l regio n of the brain . Th e onl y moto r center s o f the insec t brai n ar e situate d i n the deutocerebra l and tritocerebral lobes, from whic h are innervated th e antennal muscles, and probabl y th e muscle s of the labru m an d som e of the stomodaea l muscles. I n th e decapo d crustaceans the center s o f the oculomotor muscle s of the ey e stalks ar e locate d i n th e protocerebrum . The principa l feature s to b e distinguishe d withi n th e brai n (Fig . 251 ) are specia l group s o f cells , fibe r tracts , an d compac t bodie s forme d o f dense aggregation s o f associatio n neuritie s an d o f glomerul i o f thei r terminal arborizations .

THE NERVOUS SYSTEM

479

The Nerve s o f the Brain.—Th e principa l nerve s o f the insec t brai n are th e nerve s o f the compoun d o r simpl e lateral eyes, th e dorsa l ocelli , the antennae , th e labrum , an d th e fronta l ganglio n connectives . I n addition ther e ma y b e present a dorsa l tegumentar y nerve , connective s with the occipita l gangli a of the stomodaeal system , and sometimes othe r nerves. Nervus options. —The tru e nerve s o f the compoun d o r simpl e latera l eyes ar e th e group s o f retinal neurite s receive d i n th e oute r end s o f th e optic lobes (Fig. 251, OpNv), in which are located the optic centers

FIG. 249.—Hea d nervou s syste m o f Dissosteira Carolina, latera l view . Ao , aorta ; IBr, protocerebrum ; %Br, deutocerebrum; 3Br, tritocerebrum; CA, corpus allatum; CoeCon, circumoesophageal connective ; 3Com, tritocerebra l commissure ; Cr, crop ; FrCon, fronta l ganglion connective ; FrGng, fronta l ganglion ; HphyNv, hypopharyngea l nerve ; LbNv, labial nerve; LmNv, labral nerve; MdNv, mandibula r nerve; n, cervical nerve; OcGng, occipital ganglion; OPdcl, ocella r pedicel; OpL, optic lobe; Phy, pharynx; RNv, recurrent nerve; SID, salivar y duct ; SoeGng, suboesophageal ganglion; TgNv, dorsa l tegumentary nerve.

(/, 77 , 777). Th e opti c nerves , therefore , ar e generally very short ; bu t in insect s havin g rudimentar y opti c centers , th e opti c nerve s ma y b e long trunks , a s i n th e termite s an d i n caterpillar s (Fig . 25 0 A, OpNv). The latera l ocelli of coleopterous larvae , however , are develope d in close proximity wit h the oute r ends of the long optic lobes of the brain contain ing the center s o f the futur e compound eyes (Fig . 252) . Nervi ocellarii. —The slende r ocella r pedicel s unitin g th e facia l ocell i with the brain (Fig . 249, OPdcl) ar e commonly called the ocellar " nerves," but i t ha s been show n by Caja l (1918 ) tha t th e primar y ocella r center s lie in the enlarge d outer end s of the pedicel s (Fig . 25 8 A, OC), since it i s here tha t th e inne r end s o f the retina l fiber s (B 7 b ) are associate d wit h

480 PRINCIPLES

O F INSECT MORPHOLOGY

the terminal s (c ) of nerves from th e brai n that traverse th e stalks . Th e true ocella r nerves , therefore , ar e th e group s o f retina l fiber s (6 ) that terminate i n th e oute r end s o f the ocella r stalks . Th e ocella r pedicel s are comparable with the opti c lobe s of the compoun d eyes. Nervus ganglii occipitalis. —This i s a short , slende r nerv e connectiv e on eac h side (Figs . 249, 250 B, d) proceedin g from th e bac k of the brai n to th e occipita l ganglio n o f th e stomodaea l nervou s syste m (OcGng). The fiber s o f th e occipita l ganglio n connectives , a s show n b y Holst e (1923) in Dytiscus, originate from smal l groups of cells lying in the dorsa l part o f the protocerebrum , from whic h they travers e th e caly x glomeruli of th e corpor a pedunculat a t o mak e thei r exi t fro m th e posterio r wal l of th e brain . Nervus antennalis.—The antennal nerve s (Figs . 248, 250, 251, AntNv) have their roots in the deutocerebru m (Fig. 251) an d are the onl y nerves given of f fro m thi s par t o f th e brai n i n insects . Eac h nerv e consist s of bot h sensor y an d moto r fibers , whic h are sometime s containe d i n a single trunk , an d sometime s separate d i n sensor y an d moto r branches . The sensor y fiber s com e from th e variou s sens e organ s o f the antenna ; the motor fibers go to the antennal muscle s within the head and to thos e located i n the scap e of the appendage . Nervus tegumentalis. —A dorsa l tegumentar y nerv e arise s fro m th e posterior o r latera l surfac e o f th e brai n i n som e insect s an d goe s t o the dorsa l part of the head . Th e roots o f this nerve in Dytiscus, according t o Holst e (1923) , ca n b e trace d a s fa r a s th e fibrou s mas s o f th e deutocerebrum close to th e exi t o f the moto r nerv e of the antenna ; bu t Hanstrom (1928 ) think s tha t th e dorsa l tegumentar y nerv e mus t aris e in th e tritocerebrum , an d tha t i t belong s t o th e sam e syste m a s th e tegumentary labra l nerve . I n Acridida e a larg e tegumentar y nerv e arises clearly from the base of the tritocerebrum (Fig. 249, TgNv). It goes dorsall y clos e behin d th e brai n an d fork s befor e th e mandibula r muscles int o tw o branche s distributed t o th e epidermi s o f the fastigia l area betwee n the compoun d eyes bu t apparentl y give s no branche s t o the muscles. Nervus lateralis. —This i s a slende r nerv e presen t i n lepidopterou s larvae. I t arise s fro m th e sid e o f the brai n jus t abov e th e roo t o f th e circumoesophageal connectiv e (Fig . 25 0 A, B , a ) an d divide s int o tw o branches. On e branc h (6 ) goe s forwar d an d ventrall y t o th e facia l region o f th e hea d latera d o f th e clypea l triangle , wher e it appear s t o innervate th e mandibula r muscles; the othe r branc h (c ) turns posteriorl y and unites with the lateral occipital ganglion (B, OcGng) o f the stomodaeal nervous system . Nothin g i s know n of th e centra l connection s o f thi s nerve or o f the origi n of its fibers .

THE NERVOUS SYSTEM

481

Nervus labrofrontalis. —The labrofrenta l nerv e is a short trun k arisin g anteriorly fro m the tritocerebrum . I t soo n divides into & frontal ganglion connective (Figs . 248 , 249 , 25 0 A , FrCori) an d a labral nerve (LmNv). The fronta l connectiv e goe s anteriorl y an d mediall y t o th e fronta l ganglion (FrGng) o f th e stomodaea l system , sometime s makin g a lon g anteroventral loop , a s in Dissosteira (Fig . 248) , from whic h are give n off nerves t o th e labra l muscle s an d th e retractor s o f th e mout h angles . The labral nerve (LmNv) proceeds to the labrum and probably contains both moto r an d sensory fibers.

FIG. 250.—Brain , cerebra l nerves, and stomodaea l nervous system of a noctui d cater pillar. A , anterior view o f brain and fronta l ganglion . B , posterio r view o f right half of brai n an d postcerebra l parts o f stomodaeal nervous system.

The preora l distributio n o f th e labra l nerve, give n of f fro m th e postoral tritocerebrum , ha s le d som e entomologist s t o conclud e eithe r that th e labru m belong s t o th e tritocerebra l hea d somit e o r tha t th e roots of the labral nerve have their origins in the protocerebrum. Neithe r alternative seems to be supported b y facts. I n th e decapo d crustacean s a tegumentar y nerve , arisin g posterio r t o th e antenna l nerve , give s off a posterio r branc h whic h i s sai d t o innervat e th e epidermi s o f th e entire cephalothorax , an d als o th e nephridia l sac s (Keim , 1915) . Evi dently, therefore , tegumentar y nerve s o f th e brai n hav e n o segmenta l limitations, an d thei r distributio n i s o f n o morphologica l significance . If i t is assumed, however, that the tritocerebral brai n lobes represent th e first postoral gangli a of the primitiv e ventra l nerve cord, there is nothing incongruous in the fac t tha t the y innervat e the preora l par t of the prostomium, sinc e there is no provision fo r th e innervatio n o f this region from th e prostomia l par t o f the brai n (Fig . 24 4 B). Nervus subpharyngealis. —In som e insect s a pai r o f smal l nerve s is give n of f fro m th e suboesophagea l tritocerebra l commissure , whic h are sai d t o innervat e th e ventra l dilato r muscle s o f th e stomodaeum .

482 PRINCIPLES

O F INSECT MORPHOLOGY

The tritocerebra l commissure , however , i s sometime s include d i n th e circumoesophageal connective s an d th e suboesophagea l ganglion , an d in suc h case s th e subpharyngea l nerve s sprin g fro m th e anterio r en d of th e latte r ganglion . I n th e acridi d Dissosteira tw o media n ventra l nerves aris e fro m th e tritocerebra l commissur e (Fig . 249 , Z) , bu t the y appear to innervate the neurilemma of the circumoesophageal connectives and the suboesophagea l ganglion. Nervus postantennalis. —Nerves o f th e postantenna l appendage s ar e entirely absen t i n insects , sinc e thes e appendage s ar e represente d onl y by embryoni c rudiment s i n th e Hexapoda ; bu t i n th e Crustace a the y constitute th e principa l nerves (secon d antennal nerves ) o f the tritocere bral ganglia . The Protocerebrum.—Th e forebrain, or protocerebrum, is the dorsa l and larges t par t o f th e cerebra l mas s (Fig . 248 , IBr). I t include s th e lateral protocerebral lobes (Fig . 251 , PcrL), th e media n pars intercerebralis (Pier), an d sometime s ventra l accessory lobes (AcL), o r Nebenlappen. Within the neuropil e mass of the forebrain ar e to be distinguished group s of globuli cells, dense clusters of fibers and glomerul i forming the so-calle d "bodies" of the brain , and various fibrous tracts. The globul i cell s o f th e brai n ar e specialize d associatio n cell s char acterized by their small size, compact arrangement, an d richly chromatic nuclei. Hanstro m (1930 ) distinguishe s i n the arthropo d brai n generall y three primar y paire d group s o f globul i cells , namely , o n eac h side , a median dorsal group (Fig. 251, Gbl), a posterior lateral group (Gbll), and a ventrolatera l ventral grou p (Gblll). Th e severa l globul i group s are subjec t t o muc h variatio n i n th e exten t o f their development . I n the insect s th e dorsa l and ventra l group s ar e reduced or usually absent , while the lateral groups (Gbll) become prominent elements in the brain of mos t Pterygota . Th e neurite s o f th e globul i cell s for m som e o f th e most importan t fibrou s bodie s of the brain . The fibrou s an d glomerulou s masse s o f th e protocerebru m includ e the dorsal corpora pedunculata (Fig. 251, Cpd), the median dorsal pons cerebralis (Pncr), th e corpus centrale (Cc) , th e ventrolatera l corpora ventralia (Cv), an d sometimes dorsal corpora optica. I n additio n t o thes e bodies o f the protocerebru m proper, however , there ar e connecte d wit h the protocerebru m th e optic centers o f the compoun d eye s (7 , II, /// ) situated i n th e opti c lobes , an d th e ocellar centers locate d i n th e oute r ends o f the ocella r pedicels. Pons cerebralis. —The protocerebra l bridg e (Figs . 251 , 252 , Pncr), lying i n th e dorsa l an d posterio r par t o f th e par s intercerebralis , i s a transversely elongate body, usually horseshoe shaped with the concavity forward o r downward. Th e substance of the pons is mostly glomerulous; but accordin g to Bertschneide r (1921) , th e pon s glomerul i of Deilephila

THE NERVOUS SYSTEM

483

forms two lateral swellings of the body with a fibrous commissure between them. Th e pon s cerebrali s i s th e "posterio r dorsa l commissure " o f C. B. Thompson (1913) , but i t is evident fro m its structure that the body is a n associatio n center , sinc e fiber s ente r i t fro m man y part s o f th e brain. I n mos t arthropod s ther e ar e associate d dorsall y wit h th e pon s the tw o cell masses of the dorsa l globuli cells (Fig. 251, Gbl\ and in such cases the neurites of these cells form the body of the pons. Dorsa l globuli cells associate d wit h the pons , however , are sai d b y Hanstro m t o occur among insects onl y in Apterygota an d Ephemerida .

FIG. 251.—Diagra m o f th e interna l fibrou s bodie s and fibe r tract s o f the insec t brain . AcL, accessor y lobe; AntC, antenna l center ; 2Br, deutocerebrum , SBr, tritocerebrum ; Cc, corpus centrale; ICom, protocerebral commissural tract; 2Com, deutocerebral commissural tract; 3Com, tritocerebral commissure ; Cpd, corpus pedunculatum; Cv, corpu s ventrale ; Gbl, II, III, thre e groups of globuli cells; /, lamina ganglionaris; //, medulla externa; III, medulla interna; OpL, optic lobe; OpNv, opti c nerves; OpT, optic tract; PcrL, protocerebral lobe; Pier, pars intercerebralis; Pncr, pons cerebralis; TriC, tritocerebra l center .

Corpus centrale. —The centra l bod y o f the brai n (Figs . 251 , 252 , Cc) lies anterior o r ventral t o th e pons . I n th e insect s i t consist s o f several distinct group s o f glomeruli , whic h togethe r for m a n ova l o r flattene d mass with the lon g axis transverse. Th e subdivision o f the centra l bod y constitutes th e chie f differenc e i n interna l structur e betwee n th e brai n of insect s an d tha t o f Crustacea, in which the centra l bod y consist s o f a single mas s o f glomeruli. Th e centra l bod y ha s n o nerv e cell s directl y connected with it, but it is a most important cente r of association between the terminal s o f fibers from al l other parts of the brain . Corpora pedunculata. —The pedunculat e bodie s (mushroo m bodies , pilzformigen Korper) are situated in the dorsal part of the brain between the protocerebra l lobes and the par s intercerebralis (Figs . 251, 252, Cpd}.

484 PRINCIPLES

O F INSECT MORPHOLOGY

In thei r typica l for m thes e bodie s ar e mushroo m shaped , a s their nam e implies. Eac h consist s of an expanded cap in the upper or posterior part of th e brai n covere d wit h a mas s o f globul i cells , an d o f a large , thic k fibrous stalk, o r pedunculus, extending forward. Th e corpor a pedunculata constitute the largest an d most highl y developed association center s in the brai n o f pterygote insect s an d ar e the mos t conspicuou s features of the interna l cerebra l structure . The cellula r caps o f the pedunculat e bodie s ar e th e latera l group s of protocerebral globuli cells (Fig. 251, GbII\ but in most insects and in many other arthropods eac h primitive cel l group becomes subdivided int o

FIG. 252.—Section o f the brai n o f Dytiscus marginalia. (From

Holste, 1923. )

two or three distinc t secondar y groups of cells (Fig. 253 A). Th e pedun culus o f eac h bod y i s forme d o f th e axon s o f th e globul i cell s (B , /) . When the globul i cells are separated int o groups, therefore , eac h pedun culus contains a s many confluent bundles of fibers as there are cell groups in th e cap . Immediatel y beneat h th e cel l cap, th e axon s of the globul i cells give off short arborizing collaterals, whic h form synaptic association s (h) wit h the terminal s o f incoming fibers (g) fro m other parts of the brain . There i s thus formed , correspondin g t o eac h cell group at th e uppe r end of th e pedunculus , a cup-shape d mas s o f fibrils and glomerul i (A , B, a) , which is known as a calyx. Accordin g to the numbe r of cell groups, each pedunculus ma y b e surmounte d b y a singl e caly x o r b y tw o o r thre e calyces. The peduncul i exten d forwar d i n th e dorsa l par t o f th e brai n an d terminate i n tw o larg e roo t branche s (Figs . 252 , 25 3 A). On e branc h (Fig. 25 3 A , c) , th e median root (Balken) , goe s inwar d fro m th e mai n stalk o f th e pedunculus , an d th e tw o media n root s fro m opposit e side s usually en d i n proximit y t o eac h othe r (Figs . 251 , 252) , thoug h i n th e Isoptera each again turns posteriorl y an d is extended towar d th e bac k of the brai n beneat h th e centra l bod y an d th e pons . Th e othe r branc h (Fig. 253 , d)-, o r posterior root (cauliculus , rucklaufige Wurzel), goe s posteriorly an d dorsall y anterio r t o th e centra l bod y an d th e pons . In Lepisma eac h roo t o f th e pedunculu s end s i n a cluste r o f swelling s (Trauberi). The axon s o f th e caly x cell s enterin g th e pedunculu s procee d t o the dista l en d o f th e latter , wher e eac h appear s t o divide , on e branc h

THE NERVOUS SYSTEM

485

entering th e media n root , th e othe r th e posterior root . Withi n th e roots th e fibers are said t o end in arborizations tha t form intercommuni cating association s wit h on e another, bu t t o hav e few if any connection s here with nerves from othe r parts of the brain. I n some cases fibers have been observe d extendin g betwee n th e inne r end s o f th e media n root s of th e tw o pedunculat e bodies. Th e principa l association center s o f the corpora pedunculata , wher e nerve s intermingl e fro m al l part s o f th e brain, ar e th e glomerul i o f the calyce s (Fig . 25 3 B, h).

FIG. 253.—Structur e o f a corpu s pedunculatum , diagrammatic . A , pedunculat e body wit h two calyce s (a ) containin g th e globul i cell s (e ) of group II ; stal k (6 ) ending in two root s (c , d). B , showin g synapses (/& ) i n th e calyce s between incoming nerves (g) an d collaterals o f th e caly x cell s (e) , th e axon s of whic h (/ ) for m th e stal k an d root s o f th e pedunculate body.

The latera l globul i cells , th e axon s o f whic h compos e th e stalk s of the pedunculate bodies, occur in all arthropods an d in the annelid worms; and in both these groups the cel l axons form variously developed bundles of fibers . Th e latte r reac h thei r highes t complexit y i n th e pterygot e insects. Corpor a pedunculata , Hanstrom (1928 ) says, are first met wit h in th e anneli d famil y Hesionidae, i n whic h the neurite s o f two group s of globuli cell s for m a pai r o f stalks , whic h ben d mesall y a t thei r inne r ends an d ar e unite d b y a transvers e fibrou s commissur e (Fig . 24 3 C). With thes e bodie s ar e associate d th e root s o f th e palpa l nerves . I n the highe r annelids , a s i n Aphroditida e an d Nereida e (D) , th e corpor a pedunculata ar e well-develope d bodie s (Fig . 254) , eac h compose d o f a large ca p forme d o f tw o o r thre e group s o f globul i cell s (Gb), an d o f a thick stal k containin g th e neurite s (/ ) o f thes e cells , i n whic h ar e th e

486

PRINCIPLES OF INSECT MORPHOLOGY

principal associations (j) of the fibers (i) from the sensory root of the palpus nerve. In the Arthropoda the size and complexity of the corpora pedunculata appear in general to be correlated with the development of the compound eyes, whic h ar e th e principa l ,sens e organ s directl y associate d wit h th e protocerebrum; but, o n the othe r hand , a s in the Isoptera , the pedunculate bodie s ma y b e highl y developed , thoug h th e eye s ar e smal l o r absent. Man y comparativ e studie s o f the pedunculat e bodie s i n insect s sho w that th e relativ e siz e of the organ s gives a prett y fai r inde x o f th e developmen t of instinct s an d "intelligence" ; an d ye t complex instinct s ma y b e operativ e i n larval forms , though , a s Hanstrom (1925 ) has show n i n th e caterpillar , th e brai n centers ar e i n a rudimentar y stag e o f development. The presence of distinct corpora pedunculata i n th e brai n o f annelid worms , as demonstrated b y Hanstrom , ca n lea d only t o th e conclusio n tha t th e majo r FIG. 254.—Corpus peduncuiatum par t a t leas t o f th e arthropo d protocere of an annelid Nereis mrens. (From bmm ^as been evolved directly from a Hanstrom, 1927. ) J prostomial nerv e mas s correspondin g to th e archicerebru m of Annelida. Corpora ventralia. —The ventra l bodie s (latera l bodies , Nebenlappen, parosmatische Masseri) li e i n th e ventrolatera l part s o f th e brai n jus t above th e antenna l glomerul i o f th e deutocerebru m (Fig . 251 , Cv). Some writer s (Holste , 1923 ; Beier , 1927 ) regar d th e ventra l bodie s a s belonging t o th e deutocerebrum ; but generall y they ar e included in th e protocerebrum, an d the y ar e unite d wit h eac h othe r b y a transvers e commissural tract (ICom) tha t passes beneath th e centra l body an d th e median roots of the corpor a pedunculata. Th e ventral bodies, according to Hanstrom , ar e forme d primitivel y o f the associatio n neurite s o f th e ventral globul i cells (Gblll), bu t thes e cell s persist i n only a few arthropods, and generally the ventra l bodies consist only of masses of glomeruli. They ar e association center s having fibrous connections with the centra l body, th e corpor a pedunculata , th e pons , th e opti c lobes , the antenna l glomeruli, and othe r parts of the brain . The ventra l bodie s ar e usuall y no t wel l develope d i n th e highe r insects, thoug h the y appea r t o b e o f large size in both adul t an d larva l Coleoptera, and, according to Bretschneider (1921) , they are particularly large an d highl y elaborate d i n th e Lepidoptera , th e regio n containin g

THE NERVOUS SYSTEM

487

them formin g accessor y lobes (Nebenlappen) o f the protocerebrum . O n the oute r surfac e o f each o f these lobe s on th e bac k o f the brain , Bret schneider says, there is a mass of cells (evidently the ventral globul i cells) from which fibers stream into the ventral bodies as do those of the latera l globuli into th e stalk s o f the corpor a pedunculata. Th e relatively larg e size o f the ventra l bodie s i n th e Lepidopter a Bretschneide r regard s a s a primitive characte r i n this order , sinc e the bodie s in Deilephila ar e ver y similar t o thos e o f Forficula, an d i n bot h thes e insect s the y ar e th e best connecte d part s o f th e brain . Th e siz e an d complexit y o f th e ventral bodie s in insects generally , however, have a n invers e relation t o the developmen t o f the corpor a pedunculata , th e latter , Bretschneide r believes, supplantin g th e ventra l bodie s i n importanc e i n mos t insects . Corpora optica. —Optic bodie s are no t generall y present i n the insec t brain. I n som e o f th e Apterygota , however , accordin g t o Hanstro m (1928), ther e i s i n th e dorsa l par t o f th e brai n a pai r o f smal l bodie s lying abov e th e pon s cerebralis , whic h in Machilis ar e connecte d with the glomerul i of the ocella r nerves and with the medulla e externae of the optic lobes . Thes e opti c bodies , therefore , ar e associatio n center s of both the ocell i and the compoun d eyes. Simila r opti c centers occur also in the Branchiopod a among the Crustacea . The Opti c Centers.—Th e ganglioni c center s o f th e latera l eyes , contained withi n th e opti c lobes , ar e s o intimately associate d wit h th e protocerebral lobe s of the full y forme d brai n that the y ma y b e regarded as a part o f the protocerebrum , though they ar e distinct fro m th e latte r in thei r origin . A s described by Wheele r (1891 ) i n the Orthoptera , th e optic lobe s are forme d fro m sporadi c clusters o f cells delaminated a t a n early embryoni c stag e fro m th e oute r edges of the procephali c ectoderm. Soon the scattere d cell s arrange themselves on each side of the primitiv e head regio n i n fou r longitudina l row s similar t o th e eigh t media n row s of neuroblast s tha t ar e to for m th e media n part o f the brain , an d whic h are continuou s wit h th e eigh t row s o f neuroblasts i n th e neura l ridge s of th e postora l region . Th e cell s generated fro m th e neuroblast s o f th e optic lobes, however, Wheeler says, do not resembl e those produced fro m the neuroblasts o f the centra l strands an d appear to multiply irregularly . The opti c center s o f al l arthropods , regardles s o f th e natur e o f th e lateral eye s connected with them , hav e evidentl y ha d a commo n origin, the prototyp e o f which, or an analogous structure, i s to be found i n some of th e anneli d worms . I n th e polychaet e famil y Eunicidae, Hanstro m (1926) describes a very simple optic center, located in an optic lobe of the brain, intervening between the eye and the cerebrum proper (Fig . 255 A). The opti c nerve s (OpNv) consis t o f the shor t retina l fiber s fro m th e ey e (E) to the optic lobe (OpL). Within the latter the fibers break up into terminal arborization s that form association s wit h terminals fro m nerve s

488

PRINCIPLES OF INSECT MORPHOLOGY

of th e opti c trac t (OpT), some of whic h aris e fro m cell s located wit hin the opti c lobe, while others have their origin s in the brain itself an d send their neurite s int o th e opti c cente r (/) . Th e opti c trac t traverse s the brain between the two optic lobes and probably has connections with other parts of the cerebrum. From thi s primitiv e opti c cente r o f the Eunicida e i t i s but a step t o the mor e complicated bu t stil l ver y simpl e structur e o f the opti c cente r in the branchiopod Crustacea (Fig . 255 B). Here , as shown by Hanstrom (1926), there are in each optic lobe two ganglionic bodies, a distal lamina

FIG. 255.-—Optic lobe of an annelid worm and a branchiopod crustacean. (From Hanstrom, 1926 , 1928. ) A, sectio n o f brain , opti c lobe , an d ey e o f Leodice norwegica, with a singl e synapti c junctio n (/ ) betwee n th e opti c nerve s an d th e fiber s o f th e opti c tract. B, optic lobe of Artemia, with two synaptic regions (/, II).

ganglionaris (/ ) an d a proxima l medulla (//) , surrounde d b y ganglio n cells. Th e postretina l fiber s (OpNv)fro m th e ey e penetrat e int o th e lamina, where their thickened terminal parts are associated with terminals from tw o group s o f neurones . Th e neurocyte s o f on e grou p li e dista l to th e lamina , an d thei r axon s (a ) extend proximall y int o th e medulla , giving off arborizations in both opti c masses; the cell s of the othe r grou p (6) ar e associate d wit h th e medull a an d sen d thei r axon s distall y int o the lamina, where they end in fine terminal branches. I t is to be observed that ther e is here no crossing of the fibers between the tw o optic masses . Cells o f another se t belongin g to th e medull a (c ) hav e shor t fiber s tha t end withi n th e latter . Th e opti c center , finally, is connecte d with th e brain by neurones whose cell bodies (d) lie proximal to the medulla and give of f branchin g collateral s int o th e latter , whil e thei r axon s for m the opti c trac t (OpT)extendin g proximall y int o th e brain . Som e of th e opti c fibers end in the latera l par t o f the brain , bu t other s g o into the opti c commissure s situate d abov e an d behin d th e centra l body . The opti c lobe s o f Diplopod a an d Chilopod a contai n likewis e tw o optic masses ; bu t i n mos t Crustace a an d i n al l Insecta there ar e char acteristically thre e principa l associatio n center s i n eac h opti c lob e (Fig. 251) , namely , a dista l periopticon, o r lamina ganglionaris (7) , ? ?????? ??????????? ?? ??????? ??????? ????? ??? ? ???????? ???????

??? ??????? ?????? ??? or medulla interna (III). The connectio n betwee n th e ey e an d th e periopticon remain s essentiall y th e sam e i n al l forms ; bu t th e numbe r and variet y o f th e opti c neurones , th e structur e an d connection s of the fibrou s masses , an d th e association s o f the fiber s i n th e opti c trac t with othe r part s o f th e brain , al l becom e increasingl y comple x wit h the progressiv e evolutio n o f th e compoun d ey e an d th e functio n o f "vision." The opti c center s o f insects ar e probabl y th e mos t intricat e nervou s mechanisms develope d amon g th e arthropods . Spac e canno t her e b e

FIG. 256.-—Opti c lob e o f a moth , Deilephila euphorbiae, showin g th e thre e synapti c regions (/ , II'-, III) wit h intervenin g chiasmat a (OCh, ICh) characteristi c o f insects . (From Bretschneider, 1921. )

devoted t o a minute descriptio n o f their details , an d th e studen t shoul d consult particularl y th e wor k o f Zawarzi n (1914 ) o n th e opti c lobe s of the larva o f Aeschna, that of Bretschneider (1921 ) on Lepidoptera (Deilephila), an d tha t o f Caja l an d Sanche z (1915) , i n whic h ar e elaboratel y described th e opti c center s o f the hone y be e (Apis mellifica) an d o f th e blow fl y (Calliphora vomitoria). The grosse r structur e o f the opti c center s wil l be more easil y under stood fro m Bretsehneider' s figur e o f th e opti c lob e o f Deilephila (Fig . 256). Her e i t i s see n tha t th e postretina l fiber s (OpNv) penetrat e th e ???????? ???????? ?? ??? ??? ????? ?? ??????? ???? ????? ??? ?????? ganglionaris (/)•. Th e lamina and the medulla externa (77) are connected ?? ???????? ?????? ???? ???? ??? ????? ??????? ?????? ?????????? ????? in th e oute r par t o f th e opti c lob e sen d thei r axon s int o th e medull a

490

PRINCIPLES OF

INSECT MORPHOLOGY

externa, which has a distinctly laminate d structur e owin g to the stratified arrangement o f th e terminal s o f th e penetratin g axons . Th e medull a interna (/// ) is subdivided int o two fibrous masses, whic h are connected by fibers that cross with those from the external medulla to form the inner chiasma (ICh). Th e medull a intern a show s fou r layer s o f stratifie d fibrils withi n it s substance , an d i t ha s elaborat e fibe r connection s through th e opti c trac t (OpT) wit h variou s part s o f th e brain . On e

FIG. 257.—Example s o f structura l detail s i n th e opti c lobe s o f insects , diagrammatic . A, Aeschna larva . (From Zawarzin, 1914. ) B , Calliphora vomitoria. (From Cajal an d Sanchez, 1915. )

bundle o f fibers goes to th e corpu s ventrale, anothe r t o th e centra l bod y and th e pons , a thir d form s a unio n wit h th e bridg e an d th e corpu s pedunculatum, a fourth crosses beneath the central body in a commissural tract t o th e opposit e eye , giving terminals int o th e centra l body , an d a fifth rathe r larg e bundl e traverse s th e ventra l par t o f th e brai n goin g directly t o th e suboesophagea l ganglion . Stil l othe r fiber s en d i n th e neuropile mass of the protocerebra l lobe. The figure by Zawarzi n (Fig. 257 A), showin g diagrammatically th e relations of the nervous elements in the opti c lobes of the larv a of Aeschna, will giv e a cleare r ide a o f th e natur e o f th e association s o f th e opti c neurons withi n th e severa l opti c masses . Th e medull a intern a (///) of the Aeschna larva i s subdivide d int o fou r secondar y parts . I t wil l b e seen her e tha t th e laminate d structur e o f th e fibrou s bodies , especiall y

THE NERVOUS SYSTEM

491

of th e medull a extern a (//) , result s fro m th e alignmen t o f successiv e groups o f fibrils give n off from th e neurite s traversin g them . Th e fiber s connecting th e lamin a wit h th e medull a extern a for m a distinc t oute r chiasma (OCA) , an d thos e betwee n the medull a extern a an d th e medull a interna for m a secon d inne r chiasm a (ICh). I n additio n t o th e inter rupted fiber tracts extending fro m th e eye to the brain throug h th e thre e ganglionic centers , th e proxima l element s o f whic h hav e thei r root s i n the fou r part s o f th e medull a interna , ther e ar e als o continuou s fiber s connecting the lamin a ganglionari s an d the medull a extern a individuall y with th e brain . The opti c center s o f the blo w fly, as depicte d b y Caja l an d Sanche z (Fig. 25 7 B) , diffe r considerabl y i n detai l fro m thos e o f th e dragonfl y larva an d i n som e respect s ar e mor e complex . Mos t o f the postretina l fibers end in the lamina ganglionaris (/) , but som e of them go through th e outer chiasm a (OCh) an d terminat e i n th e medull a extern a (//) .The medulla intern a(III)is subdivide d int o tw o parts , withi n whic h ar e symmetrically distribute d th e dichotomousl y branche d terminal s o f th e neurones o f the lamin a externa . Th e opti c trac t i s compose d o f fiber s that connec t with the medulla interna and with the medulla externa, bu t not wit h th e dista l lamina . For a ful l descriptio n o f the actua l structur e o f the opti c gangli a th e student mus t hav e recours e to th e paper s abov e cited ; the figures given here ar e bu t diagrams . Afte r followin g th e wonderfu l maz e of intricate detail i n th e nerv e center s o f the compoun d eyes , however , we are stil l at a los s t o understan d ho w the effec t o f light o n th e recepto r orga n i s transformed b y th e opti c apparatu s int o specifi c reflexe s i n th e moto r mechanism o r int o a perceptio n o f variation s i n ligh t intensity , color , form, an d motion . This , fortunately , lie s outsid e th e subjec t matte r of morphology . The latera l ocell i o f endopterygot e larva e ar e connecte d wit h th e optic center s o f the compoun d eyes , th e latte r bein g firs t develope d i n the pupa . Th e latera l ocelli , therefore , appea r t o b e temporar y larva l organs, a s ar e mos t o f th e othe r specia l structure s o f th e larva . Th e compound eye s ar e no t evolve d fro m th e larva l ocell i bu t ar e newl y formed i n the epidermi s of the pupa , o r in some cases in that of the larva . The retina l part s o f the ocelli , a t th e tim e o f metamorphosis, ar e withdrawn from th e surfac e and degenerate, but i n some insects (se e Gunther, 1912; Marshall , 1928 ) remnant s o f them persis t a t th e sid e o f the opti c nerve o f th e adul t (Fig . 252 , e). Wit h exopterygot e insects , i n whic h the compoun d eye s ar e forme d i n th e embryo , the imagina l ey e is ofte n relatively large r tha n o r o f differen t shap e fro m tha t o f th e nymp h o r larva. Th e enlargemen t o f the ey e involves an increase in the siz e of the lamina ganglionari s lyin g belo w it , an d th e incremen t i n the lamina , a s

492

PRINCIPLES OF INSECT MORPHOLOGY

shown b y Zawarzi n (1914 ) in the dragonfly , i s formed fro m a part o f th e latter tha t remain s i n a n undevelope d embryoni c conditio n durin g th e larval stage . The Ocella r Centers.—Th e ganglioni c center s o f th e facia l ocell i lie i n th e dista l part s o f the ocella r pedicels (Fig . 25 8 A, OC). I t ha s been shown by Caja l (1918 ) tha t the inne r ends of the shor t retinal fibers (B, 6) are her e associated wit h the dista l terminals o f long fibers (c) that traverse th e ocellar pedicels from th e brain; these fibers were mistaken b y earlier writers for the ocella r nerves. Th e fibers from th e ocella r centers, according t o Cajal , g o t o th e lower par t o f th e brai n wher e they ar e associate d wit h th e ter minals o f branche s fro m th e opti c tracts o f th e compoun d eyes . Hanstrom (1928 ) believe s that th e neurocytes o f th e neurone s o f th e ocellar tract s ar e larg e associatio n cells lyin g mediodorsall y i n th e FIG. 258.—Inne r vation o f a n ocellus . pars intercerebralis . Thi s appear s (From Cajal, 1918. ) A , media n ocellu s of a dragonfly , Libellula, an d it s cerebra l to b e the sit e of the primitiv e opti c pedicel (OPdel). B , sectio n o f ocellar retina center o f the brain , an d i n Machi(Ret), an d ocella r cente r (OC) in dista l par t of pedice l containin g th e synapse s betwee n lis th e ocella r tract s en d her e i n the retina l nerves (b) and fiber s (c ) traversin g lateral glomeruli . Th e media n the pedice l from th e brain . facial ocellu s i s sai d t o hav e tw o strands o f fiber s i n it s pedicel , whic h i n th e Odonat a mak e a chias matic crossin g in the brain . Blackma n (1912 ) record s the occurrenc e in Melanoplus femur-rubrum o f two distinc t media n ocelli , each complete in every respect , innervate d throug h a bifurcat e media n pedicel , wit h a conical swellin g a t th e en d o f eac h branc h beneat h th e correspondin g ocellus. The Deutocerebrum.—The deutocerebru m i s th e par t o f th e brai n containing the centers of the antennal nerves. It s latera l part s generally form a pai r o f distinc t lobe s i n th e adul t brai n (Figs . 248 , 249 , 2J5r) , from whic h the antenna l nerves arise. Th e sensory fibers of the antenna l nerve trunk s terminat e i n numerous glomeruli distributed principall y i n the peripher y o f th e deutocerebra l neuropil e (Fig . 251 , AntC). Th e ganglion cell s of the moto r fibers going to the antenna l muscle s lie in th e lateral part s o f th e deutocerebra l lobes . Th e antenna l glomerul i o f opposite side s are connected by a fibrous deutocerebral commissure (2Com) which traverses th e lowe r part o f the brain . The Tritocerebrum.—Th e tritocerebral par t o f the brai n i s relativel y small in insects, owing to the absence of postantennal appendage s and th e consequent lac k o f nerve s t o thes e organs . Th e regio n o f th e trito -

THE NERVOUS SYSTEM

493

cerebrum i s usually eviden t a s a pair o f swellings or distinct lobe s (Figs . 248, 249 , 3Br) beneat h th e deutocerebra l lobes , from whic h the circum oesophageal connective s (CoeCori) procee d ventrall y an d posteriorl y around the side s o f the stomodaeu m to th e ventra l hea d ganglion . Th e tritocerebral lobe s ar e connecte d wit h eac h othe r b y a substomodaeal commissure (3Com). Th e commissur e is ofte n mor e o r les s unite d wit h the circumoesophagea l connectives , and , whe n it i s not eviden t a s a fre e trunk, i t i s probably submerge d i n th e connective s an d i n th e anterio r part o f th e suboesophagea l ganglion . Th e principa l nerve s o f th e tri tocerebrum in insects are the frontal ganglion connectives (Figs. 248 , 249, 250 A) and th e labra l nerve s (LmNv), bu t smal l nerves arisin g fro m th e commissure must als o have their roots in the tritocerebral ganglia . Morphologically, a s w e hav e seen , th e tritocerebra l lobe s o f th e brain represen t th e firs t paire d gangli a o f th e primitiv e ventra l nerv e cord and are primarily postoral in position (Fig. 245 A, B, GngI), their union wit h th e brai n bein g secondary (C , 3Br). Th e uniqu e feature of the tritocerebra l gangli a i s their connectio n wit h bot h suprastomodaea l parts o f the nervou s system, namely , wit h th e protodeutocerebra l brai n mass, an d wit h the stomodaea l syste m (Fig . 244). The Fibe r Tract s o f th e Brain.—Al l th e interna l part s o f the brai n are intricatel y connecte d wit h on e anothe r b y fibrou s tract s forme d of the axon s o f associatio n neurons . Thre e o f thes e tract s lyin g withi n the brai n ma y b e terme d commissures , becaus e som e o f their fiber s a t least g o continuousl y acros s th e brai n betwee n correspondin g part s of opposite sides . A fourt h trac t o f th e sam e natur e form s a fre e nerv e trunk. Th e firs t commissur e consist s o f fiber s o f th e opti c trac t (Fig . 251, OpT) tha t connect the medullar y bodies of the opti c lobes with each other; th e secon d (ICom) unite s th e corpor a ventralia beneat h th e root s of th e pedunculat e bodies ; th e thir d i s th e deutocerebra l commissur e (2Com) traversin g th e ventra l par t o f th e brain betwee n th e antenna l centers; th e fourt h i s th e tritocerebra l commissur e (3Com) , calle d th e suboesophageal commissur e because it is usually a free nerv e trunk pass ing beneat h th e stomodaeu m (Fig . 249) . Th e othe r tract s ru n i n al l directions an d connec t th e cerebra l bodies with on e another. Th e opti c centers, as we have seen, are connected by fibers from th e optic tract with the corpor a pedunculata , th e centra l body , th e ventra l bodies , an d th e antennal centers . Th e corpor a pedunculat a receiv e fiber s fro m al l parts o f the brai n a s wel l a s fro m th e suboesophagea l ganglion; and th e central bod y ha s connection s almos t a s extensive . Th e ventra l bodie s are mor e importan t center s i n som e insect s tha n i n others ; whe n wel l developed the y to o hav e widel y distribute d connections . Th e larges t tracts of the brain, however, go from the antenna l centers to the calyce s of the corpor a peduculata an d her e put th e antenna l sens e organs in com -

??? ?????????? ?? ?????? ??????????

munication wit h fibers from al l the othe r cerebra l center s an d fro m th e ventral nerv e cord. A stud y o f th e nerv e association s i n th e brai n suggest s tha t th e principal center s throug h whic h the sens e organ s o f the hea d exer t a n influence o n the moto r mechanism o f the res t of the bod y are the corpor a pedunculata an d th e corpu s centrale . A s yet , however , neurologist s have give n much less attention t o th e connection s between the cerebra l centers an d th e center s o f the ventra l nerv e cor d than the y hav e give n to local associations withi n the brai n itself . THE VENTRA L NERV E COR D The ventra l nervou s system , a s distinguishe d fro m th e suprastomo daeal part of the brain and the stomodaeal system , consist s o f the postora l series o f segmenta l gangli a an d thei r connectives , constitutin g th e so-called ventral nerve cord. Morphologically , as w e have seen , the ven tral nerv e cor d begin s wit h th e tritocerebra l gangli a o f th e brai n an d includes th e primitiv e gangli a o f the gnatha l regio n of the head , a s well as the gangli a o f the thora x an d abdomen . Sinc e the genera l structur e of th e ventra l nerv e cor d an d th e structur e o f the tritocerebra l gangli a have alread y bee n described , w e shal l giv e attentio n her e onl y t o th e composite suboesophageal ganglion of the head , the interna l organizatio n of a body ganglion, and the media n nerves of the thoraci c an d abdomina l ganglia. The Suboesophagea l Ganglion.—Th e ventral nerv e mass of the hea d is composed of the united ganglia of the primitive gnathal segments. Th e histology of this composite ganglion has been but littl e studied b y precis e neurological methods, whic h is unfortunate because of the long-standin g dispute a s to th e numbe r o f ganglia tha t ar e containe d i n it. Th e sub oesophageal ganglio n innervate s th e mandible s (Fig . 249 , MdNv), th e ??????????? ????????? ??? ???????? ??????? ??? ?????? ??????? the salivar y duct s (m) , an d a t leas t som e of the muscle s of the nec k (n). The nerv e trunk s contai n bot h moto r an d sensor y fibers . Th e longi tudinal nerv e tracts that ente r o r traverse th e suboesophagea l ganglio n are of great importance , since they contai n the connectiv e fibers between the sensor y center s of the hea d an d th e moto r center s o f the body , bu t we hav e littl e detaile d informatio n concernin g them . I n additio n t o being th e centra l orga n o f th e gnatha l an d cervica l nerves , th e sub oesophageal ganglio n i s als o a n inhibitor y cente r o f th e bod y ganglia , since, wit h it s removal , th e somati c reflexe s ar e foun d t o becom e more readily excited by artificial stimuli. General Structur e o f a Bod y Ganglion.—Fo r a ful l accoun t o f th e internal structur e o f th e gangli a o f th e ventra l nerv e cor d th e studen t must consul t th e detaile d wor k o f Zawarzi n (1924a) o n th e Bauchmark

??? ??????? ?????? ??? ?? ??? ????? ?? ???????? ?? ??? ???? ???? ???? ? ????? ?????? ?? Zawarzin's descriptions . A segmenta l ganglio n o f th e thora x o r abdome n i s usuall y a n ova l or polygonal mas s o f nerve tissue, continuou s anteriorly an d posteriorl y with th e interganglioni c connective s (Fig . 25 9 A) . Fro m it s side s proceed tw o o r thre e principa l lateral nerves (INv, 2Nv, 3Nv); an d i n ???? ??????? ? ?????? ????? ??????? ?????? ???????????? ?? ???? ??????????? between th e base s o f th e connectives . Th e ganglio n i s investe d i n a nucleated sheath , th e neurilemm a (Nlm), whic h is continuous ove r th e connectives an d th e nerves . Th e principa l cellula r element s o f th e ganglion (GngCls) ar e arrange d peripherally , mostl y i n th e latera l an d

FIG. 259.— A typica l ganglio n o f th e ventra l nerv e cord , an d a ganglio n moto r cell . ????? ????????? ??????? ?? ?? ????????? ???????? ??? ??? ????? ??????? ??????? position o f the nerv e ce^ls in th e ganglion . B , a moto r neurone of an abdomina l ganglion of a n Aeschna larva .

dorsal parts . Th e centra l an d ventral part s are occupied by a neuropile mass (Npt). The lateral nerves of the ganglion that contain both motor and sensor y fiber s aris e eac h fro m dorsa l an d ventra l fibrou s root s within th e ganglion , th e dorsa l roo t containin g th e moto r fiber s (Fig . ?? ? ???? ??? ??????? ???? ??? ??????? ?????? ????? Within th e neuropil e ther e ma y b e distinguishe d fiv e region s (Fig. 260) . Dorsall y i s th e regio n o f th e dorsa l interganglioni c con nective fiber s (a , 5) . Beneat h thi s i s the moto r cente r (c) , o r region of the dorsa l nerv e roots . Ventrall y i s situated th e regio n o f the ventra l connective fibers (/, g, h)9 and immediately above it the senso ry center (e), or region of the sensor y ventral root s o f the latera l nerves. Th e centra l part o f the ganglio n (d) contain s th e principa l neuropil e mass . A t th e ends of the ganglion only the dorsal and ventral fiber tracts are continued into the connectives . In eac h ganglio n ther e ar e si x principa l group s o f nerv e elements : (1) th e cel l bodie s an d root s o f th e moto r fiber s o f th e latera l nerves ; (2) the root s o f the sensor y fibers of the latera l nerves ; (3 ) the cel l bodies and fiber s o f the intraganglioni c association neurones ; (4) the cel l bodies

496

PRINCIPLES OF INSECT MORPHOLOGY

and collateral s o f th e interganglioni c associatio n neurones ; (5 ) th e cel l bodies and roots of the motor fibers of the median nerve; and (6 ) the root s of th e sensor y fibers of the media n nerve. The cel l bodies of the moto r neurone s of the latera l nerve s lie in th e dorsolateral parts of the ganglion (Fig. 260, j, &). Eac h is a large unipolar ???? ????? ??? ?? ???? ???? ? ??????? ??????????? ???????? ???? ????? ??? given off finely branching collaterals (Col) int o the dorsa l motor center of the ganglion , whil e th e mai n shaf t o f th e fiber , o r axo n (Axri), turns outward to ente r th e dorsa l part o f a lateral nerve . The sensor y fiber s enterin g th e ganglio n fro m th e latera l nerv e ?????? ????? ?? ? ??? ?? ?? ??? ?????? ?? ??? ??????? ????????? ??? ???

.*>

FIG. 260.—Diagrammati c cros s section of a n abdomina l ganglion o f a n Aeschna larva . (From Zawarzin , 1924o. ) a , 6 , dorsa l fibe r tract s o f longitudina l commissures ; c , motor neuropile; d, centra l neuropile; e, sensory neuropile; /, g , h, i, ventral fiber tracts of connectives; j, k, moto r neurocytes; Z, m, n, o, p, q , r, neurocytes of association neurones; s, t, u, sensory fibers; v, w, x, y, z, fibers entering ganglio n from connectives .

lower part of the ganglion , where some of them (s ) end in terminal arbori zations, whil e others (£ , u ) giv e of f branching collateral s an d the n tur n forward an d procee d throug h th e connective s t o som e more anterio r ganglion. Th e sensory neuropile, therefore, contains fiber endings both of the latera l sensory nerves o f the ganglio n and o f sensory fiber s fro m th e more posterio r ganglia . Th e collateral s o f som e o f th e sensor y nerve s end in the sid e of the ganglio n on which the nerv e enters (u); other s cross to th e opposit e sid e (t). The associatio n neurone s o f th e ventra l nerv e cor d includ e loca l neurones o f each ganglion and neurone s whose axons form th e principa l fibers o f th e interganglioni c connectives . Th e cel l bodies o f these neu rones are situated in the latera l parts of the ganglio n (Fig . 260, Z, m , n, o, p, q). Th e local , o r intraganglionic , associatio n neurone s ar e o f tw o types. I n on e typ e (m ) th e nerv e proces s i s T-shaped , an d th e tw o branches lie in the sam e side of the ganglio n as the cel l body, one branch going dorsally , th e othe r ventrally , t o intermediat e betwee n the dorsa l

THE NERVOUS SYSTEM 49

7

motor neuropile and the ventral sensory neuropile. I n the other typ e (Z ) the neuron e connect s the tw o halve s o f the ganglion , a collatera l bein g given off in one side, while the axon crosses to the other side, where it ends in terminal arborizations . The fiber s o f th e interganglioni c connective s originat e fro m cell s lying latera d o f th e intraganglioni c neurones . Th e axon s giv e of f collaterals i n the ganglio n of their origin , some of which branch in the moto r neuropile (Fig . 260 , n, o , p ), other s i n th e media n o r sensor y neuropil e (q, r) , bu t th e mai n neurite s procee d eithe r anteriorl y o r posteriorl y through the connectiv e tracts to othe r ganglia of the ventra l nerv e cord. Zawarzin describe s thre e type s o f connectiv e fiber s i n a n abdomina l ganglion o f the dragonfl y larva: tautomere fiber s (n , r) , whic h leave th e ganglion throug h th e connective on th e sid e o f their origi n afte r givin g off a collatera l i n thi s side ; heteromere fiber s (o , g) , whic h give of f on e collateral an d the n cros s th e ganglio n t o ente r th e connectiv e o f th e opposite side ; an d hekateromere fiber s (p ) whic h cros s th e ganglio n bu t give of f a collatera l i n eac h side . Som e o f th e connectiv e fiber s unit e successive ganglia , other s g o long distance s throug h th e ventra l nerv e cord. Th e connectiv e tract s pas s superficiall y throug h th e dorsa l and ventra l part s o f th e ganglia . I n th e dragonfl y larv a Zawarzi n distinguishes i n eac h dorsa l trac t a media n divisio n (a) , which contains fibers tha t g o lon g distance s throug h th e nerv e chain , an d a latera l division (6 ) containing shorter fibers ; an d i n eac h ventral trac t h e finds an externa l median group (/) o f long fibers, an internal media n group (g) of shor t fibers , an d a latera l grou p (h) o f shor t fibers . Beside s thes e tracts o f associatio n connectiv e fibers , ther e ar e tw o interna l ventra l tracts on each side (i) whic h contain th e sensor y fibers that traverse th e connectives. The moto r an d sensory roots o f the media n nerves lie in the posterio r parts of the ventra l ganglia , but the y wil l be described under a separat e heading treating o f the media n nerves. The structure of a thoracic ganglion in the dragonfly larva is essentially the sam e a s that o f an abdomina l ganglion , except that i t i s more complicated i n al l it s details , owin g to th e presenc e of appendage s o n th e thorax. Sinc e n o exac t stud y ha s bee n mad e o n th e histolog y o f th e thoracic gangli a o f a n adul t winge d insect , i t i s no t know n t o wha t extent th e nervou s equipmen t i s increase d i n th e imag o t o serv e th e mechanism of flight . The distributio n o f th e moto r an d sensor y fiber s fro m th e latera l nerves i n a n abdomina l segment of the larv a o f Aeschna is described b y Rogosina (1928) . Th e majorit y o f the moto r fibers in an y on e segment are derive d fro m cell s lyin g withi n th e ganglio n o f tha t segment , bu t some o f the m com e fro m th e ganglio n o f th e precedin g segment , th e

498

PRINCIPLES OF

INSECT MORPHOLOGY

muscles o f eac h segmen t thu s havin g a plurisegmenta l innervation . In th e secon d thoraci c segmen t o f the dragonfl y larva , Zawarzi n says , there ar e onl y si x pairs o f motor nerv e cell s tha t suppl y fiber s t o th e muscles of this segment. Sinc e the number of muscles in the segment and its appendage s greatl y exceed s th e numbe r o f moto r cell s innervatin g them, a single fiber must branc h t o several muscles. I t i s interesting t o observe that Rogosina finds in an abdominal segment of the same insect a corresponding numbe r o f periphera l sensor y nerv e cell s o f Typ e I I innervating the muscles, the connectiv e tissue, an d the epidermis .

FIG. 261.—Tracheation o f an abdomina l ganglion o f a noctuid caterpillar , dorsal view.

The gangli a an d nerve s o f th e ventra l nerv e cor d ar e abundantl y supplied with trachea. I n a caterpillar eac h ganglion (Fig . 261) receives a trachea o n each side (a , a) fro m th e ventra l trachea l commissur e of its segment (TraCom). Eac h ganglioni c trache a divide s a t th e roo t o f th e posterior nerv e int o anterio r an d posterio r branche s distribute d t o th e ganglion, the lateral nerves , an d the connectives. The Median Nerves.—The ventral nerv e cord of some insects, a s we have seen , include s a longitudinal median nerve lyin g between eac h pai r of interganglioni c connectives . Th e media n nerv e takes it s origi n fro m the posterio r par t o f the ganglio n lying befor e i t an d give s of f a pai r of lateral branches that extend outward to the neighborhood of the spiracles . In som e case s th e media n nerv e terminate s a t th e bifurcatio n into th e lateral branches ; i n other s i t continue s beyon d th e branche s t o th e ganglion following . Th e occurrenc e o f media n nerve s i n differen t groups of insects has not been well studied, bu t th e nerves are commonly present i n larval forms, and it is probable that where they are not presen t as independen t trunk s their fiber s ar e burie d i n th e interganglioni c connectives an d issu e fro m th e followin g gangli a i n th e anterio r nerv e trunks of the latter . The typica l arrangemen t an d distributio n o f the media n nerve s ar e well show n in a caterpilla r (Fig , 262) , in whic h there i s a media n nerv e

??? ??????? ?????? ??? for eac h of the 1 1 ganglia of the ventra l nerv e cord posterior t o th e hea d (Fig. 246) . I n th e thora x eac h media n nerv e appear s t o en d a t it s bifurcation int o the lateral branches (Fig . 262 , MedNv%), whic h are give n off fro m a small triangular swelling ; but i n the abdome n a slender median filament continue s t o th e nex t ganglion . Th e exac t termination s o f th e fibers o f the media n nerve s hav e no t bee n determined , thoug h th e end ings o f th e latera l nerve s ar e usuall y foun d t o b e distribute d t o th e tracheae an d th e spiracles . I n th e caterpillar eac h lateral branch (I) goe s outward ove r th e inne r fac e o f th e ventral muscle s in the anterio r en d of the segmen t behind th e on e contain ing th e ganglio n i n whic h th e mai n trunk o f th e media n nerv e take s it s origin. Alon g it s cours e th e latera l nerve give s of f smal l branche s an d breaks u p finall y int o termina l fiber s distributed t o th e trachea l trunk s in the neighborhoo d o f the spiracle , one of whic h innervate s th e occluso r muscle o f th e spiracle . Th e nerv e center o f eac h segmenta l pai r o f spiracles is thus located in the ganglion of th e precedin g segment . Th e branches fro m th e media n nerv e o f the prothoraci c ganglio n g o t o th e first pai r o f spiracles , whic h ar e primarily mesothoracic ; thos e fro m the mesothoraci c nerv e g o t o th e neighborhood o f th e rudimentar y 262.—Mesothoracic , metathormetathoracic spiracles ; the metathor - acic,FIG. an d firs t abdomina l ganglio n o f a acic nerve s g o to th e firs t abdomina l caterpillar, Malacosoma americana, spiracles, an d s o on. I f th e las t tw o showing median nerves (MedNv). ganglia of the ventra l nerv e cord are united, as in Malacosoma americana (Fig. 246) , th e branche s o f th e sevent h media n nerve , whic h go to th e eighth spiracles, issue from th e dorsa l surface o f the eight h ganglion. I n Malacosoma th e media n nerve s o f th e abdome n branc h clos e t o th e succeeding ganglion . I n som e caterpillar s th e media n nerve trunk continues t o th e followin g ganglion , an d it s latera l branche s ar e give n off through th e firs t pai r o f latera l nerv e trunk s o f thi s ganglio n (Fig . 261, INv). In th e larv a o f Aeschna, i t ha s bee n show n b y Zawarzi n (1924) , each median nerve contains two motor fibers (Fig. 263 A, MF) and two

500

PRINCIPLES OF INSECT MORPHOLOGY

??????? ?????? ????? ??? ????? ????? ?????? ????????? ???? ? ???? ?? large unipola r cell s (MCI) lyin g i n th e posterio r par t o f th e ganglio n from which the nerve proceeds. I n the thorax each motor axon (A, Axn), after givin g of f numerou s branchin g collateral s withi n th e ganglion , turns posteriorl y t o ente r th e media n nerv e trun k (MedNv) and , a t th e bifurcation o f th e latter , divide s int o righ t an d lef t branches , whic h go outwar d i n th e latera l nerves . Th e sensor y fiber s (SF), whic h ar e very slende r an d varicose , ente r throug h th e latera l branches , an d those fro m opposit e side s unit e i n th e media n nerv e t o for m tw o fibers that ru n forwar d int o th e ganglion , wher e the y en d i n fin e branchin g terminals.

???? ?????????????? ?? ??? ?????? ????? ????? ?? ?? ??????? ?????? ????? ????????? ?????? ?? ? ???????? ????????? ?? ?? ????????? ?????????

In th e abdome n (Fig . 26 3 B) th e cours e o f th e moto r fiber s o f th e median nerv e is quite differen t fro m tha t in the thorax . Th e two moto r ?????????? ????? ??? ??????? ????????????? ??????? ??? ?????????? ?????? and thei r axon s (Axn) procee d posteriorl y throug h th e interganglioni c connectives into th e anterio r par t o f the followin g ganglion . Her e the y turn mesall y an d forwar d int o th e posterio r en d o f th e media n nerve , which connect s th e successiv e ganglia , an d finall y branc h i n th e usua l manner wher e th e latera l nerve s ar e give n of f from th e media n trunk . The sensory fibers (SF), on the other hand, take the same course as in the thorax , enterin g th e precedin g ganglio n throug h th e par t o f th e median nerv e trun k lyin g anterio r t o th e latera l nerves . Whethe r thi s difference i n th e dispositio n o f th e moto r fiber s o f th e media n nerv e between th e thora x an d th e abdome n i s genera l i n insect s o r applie s only to th e dragonfl y larv a ha s not bee n determined.

THE NERVOUS SYSTEM 50

1

In it s origin the median nerve appears t o be derived fro m th e median strand o f nerve tissue forme d i n the embry o from th e ro w of neuroblasts at the top of the ventral groove between the neural ridges (Fig. 17 B, MC). According to Escheric h (1902) , the media n nerv e syste m i n the embry o of the blow fly Lucilia consists at first of ganglionic cell masses located over the intersegmenta l line s o f th e body , an d o f intraganglioni c strand s traversing th e segmenta l areas. Fro m th e posterio r end of each median ganglion there i s given of f a pair o f lateral nerves , presumably associate d with the tracheal invaginations . I t woul d thus appear that the definitive ventral nervou s syste m o f insects i s derive d fro m tw o distinc t sources , the primarily latera l nerv e strands and a primitive media n nerv e strand . The embryoni c ganglia of the media n nerve described by Escherich mus t eventually b e included in the posterio r part s o f the definitiv e composite segmental ganglia , an d i n man y insect s th e media n stran d i s eithe r obliterated o r entirely united with the lateral strands . 3. TH E STOMODAEA L NERVOU S SYSTE M

The stomodaea l nervou s syste m consist s o f sensor y an d moto r neurones havin g thei r center s i n smal l gangli a develope d fro m th e dorsal, o r dorsa l an d lateral , wall s o f th e siomodaeum . Th e nerv e fibers ar e distribute d t o al l th e stomodaea l part s o f th e alimentar y canal, an d i n certain insect s the y ar e continue d over the entir e lengt h of the mesenteron . Th e labra l muscle s also , i n som e cases , appea r s to receive at leas t a part of their innervatio n fro m stomodaea l nerves, as do likewise the salivar y ducts , the aorta , th e corpor a allata, and some of the mandibular muscles. Thi s system centerin g in the stomodaea l ganglia is commonly calle d th e "stomatogastric " o r anterio r "sympathetic " nervous system; but inasmuc h as its ganglioni c centers are derived fro m the stomodaeum, the term stomodaeal nervous system seems more fitting. Since the stomodaea l nervous system ha s not bee n fully studie d fro m a. comparative standpoint , i t i s impossible t o giv e a genera l descriptio n applicable t o al l it s numerou s variations i n differen t insects . Th e on e constant featur e o f the syste m i s th e presenc e of a media n precerebra l ganglion situate d anteriorl y o n the dorsa l wal l of the pharynx . Thi s is the frontal ganglion (Figs. 248, 249, 250 A, FrGng). Th e fronta l ganglion is connecte d wit h th e tritocerebra l lobe s o f th e brai n b y th e frontal ganglion connectives (FrCori), an d fro m i t ther e i s give n of f a media n recurrent nerve (RNv), whic h goe s posteriorl y o n th e dorsa l wal l o f th e pharynx beneat h th e brai n an d th e anterio r en d of the aort a (Fig . 249). Sometimes one or two nerves proceed forward from the frontal ganglion to the regio n of the clypeus . In th e bac k o f th e hea d ther e i s usuall y a secon d nerv e cente r of the stomodaea l system , whic h consist s typicall y o f a pai r o f gangli a

? ? ?????????? ?? ?????? ??????????

????? ???? ?????? ??? ????? ????? ?? ?? ??????? ????? ??????? ??? ?? ?????? ??? ????????? ???????? ?????? ???? ??? ????????? ?????? ???? "pharyngeal," "oesophageal," or " hypocerebral" ganglia. Eac h occip ital ganglion is connected wit h the bac k of the brai n b y a short occipital ganglion nerve (d ) an d communicate s wit h th e fronta l ganglio n b y a ?????? ??? ???? ??? ????????? ????? ?????? ?? ???? ??????? ??? ?????? occipital gangli a ar e unite d i n a singl e media n occipita l ganglio n (Fig . ???? ?????? ???????? ??????? ??? ????? ????? ?? ???? ????? ??? ????????? nerve end s in this ganglion, an d th e latte r ha s tw o connective s (d ) with the bac k o f the brain . Severa l nerve s ar e give n of f from th e occipita l ganglia, o r ganglion , bu t th e patter n o f th e postcerebra l stomodaea l innervation varies much in different insects . In a caterpillar , i n whic h the occipita l gangli a ar e widely separate d (Fig. 25 0 B), the larg e recurrent nerv e (RNv) proceeds posteriorly o n the dorsal wal l o f the stomodaeu m t o th e en d o f the crop , givin g of f along its cours e numerous latera l branche s t o th e stomodaea l muscles . Eac h ????????? ???????? ??????? ??? ? ?????????? ??? ???? ??? ???? ?? ??? ?????? and anothe r (h ) with th e recurren t nerve . Fro m th e secon d th e aort a (Ao) i s innervated. Laterall y the ganglio n give s of f a short nerve (e ) to the mandibular muscles, and a nerve (c) that goes forward and unites with the lateral nerve of the brain (a , b). Fro m its posterior part a small nerve (/) goe s t o th e duc t o f th e sil k glan d (SID), an d a large r nerv e (g ) t o the latera l wall of the crop . A mor e simpl e patter n o f innervatio n i n th e postcerebra l regio n i s shown i n th e acridi d Dissosteira (Fig . 249) , i n whic h ther e i s a singl e median occipita l ganglio n (OcGng) closel y associate d wit h th e open , troughlike anterio r en d o f the aorta , th e latte r bein g embrace d b y th e short connectives (d ) between the occipital ganglion and the brain. Thre e principal nerve s ar e give n of f from eac h sid e o f th e ganglion . On e (i ) goes laterally to th e corpu s allatum (CA) ; th e secon d (j ) break s u p int o branches distribute d o n th e anterio r par t o f th e crop ; th e thir d an d largest (k ) goe s posteriorl y o n th e latera l wal l o f th e crop , givin g off branches alon g its course , and end s in a lateral ingluvial ganglion (" gastric "ganglion) o n the rea r thir d o f the crop , fro m whic h th e posterio r parts o f the stomodaeu m ar e innervated . According t o th e studie s o f Orlo v (1924a ) o n th e histolog y o f th e stomodaeal gangli a of the larv a o f Oryctes nasicornis, the fronta l ganglio n and th e occipita l ganglio n contai n sensory , motor , an d association nerv e cells. Th e distal processes o f the sensory cell s extend t o the muscle s and connective tissu e o f th e stomodaeum . Th e fronta l ganglio n alone , however, contain s association s betwee n th e moto r an d sensor y fibers . The sensor y neurone s of the secon d ganglion have n o collateral s i n thi s ganglion, bu t thei r axon s exten d forwar d throug h th e recurren t nerv e

THE NERVOUS SYSTEM 50

3

into th e fronta l ganglion , wher e they for m association s wit h th e moto r neurones. Th e fronta l ganglio n no t onl y contain s th e sensory-moto r associations of the stomodaea l system bu t ha s connections with the brai n and th e ventra l nerv e centers by way of the fronta l connectives from th e tritocerebrum. I t i s said t o b e the cente r o f the peristalti c movement s of th e oesophagus . 4. TH E PERIPHERA L NERVOU S SYSTE M

The periphera l nervou s syste m include s th e nerv e trunk s radiatin g from th e ganglia , an d th e dista l branche s an d termina l organ s o f th e motor an d sensor y fibers contained i n the nerv e trunks. A full descrip tion o f th e periphera l nervou s system , therefore , shoul d contai n a n account o f th e distributio n o f al l th e nerve s i n th e body ; bu t sinc e a subject o f such magnitude coul d not b e treated i n a general text, we shall consider her e onl y th e terminal s o f th e moto r nerve s an d th e dista l endings o f the sensor y nerves . The Sensor y Neurones.—Whe n th e sensor y fiber s o f a nerv e trun k are trace d outwar d fro m th e centra l ganglia , the y ar e foun d t o en d i n cells lyin g eithe r withi n th e epidermi s o f the bod y wall , o r immediatel y beneath it , o r on the somati c muscles or the wal l of the alimentar y canal . These cell s appear t o b e the tru e neurocyte s of the sensor y neurones, for in insects there ar e no other nerve cells in the cours e of the sensor y fibers, such a s thos e o f the spina l gangli a o f vertebrates . The periphera l cell s of the sensor y nerves are eithe r bipola r o r multi polar. Th e dista l processe s o f most o f them g o direc t t o specifi c ecto derm al sens e organs . Cell s o f this kind , whic h are alway s bipolar , ar e those designate d sensory cells o f Type I (Fig . 24 2 B). Th e others , which may b e either bipola r o r multipolar, bu t whic h are typically multipolar , are provide d wit h on e or more dista l processe s that branc h elaboratel y and en d wit h fine varicose fibrils, which terminate o n th e inne r surfac e of the body wall, on the somatic muscles and connective tissue, an d on the muscles an d wal l of the alimentar y canal . Cell s o f this kin d ar e thos e distinguished a s sensory cells o f Type I I (Fig . 24 2 C). Accordin g to th e nature o f thei r neurocytes , therefore , th e sensor y neurone s themselve s may b e classe d a s o f Type I o r Typ e II . Th e tw o group s o f sensor y neurones appea r t o b e morphologicall y distinct . Thos e o f th e secon d type ar e undoubtedl y th e older , sinc e the y for m th e principa l sensor y innervation i n th e anneli d worms , which consists o f a diffus e branchin g of th e termina l processe s o f the neurocyte s o n th e inne r surfac e o f th e epidermis. Sensor y neurones of Type I are most numerous in arthropod s having scleroti c plate s i n the bod y wal l an d ar e evidentl y develope d a s a mean s o f circumventin g the los s o f sensitivity t o externa l conditions , which would otherwise result fro m th e hardenin g o f the cuticula .

504 PRINCIPLES

O F INSECT MORPHOLOGY

Sensory Neurones o f Type I. —The neurocyte s o f th e firs t typ e o f sensory neurone s ar e th e so-calle d sens e cell s of the specifi c ectoderma l sense organ s (Fig . 267 , SCI). The y ar e alway s bipolar, an d thei r dista l processes (d) ar e immediatel y connecte d with th e cuticula r part s o f th e receptors. Th e proxima l processe s are th e centripeta l axon s which end in terminal arborization s withi n th e centra l ganglia . I n mos t case s th e sense cel l (o r cells ) o f a sens e orga n lie s withi n th e epidermi s i n clos e association wit h the constructiv e cell s of the receptor (A) ; but i n various larvae th e sens e cell s o f the tactil e seta e distributed ove r th e bod y li e beneath th e epidermi s (B) , and their lon g distal processe s penetrate th e basement membran e t o ente r th e receptor . The developmenta l histor y o f the intraepiderma l sens e cells has bee n carefully studied , an d al l investigators agre e that thes e sens e cell s tak e their origi n fro m undifferentiate d ectoderma l cells . I t seem s certain , also, that the sense cells must in all cases be the neurocytes of the sensor y nerves proceedin g from them , an d ye t th e growt h o f centripeta l axon s from th e epiderma l sens e cell s ha s no t bee n demonstrated , an d som e investigators clai m that th e connectio n between the sens e cell s and th e sensory nerve s i s established secondarily , whic h would imply, therefore, that the sens e cell s of insects ar e secondary sense cellsj a s ar e mos t o f th e sensory cell s i n th e epidermi s o f vertebrates. Th e subjec t i s full y dis cussed by Hanstrom (1928) , who concludes that the arthropod sens e cells are primary sense cellsj bu t th e exac t origin of the nerves of the ectoderma l sense organ s o f the Arthropod a appear s ye t t o nee d furthe r elucidatio n from the standpoin t of development. Furthermore , the relation betwee n the intraepiderma l an d subepiderma l sens e cells of Type I ha s no t bee n determined. Sensory Neurones o f Type II. —Sensory neurone s o f thi s typ e ar e particularly abundan t i n th e Annelid a an d i n soft-skinne d arthropods , such a s th e larva e o f holometabolou s insects , bu t the y occu r als o i n arthropods, including Crustacea and insects, that have a sclerotized integu ment. Neurone s o f Typ e I I ar e neve r connecte d wit h specifi c sens e organs. Thei r neurocyte s lie on the inne r fac e o f the bod y wall, on th e muscles, o r on the wal l of the alimentary canal , and their finely branching distal processes en d in free terminal s whic h innervate th e .epidermis, th e somatic muscles , connectiv e tissue , an d th e muscle s an d epitheliu m of the alimentar y canal . Th e centripeta l axon s g o t o th e gangli a o f th e central nervou s system . Th e ontogeneti c origi n o f these neurone s fro m the ectoder m ha s not been determined . The integumentar y innervatio n o f the crayfis h i s well known. Tha t of Astacus fluviatilis has recently been studied in detail by Tonner (1933) , who find s tha t practicall y th e entir e inne r surfac e o f th e bod y wal l i s covered b y a networ k o f branchin g an d unitin g fiber s fro m numerou s

THE NERVOUS SYSTEM

505

multipolar nerve cells lying in the connective tissue beneath the epidermis. In additio n t o this multipolar cel l net, however , Tonner finds in Astacus also a n inne r integumenta l plexus o f fibers branching fro m nerve s give n off fro m th e gangli a o f the ventra l nerv e cord . Th e tw o systems , more over, are united by connecting fibers and together constitut e a n elaborat e integumentary nervous system. Among insects , sensor y cell s o f Typ e I I ar e particularl y abundan t in soft-skinne d holometabolou s larvae, where , i n som e forms, the y giv e

FIG. 264.—Example s o f terminal s of moto r an d sensor y nerv e fibers . A , moto r innervation of diffuse typ e in ventral somatic muscle of Aeschna larva. B , sensory innervation of epidermis of Melolontha larva. C , D, sensory inner vation of oesophageal muscles of Oryctes larva . E , sensor y innervation of ventricula r muscle o f Melolontha larva . F , motor innervatio n of concentrate d type i n muscl e fibe r o f rectu m o f Oryctes larva . G , sensory innervation of epidermis (e) and o f a somati c muscle (/ ) o f Aeschna larva. (A , G from Rogosina, 1928; 'B from Zawarzin, 1912a ; C , D , E , F from Orlov, 1924.)

rise t o a n elaborat e subepiderma l nerv e net . I n th e larv a o f Aeschna, Rogosina (1928 ) find s i n eac h abdomina l segmen t jus t 1 2 cell s o f thi s type, ther e bein g o n eac h sid e o f eac h segmen t on e cel l locate d o n th e sternal region , thre e o n the latera l region , an d tw o on the terga l region . The axon s of these cells enter the ventra l ganglion of the segment throug h the first and second lateral nerve trunks. Th e subepidermal innervatio n of th e larv a o f Melolontha vulgaris i s minutel y describe d b y Zawarzi n (1912a). I t consist s o f a networ k o f larg e an d smal l nerv e branche s distributed ove r th e entir e inne r surfac e o f the bod y wal l (Fig . 26 4 B) , including the appendages , bu t i s particularly develope d on the middl e of

506 PRINCIPLES

O F INSECT MORPHOLOGY

the back . Th e principa l nerve s o f th e ne t ar e th e dista l processe s of irregular bipolar an d multipolar sensory cells of Type II . Th e processes branch dichotomousl y int o th e fiber s o f th e large r meshes , an d thes e ramify t o for m th e thread s o f the fine r meshes . Th e fiber s o f the large r meshes i n Melolontha ar e relativel y smooth , bu t th e fine r branche s ar e characteristically varicose , presentin g numerou s smal l swelling s alon g their courses , a featur e note d b y mos t writer s wh o hav e studie d th e subepidermal innervatio n o f othe r insects . Th e actua l ending s o f th e fibrils have perhaps not been observed, but th e terminal branches appea r to en d fre e o n the inne r surfac e o f the basemen t membrane. Little i s know n concernin g the sensor y innervation s o f th e muscle s of the bod y wall and appendage s of insects. Orlo v (1924 ) suggested that the skeleta l muscle s may b e innervate d fro m th e subepiderma l nerves , and Rogosin a (1928 ) has foun d i n the latera l region of the abdome n of an Aeschna larva the dista l process from a sensory cell of Type I I branchin g both t o the epidermi s and to a muscle fiber (Fig. 264 G). Th e terminal s of th e sensor y muscl e nerves (/ ) withi n th e muscl e fiber diffe r b y thei r tufted structur e bot h fro m th e sensor y terminal s o f th e epidermi s (e) and fro m th e moto r nerv e ending s i n th e somati c an d viscera l muscle s (A). I n additio n t o th e muscl e innervation, Rogosin a describes also a n innervation of connective tissue in the Aeschna larva proceeding from th e two sensory cells of Type I I foun d i n the terga l region of each abdominal segment. A sensor y innervatio n o f the alimentar y cana l has bee n described b y Zawarzin (1916) i n Periplaneta americana and by Orlov (1924) in larvae of scarabaeid beetles. Rogosin a (1928) says there ar e no sensory nerves on the alimentar y trac t i n the larv a o f Aeschna. Zawarzi n finds numerous sensory cell s of Type II , mostl y multipolar, distribute d ove r the wall s of the cro p of th e cockroach . Th e dista l processe s break up int o fine varicose fiber s tha t innervat e th e epithelia l cell s o f the cro p an d th e neur ilemma o f the stomodaea l nerve s an d ganglia . Th e axon s of these cell s go to gangli a of the stomodaea l nervous system, where they terminate i n neuropile arborizations . Th e alimentar y cana l o f scarabaei d beetl e larvae, a s describe d b y Orlov , i s innervate d bot h fro m th e stomodaea l system an d fro m th e abdomina l ganglia o f the ventra l nerv e cord , ther e being in each group of nerves both motor and sensory fibers . Th e sensory neurocytes o f th e stomodaea l syste m ar e multipolar cells , th e dista l processes o f which branch into varicos e fibrils forming a network o n th e stomodaeum and ventriculus (Fig . 264, C, D, E), innervating the muscle s and connectiv e tissue . Th e sensor y nerve s o f the proctodaeum , whic h have thei r root s i n th e abdomina l ganglia , ar e distribute d principall y to the connectiv e tissue of the anterio r intestin e and the posterior narrow part o f the rectum .

THE NERVOUS SYSTEM 50

7

The Moto r Nerv e Endings. —From th e fe w studies tha t hav e bee n made o n the termination s o f the moto r nerve s in insects, i t appear s tha t there ar e two types of motor innervation of the somati c muscles. I n on e type, probably characteristic o f the mor e generalized insects, th e end s of the nerv e fibers branch diffusel y an d th e branche s ru n lengthwis e upo n the muscl e fibers or win d aroun d the m (Fig . 26 4 A). I n th e cockroach , according to Marc u (1929) , the nerves do not enter th e muscle fibers, the terminal branche s endin g fre e betwee n them ; bu t i n a dragonfl y larva , Rogosina (1928 ) says , th e nerv e terminals , a s show n i n cros s sections , penetrate the sarcolemm a an d en d among the nbrilla e in the periphera l part o f the muscl e fiber. In the secon d type of muscle innervation th e moto r fiber ends against the muscl e in a smal l flattene d o r conica l body (Fig . 26 4 F), sometime s called the "en d plate, " or "Doyeres cone." Thes e structures , however , as show n b y Marc u i n th e somati c muscle s o f Coleopter a an d Diptera , are merely the place s on the muscl e where the nerv e suddenly break s u p into a brushlike group of fine branches that enter the muscle fiber directly and penetrat e betwee n th e myofibrillae . A singl e nerv e fibe r i n thi s type o f muscle innervation goe s to bu t on e muscle fiber ; i n th e simple r type a nerv e fibe r ma y branc h t o severa l neighborin g muscl e fibers . The moto r nerv e ending s o n the muscle s of the alimentar y cana l appea r to b e th e sam e a s thos e o n th e bod y muscles . Thus , i n th e larv a of Aeschna, accordin g t o Rogosin a (1928) , th e nerve s branc h diffusel y o n the muscle s o f the alimentar y canal , a s they d o on the somati c muscles . In th e larva e o f scarabaei d Coleoptera , o n th e othe r hand , th e moto r nerves o f th e alimentar y canal , a s describe d b y Orlo v (1924) , en d i n swellings withi n whic h eac h break s u p int o a grou p o f smal l varicos e fibrils (Fig. 26 4 F). GLOSSARY O F TERM S APPLIE D T O TH E NERVOU S SYSTE M Afferent Nerve.— A nerve that conducts from the periphery toward a nerve center; the axo n of a sensory neurone. Arborizations.—The fine branching terminal fibers of axons or collaterals . Archicerebrum (Arc).—Th e ganglionic nerve mass of the prostomiu m in Annelida. (Archencephalon.) Association, o r Internuncial, Neurone.— A neuron e lying withi n th e centra l sys tem tha t intermediate s betwee n sensor y an d moto r neurones , o r betwee n othe r association neurones . Axon, or Neurite (Axri). —The principa l process, or nerve fiber, of a neurone. Brain (Br). —The cephali c nerve mass situated above the stomodaeum , including the primitivel y pos t oral second antennal ganglia in insects . Central Nervous System.—The part o f the nervou s system containin g th e moto r neurocytes and the synapti c junction s between communicating neurones. Chiasma (Ch). —The crossin g of nerve tracts within a nerve center. Circumoesophageal Connective s (CoeCon).— The connective s betwee n th e brai n and the ventral nerv e cord embracing the stomodaeum. Primitivel y th e connective s

? ? ?????????? ?? ?????? ?????????? from th e archicerebru m to the first ventral ganglia; in insects the connective s betwee n the tritocerebral gangli a and the mandibula r ganglia. Collateral (Col). —A latera l branch of an axon . Commissure (Com). —A transvers e trac t o f nerv e fiber s connectin g th e tw o ganglia o f a segment or the lateral center s within a median ganglion. Conductivity.—The propert y o f nervous o r othe r protoplasmi c tissu e b y whic h changes in metabolic activit y are propagated through it . Connective (Con). —A longitudina l cor d o f nerv e fiber s connectin g successiv e ganglia. Cyton.—See neurocyte. Dendrons, o r Dendrites.—Finely ramifying branche s given off from a nerve cell . Deutocerebrum (2Br). —The par t o f th e arthropo d brai n containin g th e firs t antennal nerve centers . Distal Process.—The peripheral branch or one of several distal branches of a sen sory nerv e cell . Effector.—One o f the organ s of the bod y activated b y nerv e stimuli, principall y a muscle or a gland . Efferent Nerve.— A nerv e that conduct s from a nerv e cente r towar d th e periph ery; th e axo n of a motor neurone. Frontal Ganglio n (FrGng). —The media n precerebra l ganglio n o f the stomodaea l system. Frontal Ganglio n Connective s (FrCon). —The connective s betwee n th e trito cerebral gangli a and the fronta l ganglion . Ganglion (Gng). —A centra l nerv e mass ; th e ter m applie d t o a singl e primitive ganglion, o r to a body formed o f two o r more united primitiv e ganglia . Glia Tissue.—The cellular supportin g tissu e o f the nervou s system . Globuli Cells.—Specialize d associatio n cell s of the brain , usuall y distinguishe d b y their small size , poverty o f cytoplasm, an d richl y chromati c nuclei. Glomerulus.—A smal l compac t mas s o f intermingle d termina l arborization s o f nerve fibers within a nerve center . Ingluvial Ganglion.—A paired ganglion of the stomodaea l nervous system i n some insects, situate d o n the sid e of the crop . (Gastric ganglion.) Median Nerve s (MedNv). —Unpaired nerve s arisin g fro m th e gangli a o f th e ventral nerve cord between the root s o f the connectives . Medullary Substance.—Th e dens e fibrou s mas s o f nerv e terminal s formin g th e ???????? ?? ? ????????? ??????????? ???????? ?????????? ?????????????? Motor Neurone.— A neurone o f which the axo n terminates in an effector . ????? ????????? ??? ?? ??? ??????? ?????? ?? ??? ?????????? ??????? ??????? whether a single fiber or a group of fibers; a nerve trunk . Nerve Fiber.—The axon or other branches of a neurocyte. Nerve Tract.— A stran d o f nerve fibers; usually applie d t o tract s within a nerv e center. Nerve Trunk.—A bundle of nerve fibers in the peripheral system; the usual nerves. Neurilemma (Nlm). —The nucleate d sheat h o f nerve tissue, coverin g the ganglia , nerve trunks, an d terminal branches. Neurite.—See axon. Neurocyte, or Cyton (NCI).—The cel l body of a neurone, usually called the "nerv e cell." Neurone.—An entir e nerv e cell , includin g th e neurocyt e an d axo n an d al l their branches. Neuropile.—The medullary substance, o r mass of fibrous tissue withi n a ganglion .

??? ??????? ?????? ? ? Occipital Ganglio n (OcGng). —A singl e o r paire d postcerebra l ganglio n o f th e ?????????? ??????? ??????? ???????????? ???????????? ?? ????????????? ?????????? Peripheral Nervou s System.—Th e outlyin g part s o f the nervou s syste m i n dis tinction t o the centra l gangli a and connectives, includin g the sensor y neurocytes an d their axons , an d th e axon s of the moto r neurones. Protocerebrum (lJ5r).—Th e firs t par t o f th e arthropo d brain , containin g th e ocular an d othe r associatio n center s lyin g anterio r o r dorsal t o th e antenna l (deutocerebral) centers . Receptor (SO). —A so-calle d sens e organ , o r specialize d structur e o f th e integu ment responsiv e to externa l stimuli . Recurrent Nerv e (RNv). —The media n stomodaea l nerv e extendin g posteriorl y from th e fronta l ganglion. Sense Cel l (SCI). —The neurocyt e o f a sensor y neurone . A sense cel l of Type I has an unbranched distal process going to a specific sense organ; a sense cell of Type I I has one or more branched processes with a diffus e distribution . Sense Orga n (SO). —A receptor . Sensitivity.—The labil e propert y o f protoplas m tha t make s i t responsiv e t o stimuli, highl y develope d i n nerve tissue . Stimulus.—Any chang e i n th e environmen t o r i n th e interna l condition s o f th e animal that produces activity in labile tissues . Stomodaeal Nervou s System.—The nervous system centering in the ganglia of the ??????????? ???????????????? ????????? ?? ???????????? ??????? ???????? Suboesophageal Commissur e (Com/).—Th e commissur e o f th e tritocerebra l ganglia, whic h goes below the stomodaeum . Suboesophageal Ganglio n (SoeGng). —The composit e ventral nerv e mas s o f th e head in insects, forme d o f the unite d primitiv e gangli a of the gnatha l somites . Synapse.—-The centra l mechanis m o f intercommunicatio n betwee n termina l fibers o f tw o o r more neurones. Tritocerebrum (3J5r).—Th e third part of the insec t brain, formed o f the gangli a of the postoral secon d antennal somite . Ventral Nerv e Cor d (VNC). —The chai n o f connected ventra l ganglia , morphologically beginnin g wit h th e tritocerebral gangli a o f th e brain ; i n entomolog y th e term usually applie d t o the thoraci c and abdomina l ganglia only .

CHAPTER XVI I THE SENS E ORGAN S The sense organs ar e ofte n poeticall y sai d t o be the " windows of th e soul." Unfortunately , however , th e simil e ha s littl e basi s i n fact , fo r neither doe s the "soul " look out o f the suppose d window s nor doe s th e external environmen t ente r b y way of them. Literally , th e sens e organs are place s on the peripher y of the anima l wher e forms o f energy existin g in the environmen t ma y activat e th e for m o f energy latent i n the nerv e tissue o f the animal . Th e activit y thus arouse d i n a nerv e termina l i s propagated centripetall y throug h the nerve to the nerve center. Here , in sentient animals , ther e ma y b e generated a form o f consciousness, which in ourselve s w e identif y wit h th e externa l energ y tha t originate d th e impulse sen t i n ove r th e receivin g nerve ; bu t wit h insect s th e onl y evidence we have of "perception" is a motor or glandular reaction of th e individual t o the external stimulus . Sense organ s ar e specificall y receptiv e t o certai n form s o f energ y because o f their physica l structure , jus t a s a telephone receiver is specif ically receptiv e o f soun d vibration s becaus e i t i s constructe d fo r thi s purpose only . A n organ of vision is stimulated b y light waves because it permits th e penetratio n o f electromagnetic vibration s o f certain length s and doe s not respond to other kinds of stimuli, though many of the latter, such a s soun d vibration s an d odo r substances, ma y constantl y imping e upon it. Th e eye membranes of a vertebrate ma y be irritated by volatile substances, t o whic h the ey e itself i s impervious; but i f these sam e sub stances fal l upon the organs of smell, they find here a receptive apparatu s specially prepare d fo r them. Th e fact , however , that a volatile irritan t may b e perceived either a s pain o r as an odo r shows that th e effec t o f a stimulus depend s also on the connection s of the receptive end of the nerv e with th e centra l nervou s syste m an d no t entirel y o n th e natur e of th e stimulus. Where ther e i s n o know n consciou s equivalen t o f a stimulus , i t i s perhaps inconsisten t t o spea k o f the recepto r a s a "sens e organ " o r t o say that th e anima l ha s a "sense " o f smell, sight , etc. , bu t thes e term s are to o convenientl y usefu l t o b e throw n ou t o n a technicality . Sinc e insects giv e definite , visibl e muscula r o r glandula r response s t o mos t of th e form s o f energ y i n natur e tha t produc e sensor y impression s i n ourselves, an d t o som e to whic h we are unconscious , insects posses s a t 510

??? ????? ?????? ??? least correspondin g mechanisms of reaction, an d henc e in entomology we use the term s "sense " and "sens e organs" with th e understandin g that they ar e not literall y significant. The differen t manifestation s o f natur e t o whic h animal s respon d through sense organs include 1. Th e energ y o f moving masses or molecule s o f matter, an d th e energ y o f contact with stationary matter, giving us the sensatio n of touch (tactile sense). 2. Vibration s of matter that giv e us the sensatio n of sound (auditory sense,). 3. Vibration s tha t giv e u s th e sensatio n o f hea t o r o f change s i n temperatur e (thermal sense). 4. Substance s in a chemically activ e state (ionization) that giv e us the sensatio n of taste (gustatory sense). 5. Th e impalpabl e state of matter that give s us th e sensatio n o f smell (olfactory sense). 6. Electromagneti c vibration s o f certai n magnitude s givin g u s th e sensatio n of light (visual sense). 7. Gravity , for which there is no equivalent in consciousness , (static, o r geotropic, 97%

To summarize more generally, we may say that animals are responsive ?? ??? ?????? ?? ???? ?? ??? ????? ?? ????????? ??? ??????????????? ??????? and (3 ) gravity. Th e sens e organs, however, are no t attune d i n all cases to the direc t receptio n o f the primar y stimul i t o whic h the anima l react s through them . Th e reaction to gravity , fo r example, is not b y means of receptors stimulate d directl y b y gravity ; th e know n sens e organ s b y which th e anima l orients itsel f i n relation t o gravity, o r maintains itsel f in gravitationa l balanc e ar e functionall y tactile organs , sinc e the direc t stimulus i s th e movemen t o f a liqui d o r o f soli d bodie s produce d b y gravity. Th e perceptio n o f sound migh t b e sai d likewis e to b e a ver y delicate sense of touch, in which the stimulu s is the alternatin g pressur e of the soun d waves. The sens e of taste and th e sens e of smell are ofte n classe d together a s chemical senses, but the y ar e no t identical . Tast e i s produce d by sub stances i n solution , partl y i n a stat e o f ionization; smell is a perception of emanations from odorous substances, ofte n traveling long distances, th e nature o f whic h i s bu t littl e understood . Th e qualit y o f tast e i s no t determined b y chemica l composition . Th e fou r tast e varieties , sweet , sour, salty, and bitter, are distinguished by insects as by man, and insects are highly sensitive to many odor s in a degree quite incomprehensible to us. The eyes , bein g light-receptiv e organs , serv e als o a s instrument s for estimatin g for m an d for measuring distance; but al l visual impressions are based o n a perception of color and differen t degree s of color intensity. Much experimenta l proo f ha s show n tha t insects , i n thei r reactions , distinguish man y o f th e color s that w e see , an d tha t the y perceiv e a s

512 PRINCIPLES

O F INSECT MORPHOLOGY

color some of the ultraviole t ray s to which we are blind. However , since black i s absenc e o f light , an d pur e whit e doe s no t occu r i n nature , al l light perceptio n i s color perception in the physica l sense. In additio n t o th e sens e organ s tha t respon d specificall y t o th e stimulus of environmental forces, there are also organs that are stimulate d by changes in the tissues of the animal itself, such as pain receptors, and in general the proprioceptor s that registe r th e interna l physiologica l conditions of the organism. The n there are also indefinite senses, such as that of muscle tension. Insect s ar e not know n to have any specifi c organ s for the perceptio n o f temperature , thoug h the y ar e highl y responsiv e t o temperature changes , no r ar e the y know n t o hav e pai n receptor s o r proprioceptors othe r than the termina l ending s of sensory nerve fibers on the skin , muscles , and other tissues . 1. GENERA L STRUCTUR E AN D CLASSIFICATIO N O F INSEC T SENS E ORGANS

The anatomica l element s o f the sens e organ s o f insect s ar e derive d from th e cellula r and cuticular parts of the integument, and all the numerous form s tha t th e organ s assum e ar e produce d b y modification s and specializations o f these primar y elements . Th e know n sense organs ar e located in the body wall or in the ectodermal parts of the alimentary canal . The Structura l Element s o f a Sens e Organ.—I n it s simples t for m a sens e orga n consist s o f a sense cell connecte d wit h th e dista l en d o f a sensor y nerve , an d s o situated a s t o b e expose d to stimul i o r t o th e secondary effect s o f primar y stimuli . Ther e ar e tw o know n kind s o f sense cells . Thos e o f on e kin d ar e th e peripherall y situate d cyton s of sensor y neurones , whic h receiv e th e stimul i eithe r directl y (Fig . 265 A , 1SCI) o r throug h a dista l proces s (B , d). Thos e o f th e othe r kind ar e ectoderma l cell s secondaril y innervate d b y dista l branche s o f a sensor y neuron e (D , 2SCI). Sens e cell s o f th e firs t kin d ar e sensor y nerve cells and ar e termed primary sense cells; those of the secon d variety are sensor y ectoder m cell s and ar e distinguishe d a s secondary sense cells. As show n i n th e precedin g chapter , al l sensor y cell s of insects, s o far a s known, appea r t o b e primary sens e cells, but the y are divisible int o two groups accordin g t o whethe r th e dista l processe s g o to a specifi c sens e organ (Fig . 24 2 B) o r ar e distribute d i n fine branches o n a n innervate d surface (C) . Thos e o f the firs t grou p are primar y sens e cells of Type I , those o f the secon d group are primary sense cells of Type II . The sensor y innervatio n o f th e epidermi s i n insect s proceed s fro m primary sens e cell s o f Typ e II . Thes e cell s have sometime s one , bu t generally several , branche d termina l processe s tha t en d i n fin e fibril s on th e inne r surfac e o f th e bod y wal l o r betwee n th e epiderma l cell s (Fig. 26 5 C). Jus t ho w the terminal s o f these nerve s ar e stimulate d i s

THE SENSE ORGANS

513

not known; but if the stimuli are transmitted t o them from th e innervate d cells o f the epidermis , these cell s (2SCls) are o f the natur e o f secondary sense cells . Typica l secondar y sens e cell s (D , 2Scl) ar e characteristi c of vertebrat e animals , in which the cyton s of the sensor y neurones (ISCl) are locate d i n the spina l gangli a nea r th e spina l cord . A sensory struc ture of this kind i s not know n to occur in insects. The sens e cells of Type I in insects, which are the receptiv e elements of th e specifi c sens e organs, are generally regarded as primary sens e cells (Fig. 26 5 A, B, 1SCI) becaus e each is directly continuou s with a sensory nerve that goe s to a ganglioni c center, an d n o other cel l bod y ha s been found i n the course of the nerve or in any way connected with it. Severa l

FIG. 265.—Various types of sensory innervation of the epidermis, diagrammatic. A, a primar y sens e cel l (1SCI) locate d i n th e epidermis , connecte d wit h a nerve cente r (Gng) by a proximal nerv e proces s (SNv). B , a subepidermal primar y sens e cel l connected wit h the exterio r b y a singl e dista l proces s (d). C , a subepiderma l sens e cel l wit h branche d distal processes (d ) innervating epiderma l cells , which become secondary sens e cells (gSCls). D, a secondary sense cell (2SCI) innervated by a long distal process (d) from a far-distant primary sense cel l (1SCI), th e usua l vertebrat e type of sensory innervation .

writers, however, studying the developmen t of insect sense organs, asser t that th e sense cells are specialized epidermal cells that make a secondary connection wit h th e nerv e b y a centripeta l process . Scho n (1911 ) describes the outwar d growth o f a sensor y nerv e throug h th e le g of th e honey bee , an d it s fina l connectio n wit h th e tibia l chordotona l organ, the latte r bein g develope d fro m th e epidermi s o f th e le g wall . Voge l (1923) report s that th e sensor y antenna l nerv e in Hymenoptera likewise penetrates distall y through the entire length of the antenna befor e there is established an y connectio n wit h th e sens e cell s o f th e antenna l sens e organs. Fro m suc h observations i t woul d appear, a s claimed by Berlese (1909), that the receptiv e cells of most insect sens e organs are specialized epidermal cell s tha t becom e sens e cell s secondaril y b y unio n wit h a sensory nerve. I f this is true, the generativ e cytons of the sensory nerves are ye t t o b e discovered . I n th e cas e of the eyes , however, there i s no doubt that the retin a cell s are primary sens e cells, since their centripeta l processes for m synapti c junction s wit h th e terminal s o f associatio n neurones in the opti c lobes .

514 PRINCIPLES

O F INSECT MORPHOLOGY

Though th e sens e cel l is the essentia l part o f a sense organ , the sens e organs o f insect s generall y includ e associate d structure s forme d o f th e cuticula an d th e epidermis . Th e cuticula r par t take s o n various form s and appear s t o b e largel y responsibl e fo r specificit y i n th e receptiv e quality o f the sens e organ , sinc e th e structur e o f the cuticula r elemen t determines th e admissio n o f the effectiv e stimulus . Th e epiderma l ele ments o f a sense organ consist o f one or more cells usually associated wit h the sens e cel l an d ar e probabl y th e chitinogenou s matri x cell s o f th e cuticular part s of the organ . Classification o f Insec t Sens e Organs.—Insect s ar e provide d wit h a grea t variet y o f external sens e organ s havin g characteristi c structura l differences i n bot h th e cuticula r an d th e cellula r parts . I t i s ver y difficult, however , to isolat e th e variou s type s o f organs for experimenta l purposes, an d for this reason we can, in most cases , only form a n opinio n as to thei r probable function base d o n a study o f their structure ; and th e structure i s ofte n s o widel y differen t fro m that o f an y orga n o f know n function i n othe r animal s tha t man y insec t sens e organ s canno t ye t b e satisfactorily identifie d a s receptor s fo r an y particula r grou p o f stimuli . The sens e organs of insects, therefore , are generally classifie d o n a purely anatomical basis . Th e recepto r comple x forme d o f th e cuticula , th e sense cel l or group of sense cells, and th e associate d chitinogenou s cell s is called a sensillum. The simples t typ e o f insect sensillu m i s a n innervate d hair ; that is , it i s a seta having a direct connectio n with the dista l process of a sensory cell. Th e externa l cuticula r part , however , ma y tak e th e for m o f a spine, a scale, or a minute peg , and i t ma y b e sunken into a pit o r deep cavity o f th e integument . Organ s bearin g typica l seta e ar e terme d sensilla trichodea; bu t i f th e externa l proces s i s spine-like , the y ar e dis tinguished a s senilla chaetica, and when scale-like as sensilla squamiformia. If th e externa l proces s is reduced to a small pe g or cone , the organ s ar e called sensilla basiconica whe n th e proces s i s freel y exposed ; bu t i f th e latter i s sunken i n a pit th e organ s become sensilla coeloconicaj an d i f th e cavity is a deep pouch they ar e sensilla ampullacea. Two othe r group s o f sens e organ s includ e sensill a whic h hav e n o external processe s corresponding to a seta , thoug h possibl y suc h organ s have bee n derive d fro m hai r organ s b y th e complet e los s o f th e seta . Some of the organ s here included are marked externally each by a minute pit in the cuticul a and are hence often calle d sense pores; but, since most of them i n section show a dome-like or bell-shaped thickenin g o f the cuticul a surrounding th e dista l proces s o f the sens e cell , they ar e mor e generall y termed sensilla campaniformia.Others are covered externally b y an oval or elliptica l plat e surrounde d b y a narro w rin g o f membrane, an d thes e organs are distinguishe d a s plate organs , o r sensilla placodea.

??? ????? ?????? ???

A third group of sense organs also lacks a specifi c externa l structure , but eac h sense cell is associated i n a special manner with two other cells, and it s dista l proces s end s i n a characteristi c rodlik e structure , th e scolops, o r scolopale . Thes e rod-bearin g sens e organs , therefore , ar e termed sensilla scolopophora. Finally, th e eyes , o r sensilla optica, constitut e a ver y distinc t grou p of receptor s havin g structurall y littl e i n commo n with th e other sens e organs. Th e externa l cuticul a form s a transparen t are a tha t admit s light to the receptive cells. Th e latter, composin g the retina, ar e specialized ectoderma l nerv e cells , havin g centripeta l axon s tha t for m th e fibers o f th e opti c nerve. 2. TH E HAI R ORGAN S

The sens e organ s i n whic h the externa l par t ha s th e for m o f a seta , or is clearl y derive d fro m a hairlike proces s of the cuticula , retai n essen tially th e structur e o f a set a wit h it s associate d cell s in th e bod y wal l (Fig. 2 8 E) , t o whic h i s adde d a sens e cel l (Fig . 266 , SCI) havin g it s ?????? ??????? ??? ????????? ???? ??? ???? ?? ??? ???? ?? ????????? ???? the hollo w o f the latter . Th e set a o r other externa l proces s is generally set o n a circula r membrane , whic h covers the oute r en d o f a cylindrica l cavity o f th e cuticula , know n a s th e pore canal, containin g th e oute r ends o f the cellula r elements o f the sensillum . The trichoge n cel l of a hair sensillum is usually larg e (Fig . 26 6 A, Trg) ; in the formativ e stages i t extend s into the cavit y o f the externa l process , but i n the matur e orga n it i s generally retracted an d more or less vacuolated (Vac). Th e sens e cell , i n mos t cases , i s associate d wit h th e tri chogen, an d i t i s possible tha t i t i s a daughte r cel l of the latter , a s ar e said t o b e th e glan d cell s o f certai n stingin g seta e (Fig . 30) . I n som e types o f hair sensilla , instea d o f a singl e sens e cell , ther e i s a grou p of sense cell s (Fig . 269 , SCls), bu t thes e cell s are evidentl y siste r cell s since they al l branch fro m a commo n nerve (B) , and thei r dista l processe s (d ) unite i n a single terminal stran d (A , TS). The singl e sense cel l in som e insects, however, is entirely remove d from th e sensillu m an d lies beneat h the basemen t membrane , thoug h i t retain s it s connection s wit h th e sense orga n by mean s of its dista l proces s (Fig . 267 , B). Thu s w e must distinguish, amon g the hai r sensilla , sens e organ s havin g intraepiderma l sense cells, and sense organs having subepidermal sense cells . Th e sens e cell, o r grou p o f sense cells , i s covere d b y a nucleate d neurilemm a con tinuous wit h tha t o f th e connecte d nerv e (Fig . 26 6 A, Nlni). A third cell (Tmg), whic h appear s t o b e th e tormogen , o r th e generativ e cel l of th e seta l membrane , i s ofte n presen t a s a par t o f the sensillum . I t embraces th e dista l en d o f th e trichogen. Surroundin g th e cellula r elements o f th e sensillu m ar e les s specialize d cell s o f th e epidermis ,

516

PRINCIPLES OF INSECT MORPHOLOGY

which graduall y merg e int o th e ordinar y epitheliu m o f the integumen t ?????? ??? ???????? ???????? ?? ??? ???? ???? ????? ???? ???? tinuously ove r th e inne r surfac e o f th e sensillum , excep t wher e i t i s penetrated b y the nerv e (Nv). The cellula r element s o f th e sensillu m d o no t alway s retai n thei r more primitiv e relation s t o on e another . Ofte n th e sensillu m become s elongate, an d its three principa l cell s take a serial arrangemen t (Fig . 266 B). Th e tormogen , being the outermos t cell , is then calle d the ca p cell

FIG. 266.—Diagrams of the relation s of the cellula r elements in a simple hair sensillum . A, th e mor e generalize d type . B , th e cell s i n axia l arrangement . BMb, basemen t membrane; c, connection of sense cell with cuticula; CpCl, cap cell (tormogen); Ct, cuticula; d, dista l proces s o f sens e cell ; ECl, envelopin g cel l (trichogen) ; Epd, epidermis ; Nlm, neurilemma; Nv , sensor y nerve; SCI, sens e cell; Set, seta; Tmg, tormogen ; Trg, trichogen; Vac, vacuole.

(CpCl), an d th e intermediat e trichoge n cel l th e enveloping cell (ECl). The innermos t cel l is the sens e cell (SCI). Though th e variou s hai r sens e organ s ar e generall y classifie d ana tomically accordin g to th e structur e o f the externa l process , o n a mor e fundamental, an d apparentl y physiological , basi s the y may be separated into tw o groups, accordin g to whethe r th e sensillu m contain s a singl e sense cel l (Fig . 267 ) o r a grou p o f sens e cell s (Fig . 269) . I n organ s of th e firs t kin d th e externa l cuticula r process, whatever it s siz e or form , has thic k wall s an d evidentl y coul d hav e onl y a mechanica l function i n stimulating the nerve . Wit h organ s of the secon d grou p the cuticula r part i s delicat e an d i s usuall y ver y small ; it s thin , nonsclerotize d wal l suggests tha t thes e organ s may b e pervious to odo r or taste substances . We may , therefore , on a structura l basis , divid e th e seta l sens e organ s

??? ????? ?????????

into tactil e organs , o r tangoreceptors, havin g a singl e sens e cell , an d chemoreceptors, havin g a group of sense cells. Sensilla trichodea.—In a sense organ of the typica l setifor m variety , the hairlik e proces s is generally freel y movabl e o n the basa l membrane . The hai r socke t ma y b e flus h wit h th e genera l surfac e o f th e cuticula , elevated o n a tubercl e (Fig . 26 7 B), o r se t int o a n alveola r cavit y (A) . The sens e cel l usuall y ha s a n intraepiderma l positio n i n adul t insect s (A, SCI), bu t i n many larval insects (B ) it i s subepidermal, lying entirel y outside th e sensillu m an d connecte d wit h th e latte r b y a lon g dista l process (d). Th e innervatio n of sensory hairs from bipolar subepiderma l

???? ???????? ????? ?? ??????????? ?? ????? ????????? ?? ????????? ???? ?????? epidermal sens e cell, organ on cercus of Gryllus. (Adapted from Sihler, 1924.) B , sensillu m with subepiderma l sens e cell , body hair of cabbage caterpillar. (Adapted from H . Schneider, 1923.)

sense cell s ha s bee n describe d i n larva e o f Odonata , Coleoptera , Lepi doptera, an d Dipter a b y Viallanes (1882), Mont i (1893 , 1894) , N. Holm gren (1896) , Hilto n (1902) , Zawarzin (1912a), H . Schneide r (1923) , an d Orlov (1924) . Sinc e the origi n an d natur e o f the sens e cell are not defi nitely known , th e significanc e o f it s subepiderma l positio n i n larva l insects i s not understood . Accordin g to Beth e (1896) , the sensor y hair s on the mout h part s of the crayfis h are likewis e innervated fro m a sub epidermal nerv e plexus. The dista l proces s o f the sens e cel l i s usuall y attache d t o th e bas e of th e set a o r t o th e seta l membran e (Fig . 26 8 A, B, C) , bu t i n certai n types o f organs it extend s into the hair (D) . A t the poin t o f attachmen t there ma y b e a smal l interna l cuticula r structure , an d som e writer s have observe d a differentiated bod y in the dista l process. It seem s probable tha t mos t seta e innervate d throug h a single sens e cell ar e organ s o f touch , th e movemen t o f th e hai r cause d b y contac t with a n object bein g the origi n of the stimulu s imparted t o the sens e cell through th e dista l proces s of the latter connected with the seta . Tactil e hairs ar e o f commo n occurrence in th e Arthropoda ; i n insect s the y ar e distributed ove r mos t part s o f th e bod y an d th e appendages . Th e

518

PRINCIPLES OF INSECT MORPHOLOGY

provision wit h innervate d movabl e hair s offset s th e los s o f surfac e sensitivity i n animal s havin g a sclerotize d integumen t an d enable s th e animal, moreover, to becom e "aware'7 o f the approac h or nearness o f an external object befor e comin g into actua l bodil y contact wit h it . Certain small , slende r hair s wit h ver y delicat e wall s hav e bee n supposed t o b e receptive to odor s an d ar e distinguishe d a s chemoreceptive hairs. Suc h hairs are said to be innervated eac h by a group of sense cells, an d i n thi s respect the y resembl e som e of the stil l smalle r peglik e organs classed as sensilla basiconic a (Fig . 26 9 A).

???? ???????????? ????? ?? ??? ????????? ????? ?? ????? ????????? ???? ??????? ????? ?? ????????? ????? ???????????? ?????? ?? ??????? ????? ?? ???? ?? ?????? ???????????? (From Freiling, 1909. ) G , club-shape d sensor y hai r o n cercu s o f Gryllotalpa. (From Sihler, 1924. )

Sensilla chaetica.—The sense organs included here are merely trichoid sensilla i n whic h the externa l proces s i s bristle-lik e o r spine-lik e rathe r than typicall y setiform . Th e thic k wall s of the proces s evidentl y pre clude any possibilit y o f penetration b y odo r or taste substances, an d th e sensilla chaetic a ar e therefor e probabl y tactil e i n function . Th e tactil e mechanism, however , as alread y pointe d out , ma y serv e i n a capacit y that i s no t functionall y on e o f touch . Stati c reaction s an arthropods , for example , ar e brough t abou t throug h organ s tha t ar e essentiall y tactile i n structure . I n insects , stati c organ s ar e no t o f general occur rence, but i n certain aquati c Hemipter a i t appear s that special groups of innervated movabl e hairs serv e to regulat e the equilibriu m o f the insect . Organs o f thi s kin d hav e bee n describe d b y Baunack e (1912 ) o n th e ??????? ?? ????? ??? ?? ??? ???????? ?? ????????? ?? ????? ???? ? 1933). Organ s on the terminal segment of certain mud-inhabiting tipuli d larvae, described by Wolf f (1922 ) as organs to enabl e the insec t to orien t

??? ????? ?????? ???

itself accordin g to th e mu d conten t o f its medium , consist o f pits o f th e integument containin g innervated hairs . Sensilla squamiformia.—Innervate d scale s hav e bee n describe d o n the wing s of Lepidoptera b y Gtinthe r (1901) , Freilin g (1909) , an d Voge l (1911). Accordin g to Vogel , sensor y scale s occu r in al l group s o f Lepi ???????? ????????? ????????? ??? ??? ????? ?? ???? ????? ?? ??? ?????? but especiall y o n th e margina l veins , an d als o o n th e win g bases . A sensory scal e is usually elongat e fusifor m i n shape , wit h fewe r rib s tha n the othe r scales , an d th e dista l par t i s draw n ou t int o a lon g taperin g point (Fig . 26 8 F). Eac h scal e is innervated b y a single large sense cell,

FIG. 269.—Structur e o f th e chemoreceptiv e typ e o f sensill a wit h thin-walle d oute r ??????? ?????? ???? ?? ???? ??? ???????? ????? ?????? ?? ???????????? ??????? ?? ????????? showing grou p o f sense cell s (SCls) , fascicl e (Fas) o f distal processes o f sense cells, terminal strand (TS) o f fiber s connecte d wit h cuticuia , an d minut e bodie s (e) a t thei r inne r ends . B, simplifie d diagra m o f separate d sens e cell s wit h dista l (d) an d proxima l (p ) processes , and termina l connection s (t) wit h th e cuticuia .

the dista l proces s of which, Vogel says, is attached t o the base of the scale. The innervate d scale s wit h thei r projectin g points woul d appea r t o b e tactile in function . Sensilla basiconica.—Sensor y peg s an d cone s ar e innervate d hair s reduced i n size , an d ther e i s n o sharpl y dividin g lin e betwee n sensill a trichodea an d sensill a basiconica , eithe r i n the characte r o f the externa l parts o r i n th e structur e o f the interna l parts . I n a typica l sensillu m basiconicum th e externa l proces s is a smal l peglik e o r conica l structur e (Fig. 26 9 A, Pg). Th e wall s of the proces s are thick o r strongly scleroti c in som e cases , whil e i n other s the y ar e thi n an d transparent , o r th e process ma y terminat e i n a delicat e membranou s cap . Organ s o f th e first typ e apparently , can respon d onl y t o mechanica l stimuli an d mus t

520 PRINCIPLES

O F INSECT MORPHOLOGY

be regarded as tactile i n function; thos e havin g thi n membranou s walls, however, ma y b e supposed to b e pervious to chemica l stimuli, an d suc h organs are usually regarded as receptors of taste or odor (chemoreceptors.) The interna l structur e o f th e tw o group s o f sensill a basiconica , a s distinguished b y th e natur e o f th e externa l process , i s als o character istically different . Th e organ s havin g thick-walle d externa l processe s are innervated b y a single sense cell, as are the typica l tactile hairs; those having thin-walled external processes, on the othe r hand, usually contai n a group of sense cells (Fig. 269) and thus resemble the thin-walled chemo receptive sensill a trichodea . The cellula r elements o f a sensillum basiconicum with multiple sens e cells (Fig. 269 A) include a distal cap cell (CpCl), a large vacuolated enveloping cell (ECl), and a compact group of sense cells (SCls). The cap cel l appears to embrac e the dista l end of the envelopin g cell, and th e latter t o be penetrated b y th e dista l part s of the sens e cells. Th e dista l processes of the sense cells form a thick fascicle (Fas) ending in a terminal strand o f fine fibers attached distall y in the ape x of the externa l process (Pg or Set). Near the middle of the bundle of terminal processes is a group of minute bodies (e), the numbe r of which appears to correspon d to the numbe r o f sense cells. Thes e bodie s have bee n particularly studie d in th e was p b y Voge l (1923) , wh o call s the m "olfactor y rods " (Riechstdbschen), bu t the y ar e s o extremel y minut e tha t nothin g satisfactor y can be determined as to their natur e or structure. Whil e they appea r t o belong t o th e sens e cel l processes , i t i s perhap s possibl e tha t th e ter minal filament s beyon d them (B , f) ar e cuticula r processes to whic h th e cell processe s ar e attached , an d tha t th e bodie s i n question , therefore , might b e themselves cuticula r structures . The possibl e chemoreceptiv e function o f organ s o f this kin d i s sug gested b y th e thinnes s an d apparen t permeabilit y o f the cuticula r wal l of the externa l process, and by the presence of a large vacuole surrounding the fascicl e of sense cell processes. Th e latter are thus bathed i n a liquid in whic h odo r o r tast e substance s migh t b e dissolve d i f the y ca n pas s through th e wall s o f th e externa l cuticula r process . Sens e peg s an d cones have been found o n all parts of the bod y and appendage s of various insects, bu t the y occu r principally on the antennae and mputh parts and in the preora l cavity . Sensilla coeloconic a an d ampullacea.—Sens e organ s o f thes e type s are pe g organ s sunken into depression s of the bod y wall (Fig . 270) . I f the depressio n i s shallow , the orga n i s termed a sensillum coeloconicum (A, B); if it i s deep or flask shaped, the orga n is distinguished a s a sensillum ampullaceum (C , D) . A s wit h th e hai r organ s an d th e expose d peg organs , som e o f th e sunke n peg s ar e thic k walle d o r soli d an d ar e innervated eac h by a single sense cell; others have thin walls and contain

THE SENSE ORGANS 52

1

each a group of sense cells. Th e secon d are regarded as chemoreceptors, but th e functio n o f the forme r i s no t clea r sinc e th e externa l part s ar e removed from contac t wit h mechanical stimuli. I n som e cases a number of peg s ma y occu r i n a singl e pit . Organ s o f thi s kin d occu r o n th e antennae o f certai n Diptera . Thos e o f th e hous e fly , a s describe d b y Rohler (1906) , ar e i n som e cases simple cavities containing 1 0 to 2 0 sensory pegs , an d other s ar e compound , eac h cavit y bein g divide d int o several shallo w compartments, eac h with it s grou p of pegs. Th e labia l palpus o f th e cabbag e butterfl y (Pieris) ha s a dee p apica l cavit y con taining man y sens e pegs. Sensill a ampullacea occu r particularl y o n th e

FIG. 270.-—The cuticular parts of several varieties of sensilla coeloconica (A , B) an d ampullacea (C , D).

antennae o f Hymenoptera. I n som e the cavit y containin g the sens e peg is distinguished fro m tha t o f a typical sensillu m coeloconicum simply b y being deepe r an d mor e flasklike in for m (Fig . 27 0 C); in other s (D ) th e "flask" i s connected with the exterior by a long tubular neck. 3. TH E CAMPANIFOR M ORGAN S

The sens e organ s include d i n thi s clas s hav e bee n calle d vesicles, organs of Hicks, papillae, cupola organs, dome organs, umbrella organs, bell organs, an d sense pores. I n vertica l sectio n th e cuticula r part s of the organ s generall y have the for m o f a small dome into whic h the sens e cell proces s i s inserte d lik e th e clappe r o f a bell . Fo r thi s reaso n th e organs ar e appropriatel y terme d i n genera l sensilla campaniformia (Berlese, 1909) . The externa l part s o f th e campanifor m organ s are , i n som e cases , small dome-lik e papillae , o r bu t slightl y conve x swellings , usuall y les s than 2 5 microns i n diamete r (Fig . 271) ; others (Fig . 27 2 B) ar e minute discs. slightly sunke n int o th e bod y wall , resemblin g i n surfac e vie w vacant hai r follicles , thoug h the y ar e usuall y distinguishabl e fro m th e circular hair socket s by a more elliptical o r oval form. Th e dome or disc in typica l example s consists o f a ver y thi n oute r lamell a o f the cuticul a (Fig. 27 1 A , a) , an d o f a n endocuticula r layer (b) generall y havin g th e

522

PRINCIPLES OF INSECT MORPHOLOGY

form o f a n inverte d cu p o r saucer . Th e inne r laye r i s perforated b y a central openin g o r b y a n axia l sli t throug h whic h the dista l en d o f th e sense cell process (d) i s inserted o n the unde r surface o f the oute r lamella. Beneath th e ca p is the usua l canal of the cuticula . Each campanifor m orga n i s innervate d throug h a singl e sens e cel l (Fig. 27 2 A,-SCI); bu t sinc e the organ s ofte n occu r in groups, there ma y be a compac t mas s o f sense cell s in th e neighborhood . Th e sens e cell s are long and narro w and generall y project into the bod y cavity , thoug h they are covere d by the basemen t membran e of the epidermis . Accord ing t o Newto n (1931) , th e sens e cell s of the campanifor m organs o f th e

FIG. 271.—Structur e o f the euticula r part s o f various type s o f sensilla campaniformia. A, diagrammatic sectio n showing outer (a ) an d inne r (6 ) lamella o f the dome , an d euticula r connection (c ) o f dista l proces s (d) o f sens e cell . B , C , section s throug h shor t an d lon g diameters o f organ on halter o f Calliphora. D , organ on halter of Syrphus.E, fro m cercus of Periplaneta. F , sunke n orga n fro m labiu m o f Dytiscus. G , sunke n orga n o n mandibl e of Dytiscus with n o external opening . (B , C, D from Pflugstadt, 1912 ; E from Sihler, 1924 ; F, G from Hochreuther, 1912. )

honey bee are doubly fusiform (B, SCI) and each swelling contains a nucleus. I n the dista l proces s of each cell is a "sense fiber " (/) , the ape x of which , Newto n says , end s i n a slightly swollen refractive body, which lies directl y agains t th e inne r surfac e o f th e thi n oute r lamell a o f th e external dome . I t i s possibl e tha t th e presenc e o f thi s bod y i n th e center o f the ca p has given rise to the idea that the nerve fiber penetrates to th e exterio r (Mclndoo , 1914) . I n som e insect s th e termina l bod y appears t o b e represented b y a relatively larg e thickening of the cuticul a (Fig. 27 1 C, c), while in others it takes the for m o f a small capsule receiving th e en d o f the sens e cel l process (Figs . 27 1 A, B , D , 27 2 A, c) . I n any cas e i t i s probabl y onl y a euticula r modificatio n a t th e poin t of attachment o f the sens e cell. Special epiderma l cell s associated wit h th e sens e cel l have no t gener ally been observed in the campaniform organs. Newto n (1931) expresse s the opinio n tha t th e sens e cel l i s bot h chitinogenou s an d receptiv e i n

??? ????? ????????? function. Variou s investigators , however , hav e foun d trace s o f othe r cells i n th e campanifor m sensilla , an d Sihle r (1924 ) describe s i n Periplaneta a larg e cel l (Fig . 27 2 A, C7 ) endin g in th e cana l o f th e cuticula , which is traversed b y th e dista l proces s of the sens e cell (d). Campaniform organ s occu r o n th e head , thorax , abdomen , th e antennae, mout h parts , legs , win g bases, cerci , an d oviposito r o f adul t insects an d hav e bee n observe d i n al l th e principa l orders ; the y hav e also bee n foun d o n th e larva e o f som e species . Th e functio n o f th e organs i s a subjec t o n whic h ther e i s som e differenc e o f opinion , bu t Mclndoo (1914 , 1915 ) ha s give n much experimental reason for believin g

FIG. 272.—Section s o f cuticula r an d cellula r part s o f sensill a campaniformia . A , organ o n cercu s o f Blatta orientalis wit h singl e large cell (C7 ) associate d with the sens e cell (SCI). (Diagrammatic from Sihler, 1924. ) B , organ s fro m bas e o f hin d win g o f Apis mellifica wit h binucleate sense cells. (From Newton, 1931. )

that th e organ s ar e receptor s o f odo r stimuli , sinc e ther e i s scarcel y any questio n tha t insect s retai n a " sense o f smell " afte r remova l of the antennae . Structurally , however , th e campanifor m organ s woul d appear t o offe r bu t littl e surfac e fo r th e penetratio n o f odor substances , and, since each is innervated b y a single sense cell, their receptiv e power must b e o f a lo w order , considerin g that mos t o f th e apparen t chemo receptors hav e multiple sense cells. Th e campanifor m organs, however, usually occur in groups. 4. TH E PLAT E ORGAN S

The sensill a placode a presen t externall y eac h a thi n cuticula r plate , elliptical, oval , or elongate in form, se t over a large cavity i n the cuticul a (Fig. 273) . Th e interna l structur e o f thes e organ s (Fig . 274 ) closely resembles that o f sensilla basiconic a havin g multipl e sens e cells . Plate -

524

PRINCIPLES OF INSECT MORPHOLOGY

like organs occur on the antenna e of certain Homoptera, Coleoptera, an d Hymenoptera. Among the Homoptera antenna l plat e organs are present in Aphididae, Aleurodidae, an d Psyllidae , bu t th e "plate " is here a thin membranou s disc (Fig . 27 3 A). I n Coleopter a (B , C , D ) an d Hymenopter a (E-K ) the plate , thoug h thin , i s usually sclerotize d and i s generall y separate d from th e surroundin g antenna l wal l by a membranous ring o r a n inflec tion. Accordin g to Hochreuthe r (1912) , ther e ar e fro m 4,50 0 t o 5,00 0 plate organ s o n th e antenna e o f Dytiscus, the plate s bein g ver y small , measuring onl y 6 t o 8 micron s in diameter . I n th e Hymenopter a th e

FIG. 273.—Th e cuticula r part s o f variou s form s o f sensill a placodea . A , sensor y plates o n antenn a o f a n aphis . B , sectio n o f antenna l plat e organ o f Dytiscus. C , sam e of Cetonia aurata. D , sam e o f Necrophorus vespillo. E , surfac e view of antenna l plate of Cynips gallae. G , sectio n o f E . H , I , surfac e vie w an d sectio n o f antenna l plate s o f Vespa crabro. J , K , surfac e view an d sectio n o f antennal plat e o f Apis mellifica: a , oute r ring of clear cuticula ; b, inner groove . ( B from Hochreuther, 1912; C from vo n Rath, 1888 ; D-I from Ruland, 1888. )

plates ar e usually muc h larger an d var y i n for m fro m a n ellips e (J ) t o a narrow elongate oval (E, H). The y are generally flush with the antenna l surface (K ) but ar e sometimes elevated (F ) and may be surrounded by a deep groov e (I) . In the hone y bee it is estimated tha t there are abou t 30,000 plate organ s on both antenna e o f the drone , 5,00 0 to 6,00 0 in th e worker, and 2,000 to 3,000 in the queen. Eac h plate is elliptical in shape, and fro m 1 2 to 1 4 microns in it s longes t diameter , whic h is lengthwise on th e antenna . Surroundin g the plat e i s a narro w membranou s rin g (J, K , a) , withi n whic h i s a concentri c lin e forme d b y a submargina l groove (K , 6 ) o n the inne r surfac e o f th e plate . The interna l structur e o f a sensillu m placodeu m i n Hymenopter a (Fig. 27 4 A ) include s a larg e plat e cel l (CpCl), a n elongat e cel l (ECl) enveloping th e dista l stran d o f th e sens e cells , an d a compac t mas s of numerous sense cells (SCls). Th e envelopin g cell (ECl) is attached b y a slender nec k to th e externa l plate a t a definit e poin t i n the submargina l

THE SENSE ORGANS

525

groove of the latter. It contains a large vacuole (Vac) surrounding the dista l processes o f the sens e cells (Fas) an d th e termina l stran d (TS) connecting th e latte r wit h the plate . I n a tangential sectio n jus t below the plate , therefore , th e larg e plat e cell , o r ca p cel l (B , CpCl), appear s to surroun d th e nec k o f the envelopin g cel l (ECl), which , i n turn , con tains the terminal strand (TS) of the sense cells. At a lower level (C) the fascicl e o f distal sense cel l processes (Fas) is seen within th e vacuol e of th e envelopin g cell. Th e grou p of sense cells in each organ lies agains t the basemen t membran e o f th e epidermi s (A , SCls), an d th e dista l processes form a compact fascicle (Fas), whic h contracts int o the termina l strand (TS) attache d t o th e plate . A t the dista l en d of the fascicl e i s a group o f minut e refringen t bodie s (e) a s i n th e sensill a basiconica . In th e antenna l plat e organ s o f Aphididae, a s figure d b y Floge l (1905), th e dista l processe s o f th e sense cell s ar e shor t an d thei r attachments ar e distribute d ove r the entir e inne r surface s o f th e membranous plates . The plat e organ s are commonly regarded a s olfactor y i n function , and thos e o f th e aphid s woul d appear to be well constructed for „ ^^ t FIG. 274.—A sensillum placodeum of

the reception of odor Stimuli. In Apis mellifica, diagrammatic. A, vertical Coleoptera and Hymenoptera , how- sectio n showing large ca p cell (CpCl) beneat h . ^ . outer plate, enclosing distal end of vacuolated ever, the relatively thick sclerotic enveloping cell (ECl), which contains the plates Canno t b e Suppose d t o b e fascicl e (Fa S) o f sense cel l processe s an d the ^ ^^ termina l stran d (TS). B , horizonta l sectio n pervious t o Odo r Substance, those o f jus t below outer plate. C , horizontal section

the hone y be e bein g abou t 1. 5 throug h bas e of enveloping cell. microns i n thickness , an d th e narrowe d distal stalk o f the receptiv e cell s attached i n the groove of the plate presents a very restricted are a at which stimuli coul d be effective . I f the organs , nevertheless, ar e olfactory, th e plate woul d appear a t leas t t o b e an entirely superfluou s adjunct. 5. TH E SCOLOPOPHOROU S ORGAN S

The scolopophorou s organs ar e usuall y compoun d sense organs , eac h consisting o f a bundl e o f simpl e sensill a havin g a commo n poin t o f attachment o n the bod y wall. Ther e i s no specifi c differentiatio n of th e cuticula that forms structurall y a part of the sensillum , a s with the othe r sense organs , thoug h th e poin t o f attachmen t o n the cuticul a ma y b e marked b y a pit, a thickened disc , o r a sclerotic nodule. I n som e cases , however, a scolopophorou s orga n i s immediatel y associate d wit h a

526

PRINCIPLES OF INSECT MORPHOLOGY

membranous are a o f th e integument , know n a s a tympanum, an d th e latter the n become s functionall y a n essentia l par t o f th e receptiv e apparatus. Frequentl y th e inne r en d of the orga n is united t o th e bod y wall b y a ligament , an d i n suc h case s th e entir e orga n ma y hav e th e form o f a thick cor d stretched betwee n two points o n the bod y wall (Fig . 275 B). Sinc e th e scolopophorou s organs firs t studie d wer e of this type or wer e thos e foun d associate d wit h tympana l membranes , the y wer e given th e nam e o f chordotonal organs o n th e assumptio n tha t the y wer e

FIG. 275.—Sensill a scolopophora . A , diagrammati c structur e o f a singl e scolopo phorous sensillum . B , chordotona l typ e o f scolopophorou s organ i n abdomina l segmen t of larv a o f Monohammus confusor. (From Hess, 1917. ) C , scolopophorou s organ s i n tibia o f righ t forele g o f Decticus. (Adapted from Schwabe, 1906. ) AB, apica l body ; aTm, anterio r tympanum ; aTra, anterio r trachea ; AxF, axia l fiber ; cr , crest; g , posterio r attachment o n cuticula ; h, chordotona l ligamen t attache d t o cuticul a a t i~ ImO, inter mediate organ; /, pleural disc of body wall; LW, leg wall; pTm, posterior tympanum ; pTra, posterior trachea ; SgNv, subgenua l nerve ; SgO, subgenual organ ; TC , tympana l cavity ; TmNv, nerv e o f tympanal organ; TmO, tympana l organ .

sound receptors . A wider knowledge of the organs , however , now shows that this term ca n apply literall y only to certai n type s of scolopophorous organs. Each scolopophorou s sensillu m (Fig . 27 5 A ) consist s o f three dis tinct cells : a dista l ca p cel l (CpCl) attache d t o th e cuticula , a n inter mediate envelopin g cel l (ECl), an d a basa l sens e cel l (SCI). The i nner end o f th e ca p cel l embrace s th e oute r en d o f th e envelopin g cell , an d the latte r surround s th e elongat e dista l proces s o f th e sens e cel l (d) , the ape x o f whic h i s connecte d wit h th e cuticul a b y a termina l fibe r (t) traversin g th e ca p cell . The distinguishin g featur e o f sensill a scolopophor a i s th e presenc e of a well-differentiated , peg-shaped " sense rod, " o r scolops, a t th e ape x of eac h sens e cel l (Fig . 27 5 A , Sco). Th e sens e ro d (corpus scolopale, Stift o f Germa n writers ) i s ofte n calle d a scolopale. Th e ter m i n thi s

THE SENSE ORGANS 52

7

form, bein g a n adjectiva l derivative , shoul d b e pluralize d a s scolopalia. The form s "scolopala " an d "scolopalae " (Snodgrass , 1926 , an d others ) have no grammatical standing . The scolope s var y i n lengt h fro m a fe w microns t o a s muc h a s 2 3 microns i n differen t organs . I n for m som e are slende r an d cylindrical , others ar e shor t an d bulblike , bu t th e typica l ro d (Fig . 27 5 A, Sco) i s elongate, somewha t expande d towar d th e dista l end , an d the n sharpl y tapering to an acute point. I n the distal part is an " apical body" (AB), always conspicuou s in staine d specimens , t o whic h is attached a n axia l fiber (AxF) o f th e sens e cell . Th e wall s o f th e ro d ar e usuall y ribbe d internally. Ofte n ther e i s no apparent connectio n between the ro d an d the body wall, but in most organs a fine terminal strand (f) extends from the ape x o f the ro d t o th e cuticula . Th e connectio n wit h th e cuticul a suggests that the scolop s itself may be a cuticular structure, but, accord ing to th e vie w of Eggers (1923) , the scolop s is a part o f the wal l of th e sense cell, and its ribs are fibrous thickenings of the latter continued into the termina l strand , whic h ma y b e likene d t o th e tonofibrilla e o f a muscle. Scolopophorous sens e organ s ar e widel y distribute d i n insects , bu t until recentl y the y hav e no t bee n reporte d i n othe r arthropods . I n adult insect s they have been found in the head, the thorax, the abdomen, the antennae , th e legs , and the win g bases. Organ s of the cordlik e type of structure (Fig . 275 B) occur only in the abdome n of certain larvae, bu t organs of the usua l form hav e been described in the labium, the legs, and even in the tars i of larval insects . Tympanal scolopophorou s organs ar e wel l know n i n severa l group s of insects, as those on the base of the abdomen in Acrididae and Cicadidae , on the for e tibiae of Tettigoniidae and Gryllidae, on the thorax of Notonectidae, an d on the thora x and abdomen of Lepidoptera. Mos t interestin g and comple x of these organ s are thos e i n the tibia e of the Tettigoniida e (Fig. 27 5 C). Ther e ar e her e in each leg three separat e organ s lying on the outer face of the leg trachea (Trd) between two tympana (aTm, pTm) concealed i n smal l cavitie s (TC) o f the tibia l wall . Proximally , belo w the "knee, " i s a larg e fan-shaped subgenua l orga n (SgO), the n a smal l intermediate orga n (ImO), an d finall y a lon g tympana l orga n (TmO). The ca p cell s o f the las t ar e arrange d i n a graduate d serie s formin g a crest (cr) on the outer wall of the trachea, with the sense cells (SCI) lying laterally alon g the nerve trunk (TmNv). It seem s probabl e tha t i n genera l th e scolopophorou s organ s ar e receptors o f vibrator y stimuli , an d tha t thos e associate d wit h tympana l membranes are organs of " hearing," tha t is, phonoreceptors. The sens e orga n locate d i n th e secon d segmen t or , pedicel , o f th e antenna o f nearl y al l insects , know n a s th e organ o f Johnston, is t o b e

??? ?????????? ?? ?????? ??????????

classed wit h th e scolopophorou s organs , thoug h th e sens e rod s o f thi s organ ar e no t typicall y scolopoi d (Fig . 276) . Th e numerou s sensilla , ?????????? ???? ??? ??????? ???????? ????? ????? ??? ???????? ?? ??? form o f a cylinde r withi n th e pedicel , an d thei r dista l end s ar e attache d in a circl e to th e articula r membran e betwee n th e pedice l an d th e nex t antennal segment , whic h is the bas e o f the flagellu m (A) , or to scleroti c processes radiatin g fro m th e latter (B , a, &) . I n size and complexit y th e organ o f Johnston varie s muc h in differen t insects ; it attain s it s highes t development i n Gyrinidae , Chironomidae , an d Culicida e (B) . Ther e

FIG. 276.—Th e orga n o f Johnsto n i n simpl e an d comple x form . (From Child, 1894. ) A, orga n in secon d segmen t of antenna o f Melolontha vulgaris showin g dista l end s of sens e cell processe s attached i n pit s (Pi) i n articula r membran e at bas e o f third segment . B , organ i n secon d segmen t o f antenn a o f Corethra showin g distal processe s o f sens e cell s attached t o prong s (& ) fro m circula r plate (a ) on bas e o f third segment .

appears t o b e little doub t tha t th e orga n serve s t o registe r movement s of th e dista l par t o f the antenna , whic h is freely implante d o n the en d of the pedice l (se e Eggers 1923-1929) . A ver y simpl e orga n o f the " chordotonal" typ e o f structure, thoug h it contain s n o sens e rod , o r scolops , ha s recentl y bee n foun d i n th e antennule o f an amphipo d crustacean Caprella dentata b y Wetze l (1934) . It consist s o f a serie s o f thre e cell s attache d basall y t o th e epidermi s in the dista l end of the secon d segment of the antennule , an d distally b y a long stran d t o th e wal l of the thir d segment . Th e proxima l cel l is th e sense cell , with a fiber entering th e antennula r nerve ; the othe r tw o cells appear t o b e an enveloping cell and a cap cell. 6. TH E EYE S

The wor d "eye " i s used i n genera l for an y orga n tha t i s specifically sensitive t o ligh t ray s impingin g upo n it , an d capabl e o f transmitting the effec t o n its sens e cell s to th e centra l nervou s system . A primitiv e eye, therefore , i s merel y a light-perceivin g organ , o r photoreceptor ,

THE SENSE ORGANS 52

9

and it is not to be assumed that all eyes are capable of registering impressions o f form , colo r differences , o r motio n i n externa l objects . Th e effect o f the ligh t stimulu s mus t depen d o n the developmen t o f the ey e and o f the opti c center s of the nervous system . In som e of the Annelid a photoreceptive cells occur scattered throug h the epidermi s an d giv e the anima l a genera l sensitiveness t o degree s of light intensity . Amon g insects , th e Collembol a ar e sai d t o exhibi t a reaction t o ligh t throw n o n th e genera l bod y surface ; bu t sinc e th e integument contain s n o photoreceptors , th e sensitivenes s probabl y arises fro m th e effec t o f ultraviolet ray s o n th e bod y tissues. Th e eye s of insect s ar e alway s locate d o n th e head , an d th e opti c center s li e i n the protocerebra l parts of the brain . Th e eye s belong, therefore, to th e preantennal regio n o f th e procephalo n an d ar e probabl y prostomia l organs in their origin . The receptiv e element s o f an ey e sensillum , a s o f an y othe r sensor y organ, ar e specialize d cell s of the epidermis . A n essentia l featur e o f a photoreceptor, therefore, i s a transparency i n the cuticul a over the sens e cells. I t i s conceivabl e tha t an y sensor y cell s migh t b e stimulate d b y light i f ligh t ray s ar e abl e t o reac h the m i n sufficien t strength , an d i t appears that certai n organ s on the hea d o f the larva e of higher Diptera, having essentially the structure of the supposedly chemoreceptive organs, are photoreceptive in function b y reason of the transparenc y an d lenticular for m o f th e externa l cuticula r part . Th e sens e cell s o f th e usua l optic organ s o f insect s diffe r fro m th e sensor y cell s o f th e othe r sens e organs i n tha t the y lac k termina l processes , an d i n tha t th e receptiv e part o f each cell is finely striated perpendicula r to th e receptiv e surfac e (Fig. 27 8 A , a). Th e striation s appea r t o b e termina l thickening s o f neurofibrillae traversin g th e sens e cell . Usuall y th e striate d part s of two o r mor e adjacen t cell s are unite d t o for m a n opti c rod , o r rhabdom (H, I , Rhb) m, th e componen t element s o f a rhabdom , representin g th e striated parts of the opti c cells, ar e rhabdomeres. The photoreceptor s o f insects , a s o f arthropod s generally , includ e dorsal an d latera l eyes . Th e dorsa l eye s ar e alway s simpl e ocelli , bu t the latera l eye s may b e eithe r simpl e or compoun d in structure , an d i n many case s i t i s difficul t t o distinguis h o n a n anatomica l basi s th e tw o kinds o f simpl e eyes . Th e usua l opti c organs , however , fall int o thre e groups that are ontogenetically distinct i n the individual an d for descrip tive purpose s ma y b e distinguishe d a s dorsal ocelli, lateral ocelli (stem mata), an d compound eyes. I n actua l positio n th e morphologicall y dorsal ocell i ar e dorsal , anterior , o r ventra l accordin g t o th e positio n of th e facia l are a o f the head . Th e apparen t photoreceptor s o f muscoid maggots constitute a separate clas s of light-perceiving organs, since they have none of the characteristi c features o f true eyes .

530 PRINCIPLES

O F INSECT MORPHOLOGY

The Photoreceptiv e Organ s o f Muscoi d Larvae.—Th e maggot s o f the highe r Dip t era ar e negativel y phototropic . Experiment s sho w that the light-sensitiv e part o f the anima l is its extreme anterior end . Ther e are her e locate d o n th e ape x o f th e larva l "head " tw o pair s o f smal l papillae, eac h bearin g on e o r severa l minut e cuticula r processes , an d containing th e oute r en d o f a large body o f sense cell s (se e Lowne, 1890 1895). Th e structure o f these organs in Lucilia sericata has recently been investigated b y Ellsworth (1933) . Th e externa l part o f each organ (Fig . 277) is a transparent cuticular cone (In) resembling a strongly biconvex lens o f a n ocellus . Th e interna l par t consist s o f a larg e ovat e mas s of

FIG. 277.—Sens e organs , probabl y photoreceptors , i n th e cephali c lob e o f th e larv a o f Lucilia sericata. (From Ellsworth, 1933. )

elongate, bipola r sens e cell s (SCls), th e dista l processe s of which form a thick cylindrica l fasciculu s (Fas) attached t o a thi n membran e beneat h the cuticula r lens. Th e proximal processes of the cell s come together in a nerve (Nv). These organs appear to lack the characteristic features of photoreceptive organs , namely , th e presenc e o f striate d band s o n th e receptive end s o f the sens e cell s and th e formatio n o f rhabdoms amon g the dista l processes . O n th e othe r hand , eac h orga n clearl y ha s th e internal structur e o f a chemorecepto r with multipl e sens e cells . Hence , accepting th e evidenc e tha t th e organ s ar e th e photoreceptor s o f th e maggot, we must conclud e that they presen t a unique example of a sens e organ havin g th e usua l chemoreceptiv e typ e o f structure i n it s cellula r parts that ha s become functionally a photoreceptor b y a n adaptatio n o f the cuticula r par t fo r th e transmissio n an d condensatio n o f light rays . General Structur e o f the Opti c Organs.—Regardless o f the diversit y in structur e o f th e severa l kind s o f eyes , ther e ar e alway s t o b e dis tinguished i n eac h orga n tw o functionall y distinc t parts , namely , a dioptric apparatus an d a receptive apparatus. Th e firs t transmit s an d

THE SENSE ORGANS 53

1

usually als o condense s th e impingin g ligh t ray s upo n th e receptiv e surface; the second consists of the sensory cells composing the retina, which present specialize d receptive surface s a t th e point s wher e the ligh t ray s are focused , an d ar e continued proximally into the opti c nerve . Usuall y the photorecepto r include s als o pigmen t cells , supportin g connectiv e tissue cells , an d tracheae . Morphologicall y th e tw o principa l part s of the opti c orga n ar e no t necessaril y o f different origin , sinc e the dioptri c apparatus, thoug h i t alway s include s th e cuticula , ma y b e i n par t o r largely forme d b y specialize d cell s of the opti c epidermi s o r o f product s of thes e cells . Th e receptiv e apparatus , o n th e othe r hand , i s entirel y epidermal. Th e optic center s of the brain , describe d in the las t chapter , lie generall y i n extension s o f the protocerebru m formin g lobe s o r stalk s capped b y th e eyes , in th e oute r end s o f which are th e terminal s o f th e optic nerves . I n som e larval insects , however , the opti c nerves ar e long trunks extendin g to the protocerebrum . Simple -eye s an d compoun d eye s ar e no t fundamentall y differen t i n their structure , an d al l type s o f eye s ar e develope d fro m th e ectoder m in practically th e sam e way. A simple eye, or ocellus, is a photorecepto r having a single dioptric apparatus fo r all the sense cells. A compound eye has numerous individualized groups of sense cells, and a separate dioptri c apparatus fo r eac h group . Th e anatomica l element s o f a n insect' s organ of vision include the followin g parts. The Cornea. —The cornea is the cuticula r coverin g of the eye , which is always transparen t i n orde r t o admi t ligh t ray s bu t i s o f a thicknes s presumably sufficien t t o exclud e ultraviole t i n harmfu l amount . Th e cornea may be but littl e differentiate d fro m the surroundin g cuticula , except fo r th e lac k o f pigment (Fig . 27 9 A, Cor) , but usuall y i t i s mor e or less dome shaped, o r it i s thickened t o for m a corneal lens (Figs . 27 9 F, 283 A, Cor), which may b e eithe r planoconve x o r biconvex . Generall y the len s shows a laminated structur e i n section, an d it includes the entir e thickness o f the cuticula . The Corneagenous Cells.—The epidermal matrix o f the corne a consist s of a laye r o f cells, which, in th e formativ e stage o f the eye , is a distinc t outer stratu m of the opti c orga n that secretes the corne a (Fig . 27 9 D, E , CgCls). I n th e matur e ey e th e corneagenou s cells ar e reduce d i n siz e and usuall y appea r a s a transparen t (vitreous ) epitheliu m underlyin g the corne a (F , CgCls), thoug h they ma y be withdrawn fro m beneat h th e latter (Figs. 281 B, D, 283 B) and converte d int o pigmen t cells . I n som e cases th e corneagenou s cell s for m beneat h th e corne a a transparen t crystalline bod y that becomes a part of the dioptri c apparatus . The Crystalline Body. —Many simpl e eye s an d mos t compoun d eye s have a transparen t vitreous, o r crystalline, body beneat h th e cornea , which serves as an adjunct to the cornea l lens or functionally replaces th e

532

PRINCIPLES OF INSECT MORPHOLOGY

latter whe n the corne a is not lenticula r i n form . Th e crystallin e bod y generally has an oval shape in simple eyes (Fig. 281 A, B, D, CB), bu t i n compound eye s it i s typically conica l with the ape x directed towar d th e inner part o f the ey e (Fig . 283 A, Cri). I t i s a functional rathe r tha n a morphological part of the ey e and is variously produced. I n th e ocell i of some insects a vitreous body is formed of elongate transparent corneagenous cell s intervening betwee n the corne a an d th e retina . A large ova l crystalline body in the ocell i of Ephemerida consists of a compact mass of small vitreou s cell s lying beneath a subcornea l epithelium o f corneagenous cells. Th e crystalline body here serves as the lens of the eye . I n th e lateral ocelli of certain larval insects an oval crystalline body lies beneath

FIG. 278.—Structure of optic sense cells, diagrammatic. A , receptive pole of a sense cell: a , striate d zon e (rhabdomere ) forme d o f end s o f neurofibrillae ; b, basa l bodies ; c, clear zone . (From Hesse, 1901. ) B-J , differen t position s of the striate d zone s on end s of sense cells . H , I , unio n o f the striate d zone s o f adjacent cell s to form , a rhabdom (Rhb). (B-J from Weber, 1933. )

the cuticula r len s (Fig . 281 A, B, D, CB)t which may be a cellular structure, a s i n certai n dipterou s larva e (Fig . 28 0 C, F , CB), thoug h more commonly it appears to be a secretion product of some of the surrounding cells. Th e crystalline con e of compound eyes (Fig. 283 B, Cn) i s usually formed o f four unite d transparen t cells, but i n some cases it i s a vitreous transformation o r secretion product o f the cells . The Retina. —The retin a i s compose d o f th e opti c sens e cells . I t lies in the deepe r part o f the ey e and usuall y consists o f a single layer of elongate cells, the narrowed inner ends of which penetrate th e basemen t membrane o f th e ey e an d becom e th e fiber s o f the opti c nerve . Th e receptive part of each optic sense cell (Fig. 278 A, a) is the striate d ban d (Stiftchensaum), th e striation s o f which appear to b e rodlike thickenings of neurofibrilla e traversin g th e cell . A t th e bas e o f each rod i s a smal l nodule (6) . Proxima l to th e rod s is a pale intermediate zon e (c) separating the striate d ban d fro m th e plasmatic basal part of the cell, which contains the cel l nucleus (Nu). Th e primary position of the striated band is

??? ????? ?????? ???

probably o n the dista l surfac e o f the sens e cell (A, B), an d th e ban d ma y envelop th e en d o f the cel l (C) > bu t mor e generall y i t i s transferre d t o one side (D , E, F). I n mos t case s the latera l striate d band s o f adjacent cells are united to for m a rhabdom (H, I, Rhb). The rhabdo m may be cylindrical, but ofte n i t shows a stellate o r branched for m i n cross sectio n due t o th e componen t elements , o r rhabdomeres , contribute d b y th e several encirclin g cells. Pigment Cells. —A dar k pigmen t usuall y occur s i n som e par t o f th e eye and may b e contained i n various cells; it i s commonly present i n th e retina cells , an d generall y ther e ar e specia l pigmen t cell s surroundin g the eye , o r th e separat e element s o f a compoun d eye , whic h opticall y isolate th e sensillu m an d absorb th e ligh t tha t doe s not ente r a t suc h an angle as to reach the sens e cells. Th e pigment cells in most case s are th e epidermal cell s surrounding the eye , or enclosing each group of rhabdomforming retin a cells , but i n the compoun d eye s an d i n som e simple eye s the corneageiiou s cells become pigment cell s in the mature organ . The Tapetum. —A reflectin g surface , know n a s th e tapetum , i s variously develope d i n differen t eye s t o giv e greate r effectivenes s t o smal l amounts o f ligh t an d i s therefor e mor e usuall y presen t i n nocturna l species. I t reflect s th e ligh t fro m th e dept h o f the ey e bac k int o th e retina an d give s a shinin g appearanc e t o the eye s of many insect s whe n seen i n di m light . A tapetu m i s forme d i n som e ocell i b y a shee t o f connective tissue through the base of the retina containin g light-reflectin g substances (Fig . 279 A, Tap). I n the compoun d eyes the usua l tapetum consists o f dens e masse s o f glistenin g air-fille d trachea e tha t penetrat e between the retina l elements an d form a reflecting sheath enclosin g each group of retinal cells . The Basement Membrane. —The inne r surfac e o f th e ey e i s alway s covered by a basement membran e continuou s wit h that o f the surround ing epidermis. Th e membrane is penetrated b y the proximal processes of the retina l cells , which become the fiber s o f the opti c nerve . Th e base ment membran e o f the compoun d eye is sometimes calle d th e membrana fenestrata. The Dorsa l Ocelli. —The dorsa l ocell i ar e th e usua l simpl e eye s o f adult insect s an d o f exopterygot e larva e an d nymphs . Typicall y there ar e thre e o f them, on e median , locate d o n th e uppe r par t o f th e frons o r th e fronta l are a o f th e head , th e othe r tw o mor e latera l o n the postfronta l region . Ther e i s evidenc e tha t th e media n ocellu s ha s been forme d b y th e unio n o f two primitiv e fronta l ocelli. Th e media n ocellus i s ofte n suppressed , seldo m i s i t retaine d alone , bu t frequentl y all three ocell i are absent . The dorsa l ocell i hav e a relativel y primitive structur e i n som e of the Apterygota , a s in Orchesella and Machilis. I n MachiliSj a s described

534 PRINCIPLES

O F INSECT MORPHOLOGY

by Hess e (1901) , th e cornea l cuticul a i s elevate d ove r eac h ocellu s (Fig. 27 9 A, Cor) , but i t i s not thickene d t o for m a lens . Beneat h th e cornea ar e th e cellula r element s o f th e eye , whic h consis t o f tw o sets o f cells . Th e large r cell s (SCls) ar e th e sens e cell s formin g the retina , an d i n thi s cas e som e o f th e retina l cell s exten d fro m the cornea to the base of the eye. The smaller cells (CgCls), intercalated betwee n th e oute r end s o f the retina l cells , are the corneagenous cells. I t i s evident fro m th e arrangemen t o f the cellula r elements in this eye that the tw o sets o f cells have been differentiated side by sid e in th e ocellular epidermis . Th e retina l cell s ar e arrange d i n group s o f fou r cells each, the rhabdomeres of which form rhabdoms (Rhb) irregularly X-shaped i n cross section. Th e inner part o f the retin a i s traversed b y a sheet of reflecting connective tissue forming a tapetum (Tap), beneath which is a layer o f dark pigment (Pig) a t th e proxima l ends of the retinal cells. Th e ey e i s limite d internall y b y a basemen t membran e (BMb) continuous with that o f the surrounding epidermis (Epd). In th e usua l dorsa l ocell i o f pterygot e insect s th e corneagenou s cells and the retinal cell s form two distinct layers, the forme r intervenin g completely betwee n th e corne a an d th e retin a (Fig . 27 9 F) . Th e cornea may be a simple dome over the eye , but generall y it i s thickene d to for m a strongly biconve x lens (Cor). Th e corneagenou s cells (CgCls) are transparen t i n orde r no t t o imped e th e transmissio n o f light t o th e retinal cells, and whe n a corneal lens is developed the corneagenou s cells are usually smal l an d appea r a s a vitreou s epitheliu m beneat h th e lens . In th e absenc e o f a cornea l lens, however , the corneagenou s cell s ma y be enlarged to form a crystalline dioptric body. I n some cases a vitreous mass o f refractive flui d occurs between th e epithelia l corneagenou s cells and th e retin a cells . Th e periphera l cell s o f th e corneagenou s layer , where the latter merge into the normal epidermis, usuall y contain a dark pigment forming an "iris" about the sensory elements. Rhabdoms (Rhb) produced b y adjacen t retina l cell s ar e containe d i n th e oute r par t of the retina , proxima l t o whic h th e retina l cell s ar e usuall y pigmented , and th e retina l nucle i li e in th e basa l part s o f the cells . Intersperse d between th e sensor y cell s o f th e retin a ar e supportin g cell s tha t ar e purely mechanical elements in the structur e o f the eye . The developmen t o f an ey e of this type follow s a very simpl e course and i s wel l illustrate d i n th e ant , a s describe d b y Caesa r (1913) . Th e first rudimen t o f the ocellu s is a thickening o f the epidermi s in a fol d a t the bac k of the larva l hea d (Fig . 27 9 B, 0). Accordin g to Caesar , ther e are fou r ocella r rudiment s i n th e an t larva , tw o o f which subsequentl y unite t o for m th e definitiv e media n ocellus . Th e cell s o f eac h ocula r area soo n becom e differentiate d int o tw o set s (C) , th e cell s o f which, though at first interpolated amon g one another, soo n draw apart towar d

THE SENSE ORGANS

535

opposite end s o f the ey e (D) . Th e oute r cell s become the corneagenou s cells (CgCls), th e inne r one s th e retin a cell s (SCls). Th e corneagenou s cells nex t becom e greatl y enlarged , whil e the retin a cell s condens e t o a compact grou p (E) . The n th e corneagenou s cell s begi n a n activ e secretion of chitin, which increases the thickness of the superlyin g cuticula until th e latte r take s th e for m o f a large biconvex lens (F , Cor) ; bu t th e

FIG. 279.—Example s o f th e structur e an d developmen t of dorsa l ocelli. A , dorsal ocellus o f Machilis. (From Hesse, 1901. ) B-F , stage s i n th e developmen t o f a dorsa l ocellus o f mal e of Formica pratensis, an d matur e median ocellus o f same . (From Caesar, 1913.) BMb, basemen t membrane; Br, brain ; CgCls, corneagenou s cells ; Cor, cornea; Ct, cuticula ; Epd, epidermis ; Nv, nerve ; O, ocellar rudimen t in epidermis ; Pig, pigment; Rhb, rhabdom; SCls, sense cells; Tap, tapetum.

secretion activit y a t las t reduce s the corneagenou s cell s to a thin transparent epitheliu m separatin g th e len s fro m th e retina , an d th e cell s remain thu s in the full y matur e ey e (F , CgCls). The ocell i o f Ephemerida , a s describe d i n Cloeon b y Hess e (1901) , present a n exceptional structure i n that each eye contains a large lenslike cellular bod y enclose d between a thi n oute r epitheliu m o f corneagenous cells lyin g immediatel y beneat h th e dome-shape d cornea , an d a thic k inner laye r o f elongate vitreous cell s distal to th e retina . Th e lenticula r body i s composed o f many distinct , transparent , nucleate d cells , and a t one poin t o n it s oute r surfac e it i s continuou s wit h th e subcornea l epi -

536 PRINCIPLES

O F INSECT MORPHOLOGY

thelium, suggestin g tha t i t i s a produc t o f the latter . Th e retin a ha s the usua l structure , wit h shor t rhabdome s i n th e oute r end s o f its cells , but i t contain s n o pigment . Th e entir e ey e i s investe d i n a densel y pigmented epithelium . The Latera l Ocell i o f Holometabolou s Larvae.—The eye s o f holo metabolous larvae ar e o f the ocella r type o f structure an d ar e precursor s of th e compoun d eye s o f the adult . Th e larva l ocelli , however , do no t become th e ommatidia l element s o f th e compoun d eye ; th e latte r ar e developed independentl y fro m th e epidermis , whil e th e larva l ocell i usually degenerate , thoug h i n a few insects the y ar e sai d t o b e retaine d in the imag o along with the compoun d eyes. I n som e cases rudiments of the degeneratin g larva l ocell i are t o b e foun d withdraw n int o th e hea d and attache d t o nerv e strands o f the compoun d eyes (Fig . 28 1 F, 0) . The larva l ocell i generall y occu r o n th e side s o f th e hea d i n th e neighborhood wher e the compoun d eyes will be developed . I n numbe r they var y fro m on e to si x and sometime s seve n on each side. The y ar e often terme d "stemmata " t o distinguis h the m fro m th e dorsa l ocelli , probably i n allusio n t o th e fac t tha t thos e o f each lateral grou p are frequently arrange d i n a circl e o r "wreath " about a centra l pigmente d area, thoug h i n thi s sens e th e ter m "stemma " woul d more properl y apply t o the entir e circle of ocelli than to a single organ. Ther e is much variation i n the structur e o f the larva l ocelli in different group s of insects, but i t i s probable that these eye s are homologous structures i n all cases, and tha t the y hav e variousl y diverge d i n thei r evolutionar y develop ment. Sinc e th e larva l ocell i and th e compoun d eyes o f the adul t ar e both innervate d fro m th e opti c lobe s of the brain , i t woul d appear tha t the tw o sets of eyes are genetically related , thoug h ontogeneticall y the y are distinc t development s fro m th e latera l ocula r rudiment s i n th e epidermis. In thei r simple r forms , an d i n their growth , th e latera l ocelli do not differ essentiall y fro m the dorsa l ocelli. Eac h is developed by differentia tion o f the cell s in the ocula r region of the epidermi s into a corneagenous layer an d a retinal layer. Th e cornea is sometimes fla t o r dome shaped, but usuall y i t ha s th e for m o f a biconve x lens. Th e corneagenou s cells may form a thick vitreou s layer between the len s and the retina, o r they may b e reduce d t o a thi n epithelium ; i n certai n type s o f larva l ocell i they ar e withdraw n completel y fro m beneat h th e corne a i n th e matur e eye. Usuall y there is a dioptric body below the cornea , either composed of vitreou s corneagenou s cell s o r produce d a s a vitreou s secretio n of th e cells . Rhabdom s ar e generall y presen t i n th e oute r par t o f the retina , forme d i n th e usua l manne r betwee n the dista l end s o f the retinal cells . Pigmen t i s variously distribute d i n the eye , or sometime s absent.

??? ????? ?????? ???

A simple type o f lateral ocellu s is found i n the larva e o f Tipulidae, i n which there i s a group of five very small ocelli oh each side o f the head . Each ocellus , a s described by Constantinean u (1930) , consists o f a thic k corneal len s (Fig . 28 0 A, Cor), a subcuticula r epitheliu m o f transparen t corneagenous cells (CgCls), and an inner mass of sense cells (SCls) containing rhabdoms (Rhb) in their outer ends. In certain other nematocerous larvae, in which the corne a is not lenticular, some of the corneagenous cells take o n a vitreous character an d for m a dioptric apparatus beneat h

FIG. 280.—Latera l ocell i o f dipterou s an d tenthredini d larvae . A , Tipula. B , Culex pipiens. C , Melusina (Simulium). D , Allantus togatus (Tenthredinidae) , pigmen t removed. E , detai l o f corneagenou s an d retina l layer s o f same . F , Tendipes (Chironomus), oute r pigmente d ocellu s (0) , inner pigment-fre e ocellu s (O'). G , horizontal sectio n of pigmente d ocellu s o f same. (A-C , F , G from Constantineanu, 1930; D , E from Cornell, 1924.)

the cornea , which, as in the mosquit o larv a (B , VCls), ma y be but littl e differentiated fro m th e res t o f the corneagenou s layer, o r which , a s i n Simulium (C , CB), ma y b e a well-define d crystallin e bod y o f larg e transparent cells . A crystalline bod y is shown by Constantinean u t o b e particularly wel l develope d i n th e pigmente d ocell i o f variou s gener a of Chironomidae , in which it i s a compact lenslike structure compose d of four o r five large vitreous cells (F, CB). Accompanyin g each pigmente d ocellus (0 ) ther e ma y b e a n imperfect , nonpigmented accessor y ocellu s (Or) lackin g a crystallin e body . Th e chironomi d larva l eyes , whic h were formerl y suppose d t o b e ver y primitiv e photoreceptiv e organs , are thus show n by Constantinean u t o b e well developed and specialized in their structure .

??? ?????????? ?? ?????? ??????????

The larva e o f Tenthredinida e an d relate d familie s hav e a charac teristic an d distinctiv e appearanc e by reaso n o f the singl e large ocellus on eac h sid e o f the head . Thi s ey e in it s structur e (Fig . 28 0 D) muc h resembles a typical dorsa l ocellu s (Fig. 27 9 F). Th e corneagenou s cells form a thick layer (CgCls) betwee n the larg e biconvex corneal lens (Cor) and th e retina , an d th e sens e cells (SCls) ar e united i n retinular group s forming rhabdoms in their dista l end s (E, Rhb). Th e single tenthredinid larval eye , however , i s i n n o respec t structurall y differen t fro m th e simple latera l ocell i of a tipuli d larv a (A) , and i n it s development , a s described by Cornel l (1924) , it follow s th e usua l course of differentiation of th e primitiv e ocella r epidermi s int o tw o superpose d layer s o f cells . The larv a o f the antlio n Myrmeleon ha s severa l minute ocelli on each side o f the head . Th e structur e o f these eye s (Fig. 28 1 A), a s shown b y Hesse (1901) , is relatively simple in that each organ has a well-developed outer layer of corneagenous cells (CgCls) beneath the corneal lens, while the rhabdom-bearing sense cells (SCls) form a compact inner layer. In a space between the corneagenou s cells and the cup-shape d outer surfac e of th e retin a lie s a larg e ova l crystallin e bod y (CjB) . Th e peripher y of the crystallin e body is surrounded by long corneagenous cells that exten d from th e cornea to the sides of the retina. Th e rhabdoms have the usua l form o f rods converging toward th e dioptri c apparatus . Ocelli in whic h the corneagenou s cells are withdraw n fro m th e inne r surface o f the corne a in the matur e ey e have, in section, th e appearanc e of a cellula r invagination beneat h th e cornea , an d eac h ma y contai n a deep lumen . Ocell i of this typ e occu r i n th e larv a o f Dytiscus an d i n the larva e o f Trichoptera an d Lepidoptera . The larv a o f Dytiscus has o n eac h sid e o f th e hea d si x functiona l ocelli and one rudimentary ocellus , or "eye spot." Eac h fully develope d ocellus has th e for m o f a dee p cellular sac beneath th e lenticula r corne a (Fig. 28 1 B), th e lume n o f which is a narro w clef t throug h th e longe r axis of the eye . Jus t beneath th e cornea l lens (Cor ) is a large crystallin e ???? ????? ??? ????? ?? ??? ?????? ???? ?? ??? ??????? ??? ???? ?????? from th e epidermi s surroundin g th e crystallin e bod y an d for m a pig mented "iris" (Ids). Continuing from the latter are the sensory retinal cell s o f th e deepe r par t o f th e eye . Th e retina l cell s includ e ???????? ?????? ????? ??????? ???????? ?? ??? ????? ???? ?? ??? ?????? of th e ocella r sac , an d lon g periphera l cell s (pSCls) convergin g distally beyond th e oute r end s o f th e media n cells . Bot h set s o f retina l cell s ??????? ???????? ?? ????? ??????? ???? ?????? ????? ?? ??? ???????? ????? forming tw o paralle l row s a t th e botto m o f the sac , thos e o f th e con vergent periphera l cell s being directe d towar d on e another i n the latera l walls o f the ey e beyond th e vertica l rhabdoms . Th e taperin g proxima l ends of all the retinal cells come together to form the ocellar nerve (Nv).

THE SENSE ORGANS

539

The rudimentar y ocellus in each lateral grou p of ocelli in the Dytiscus larva, a s shown by Giinthe r (1912 ; Korschelt, 1924) , is more primitive in structure tha n th e othe r ocelli . Th e orga n lie s immediatel y beneat h the cuticul a (Fig . 28 1 C) an d ha s n o cuticular lens . Th e corneagenous cells (CgCls)j however, form a continuous layer between the cuticula and th e sens e cells (SCls) an d enclos e a small mass of vitreous cell s (C.B) evidently representin g th e crystallin e body . Th e structur e o f thi s ocellus ma y b e suppose d t o represen t a n arreste d earl y stag e i n th e development o f the othe r ocelli.

FIG. 281.—Latera l ocell i o f holometabolou s larvae . A , Mermeleon. B , Dytiscus, section throug h shor t axi s o f ocellus . C , rudimentar y ocellu s o f Dytiscus larva . D , diagram o f ocellu s o f a caterpillar . E , horizonta l sectio n o f ocellu s o f Arctia caja. F , degenerating larval ocellus of Dytiscus attached to nerve strand of compound eye. (A, E from Hesse, 1901 ; B , G , ~F from Giinther, 1912 ; D , based o n Pankrath, 1890 , an d Hesse, 1901. ) CB, crystallin e body ; cSCls, centra l sens e cells ; Ids, iri s cells ; MnCl, mantl e cell ; pSCls, peripheral sens e cells .

The latera l ocell i of trichopterous an d lepidopterou s larvae hav e th e same typ e o f structur e a s thos e o f the Dytiscus larva, bu t th e cellula r elements o f each ey e are fewe r i n number , an d th e lume n i s obliterate d by a contiguity o f the inne r ends of the retina l cell s (Fig. 281 D). Lyin g immediately beneath the thick cuticular lens (Cor) is a small oval crystal line bod y (CB). Th e corneagenou s cells (CgCl) li e a t th e side s o f th e crystalline body , an d thos e a t th e peripher y for m lon g mantl e cell s (MnCl) surroundin g the retina . Th e retin a consist s o f a centra l grou p of vertica l cell s (cSCl), an d o f peripheral cells (pSCl) convergen t distall y over th e centra l cells . Th e rhabdomere s of all the retina l cell s form a n axial rhabdom (Rhb) beneath the crystalline body. In a horizontal section throug h th e dista l part o f the retin a (E) , th e retina l cell s appear as two concentric cell groups surrounding the rhabdom , there being fou r cells i n th e centra l grou p (cSCls), an d thre e i n th e periphera l grou p

540 PRINCIPLES

O F INSECT MORPHOLOGY

(pSCls). Surrounding the sensory cells are three large corneagenous mantle cell s (MnCl) ensheathin g th e retina . Th e eye s of the caterpilla r closely resembl e i n structur e th e latera l eye s o f certai n Apterygota . Simple Latera l Eye s o f Adul t Insects.—Certai n adul t insects , bot h apterygote an d pterygote , ar e provided with simpl e lateral eye s in place of compoun d eyes. Thes e eye s usually occur in groups , but thei r struc ture, though variable, never closely resembles that of an ommatidium of a typical compoun d eye. Among th e Apterygot a simpl e latera l eye s ar e foun d i n Collembol a and i n Lepismatidae . Thos e o f Collembol a for m a grou p o n eac h sid e of th e head , th e numbe r o f ocelli in eac h group being variable but neve r more tha n eight . Th e cuticul a i s elevate d i n a simpl e dome-shape d cornea over each eye (Fig. 282 A, Cor). Beneat h th e corne a are two fla t corneagenous cells (CgCl), which cover the outer surface of a large spherical crystallin e bod y (CB). Th e crystallin e bod y o f Orchesella, Hesse (1901 ) says , show s n o evidenc e o f cellula r structur e excep t fo r indistinct trace s o f nuclei, bu t i n Podura, accordin g t o Wille m (1900) , the bod y consist s o f four crystallin e cells . Th e retin a ha s th e for m o f a cellular pocke t beneat h th e crystallin e body , compose d o f fou r distal , or peripheral, cells (pSCl) and of three proximal central cells (cSCl). The retina l cell s hav e striate d border s directe d towar d th e lume n of th e eye , bu t the y d o no t for m a tru e rhabdom . Dens e masse s o f pigment (Pig) surround th e crystallin e bod y an d th e receptiv e part s of the retina l cells . In Lepisma, a s describe d b y Hess e (1901) , eac h latera l ey e consist s of a comple x o f 1 2 simpl e eye s separate d b y a fe w intermediat e pig mented epiderma l cells . Th e corne a form s a biconve x len s ove r eac h eye (Fig . 28 2 B, Cor). Immediatel y beneat h th e cente r o f the len s is a crystalline bod y (CB) compose d o f fou r distinc t vitreou s cells . Tw o corneagenous cell s (CgCl) surroun d th e crystallin e bod y bu t ar e onl y slightly inserte d betwee n th e latte r an d th e lens . Th e retin a i s mad e up o f four peripheral cells (pSCl) an d three central cells (cSCl), th e forme r being convergen t distall y beyon d th e centra l cells . Eac h retina l cel l has a rhabdomer e (a ) o n it s inne r surface , an d th e seve n rhabdomere s compose a hollow axial rhabdom. The adul t eye s o f Collembola and Lepisma are thu s see n to b e ver y similar to the larva l eye s of Lepidoptera (Fig . 281 D), with the exceptio n that the latter have three cells in the periphera l group of retinal cell s and four i n the centra l group (E) , the numbe r being the revers e in the apter ygote forms. Th e eye s of both Collembol a and Lepisma , however , have two feature s characteristi c o f compoun d eyes , namely , th e presenc e of onl y two corneagenou s cells in eac h eye , an d o f four componen t cells in th e crystallin e body . Fo r thi s reaso n som e writers clai m that thes e

THE SENSE ORGANS

541

composite simple eyes are dissociated an d more or less degenerate ommatidia o f compound eyes. I n th e Arachnid a and Myriapod a also , groups of simpl e lateral eye s replace the compoun d eyes o f the mor e primitiv e xiphosurans, eurypterids , an d certai n extinc t myriapods . Th e relatio n of th e variou s form s o f latera l arthropo d eye s t o on e anothe r i s no t known, bu t i t seem s probable that the y ar e simpl y differen t mode s of development proceedin g from a common lateral ey e fundament with it s nerve roots in the latera l opti c lobe of the brain .

FIG. 282.—Latera l eye s o f adul t arthropods . A , singl e ey e o f lateral grou p o f eye s Orchesella rufescens. B, single eye of composite lateral eye of Lepisma saccharina. C, single ey e o f composite lateral ey e o f Xenos rossii. D , tw o ommatidi a o f compound eye of Machilis. E , tw o ommatidi a o f compoun d ey e o f Astacus flumatilis . F , tw o ommatidi a of compoun d ey e o f Osmylus chrysopa. G , diagra m o f ommatidi a o f appositiona l com pound ey e o f diurna l Lepidoptera . H , diagra m o f ommatidi a o f superpositiona l ey e o f nocturnal Lepidoptera . (A , B, D from Hesse, 1901 ; C from Strohm, 1910 ; E from Bernhards, 1916 ; F from Ast, 1920 ; G , H from Nowikoff, 1931. )

The Siphonapter a have two small simple eyes in the adul t stage , on e on eac h side of the head , near th e usua l position o f the compoun d eyes, the latte r bein g absent i n the fleas . Th e structur e o f these latera l eye s of th e flea , a s shown by Hess e (1901) , resembles that o f the usua l dorsa l ocelli o f othe r insects . Eac h ey e ha s a singl e biconve x lens , beneat h which are long cells reaching, as in the ocellus of MachiliSj from the base of th e ey e to th e lens . Thes e cells are evidentl y th e sens e cells, though they d o not for m rhabdoms . Betwee n their oute r end s are a few small corneagenous cells . Th e entir e inne r par t o f th e ey e i s investe d i n a

542 PRINCIPLES

O F INSECT MORPHOLOGY

pigmented sheath . I t ha s bee n suppose d tha t thes e latera l ocell i of the adul t flea are th e latera l dorsa l ocell i transposed t o th e side s o f the head, bu t thei r tru e natur e canno t b e decide d unti l th e sourc e o f their innervation is known. Lateral eye s unquestionabl y o f a n ocella r structur e occu r i n som e adult Strepisiptera . Th e male s o f Xenos rossi, as describe d b y Stroh m (1910), hav e a larg e projectin g composit e ey e o n eac h sid e o f the hea d composed o f abou t 5 0 closel y appresse d simpl e eyes . Th e individua l lenses ar e larg e an d eac h ha s a lon g peglik e projectio n o n th e inne r surface (Fig . 28 2 C) surrounde d b y a pigmente d fol d o f the epidermis . Beneath th e len s is a nucleated corneagenou s epithelium (CgCl) i n which cell boundarie s ar e lost . Th e retin a consist s o f abou t 5 0 to 5 5 slende r cells with hexagonal rhabdoms among their distal ends converging towar d the inne r surfac e o f the lens . Th e ey e element s o f Xenos, Stroh m con tends, ar e no t ommatidi a bu t ar e " ocellar compoun d eyes, " sinc e th e structure of each is that o f a typical ocellus . Simple eye s o f the structur e abov e describe d shoul d no t b e confuse d with reduced compound eyes, which may consis t o f only a few ommatidia, or even a single ommatidium , as in certain worke r ants. The Compoun d Eyes.—Compound eyes are an ancient heritag e o f the Arthropoda. The y wer e possessed by the trilobites an d the eurypterids ; among modern form s they have descende d to the xiphosurans, on the one hand, an d to the crustacean s an d the insects , o n the other . I n moder n arachnids an d myriapod s th e compoun d eye s hav e bee n replaced b y groups o f lateral simpl e eyes , bu t certai n Permia n diplopod s ar e sai d t o have ha d well-develope d compoun d eyes . I t i s no t t o b e supposed , however, tha t th e compoun d eye s o f primitiv e arthropod s closel y resembled th e highl y organize d compoun d eye s o f moder n crustacean s and insects . Th e compoun d eye s o f Xiphosura ar e comparativel y ver y simple structures, each consisting o f a large cornea with nurnerous peglike processes o n it s inne r surface , beneat h whic h ar e correspondin g group s of rhabdom-formin g epiderma l sens e cells . Th e origi n o f the compoun d eye i s los t i n antiquity , sinc e th e trilobite s ar e th e oldes t o f know n arthropods. I n it s ontogen y th e compoun d ey e is forme d directl y b y the differentiatio n of its cellula r element s fro m th e latera l ocula r regio n of the epidermis ; in its growth it enlarge s by the additio n o f new element s to its margin . A typical, full y develope d compoun d ey e is present only in Crustace a and Insecta . Amon g the insect s suc h eye s occu r in Machilida e an d i n nearly al l th e Pterygota . I n Collembola , Lepismatidae , Strepsiptera , Siphonaptera, an d holometabolou s larvae, th e sit e of the compoun d eye s is occupied b y simpl e latera l eyes . I n a few apterygote an d pterygot e insects latera l eye s ar e entirely suppressed .

THE SENSE ORGANS 54

3

A compoun d ey e i s mad e u p o f individua l ey e element s know n a s ommatidia (Fig . 28 3 A , B) . Eac h ommatidiu m include s a cylindrica l group o f elongate sens e cells, the retinula (A , Ret), enclosing a long axial rhabdom (Rhb), an d ha s a n individua l dioptri c apparatu s forme d o f a corneal len s (Cor ) an d a vitreou s body , th e latte r bein g typicall y a crystalline cone (Cn). Th e con e is surrounded by tw o pigmented cornea genous cell s (CgCl), an d th e entir e ommatidiu m i s more o r less isolate d by a sheath o f epidermal pigment cell s (PgCls). Th e taperin g proxima l ends o f the retinul a cell s penetrate th e basemen t membran e (membran a fenestrata) and become the optic nerve fibers (Nv), which end in the lamina ganglionari s (I ) o f th e opti c lob e o f th e brain . Generall y th e inner surfac e o f the ey e ha s a n eve n contou r limite d b y th e basemen t membrane, but i n th e dorsa l eye s of the mal e of Simulium, as describe d by Dietric h (1909) , each retinula protrude s a s a long free proces s a t th e base o f the eye . The margi n o f th e cornea l surfac e o f th e ey e i s usuall y surrounde d by a n apodema l inflectio n o f th e hea d wall , whic h form s a collarlik e ocular ridg e surroundin g th e retin a (Fig . 28 3 A , OR). Th e externa l groove o f the ridg e is the ocula r sutur e (os) , an d th e narro w rim o f th e head wall next to the ey e (OSc) i s the so-calle d ocular sclerite (Fig . 57 A). The lense s o f th e corne a ar e sixside d i n insects , thoug h the y ma y b e quadrate i n Crustacea , an d giv e the corne a its facete d appearance . I f the ommatidia l lense s ar e conve x externally , the y for m elevation s o n the genera l corneal surfac e correspondin g to th e position s o f the omma tidia, but i n some insects th e oute r surfac e o f the corne a is smooth, an d the lenticula r swelling s ar e internal . Th e numbe r o f facet s i n th e cornea varie s greatl y i n differen t insect s accordin g t o th e numbe r o f ommatidia i n the eye . Usuall y there ar e from a few hundred t o severa l thousand; the maximum, in Odonata, has been estimated a s being close to thirty thousand. Th e surface of the compoun d eye is generally bare, bu t in som e insects setifor m hairs ar e born e o n the interspace s betwee n th e lenses. The severa l element s o f a n ommatidiu m var y considerabl y i n thei r structure, bu t thei r principal modifications may be briefly stated . The Lens.—Each ommatidial len s is typically biconvex, but i t may be planoconvex an d i s sometimes bu t littl e rounde d o n either surface . I t is ofte n ver y thic k an d probabl y ha s a protectiv e a s wel l a s a dioptri c function. I n many Lepidoptera the inner surface of each lens is produced in a lon g transparen t cornea l proces s (Fig . 28 2 G , cp). Th e len s i s a cuticular produc t o f the corneagenou s cells and generall y show s a lami nated structur e i n cross section. The Corneagenous Cells. —The corneagenou s cell s o f th e compoun d eyes ar e invariabl y foun d t o b e onl y tw o i n number . Usuall y i n th e

544 PRINCIPLES

O F INSECT MORPHOLOGY

mature ey e they li e at th e side s o f the crystallin e con e and ar e densel y pigmented, fo r whic h reaso n the y ar e know n a s th e primar y pigmen t cells (Hauptpigmentzellen). In Machilis (Fig. 282 D) and in Crustacea (E), however , th e corneagenou s cell s (CgCl) ar e inserte d betwee n th e lens an d th e bas e o f th e cone . Thi s i s undoubtedl y thei r primitiv e position, an d i t ha s been observed that i n developmental stage s o f exopterygote insects the corneagenou s cell s temporarily assum e thi s position at th e beginnin g o f each instar i n orde r t o regenerat e th e cornea . Th e Crustacea moul t throughou t lif e an d thei r corneagenou s cells, therefore , retain the norma l position beneat h th e lens . The Crystalline Cone. —The crystallin e bod y o f th e compoun d eye , both i n insect s an d i n Crustacea , i s forme d o f fou r cells . Thoug h th e structure i s typicall y a crystallin e cone , wit h it s bas e agains t th e len s and its ape x in proximity t o th e dista l end of the rhabdo m (Fig . 28 3 B), it i s not produce d in the sam e way in all insects, an d in some it i s imperfectly developed . Th e con e cells, sometime s calle d th e cell s o f Sempe r after thei r discoverer , ar e probabl y specialize d corneagenou s cells . The con e varies somewha t i n relativ e siz e among insects, i n som e Crus tacea it is very long as compared with the length of the retina (Fig. 282 E), and i t i s not alway s strictly conical in shape . According to whethe r th e con e cells form a tru e conica l structur e o r not, an d accordin g t o th e manne r i n whic h th e cone , whe n typicall y present, i s produced by th e con e cells, the compoun d eyes of insects ar e classed, a s acone eyes, eucone eyes, an d pseudocone eyes. Th e charac teristic and variabl e features of these thre e kind s of eyes are summarized by Webe r (1933 ) a s follows: (1 ) In acon e eyes the con e cells are presen t but they d o no t for m a tru e cone . Eithe r th e plasm a o f th e cell s i s entirely unifor m (man y Heteroptera, Nematocera , Brachycera , and some Coleoptera) o r a par t o f it i s transformed int o a vitreou s mas s (Redu viidae, Silphidae , Bibionidae) . (2 ) I n th e eucon e type o f eye th e con e cells ar e almos t completel y transformed into a four-part crystallin e con e (Fig. 28 3 B, C , Cri), leavin g onl y a smal l plasmati c zon e at th e bas e of the latte r containin g th e nuclei , an d a thi n plasmati c sheat h investin g the cone . Thi s i s th e usua l structur e o f th e con e i n th e majorit y o f insects. (3 ) In eye s of the pseudocon e type th e con e is an extracellula r body forme d b y a vitreou s secretio n o f th e con e cells , th e nucleate d remnants of which in this case lie at the ape x of the cone . Th e substance of the pseudocon e either remains soft an d clearly distinguishable fro m th e cornea (man y Cyclorrhapha ) o r becomes har d an d opticall y continuou s with th e cornea , givin g the impressio n tha t th e ey e is of the acon e type (many Dipter a and som e Coleoptera) . It i s evident that the degree of development o f the cone and its manner of formation are not dependent on phylogenetic relationships. The

THE SENSE ORGANS

545

eucone ey e an d th e pseudocon e eye , a s Webe r point s out , ar e funda mentally different , sinc e each forms the con e in a different way , but ther e is no sharp distinctio n betwee n either o f these types an d th e acon e type . The Eetinula. —The retina l par t o f eac h ommatidiu m consist s pri marily of 8 elongate sense cells arranged in a cylindrical fascicle between the ape x of the con e and th e basemen t membrane of the ey e (Fig . 283 A, Ret, B, SCI), though in rare cases as many as 10 and even 12 nuclei have been observe d i n th e retinula r fascicle . Th e rhabdomere s ar e o n th e apposed surface s o f the cell s and togethe r for m a n axia l rhabdom (Rhb). In cros s sectio n a retinul a appear s a s a rosett e o f cells surrounding th e rhabdom (D) . Generall y th e rhabdo m i s narro w an d cylindrica l i n

FIG. 283.—Structur e o f a compoun d eye , diagrammatic . A , vertica l sectio n o f part of eye. B, typical structur e of an ommatidium. C, horizontal section of ommatidium through cone . D , sam e throug h retinula . a , eccentri c retinul a cell ; BMb, basemen t membrane (membran a fenestrata) ; CgCl, corneagenou s cell ; Cn, crystallin e cone ; Cor, corneal lens; I, lamina ganglionaris ; IPgCl, iris pigment cell ; Nv, nerve; OCh, outer chiasma ; OR, ocular ridge ; os , ocular suture ; OSc, ocular sclerite ; PgCls, pigment cells ; Ret, retinula; Rhb, rhabdom; RPgCl, retinal pigment cell ; SCI, sens e cel l (retinul a cell) .

form (Fig . 28 2 D, G) , bu t i t i s sometimes clu b shaped , o r ellipsoi d (E , F, H), an d it may have a laminated appearanc e in sections. The eigh t cell s o f th e retinul a ar e usuall y no t equall y develope d along th e entir e lengt h o f the rhabdom . Generall y one o r tw o o f the m are reduce d in th e dista l par t o f the retinul a an d ar e thu s mor e or les s restricted t o th e basa l part , wher e at leas t th e nucle i remain visibl e i n sections. I n man y insects , however , on e o r bot h o f th e reduce d cells disappear completely , an d th e retinul a the n contain s onl y seve n o r si x cells. I n som e o f the Coleopter a on e cel l becomes reduced an d finall y eliminated fro m th e retinula , whil e another take s a proximal position i n the axi s of the retinula , surrounde d by th e si x other cells , and th e bas e of th e rhabdo m is imbedded in the dista l en d of the axia l cell. O r again, in certai n insects , th e retinul a cell s ar e mor e equall y separate d int o a distal group and a proximal group.

546 PRINCIPLES

O F INSECT MORPHOLOGY

In th e Neuropter a an d Lepidopter a th e retinul a o f th e compound eye becomes differentiated int o two types of structure, on e characteristic in genera l o f day-flyin g species, the othe r o f crepuscula r and nocturna l species (se e Johnas, 1911 ; Ast, 1920 ; Nowikoff, 1931) . Eye s o f the firs t type hav e cylindrica l retinula e o f th e usua l form , an d lon g slende r rhabdoms reaching from th e ape x of the con e to the bas e of the ey e (Fig . 282 G) . I n eye s o f the secon d type th e retinul a become s more or les s constricted betwee n a distal par t containin g most of the retinula r nuclei, and a proximal part containing one nucleus and the short, thick rhabdom (F). Th e dista l part o f the retinula , i n such eyes, may be reduced to a mere filamen t (H) , with a fusifor m swellin g in its cours e containing th e distal nuclei . Thi s modificatio n i s foun d i n th e eye s o f nocturna l species an d i s an adaptatio n t o th e " superposition" principl e o f vision by compound eyes, since it allows the obliqu e rays of light from eac h cone to b e thrown upon the rhabdom s of several surrounding ommatidia. The Pigment Cells. —Each ommatidium is enclosed in a sheath o f cells containing a n abundanc e of dark pigmen t (Fig . 283 A, PgCls), b y which it i s opticall y mor e or less isolated fro m it s neighbors . Thes e cell s ar e probably undifferentiated epiderma l cells of the part s of the ey e between the ommatidia . Usuall y there are two sets of them, an outer set havin g their base s o n the corne a and reachin g to th e dista l en d of the retinul a (B, IPgCl), an d a n inner set (RPgCl) restin g on the basemen t membrane and extendin g a varyin g distanc e toward th e dista l en d o f the retinula . In cros s sections, therefore, th e oute r pigment cells , or iris pigment cells, form a circl e aroun d th e corneagenou s cells (C , IPgCl), an d th e inne r cells, o r retinal pigment cells, encircl e th e retinul a (D , RPgCl). Th e number o f pigmen t cell s surroundin g a n ommatidiu m varie s fro m a few t o many . I n additio n t o thes e interommatidia l pigmen t cells , as we have already observed, the corneagenou s cells of the matur e eye in pterygote insect s become pigment cell s ensheathing the crystallin e cone (B, C , CgCl). Th e corneagenous pigment cells ar e ofte n termed th e primary pigmen t cell s (Hauptpigmentzellen) o f th e iris , an d th e oute r iris cell s distinguishe d a s th e secondar y o r accessor y pigmen t cell s (Nebenpigmentzellen). Pigment also occurs abundantly in the retinula cells themselves. Tracheae fro m th e opti c lob e penetrate th e basemen t membran e of the ey e and ramif y betwee n the retina l pigmen t cells . Here , i n crepuscular an d nocturna l species , the y ma y for m a dens e networ k o f fin e branches (Fig . 282 H, Tra), constitutin g a tracheal tapetu m that reflect s light fro m th e wall s of the ommatidi a back into the retina . I n som e of the Crustacea, as in Astacus (E), the retinal pigment cells (RPgCl) contain dens e masses o f a golden-brow n pigment, which , however, reflect s the light an d thus forms a pigment tapetum.

THE SENSE ORGANS 54

7

In th e eye s o f diurna l insect s th e retinula e ar e generall y entirel y surrounded by pigment, so that each ommatidium is an optically isolated element o f the eye . I n suc h eyes only perpendicular light ray s passing the dioptri c apparatu s affec t th e retin a throug h th e rhabdom , oblique rays bein g absorbed i n th e pigmente d wall s of the ommatidium . Thu s the light from an y part of an external object stimulates a retinula directly opposite t o i t i n th e eye . Th e resul t i s tha t th e stimulate d are a i n the retina represents a mosaic pattern of the point s of the objec t reflected into the eye . A n eye that works on this principle is termed an apposition eye. O n the othe r hand , i n eye s that functio n i n di m light, pigmen t is restricted t o the iris region and to the basal parts of the retina. I n such eyes ther e i s no ligh t waste d b y absorption , sinc e oblique rays enterin g any on e ommatidiu m ma y pas s int o neighborin g ommatidia an d ther e become effectiv e stimuli . Eye s o f this typ e ar e distinguishe d a s superposition eyes. The eye s of insects hav e n o focusin g mechanis m an d n o mechanical means o f adaptatio n t o differen t degree s o f light . I n man y insects , however, a n adaptatio n t o ligh t intensitie s i s effecte d b y change s i n the exten t o f the pigmente d areas. Whe n the insec t is brought from th e dark int o th e light , th e pigmen t move s outward in th e retina l pigmen t cells o r extend s inwar d i n th e iri s cell s (Fig . 28 2 F, right) bu t retreat s again when the insect is placed in the dark (F, left). The eye thus becomes accommodate d t o th e amoun t o f ligh t b y changin g fro m th e appositional t o th e superpositiona l condition . A permanent structura l adaptation t o di m ligh t ma y b e brough t about , a s show n in nocturna l Lepidoptera (Fig . 28 2 H) , b y th e differentiatio n o f th e retin a int o a distal nonsensitiv e part i n a n unpigmente d region of the ey e and int o a proximal sensitiv e par t containin g th e rhabdoms . I n th e clea r dista l part o f th e ey e ther e i s a fre e crossin g ove r o f obliqu e ligh t ray s fro m one ommatidium to another . The ommatidi a ar e ofte n differen t i n tw o area s o f th e sam e eye . They ma y diffe r i n diamete r an d i n optica l construction , an d th e tw o sets ma y b e quit e apparen t o n th e exterio r o f the eye , o r eve n s o dis tinctly separated tha t th e ey e becomes double. I n som e cases one part of a divided eye is dorsal and th e othe r ventral . The optica l mechanis m o f the compoun d eye is a subjec t that ha s been muc h discusse d an d i s perhap s fairl y wel l known , but w e ar e fa r from understandin g th e physiologica l an d " psychological" effect s o f light o n the insect . Experiment s sho w that insect s distinguis h i n thei r reactions variation s o f ligh t intensity , color , form , an d distance , bu t there i s no doub t that th e compoun d eyes function mos t effectivel y fo r the perceptio n of motion in external objects.

548 PRINCIPLES

O F INSECT MORPHOLOGY

GLOSSARY O F TERM S APPLIE D T O TH E SENS E ORGAN S Cap Cel l (CpCl). —The dista l or periphera l cel l of a sens e organ , o r o f one o f th e component unit s o f the organ , probabl y correspondin g to th e tormogen , o r socket forming cell of a seta. (Deckzelle.) Chemoreceptor.—A sens e orga n supposedl y sensitiv e t o chemica l propertie s o f matter ( a "taste" receptor or an "odor" receptor). Chordotonal Organ.— A sens e organ o f the scolopophorou s type, th e cellula r ele ments forming a n elongate structure attache d a t bot h end s to the body wall, but no t necessarily containin g sens e rods, or scolopes . Compound Eye (E). —A composit e opti c orga n havin g a dioptri c apparatu s for each receptive unit . Cone (Cn). —The usuall y conica l crystalline bod y of a compound eye. Cornea (Cor). —The cuticula r part o f an eye . Corneagenous Cells (CgCls). —The epiderma l cells that generat e the cornea . Crystalline Body.— A transparent subcornea l part o f the dioptri c apparatu s o f an eye, forme d o f cell s or o f cel l products, havin g a n ova l o r conica l shape. (Vitreous body, crystalline cone.) Dioptric Apparatus.—The outer transparen t par t o f an opti c organ, consistin g of the corne a and usuall y o f a subcorneal crystalline body . Distal Process.—Th e peripheral process of a sense cell. Enveloping Cel l (ECl). —The intermediat e cel l of a sens e organ , o r o f on e o f th e component sensor y units o f the organ , probabl y correspondin g to th e trichoge n o f a seta. (Hullzelle.) Eye.—In genera l a photoreceptor, bu t usuall y meaning on e of the mor e complex types o f light-perceptive organs . Iris.—Dark pigment surrounding the dioptri c apparatu s o f an eye . Iris Pigment Cells.—Cell s containing the iri s pigment. Johnston!an Organ.—A n orga n o f the scolopophorou s type locate d i n the secon d segment, o r pedicel, of the antenna e o f nearly al l insects . Lens (Ln). —A lenticula r outer par t of the eye , generall y forme d o f the cornea . Ocellus (O).— A simpl e eye, or photoreceptor havin g a single dioptric apparatus , including th e dorsa l ocelli , and lateral ocelli (stemmata) . Ommatidium.—One of the componen t units of a compound eye . Phonoreceptor.—A sense organ responsive to sound . Photoreceptor.—A sense organ responsive t o light . Pore Canal.—Th e channel of the cuticul a beneat h th e set a or other externa l par t of man y sens e organs. Receptive Apparatus.—Th e par t o f a sens e orga n primaril y responsiv e t o th e stimulus transmitte d b y or through th e periphera l parts , forme d o f the sens e cel l or cells. Retina (Ret). —The receptiv e apparatu s o f an eye . Retina Cells.—Th e cells composing the retina . Retinal Pigmen t Cell s (RPgCls). —Pigment cell s in the retina l region of the eye . Retinula.—The group of retinal cell s in a single ommatidium o f a compound eye. Rhabdom (Rhb). —A rodlik e structur e forme d o f th e unite d sensor y border s of adjacent retin a cells . Rhabdomere.—The receptiv e are a o f a retin a cel l that i s on e o f the componen t parts of a rhabdom. Scolops, Scolopal e (SR). —A "sens e rod, " o r minut e rodlik e capsul e envelopin g the distal end of the sense cell in certain sense organs. (Stift.)

??? ????? ?????? ??? Sense Cel l (SCI}. —The receptiv e cel l o f a sens e organ , wit h a proxima l nerv e process goin g to a nerve center . Sensillum.—A simpl e sens e organ , o r on e o f the structura l unit s o f a compound sense organ. Sensillum basiconicum.— A sens e orga n o f which the externa l par t ha s th e for m of a minute projectin g con e or peg. Sensillum campaniformium.— A sens e organ without a n external process, in which the cuticula r par t ha s typically th e for m o f a bell or hollow cone receiving the dista l process of the sens e cell. Sensillum chaeticum.—A sense organ of which the externa l part is spine-like . Sensillum coeloconicum.— A sens e orga n i n whic h the externa l proces s is sunken in a cavity o f the bod y wall. Sensillum opticum.— A photorecepto r o f the ey e type o f structure, o r on e o f th e ommatidia o f a compoun d eye. Sensillum placodeum.— A sense organ having a flat, plate-like externa l part. Sensillum scolopophorum.— A sens e orga n in which the sens e cells contain "sense rods" of the scolop s type . Sensillum squamiformium.— A sens e orga n o f whic h the externa l par t i s scale like in shape. Sensillum trichodeum.— A sense organ of which the externa l part is a seta. Stemmata.—A nam e sometime s give n t o th e latera l ocell i o f holometabolou s larvae, whic h are ofte n arrange d in a circle. Tangoreceptor.—A sense organ responsive to touch. Tapetum (Tap). —A reflectin g surfac e withi n a n eye , formed eithe r o f pigment or of densel y massed tracheae . Vitreous Bod y (CB). —See crystalline body.

CHAPTER XVII I THE INTERNA L ORGAN S O F REPRODUCTIO N The reproductiv e organ s diffe r fro m al l th e othe r organ s o f th e body i n that thei r function s d o not contribut e primaril y t o th e welfar e of th e individua l o f which they ar e a part ; their chie f concer n lies wit h the succeedin g generation . O n the othe r hand , man y o f the activitie s of th e organis m mus t b e correlate d wit h th e reproductiv e functions . This correlatio n i s largel y brough t abou t i n vertebrat e animal s b y th e secretion o f hormone s i n th e gonads , bu t wit h insect s ther e i s n o evidence tha t th e reproductiv e organ s hav e a regulator y effec t o n th e activities of the bod y organ s or o n the developmen t o f secondary sexua l characters. The reproductiv e syste m o f insect s i s a comple x o f organ s derive d from thre e anatomica l sources . It s parts , therefore , may b e classe d i n three morphologica l groups as follows : (1 ) primary interna l mesoderma l organs, (2 ) secondary ectoderma l parts , produce d fro m invagination s o f the bod y wall, forming th e usua l exi t apparatus ; and (3 ) external appen dicular structures . I n a stud y o f the reproductiv e system , however , it will be found more convenient to divide the subjec t into internal genitalia and external genitalia, eac h divisio n includin g som e o f the part s classe d above i n th e secon d group . Th e interna l genitali a serv e t o lodg e th e germ cells, to provide for their nutrition, an d to furnish the m a protecte d space withi n th e bod y wher e the y ma y underg o a par t o r al l o f th e development tha t bring s the m t o a stag e read y fo r conjugation ; the y include device s fo r insurin g fertilization , an d gland s fo r th e productio n of mucou s o r a n adhesiv e o r protectiv e medium ; and , finally , the y dis charge th e ger m cells from th e bod y a t th e prope r time . Th e externa l genitalia accomplis h th e unio n between the sexe s and enabl e the femal e to deposi t th e egg s according to th e manne r fixed in the instinc t o f each particular species . Insects, with rar e exceptions , ar e bisexual , i n so far a s the mal e an d female germ cells are matured i n separate individuals; but i n some species males ar e no t know n to exist , an d parthenogenesi s i s of frequent occur rence, th e unfertilize d egg s i n som e case s producin g males , i n othe r females. A conditio n approachin g hermaphroditis m i s sai d t o occu r in a specie s o f Plecoptera, Perla marginata. I n th e mal e organ s o f this insect, illustrated b y Schoenemun d (1912) , th e genita l duct s ar e unite d 550

THE INTERNAL ORGANS

O F REPRODUCTION 55

1

in a transverse arch , a s they ar e als o in the female , an d i n young males the media n par t o f the arc h bears , betwee n two latera l group s of sperm tubules, a numbe r o f smalle r tubule s containin g egglik e cells . Thes e "male eggs/' however , in the diploi d stage , accordin g to Junke r (1923) , have th e sam e chromosom e formul a a s th e sper m cells , eac h bein g provided with two unlike heterochromosomes, and, though they undergo a partia l developmen t lik e that o f a norma l eg g cell, the y neve r reac h maturity, an d th e tubule s containin g the m degenerat e whe n th e insec t approaches the tim e for transformation to the adult . The occurrenc e of functional hermaphoditis m has bee n demonstrate d by Hughes-Schrader to occur in the cottony-cushion scale Icerya purchasi, in which , sh e say s (1930) , the so-calle d female s "ar e i n realit y herma phrodites capabl e o f self-fertilizatio n o f thei r ow n egg s b y thei r ow n sperm." Th e gona d of these hermaphroditi c females , whic h are diploi d in chromosom e constitution, i s primaril y a pai r o f ovarie s unite d ante riorly above the alimentar y canal . Th e organ at a n early stage contains no lumen, and i n the firs t insta r som e of its centra l cells become reduced to th e haploi d conditio n an d proliferat e a soli d centra l cor e o f haploi d cells. Th e oute r cells of the gona d remain diploid and for m the ovariole s with thei r containe d oocyte s and nurs e cells . Th e centra l haploi d cell s develop int o spermatozoa . Th e oocyte s underg o norma l maturation , resulting in haploid ova , an d when the latte r ar e fertilized the y giv e rise to diploi d hermaphrodites ; unfertilized eggs develop parthenogeneticall y into haploi d males . Male s o f thi s specie s ar e rare ; the y hav e bee n observed t o copulat e wit h th e females , bu t i t i s no t know n that the y accomplish fertilization. The majorit y o f insect s ar e amphigonou s an d oviparous . Th e spermatozoa are stored in a receptacle of the femal e at the time of mating. The eggs in most cases are fertilized as they are extruded from the oviduc t and thu s underg o thei r entir e developmen t outsid e th e bod y o f th e female. Viviparity , however , is of frequent occurrenc e among insects of various orders , th e eggs , o r eve n th e larvae , bein g retaine d withi n th e body of the female where they complete a part or all of their development. Usually, in such cases, the eg g is deposited just befor e th e tim e o f hatching and almost immediately gives issue to an active young insect. Hatch ing may take place, however, within the eg g passage of the female , where the larv a the n spend s a varyin g lengt h o f tim e befor e it s extrusion , as in the viviparou s Dip t era that giv e birth t o livin g maggots . I n th e tsetse fly and Hippoboscidae the larva complete s its developmen t withi n the bod y o f the paren t femal e an d pupate s o n emergence. In th e viviparou s Dipter a th e developin g eg g o r larv a i s retaine d in a dilatatio n (uterus ) o f the vagina l section o f the media n eg g passage (Fig. 290 E, Utrs), where in pupiparous forms the larv a is nourished fro m

552 PRINCIPLES

O F INSECT MORPHOLOGY

special gland s (Fig . 291) . I n othe r cases , however , a s i n viviparou s Aphididae and Coccidae, and in the viviparou s beetle Chrysomela varians, the embryo s are developed within the eg g tubes of the ovaries . Probabl y in most case s development within the ovarie s is parthenogenetic, bu t th e eggs o f Chrysomela varians, according to Rethfeld t (1924) , are fertilize d in th e eg g follicles b y spermatozo a that trave l upwar d into th e ovaria l tubes. Th e female o f this beetle has no sperm receptacle, and copulatio n takes plac e during immature stages . 1. TH E FEMAL E ORGANS

The essentia l part s o f th e reproductiv e syste m i n femal e insect s (Fig. 28 4 A ) consis t o f a pai r o f ovaries (Ov), tw o lateral oviducts (Odl) converging posteriorl y fro m th e ovaries , an d generall y a median oviduct, or oviductus communis (Ode) , receivin g the latera l duct s anteriorly, an d opening posteriorly to the exterio r at th e gonopore (Gpr). I n additio n t o these primar y parts , there ar e usually a saclik e receptaculum seminis, or spermatheca (Spt), for th e receptio n an d storag e o f th e spermatoz oa, a pair of accessory glands (AcGl) havin g various functions, and a copulator y pouch o f th e bod y wall , which i s eithe r a n ope n genital chamber (GC), or a tubular exi t passage from th e media n oviduct, known as the vagina. The Ovaries.—Eac h ovary , i n mos t insects , consist s o f a grou p of cylindrical o r tapering units , th e ovarioles (Fig . 28 4 A, Ovl), whic h ordinarily converge upon the anterior end of the corresponding lateral oviduc t (Odl), thoug h i n man y o f th e mor e generalize d insects th e ovariole s of each ovar y aris e serially fro m on e side of the oviduc t (Figs . 287 , 290 A). The anterio r part s o f the ovariole s consis t o f threadlike filaments , an d usually all the ovariole filaments in each ovary are united distally with one another i n a suspensory ligament (Fig . 28 4 A, Lg). Th e ligamen t ma y end i n th e neighborin g fat tissue , bu t generall y it i s attache d eithe r t o the bod y wall or to th e dorsa l diaphragm. I n som e cases the ligament s from th e tw o ovarie s are combine d in a single median ligament, which is inserted i n the ventra l wal l of the dorsa l blood vessel. I n youn g stage s the entir e ovar y i s usually incase d i n a peritoneal sheath o f adventitiou s connective tissue ; i n th e adul t thi s sheat h i s generall y absent , thoug h it ma y b e retained , a s i n som e Dipter a (Fig . 29 0 F), i n th e for m o f a membrane envelopin g the ovarioles . The numbe r o f ovariole s i n a n ovar y varie s greatl y i n differen t insects. Usuall y it is not large, four, six , or eight ovariole s being perhaps typical, bu t i n som e Hymenopter a an d Dipter a th e numbe r ma y b e increased t o 10 0 or eve n 200 , an d i n th e Isopter a i t i s sai d t o reac h 2,400 o r more . O n th e othe r hand , th e numbe r o f ovariole s ma y b e reduced to tw o o r to on e (Fig. 290 B, E) .

THE INTERNAL ORGANS OF REPRODUCTION

553

Structure o f a n Ovariole.— A typica l ovariol e (Fig . 28 4 B ) consist s of three parts: a terminal filament (TF), an egg tube (ET), and a supporting stalk, or pedicel (Pdcl). The principal part is the egg tube, which contains th e ger m cells and their derivatives ; the termina l filament continued fro m th e anterio r en d of the eg g tube i s a part o f the suspensoria l apparatus o f the ovary ; the pedice l is the ovariol e duc t unitin g th e eg g tube wit h the latera l oviduct .

FIG. 284.—Structure o f the femal e reproductiv e organs. A , diagra m o f the ovaries , exit ducts , an d associate d structures . B , diagra m o f a n ovariole . AcGl, accessory gland; Clx, calyx; ET, eg g tube; Fol, follicle, o r egg chamber; GC, genital chamber (vagina) ; Gpr, gonopore ; Grm, germarium ; Lg , ovaria l ligament ; Ode, oviductu s communis ; Odl, oviductus lateralis ; Ov, ovary ; Ovl, ovariole ; Pdcl, ovariol e pedicel ; Spt, spermatheca ; SptGl, spermatheca l gland; TF , terminal filament; Vtl, vitellarium .

The usua l coverin g of an ovariol e is a thi n structureles s membrane , known a s th e tunica propria (Fig . 28 5 C, Tp), whic h stretches ove r th e terminal filament , th e eg g tube, an d th e pedicel . I n som e insects, how ever, ther e i s t o b e see n outsid e th e tunic a a n epithelial sheath o f fla t cells. Thi s oute r cellular sheath i s generally better developed in younger stages of the ovary (B, ESk). It is sometimes regarded as a connective tissue layer , bu t accordin g to K . Schneide r (1917 ) it s cell s in the mot h Deilephila originat e withi n th e bod y o f th e embryoni c gonad . Th e epithelial sheath , therefore , is probably o f mesodermal origin and repre sents the origina l mesodermal wall of the gonad. I n early developmental stages i t i s continuou s ove r th e entir e ovaria l rudiment ; bu t whe n th e latter becomes divided into compartment s tha t are to form th e definitiv e

554

PRINCIPLES OF INSECT MORPHOLOGY

ovarioles, the epithelia l sheat h i s inflected between the lobe s and eventu ally form s a covering abou t eac h ovariole . The Terminal Filament.—The slender, threadlik e filamen t tha t form s the anterio r par t o f an ovariol e (Fig . 24 8 B, TF) i s a solid strand o f cells (Fig. 285 C) ensheathed in the tunica propria (Tp). The group of terminal filament s i n eac h ovar y i s derived fro m th e embryoni c shee t of mesoderm b y whic h the primitiv e ovaria l rudimen t wa s suspended fro m the splanchnic wall of the coelo m (Fig. 294 E, a), and, as we have seen, the

FIG. 285.—Histolog y o f a n ovariole , mostly diagrammatic . A , B , C , stage s i n th e development o f an ovariole . D , ovariol e egg chambers containing oocyte s only. E , eac h egg chambe r containin g a n oocyt e an d nurs e cells . F , alternatio n o f eg g chamber s an d nutritive chambers . G , lower end o f an eg g tube fro m whic h an eg g has bee n discharged, Dytiscus marginalia. (From Demandt, 1912. ) Cho, chorion ; Clt, corpu s luteum ; EC , egg chamber ; ESh, epithelia l sheath ; FCl, follicl e cell ; GCls, primar y ger m cells ; Mpl, micropyle; NrC, nutritiv e chamber ; NrCl, nurs e cell ; nrcl, remnant s o f nurs e cells ; Ooc, oocyte; Oog, oogonium; pig, epithelia l plug ; Tp , tunic a propria . (Othe r letterin g a s o n Fig. 284. )

terminal filament s for m i n th e adul t th e suspensor y apparatu s o f th e mature ovary . I n som e insect s th e termina l filament s ar e no t unite d with one another, and in a few cases they are absent . The Eg g Tube. —The middl e sectio n o f a n ovariole , th e eg g tub e (Fig. 284 B, ET)j represents the intermediate part of the embryonic gonad, whic h contain s th e ger m cell s (Fig . 29 4 E, Grm). I n th e adul t organ it i s an elongat e sa c filled with a mass o f cells (Fig . 28 5 C) derive d from th e germina l elements , som e o f which become the ova . Th e w Tall of th e eg g tube i s formed o f the tunic a propri a (Tp), outsid e whic h ther e may be a cellular epithelial sheath , but th e latter, as we have observed , is

THE INTERNAL ORGANS O F REPRODUCTION55

5

often no t eviden t i n adult insects. Mos t o f the lengt h o f the eg g tube i s lined insid e th e tunic a propri a b y a laye r o f follicular epithelium (FCl). In eac h matur e eg g tube ther e ar e to b e distinguishe d tw o principa l parts. A t th e anterio r en d i s a regio n containin g th e ger m cell s i n a n active stat e o f divisio n an d incipien t differentiation . Thi s regio n i s the end chamber, or germarium (Fig. 284 B, Grm). Beyon d the germariu m is th e regio n i n whic h th e eg g cells gro w an d attai n thei r matur e size . This part o f the eg g tube is the zone o f growth, or vitellarium (Vtl). The germariu m represent s the primar y egg tube of the youn g ovar y in whic h th e ger m cell s ar e lodge d (Fig . 28 5 A, Grm). Th e ger m cells ^ however, soon develop into oogonia (C,.0og), and from the latter are formed th e oocytes, o r youn g eg g cells (Ooc), whic h are generall y accompanied by nutritive nurse cells (NrCl). In the germarium there are usually als o smalle r cells , whic h becom e th e follicle cells (FCl) i n th e lower par t o f the eg g tube. I n mor e genera l term s w e ma y designat e the follicl e cell s cystocytes, an d th e nutritiv e cell s trophocytes, sinc e bot h types o f cell s accompanyin g th e tru e reproductiv e cell s ar e foun d i n various form s i n both th e ovar y an d the testis . The vitellariu m i s forme d durin g th e developmen t o f th e insec t as an extensio n o f the eg g tube beyon d the germariu m (Fig . 28 5 B). I t enlarges rapidly a s the oocyte s multiply an d mature an d varies i n lengt h according to th e numbe r an d siz e o f the egg s it contains . A t the uppe r end o f th e vitellariu m th e follicl e cell s tak e a periphera l positio n i n the tub e an d sho w th e beginnin g o f a definit e epithelia l arrangemen t (C, FCl), while the oocytes (Ooc) and nurse cells (NrCl) assume an axial position. The growt h o f th e oocyte s distend s th e vitellariu m int o a serie s of eg g chambers, o r follicles (Fig . 28 4 B , Fol), whic h become successively larger towar d th e posterio r en d o f th e tube . Th e follicula r wall s ar e formed b y th e follicl e cells , which enclose each egg , o r eac h eg g and it s accompanying nurse cells, in a cystlike sac. Beyon d the last egg chamber a mas s o f follicl e cell s form s a "plug " tha t close s th e rea r en d o f th e egg tube (Fig . 28 5 E, pig). Differen t type s o f ovarioles resul t fro m th e presence or absence of nutritive cell s or from difference s i n their positio n in the eg g tube, bu t thes e modification s will be noted later . Since th e oocyte s ar e produce d continuousl y fro m th e oogonia , th e first encyste d oocyt e (Fig . 28 5 B, Ooc) becomes th e lowermos t an d th e first matur e oocyt e i n th e vitellarium . Wit h eac h successiv e additio n of a n oocyt e t o th e serie s the eg g tube become s lengthened betwee n th e last formed eg g chamber an d the germariu m by a rapid multiplicatio n o f the follicl e cell s i n thi s region . Th e ovariole s thu s underg o a grea t increase i n siz e durin g th e growt h perio d o f th e eggs , whic h i s usuall y the earl y part o f the imagina l stag e o f the insect .

556 PRINCIPLES

O F INSECT MORPHOLOGY

The las t eg g in each egg tube, when fully formed , i s usually abruptl y larger than an y o f those precedin g it an d i s enclosed in the chorion , bu t it i s still i n th e oocyt e stage , sinc e maturation doe s not ordinaril y tak e place until the eg g is laid. Henc e it is not strictl y prope r to speak of the full-grown ovaria l oocyte s as "mature " o r "ripe " eggs. I f a n oocyt e is fully forme d i n all the ovariole s at th e sam e time, the insec t may deposi t at on e laying as many egg s as there ar e ovariole s in the ovaries . Som e insects, however , such a s the quee n bee and the quee n termite, eject th e eggs singl y i n a continuou s succession . Wit h a fe w insect s th e full grown oocytes are produced alternately first in one ovary and then in th e other. Finally , a larg e numbe r o f egg s ma y accumulat e i n th e exi t passages before they are discharged. Thu s in Ephemerida and Acrididae the elongat e calyx and wide lateral duct s (Fig . 287), prior to oviposition , become distended with eggs ready to be laid, and in some of the Lepidop tera th e greatl y lengthene d pedicel s of the ovariole s for m storag e tube s filled with long series of eggs, in which the latter are held until the mot h is inseminated (Fig . 288). The Ovariole Pedicel. —The pedicels , or stalks , o f th e ovariole s (Fig . 284 B , Pdcl) ar e shor t duct s connectin g the eg g tubes wit h th e latera l oviduct. The y are formed fro m th e stran d o f mesodermal cells along the lower margin of the embryoni c gonad (Fig . 294 E, 6) , which is continuous posteriorly wit h th e primitiv e oviduct . Eac h pedice l develops a lumen communicating with that of the duct but close d at the upper end beneath the epithelia l plu g o f the.egg tub e (Fig . 28 5 E). A t th e tim e th e firs t egg is ready t o b e laid th e cell s of both th e plu g an d th e retainin g wal l of th e pedice l are dissolve d t o ope n a passag e through th e pedice l fro m the eg g tube into the oviduct . Th e walls of the matur e pedicels consist of a simpl e elasti c epithelium , an d i n som e insects th e muscl e sheath of the oviduc t is continued upon them. The Trophi c Function of the Eg g Tubes.—The growth of the egg s i n the ovary , which is mostly the accumulatio n of yolk, involves the utiliza tion o f much nutrien t material . Thi s materia l i s supplie d eithe r fro m the dail y foo d o f th e insec t absorbe d int o th e bloo d o r fro m th e foo d reserves store d i n th e body , principall y i n th e fa t tissue . Wit h insect s that d o not fee d i n the imagina l stage, al l the adde d eg g material mus t be drawn from th e latte r source. Since th e egg s i n th e eg g tube s d o no t hav e direc t acces s t o th e nutrient element s o f th e blood , a n importan t functio n o f th e ovar y i s that o f an intermediat e trophi c orga n betwee n the bloo d an d th e eggs . It discharge s this function i n various ways . I t i s possible that the egg s in some cases utiliz e materia l tha t passe s b y diffusio n directl y throug h the wall s o f th e ovarioles ; bu t observation s o n th e histolog y o f th e epithelial laye r o f follicl e cell s indicat e tha t mor e generall y th e wall s

THE INTERNAL ORGANS

OF REPRODUCTION

557

of th e eg g chamber s pla y a n activ e physiologica l rol e i n th e nutritio n of th e eggs . O n th e on e hand , th e follicula r cell s apparentl y absor b food materia l fro m th e bloo d and elaborat e i t i n their cytoplasm , while , on the othe r hand , eithe r the y discharg e the products into the eg g tubes, where the materia l i s directly o r indirectly take n u p b y th e eg g cells, or the highl y nutritiou s plasma itsel f o f the follicl e cell s is absorbed b y th e egg cells . Th e firs t proces s i s th e more usual one. The structur e o f th e eg g tube s differs somewha t accordin g t o th e manner i n whic h th e oocyte s ar e nourished. I n genera l two principal types o f structur e ar e distinguishe d by the presenc e or absence of special nutritive cell s withi n th e follicula r tubes. Whe n there are no nutritiv e cells, a n eg g tube is classed as panoistic (that is, " all eggs"); when nutritive cell s ar e presen t th e tub e i s meroistic ("par t eggs") . Th e meroistic typ e i s agai n subdivide d int o two groups known as polytrophic and FIG. 286.—Thre e principa l type s o f acrotrophic accordin g to th e positio n egg tubes , diagrammatic . A , panoisti c type. B , polytrophi c type . C , acro of th e trophocyte s i n th e eg g tube. trophic type . D , uppe r en d o f a n acro The Panoistic Type of Egg Tube.— trophic ovariol e o f Pseudococcus. (From In a n eg g tub e o f thi s typ e ther e Shinji, 1919. ) are n o specia l nutritive cell s differentiated fro m th e eg g cells; the foo d products elaborate d b y th e follicula r epitheliu m ar e absorbe d directl y by the oocytes . Ther e is within th e vitellarium , therefore , onl y a series of eg g cell s (Fig . 28 6 A , Ooc), each of which is generall y contained i n a distinct follicular egg chamber. Insect s havin g panoistic ovariole s occur in the Apterygota , Ephemerida, Odonata, Orthoptera, and Siphonaptera. The Polytrophic Type o f Eg g Tube. —An eg g tub e o f th e polytrophi c type contain s a n alternatin g successio n of oocytes and trophocyte s (Fig . 286 B). I n mos t case s the trophocytes , o r nurse cells , ar e descendant s along wit h th e oocyte s fro m th e oogonia , bu t i n som e insects the y ar e said t o b e derived fro m th e follicl e cells . Where the trophocyte s ar e o f germ-cell origin, the cell s produced b y the divisio n o f a single oogonium adhere in compact groups and maintai n connections with one another in the for m o f protoplasmic strands. Onl y the mos t posterio r cel l in eac h group becomes a functional oocyte (Fig . 286 B , Ooc) ; th e other s specializ e i n th e trophi c functio n an d becom e differentiated a s the nutritiv e cells (NrCls).

558 PRINCIPLES

O F INSECT MORPHOLOGY

Usually ther e i s a definit e numbe r o f nurs e cell s t o eac h oocyte . In muscoi d Diptera , accordin g to Verhei n (1921) , the proportio n i s 16 to 1, while in the hone y bee, according to Paulck e (1901) , the ratio is 48 to 1. In th e firs t cas e it woul d seem that th e eg g cell and th e nurs e cell s ar e differentiated fro m th e cell s o f th e thir d generatio n produce d fro m a single oogonium ; in th e second , th e eg g is on e o f the daughte r cell s of the firs t division , an d th e nurs e cell s th e descendant s o f the othe r b y four succeedin g divisions . I n Deilephila euphorbiae, i t i s sai d b y K . Schneider (1917 ) that, of the five nurse cell s accompanying eac h oocyte , four ar e daughter cell s of the oogoniu m and on e a sister cel l of the oocyte . On the othe r hand, the numbe r of nurse cells accompanying each egg may be highl y variable , a s i n Carabidae , describe d b y Ker n (1912) . I n Carabus violaceus Ker n foun d a maximu m o f 12 7 nurs e cell s wit h a single eg g cell, and th e tota l o f 128 cells he presume s t o hav e bee n pro duced by seve n successive divisions fro m on e oogonium. The polytrophi c typ e o f eg g tub e i s characteristi c o f Anoplura , Neuroptera, Coleoptera , Lepidoptera, Hymenoptera , an d Diptera. In som e insect s wit h polytrophi c ovarioles , suc h a s Coleopter a an d Lepidoptera, eac h oocyt e an d it s accompanyin g nurs e cell s occup y th e same ovariole chamber (Figs . 285 E, 286 B); in others there is an alternation o f egg chambers an d nutritiv e chamber s (Fig . 28 5 F), th e nutritiv e chambers i n such cases being the large r i n the uppe r part s o f the tubes , while in the lower parts the size relation i s the reverse owing to the growth of th e eg g a t th e expens e o f it s nurs e cells . Th e nurs e cell s a t firs t increase in size presumably b y absorbing material elaborate d b y the cell s of th e egg-tub e walls . Fro m histologica l studie s i t i s generall y eviden t that th e oocyte s ar e nourishe d b y a n activ e streamin g o f the plasmati c contents o f th e nurs e cell s int o th e cytoplas m o f th e egg s alon g th e strands originall y connectin g th e cell s i n th e grou p produce d fro m a primary oogonium . Whe n th e oocyt e i s mature , it s nurs e cell s ar e exhausted and reduced to mere remnants i n a state of degeneration (Fig . 285 E, nrcl). The productio n o f special groups o f nutritive cell s from th e follicula r epithelium ha s bee n describe d i n Apterygot a b y Wille m (1900) , i n th e May beetle Melolontha vulgaris by Molliso n (1904) , in a moth, Deilephila euphorbiaej b y K . Schneide r (1917) , an d i n Tenthredinida e b y Peacoc k and Gresso n (1928) . I n som e o f the Apterygota , accordin g t o Willem , the epithelia l nurs e cell s for m larg e protoplasmi c masse s o f nutritiv e material i n th e eg g tubes alternatin g wit h group s o f oocytes . Earlie r students o f thes e insects , Wille m claims , mistoo k th e eg g cell s fo r th e nurse cell s an d regarde d th e masse s o f epithelia l nutritiv e cell s a s th e oocytes. I n Melolontha, a s describe d b y Mollison , th e follicl e cell s of the eg g tubes for m a mas s o f nutritiv e cell s i n th e uppe r par t o f eac h

THE INTERNAL ORGANS O F REPRODUCTION55

9

egg chamber, many of which become connected with the oocyt e by proto plasmic strand s throug h whic h their content s ar e passe d int o th e cyto plasm o f th e oocyte . A simila r conditio n i s reporte d b y Peacoc k an d Gresson in Tenthredinidae, wher e certain cell s of the eg g follicles appea r to becom e nutritive cells since the chromati n o f their nucle i is discharged into the oocytes . The Acrotrophic Type o f Eg g Tube. —In a fe w insects , particularl y in th e Hemipter a an d som e Coleoptera , th e cell s produce d wit h th e oocytes fro m th e oogonia , bu t whic h are destine d t o becom e nurse cells, remain i n th e uppe r par t o f the eg g tube (Fig . 28 6 C, D , NrCls), whil e the oocyte s become removed from the m a s the serie s of egg cells increases in th e vitellarium . Th e origina l protoplasmi c connection s betwee n th e two set s o f cells are maintained , however , as long plasmatic strand s (us)

FIG. 287.—Femal e reproductiv e system o f a grasshopper , Dissosteira Carolina, latera l view. eg , egg guide; GC, genital chamber; Spr, spermathecal aperture; Spt, spermatheca.

through whic h th e oocyte s i n th e eg g tub e continu e t o receiv e th e yolk-forming materia l fro m the nurs e cells . The germariu m in the acrotrophic typ e o f egg tube, therefore, is also an apica l feeding chambe r for th e oocytes . A good illustration o f th e acrotrophi c type of egg tube in Hemiptera i s given by Malou f (1933) . Origin an d Relatio n o f th e Cellula r Element s o f th e Eg g Tubes. — There ha s bee n muc h differenc e o f opinio n a s t o th e derivatio n o f th e various cell groups composing an ovariole and foun d withi n the eg g tube. The terminal filament , th e oute r epithelial sheat h o f the eg g tube (whic h may b e absen t i n th e adul t organ) , an d th e pedice l ar e withou t doub t mesodermal parts of the ovariol e derived fro m th e primitiv e mesoderma l covering of the ger m cells. Th e tunica propria is probably a product o r a remnant o f the oute r epithelia l wal l o f the eg g tube. Th e oogoni a ar e direct descendant s of the primar y ger m cells. Th e oocytes and the usua l trophocytes, o r nurs e cell s tha t accompan y th e oocytes , ar e produce d from the oogonia by mitotic divisio n and nuclear changes. Th e principa l question o f origin, then , pertains to th e derivatio n of the follicula r cells. It i s claimed by K. Schneide r (1917 ) tha t in the mot h Deilephila th e ovarial follicl e cell s are proliferate d anteriorl y durin g th e growt h o f th e

560 PRINCIPLES

O F INSECT MORPHOLOGY

egg tube from th e mas s o f cells that close s the posterio r en d o f the tube . The origi n o f th e posterio r cell s Schneide r di d no t discover , bu t h e asserts that onl y the oocyte s an d th e nurs e cell s ar e produced fro m th e oogonia. Ther e seem s t o b e no question , however , that th e generativ e cells of the follicl e cells referred to b y Schneide r are of mesodermal origin. Seidel (1924 ) states definitely that the follicl e cell s of Pyrrhocoris apterus are forme d fro m th e epithelia l cell s o f th e uppe r end s o f th e pedicels , and mor e recentl y Lautenschlage r (1932) , i n a stud y o f the developin g female organ s o f Solenobia triquetrella (Psychidae) , find s tha t bot h th e follicle cell s and the posterio r masse s of cells that give rise to the ovariol e pedicels ar e derived fro m th e mesoderma l sheat h of the primitiv e gonad. It seem s mos t probable , therefore , tha t th e follicula r eg g tube, which , during th e multiplicatio n o f th e oocytes , i s interpolate d betwee n th e germarium an d the pedice l of the ovariole , is formed b y cell s proliferated from th e uppe r en d o f th e pedice l an d i s henc e o f mesoderma l origin . Earlier investigator s believe d tha t th e follicl e cell s ar e descendant s o f the oogoni a alon g wit h th e oocyte s an d th e usua l trophocytes , thoug h some claime d tha t the y ar e derive d fro m th e mesoderma l sheat h o f th e gonad. Formation o f th e Chorio n an d th e Discharg e o f th e Eggs. —The mouth o f each egg tube, a s we have seen, is closed behind th e las t oocyt e by a plu g o f the follicula r epithelial cells , an d th e plu g abut s agains t a transverse septu m forme d o f th e termina l wal l o f th e ovariol e pedice l (Fig. 285 E). Th e last oocyte is thus completely enclosed in the follicula r egg chambe r an d i s prevente d fro m escapin g prematurel y int o th e oviduct. Whe n th e eg g is fully formed , th e epitheliu m o f the chambe r begins a secretiv e activit y producin g a substanc e whic h i s discharge d upon th e eg g and there hardens t o for m th e eg g shell. Thi s shell i s th e chorion (Cho). Th e substanc e o f th e chorio n resemble s i n appearanc e the harde r part s o f the bod y wal l cuticula, bu t i t i s invariably foun d t o be nonchitinous . O n it s oute r surfac e th e chorio n retain s th e mark s of th e cell s that produce d i t i n th e for m o f a honeycom b patter n o f fine ridges reproducin g th e outline s o f the cell s o f the follicula r wall. Onl y at th e uppe r en d o f the eg g is th e chorio n incomplete , ther e bein g lef t here a poin t no t covere d b y th e chorio n deposit , whic h become s th e micropyle o f the eg g (E, Mpl), a n opening in the shel l through whic h th e spermatozoa gai n entranc e t o th e interio r o f the egg . I n som e insect s there are several aperture s i n the micropyl e area . When th e eg g is finally ready t o b e discharge d fro m th e ovary , th e epithelial plug behind it an d th e adjoinin g wall of the pedice l degenerat e and ope n a passag e throug h whic h th e eg g slips int o th e lume n o f th e pedicel an d the n goe s throug h th e calyx , throug h th e latera l oviduct , into th e media n oviduct , an d finall y reache s th e exterior . A s th e

THE INTERNAL ORGANS O F REPRODUCTION56

1

egg passes th e mout h o f the spermatheca l duc t a smal l mas s o f sperma tozoa is discharged upon its micropyla r surface , an d som e of the sperma tozoa her e ente r th e egg . Thu s th e eg g is inseminated jus t a s it leave s the oviduct . I t i s usually deposite d a t once , but wit h som e insects i t is held a varying length of time in an external genital chamber of the female . The eg g now undergoes its maturation divisions , an d shortly thereafte r a sperm nucleus unites with the nucleu s of the ovum , and the fertilize d egg is ready for development whe n external condition s are favorable.

FIG. 288.—Female reproductive system of a moth, Malacosoma americana, lateral view. An, anus ; AcGl, accessor y glands; bcpx, burs a copulatrix ; Ode, oviductu s communis ; Opr, oviporus (eg g exit); Ov, ovary; Res, reservoir of accessory glands; Spt, spermatheca ; Vag, vagina; Vul, vulv a (copulator y opening).

After a n eg g has lef t th e follicle , th e wall s o f it s chambe r collapse . The epithelia l cells , includin g th e remnant s o f thos e tha t forme d th e closing plug, degenerate (Fig . 285 G) and are at last mostly dissolve d an d absorbed. Th e mass of degenerating cells in the lowe r end of an egg tube (Clf) i s sometime s calle d a corpus luteum i n referenc e t o it s likenes s t o the degeneratin g Graafia n follicl e o f a vertebrat e ovary . Wit h th e disappearance o f the posterio r chambe r i n the eg g tube, th e nex t oocyt e and it s investin g follicl e assum e a terminal positio n a s th e eg g tube i s lengthened b y a growt h i n it s cellula r walls . Th e successiv e egg s thus probably do not literall y pas s dow n the eg g tubes, a s they ar e ofte n sai d to do ; more exactly, the tub e shorten s posteriorl y b y the degeneratio n of each emptie d chambe r an d increase s it s lengt h anteriorl y b y renewe d growth to accommodat e th e newl y forming oocytes. The Latera l Oviducts.—Th e duct s tha t lea d posteriorl y fro m th e ovaries ar e probabl y fo r th e mos t par t th e primar y mesoderma l exi t tubes o f the gonads , though in some of the highe r insects th e mesoderma l ducts ar e largel y o r entirel y replace d b y ectoderma l tube s forme d a s

??? ?????????? ?? ?????? ??????????

branches o f the media n oviduct . I n th e earl y developmenta l stage s of many insects the lateral oviducts are attached posteriorl y to the body wall at th e posterio r margi n o f the sevent h abdomina l venter , bu t i t i s only in th e Ephemerid a that th e lateral duct s have their permanent openings in thi s position . Wit h al l othe r insect s th e latera l oviduct s discharg e into a median invagination of the body wall, which becomes the oviductu s communis (Fig . 28 4 A, Ode). Th e anterio r en d o f eac h latera l duc t i s generally somewha t expanded, forming a receptacle known as the caly x (Clx), int o which open the pedicel s of the ovarioles . Whe n the ovariole s open serially into the oviduc t the caly x is lengthened and may be greatly enlarged fo r th e receptio n of the egg s (Fig . 287 , Clx). Othe r tha n thi s the oviduct s ar e generall y simple tube s withou t accessor y structure s o f any kind , though i n the Acridida e the anterio r en d o f each is prolonged into a tubular glan d (Fig . 287, Old). Th e cellula r walls of th e duct s ar e usually covered by a muscular sheath consisting of circular or longitudinal fibers or both. The Oviductu s communis.—The median oviduct is not a part o f th e primitive genita l system ; i t belong s t o th e secondar y exi t apparatu s formed a s a series of invaginations o f the bod y wall. Th e firs t rudimen t of th e oviductu s communi s is a n ectoderma l pouch behin d th e sevent h abdominal sternu m receivin g the tw o approximate d mesoderma l lateral ducts (Fig . 28 9 A, Ode'). Th e media n duct retains this primitive condi tion i n Dermaptera , bu t i n othe r insect s i t ha s bee n extende d into th e eighth segmen t (B , Ode) b y th e closure of a groove continued from i t o n the vente r o f the eight h segment. Generall y the definitiv e median ovi duct open s into a n invagination o f the bod y wall on the eight h segment , which is the genita l chamber (Figs. 284 A, 287, 289 C, GC) or a derivative of th e latte r known as the vagin a (Fig . 28 9 D, Vag). The posterio r openin g of the media n oviduc t i s th e female gonopore. Primarily i t is located on the rear part of the seventh abdominal segment ????? ??? ?? ??????? ???? ?? ??? ????????? ????????? ?? ??? ???????? ?? occurs generally at the posterior end of the eighth segment (B, Gpr), where usually it is concealed in the genital chamber (C, GC) or the vagin a (D, Vag). Th e gonopor e serves fo r th e discharg e o f the egg s fro m th e oviduct an d i s no t a copulator y opening . I t shoul d b e distinguished , therefore, fro m th e vulva, whic h i s th e externa l openin g o f th e genita l chamber (C, VuT) or of the vagina (D, F). The oviductu s communi s has a cuticula r linin g continuou s wit h th e cuticula o f the bod y wall , an d th e entir e epithelia l tub e i s surrounded by a strong muscular sheath consisting of circular and longitudinal fibers. The lengt h o f the tub e varie s much in differen t insects , an d it s anterio r end i s sometime s bifurcate . I n som e insect s branche s o f th e media n duct partiall y o r entirely replace the mesodermal lateral ducts .

??? ???????? ?????? ?? ???????????? ???

The Genita l Chambe r an d It s Derivatives.—Th e media n oviduc t extended int o th e eight h segmen t doe s no t ordinaril y ope n directl y t o the exterior . It s aperture , th e gonopore , is generall y conceale d i n a n inflection o f the bod y wall behind the eight h sternum. Th e cavit y thu s formed i s th e genital chamber (Figs . 287 , 28 9 C , GC). Th e genita l chamber receives the media n oviduct (Ode) an d th e duc t o f the sperma ????? ????? ???? ??? ???????? ???? ?? ?????? ?? ? ?????????? ????? ?????? ?????? ??? ?? ????????? ???????? ?????? ??? ????? ??????????? ??? external openin g i s th e vulva (Fig . 28 9 C, Vul). Th e genita l chambe r in its mor e primitive for m i s an ope n pocket of the bod y wall (Fig. 287, GC); bu t i n many insects it becomes an internal pouch or takes the for m of a tubular passag e continuous with the media n oviduct, i n which case it i s distinguished fro m th e latte r as the vagina (Fig . 289 D, Vag). Since th e vagin a i s a derivativ e o f th e genita l chamber , i t open s primarily o n th e posterio r par t o f the eight h abdomina l segment . I n many insects, however, as in some Cicadidae, Panorpidae, most Trichop tera, Lepidoptera, an d Coleoptera , th e vagin a i s continued through th e ninth segmen t an d ha s acquire d a n openin g o n this segmen t (Fig . 289 E, F) . I n mos t suc h case s the primar y anterio r openin g on the eight h segment i s closed , an d th e posterio r openin g o n th e nint h segmen t becomes the functional vulva (F, Vul), serving both for copulation and for th e discharg e of the eggs . I n th e majorit y of Lepidoptera, however, the anterio r apertur e i s retained a s a copulatory opening (Figs. 288, 289 ?? ????? ??? ????????? ??????? ???????? ??????? ???? ??? ??? ????????? of th e eggs , ma y b e distinguishe d i n thi s cas e a s th e oviporus (Opr). The vulv a o f Lepidopter a havin g tw o genita l opening s lead s int o a passage connecte d wit h th e vagina , whic h usuall y ha s a diverticulu m ?????? ??????? ?? ? ?????????? ?????? ??? ??????? ??? ??????? ???????? and th e copulator y pouc h of the latte r collectivel y represent th e genita l chamber o f mor e generalize d insects. Th e vagin a i s continuou s wit h the median oviduct (Ode), and the spermatheca (Spt) opens dorsally into its anterio r end. A similar conditio n exist s in certain specie s of Cicadi dae. I t shoul d b e observe d that , i n th e continuou s eg g passage, th e point o f unio n betwee n th e tru e oviduc t an d th e vagin a i s marke d approximately b y th e openin g of the spermathec a int o th e anterio r en d of th e latter . Some confusio n ha s arise n i n th e terminolog y o f th e femal e exi t apparatus owin g to a failur e t o distinguis h betwee n the tru e oviductu s communis and the vagina, and because it has not been perceived that the vagina i s a direct derivativ e o f the genita l chamber and no t a continua tion o f th e oviduct . Whe n th e genita l chambe r i s a n ope n externa l pocket of the body wall (Fig. 289 C, GC), the fact s are clear, and it should be recognized that a n internal pouc h or tube receivin g both th e oviduc t

564

PRINCIPLES OF INSECT MORPHOLOGY

and th e spermatheca , thoug h called the " vagina" (D , E, F, Vag\ i s still the genita l chambe r o r a part o f it. Thus , i n the Diptera , th e saclike posterior par t o f th e media n eg g passage (Fig . 29 0 F , Fag) , receivin g the spermatheca l duct s (Spt) int o it s dorsa l wall, an d continuou s anteriorly with the median oviduct (Ode), is the homologue of the open genital

FIG. 289.—Diagram s illustratin g th e evolutio n o f th e media n exi t apparatu s o f th e female reproductive system. A, primitive median gonopore (Gpr') behind seventh abdominal segment . B , oviduc t extende d throug h eight h segment , definitiv e gonopore (Gpr) a t en d of this segment. C , genital chamber (GC) invaginate d behin d eighth segment. D, genita l chambe r converte d int o a vagin a (Vag). E , vagin a extende d throug h nint h segment, bu t copulator y openin g (Vul) retaine d o n eighth . F , vagin a extende d throug h ninth segment, anterio r openin g lost, posterior openin g becomes the vulva (Vul). AcGl, accessory gland ; bcpx, burs a copulatrix ; d, vagina l pouch ; GC , genita l chamber ; Gpr, definitiv e gonopore ; Gpr', primitiv e media n gonopore ; Ode, oviductu s commu nis; Ode', primitiv e media n oviduct ; Odl, oviductu s lateralis ; Opr, oviporus ; Spt, spermatheca; Vag, vagina (genital chamber); Vul, copulatory opening, or vulva.

chamber of the Orthopter a (Fig. 287, GC). I n the honey bee (Fig. 290 D) there is a shallow genital cavity conceale d abov e the sevent h abdominal sternum a t th e bas e of the sting , but fro m thi s external depression there open a larg e interna l median pouch (Vag) and tw o lateral pouches (P). The median pouch is functionally the vagina , but th e fac t tha t both th e spermatheca (Spt) an d the oviduc t (Ode) discharg e into it shows that it is a part o f the genita l chamber (GC), a s are also the latera l pouches. In th e viviparou s Dipter a th e anterio r par t o f the genita l chamber forms a pouch known as the uterus (Fig . 29 0 E, Utrs), int o the anterio r end of which open the oviduct (Ode), the ducts of the spermathecae (Spt), and the accessory glands (AcGl). Th e egg is fertilized and in some species

THE INTERNAL ORGANS

OF REPRODUCTION

565

hatched, and the larv a retained a varying length of time, eve n to matur ity, withi n th e uterus , wher e it ma y b e fe d fro m th e secretio n o f th e accessory gland s (Fig . 291) . Keili n (1916 ) distinguishe s two group s of viviparous flies according to whethe r the larv a receive s no nourishment from th e mothe r o r i s fe d fro m uterin e glands . Wit h som e specie s

FIG. 290.—Example s o f unusua l type s o f reproductiv e organs . A , Heterojapyx gallardi, female. (From Tillyard, 1930. ) B , Acerentulus confinis, female . (From Berlese, 1910.) C , stonefly , Leuctra prima, male . (From Mertens, 1923. ) D , hone y bee , queen , diagrammatic. E , Mesembrina meridiana, female . (From Keilin, 1916 , after Cholodkowsky.) F , Rhagoletis pomonella, female.

of th e firs t grou p th e eg g is extrude d fro m th e uterus , bu t th e larv a hatches a s ovipositio n occurs ; in other s hatchin g take s plac e within th e uterus an d the larv a passes the first stage or several stages of its life in the uterine chamber . I n th e secon d group, including the tsets e fly Glossina and th e Hippoboscidae , i n whic h th e larv a i s nourishe d withi n th e uterus (Fig . 291) , th e youn g insec t i s deposite d a s a full-grow n larv a or as a pupa. The Spermatheca , o r Receptaculu m seminis.—Sinc e wit h insect s insemination o f the egg s is not generall y accomplished during the ac t of

566

PRINCIPLES OF INSECT MORPHOLOGY

mating bu t take s place a varying lengt h o f time afterward , most femal e insects ar e provide d wit h a sper m receptacl e i n whic h the spermatozo a are stored , an d fro m whic h the y ca n b e ejecte d upo n th e egg s a s th e latter ar e extrude d fro m th e oviduct . Thi s orga n i s th e receptaculum seminis, or spermatheca (Fig . 28 4 A, Spt). The spermathec a i s primaril y a n invaginatio n o f the integumen t a t the posterio r en d o f the vente r o f the eight h abdomina l segmen t (Fig . 289 B, Spt). It s opening , therefore, comes to b e enclosed in the genita l chamber whe n a copulator y pouc h is formed behin d th e eight h sternu m (Figs. 287 , 28 9 C, Spf), o r i t lie s i n th e dorsa l wal l of the vagin a whe n the genita l chambe r ha s th e for m o f a vagina l tub e (Fig . 28 9 D, E , F) . Usually th e spermatheca i s a single organ; bu t sinc e i t i s sometime s double o r consist s o f two branche s of unequa l size, it is possible that it is primitivel y bifurcat e o r paired , though i n som e Dipt era it i s triple FIG. 291.—Larv a o f Glossina palpalis (Fig. 29 0 E, F) . in uteru s o f th e fly . (Fro m Keilin , 1916 , The size , shape , an d structur e after Roubaud , figur e reverse d an d relet of th e usua l single spermatheca ar e tered.) AcGl, accessor y gland ; An , anus ; Int, intestine ; Lar, larva ; Mth, mouth ; Ode, highly variabl e i n differen t insects , oviductus communis ; Oe , oesophagus; SpPl, spiracular plat e o f larva ; Spt, spermatheca ; but generall y the orga n is saclike in Utrs, uterus ; Vag, vagina ; Vent, ventriculus ; form with a slender duct (Figs . 287, VNC, ventra l nerve cord. 288, 29 0 D, Spt). Ver y commonly a diverticulu m o f the duc t form s a tubular spermathecal glan d (Fig . 284 A, SptGl), whic h secrete s a flui d i n whic h th e sper m ar e discharged . On th e oute r surfac e o f th e duc t ther e i s a muscula r sheath , an d th e muscle fiber s ar e sometime s s o arrange d a s t o for m a specia l pumpin g apparatus fo r ejecting the sperm , or a certain quantit y of sperm-containing fluid , upo n eac h eg g as i t issue s fro m th e oviduc t int o th e genita l chamber o r vagina . The Femal e Accessor y Glands.—The pai r o f glands associate d wit h the exi t apparatus of the femal e genital organs (Fig . 284 A, AcGl) usuall y has som e function connecte d with the layin g o f the eggs . Thes e glands , therefore, ar e known as the accessor y glands of the reproductiv e system . In th e mor e generalize d insects th e openin g of the accessor y gland s lie s on th e vente r o f th e nint h abdomina l segmen t (Fig . 28 9 B , C , AcGl) between th e base s o f the secon d valvifer s of th e ovipositor . Whe n th e genital chambe r i s converte d int o a vagina l pouc h o r tube , however , the accessor y glands ope n either a t th e en d of the latte r o r in its dorsa l wall (Figs . 288 , 28 9 E , AcGl). Thei r proxima l part s ar e sometime s enlarged t o for m reservoir s (Fig . 288 , Res). Th e accessor y glands an d their duct are the only ectodermal parts of the female reproductive system

THE INTERNAL ORGANS

O F REPRODUCTION56

7

that ma y possibl y b e homologou s wit h part s o f th e mal e system ; i n position a t leas t the y correspon d t o th e ejaculator y duc t an d accessor y glands of the male . In th e majorit y o f femal e insect s th e accessor y gland s produc e a n adhesive substance fo r attaching th e egg s to a support o r for gluing them together i n a mas s a s the y ar e laid , an d fo r thi s reaso n the y ar e com monly terme d colleterial glands. Bu t i n som e insect s a n abundanc e o f secretion fro m th e accessor y gland s i s used t o for m a coverin g ove r th e egg mass, o r a n eg g case. Th e egg-coverin g material, however, may b e secreted i n specia l glandula r part s o f the oviducts , a s i n th e Acridida e (Fig. 287 , Old). A t leas t on e o f th e accessor y gland s i n th e stingin g Hymenoptera produce s a n irritatin g o r toxi c liqui d an d ha s becom e an essential adjunc t t o th e stingin g apparatu s evolve d fro m th e ovipositor . In som e viviparous Diptera , a s we have seen, there ar e accessor y gland s having a nutritiv e function , bu t sinc e they open into the anterior en d of the uteru s (Figs . 29 0 E, 291 , AcGl) i t i s possibl e tha t the y ar e specia l food gland s no t homologou s wit h th e usua l accessor y gland s o f th e ninth segment. 2. TH E MAL E ORGAN S

The interna l organ s o f reproduction i n mal e insect s havin g a singl e genital openin g are in many respect s similar t o thos e o f the female . I n an adult insect the essential parts of the male reproductive syste m include a pair o f testes (Fig. 292 A, Tes), a pair o f lateral ducts, the vasa deferentia (Vd), corresponding to the lateral oviducts of the female, and a median ectodermal exi t tube , o r ductus ejaculatorius (Dej), functionall y compar able wit h th e media n oviduc t o f th e female . Beside s thes e constan t parts ther e ar e generall y presen t als o accessor y structure s o f a mor e variable nature. Frequentl y a section o f each vas deferens, fo r example, is enlarged to serve as a sperm reservoir, or vesicula seminalis (Vsni), or, again, a considerabl e lengt h o f the duc t i s throw n int o a compac t coi l of irregula r convolutions , formin g a n epididymis. Ectoderma l accessory glands (AcGls) ar e commonl y presen t i n th e for m o f pouche s o r blin d tubes branchin g fro m th e uppe r en d o f th e ejaculator y duct . Th e external openin g o f th e exi t duct , o r male gonopore (Gpr), i s generall y situated o n o r withi n a media n intromitten t organ , th e penis (Pen), or phallus. The Testes.—Eac h testi s consist s typicall y o f a grou p o f shor t sperm tubes (Fig. 292 B, SpT). The tubes contain the male germ cells in successiv e stages o f development, an d othe r cell s associate d wit h th e germ cell s i n variou s capacities . Th e sper m tube s i n thei r origi n an d development correspon d t o th e eg g tubes o f th e ovaries , bu t the y ar e usually called the testicula r "follicles."

568

PRINCIPLES OF

INSECT MORPHOLOGY

General Structure of a Testis. —The testis in some of the mor e generalized insect s closel y resemble s a n ovar y i n tha t th e sper m tube s aris e serially fro m th e dista l part o f the exi t duc t (Fig . 29 2 B). Eac h tubul e is attache d t o th e va s deferen s b y a smal l stalklik e va s efferens (Ve), but th e testicula r tube s hav e n o termina l filaments . I n som e insects , as i n Apterygot a an d Plecoptera , th e sper m tube s ar e fre e fro m on e another i n the adul t stag e (Fig . 290 C), as are the ovariole s o f the ovary , but generall y the y ar e al l containe d i n a n investin g peritoneal sheath (Fig. 29 2 B , PSh). Frequentl y th e tw o teste s o f opposit e side s ar e united in a single median organ. I n the higher insects the teste s usuall y

FIG. 292.—Genera l structur e o f th e mal e reproductive organs, diagrammatic. A , the mal e reproductive system. B , structur e of a testis. C , sectio n of a testis an d duct . AcGls, accessor y glands; Dej, ductu s ejaculatorius; ESh, epithelial sheath; Gpr, gonopore; Pen, penis; PSh, peritoneal sheath; SpT, spermati c tube; Tes, testis; Vd, va s deferens; Ve vas efferens ; Vsm, vesicul a seminalis.

have a mor e compact structur e becaus e of the incomplet e separation o f the sper m tube s withi n th e peritonea l sheat h (C) . Th e numbe r o f sperm tubes in a testis varie s in different insects ; it i s generally less than the numbe r of egg tubes in the ovary , bu t i n most Lepidoptera ther e ar e four tubule s i n eac h sex . I n certai n insects , a s i n som e Apterygota , Coleoptera, an d Diptera , eac h testi s i s a simpl e saclik e organ , whic h in mos t case s i s probably a singl e sper m tube , thoug h i n som e Dipter a it i s said t o be partially subdivided . Structure o f a Sperm Tube. —The wal l of a testicula r tubul e consist s of a cellula r epithelial sheath (Fig . 29 3 A , ESh), whic h i s sometime s divided int o tw o layers , formin g a n oute r epitheliu m an d a n inne r epithelium. Th e sper m tubes , however , d o no t hav e a tru e follicula r

THE INTERNAL ORGANS O F REPRODUCTION56

9

epithelium suc h a s tha t whic h form s th e wall s o f th e eg g chamber s in the ovary . I f the testis is but incompletel y divided into sperm tubes , as in Lepidoptera, th e sept a betwee n the compartment s appea r a s fold s of th e epithelia l sheat h extendin g posteriorl y towar d th e mout h o f th e duct (Fig . 292 C, ESh), while the entire organ is invested in the peritonea l sheath (PSh). The wall s o f the testicula r tubule s probabl y serv e a s trophi c intermediaries betwee n the bloo d surroundin g th e gonad s an d th e ger m cells within them , a s d o th e wall s o f th e ovarioles . Th e singl e long , coile d tube formin g th e testi s o f Dytiscus, accordin g t o Demand t (1912 ; Kor schelt, 1924) , is covere d b y tw o epithelia l sheath s insid e th e peritonea l sheath. Th e thic k oute r epitheliu m consist s o f a spongy , granula r plasma i n whic h cel l boundarie s ar e no t visible , bu t th e cytoplas m i s vacuolated b y numerou s smal l cavities , indicatin g that th e oute r epi thelium ha s a secretor y functio n an d probabl y elaborate s nutritiv e products discharge d int o th e lume n o f the tube . Th e oute r epitheliu m is bounded externally by a basement membrane. Th e inner epithelium is a thi n elasti c laye r havin g a fibrillate d appearanc e an d containin g a large numbe r o f smal l nuclei . Th e singl e saclik e testis o f Dipter a als o is sai d t o b e surrounde d b y tw o envelopes , distinguishe d a s th e tunica externa an d tunica interna by Keucheniu s (1913) . Th e oute r tunic , a s described b y Lomen (1914 ) i n the mosquito , i s a thick connectiv e tissu e layer abundantl y vacuolate d b y smal l space s fille d apparentl y wit h stored nutritiv e material . Th e inner tunic forms th e linin g of the testis. In Lepidoptera , accordin g t o Rucke s (1919) , th e wall s o f th e incom pletely separate d testicula r compartment s hav e th e appearanc e o f connective tissu e an d apparentl y serv e fo r storag e o f reserv e materials , including fa t i n som e cases. I n specie s wit h colore d testes this coa t i s the repositor y o f the pigmen t granules . Within eac h sper m tub e ther e ar e t o b e distinguishe d successiv e regions accordin g t o th e stat e o f developmen t o f th e ger m cells . Th e upper part containing th e primar y spermatogonia is the germarium (Fig . 293 A, Grm), a s i n th e ovary ; beyon d th e germariu m is a zone o f growth (7) i n whic h the spermatogoni a ente r a stag e o f multiplication an d ar e usually encysted ; next i s the maturation zone (II) i n whic h the matura tion division s tak e place ; an d lastl y come s th e zone o f transformation (III) wher e the spermatocyte s develo p into spermatid s an d finally into mature spermatozoa . Th e entir e proces s o f spermatogenesis thu s takes place regularly within th e tube s o f the testis . The Cellular Elements within a Testicular Tube. —A characteristi c feature o f the testicula r tubes i s the presenc e of a large cell or nucleated mass of protoplasm in the ape x of the germarium . Thi s cel l is known a s the Versonian cellj o r apical cell (Fig , 29 3 A, B, ApCl). Earlie r invest! -

570

PRINCIPLES OF INSECT MORPHOLOGY

gators believe d the apica l cel l to b e the primar y spermatogoniu m o f th e tube, fro m whic h by divisio n al l the othe r spermatogoni a ar e produced. The majorit y o f recen t workers , however , regar d th e apica l cel l a s a spermatogonium specialize d as a trophocyte. The apica l cel l i s particularl y wel l develope d i n th e Lepidoptera , where i t consist s o f a larg e mas s o f cytoplas m containin g a nucleu s ????? ??? ?? ?????? ??????????? ??? ?????? ???? ??? ??????? ??????????

???? ??????????????? ?? ??? ??????? ?? ?????????? ??????? ?? ? ????? ????? ???????? matic. B , section of a larval sperm tube of Pieris. (From Knaben, 1931, after Zick, 1911. ) C, diagra m o f th e typica l structur e o f spermatogonia l cysts . ApCl, apica l cell ; CCl, cyst cell ; Cst, sperm cyst ; ESh, epithelia l sheath ; Grm, germarium; /, zone of growth; II , maturation zone ; 7/7 , transformatio n zone ; Spd, spermatids ; Spg, spermatogonia ; Spz, mature spermatozoa; Ve, vas efferens .

???? ?? ????????????? ?????? ?? ????? ????? ??????? ??? ?????? ???? ??? seen to be connected with the latte r b y protoplasmi c strand s containin g dark granule s tha t appea r t o originat e i n th e apica l cell . Th e sperma togonia o f Lepidoptera, therefore , a s pointe d ou t b y Grtinber g (1903) , must be nourished directl y fro m the apica l cell . The nutritive materia l utilized b y th e apica l cell , it i s claime d bot h b y Grlinber g an d b y Zic k (1911), is derived from certai n spermatogonia in the immediate neighbor hood o f the apica l cel l whic h ar e dissolve d an d absorbe d b y th e latter . According t o thi s view , then , th e apica l cel l o f a testicula r tub e i n the Lepidopter a i s a spermatogonia l nurs e cell , which, on the on e hand, dissolves an d absorb s som e o f th e adjacen t spermatogoni a and , o n th e other hand , feed s a second set o f spermatogonia wit h th e materia l fro m the firs t se t elaborate d i n it s cytoplas m an d delivere d t o th e recipien t

THE INTERNAL ORGANS O F REPRODUCTION57

1

cells throug h th e connectin g cytoplasmi c strands . Thes e speciall y nourished spermatogoni a becom e spermatocyte s an d develo p int o th e spermatozoa. I n th e firs t place , however , the nutrien t materia l o f th e male ger m cells , a s i n th e cas e o f th e femal e ger m cells , come s fro m the bloo d and is primarily elaborated in the trophic sheath of the gonadial tube. An apica l cel l i s als o wel l develope d i n Diptera . Accordin g t o Friele (1930) , it is a conspicuous structure in the uppe r end of the testicular lumen in Psychoda alternata during the larval stages, but it degenerates and disappear s i n th e pup a an d i s not t o b e foun d i n th e adul t testis . Friele claim s tha t th e apica l cel l o f Psychoda i s a secretio n cel l only , the product s o f whic h dissolv e th e spermatogoni a adjacen t t o i t an d convert them into a fluid plasma, which is dispersed through the testicula r lumen where the developin g sperm cells can make use of it . The spermatogonia that are destined to become spermatozoa undergo a serie s o f divisions , bu t th e cell s produce d fro m eac h primar y sper m cell usuall y remai n attache d t o on e anothe r centrall y b y protoplasmi c strands an d assum e a radia l position , givin g a rosett e patter n t o th e spherical mas s i n cros s sectio n (Fig . 29 3 C) . Eac h spermatogonia l group i n mos t insect s soo n become s enclose d i n a cellula r envelope , known a s a sperm cyst (A , B, Cst, C) . The origi n o f th e mal e cys t cell s ha s no t bee n definitel y observe d in man y insects , but , whil e some investigators hav e believe d that thes e cells ar e derive d fro m th e sheat h o f the sper m tube, mos t writer s regard them a s product s o f th e ger m cells . Accordin g to Zic k (1911) , th e differentiation betwee n the secondar y spermatogonia and the cys t cells in Lepidoptera i s a matter o f nutrition. Th e primar y spermatogoni a (Fig . 293 B, Spg) immediatel y surroundin g the apica l cel l (ApCl), an d whic h are directl y nourishe d b y th e latter , h e says , becom e th e functiona l spermatogonia; those o f the nex t rank , receiving little nourishment fro m the apica l cell , become the cys t cell s (CCl). Adjacen t cells o f these tw o groups attach t o each other in pairs; the poorly nourished cell by division forms th e cyst , th e othe r produce s the enclose d group of spermatogonia. On comparin g th e interna l cellula r organizatio n o f a testicula r tube wit h that o f an ovaria l tube , i t become s evident tha t th e cys t cell s of th e forme r correspon d functionally at leas t t o th e follicl e cell s of th e latter, thoug h investigation s see m t o sho w that th e ovaria l follicl e cells an d th e testicula r cys t cell s hav e differen t origins . I n th e ovar y the cystocyte s produc e a continuou s epitheliu m linin g th e eg g tube ; in th e testi s the y inves t group s o f spermatogonia separately . I n som e insects i t ha s bee n observe d tha t th e testicula r cyst s ar e connecte d b y strands o f cell s formin g a networ k throughou t th e lume n o f th e sper m tube.

572 PRINCIPLES

O F INSECT MORPHOLOGY

The spermatogoni a withi n eac h cys t continu e t o multiply , formin g successively spermatocytes , spermatids , an d finall y spermatozo a i n advancing stages of development (Fig. 293 A). Th e cyst s finally degenerate an d ar e dissolved , bu t th e spermatozo a (Spz) generall y remai n grouped in bundles as they were in the cysts. Th e details of the matura tion of the mal e germ cells, the differentiatio n of the tw o sex-determinin g types o f sperm cells, and th e developmen t of the spermatozo a have been described in many special papers o n spermatogenesis i n insects an d need not b e give n here . A s the sper m cell s mature, other s ar e produce d i n the apica l part of the sperm tube, an d the sperm tube increases in length between it s tw o ends . Th e rip e spermatozo a ar e thu s alway s locate d at the posterior ends of the testicular tube s in proximity to the exit ducts . The Vas a deferentia.—The duct s leading from th e testes (Fig . 292 A, Vd) ar e usuall y simpl e tubes , eac h havin g a thic k cellula r epitheliu m limited b y a basemen t membrane , outsid e whic h i s a stron g muscula r coat o f circular fibers . Frequentl y a par t o f each duc t i s much convoluted, an d th e coil s ma y for m a definit e epididymis . I n som e insect s an enlargemen t o f the duc t i n th e shap e o f a dilatatio n o r diverticulu m constitutes a vesicul a seminali s (Vsm) fo r th e storag e o f th e matur e sperm a s th e latte r leav e th e testis . Th e spermatozo a ar e generall y found closel y massed i n th e vesicula , wit h thei r head s imbedde d i n th e epithelial wal l and their vibratile tail s projecting like cilia into the lumen. The vas a deferenti a ar e primaril y o f mesoderma l origin , bu t the y may b e extended posteriorly b y ingrowths from th e ectoder m or more or less replaced by the latter. Thus , i n Ephemerida, a s shown by Wheeler (1893), the termina l part s o f the mal e ducts ma y be lined wit h a n ecto dermal cuticula, and in the Diptera , accordin g to Brtiel (1897 ) and Friel e (1930), th e definitiv e duct s o f the teste s ar e forme d entirel y a s latera l diverticula fro m th e ectoderma l ductu s ejaculatorius . Th e latera l genital duct s o f th e mal e ope n separatel y t o th e exterio r i n Protura , Ephemerida, an d som e Dermaptera ; i n othe r insect s the y unit e wit h the anterio r en d of a median ejaculatory duct . The Ductu s ejaculatorius.—The usua l common exit tube o f the mal e genital system is formed as a median ventral invagination o f the ectoderm at th e posterio r en d o f the nint h abdomina l segment , wit h whic h th e vasa deferenti a becom e connected . Th e ejaculator y duct , therefore , and al l part s derive d fro m i t hav e a cuticula r linin g continuou s wit h that of the body wall. Th e epithelial wall of the duc t is surrounded by a strong muscula r sheath, usuall y consistin g o f circular fibers , an d some times containin g longitudina l fibers , bu t th e relatio n o f the tw o set s of fibers to eac h othe r appear s no t t o b e th e sam e i n al l cases . Thoug h the ejaculator y duc t i s alway s describe d a s unpaire d i n it s embryoni c origin, it s anterio r en d i s frequentl y forked , especiall y whe n accessor y

THE INTERNAL ORGANS O F REPRODUCTION57

3

glands aris e fro m it . Th e externa l apertur e o f th e exi t duct , o r mal e gonopore, i s usually situate d o n an intromittent organ , the phallus , bu t the latte r ofte n contain s a n invaginatio n cavity , o r endophallus, withi n which th e tru e gonopor e i s concealed . Th e endophallus , whic h i s either a permanentl y enclose d cavit y o f th e phallu s o r a n eversibl e vesicle or tube, i s sometimes mistaken fo r the termina l part of the ductu s ejaculatorius. The Mal e Accessor y Glands.—Glandular structure s associate d wit h the mal e organs usually have the functio n o f secreting a mucous or viscid substance, whic h either i s discharge d a s a liqui d wit h th e spermatozo a or hardens about the m t o for m a covering or capsule known as a spermatophore. Th e mal e accessor y gland s generall y aris e fro m th e anterio r end o f th e ejaculator y duc t o r fro m shor t divergen t anterio r branche s of th e duct . Typicall y the y hav e th e for m o f elongat e sac s o r tubes , and th e tubula r variet y i s ofte n greatl y loope d an d coiled . Usuall y there i s but on e pair o f accessory glands (Fig . 29 2 A, AcGl)j bu t i n some insects a serie s o f gland s arise s fro m eac h sid e o f the ejaculator y duct , while i n other s ther e ma y b e a compac t mas s o f tubules abou t th e ter minus of each vas deferens . Th e walls of the accessor y glands may have a muscula r sheath continuou s with that o f the ejaculator y duct . Glands associate d wit h th e externa l openin g o f th e ductu s ejacula torius ar e terme d preputial glands. Groups o f one-celle d preputia l glands ar e describe d b y Demand t (1912 ; Korschelt , 1924 ) i n Dytiscus marginalis. 3. GENERA L MORPHOLOG Y O F TH E REPRODUCTIV E ORGAN S

The gonad s o f th e simple r metazoi c animal s ar e merel y epithelia l swellings o r simpl e capsule s o f somatic cell s surrounding the ger m cells and retainin g the m unti l th e latte r ar e read y t o perfor m thei r destin y independent o f the soma . Th e matur e ger m cells either ar e discharge d directly int o th e surroundin g mediu m o r ar e liberate d int o th e bod y cavity. I n th e secon d case the ger m cells must find an exit through th e body wall to the exterior . In th e Annelida , th e gonad s ar e develope d fro m th e coelomi c epi thelium, an d frequentl y the y occu r o n th e posterio r surface s o f th e intersegmental sept a tha t divid e th e bod y cavity . Typically , th e rip e germ cell s fal l int o th e coelomi c cavities, wher e they matur e an d fro m which th e ov a an d spermatozo a finall y escap e t o th e exterior , i n som e species throug h rupture s o f the bod y wall , but generall y by wa y o f th e nephridial tube s o r throug h specia l genita l ducts . I n som e form s th e genital ducts are continuous with the wall s of the gonads , and the y ma y be united distall y i n a common outlet tube .

574 PRINCIPLES

O F INSECT MORPHOLOGY

The reproductiv e syste m o f th e Arthropod a i s entirel y closed , th e exit duct s bein g continuou s wit h th e wall s o f th e ovarie s o r testes . The duct s eithe r ope n separately t o th e exterior , generall y on the base s of segmenta l appendages , o r unit e i n a media n termina l duct . Con sidering th e eviden t relationshi p betwee n th e arthropod s an d annelids , as expresse d i n variou s fundamenta l feature s o f their structure , i t wil l be interesting t o learn if in the development o f the arthropo d reproductiv e organs there is any suggestion of an early condition resembling that foun d in adult annelids . Though th e ger m cell s o f insect s ar e know n i n som e case s t o b e differentiated fro m th e somati c cell s a t th e tim e o f cleavage, thi s earl y origin o f th e reproductiv e element s canno t b e demonstrate d i n al l species. Visibl e difference s betwee n th e ger m cell s an d th e somati c cells appea r i n mos t insect s onl y afte r th e formatio n o f the ger m layer s of th e embryo , when the ger m cells are found imbedde d in the splanchni c layer o f th e abdomina l mesoderm . Th e rudiment s o f th e gonad s no w appear a s thickening s o f th e mesoderm , know n a s th e genital ridges, in th e anterio r part s o f whic h th e ger m cell s ar e lodged . Wit h th e dorsal extensio n o f th e ger m layers , th e genita l ridge s ar e carrie d t o the dorsa l par t o f the abdomina l cavity , wher e they protrud e fro m th e splanchnic wall. According to th e detaile d stud y b y Heymon s (1892 ) o n the develop ment o f the femal e reproductiv e organ s in th e roac h Blatella germanica, the ger m cell s ar e a t firs t mostl y aggregate d i n group s opposit e th e intersegmental grooves , so that, when the coelomi c sacs appear, th e cell s lie dorsall y i n th e intercoelomi c walls . Thi s earl y positio n o f the ger m cells i n th e roac h i s thus , a s Heymon s point s out , suggestiv e o f th e condition in many Annelida , in which the ger m cells are groupe d in th e intersegmental septa. Whereve r the germ cells assemble in the Annelida, the gona d i s forme d a s a mesoderma l coverin g ove r them , fro m whic h the ger m cells are eventually liberated int o the coelomi c cavities. I n the female roach , Heymon s finds , th e ger m cell s a t a n earl y stag e ar e likewise extrude d fro m the mesoder m int o the coelomi c cavities , but , though an occasional cell may be entirely liberated, mos t o f them remai n attached t o th e dorsa l coelomi c walls , wher e the y migrat e posteriorl y and ar e soon again overgrow n by the mesoderma l epithelium. Afte r th e disappearance o f th e intercoelomi c septa , th e ger m cell s li e i n a con tinuous serie s alon g eac h side o f the body , imbedde d i n th e splanchni c mesoderm, whic h grows out a s th e genita l ridges , o r firs t rudiment s of the reproductiv e organs (Fig . 29 4 A). As developmen t proceeds , th e ger m cell s becom e limite d t o th e anterior par t o f eac h genita l ridge , whic h increase s i n thicknes s unti l it form s a fol d hangin g from th e dorsa l are a o f the splanchnopleur e and

THE INTERNAL ORGANS

OF REPRODUCTION

575

eventually become s the ovary or testis, whil e th e posterio r narro w par t of th e ridge becomes the latera l genital duct . I n th e gonadia l rudiment

FIG. 294.—Developmen t of the gonads , the ovary , and th e genita l ducts. A , section of coelomi c sac o f Blattella germanica wit h grou p o f germ cells in splanchni c wall. (From Heymons, 1892. ) B , sectio n o f posterio r en d o f embry o o f Conocephalus, wit h termina l ampullae o f male ducts in appendages of tenth abdominal segment. (From Wheeler, 1893. ) C, differentiatio n of femal e gona d o f Blattella into eg g tube s o f ovary . (From Heymons, 1892.) D , showin g branchin g o f gonadia l duct s i n embry o o f Forficula t o sevent h an d tenth abdomina l segments . (From Heymons, 1895. ) E , coelomi c sa c o f embry o o f Leptinotarsa wit h rudimen t o f gonad attache d t o splanchni c wall . (From Wheeler, 1889. ) F, G , tw o lat e stages i n developmen t o f ovary o f Blattella. (From Heymons, 1892.) a , suspensorium o f gonad; . Am, amnion ; b , ventra l stran d o f gonad ; c , anterio r branc h of gonadial duct ; Cdbl, cardioblasts ; Clx, calyx ; Coel, coelomi c cavity; d, posterio r branc h of gonadial duct ; e , terminal ampulla o f gonadial duct ; Ecd, ectoderm; EMsd, somatopleure ; ET, eg g tube; GCls, germ cells ; Gd, gonad; IMsd, splanchnopleure ; Msd, mesoderm ; NIR, neural ridge ; Odl, oviductu s lateralis ; Ovl, ovariole ; Pdcl, ovariol e pedicel ; Proc, procto daeum; Sp , spiracle ; TF, termina l filament ; VNC, ventra l nerv e cord ; XApd, appendag e rudiment o f tenth abdomina l segment .

there is soon to be distinguished a suspensorium (Fig . 294 E, a), or dorsal strand o f cells by whic h the gona d i s attache d t o th e coelomi c wall , a median part , o r germarium (Grm), containin g th e ger m cells , an d a

576 PRINCIPLES

O F INSECT MORPHOLOGY

cellular ventral strand (b) , whic h is continuous posteriorl y wit h the duct . During the embryoni c period th e ger m cells increase in number withou t other chang e from thei r earlie r condition . The nex t stag e i n th e developmen t o f th e gona d i s tha t h i whic h the singl e orga n becomes subdivided int o a series o f compartments—the ovarioles o f the ovar y o r th e sper m tube s o f the testis . I n th e femal e roach, accordin g t o Heymons , th e cell s o f th e gonadia l suspensoriu m become flattene d an d arrange d i n vertica l serie s (Fig . 29 4 F, a) , there being finall y thu s forme d abou t 2 0 columns o f regularly stratifie d cells , which becom e the termina l filament s (TF) o f the matur e ovary . Now , the intermediat e par t o f the gonad , containin g th e ger m cells , lose s it s even contou r an d i s produce d dorsall y int o a serie s o f swelling s cor responding to the bases of the filament columns (C), while the intervenin g depressions ar e extende d ventrally . Whe n th e cleft s reac h th e ventra l strand of the gonad, the ovary is divided into its definitive egg tubes (ET), each surmounte d b y a terminal filament , an d i s connected basall y wit h the ventra l stran d (b) , which i s continuou s wit h th e duc t (Odl). Th e final inclined o r horizontal positio n o f the ovariole s (G ) is attained b y a shortening o f the ventra l stran d an d a compensatin g differentiatio n i n the lengt h o f the termina l filaments . In othe r insect s th e formatio n o f the ovaria l o r testicular tube s ma y be a mor e simplified process , consistin g merel y o f the outgrowt h o f th e tubes fro m th e primitiv e orga n or of a division o f the latte r by a n inflec tion o f its epithelia l walls . I n an y case , i t i s eviden t tha t th e eg g or sperm tube s ar e bu t secondar y outgrowth s o r subdivision s o f a primi tively simpl e gonad, and that there is no basis in ontogeny for regarding the gonadia l tubes a s a series of primitive segmenta l reproductive organs secondarily unite d b y a commo n lateral duct . O n the othe r hand , th e segmental arrangemen t o f th e gonadia l tube s i n certai n Apterygot a (Fig. 29 0 A) might b e take n a s suggestive tha t th e tube s originate d a s segmental pouche s o f a primitivel y continuou s gonad . However , i n many o f the Apterygot a the arrangemen t of the genital tubes has no close relation t o th e metameris m of the body , an d th e gona d may consis t o f a single large saclike ovary o r testis on each side. In the Protur a both th e ovary an d the testis, as described by Berlese (1910), consis t o f a simpl e elongat e sac , th e wall s o f whic h consis t o f a laye r o f epithelia l cell s covere d externall y b y a sheat h o f connectiv e tissue. Eac h ovar y (Fig . 29 0 B , Ov) ha s th e structur e o f a typica l panoistic ovariole , except that it lacks a terminal filament . Th e anterio r end, which is deflected, contain s the germarium , and there are from 1 3 to 15 oocytes i n th e res t o f th e tub e i n successiv e stages o f growth. Th e eggs attain thei r matur e siz e first in on e ovary, the n i n the other . Th e oviducts unite in the eight h abdomina l segment to for m a median duct .

THE INTERNAL ORGANS

O F REPRODUCTION 57

7

The testi s is almost a replica of the ovary ; the mal e ducts, however , are long an d coiled , and the y ope n separatel y throug h th e termina l spine s of th e externa l genita l orga n exserte d fro m betwee n th e elevent h an d twelfth abdomina l segments. Among apterygot e insect s th e gonad s have likewis e a simpl e saclike form i n Collembola, in Campodea, an d i n the mal e o f Japyx. I n Campodea, eac h testis an d ovary , a s describe d by Grass i (1887) , consist s o f a single long tube extendin g forward int o the posterio r part o f the thorax . In Japyx th e teste s ar e simila r t o th e teste s an d ovarie s o f Campodea, but th e vasa deferenti a are long and convoluted. Posteriorl y the latera l ducts unite in a very short media n ejaculatory duct, which opens between the nint h and tenth segments of the abdomen. In othe r Diplur a an d Thysanura , includin g th e femal e o f Japyx, each gona d consist s o f severa l o r numerou s tubules , whic h i n som e cases ar e segmen t ally arrange d o n th e latera l ducts . Thus , i n Japyx, there ar e seve n ovariole s i n eac h sid e o f th e body , on e i n eac h o f th e first seve n abdomina l segments , openin g int o a lon g latera l oviduc t (Fig. 29 0 A) . Th e sam e typ e o f structur e occur s i n th e ovarie s o f Machilis, thoug h ther e ar e her e onl y si x pair s o f ovarioles . I n youn g specimens o f Lepismatidae, accordin g t o Grassi , th e ovariole s ar e als o segmentally disposed , bu t i n adult s th e metameri c arrangemen t i s lost. In th e male s o f these form s th e numbe r o f testicular tube s i s variable , and they d o not have a segmental arrangement . Since the ovaries and the testes in Protura, Collembola , and Campodea consist eac h o f a n undivide d sac , i t migh t b e suppose d tha t th e adul t organs i n these form s represen t th e simpl e undivided embryoni c gonads of th e highe r insects. However , if the fac t i s considered that the ovaria l or testicular sa c in these lower forms has the structur e of a single ovariole or sper m tub e an d no t tha t o f a n embryoni c gonad , and i t i s recalled that the ovar y of some of the more specialized insects may likewise consist of bu t on e ovariole, it seems more probable that the simpl e gonads of the groups abov e mentione d ar e case s of reduction rathe r tha n example s of primitive structure. O n the other hand, there can be little doubt that the primary compoun d ovary o r testi s consiste d o f only a fe w egg tube s o r sperm tubes, an d it may be conceded that the tubes were perhaps at first segmentally arrange d o n the duct . Th e furthe r evolution o f the gona d has been generally in the directio n of an increase in the numbe r of tubes, which hav e kep t thei r seria l arrangemen t i n th e ovar y o f som e o f th e more generalize d insects bu t i n mos t form s hav e assume d a clustere d arrangement owin g to the shortenin g of the duct . The primar y exi t apparatus o f th e reproductiv e syste m consist s in the femal e o f the pedicel s of the ovarioles , the calyces , and th e latera l oviducts; i n th e mal e it include s the vas a efferenti a an d th e vas a defer -

578 PRINCIPLES

O F INSECT MORPHOLOGY

entia. Th e ovariol e pedicels, th e vas a efferentia , an d the oviducal calyx are developed fro m th e cellula r ventral strand s o f the gonads ; the latera l ducts ar e direc t derivative s o f th e posterio r unspecialize d parts o f th e genital ridges , whic h ar e continuou s wit h th e ventra l strand s o f th e gonads. There is no evidence that the lateral reproductive duct s of arthropod s ever serve d an y othe r tha n thei r presen t purpose . I t i s probable tha t their mesoderma l part s represen t close d groove s o f th e splanchni c mesoderm tha t primaril y conducte d th e reproductiv e element s t o openings in the body wall. Thei r lumin a are to be regarded, therefore, as parts o f the coelomi c cavity. I n immatur e stage s o f many insect s th e female duct s ar e attache d posteriorl y t o th e ectoder m o f th e sevent h abdominal segment , an d thos e o f the mal e t o th e ectoder m o f the tent h segment. A s described in Orthoptera by Wheeler (1893), and in Dermaptera an d Orthopter a b y Heymon s (1892 , 1895) , th e embryoni c duct s terminate i n hollo w swellings , or ampullae , withi n th e rudiment s o f th e appendages of these segment s (Fig . 19 4 B, e). I t seem s probable, there fore, tha t th e primitiv e reproductiv e duct s o f insect s opene d o n th e bases o f segmental appendages . The media n exi t apparatu s o f th e genita l syste m o f modern insect s is a secondar y developmen t fro m th e ectoderm . I n som e of the highe r insects branche s from th e median duct have partly o r entirely supplanted the primar y mesodermal lateral ducts . GLOSSARY O F TERM S APPLIE D T O TH E INTERNA L REPRODUCTIV E ORGANS

Accessory Gland s (AcGl)'. —In th e female , a pai r o f glands openin g primarily o n the vente r o f th e nint h abdomina l segment , secretin g a n adhesiv e substanc e o r material formin g a coverin g or a cas e (ootheca ) fo r th e eggs ; in th e male , mucous glands opening into the ejaculator y duct . Acrotrophic Egg Tube.—A type o f egg tube in whic h the trophi c cell s remain i n the apica l chamber. (Telotrophic type.) Apical Cel l (ApCl). —A special , usually large , trophic cell in th e uppe r end o f the testicular tub e i n some insects. (Versonian cell.) Bursa copulatri x (Bcpx). —A copulator y pouch of the female , usuall y th e genita l chamber or a part o f the latter . Calyx (Clx).—The widene d anterior end of the lateral oviduct receiving the ovariole pedicels. (Eierkelch.) Chorion (Cho). —The eg g shel l secrete d b y th e follicl e cell s o f th e ovaria l eg g chamber. Colleterial Glands. —Accessory gland s o f th e femal e secretin g a n adhesiv e sub stance used to fasten th e egg s to a support . Corpus luteu m (Clt). —The mas s of degenerating follicle cell s left i n an eg g chamber afte r th e discharg e of the egg. Cystocytes.—The cell s enclosin g th e ger m cell s i n a gonadia l tube ; th e follicl e cells of the ovary , the cys t cell s of the testis .

THE INTERNAL ORGANS O F REPRODUCTION 57

9

Ductus ejaculatoriu s (Dej). —The media n ectoderma l outle t tub e o f th e mal e genital system . Egg Chambe r (EC). —One o f th e compartments , o r follicles , o f a n ovaria l eg g tube, forme d o f the follicl e cells , containing an oocyte . (Eikammer.) Egg Tub e (ET). —The tubula r par t o f an ovariole containing the ger m cells, th e oocytes, the nurs e cells, and th e follicl e cells. Ejaculatory Duc t (Dej). —See ductus ejaculatorius. End Chambe r (Grm). —The germariu m of a gonadial tube. Epididymis.—A convoluted part of the vas deferens. (Nebenhode.) Follicle (EC).—See egg chamber. Follicle Cell s (FCls).* —The inne r epithelial cell s of an ovaria n egg tube. Genital Chambe r (GC). —In th e female , primaril y a n i n vagina tion cavit y behind the eight h abdomina l sternu m containin g th e gonopor e and th e spermatheca l aper ture, ofte n converte d into a vagina or uterus, and in some insects openin g secondarily on or behind th e ninth sternum; in the male , a n invagination cavit y behind (above ) the ninth sternum containin g the intromittent organ. Genital Ridge.—On e of the embryoni c gonadial rudiments, a ridge-like swelling of the splanchnopleur e wall of the mesoder m containing the ger m cells. Germ Cell s (GCls). —The reproductiv e cell s a s distinguishe d fro m th e somati c cells, or, more specifically, the early undifferentiated reproductiv e cells. (Keimzellen.) Germarium (Grm). —The en d chamber of an ovarial or testicular tube, containin g the primary oogonia or spermatogonia. (Endkammer.) Gonad (Gd).— The ovar y or testis, or the embryoni c rudiment of either, forme d of splanchnic mesoderm cells enveloping the ger m cells. Gonopore (Gpr).— The externa l openin g of a genital duct . Lateral Oviduc t (Odl). —See oviductus later alls. Median Oviduct (Ode).—See oviductus communis. Meroistic Eg g Tube.— A typ e o f eg g tube containin g bot h oocyte s an d tropho cytes, including the acrotrophi c and poly trophic types . Nurse Cell s (NrCls). —The trophocyte s o f the ovar y or testis. Oocytes (Ooc). —The eg g cell differentiated fro m the oogonium, before maturation. Oogonium (Oog). —The firs t stag e i n th e differentiatio n of an eg g cell from a primary female ger m cell. Ovarial Ligamen t (Lg). —A ligamentou s stran d attachin g th e termina l filaments of a n ovary to th e dorsa l diaphrag m or to the bod y wall, sometimes united with that from th e opposite side in a median ligament attached t o the ventral wal l of the dorsa l blood vessel . Ovariole (Ovl). —One o f th e secondar y division s o f th e ovary , compose d o f a terminal filament , a n eg g tube, and a pedicel. Ovary (Ov). —The femal e reproductive organ containing the eg g cells. (Eierstock.) Oviductus communi s (Ode). —The media n ectoderma l outle t duc t o f the femal e genital system , usually opening into a genital chamber , or vagina. (Eiergang.) Oviductus laterali s (Odl). —One o f the paire d lateral ducts o f the femal e syste m connected wit h th e ovary , mesoderma l in origin , bu t sometime s partl y o r entirel y replaced by a n ectoderma l branch of the media n duct. (Eileiter.) Ovum.—The mature (unfertilized ) eg g cell. Panoistic Egg Tube.—A type o f egg tube in whic h the vitellariu m contain s eggs only. Pedicel (Pdcl). —One o f the ovariol e stalks (Eirohrenstielen), o r shor t duct s fro m the eg g tubes to the oviduct . Poly trophic Egg Tube.—A type of egg tube in which group s of trophocytes accompany th e oocytes .

580 PRINCIPLES

O F INSECT MORPHOLOGY

Sperm Cys t (Cst).' —One o f th e cellula r capsule s i n th e testi s containin g th e spermatocytes. Sperm Tub e (ST). —One o f the secondar y division s o f the testis . Spermatheca, o r Receptaculu m semini s (Spt). —The sper m receptacl e o f th e female. Spermatid.—An immature spermatozoon . Spermatocyte (Spc). —The sper m cel l differentiate d fro m a spermatogonium , before maturation . Spermatogonium (Spg). —The firs t stag e i n th e differentiatio n o f a sper m cel l from a primitive mal e germ cell. Spermatozoon Spz).—The matur e sper m cell . Terminal Filament (TF).—The cellular end thread of an ovariole. (Endfaden.) Testis (Tes). —The mal e reproductiv e orga n containin g th e primar y ger m cells , and i n which the sper m cell s undergo maturation. (Hode.) Trophocytes (NrCls). —The nutritiv e cells , o r nurs e cells , of the ovar y o r testis. (Ndhrzellen.) Uterus (Utrs). —A compartmen t o f the .genital chamber , o r vagina, i n whic h th e embryonic an d a par t o f the postembryoni c developmen t o f the youn g insec t ma y take place . Vagina (Vag). —A par t o f the definitiv e egg passage i n man y insect s posterio r t o the tru e oviductu s communis , derived from th e genita l chamber . (Scheide.) Vas deferen s (Vd). —One o f the latera l duct s o f the mal e reproductiv e system . (Plural, vasa defer entia.) Vas efferens.—On e o f the shor t duct s connectin g th e sper m tube s o f the testi s with the va s deferens , correspondin g to the pedice l of an ovariole . Versonian Cell.—Se e apical cell. Vesicula seminali s (Vsm). —A dilatatio n o f the va s deferen s i n which the sperm atozoa may be retained . Vitellarium (ViT). —The par t o f an ovariol e eg g tube i n whic h the oocyte s gro w by th e accumulatio n of yolk and attai n their matur e size. (Zone o f growth.)

CHAPTER XI X THE ORGAN S O F COPULATIO N AN D OVIPOSITIO N The organ s specifically concerne d with sexual mating an d the deposi tion o f th e egg s ar e know n collectivel y a s th e external genitalia. Th e copulatory organ s pertai n t o bot h sexes , thoug h the y ar e particularl y developed i n th e male ; th e femal e organ s o f ovipositio n ar e externa l genitalia in the sense that they are accessory to the reproductive function . The copulator y apparatu s o f th e mal e include s primaril y a n orga n for conveyin g th e spermatozo a int o a sper m receptacl e o f th e female , and usuall y a grou p o f associate d structure s adapte d fo r graspin g an d holding th e female . Th e recipien t orga n o f the femal e i s a copulator y pouch (genita l chambe r o r vagina ) o r a spermatheca l diverticulu m of the latter. The principa l claspin g organ s o f th e mal e ar e generall y movabl e appendicular structure s o f the nint h segment , whic h serve a s a pai r of grappling hook s (harpagones) , though accessor y copulator y processe s of various forms may occur on the same segment or on any of the neighboring segments, an d i n som e case s th e cerc i ar e transforme d int o graspin g organs. Coition , i n mos t insects , i s effecte d b y a media n intromitten t organ located o n the conjunctival membrane behind the ninth abdomina l sternum, wit h which there may be associated various accessory structure s forming a grou p o f phalli c organs ; bu t i n som e o f th e Apterygot a a n intromittent orga n i s absent, an d i n Odonat a i t i s functionally replace d by a secondary copulatory structure on the anterior part of the abdomen . In certai n lowe r pterygote insect s ther e i s a pair o f intromittent organs . The externa l genitali a o f the female , i n additio n t o th e copulator y pouch, consis t o f structura l adaptation s fo r th e disposa l o f th e eggs . In th e Thysanur a (Machilida e an d Lepismatidae ) an d i n man y o f the pterygot e orders , th e femal e i s provide d wit h a specia l egg-layin g organ, known as the ovipositor, which appears t o be formed of the append ages o f th e eight h an d nint h abdomina l segments . B y mean s o f th e ovipositor th e femal e i s enable d t o deposi t he r egg s i n th e ground , in th e leaves , stems , an d woo d o f plants , o r int o th e bodie s o f othe r insects. Wit h man y insects , however , especiall y i n thos e havin g th e genital apertur e fro m whic h th e egg s ar e discharge d locate d o n th e ninth abdomina l segment , th e oviposito r i s reduce d o r absent , an d i n such case s th e termina l segment s o f th e abdome n ar e usuall y slende r 581

582 PRINCIPLES

O F INSECT MORPHOLOGY

and taperin g and capabl e of being protracted a s a tube , fro m whic h the eggs are extruded and may be attached t o smooth surfaces or concealed in crevices. 1. TH E MAL E GENITALI A

The morpholog y o f th e mal e organ s o f copulatio n i s no t definitel y known, notwithstanding the effort s tha t various investigators hav e given to th e subject . I n th e followin g discussions , therefore , a minimu m of attention wil l be given to theoretica l view s that d o not appea r t o b e in harmony wit h anatomica l facts . Moreover , i n orde r t o avoi d th e nomenclatural confusio n tha t ha s resulte d fro m th e lac k o f a n under standing o f the fundamenta l nature o f the mal e organs, an d i n orde r t o present simpl y th e fact s o f structure, a terminolog y ha s bee n adopte d that can be applied consistently to the major structural element s regardless o f what ma y b e th e morphologica l relations o f the latter . I n eac h order, however , man y specia l structure s mus t b e name d individually , since i t i s clea r tha t ther e ar e numerou s modification s o f th e genita l organs that have only a local significance. The primar y mesoderma l outlet tubes o f the mal e genita l system , a s we learne d i n th e las t chapter , ar e attache d durin g embryonic development i n som e orthopteroi d insect s t o th e ectoder m o f the ventra l wal l of the tenth abdominal somite, or in some cases they terminate in ampullae located within the appendag e rudiments o f this segment. I n certai n hexapods (Protura , Ephemerida , Dermaptera ) th e vas a deferenti a retain thei r separat e opening s in the adult , thoug h eac h may terminate in a n exi t duc t o f ectodermal origin, an d i n such cases the gonopore s are borne on a pair o f penes or on paired processe s of a single organ. Since , however, the positio n o f the intromitten t organ in these several hexapod groups varies fro m th e nint h to the elevent h segment , it seem s doubtful that the paire d adult structure s represen t rudimentar y limbs , though we might conclud e from th e embryologica l evidenc e given abov e that th e primitive mal e duct s opened on the base s of the appendage s of the tent h abdominal segment. With th e majorit y of adult insect s the vas a deferenti a open into th e proximal parts of the ectoderma l accessory glands (Fig . 292 A), an d th e latter the n unit e in a commo n outle t tube , the ductus " ejaculatorius. The ejaculatory duct opens usually on a median intromittent organ arising fro m th e ventra l conjunctiva l membran e betwee n th e nint h an d tenth abdominal segments; this membrane is probably the posterior par t of th e vente r o f the nint h primar y somite . I t woul d appear , therefore, that the original orifices of the vasa deferentia in such cases have migrated forward t o ope n i n commo n wit h th e accessor y gland s o n th e nint h abdominal venter , an d tha t a media n invaginatio n o f th e bod y wal l

THE ORGANS OF COPULATION AND OV1POSITION

583

at thi s poin t ha s formed th e commo n ejaculatory duct . Thi s theoretical origin of the male exit apparatus, however , is not necessarily recapitulated in ontogeneti c development , sinc e i n man y insect s th e embryoni c vas a deferentia hav e no t bee n trace d beyon d th e eight h o r nint h abdomina l somite, wher e they ar e said t o unit e directl y wit h th e ectoderma l ductus ejaculatorius. Th e media n gonopore, or external openin g of the ejacula tory duct, may remain flush with the surfac e of the membrane in which it is situated; but, with>the majority of insects, it is situated on a tubula r intromitten t organ , th e median penis, o r phallus. Male Genitali a o f Hexapod a Havin g Paire d Gono pores.—Here ar e include d the Protura , th e Ephemerida, and th e Dermaptera , thoug h som e o f th e las t hav e secondarily a single gonopore. Protura.—The male s o f Protura , a s describe d b y Berlese (1910 ) an d b y Prel l (1913) , hav e a n elaborat e bipartite intromitten t orga n (Fig . 295 ) eversibl e fro m between th e sterna l plate s o f th e elevent h an d twelft h abdominal segments , bu t the y hav e n o accessory latera l copulatory structures . Th e bifi d dista l par t o f the gen ital orga n end s i n a pai r o f lon g hollo w processe s o n which th e tw o genita l duct s ope n separatel y throug h FIG. 295. — subterminal aperture s (Gprs). Th e positio n o f the mal e Male genital organ of Protura (Eosenorgan i n Protur a woul d appear t o preclud e the possibil - tomon germaniity o f an y homolog y wit h th e usua l intromitten t orga n cum), ventral view. (From Prell, 1913.) of insects , locate d o n the nint h abdomina l segment . Ephemerida.—The vas a deferenti a o f Ephemerid a terminat e i n exi t ducts o f ectoderma l origi n tha t ope n separatel y o n a pai r o f penes . The ephemeri d pene s ar e small , flattened , conica l processe s arisin g ventrally a t th e bas e o f the tent h abdomina l segmen t (Fig . 296 , B , C , Pen). I t i s no t clea r whethe r th e membran e supportin g th e organ s (C) belong s t o th e nint h segmen t o r represent s th e vente r o f the tent h segment. Associate d wit h th e intromitten t organ s i s a pai r o f latera l segmented clasper s (B , Sty), arisin g eithe r separatel y fro m basa l lobe s or from a common basal plate (B , Cxpd) born e by the ninth sternum. Thes e claspers, which are individually movable by basal muscles (smd), are evidently th e styl i o f th e nint h segmen t an d therefor e belon g t o th e appendages o f thi s segment . Th e presenc e o f paire d pene s i n mal e Ephemerida i s correlate d wit h th e presenc e o f tw o separat e oviduca l openings in the female . Dermaptera.—The termina l ectoderma l part s o f th e genita l duct s of mal e Dermapter a i n som e forms ope n separatel y o n a pai r o f penes, while i n other s the y unit e i n a commo n exit duc t tha t open s on a single

584

PRINCIPLES OF

INSECT MORPHOLOGY

median peni s (Fig . 29 7 A , C) . Th e ovaria l duct s o f th e femal e com e together i n a short media n pouch with a single opening. Th e male organ in this order, however, is basally a n unpaired structure, since the terminal

FIG. 296.—Male genitalia of Ephemerida (Hexagenia). A, end of abdomen, dorsal view. B, same, ventral view. C, segments beyond the ninth, ventral view. An,, anus; Cer, eercus; cf, caudal filament; Cxpd, basal plate of styli; Eppt, epiproct; Papt, paraproct; Pen, penis; smcl, stylus muscles; Sty, stylus .

parts, whethe r doubl e o r single , aris e fro m a commo n media n plate . The basa l plat e i s situate d anterio r t o th e tent h abdomina l sternu m in th e membranou s floor of a genital chambe r abov e th e nint h sternum . Proximally it is produced into a long apodemal process (Ap). Styli are absen t i n Dermaptera , an d there ar e usuall y n o othe r accessor y structures associate d wit h the intromitten t organ .

FIG. 297.—Mal e genitali a o f Dermaptera . A , Anisolabis maritima, ventra l view . B , same, last instar nymph . C , Forficula auricularia.

The bipartit e typ e o f intromittent orga n i s well shown in Anisolabis maritima (Fig . 29 7 A), in which there ar e two elongat e pene s (Pen, Pen) arising fro m a common basal apodema l plate (Ap). Eac h peni s end s in

THE ORGANS OF COPULATION AND OVIPOSITION

585

a dista l lob e (a ) and bear s a stron g latera l process , o r "paramere " (6) , arising a t th e bas e of the lobe . Th e lef t lob e is turned proxirnall y in the usual condition . A strongl y musculate d ejaculator y duc t (Dej) pene trates each penis and opens into an eversible sac at the end of the termina l lobe. Th e immatur e intromitten t orga n o f Anisolabis, eve n i n a full grown nymph (B) , has th e for m o f four smal l simple lobes arising fro m a long thi n apodema l plate . Th e doubl e o r deepl y bipartit e peni s wit h two ejaculatory ducts opening separately t o the exterior i s characteristic , according t o Walke r (1922) , o f th e superfamilie s Protodermapter a an d Paradermaptera. The unpaire d typ e o f orga n occur s i n Eudermaptera , where , a s shown i n Forfícula (Fig . 29 7 C) , ther e i s a singl e media n peni s (Pen) with one terminal lob e (a ) but provide d with two lateral processes (6 , 6). The vasa deferentia of Forficuldj Walke r says, unite in a single ejaculatory duct (Dej) that open s o n th e media n lobe , bu t ther e i s presen t als o a vestigial second duct with no external orifice, suggesting that one lobe of a primitively doubl e penis has been suppressed . It i s difficul t t o estimat e th e significanc e o f th e presenc e o f paire d penes i n tw o suc h unrelated order s o f the Pterygot a a s th e Ephemerid a and Dermaptera , whe n ther e i s n o suggestio n o f a doubl e origi n o f th e penis in an y o f the mor e primitiv e apterygot e insects , no r an y evidenc e that th e singl e orga n o f othe r Pterygot a i s forme d b y th e unio n o f a pair o f primitive penes . Th e essentia l difference s i n structur e betwee n the paire d organ s o f Ephemerid a an d thos e o f Dermapter a shoul d no t be overlooked. The Mal e Genitali a o f Thysanura.—Th e genita l equipmen t o f male Thysanur a i s deservin g o f specia l attentio n becaus e i t present s in a simple form a structural complex of the type found in many pterygot e insects, consistin g o f a n unpaire d media n intromitten t orga n an d o f paired latera l accessories . In Machilida e an d Lepismatida e th e intromitten t orga n i s a simpl e tubular media n penis , o r phallus (Fig . 29 8 B, C) , arisin g fro m th e mem brane behin d th e narro w membranou s vente r o f th e nint h abdomina l segment. Th e orga n i s somewha t differentiate d int o a proxima l part , or phallobase (Phb), and a distal part, or aedeagus (Aed). In Diplura the peni s is rudimentary . Closely associate d wit h th e peni s ar e th e appendage s o f th e nint h segment, whic h ar e wel l develope d i n th e Thysanura . Eac h genita l appendage, o r gonopod , consist s o f a large , fla t coxopodit e (Fig . 29 8 B, C , Cxpd) an d o f a slende r dista l stylu s (Sty) movabl e b y muscle s arising i n th e coxopodite . I n som e specie s ther e aris e fro m th e mesa l angles o f th e base s o f th e coxopodite s o f th e nint h segmen t a pai r of short gonapophyse s (B , 2Gon), which closely embrace th e penis. Certai n

586

PRINCIPLES OF INSECT MORPHOLOGY

species o f Machilis hav e als o a pai r o f smalle r anterio r gonapophyse s arising a t th e correspondin g angles of the coxopodite s of the eight h seg ment (A , IGori). Wit h suc h species , therefore , th e appendage s o f th e eighth an d nint h abdomina l segments o f the mal e are identical in struc ture wit h thos e o f th e sam e segment s i n th e femal e (Fig . 31 3 B , C) , except fo r th e greate r lengt h o f the gonapophyse s i n th e latter . Bot h pairs o f gonapophyses, however, may b e absent i n the mal e (Fig . 298 C). The ontogeneti c origi n o f th e media n peni s o f Thysanur a ha s no t been carefull y studied , bu t th e simpl e structure o f the adul t orga n sug gests tha t th e latte r i s merel y a tubula r outgrowt h o f th e bod y wal l around th e mout h of the ejaculator y duct.

FIG. 298.—Mal e genitalia o f Thysanura. A , Machilis variabilis, dorsal vie w of firs t gonopods, showin g gonapophyse s o f eight h segment . B , same , dorsa l vie w o f secon d gonopods an d media n copulator y organ . C , Nesomachilis maoricus, nint h an d termina l segments, ventra l view .

General Structur e o f th e Mal e Genitali a o f Pterygot e Insects. — The primar y externa l genital organ s of male pterygote insects are locate d medially o n the vente r o f the nint h abdomina l segment. Thi s segment , therefore, i s th e male genital segment, o r gonosomite. Accessor y geni tal structures , however , may b e presen t o n the peripher y o f the genita l segment o r o n th e pregenita l o r postgenita l segments . Th e genita l parts, therefore , ca n b e classe d i n tw o distinc t group s o f structures . Those o f one group constitute th e media n intromittent apparatu s o f th e ninth segmen t an d ma y b e designate d th e phallic organs. Thos e o f th e other grou p are the periphera l accessory structures o f the nint h o r othe r segments an d ma y b e terme d collectivel y the periphallic organs. Th e phallic organ s are immediatel y concerne d with the functio n of coition ; they includ e the phallu s an d variou s accessor y o r supporting structure s associated wit h the latter. Th e periphallic organs are movable or immovable lobes or processes that have for the most part a grasping or clasping role in th e functio n o f copulation. The Male Genital Segment. —The genita l segmen t o f the mal e (nint h abdominal somite) ma y b e a simple segmental annulus resembling thos e

THE ORGANS OF COPULATION AND OVIPOSITION

587

that precede it; but usuall y it is more or less modified, an d it is sometimes greatly distorted , asymmetrical , turne d upo n its axis , o r even inverted . Generally the tergum and the sternum of the genital segment are distinct plates. The y ar e sometimes separated b y lateral pleural plates bearing a pair o f appendicular lobes , but mor e usually the pleura l and sternal areas of th e genita l segmen t ar e united i n a definitive pleurosternal plate , an d frequently th e entir e segmen t i s a continuousl y sclerotize d annulus . The phalli c organ s (Fig . 29 9 A , Phi) aris e fro m th e conjunctiva l membrane behin d th e nint h sternum , bu t thi s membran e i s usuall y invaginated withi n th e nint h segmen t t o for m th e male genital chamber (GC), i n which the phalli c organ s ar e ordinaril y mostly concealed . Th e ninth sternu m (IXS) i s thus , i n mos t cases , th e male subgenital plate ("hypandrium"), bu t ofte n th e externa l plat e beneat h th e mal e genital apparatus is the eight h o r the sevent h sternum . The periphalli c organ s o f th e genita l segmen t ma y includ e a pai r of latera l movabl e clasper s (Fig . 29 9 B , Hrp) an d variou s immovabl e lobes o r processe s arisin g fro m th e tergu m o r th e sternum . I n som e of th e mor e generalize d pterygot e insect s a pai r o f styl i i s born e o n the sternu m o r o n coxosterna l lobe s o f th e nint h segmen t o f th e mal e (A, Sty), bu t i t shoul d b e observe d tha t typica l styl i neve r occu r i n conjunction wit h movable claspers (harpagones) . The Phallic Organs.—The phalli c organ s o f mos t insect s othe r tha n Ephemerida an d Dermapter a ar e th e media n genita l outgrowth s o f th e ninth segment, surrounding or containing the gonopore , that, as already defined, ar e immediatel y concerne d with th e functio n o f coition. The y take th e for m eithe r o f lobes, phallomeres, or o f a media n tubular penis , the phallus , an d variou s accessor y processes or supportin g plate s asso ciated wit h the latter. In th e Blattida e an d Mantida e th e phalli c organ s consis t o f thre e phallomeres arisin g clos e to th e gonopor e from th e anterio r wal l of th e genital chamber . Thes e structure s i n th e youn g nymp h o f Blatta ar e simple membranous lobes (Fig . 302 C), tw o o f which are latera l an d th e other ventra l wit h respec t t o th e gonopore . I n th e adult s o f bot h families, however , th e phallomere s become greatly enlarged , somewha t altered i n position, an d take o n highly irregular forms (F , G , H) . Studies o n th e developmen t o f th e mal e genitali a i n Trichoptera , Lepidoptera, an d Hymenopter a hav e show n tha t th e tubula r phalli c organ o f these insect s is formed durin g larval developmen t by th e unio n of a pai r o f genita l lobe s tha t gro w ou t a t th e side s o f th e gonopor e (Zander, 1900 , 1901 , 1903 ; Sing h Pruthi , 1924 , 1925 ; Mehta , 1933) . I t is possible, therefore , that thes e larval phalli c lobes of the highe r insect s are homologue s o f th e latera l phallomere s o f Mantida e an d Blattidae . According t o Zander , th e primitiv e phalli c lobe s divid e eac h int o a

588

PRINCIPLES OF INSECT MORPHOLOGY

median lob e an d a latera l lobe , th e tw o media n lobe s unitin g t o for m the intromittent organ , while, in Trichoptera and Lepidoptera, the latera l lobes mov e t o th e side s an d becom e articulated t o th e margin s o f th e annulus o f th e nint h segment , wher e the y develo p int o th e movabl e claspers o f this segment. W e might, therefore , regard th e media n lobes as gonapophyses o f th e gonopods , and the latera l lobe s (valvae, or harpagones) a s th e styli . However , sinc e i t i s claime d b y Meht a tha t th e lateral lobe s i n Lepidopter a aris e separatel y fro m th e media n lobes , we cannot accep t i t a s establishe d tha t th e gonopod s o f th e mal e insec t take an y par t i n th e formatio n of the intromitten t organ , thoug h ther e appears to be little doubt that they give rise to the styli or to the movable claspers o f the genita l segment.

FIG. 299.—Diagram s o f the basi c structur e o f the mal e genitali a o f pterygote insects A, en d o f abdomen , wit h phalli c orga n i n genita l chamber , latera l view . B , same , en d view, wit h clasperlik e modification s o f styli . AcGl, accessor y gland ; Aed, aedeagus ; Dej, ductu s ejaculatorius ; GC , genita l chamber ; Gpr, gonopore ; Hrp, harpag o (stylus) ; Phb, phallobase; Phi, phallus (median penis) ; Pmr, paramere; Sty, stylus; Vd, va s deferens.

Some male Thysanura , a s we have see n (Fig . 29 8 A, B), hav e gona pophyses that ar e withou t doub t homologou s with th e processe s o f th e female ovipositor , bu t ther e i s here present als o a well-developed median phallic orga n betwee n th e gonapophyse s o f th e nint h segment , an d the gonapophyse s ma y b e entirel y absen t (C) . Th e latera l phalli c lobes o f nymphal blattid s (Fig . 30 2 C ) ar e fa r remove d fro m th e styl i (Sty) carrie d o n th e margi n o f th e nint h sternum , an d i n th e adul t stage (F , G ) the y ar e widel y differen t i n bot h for m an d musculatur e from th e gonapophyse s of the female . The typica l phallu s i s a conica l or tubular structur e containin g th e terminus of the ejaculator y duct. Th e organ, however, is highly variable in form and in the extent of its secondary modifications. It s musculatur e is simple o r comple x and s o variable tha t i n mos t case s it i s impossible to trace any consistent schem e of homology in the various muscle patterns of th e organ . Th e curren t vie w that mal e gonapophyse s ar e involve d in th e formatio n o f the phallu s i s disregarde d i n th e presen t discussio n because o f th e absenc e o f positive ontogeneti c evidence , and becaus e of

THE ORGANS OF COPULATION

AND OVIPOSITION

589

the entir e lac k of conformity in the phalli c musculature. I t i s assumed tentatively that th e phallu s i s an independen t media n outgrowt h of the body wall, independently musculated, and bound by no phylogenetic influences t o confor m wit h th e structur e o f an y ancestra l appendage . On the othe r hand , th e gonopod s of the nint h segment appea r undoubt edly t o contribut e th e movabl e clasper s t o th e mal e genita l complex . A simple tubular phallus , similar to that of the Thysanura , i s present in Plecoptera , thoug h i n th e latte r orde r th e orga n ma y hav e scleroti c plates i n it s walls , an d i t i s ordinaril y retracte d int o a pouc h o f th e conjunctival membran e abov e th e nint h sternum , fro m whic h i t i s exserted durin g copulation . With the majorit y of insects th e phallu s is differentiated into severa l more or less distinct parts, an d it may be provided with various accessory structures. Externall y ther e i s ver y commonl y to b e distinguishe d a proximal part, or phallobase (Figs . 299 B, 300 A, Phb\ and a more slender terminal part, the aedeagus (Aed). When the basal differentiation is not evident th e entir e orga n i s generall y calle d th e aedeagus . Th e wall s of the phallobase and the aedeagus constitute together the ectophallus in distinctio n t o a n inne r chamber , o r endophalluSj whic h i s usuall y invaginated a t th e en d of the orga n (Fig . 300 A, Enph) an d contain s th e true gonopor e (Gpr), o r apertur e o f th e ductu s ejaculatoriu s (Dej). The endophallu s is sometime s eversibl e (B ) an d i s sometimes a perma nently interna l structure . It s openin g at th e en d of the aedeagu s is the phallotreme (A , Phtr). The phallobas e i s a n importan t par t o f th e phalli c orga n i n man y insects, bu t i n som e it i s reduced or represente d onl y b y basa l sclerite s supporting the aedeagus , an d agai n i t ma y be entirely absent . Usuall y the phallobas e give s attachment t o phalli c muscle s from th e bod y wall, as well as to muscle s of the aedeagus , and ma y b e provided wit h a basal apódeme. Sclerite s in its wal l are termed th e basal plates of the phallus . Very commonly the dista l part o f the phallobas e form s a fol d abou t th e base o f the aedeagu s (Fig . 300 D), an d thi s fol d i s sometimes produced into a tubula r sheath , th e phallotheca, whic h partly o r wholl y encloses the aedeagu s (E , The). I n suc h case s th e aedeagu s ma y b e reduce d (G, Aed) o r entirel y suppresse d (H) ; the phalli c tub e i s then th e thec a (The), and its lining is the endotheca (Enth). In some insects in which there is no evident phallobase , th e aedeagu s is more or less sunken into a phallocrypt (C, Crpf), or pocket in the genital chamber wall, which possibly represent s th e endotheca . Th e wall s o f th e cryp t ma y b e membranous or variously sclerotized; the scleroti c part sometime s forms a ring or tube fro m whic h the aedeagu s projects. Lobes o r processe s arising from th e phallobas e are o f frequent occur rence. Latera l basa l lobe s ar e particularl y characteristi c genita l struc -

590

PRINCIPLES OF INSECT MORPHOLOGY

tures i n th e Coleóptera , wher e the y ar e commonl y calle d parameres (Fig. 303 C, Pmr). Th e term, however , is applied als o to othe r processe s of th e genita l armatur e an d ha s bee n use d b y Walke r (1922 ) an d othe r writers as synonymous with "male gonapophyses," whic h are supposed t o be component s o f th e phallus . Since , however , a s alread y shown , i t appears doubtfu l tha t th e gonapophyse s ar e retaine d i n th e male s of pterygote insects , th e ter m "paramere " is here define d accordin g t o th e

FIG. 300.—Modification s o f th e phallus , diagrammatic . A , simpl e structure . B , endophallus everted . C , aedeagu s partl y retracte d int o a phallocrypt . D , aedeagu s partly retracte d int o phallobase . E , aedeagu s enclose d i n a phallotheca . F , phallu s extended by eversión of endophallus and endotheca. G , aedeagus reduced, theca enlarged. H, aedeagu s suppressed an d replace d b y th e theca . Aed, aedeagus; Crpt, phallocrypt ; Dej, ductu s e jaculatorias; Enph, endophallus ; Enth, endotheca ; Gpr, gonopore ; Phb, phallobase; Phtr, phallotreme; The, phallotheca.

current usag e b y coleopterist s a s latera l proces s o f th e phallobase . Dorsal an d ventra l processe s o f th e phallobas e ma y the n b e named , respectively, epimeres or hypomeres. The aedeagu s i s usuall y tubula r i n for m (Fig . 30 3 D , Aed)j thoug h it assume s a grea t variet y o f shapes, an d it s wall s ar e characteristicall y strongly sclerotized , excep t ofte n fo r a terminal membranou s part known as the preputial sac, or vesica (Vsc). Th e orga n is usually provide d wit h its own muscles, and its base may be produced into one or more apodemal processes (Apa). The distal extremity is frequently armed with spines, small plates , o r slende r processe s ofte n calle d titillators. Thoug h th e

THE ORGANS OF COPULATION AND OVIPOSITION

591

aedeagus i s generall y th e conspicuou s part o f the phalli c organ , i t ma y be muc h reduce d o r eve n practicall y obliterated , an d i n suc h cases , as already mentioned, a tubular thecal extension from the phallobase may replace the aedeagu s (Fig . 30 0 G, H, The). The endophallu s varie s fro m a smal l invaginatio n i n th e en d o f the aedeagus , containing the gonopore (Fig. 300 A, Enph), to a long inner tube (D ) or an elaborate interna l chambe r of the phallus . I n its tubula r form th e endophallu s is often mistake n fo r th e ejaculator y duct . Whe n its wall s ar e membranous , th e endophallu s i s usuall y eversibl e (B , F) , and, when everted durin g copulation, it becomes the functional intromittent orga n (sometime s calle d "th e penis") , sinc e i t i s th e par t o f the phallu s projecte d int o th e copulator y receptacle of the female . Th e length o f the endophallu s i n suc h case s i s probably correlate d wit h th e length of the genita l tract of the femal e betwee n the copulator y entranc e and th e mout h o f the spermatheca . Th e inne r wall s of the endophallu s are ofte n arme d with spicules , spines , an d plates, whic h become external with eversió n (F) . Whe n th e phallobase , th e aedeagus , an d th e endo phallus ar e eac h retracted on e within th e other , th e full y everte d orga n may becom e a n extraordinaril y lon g slende r tub e (F) . O n th e othe r hand, when the endophallus is a permanently internal part of the phallus , it sometime s attains a hig h degree of development. A n endophallu s of this typ e i s characteristi c o f th e Acrididae , wher e i t form s a complex , strongly musculate d apparatu s fo r dischargin g th e spermatophore s through th e relativel y smal l aedeagus. The Periphallic Organs. —The periphalli c organs , i n contras t t o the media n phallic organs, aris e peripherally, generall y from th e annulu s of th e nint h abdomina l segmen t bu t als o fro m th e othe r segment s ofte n closely associated wit h th e latter in the genita l complex . I n th e Thysa nura th e periphalli c organ s ar e th e gonopod s o f th e nint h abdomina l segment; i n th e Pterygot a the y includ e movabl e lobe s (harpagones ) and variou s accessor y immovable genital processes . The gonopod s o f th e Thysanur a (Fig . 298) , a s w e hav e alread y observed, consist each of a large basal coxopodite (Cxpd) an d o f a slender distal stylu s (Sty) movable by muscles arising in the coxopodite . Gona pophyses ma y b e presen t o n th e appendage s o f th e nint h segmen t (B , 2Gon) o r absen t (C) ; rarely the y occu r on the appendage s o f the eight h segment (A) . Whe n present o n the nint h segmen t they closely embrace the peni s but d o not for m a part of the phalli c organ . The movable periphallic genital lobes of pterygote insect s are typicall y lateral appendage s and , whe n present, alway s pertai n t o th e annulu s of the nint h abdomina l segment (Fig . 299 B, Hrp). The y ar e to be identi fied by the fact that they ar e individually provided with muscles inserted on thei r bases . I n for m the y var y fro m slende r processe s t o broa d

592

PRINCIPLES OF INSECT MORPHOLOGY

lobes o r fro m smal l hook s t o lon g falcifor m arms , an d frequentl y the y are armed with secondary outgrowths. I n general these movable periphallic organ s serve as copulatory claspers, an d fo r this reason they ar e here designated the harpagones (from apirayrj, a "grappling hook")- They occur principall y i n Ephemerida , Hemiptera , Neuroptera , Mecoptera , Lepidoptera, an d Dipter a bu t ar e frequentl y absen t i n member s of groups in which they ar e typically present . The harpagone s always arise from som e part o f the latera l o r ventra l walls o f th e genita l segment , neve r fro m th e terga l region . I n som e insects the y ar e born e o n independen t basa l plates , whic h eithe r ar e interpolated laterall y betwee n th e tergu m an d sternu m o f th e genita l segment o r are attached t o th e sternum . Whe n such plates ar e presen t the muscle s of the clasper s take thei r origin s upon them ; otherwis e th e claspers ar e born e on th e sterna l (coxosternal ) plate of the segmen t and their muscle s arise o n this plate . Th e positio n an d musculatur e o f th e harpagones at onc e suggest that these movable genital claspers represent the styl i o f the mor e generalize d insects, an d tha t th e basa l plate s o n which they ar e sometimes supported ar e the coxopodite s of the gonopods. When th e basa l plate s o f the clasper s ar e no t individuall y evident , i t i s to b e supposed that the y hav e unite d wit h th e sternum , producin g the same conditio n a s i n som e Thysanur a an d Orthopter a wher e th e styl i arise directl y fro m a coxosternal plate of the genita l segment. Movabl e genital lobes and typical styli, as we have observed, do not occur together in any insect, though both may be absent . The numerous fixed or merely flexible processes arising from th e walls of th e genita l segment , o r als o fro m th e othe r segment s associate d i n the genita l complex , are s o variable i n thei r occurrenc e tha t ther e ca n be little homology between those occurring in the differen t majo r group s of insects, though their presence and form are often highly characteristic of smaller groups an d o f species . Suc h structure s ar e neve r specificall y provided wit h muscles , though i n rar e case s the y ma y b e movabl e b y segmental muscle s attache d a t thei r bases . Th e immovabl e genita l processes ar e o f suc h divers e form s tha t the y ca n hav e n o constan t function; i n genera l they appea r t o b e adapte d fo r graspin g o r holdin g various part s of the femal e apparatus , bu t onl y a clos e study o f insect s in copulation will reveal their exac t uses in the genita l mechanism. Characteristics o f th e Mal e Genitali a i n th e Principa l Pterygot e Orders.—It i s impossibl e t o giv e a n adequat e treatmen t o f th e man y modifications i n th e copulator y apparatu s o f mal e insect s withi n th e space tha t ma y b e allotte d t o th e subjec t i n a genera l text. Th e following descriptions , therefore , presen t onl y a sketc h o f th e salient o r distinctive feature s o f th e mal e genitali a i n th e principal orders , wit h suggestions a s to ho w the variou s part s ma y b e related t o on e another.

THE ORGANS OF COPULATION AND OVIPOSIT ION

593

Odonata.—The Odonat a ar e o f particula r interes t i n a stud y o f th e male genitali a becaus e o f th e developmen t o f a secondar y copulator y organ o n th e anterio r par t o f the abdomen . Th e tru e gonopor e of th e male (Fig . 30 1 C, Gpr) i s situated o n a rudimentar y peni s o f the nint h abdominal segmen t conceale d beneat h tw o smal l plate s (e), whic h possibly represen t th e gonopods . A large postgenita l plat e (/ ) appear s to b e a secondar y sclerotizatio n o f the intersegmenta l are a behin d th e genital organ . The functiona l intromitten t orga n o f th e Odonat a i s a secondar y structure situate d i n a median depression, or genital fossa, o n the ventra l

FIG. 301.—Male genitalia of Odonata (Plathemis lydia). A, base of abdomen, under surface, showin g secondar y copulator y organs . B , secondar y peni s o f secon d abdomina l segment, latera l view. C , posterior en d of abdomen, under surface , showin g true gonopore (Gpr) on ninth segment between two valve-like plates.

wall o f th e secon d abdomina l segmen t (Fig . 30 1 A , Pen). Thi s orga n is a strongl y sclerotize d tubular structur e compose d o f several segment like part s movabl e upo n eac h othe r (B) . Variou s accessor y lobe s (A, a , 6) , differin g muc h i n differen t species , ma y aris e fro m th e sur rounding wall s o f th e firs t an d secon d abdomina l segments . Th e copulatory orga n contains a chamber open to the exterio r which serves as a sper m receptacle . Befor e copulatio n th e mal e dragonfl y transfer s spermatozoa fro m th e genita l openin g o n th e nint h segmen t t o th e receptacle o f th e intromitten t orga n b y flexing the abdome n ventrall y and forwar d unti l th e tw o apertures ar e in contact . I n copulatio n th e male grasp s th e femal e wit h th e cerc i b y th e nec k o r th e bac k o f th e thorax, o r sometimes b y th e head , an d th e femal e bring s the en d o f her abdomen forwar d beneat h tha t o f the mal e t o effec t a unio n wit h th e anterior genitali a o f th e latter . Detail s o f th e structur e o f th e mal e organs in Anisoptera an d Zygoptera have been described by Ingenitzsk y (1894), Backhof f (1910) , E . Schmid t (1916) , an d Kenned y (1917 , 1922) .

594

PRINCIPLES OF INSECT MORPHOLOGY

Orihoptera.—The externa l genitali a o f mal e Orthopter a ar e mostl y phallic structures . Styl i o f th e nint h segmen t ar e presen t i n severa l families, bu t the y tak e n o part i n the genita l apparatu s an d depart bu t little from th e typica l stylu s for m (Fig . 302 A-E, Sty). Th e coxopodites of th e styl i are plates distinc t from th e nint h sternum in Grylloblattida e (A, Cxpd), bu t otherwis e th e genita l coxopodite s ar e unite d wit h th e sternum i n the definitiv e sternal plat e o f the nint h abdomina l segment . In Mantida e an d Blattida e th e mal e organ s consis t typicall y o f three phalli c lobe s surroundin g th e gonopore , containe d i n a genita l chamber between the ninth sternum an d the paraprocts (Fig . 302 E). O f the thre e phallomeres , tw o are situated abov e the gonopore , one to th e left (E , F) , th e othe r t o th e righ t ((?) , whil e the thir d (H ) lie s ventra l to the genita l opening. Th e right lob e usually assumes a position dorsa l to the other s an d its base may extend almos t completel y across the wall of th e genita l chamber. I n th e adult s o f most members of these familie s the latera l phalli c lobe s becom e extraordinaril y comple x i n structur e by th e developmen t o f secondary lobes an d processe s of variou s form s (F, G) . Th e ventra l lob e (H ) i s usuall y mor e simple , and , sinc e th e ejaculatory duct (Dej) opens in a membranous fold at its base (Gpr), it i s ofte n calle d th e "penis. " A n intricat e syste m o f muscles arisin g in the nint h segmen t is inserted o n the base s of the phallomeres , and i n addition there ar e numerous muscles within th e latera l lobe s inserted o n their variou s secondary parts. I n som e of the roaches the phalli c organs are simpler , an d i n certai n forms , a s i n Ectobia, th e latera l lobe s ar e retracted int o dee p pouches of the genita l chamber . In th e nymph of Blatta the highl y complex genital organs of the adul t are represente d b y thre e simpl e lobes projectin g from th e anterio r wal l of th e genita l chamber (Fig . 302 C), tw o o f which (F, G) lie immediately laterad o f the gonopore , and the third (H ) below it. Thes e three simple phallic structures appear to be merely outgrowths of the genita l chamber wall i n th e immediat e neighborhoo d o f th e gonopore . Th e latera l lobes have bee n regarded a s gonapophyses of the nint h segment , but i n the nymp h ther e i s nothing t o sugges t tha t the y hav e an y relatio n t o the gonopo d bases incorporated i n the ninth sternum . Among othe r Orthopter a th e mal e genita l structure s ar e ver y dif ferent i n th e severa l families . Th e Phasmida e hav e a shor t compac t intromittent organ , whic h possibly i s forme d b y th e unio n o f primitiv e phallic lobe s abou t th e gonopore . I n Tettigoniida e an d Gryllida e th e lateral lobe s ar e reduced an d retracted , whil e the ventra l lob e becomes enlarged an d ma y b e th e onl y phalli c structur e ordinaril y visible . I n Acrididae the phallus is a large conical structure distinctl y divide d int o a phallobase an d aedeagu s an d contain s a highl y develope d endophallu s forming a sper m ejectio n pum p int o whic h opens the ejaculator y duct .

THE ORGANS OF COPULATION AND

OVIPOSITION

595

A large, often comple x sclerite, known as the epiphallus, o r "pseudosternite," lie s dorsall y a t th e bas e o f th e phalli c organ s i n Tettigoniidae ,

FIG. 302.—Male genitalia of Orthoptera. A, Grylloblatta campodeaformis, end of abdomen, ventra l view . (From Walker, 1922. ) B , Blatta orientalis, en d o f abdomen , ventral view . C , same , youn g nymph , genita l chambe r abov e nint h sternum , showin g phallic lobe s (F , H, G ) surrounding gonopore . D , same , adult , en d o f abdomen, termina l view. E , Paratenodera cinensis (Mantidae), en d o f abdomen , dorsa l view , wit h phalli c lobes i n place . F , Blatta orientalis, adult, lef t phalli c lobe , dorsal surface . G , same, righ t phallic lobe , dorsa l surface . H , same , ventra l phalli c lob e an d gonopore , dorsa l view .

Gryllidae, an d Acridida e and form s a n importan t elemen t o f the copula tory mechanism.

596

PRINCIPLES OF INSECT MORPHOLOGY

Coleóptera.—The mal e genitali a i n Coleóptera , a s i n Orthoptera , are phallic structures only, there being in general no accessory or periphallic armatur e o n the genita l segments. Ther e are, therefore, n o elements of th e genita l comple x tha t ca n b e referre d directl y t o th e gonopods ; movable clasper s (harpagones ) of the nint h segmen t ar e alway s absent , and, in the mal e at least, styli are never present in any form. Th e nint h and tent h segment s o f th e abdome n ar e usuall y muc h reduce d an d retracted int o th e eight h segment , an d i n som e form s th e eight h i s concealed withi n th e seventh . Th e phalli c organ s consis t essentiall y

FIG. 303.—Abdomen and mal e genitalia o f Coleóptera (Phyllophaga chiriquiana). A , male, abdomen . B , genita l chambe r an d phallu s o f rudimentar y nint h segment , an d anogenital vestibul e (a ) inflected from eight h segment . C , the phallu s and en d o f ejaculatory duct , showin g apódem e (Apb) and paramer e (Pmr) of phallobas é (Phb), and dista l aedeagus (Aed). D , the aedeagus, with aedeagal apódeme (Apa) an d terminal vesica (Vsc).

of a tubula r aedeagu s an d a variousl y develope d phallobasé usually provided wit h parameres . Th e innumerabl e variation s i n th e genita l apparatus o f male Coleópter a hav e bee n describe d by Shar p an d Mui r (1912), bu t th e fundamenta l structur e o f th e part s involve d ma y b e understood fro m a few typical examples . The externa l par t o f the abdome n consist s usually , a s illustrated i n Phyllophaga (Fig. 303 A), of the first eight segments. Within the eighth segment is an invagination cavit y (B , a) into whic h are retracte d the reduce d ninth an d tent h segments , bu t whic h is continued forwar d through th e narro w annulu s o f th e nint h segmen t a s a larg e genita l chamber (GC) containin g the phalli c organ s (Phi). Th e tent h segmen t (X) appear s a s a smal l projectio n fro m th e dorsa l wal l of the entranc e chamber, bearing the anus (An). The region of the ninth segment that encircles th e mout h o f th e genita l chambe r ma y contai n a complete, though narrow , scleroti c annulus , a s i n Carabidae , bu t generall y onl y

THE ORGANS OF COPULATION AND OVIPOSITION

597

a sterna l sclerit e i s present , an d this , a s i n Phyllophaga an d variou s other beetles, is reduced to a U-shaped or V-shaped bar (B, IXS), often provided wit h a ventral apódem e (ap), whic h gives it a Y-shape d form . The phallic organs arise from th e anterio r wal l of the genita l chambe r (Fig. 303 B, Phi). Th e phallobase (legmen o f Sharp an d Muir ) is usually well differentiate d fro m th e aedeagu s (C , Phb) but varie s muc h i n for m and character ; i t ma y b e a membranou s fol d abou t th e bas e o f th e aedeagus containin g severa l basa l plates , bu t ofte n i t i s a scleroti c rin g and sometime s form s a cylindrica l thec a completel y investin g th e aedeagus. Generall y th e phallobas e bear s a pai r o f paramere s (Pmr) and i n som e case s a media n dorsa l lobe , o r epimere ; fro m it s bas e a n apódeme (Apb) project s int o th e bod y cavit y fo r muscl e attachments . The aedeagus (C, D, Aed) is typically a sclerotic tube with a membranous distal part (Vsc); from its base an apódeme (D, Apa) projects into the body cavit y beneat h th e apódem e o f th e phallobase . A n endophalli c chamber o r tub e i s usuall y presen t an d i s generall y eversible ; whe n everted i n coitio n i t become s the functional intromittent organ . In som e beetles the eighth segment also is partly o r entirely retracted , and, a s in Oedemeridae , it ma y tak e th e for m o f a sheat h enclosin g th e ninth an d tenth segments , th e genita l chamber , an d the phalli c organs , the last , finally , consistin g o f a phallobase , a n aedeagus , an d a n endo phallus. I n suc h case s the genita l apparatu s assume s th e complicate d form o f numerou s fold s successivel y ensheathin g eac h other , al l o f which probably ar e protracted durin g the ac t o f copulation . Hemiptera.—In th e Hemipter a ther e ar e present , i n additio n t o well-developed phalli c organs , variou s periphalli c structure s havin g the for m o f lobes or processes arisin g fro m th e eighth , ninth , an d tent h abdominal segments . Amon g thes e structure s ther e i s usuall y on e pair movabl y articulate d t o som e par t o f the nint h segmen t an d indi vidually provide d wit h muscles . Thes e movabl e clasper s ar e thu s t o be identified a s the harpagones , that is, as derivatives presumabl y o f the styli o f the gonopod s of the nint h segment. Th e harpagone s var y fro m small hook s to lon g slender processe s or broad spatulat e lobes , ofte n of irregular shapes . I n som e case s the y ar e absent. I n Homopter a the y arise fro m th e floor of the genita l chamber , where their base s ar e usuall y associated wit h one of the sclerite s o f the phallobas e or supporting plate s of th e aedeagu s (Fig . 30 5 E, Hrp). I n Heteropter a the harpagone s ar e small bu t strongl y musculate d processe s (Figs . 30 4 B, C , 30 5 G, Hrp) articulated t o th e scleroti c wal l of the nint h segmen t inflecte d int o th e genital chamber . The principa l segmen t involve d i n th e genita l modificatio n o f th e hemipterous abdome n i s th e nint h (Figs . 304 , 30 5 A , H , I X ) , bu t i n Heteroptera th e eight h i s often reduced and closel y associate d wit h th e

598

PRINCIPLES OF INSECT MORPHOLOGY

ninth (Fig. 304 A, B, VIII). In most Homoptera the tenth and eleventh segments are distinct annuli (Figs. 304 D, 305 H, X, XI), the tenth being sometimes provided with accessory genital processes in the for m of lateral lobes (Fig . 30 4 D , c) . I n Heteropter a th e tw o postgenita l segment s apparently ar e united i n a tubular proctige r (A , B, C, Ptgr). Th e nint h segment i n Homopter a ofte n bears , i n additio n t o th e harpagones , accessory periphallic structures havin g the for m o f short processe s (Fig . 304 D , a , b ) o r o f lon g arm s o r broa d lobe s (Fig . 30 5 H , a , b) . Suc h

FIG. 304.—Mal e genitali a o f Hemiptera . A , Anasa tristis, genita l segment s an d proctiger, latera l view . B , same , dorsa l view , showin g tips o f harpagones. C , Notonecta variabilis, nint h segmen t an d proctiger . D , Magicicada septendecim, sectio n o f nint h segment an d genita l chamber , wit h tent h an d eleventh segment s attached , showin g theca l tube, which replace s the tru e aedeagus, supporte d on basal plate (BP) .

structures ma y b e flexible at thei r base s an d the y hav e bee n mistake n for th e harpagone s (styli) , bu t the y ar e t o b e distinguishe d fro m th e latter by the fac t tha t they ar e never provided with muscles. The phallu s o f the Hemipter a comprise s i n mos t case s a phallobas e and a n aedeagus , thoug h eithe r on e o r th e othe r ma y b e suppressed . The aedeagu s i n it s simples t developmen t i s a tubula r structur e (Fig . 305 E, Aed), bu t mor e usually it takes o n an irregular shape (Figs . 304 C, 305 F , Aed), whic h by exaggeratio n ma y produc e bizarr e forms , ofte n with curious terminal outgrowt h (Fig . 30 5 D, g). I t i s usually provided with apodema l processe s fo r muscl e attachmen t (Fig . 30 5 D , h , i) . The phallobas e i s variousl y developed . I t ma y consis t merel y o f on e or tw o basa l plate s i n th e wal l o f the genita l chambe r (Fig . 30 5 E, F , IBP, 2BP) supportin g th e aedeagu s (Aed) an d givin g attachmen t t o phallic muscles. I n th e cicad a the singl e large basal plate o f the phallu s (I, J , BP ) i s articulate d upo n fulcra l arm s (I) o f th e sterna l margi n of the nint h segment . I n som e case s th e phallobas e bear s paramera l processes (F , Pmr), o r agai n i t ma y b e produce d int o a theca l sheat h

THE ORGANS OF COPULATION AND OVIPOSIT ION

599

more or less investing the aedeagu s (C , The) an d subjec t t o man y varia tions i n form . Whe n the thec a i s well develope d the aedeagu s is sometimes greatl y reduced , a s i n Fulgorida e an d Cicadellida e (D , Aed), except fo r termina l processe s (g ) that ma y protrud e throug h th e thec a (C,.0r). I n th e Cicadida e it is evident that the aedeagu s has been almost entirely suppressed , an d tha t th e lon g tubula r intromitten t orga n is the thec a (Figs . 30 4 D, 30 5 J, The). Th e inne r tub e o f the orga n is

FIG. 305.—Male genitalia of Hemiptera. A, Poblicia fuliginosa (Fulgoridae), ninth and tenth abdominal segments. B , same, eleventh segmen t removed from tenth. C , same, the phalli c thec a wit h end s of aedeagal cornua projecting. D , same , the reduce d aedeagu s exposed b y remova l o f theca , showin g aedeagal cornu a (g ) and apodeme s (h , i). E , Idiocerus atkinsoni (Cicadellidae) , phallus an d harpagones , ventra l view . F , Amblydisca gigas (Cicadellidae), phallus wit h paramere s arisin g fro m basa l plates. G , Euschistus variolarius (Pentatomidae), lef t harpag o an d muscle . H , Idiocerus atkinsoni, ninth , tenth , an d eleventh segments , showin g accessor y lobe s (a , b) o f nint h segment . I , J , Magicicada septendecim, bas e o f phallus , ventra l an d latera l views , showin g theca l an d endotheca l tubes; aedeagu s obliterate d excep t fo r apódem e (i ) supportin g endothec a a t unio n wit h ductus ejaculatorius .

therefore th e endothec a (Fig . 30 5 J, Enth). Th e onl y remnan t o f th e aedeagus i n the cicad a is an apodema l proces s (i ) attache d t o th e inne r extremity o f the endothec a wher e the ejaculator y duct (Dej) open s int o the latter . Th e homologie s o f th e variou s genita l structure s o f mal e Hemiptera ca n be determined only by a very clos e study o f the relation s of th e part s to on e another . Mecoptera.—In th e Mecopter a th e periphalli c genita l clasper s o f th e ninth abdomina l segmen t ar e distinctl y tw o segmented . Th e basa l segments appea r t o b e th e coxopodites , th e dista l segment s th e tru e harpagones (styli) . Eac h segmen t i s individually movabl e b y muscles . In Panorpa the organs are strongly developed, but the harpagones extend

600

PRINCIPLES OF INSECT MORPHOLOGY

but little beyond the ninth segment; in Merope tuber they are long slender? weakly sclerotize d appendages , eac h o f whic h i s bifi d terminall y an d bears a suckerlike disk on its inne r surface . The abdomen of the typical scorpionflies ends with a recurved bulbous structure forme d o f the nint h segmen t an d it s appendage s (Fig . 30 6 A, IX), whic h contain s th e phalli c organ s an d mostl y conceal s the tubula r proctiger (B , Ptgr). I n Panorpa th e genita l segmen t i s a continuousl y sclerotized annulus , th e terga l regio n o f whic h i s greatl y prolonge d posteriorly (B, C, I XT), and the short sternal region produced into two long lobe s (B , a) . Th e larg e latera l coxopodite s ar e broa d ova l lobe s

FIG. 306.—Male genitalia of Mecoptera (Panorpa consuetudinis). A, end of abdomen. B , genital segment and proctiger, latera l view, showing 2-segmented gonopo d (Cxpd, Hrp) an d accessor y lobe s (a ) of ninth sternum. C , same, dorsa l view . D , phallus , dorsa l view. E , gonopod , showin g muscle s o f harpago arisin g in coxopodite .

(B, C, Cxpd) articulated basally to the annulus of the ninth segment. Distally each bears a strong, hooked harpago (Hrp), which is individually movable b y antagonisti c muscle s arisin g i n th e coxopodit e (E) . Th e aedeagus, a fla t irregula r structur e (D , Aed) wit h a pai r o f distal arm s (g)j arises from the wall of the genital pouch between the bases of the claspers. A t its side s ar e tw o slende r paramere s (Pmr) supporte d o n a U-shaped bar (BP) i n the ventra l wall of the genita l chamber proximal t o the bas e o f the aedeagus . Th e morpholog y o f the phalli c structure s i s difficult t o understan d i n Panorpa. A mor e comparativ e stud y o f th e mecopteran genitalia , a s give n b y Issik i (1933) , show s muc h variatio n in the structur e o f the organ s and suggest s that the phallu s i s composed primarily o f two unite d latera l phalli c lobes , provide d wit h protracto r and retracto r muscles , betwee n whic h open s th e larg e saclik e ductu s ejaculatorius.

THE ORGANS OF COPULATION AND OVIPOSITION

601

Trichoptera.—In mos t o f the Trichoptera , a s i n th e Mecoptera , th e genital clasper s ar e tw o segmented , eac h consistin g o f a proxima l cox opodite an d a dista l stylus , o r harpag o (Fig . 30 7 A , B , Cxpd, Hrp). The coxopodit e i s movabl e o n th e annulus o f th e nint h segmen t b y muscles inserte d o n it s bas e (B) , an d th e harpag o i s movabl e o n th e coxopodite b y muscle s arisin g withi n th e latter . I n som e form s eac h clasper consist s o f a single segment, whic h appear s t o b e the coxopodit e rather tha n th e harpago , sinc e th e harpag o i s ofte n reduce d an d i s sometimes no t separate d fro m th e coxopodite , thoug h i t retain s it s basal muscles . Th e coxopodite s ar e generall y unite d wit h eac h othe r medially b y a transvers e bridge , o r pons coxalis (C , Pncx), lyin g i n th e

FIG. 307.—Male genitalia of Trichoptera (Neuronia semifasciata). A, ninth and tenth abdomina l segment s an d appendages . B , righ t gonopod , inne r view , showin g muscles. C , ventral vie w of gonopods united by pons coxalis (Pncx) supportin g basal plat e of phallus. E, aedeagus (Aed) and basal crypt (Crpt).

floor of th e genita l chamber , an d fro m th e bridg e a median process (BP) extends upward in the genital chambe r wall to give support t o the sheat h of the aedeagus (E). The aedeagus is tubular (Aed); its base may be sunken in a crypt of the genital chamber (Crpf), or the entire organ may be ensheathed i n a thecal fold . The singl e postgenital segmen t presen t i n Trichopter a appear s t o b e the tent h (Fig . 30 7 A, X). I t varie s greatl y i n shap e an d i s often pro vided with terminal processe s of various forms. Paire d latera l processe s (Soc) sometime s arisin g fro m th e bas e o f th e segmen t appea r t o b e homologues o f the so-calle d soci i o f mal e Lepidopter a an d ar e possibl y the pygopod s of the tent h segment . Lepidoptera.—The genita l comple x o f mal e Lepidopter a include s the eighth , ninth , an d tenth abdomina l segments . Th e eight h segmen t forms a t leas t a protractil e bas e fo r th e copulator y apparatu s (Fig . 308 H), an d i n som e case s i t bear s accessor y genita l lobes . Th e nint h egment may be a simple sclerotic ring (H, IX), but usuall y it is irregular n form , wit h distinc t terga l an d coxosterna l area s o f sclerotization (D) .

602

PRINCIPLES OF

INSECT MORPHOLOGY

??? ?????? ????? ?? ??? ??????? ?? ??????????????? ??? ??? ??????????? ??? ??? ????????? ??? ??????? ?????? ?? ??? ???????? ?? ????? ???????? forward i n a dee p inflection known as the saccus (A , D, E , c) , th e mem branous ventra l wal l o f whic h (E , d ) i s usuall y adnat e with the sterna l surface (Stn). Th e tent h segmen t ma y b e a simpl e membranou s tub e (H, X)j bu t usuall y i t present s a variousl y modifie d terga l structur e called th e uncus (B , C , D , e) an d a mandible-lik e ventra l lobe , th e gnathos (/). A pair of lateral processes, termed the socii, often arise from th e bas e o f the tent h segment ; they ar e usually membranous hair y appendages, an d ther e i s some evidenc e that the y ar e derive d fro m th e pygopods (postpedes ) o f the tent h segment o f the larva . Movable genital claspers (the harpes of lepidopterists) are characteristic o f the mal e genitali a o f Lepidoptera an d assum e a grea t variet y ?? ????? ????? ? ? ?? ?? ?? ????? ????????? ???? ??? ????? ?? ??? coxopodite area s o f th e vinculum , i n whic h their muscle s tak e thei r origin (D , E). I n suc h case s ther e ca n be little doub t tha t th e organ s are th e harpagone s (stylu s derivatives ) o f other insects . I n som e of the Microlepidoptera, however , th e clasper s ar e articulate d ventrall y t o a median triangula r plat e (I , BP) tha t supports the sheath of the aedeagus , and thei r muscle s (6 ) tak e thei r origin s mediall y o n a sclerotizatio n o f the aedeaga l sheath . Her e th e intimat e relatio n betwee n th e clasper s and th e phallobas e suggest s tha t th e claspin g organ s i n suc h case s ar e parameres, and that they are not homologous with the typical dorsolatera l harpagones of other Lepidoptera (D, Hrp), the muscles of which arise in the coxopodit e area s o f the vinculu m (E) . The phallic organs of the Lepidoptera includ e an aedeagus (Fig. 308 A, D, F , I , Aed)j usuall y a n eversibl e endophalli c tub e ofte n o f muc h greater lengt h tha n th e aedeagu s (F , Enph), an d variou s supportin g structures tha t ma y b e referre d t o th e phallobase . I n it s simples t development th e phallobas e i s a mer e inflectio n o f the genita l chambe r wall forming a pocket (phallocrypt ) containing the bas e of the aedeagus ; but th e lip s o f the pocke t ma y b e produce d a s a tubula r thec a (D , F , The) more or less enclosing the aedeagus. The inner walls of the sheath, however, ar e usuall y variousl y sclerotized , formin g i n som e case s a sclerotic rin g o r tube , th e anellus, fro m whic h th e aedeagu s protrude s (I, Anl). Th e ventra l lip of the anellus may be continuous with a median arm from a supporting basal plate (BP) in the ventral wall of the genital chamber. ???????????????? ???? ????????? ?? ??????????? ?????? ?? ?? phallic structure s only , periphallic appendicula r organ s bein g absen t i n most cases . Th e phallu s i s usuall y a large , highl y comple x structur e (Fig. 30 9 C, D ) arisin g fro m th e wal l of the genita l chambe r abov e th e ninth sternu m (A) . I t consist s o f a centra l aedeagu s (C , D , Aed),

THE ORGANS OF COPULATION AN D

OVIPOSITION

603

often provide d wit h latera l o r termina l processes , an d o f a larg e two segmented phallobas e (Phb) bearin g variou s lobe s an d processe s sur rounding the aedeagus . Th e proximal segment, or basal ring ("cardo") ,

FIG. 308.—Mal e genitalia o f Lepidoptera. A , Bombyx mori, ventral vie w of genital segment. B, same , ninth tergu m and tent h segment . C, same , inner vie w of right half . D, same , genita l an d tent h segment s wit h phallus , appendages , an d muscles . E , same , right harpag o and half o f ninth sternum, inner view. F , same, phallus and phallic muscles. G, Carpocapsa pomonella, abdomen. H , same , eighth , ninth , an d tent h segments . I , same, genital segment and aedeagus, ventral view. J , a lepidopterous spermatophor e fro m bursa copulatri x o f female .

of th e phallobas e (BR) open s fro m th e bod y cavit y b y a larg e forame n (D) tha t give s passag e to the ejaculator y duct . The dista l segmen t bears usuall y on e o r tw o pair s o f movabl e lobe s provide d individuall y

604

PRINCIPLES OF INSECT MORPHOLOGY

with muscles . O f these th e mor e latera l ventra l pai r (C , D , G , Pmr) may b e terme d parameres, sinc e the y ar e a t leas t analogou s wit h th e parameres o f Coleópter a (Fig . 30 3 C , Pmr), thoug h th e dorsa l lobe s (Fig. 30 9 C, G , Z> ) ar e accessor y structures o f the sam e nature. Hymen opterists generally cal l the ventra l lob e on each side the volsella, and th e dorsal lob e th e squama. Some , however , regard th e basa l part s o f th e genital organ as being formed o f the unite d coxopodite s of the nint h seg ment, an d therefor e regard th e appendicula r lobe s as stylus derivatives .

FIG. 309.—Male genitalia o f Hymenoptera (Pteronidea ribesii) . A , abdomen and base of thorax. B , ninth an d tenth abdomina l segments. C , phallus, dorsal surface. D , same, ventral surface . E , righ t latera l plat e o f aedeagu s an d muscles . F , lef t latera l plat e of aedeagus. G , phallus an d muscles, lateral view .

The aedeagu s i s a relativel y simpl e structure . A s represente d i n Pteronidea (Fig . 30 9 C , D , Aed) i t i s mostl y membranou s bu t contain s two latera l plate s (E , F , d ) produce d proximally a s apodeme s on which the aedeaga l muscles are attached (E , G). A median dorsal groove leads into a proximal aperture (C , /) fro m whic h a membranous endophallus is eversible. I n the Apidae the aedeagus commonly bears a pair of proximal lateral processes (sagittae), and often a dorsal lobe (spatha). In the honey bee the entir e phalli c organ is much simplified; the basal structure s so characteristic o f other Hymenopter a ar e absent, an d the orga n appear s to consis t o f the aedeagu s with a highly develope d eversible endophallus . In th e ontogeneti c developmen t o f the Hymenoptera , a s show n b y Zander (1900) , the entir e grou p o f phallic structure s proceed s from tw o primary genita l lobe s a t th e side s o f th e gonopore . First , eac h lob e divides int o two , the n thos e o f the resultin g media n pai r unit e t o for m

THE ORGANS OF COPULATION AND OVIPOSITION

605

the aedeagus , whil e those o f the oute r pair becom e the latera l lob es of the phallobase . Th e basa l rin g i s said t o b e differentiated a s a circula r fold o f the wal l of the genita l chamber . Only on e postgenita l segmen t i s presen t i n Hymenoptera , which , judging from the larva, is the tenth abdominal segment (Fig . 309 A, B, X) . In som e o f th e lowe r familie s i t bear s a pai r o f cercuslik e appendage s (B, Soc), which, since they occur on the tenth segment, ar e perhaps to be identified with the soci i of Trichoptera an d Lepidoptera (Fig . 307 A, Soc). Díptera.—The mal e genitali a o f Dipter a sho w a grea t proclivit y toward th e developmen t o f secondary lobe s an d processes , bot h phalli c and periphallic . I n th e mor e generalize d families the genita l segment s have a tendenc y t o for m a terminal enlargemen t (hypopygium ) o f th e abdomen (Fig . 31 0 A) ; i n highe r familie s th e distinctio n betwee n th e genital an d viscera l region s o f the abdome n become s accentuated b y a reduction o f th e sixt h an d sevent h segment s an d a clos e associatio n between the eighth , ninth, and tenth segments to form a genital complex (F), whic h become s mostl y conceale d withi n th e fift h segmen t (E) < Asymmetry i s o f frequen t occurrenc e i n th e genita l region , an d th e ninth segmen t i s sometimes partl y revolve d upo n it s axi s or completel y inverted. The harpagone s ar e wel l develope d i n lowe r Dipter a (Fig . 31 0 A , Hrp); the y ar e ofte n bilobe d (B ) and sometime s ar e bipartite. I n som e of th e Tipulida e th e genita l coxopodite s ar e distinc t pleura l plate s o n the side s o f the nint h segmen t (A , D, Cxpd), an d the y ma y b e partl y exserted fro m betwee n th e tergu m an d sternu m a s i n Mecopter a an d Trichoptera; but more commonly the coxopodites unite with the sternu m or entirel y los e thei r identit y i n th e continuousl y sclerotize d annulus of the nint h segment . Th e venter of the genita l segment may be entirel y membranous (C, V), but usually it contains a sternal plate. The greatl y specialize d typ e o f abdome n characteristi c o f highe r Diptera is well exemplified in Pollenia rudis (Fig. 310 E-J). Th e viscera l part o f the abdome n consist s of segments I-V (E) , but th e firs t segmen t is usually obscured by reduction and union with the second . I n Pollenia, segment V I appear s t o b e obliterated, an d segmen t VI I contain s onl y a small tergal plate (F, VIIT). Segment VIII has a well-developed tergum (VII IT), but its sternum is reduced to a narrow sclerotic band (VII IS), whic h is incomplete o n the righ t side . Segmen t I X present s externally a smal l terga l plat e (IXT) behin d an d belo w th e eight h tergum; the sternum of the ninth segment (IXS), however, ordinarily projects forwar d an d upwar d fro m th e lowe r angle s o f th e tergu m i n the dorsal wall of a large pouch with membranous walls (g) invaginated within th e eight h sternum . Thi s pouc h contain s th e phalli c organs . Seen fro m below , the nint h sternu m i s a broa d plat e (G , IXS) havin g

606

PRINCIPLES OF INSECT MORPHOLOGY

its posterio r angle s produce d a s tw o arm s (h) i n th e membranou s wal l beneath the ninth tergum, from which a pair of lateral bars (i) extends to the tenth segment. Tw o median plates (a, a) arise from the posterio r

FIG. 310.—Mal e genitali a o f Diptera . A , Nephrotoma ferruginea (Tipulidae) , en d of abdomen . B , same , harpag o an d muscles . C , same , genita l segment , ventra l view . D, same , latera l view . E , Pollenia rudis (Calliphoridae) , ventra l vie w of abdomen . F , same, segment s o f genita l complex , latera l view . G , same , nint h segmen t (aedeagu s removed), ventra l view . H , same , ninth an d tenth segment s with aedeagus . I , Phormia regina, ejaculatory bulb . J , Pollenia rudis, phallus, wit h basa l apódem e and muscles .

margin o f th e nint h sternu m an d suppor t a pai r o f fre e lobe s (6 ) that embrace th e bas e of the aedeagus . Movable clasper s tha t ca n b e identifie d wit h th e harpagones , o r styli o f th e gonopods , ar e absen t i n muscoi d Diptera , bu t th e nint h tergum commonl y bear s on its lowe r posterio r angles a pair of long lobe s

THE ORGANS OF COPULATION A ND OVIPOSITION

607

(Fig. 31 0 F, H , fc), which ma y b e flexible at thei r base s bu t ar e no t provided with muscles. The small flat membranous tenth segment (X}9 in which the anu s appears as a median slit (An), als o is usually provide d with a pair o f lateral lobe s (Z ) associate d wit h those of the nint h tergum . On th e base s o f thes e lobe s o f th e tent h segmen t i s inserte d a pai r of muscles from th e ninth tergum. The phalli c organ s o f Dipter a consis t principall y o f a variousl y developed aedeagus , thoug h supportin g basa l structure s als o ma y b e present. Th e aedeagus in its simpler forms varie s from a short taperin g process (Fig . 310 C, Aed) t o a long slender tube usually curved or coiled. The typica l muscoi d aedeagu s i s a larg e irregula r structur e (J , Aed) with basal , lateral , an d ventra l lobe s o r processes . Th e phallobas e (Pkb) i s represente d b y a lo w thecal fol d surroundin g the bas e o f th e aedeagus, i n th e wall s o f whic h are tw o smal l plate s (p) supportin g a large basal apódeme (Apb) for muscle attachments. Ordinarily the aedeagus is turned forwar d in the phallic pouch above the eighth sternu m (F). Th e ductu s ejaculatoriu s i s provide d wit h a n ejaculator y bul b (I), a syringe-lik e structur e wit h a thick muscula r sheat h supporte d on a flat central apódeme (ap) arising from one side of the cuticular lining o f the duct . Dista l t o th e bul b th e duc t enter s th e bas e o f th e phallus (J , Dej). 2. TH E FEMAL E GENITALI A

The primitiv e individua l opening s of the latera l duct s o f the femal e genital system, situate d o n the sevent h abdomina l segment, are retained in moder n adul t insect s onl y i n th e Ephemerida , th e female s o f thi s order havin g a pai r o f gonopores located i n th e conjunctiva l membrane behind th e sevent h sternum , fro m whic h the egg s issue i n tw o distinc t masses (Fig . 311 B). I n Dermaptera the lateral oviducts unite in a very short media n oviductus communis (A, Ode') openin g immediately behind the sevent h abdomina l sternu m (Gpr). I n al l other insect s th e media n egg passag e i s extende d posteriorly , an d th e exi t apertur e i s locate d either o n th e eight h abdomina l segmen t o r o n th e ninth . Whe n th e genital openin g is establishe d o n the eight h segment , ther e i s generally associated wit h i t a n orga n forme d o f appendicular parts of th e eight h and nint h segment s serving for th e depositio n o f the eggs . Thi s orga n is the ovipositor (Fig . 311 C, Ovp). An oviposito r havin g a unifor m basi c pla n o f structur e i s o f wid e occurrence amon g pterygot e insects , an d a n orga n o f th e sam e typ e though o f mor e primitiv e structur e occur s in th e Thysanura . Henc e there is little reason to doubt that the common ancestors of the Pterygota and Thysanur a wer e equippe d with an egg-layin g organ from whic h th e modern oviposito r ha s bee n evolved . Th e oviposito r o f present-da y

608

PRINCIPLES OF

INSECT MORPHOLOGY

insects i s nearl y alway s rudimentar y o r suppresse d i n insect s havin g the eg g exit o n the nint h segment , an d i t i s often reduce d o r absen t i n forms havin g th e genita l openin g o n th e eight h segment . Insect s i n which th e oviposito r i s absen t o r neve r full y develope d includ e Col lembola, Diplura , mos t Odonata , Ephemerida , Plecoptera , Mallophaga , Anoplura, th e tubuliferou s Thysanoptera, Coleóptera , mos t Neuroptera , Mecoptera, Trichoptera , Lepidoptera , an d Díptera. Insects lackin g an ovipositor, o r in which the organ is not functionall y developed, ma y hav e n o specia l provisio n fo r placin g th e eggs ; bu t with many of them the posterio r segment s of the abdome n are so modified that they can be protracted i n the form of a slender telescopic tube havin g

FIG. 311.—Thre e type s o f externa l genital structur e o f th e femal e abdomen . A , genital and terminal segments of a dermapteron, Anisolabis marítima, with median gonopore immediately behin d sevent h abdomina l sternum. B , abdome n of a mayfly , Hexagenia, with paired gonopores behind seventh sternum, eggs issuing in tw o masses. C , nymph of Scudderia, illustrativ e o f insect s wit h a n appendicula r oviposito r an d gonopor e behind eighth abdominal sternum.

the openin g o f th e eg g passag e nea r it s dista l end . A substitutiona l " oviposit or" of this kind i s characteristic of the tubuliferou s Thysanop tera, th e Mecopter a (Fig . 31 2 D), th e Lepidopter a (A) , the Coleóptera , and th e Dípter a (B , C). Mos t insect s of these group s insert thei r egg s into crevices or attach them to smooth surface s by a cementing substanc e discharged fro m th e colleteria l glands . I n som e cases , however , th e end o f the abdome n form s a piercing o r cutting organ , o r it ma y hav e a simple modification for manipulating o r placing the eggs . Th e abdome n of th e frui t flie s (Trypetidae , Lonchaeidae) , th e dista l par t o f whic h i s narrowed and tapering (B ) or sometimes greatly elongat e (C) , terminate s in a shar p poin t tha t enable s th e insect s t o pierc e th e ski n o r rin d o f fruit i n which they deposit their eggs . I n mos t Lepidopter a th e femal e abdomen i s provided wit h tw o terminal lobe s at th e sid e of the eg g exit, which serve to grasp the issuing eggs, or which, when spread out flat, form a disc for pressing the egg s against th e surface on which they ar e attached by th e secretio n of the cemen t glands .

??? ?????? ?? ?????????? ??? ??????????? ? ?

The Oviposito r o f Thysanura.—Th e oviposito r o f th e Thysanur a presents i n a simpl e for m structura l element s fro m which , ther e ca n be littl e question , th e mor e highl y perfecte d oviposito r o f pterygot e insects ha s bee n evolved . Mor e tha n this , it s basa l part s sho w a n exact seria l identit y wit h ventra l plate s o f the pregenita l segments tha t are almos t certainl y rudiments of abdominal limbs. The unde r surface s o f th e pregenita l segment s o f Machilidae , a s was shown in Chap . XI , hav e each three plate s (Fig . 13 8 A), namely, a

FIG. 312.—Example s o f a n "ovipositor " forme d o f th e termina l segment s o f th e ???????? ?? ? ????? ????????? ???????? ????? ???????? ?????? ?? ? ????? ???? Paracantha culta. C , a frui t fly , Toxotrypania curmcauda. D , Panorpa consuetudinis.

????? ?????????? ?????? ?????? ????? ?? ??? ???? ??????? ??????? ??? ??? ????? ??????? ?????? ??????? ????? ?????? ?? ?? ??? ??????????? ?? ??? otherwise rudimentar y limbs . Th e coxopodite s of each pair ar e unite d medially behin d th e sterna l plate , an d eac h bear s a slende r appendag e known a s a stylu s (Sty), whic h i s movabl e b y muscle s arisin g i n th e supporting basa l plate . In th e genita l segment s o f femal e Thysanur a th e coxopodite s ar e ???? ?????? ?? ???? ????? ????? ??? ?? ?????? ??? ??????? ?????? ??? ??????? Each coxopodite bears distally a stylus (Sty), but in addition it has a ???? ??????? ??????? ???????? ?? ????????????? ??????? ???? ??? ????? angle o f its bas e (B , C , IGon, 2Gon), th e fou r o f which are closel y associated to form the shaft of the ovipositor (A, Ovp). Since the gonapophyses are never represented o n the pregenita l segment s they would appear to b e specia l development s o f the gonopods . I f so , judgin g from thei r position, the y ar e o f the natur e o f coxal endites. Eac h i s provided with a short muscle, arising in the supporting coxopodite (B, C, F, gmcl).

610

PRINCIPLES OF INSECT MORPHOLOGY

The principa l movement s o f th e gonapophyses , however , ar e probabl y brought abou t b y muscle s o f th e coxopodites , whic h aris e o n th e cor responding terg a o f the genita l segment s (F) . I n som e cases the secon d gonapophyses ar e fuse d a t thei r base s (C) , and i n other s the y ma y b e united throughou t thei r length , bu t th e firs t gonapophyse s ar e alway s entirely fre e fro m eac h other an d ar e apparentl y capabl e of independent movement. The tw o pair s o f gonopods ar e practicall y alik e i n Machilida e (Fig . 313 D, E); but in Thermobia (G), and probably in other Lepismatidae,

FIG. 313.—Th e ovipositor of Thysanura. A , Thermobia, eighth and ninth abdominal segments, ventral view. B , Nesomachilis maoricus, dorsal view of first gonopods. C , same, second gonopods . D , E , Machilis, latera l vie w o f right gonopod s and supportin g terga l plates. F , Thermobia, muscles of first gonopod, diagrammatic. G , same, second gonopod, showing subdivision of coxopodite.

the coxopodite s o f th e secon d pai r ar e eac h divide d int o a n anterio r plate (a ) bearing the gonapophysis , an d a posterior plat e (6 ) supporting the stylus . Th e terga l muscle s o f th e coxopodit e ar e inserte d o n th e anterior sclerite . Thi s division o f the secon d coxopodite occurring in th e Lepismatidae i s most interestin g sinc e i t suggest s th e simila r conditio n that is characteristic o f most pterygot e insects . General Structur e o f th e Oviposito r o f Pterygot e Insects. —The ovipositor of pterygote insects, in its typical form, consists of a shaft and a basal apparatus an d usuall y include s a pai r o f accessor y lobe s (Fig. 31 4 A) . Th e shaft , i n mos t insects , i s compose d o f two pair s o f closely appresse d elongat e processes , th e firs t an d th e second valvulae (1FZ, 2FZ) , th e firs t valvula e bein g usuall y ventral , th e secon d dorsal .

THE ORGANS OF COPULATION AND OVIPOSITION

611

The secon d valvula e ar e ofte n unite d i n a singl e media n dorsa l piece . The basal apparatus consist s essentially o f two pairs of lobes or plates, th e first an d th e second valvifers (1YZ/ , 2FZ/) , whic h support th e oviposito r shaft b y th e base s o f the valvulae . Th e proxima l parts of the valvulae , by whic h th e latte r ar e attache d t o thei r respectiv e valvifers , ar e dis tinguished as the rami of the valvulae. The accessory lobes, or third valvulae (3FZ) > ar e born e o n th e posterio r end s o f th e secon d valvifers .

FIG. 314.—Structur e o f th e oviposito r o f pterygot e insect s (A-D , diagrammatic) . A, showin g segmenta l relations of the part s o f the ovipositor . B , ventra l vie w of genita l segments an d ovipositor . C , D , latera l vie w of genital segment s and part s o f oviposito r dissociated. E , nymp h of Blatta orientalis, ventral vie w o f genital segments with lobes of ovipositor. aiv, anterior intervalvula; Apr, apertur e of accessory glands; Gpr, gonopore; Ode., oviductu s communis ; piv, posterio r intervalvula ; Spr, spermatheca l aperture ; Sty, stylus; 1VI, 2VI, 3FZ , first, second, and thir d valvulae ; IVlf, 2Vlf, firs t an d secon d valvifers.

They usuall y ensheat h th e dista l par t o f the shaft , bu t i n Orthopter a they for m a thir d pai r o f blades i n th e shaft . Th e ventra l bod y wal l between th e base s o f the secon d valvifer s sometime s contain s sclerites , which are terme d intervalvulae (B , aiv, piv). The principa l muscle s of the oviposito r ar e inserte d o n the valvifers . The dorsa l muscle s of the firs t valvifer s tak e thei r origi n o n the tergu m of th e eight h abdomina l segment , thos e o f th e secon d valvifer s o n th e tergum o f the nint h segmen t (Fig . 31 4 A). I t i s thus eviden t tha t th e first valvifers pertai n t o th e eight h segmen t an d th e secon d valvifers t o the nint h segment . Th e secon d valvifers usually retai n a clos e connection with, the tergum of the ninth segment, but th e first valvifers are ofte n

612

PRINCIPLES OF INSECT MORPHOLOGY

more o r les s dissociate d fro m th e eight h segment . A n exceptio n t o the genera l structure and musculature of the oviposito r occurs only in the Acrididae and Tridactylidae. When th e separate d element s o f the pterygot e oviposito r (Fig . 314 B, C , D) ar e compared with those of the thysanura n oviposito r (Fig . 313 D, E), it is to be seen at once that there is almost an exact correspondence between th e part s o f the orga n i n th e tw o cases . I t become s evident , therefore, that the firs t an d second valvifers (1FZ/ , 2VIJ) o f the pterygot e ovipositor ar e derived from th e coxopodite s of the gonopod s respectively of th e eight h an d nint h abdomina l segments , an d tha t th e firs t an d ?????? ???????? ????? ???? ??? ???????????? ??? ????? ??? ?????? ????? pophyses. Th e thir d valvula e o f th e Pterygot a (Fig . 31 4 D , 3VI), which ar e carrie d b y th e secon d valvifers , ar e als o derivation s o f th e second coxopodites ,(Cxpd), but they are not the styli (Sty). True styli are presen t o n th e secon d coxopodite s of a fe w pterygote insects , a s i n nymphs o f Blattidae (E , Sty) an d i n th e adult s o f some Odonata. W e ??? ??? ?????? ???? ?? ????????? ????? ??? ?? ???? ?????????? ?? ??? ninth segmen t i s divide d int o a proxima l plat e (a ) carryin g th e gona pophysis an d a distal stylus-bearing plat e (b). Styl i of the coxopodite s of the eight h segmen t are never present i n Pterygota . The developmen t o f th e pterygot e oviposito r i s entirel y i n accor d with th e homolog y of its part s suggeste d b y th e structur e o f the adul t ?????? ??? ????? ???????? ????? ??? ?? ???? ??? ?????? ?????????? ?? the coxal areas of the eighth abdominal segment (VHICxpd), the latter developing int o th e firs t valvifers . Th e primar y genita l processe s of the nint h segmen t are the thir d valvulae , which are distal outgrowth s of the coxopodites of the second gonopods (IXCxpd), which become differentiated proximall y int o th e secon d valvifers , whil e mesa l (endite ) processes becom e th e secon d valvulae . Styl i neve r appea r o n th e coxopodites o f th e eight h segment , an d thos e o f th e nint h segmen t (Sty), i f presen t i n immatur e stages , ar e los t wit h th e transformatio n to th e adult , excep t i n som e Odonata . Whil e the theor y o f th e origi n of th e part s o f th e oviposito r fro m th e primitiv e limb s o f th e genita l segments ma y thus see m to b e well substantiate d b y ontogeneti c devel opment, du e consideratio n shoul d b e give n t o th e eviden t fac t tha t all outgrowth s o f th e bod y wal l mus t necessaril y loo k muc h alik e i n their earl y stages of growth, whether they represent primary or secondary organs. Similarit y i n suc h structures, therefore , is no proof o f identity. The principa l group s o f pterygot e insect s i n whic h th e oviposito r is wel l develope d ar e th e Orthoptera , Hemiptera , Thysanoptera , an d Hymenoptera. The Oviposito r o f Orthoptera.—Th e oviposito r o f Orthopter a i s in som e respect s mor e generalize d tha n tha t o f othe r insects , while ,

THE ORGANS OF COPULATION AND OVIPOSITION

613

on th e othe r hand , i t ha s on e uniqu e featur e an d certai n structura l specializations tha t adap t i t t o th e particula r mechanis m develope d in this order . The orthopteroi d oviposito r i s generalize d i n tha t th e coxopodite s of the nint h segment are not anatomicall y separated into second valvifers and thir d valvulae , thoug h eac h is distinctly differentiate d into a basa l valvifer region (Fig. 315 A, C, E, 2Vlf) and a distal valvular process (3FO- Als o there ar e presen t i n som e familie s anterio r an d posterio r

FIG. 315.—Th e oviposito r o f Orthoptera . A , Scudderia (thir d válvul a cu t of f near base). B , Locusta viridissima, cros s sectio n o f shaf t o f ovipositor . (From Weber, 1933. ) C, Gryllus assimilis, base of ovipositor. D , E, same, first and second valvifers with muscles, inner vie w of right side. F , same , base of ovipositor wit h muscles , inner view of right side. G, same, diagram o f mechanism o f ovipositor. H , same , cros s section of shaft of ovipositor .

intervalvular sclerite s i n th e vente r o f the nint h segmen t betwee n th e second valvifer s (F, aiv y piv), an d muscle s from th e nint h tergu m (5 , 8) and fro m th e valvifer s (9 , 10 ) tha t d o no t generall y occu r i n highe r insects ar e inserted o n these sclerites . The uniqu e featur e o f th e orthopteroi d oviposito r i s th e inclusio n of th e thir d valvula e a s a pai r o f blades in th e shaf t o f the orga n (Fig . 315 A, C, 3F7)- Whe n the thre e pairs o f valvulae, therefore, are al l well developed, th e shaf t consist s o f thre e pair s o f valvula r lobe s o r blade s (A, B) . I n suc h cases , th e firs t valvula e (1FZ ) ar e ventral , th e thir d valvulae (3YZ ) dorsal, and the secon d valvulae (2VI) media n and usually concealed betwee n the others . Th e firs t valvula e ma y b e movabl e on both th e secon d an d th e thir d valvula e b y interlockin g groove s an d

614

PRINCIPLES OF INSECT MORPHOL OGY

ridges (B) . Th e secon d valvulae , however , are i n som e case s reduce d or rudimentary, as in Acrididae and Gryllidae (E, F, 2VI), and the shaft then consist s o f only two pair s o f blades (H) , which, it shoul d be noted , are th e firs t an d thir d valvulae , an d no t th e firs t an d secon d a s in th e usual four-valve oviposito r o f other insects . The particula r mechanica l features characteristic o f the orthopteroi d ovipositor ar e in the relation s o f the valvifer s t o th e terg a o f the genita l segments, an d i n th e interrelatio n o f the valvifer s to eac h other . Th e first valvifer is never closely attached t o the eighth tergum, and it may be displaced so far posteriorly that it appears to belong to the ninth segment. By it s posterio r angl e i t articulate s wit h th e secon d valvife r (Fig . 31 5 A, C, 6), and it may also have a strong articulation wit h the ninth tergum (C, D , a) . Internall y th e tw o valvifer s o f Gryllus osculat e b y specia l articular processe s (D , d y E , h , G , dh) —a featur e o f muc h importanc e in the mechanis m of the grylli d ovipositor . Th e secon d valvifer has n o direct articulatio n wit h the ninth tergum , its fulcrum o f movement being the articulation (E , h) with the first valvifer (G, dh). I t i s interesting t o note tha t thi s sam e anatomica l relatio n o f the valvifer s to eac h othe r and t o th e nint h tergu m i s characteristi c o f Hymenoptera also , thoug h the structur e an d mechanism are not exactl y the sam e in the tw o orders. The definitiv e first valvifer of Gryllidae is a composite structure forme d of th e tru e firs t valvife r and a smal l plat e derive d fro m th e coxopodit e of th e nint h segment . The musculatur e o f th e ovipositor , a s illustrate d i n Gryllus (Fig . 315 F), is somewhat more complex than i n insects of other order s owing to th e presenc e o f th e muscle s inserte d o n the intervalvula r sclerites . Each firs t valvife r ha s a muscl e fro m th e sevent h sternu m (1 ) and a large muscl e (3 ) fro m th e eight h tergum , bot h inserte d o n a n anterio r apodemal ar m o f the sclerit e (C , D , e). I n it s musculature , therefore, the firs t valvife r asserts it s relatio n wit h th e eight h abdomina l segment regardless o f it s mechanica l connections . Eac h secon d valvife r ha s a pair of large antagonistic muscle s (E, F, 6, 7) arising on the ninth tergum . This pair of muscles recurs in nearly all insects provided with an oviposi tor. Th e muscle s o f th e intervalvula r sclerite s (F , aw, piv) includ e a pair o f tergosternal muscle s (5, 8) of the nint h segment , and tw o pair s of muscles (9,10) between the posterior intervalvula and the first and second valvifers, respectively . The motio n o f the valvifer s produced by th e terga l muscle s inserted on them results in an alternate back-and-fort h movement of the valvula e on each other (Fig . 315 G). Th e articulation s o f the firs t valvifer s with the nint h tergu m (a) , an d th e articulatio n o f the two valvifer s on each side with eac h other (dh) constitute a mechanism o f such a nature tha t any motio n o f either pai r o f valvifers i s communicate d reversely t o th e

THE ORGANS OF COPULATION AND OVIPOSITION

615

other pair , wit h the result that the corresponding valvulae have opposit e movements. The Phasmidae , Mantidae , Blattidae , Grylloblattidae , an d Tet tigoniidae retai n th e thre e pair s of valvulae in the ovipositor , bu t other wise the oviposito r ha s essentially th e sam e structure i n these familie s as in Gryllidae , thoug h i t i s reduced and mor e or less modifie d i n th e firs t four an d attain s its highes t mechanica l perfection in Gryllidae . I n th e Acrididoidea an d Tridactylidae , however , the orga n depart s widel y in structure, musculature , an d mechanism , no t onl y fro m th e oviposito r of othe r Orthoptera , bu t fro m tha t o f al l othe r insects , sinc e th e fou r terminal processe s work by a divergen t motio n instea d o f sliding upo n each other . The Oviposito r o f Hemiptera.—Th e morpholog y o f th e pterygot e ovipositor i s perhap s bes t show n i n th e Hemiptera , becaus e her e th e valvifers mor e nearl y tha n i n th e othe r order s retai n thei r prope r seg mental connections . I n mos t o f th e Heteropter a th e oviposito r i s reduced o r rudimentary , bu t i n Homoptera , excep t parasiti c forms , i t is generall y wel l developed . Th e eight h abdomina l sternu m i s reduced or practicall y obliterate d i n al l Hemiptera , an d th e subgenita l plat e is the sevent h sternum . The shaf t o f the oviposito r i n Homoptera, a s illustrated i n the cicad a (Fig. 31 6 A), issues at th e bas e of the nint h segmen t between the eight h tergum an d th e sevent h sternum . Whe n dissecte d (B , C ) th e part s belonging t o th e tw o genita l segment s ar e easil y distinguished . Th e first valvifer s (IVlf), implante d i n th e membranou s ventrolatera l part s of th e eight h segmen t (A) , are smal l triangular plate s carryin g th e firs t valvulae. Eac h articulate s posteriorl y wit h th e nint h tergum , an d a small plate on the inner surface (F , m), continuous with the dorsal margin of th e firs t válvula , i s directly fused wit h a lobe (n) o f the nint h tergum . This feature is characteristic o f all Hemiptera, an d generall y the bas e of the first válvula is produced into two rami, the oute r of which is attached to th e valvifer, while the inne r on e is fused wit h the nint h tergum . I n the cicad a a short muscl e extends between the oute r an d inne r plate s of the firs t valvife r (F , £) • Othe r muscle s o f th e firs t valvife r compris e a muscl e fro m th e sevent h sternum (H , 1 ) an d tw o muscle s fro m th e eighth tergu m (2 , 8), al l inserted o n an apódem e (e ) of the dorsa l margi n of th e valvifer . The secon d valvifer s o f th e cicad a ar e elongat e plate s (Fig . 31 6 A, C, G, 2Vlf) mostly concealed within the projecting lower parts of the ninth tergum (A). A t their anterio r ends the second valvifers are directly continuous with the bases of the second valvulae (C, G, 2VI), and the third valvula e (3FZ ) aris e a t thei r posterio r ends . Eac h secon d valvife r is articulate d a t a poin t nea r th e middl e o f it s dorsa l margi n wit h a

616

PRINCIPLES OF INSECT MORPHOLOGY

condyle o n th e ventra l margi n o f th e nint h tergu m (F , G , p) . Tw o large muscles (G, 6, 7 ) arising on the nint h tergu m are inserte d o n each second valvife r respectivel y anterio r an d posterio r t o th e articula r fulcrum. Th e second valvifers of the Hemiptera , therefore, rock directly on the nint h tergum , and not o n the firs t valvifer s as in Orthopter a an d Hymenoptera. Ther e ar e n o othe r muscle s in th e nint h segmen t con -

FIG. 316.—Th e ovipositor o f a cicada, Magicicada septendecim. A , end o f abdomen , with ovipositor , genita l chambe r (GC) an d spermathec a (Spt) exposed . B , C , part s of ovipositor dissociated . D , ti p o f ovipositor , ventra l view . E , cros s sectio n o f shaf t of ovipositor. F , firs t valvife r an d associate d parts , inne r vie w o f right side . G , muscle s of secon d valvife r an d termina l segments , inne r vie w o f right side . H , bas e o f ovipositor , showing muscle s o f conceale d firs t valvifer .

nected with the ovipositor, since intervalvular sclerite s and their muscles, such as those of the cricke t (Fig . 315 F), ar e absen t i n Hemiptera . The shaf t o f th e cicadi d oviposito r consist s o f onl y thre e distinc t parts, sinc e th e secon d valvula e ar e unite d wit h eac h othe r (Fig . 31 6 E, 2VI) t o for m a stron g media n rod , t o th e side s o f whic h th e firs t valvulae (1FZ ) ar e attached b y th e usua l ridge-and-groove device. Th e lateral positio n o f the firs t valvula e wit h respec t t o th e secon d valvulae is characteristi c o f Homoptera, bu t mor e commonl y the valvula e hav e the for m o f flattene d blades . Whe n th e oviposito r i s no t i n us e i t i s ensheathed betwee n th e concav e inne r surface s o f th e thir d valvulae . The Oviposito r o f Hymenoptera.—Th e oviposito r o f Hymenoptera , in it s genera l for m an d i n th e compositio n o f it s shaft , resemble s th e

THE ORGANS OF COPULATION AND OVIPOSITION

617

ovipositor o f Hemipter a mor e closel y tha n tha t o f Orthoptera , bu t i t has one special character, namely, the articulatio n o f the secon d valvifers with th e first valvifers an d no t wit h th e nint h tergum , tha t i s a highly developed feature in the mechanis m of the oviposito r o f Gryllidae . The basi c structur e o f th e hymenopterou s oviposito r i s wel l shown in th e Tenthredinidae , thoug h th e orga n her e doe s no t hav e th e for m typical o f th e oviposito r o f clistogastrou s Hymenoptera , i n whic h i t i s

FIG. 317.—Th e ovipositor o f a tenthredinid , Pteronidea ribesii. A , en d o f abdomen. B, showing relation o f basal parts of ovipositor t o each other an d t o ninth tergum . C , first valvifer an d válvula . D , secon d válvula . E , secon d valvife r wit h secon d an d thir d valvulae.

usually lon g an d slender . Th e shaf t o f th e tenthredini d ovipositor , a s illustrated i n Pteronidea ribesii (Fig. 317 A, B), is short an d broad with an acute ape x an d stron g latera l ridges . I t i s compose d o f th e firs t an d second valvula e (B , 1FZ , 2FZ) , and i s ordinarily ensheathe d betwee n th e broad thir d valvula e (3FÍ) . Th e basa l par t o f the oviposito r consist s of the first and second valvifers (B, IVlf, 2Vlf) lying beneath the long lower margin of the ninth abdominal tergum (IXT). The first valvifer (B, C, IVlf) is a small triangular plate articulated by its dorsal angle (a) wit h th e nint h tergu m (B ) an d b y it s posterio r ventra l angl e (6) with th e secon d valvifer . Anteriorl y th e firs t valvife r i s continuou s with the narro w ramus of the first válvula (C) . Th e second valvifer is a relatively large , elongate plate (B , E, 2Vlf) wit h the ramu s of the second válvula (E , r2vl) attache d t o its anterior extremity , an d the third válvula (3FZ) formin g a broa d lob e a t it s posterio r end . Th e secon d valvife r

618

PRINCIPLES OF INSECT MORPHOLOGY

has n o articula r connectio n with th e nint h segment , it s poin t o f movement bein g th e articulatio n (B , b) wit h th e firs t valvifer . Th e tw o second valvula e ar e unite d t o eac h othe r b y a media n membran e (E) . It shoul d b e observe d tha t i n th e Tenthredinida e th e terg a o f th e eighth an d nint h abdomina l segment s are normally develope d an d full y exposed plates (Fig. 317 A). Ther e are, however, as in all Hymenoptera , no sterna l plate s i n thes e segments , th e subgenita l plat e bein g th e seventh sternum . I n the highe r hymenopterous families th e eight h an d ninth segment s ar e retracte d int o th e seventh , an d th e terg a o f thes e segments are progressively reduced, until, i n the bees , they consis t onl y of tw o pair s o f latera l sclerite s associate d wit h th e bas e o f th e sting .

FIG. 318.—Th e oviposito r o f Megarhyssa. A , M. atraía, en d o f abdome n an d bas e of oviposito r wit h part s in usua l position . B , M. lunator, showin g position o f abdomina l segments an d bas e o f ovipositor durin g oviposition.

In th e mor e typical for m o f the hymenopterou s oviposito r th e shaf t is long and slender , and th e ensheathin g third valvula e ar e correspond ingly lengthened , bu t th e genera l structur e o f th e orga n i s i n n o wa y essentially differen t fro m tha t o f th e Tenthredinidae . A n extrem e development o f the slende r typ e o f shaft occur s in som e of the Ichneu monidae, a s i n Megarhyssa (Fig . 318 ) an d relate d genera . Durin g oviposition b y suc h species the termina l par t o f the abdome n i s turne d downward (B), exposing the wide conjunctival membrane (Mb) between the sevent h an d eight h terga. Th e bas e o f the oviposito r i s now dorsa l and is contained in a large pouch formed b y inflection o f the membranou s ventral par t o f th e bod y wal l includin g the sevent h sternu m (VIIS). The ovipositor shaft (Ovp) protrudes from the pouch at right angles to the lengt h of the abdome n and is ensheathed distall y i n the slende r third valvulae (3FZ) ; but, a s the shaft penetrates th e woo d in which the femal e lays her eggs , the sheat h valve s separate fro m th e shaf t an d ma y curv e upward at the sides of the body, as shown in various familiar illustrations. In th e stingin g Hymenoptera th e oviposito r loses its egg-layin g func tion an d i s converted int o a poison-injecting instrument, bu t eve n her e there i s little chang e in the structur e o f the orga n except in the develop ment o f valvular lobes on the firs t valvula e for driving the poiso n liquid

THE ORGANS OF COPULATION AND OVIPOSITION

619

through th e shaft . Th e stingin g apparatu s o f th e bee s (Fig . 31 9 A ) includes no t onl y th e usua l part s o f the oviposito r bu t als o th e latera l sclerites (quadrate plates) of the ninth tergum (IXT) and the lateral spiracular plates of the eighth tergum (D, VII IT). The united second valvulae for m a n inverted trough , the enlarge d proximal part of which is the bulblik e swellin g o f the shaf t (A , bib), an d th e taperin g dista l par t the median stylet (stl). The slender first valvulae, or lancets (Let), slide on the lowe r margins of the bul b and stylet . Proxima l to th e bul b

FIG. 319.—The ovipositor (sting) of Hymenoptera. A, Apis mellifica, shaft and basal apparatus of sting, showin g relation of valvifers t o ninth tergal plate. B , Atanycoins rugosiventris (Braconidae) , basa l par t o f oviposito r an d muscles , dorsa l view . C , Apis mellifica, muscle s o f sting , inne r vie w o f righ t side . D , same , positio n o f part s o f stin g when shaf t i s extruded , diagrammatic .

the ram i o f the valvula e (rlvl, r2vl) diverg e on each side to thei r attachments wit h th e valvifers . Th e reservoi r o f th e poiso n glan d (PsnSc) discharges directl y int o the bas e o f the bulb ; the tubula r lef t accessor y gland open s ventrall y betwee n th e ram i o f th e valvulae . Whe n th e sting i s in repos e i t i s concealed in a stin g chambe r withi n th e sevent h abdominal segment , wher e it i s ensheathed betwee n th e thir d valvulae . To b e use d effectively , th e stin g mus t b e protracte d fro m th e stin g chamber an d deflected at righ t angle s to the basal plate s (D) . The protractio n o f th e bee' s stin g i s brough t abou t b y pressur e engendered in the anterio r par t o f the abdomen , which causes the entir e organ to swing backward on the connections between the eighth and ninth

620

PRINCIPLES OF

INSECT MORPHOLO GY

tergal sclerites (Fig. 319 D) as shown by the arrow at x. At the same time th e shaf t i s depresse d i n th e directio n o f the arro w a t y, an d th e membranous venter o f the nint h segmen t (IXV) i s protruded a s a hood over th e bas e o f th e bulb . Th e depressio n an d elevatio n o f th e shaf t are effecte d b y tw o pair s o f muscles common to al l Hymenoptera , bu t which appea r t o hav e n o homologue s in othe r insects . Th e depresso r muscles (B , C , 19 ) arise o n the secon d valvifers and ar e inserte d o n th e base o f th e bul b o f th e shaft ; th e levator s (20 ) stretc h betwee n th e extremities o f the ram i of the secon d valvulae. The principa l mobil e element s o f the shaf t o f the stin g ar e th e firs t valvulae, o r lancets , whic h are move d b y motion s o f the firs t valvifer s produced b y th e muscle s o f th e secon d valvifers . Th e firs t valvifer s have eac h onl y a singl e small muscle arising o n th e terga l plat e o f th e eighth segment (Fig . 319 D, 3). Th e second valvifers are provided with the usual antagonistic muscles arising on the ninth terga l plates (C , 6, 7); but sinc e these plate s themselve s ar e movabl e o n accoun t o f the mem branization o f th e media n par t o f th e dorsum , th e pul l o f th e muscle s on th e secon d valvifer s vibrate s th e terga l plates , an d th e motio n of the latter is communicated t o the first valvifers, whic h in turn move the lancets bac k an d fort h i n th e shaf t o f th e sting . Th e valve s o n th e bases o f th e lancet s driv e th e poiso n liqui d throug h th e channe l o f the unite d secon d valvulae , fro m whic h i t escape s ventrall y betwee n the tip s of the lancets . With mos t Hymenopter a th e egg s traverse th e channe l o f th e ovi positor shaft , regardles s o f th e diamete r o f th e latter . I n th e ver y slender bristle-lik e ovipositor s o f som e parasiti c forms , a s show n b y Fulton (1933) , the eggs are compressed and stretched to an extreme degree in their transi t throug h the narrow passage. In the stingin g Hymenoptera th e egg s ar e ejecte d fro m th e openin g o f th e genita l chambe r a t the bas e of the ovipositor . GLOSSARY O F TERM S APPLIE D T O TH E EXTERNA L GENITALIA

THE MAL E ORGAN S Aedeagal Apódeme (Apa).—An apódem e of the aedeagus . Aedeagus (Aed). —The dista l part o f th e phallus , usuall y th e principa l par t of the intromittent organ , typically a sclerotic tube . Anellus (Anl). —A sclerotizatio n o f the inne r wal l o f th e phallocryp t o r phallo theca, ofte n forming a ring or tube abou t th e bas e of the aedeagus . Basal Apódem e (Apb).—An apódem e of the phallobase . Basal Plate s (BP). —Sclerites o f the phallobase . Ectophallus.—The outer phalli c wal l in distinction t o the endophallus . Ejaculatory Duc t (Dej). —The ectoderma l outle t duc t o f the mal e genital system .

THE ORGANS OF COPULATION AND OVIPOS ITION

621

Endophallus (Enph).— The inne r chambe r o f the phallu s invaginate d a t th e en d of th e aedeagus , int o whic h the ejaculator y duc t opens ; typically an eversibl e sa c or tube, bu t sometime s a permanently interna l phalli c structure . Endotheca (Enth). —The inne r wall of the phallotheca . Epimere.—A dorsal process of the phallobase . Epiphallus.—A sclerit e i n som e Orthopter a i n th e floo r o f th e genita l chambe r proximal to the base of the phallus. (Pseudosternite.) Genital Chambe r (GC). —A ventra l invaginatio n o f th e conjunctiva l membran e between the ninth and tenth abdominal segment s containin g the phallic organs . Genital Segment.—Specificall y th e nint h segmen t o f the abdome n i n th e male , though othe r segment s ar e frequently associate d wit h th e nint h in the genita l com plex. (Gonosomite.) Genitalia.—The genital organs collectively, but usually applied only to the external genitalia. Gonapophyses (Gon). —Median proxima l processe s o f th e coxopodite s o f th e gonopods, present i n the mal e in some Thysanura . Gonopore (Gpr). —In th e mal e th e externa l openin g o f th e media n ejaculator y duct, usually concealed in the endophallus , or one of the aperture s of paired exit ducts . Gonosomite.—See genital segment. Gonostyli.—The styli of the nint h segment , whe n present , generall y modifie d t o form claspin g organ s (harpagones) . Harpagones (Hrp). —Movable periphalli c processe s o f th e nint h segmen t indi vidually provide d with muscles, probably derivative s .o f th e gonostyli , usuall y having a claspin g function. (Terme d harpes i n Lepidoptera. ) Hypandrium.—See subgenital plate. Hypomere.—A ventral process of the phallobase . Parameres (Pmr). —Lateral processe s o r lobe s o f the phallobase . (Th e ter m i s here defined a s used in Coleóptera, but i t i s also applied t o th e gonapophyses. ) Penis (Pen) .—One of the paire d intromittent organs of certain insects, o r the usua l median phallic organ. (Se e phallus.) Periphallic Organs.—Periphera l genital processe s o f the nint h segment, o r also of other segment s in the genita l complex , including the movabl e claspers, or harpagones. Phallobase (Phb). —The proxima l par t o f th e phallus , highl y variabl e i n it s development, sometime s a larg e structur e supportin g th e aedeagus , ofte n produce d into a thecal fol d o r sheath about th e aedeagus , sometimes represente d onl y by basal phallic sclerite s in the wal l of the genita l chamber . Phallocrypt (Crpt). —A pocke t o f th e phallobas e o r o f the genita l chambe r wal l containing th e bas e of the aedeagus . Phallomeres.—Genital lobes formed a t th e side s o f th e gonopor e in the ontogen y of som e insects; i n mos t case s they unit e t o for m th e phallus , bu t i n Blattida e an d Mantidae they develo p separately int o comple x genital organs of the adult . Phallotheca (The). —A fol d o r tubula r extensio n o f th e phallobas e abou t th e aedeagus. Phallotreme (Phtr). —The dista l opening of the endophallus , usually a t th e en d of the aedeagus . Phallus (Phi). —The unpaire d penis , o r median intromitten t organ , including th e phallobase, th e aedeagus , th e endophallus , an d variou s processe s o f th e phallobas e and th e aedeagu s if present. Sagittae.—Proximal lateral processe s of the aedeagu s in Hymenoptera. Socii (Soc). —Lateral appendicula r processes of the tent h segmen t i n Trichopter a and Lepidoptera , possibl y homologou s with th e cercuslik e appendage s o f th e tent h segment in lowe r Hymenoptera .

622

PRINCIPLES OF INSECT MORPHOLOGY

Spatha.—A dorsal lobe of the aedeagu s in Hymenoptera . Squama.—A dorsal lateral lobe of the phallobas e in Hymenoptera . Subgenital Plate.—Usuall y th e nint h abdomina l sternu m o f the mal e extende d beneath th e genita l chamber , bu t sometime s th e eight h o r th e sevent h sternum . (Hypandrium.) Titillators.—Terminal processes of the aedeagus . Uncus.—A proces s o f th e tent h abdomina l tergu m overhangin g th e anus i n Lepidoptera. Vesica (Vsc). —A termina l membranou s par t o f th e aedeagus . (Preputial membrane.) Vinculum.—The entir e coxosterna l plat e o f th e nint h abdomina l segmen t i n Lepidoptera. Virga.—A terminal phalli c spine, usually arisin g from th e endophallus . Volsellae.—Ventral latera l processe s (parameres ) o f th e phallobas e i n Hy menoptera.

THE FEMAL E ORGAN S Basivalvulae.—Small sclerite s sometime s occurring at th e base s o f the firs t val vulae, often confuse d wit h the first valvifers. Egg Guide.—A median process of the subgenita l plate behind the genita l opening, particularly develope d in Acrididae. Genital Chambe r (GC). —In th e femal e a copulator y invaginatio n cavit y behin d or abov e th e eight h abdomina l sternu m containin g th e gonopor e and th e orific e of the spermatheca, often narrowed to form a pouchlike or tubular vagina. (Bursa copulatrix.) Gonapophyses (Gon). —Mesal processe s o f th e base s o f th e gonopods , perhap s endites, formin g the firs t an d second valvulae o f the ovipositor . Gonopore (Gpr). —In th e femal e either one of the paire d primitive openings of th e lateral oviducts, o r the media n opening of the oviductu s communis. Intervalvulae (iv). —Sternal sclerite s in the venter of the ninth abdominal segment between the secon d valvifers. Lancets (Let, IF/).—Th e first valvulae. Ostium bursae (ob). —The openin g of the bursa copulatrix in Lepidoptera, equiva lent to the vulv a o f female insects having the genita l opening on the eight h segment . Oviporus (op). —The posterio r opening of the vagin a in most Lepidoptera, servin g only for the discharg e of the egg s when there ar e two genital apertures . Ovipositor (Ovp). —The egg-layin g organ formed of the gonopod s of the eight h an d ninth abdomina l segments ; or also, in a functional sense, the egg-layin g tube of some insects formed o f the protractil e termina l segment s of the abdomen . Rami valvularum.—Th e proximal, , ofte n slender , part s o f th e firs t an d secon d valvulae by which the latter are attached to the valvifers. Stylet (stl). —A media n dorsal element in the shaf t o f the oviposito r formed o f th e united secon d valvulae. Subgenital Plate.—I n the femal e th e eight h abdomina l sternum, or the sevent h when the eight h is reduced or obliterated . Valvifers (Vlf). —The basa l plate s o f the ovipositor , probabl y derive d fro m th e coxopodites of the gonopods , carrying the valvulae , includin g first valvifers (IF// ) of the eight h abdomina l segment, and second valvifers (2F// ) of the nint h segment. Valvulae (VI). —The thre e pair s o f processes forming th e blade s an d ensheathing lobes of the ovipositor . Th e firs t an d second válvulae (IF/ , 2VI) ar e gonapophyse s of

THE ORGANS OF COPULATION AND OVIPOSITIO N

623

the gonopods; the third valvulae (3VI) are distal outgrowths of the coxopodites of the ninth abdominal segment . Vestibulum (Vst). —An externa l genital cavit y forme d abov e the seventh abdomi nal sternum whe n the latter extends beyond the eighth . Vulva (Vul). —The externa l openin g of the genita l chambe r or vagina servin g i n most case s for both copulation and the discharg e of the eggs ; sometimes on the eight h abdominal segment, sometimes on the ninth.

This page intentionally left blank

REFERENCES The followin g lis t o f reference s is b y n o means a bibliograph y o f th e subject s included i n th e text . I t contain s th e work s cite d an d a fe w other s o f genera l importance. ABBOTT, R . L . (1926 ) Contribution s t o th e physiolog y o f digestio n i n Periplanet a australasiae. Journ. Exp. ZooL, 44: 219-253. ALDRICH, J . M . (1922 ) A new genu s o f two-winged fly with mandible-lik e labella. Proc. Ent. Soc. Washington, 24 : 145-148. ALT, W . (1909 ) Übe r den Ba u de r Stigme n vo n Dytiscu s marginalis . ZooL Anz., 34:793-799. ————. (1912) Über das Respirationssystem von Dytiscus marginalis. Zeitschr. wiss. ZooL, 99: 357-413. ————. (1912a ) Übe r da s Respirationssyste m de r Larve vo n Dytiscu s marginalis . Ibid., 99:414-443. AST, F . (1920 ) Übe r de n feinere n Bau de r Facettenauge n be i Neuropteren . ZooL Jahrb., Anat., 41: 411-458. ATHANASIU, J., and DRAGOIU , J . (1913 ) Su r les capillaires aériens des fibres musculaires chez les insectes. C . R. Soc. BioL Paris, 75: 578-582. ————. (1915 ) La structure de s muscles striés de s insectes et leurs rapports avec les trachées aériennes . Arch. Anat. Micr., 16 : 345-361. BABÁK, E. , an d FOUSTKA , O . (1907 ) Untersuchunge n übe r de n Auslosungsrei z de r Atembewegungen be i Libellulidenlarven . Pflügers Arch. ges. PhysioL, 119 : 530-548. BACKHOFF, P . (1910 ) Di e Entwicklun g de s Copulationsapparate s vo n Agrión . Zeitschr. wiss. ZooL, 96: 647-706. BALFOUR, F . M . (1883 ) Th e anatom y an d developmen t o f Peripatu s capensis . Quart. Journ. Micr. Sci., 23: 213-259. BARRATT, J . O . W. , an d ARNOLD , G. (1911 ) A study o f the bloo d of certain Coleóptera. Quart. Journ. Micr. Sci., 56: 149-165. BAUNACKE, W . (1912 ) Statisch e Sinnesorgan e be i de n Nepiden . ZooL Jahrb., Anat., 34: 179-346. BECK, H . (1920 ) Di e Entwicklun g de s Flügelgeáder s be i Phyllodromi a (Blatta ) germánica. ZooL Jahrb., Anat., 41: 377-410. BEIER, M . (1927 ) Vergleichend e Untersuchunge n übe r da s Zentralnervensyste m der Coleopterenlarven . Zeitschr. wiss. ZooL, 130: 174-250. BENECKE, W. (1905 ) Über Bacillus chitinovorus, eine n Chitin zersetzende n Spaltpilz. Boianische Zeitung, 63: 227-242 . BERLESE, A. (1909 ) Gli insetti, vol. I , Milan . ————. (1910) Monografía dei Myrientomata. Redia, 6: 1-182. . ZeitBERNHARDS, H . (1916 ) De r Ba u de s Komplexauge s von Astacu s fluviat schr. wiss. ZooL, 116: 649-707. BETHE, A . (1896 ) Ei n Beitra g zu m Kenntni s de r periphere n Nervensystem s vo n Astacus fluviatilis . Anat. Anz., 12: 31-34. BETTS, ANNI E D . (1923 ) Practical be e anatomy. Th e Api s Club. ————. (1933) How bees fly. The Bee World, 14: 50-55. 625

626

PRINCIPLES OF INSECT MORPHOL OGY

BIEDERMANN, W. (1903 ) Geformte Secrete. Zeitschr. allg. PhysioL, 2: 395-481. BISHOP, G. A., BBIGGS , A. P., an d RONZONI , E. (1925 ) Body fluids of the hone y be e larva. II . Journ. Biol. Chem., 6: 77-88. BLACKMAN, W . M . (1912 ) O n a supernumerar y media n ocellu s i n Melanoplu s femur-rubrum. Psyche, 19 : 92-96. BLTJNCK, H . (1916 ) Da s Lebe n de s Gellbrand s (Dytiscu s marginalis ) (ohn e di e Metamorphose). Zool. Anz., 46 : 271-286, 289-300. 47: 18-31, 33-42. ————. (1916a ) Di e Metamorphose des Gelbrands. Ibid., ————. (1918 ) Di e Entwicklung des Dytiscus marginali s vo m Ei bis zur Imago . 2. Die Metamorphose . Zeitschr. wiss. Zool., 117: 1-129. BOBINE, J . H . (1926 ) Hydroge n io n concentratio n i n th e bloo d o f certain insect s (Orthoptera). Biol. Bull, 61: 363-369. BOELITZ, E . (1933 ) Beitrág e zu r Anatomi e und Histologi e der Collembolen . Zool. Jahrb., Anat., 57:375-432 . DE BOISSEZON, P . (1930 ) Contribution a Fétude de la biologic et de Thistophysiologie de Culex pipiens. Arch. Zool. Éxp. Gen., 70: 281-431. ———— . (1930a ) Le s reserves dan s l e corps gras de Culex pipiens et leur role dans la maturation de s oeufs. C . R. Soc. Biol. Paris, 103: 1232-1233. ———— . (19306 ) Le role du corps gras comme rein d'accumulation chez Culex pipiens et che z Theobaldia annulata . Ibid., 103: 1233-1235. ———— . (1932 ) Localisation d u glycogéne et du fer chez Cule x pipiens. Ibid., Ill: 866-867. BURNER, C . (1921 ) Di e Gliedmasse n de r Arthropoden . I n Lang' s Handbuch de r Morphologie de r wirbellosen Tier e, 4: 649-694. Jena . BOVINO, A . G . (1910 ) Natura l histor y o f th e larva e o f Donaciinae . Internal. Revue ges. Hydrobiol. u . Hydrogr., 1910 : 108 pp. BOVINO, A. G., and CRAIGHEAD , F. C. (1931 ) A n illustrated synopsis of the principa l larval forms o f the orde r Coleóptera. 35 1 pp. Brookly n Ent . Soc . BRACH, H . (1912 ) Untersuchunge n übe r de n chemische n Aufba ú de s Chitins . Biochem. Zeitschr., 38: 468-491. BRADLEY, J. C. (1931 ) A laboratory guid e to the study of the wing s of insects, 41 pp. Ithaca, N.Y . BRANCH, HAZE L E . (1922 ) A contribution t o th e knowledg e of the interna l anatom y of Trichoptera . Ann. Ent. Soc. America, 15: 256-275. BRAUN, M. (1912 ) Das Mitteldarmepithel de r Insektenlarven wáhrend der Háutung . Zeitschr. wiss. Zool, 103 : 115-169. BRETSCHNEIDER, F . (1921 ) Über das Gehir n de s Wolfsmilchschwármers (Deilephil a euphorbiae). Jen. Zeitschr. Naturwiss., 57 : 423-462. BROCHER, F . (1909 ) Su r l'organ e pulsatile , observ é dans les pattes des Hémiptére s aquatiques. Ann. Biol. lacustre, 4: 33-41. ————. (1916 ) Nouvelle s observation s biologique s e t physiologique s sur le s Dyticides. Arch. Zool. Exp. Gen., 55: 347-373. ———— . (1917 ) Étud e experiméntal e sur le fonctionnement du vaisseau dorsal et sur la circulatio n d u san g che z le s insectes . 1 . Dytiscu s marginalis . Ibid., 56 : 347-358. ———— . (1917a ) 2 . Les larves des Odonates. Ibid., 56 : 445-490. ———— . (1919) Les organes pulsátiles meso- et métatergaux des Lépidoptéres. Ibid., 58: 149-171 . ^ ————. (1920 ) Étud e experiméntale , etc . 3 . L e Sphin x convolvuli . Ibid., 60 : 1-45. ———— . (1921 ) 4 . La Vesp a crabro. Ann. Soc. entom. France, 89 : 209-232 . ————. (1922 ) 5. La Periplaneta orientalis. Ibid., 91: 156-164.

REFERENCES

627

BROWN, J . M . (1910 ) Som e points in th e anatom y o f the larv a o f Tipula maxima . A contributio n t o ou r knowledg e of th e respiratio n an d circulatio n o f insects . Trans. Linn. Soc. London, 2d ser., Zool. , 11: 125-135. BRÜEL, L . (1897 ) Anatomi e un d Entwicklungsgeschicht e de r Geschlechtsausführ wege sam t Annexe n vo n Calliphor a erythrocephala . Zool. Jahrb., Anat., 10 : 511-618. BRUNTZ, L . (1904 ) Contributio n a F étude d e Fexcrétio n che z le s arthropodes . Archives de Biologie, 20: 217-422. ————. (1908 ) Le s reins labiaux e t les glandes céphaliques des Thysanoures. Arch. Zool. Exp. Gen., ser. 4, 9: 195-238. VON BUDDENBROCK , W . (1930 ) Beitrag zu r Histologie un d Physiologi e der Raupen háutung, mi t besondere r Berücksichtigun g de r Versonche n Drüsen . Zeitschr. Morph. Ókol. Tiere, 18: 701-725. ————. (1931 ) Untersuchunge n libe r di e Hautungshormon e de r Schmetterlings raupen. Zeitschr. vergl. PhysioL, 14: 415-428. VON BUDDENBROCK , W. , an d VO N ROHR, G . (1923 ) Di e Atmun g vo n Dixippu s morosus. Zeitschr. allg. PhysioL, 20: 111-160. BUGNION, E . (1921 ) Hexapoda . I n Lang' s Handbuch de r Morphologic de r wirbellosen Tiere, 4: 415-586. ————. (1925 ) Nouvell e étud e de s organe s buccau x d e la Scolie . Bull. Soc. Roy. Entom. d'Égypte, 18 : 291-380. ———— —. (1927 ) Les piésce s buccales , l e sa c infrabucca l e t l e pharyn x de s fourmis . Folia Myrmecologica e t Termitologica, 1 , 91 pp . ———— -. (1929 ) Les organes bucco-pharyngés de deux Sphégiens. Bull. Soc. Entom. Suisse, 14: 139-172. ————. (1929a ) L e ver-luisant provenga l e t l a luciól e nigoise . Mem. Assoc. Nat. Nice et Alpes-marit., 13 1 pp. ————. (1930 ) Le s piésces buccales , l e sa c infrabucca l et l e pharyn x de s fourmis. Bull. Soc. Roy. Entom. d'Égypte, n.s. , 14 : 85-210. Du BUISSON , M . (1924 ) Observation s su r l a ventilatio n trachéen e de s insectes . Bull. Sci. Acad. Roy. Belgique, ser. 5, 10:373-391. ————. (1924a ) Observations su r le méchanisme de la ventilation trachéene chez les insectes. Ibid., 10 : 635-656. ————. (1926 ) Observation s su r l a ventilatio n trachéen e de s insectes. Ibid., 12 : 127-138. BULL, L . (1904 ) La chronophotographi e des mouvements rapides. Bull. Soc. Philomath. Paris, ser. 9, 6: 192-199. BURGESS, E . (1880 ) Contribution s t o th e anatom y o f th e milk-wee d butterfly , Dañáis archippus . Anniversary Memoirs Boston Soc. Nat. Hist., 1880: 16 pp. ———— —. (1883 ) Th e structur e o f the mout h i n the larv a o f Dytiscus. Proc. Boston Soc. Nat. Hist., 21: 223-228. BUSCK, A. , an d BÓVING , A . G . (1914 ) O n Mnemónic a auricyani a Walsingham . Proc. Ent. Soc. Wash., 16 : 151-163. CAESAR, J . (1913 ) De r Stirnaugen de r Ameisen. Zool. Jahrb., Anat., 36: 161-242. CAJAL, S. R. (1918 ) Observacione s sobre l a estrutur a d e los ocelos y vias nerviosa s ocelares de algunos insectos. Trabajos Lab. Invest. Biol. Univ. Madrid, 16: 109-139. CAJAL, S . R., and SÁNCHEZ , D . (1915) Contribució n a l conocimient o d e los centros nerviosos de los insectos. Trabajos Lab. Invest. Biol. Univ. Madrid, 13: 1-164. CAMPBELL, F . L . (1929 ) Th e detectio n an d estimatio n o f insec t chitin ; an d th e irrelation o f chitinization t o hardnes s an d pigmentatio n o f the America n cock roach, Periplanet a americana . Ann. Ent. Soc. America, 22: 401-426.

628

PRINCIPLES OF INSECT MORPHOLO GY

GARRIERE, J. , an d BURGER , O . (1897 ) Die Entwicklungsgeschichte der Mauerbien e (Chalicodoma muraría) im Ei. Nova Acta Leop.-Carol. Deut. Akad., 69: 253-420. CASPER, A . (1913 ) Di e Korperdeck e un d di e Drüse n vo n Dytiscu s marginalis, ei n Beitrag zur feineren Bau der Insektenkórpers. Zeitschr. wiss. ZooL, 107: 387508. CECIL, R . (1930 ) Th e alimentar y cana l o f Philaenu s leucopthalmus . Ohio Joum. Sci., 30: 120-128. CHAPMAN, R. N. (1918 ) The basal connections of the trachea e of the wing s of insects. In Comstock' s Th e wings o f insects, pp. 27-51 . CHILD, C . M . (1894 ) Beitrág e zu r Kenntni s de r antennale n Sinnesorgan e de r Insekten. Zeitschr. wiss. Zool., 58: 475-528. CHILDS, L . (1914 ) Th e anatom y o f th e diaspin e scal e insect , Epidiaspi s piricola . Ann. Ent. Soc. America, 7: 47-57. CHINA, W. E. (1931 ) Morphologica l parallelism in the structur e of the labiu m in th e hemipterous gener a Coptosomoide s and Boziu s in connectio n with mycetopha gous habits. Ann. Mag. Nat. Hist., ser. 10 , 7: 281-286. CHOLODKOWSKY, N . (1891 ) Di e Embryonalentwicklung von Phyllodromi a (Blatta ) germánica. Mém. Acad. Sci. St. Pétersbourg, ser. 7, 38, No. 5 , 120 pp. ————. (1904 ) Zur Morphologie der Pediculiden. Zool. Am., 27: 120-125. CLAUSEN, C . P . (1932 ) Th e biolog y o f Encyrtu s infidus , a parasit e o f Lecanium kunoensis. Ann. Ent. Soc. America, 25: 670-686. CLAYPOLE, AGNE S MARY . (1898 ) Th e embryolog y and oogénesi s of Anurid a marítima. Journ. Morph., 14: 219-300. COMSTOCK, J . H . (1918 ) Th e wing s of insects. Ithaca , N . Y . COMSTOCK, J. H., and NEEDHAM , J. G . (1898 , 1899 ) The wings of insects. American Nat., 32, 33. CONSTANTINEANU, M . J . (1930 ) Di e Aufba u de r Sehorgan e bei de n i m Süsswasse r lebenden Dipterenlarve n un d be i Puppe n un d Imagine s vo n Culex . Zool. Jahrb., Anat., 52 : 251-346. CORNELI, W . (1924 ) Vo n de m Aufba u de s Sehorgan s de r Blattwespenlarve n un d der Entwicklun g des Netzauges. Zool. Jahrb., Anat., 46: 573-608. CRAGG, F . W . (1912 ) Studie s o n th e mouth-part s an d suckin g apparatu s i n th e blood-sucking Diptera . Scient. Mem., Med. an d Sanit. Dept. Gov. India, n.s. , No. 54, 17pp. ———— . (1920 ) Secretion and epithelial regeneration in the mid-intestine of Tabanus. Ind. Journ. Med. Res., 7: 648-663. CRAMPTON, G . C . (1914 ) Th e groun d plan o f a typica l thoracic segment in winged insects. ZooL Anz., 44 : 56-67. ————. (1917 ) The nature o f the veracervi x o r neck region in insects. Ann. Ent. Soc. America, 10: 187-197. ———— . (1923 ) A comparison of the labiu m i n certai n holometabolous insects fro m the standpoin t o f phylogeny. Proc. Ent. Soc. Washington, 25 : 171-180. ———— . (1925 ) A phylogeneti c stud y o f the thoraci c sclerite s o f th e non-tipuloi d nematocerous Diptera. Ann. Ent. Soc. America, 18: 49-67. ————. (1925a ) Evidenc e o f relationship indicated by the thoraci c sclerite s of certain eriopterine Diptera. Insecutor Inscitiae Menstruus, 13 : 197-213. ———— . (19256 ) A phylogenetic study o f the labiu m of holometabolous insects, with particular referenc e t o the Diptera . Proc. Ent. Soc. Washington, 27 : 68-91. ————. (1926 ) A comparison of the nec k and prothoraci c sclerites throughou t th e orders of insects fro m th e standpoin t of phylogeny. Trans. American Ent. Soc., 52: 199-248.

REFERENCES

629

. (1928 ) Th e eulabium , mentum , submentu m an d guia r regio n o f insects . Journ. Ent. an d Zool., 21: 1-15 . ———— (1932) A phylogeneti c stud y o f th e hea d capsul e i n certain orthopteroid , psocid, hemipteroi d an d holometabolou s insects . Bull. Brooklyn Ent. Soc., 27 : 19-49. CRAMPTON, G . C. , and HASEY , W. H. (1915 ) The basal sclerites of the le g in insects . Zool Jahrb., Anat., 39: 1-26. CUÉNOT, L . (1891 ) Etude s su r l e san g e t le s glande s lymphatique s dan s l a seri e animale. Arch. Zool. Exp. Gen., ser. 2, 9: 13-90, 365-475, 593-670 . ————. (1896 ) Etudes physiologique s sur les Orthoptéres. Arch. BioL, 14: 293-341. DAVIDSON, J . (1925 ) Biologica l studies o f Aphis rumiéis. Ann. Applied BioL, 12: 472-507. DAVIES, W . M . (1927 ) On the trachea l syste m o f Collembola, with specia l referenc e to that of Sminthurus viridis. Quart. Journ. Micr. Sci., 71: 15-30. DAVIS, A. C. (1927 ) Studies o n the anatom y an d histolog y of Stenopelmatus fuscus . Univ. o f Calif. Pubs, in Ent., 4: 159-208. DAWYDOFF, C . (1928 ) Traite d'embryologie comparée des Invertébrés . Paris . DEEGENER, P . (1904 ) Entwicklun g de s Darmkanal s de r Insekte n wáhren d de r Metamorphose. Zool. Jahrb., Anat., 20: 499-676. ————. (1910 ) Beitrág e zu r Kenntnis der Darmsekretion. II . Macrodyte s (Dytis cus) circumcinctus . Archiv Naturg., 76: Bd. 1 , Heft 2: 27-43. VON DEHN , MADELEINE . (1933 ) Untersuchunge n übe r Bildun g de r peritropische n Membran bei den Insekten. Zeitschr. Zellforsch. u. mikr. Anat., 19: 79—105. DEMANDT, C . (1912 ) De r Geschlechtsappara t vo n Dytiscu s marginalis . Zeitschr. wiss. Zool, 103 : 171-299. DEMOLL, R. (1918 ) De r Flu g der Insekte n un d de r Vógel . 6 7 pp. Jena . ———— -. (1927 ) Die Atmung der luftatmenden Insekten . Zool. Anz., 69 : 8-16. ———— . (1927a ) Untersuchunge n über die Atmung der Insekten . I . Zeitschr. BioL, 86:45-66. ————. (1928 ) II. Ibid., 87: 8-22 . ————. (1928o ) III . Zool. Jahrb. allg. Zool., 45: 515-534. DIETRICH, W . (1909 ) Di e Facettenauge n de r Dipteren . Zeitschr. wiss. Zool., 92 : 465-539. DIRKS, ELIZABETH . (1922 ) Liefer n di e Malpighische n Gefáss e Verdauungssekret e ? (Fermeiitstudien an Insekten.) . Arch. Naturg., 88 : Abt. A, Heft 4 : 161-220. DTJNAVAN, D . (1929 ) A study o f respiration an d respirator y organ s of the rat-taile d maggot, Eristalis arbustorum. Ann. Ent. Soc. America, 22: 731-739. Du PORTE , E . M . (1918 ) O n th e structur e an d functio n o f th e proventriculu s of Gryllus pennsylvanicus. Psyche, 25 : 117—121 . EASTHAM, L . E . S . (1925 ) Peristalsis i n th e Malpighia n tubule s o f Diptera , wit h a note o n th e eliminatio n o f calciu m carbonat e fro m th e Malpighia n tubule s o f Drosophila funebris . Quart. Journ. Micr. Sci., 69: 385-398. ————. (1927 ) A contributio n t o th e embryolog y o f Pieri s rapae . Quart. Journ. Micr. Sci., 71:353-394. ————. (1929 ) The post-embryoni c developmen t o f Phaenoserphus viator , a para site o f the larv a o f Pterosticu s niger , wit h note s o n the anatom y o f the larva . Parasitology, 21 : 1-21 . ————. (1930 ) Th e formatio n o f the ger m layer s i n insects. BioL Rev., 5: 1-29 . EGGERS, F . (1923 ) Ergebniss e vo n Untersuchunge n a m Johnstonche n Orga n de r Insekten un d ihr e Bedeutun g fu r di e allgemein e Beurteilung de r stiftführenden Sinnesorgane. Zool. Anz., 57: 224-249 .

————

630

PRINCIPLES OF

INSECT MORPHOLOGY

. (1924 ) Zu r Kenntni s de r antennale n Stiftführende n Sinnesorgan e de r Insekten. Zeitschr. Mor ph. Okol. Tier e, 2: 259-349. ————. (1929 ) Di e stiftführende n Sinnesorgane . Morphologi c un d Physiologi c der tympanale n Sinnesapparat e de r Insekten . Zool. Bausteine —Gesamtgebiet der Zoologie, 2: 353 pp. EIDMANN, H . (1922 ) Di e Duchlássigkei t de s Chitin s be i osmotische n Vorgangen . Biol. Zentrbl, 42: 429-435. ———— . (1924 ) Untersuchungen über die Morphologic und Physiologi c des Kaumagens von Periplanet a orientalis . Zeitschr. wiss. Zool., 122: 281-307. ————. (1924a ) Untersuchunge n übe r Wachstu m un d Háutun g de r Insekten. Zeitschr. Morph. Okol. Tiere, 2: 567-610. ————. (19246 ) Untersuchunge n übe r de n Mechanismu s de r Háutun g be i de n Insekten. Archiv mikr. Anat. u. Untwicklungsmechanik, 102 : 276-290. ———— . (1925 ) Vergleichenden-anatomisch e Studie n übe r di e Pharynxmuskulatur der Insekten. Zool. Am., 62: 49-64. ———— . (1929 ) Morphologisch e und physiologisch e Untersuchunge n am weiblichen Genitalapparat de r Lepidopteren . I . Morphologische r Teil . Zeitschr. ang. Entom., 16: 1-66 . ELLSWORTH, J . K . (1933 ) Th e photoreceptiv e organ s o f a fles h fl y larva , Lucili a sericata. Ann. Ent. Soc. America, 26: 203-214. EMERY, C . (1888 ) Übe r de n sogenannte n Kaumage n einige r Ameisen . Zeitschr. wiss. Zool., 46: 378-412. ENDERLEIN, G . (1905 ) Übe r di e Morphologic , Klassifikatio n un d systematisch e Stellung de r Anopluren nebst Bemerkungen zur Systematik der Insektenordnungen. Zool. Anz., 28 : 121-147. ————. (1905a ) Zu r Morphologic des Láusekopfes. Zool. Anz., 28: 626-638. ENGEL, E . O . (1924 ) Da s Rectu m de r Diptere n i n morphologische r un d histolo gischer Hinsicht . Zeitschr. wiss. Zool., 122: 503-533. ERTOGROTJL, T. (1929 ) Sur Torigin de la membrane péritrophique che z le ver á soie. C. R. Acad. Sci. Paris, 188: 652-654. ESCHERICH, K . (1902 ) Zu r Entwicklun g de s Nervensystem s de r Musciden , mi t besonderer Berücksichtigun g de s sog. Mittelstranges . Zeitschr. wiss. Zool., 71 : 525-549. EVENIUS, CHRISTA . (1933 ) Übe r di e Entwicklun g de r Rektaldruse n vo n Vesp a vulgaris. Zool. Jahrb., Anat., 66: 349-372. EWING, H. E. (1928 ) The legs and leg-bearing segments of some primitive arthropo d groups, wit h note s o n leg-segmentatio n i n th e Arachnida . Smithsonian Misc. Coll., 80: No. 11, 41pp. EWING, H . Z . (1904 ) Th e functio n o f th e nervou s system , wit h specia l regar d t o respiration, i n Acrididae. Kan. Univ. Sci. Bull., 2: 305-319. FAUSSEK, V. (1887 ) Beitráge zur Histologie de s Darmkanals der Insekten. Zeitschr. wiss. Zool., 45: Q94r-712. FERNANDO, W . (1933 ) Th e developmen t an d homologie s of the mouth-part s o f th e head-louse. Quart. Journ. Micr. Sci., 76: 231-241. FERRIS, G . F . (1931 ) Th e lous e o f elephants , Haematomyzu s elephantis . Parasitology, 23: 112-127 . FLETCHER, F . W . (1930 ) Th e alimentar y cana l o f Phyllophag a gracilis . Ohio Journ. Sci., 30: 10&-117 . FLÓGEL, J . H . L . (1905 ) Monographi c de r Johannisbeeren-Blattlaus , Aphi s ribes . Zeitschr. wiss. InsektenbioL, 1: 49-63. FLORENCE, LAURA. (1921 ) The ho g louse, Haematopinu s suis : its biology , anatomy, and histology. Cornell Univ. Agrie. Exp. Sta., Memoir 61: 637-742. ————

REFERENCES

631

FOLSOM, J . W. , an d WELLES , MIRIA M U . (1906 ) Epithelia l degeneration , regenera tion, an d secretio n in th e mid-intestin e o f Collembola. Univ. of Illinois Studies, 2, No . 2 , 31 pp. FORBES, W . T . M . (1933 ) Th e axillar y venatio n o f the insects . V e Cong. Internal. Entom., 1932: 277-284. FORD, NORMA . (1923 ) A comparativ e stud y o f th e abdomina l musculatur e o f orthopteroid insects . Trans. Roy. Canadian Inst., 14 : 207—319. Fox, H. M. (1921 ) Method s of studying the respirator y exchang e in smal l aquati c organisms, wit h particula r referenc e t o th e us e o f flagellate s a s a n indicato r fo r oxygen consumption . Journ. Gen. Physiol., 3: 565-573. FRAENKEL, G . (1932 ) Da s proble m de s gerichtete n Atemstrome s i n de n Trachee n der Insekten . Zeitschr. vergl. Physiol., 16 : 418-443. ————. (1932a ) Untersuchunge n libe r di e Koordinatio n vo n Reflexe n un d auto matische-nervosen Rhyth m en bei Insekten. Ibid., 16 : 444-462 . FREILING, H . H . (1909 ) Duftorgane der weibliche n Schmetterlinge nebs t Beitráge n zur Kenntni s de r Sinnesorgan e au f de n Schmetterlingsflüge l un d de r Duftpinse l der Mánnche n von Dañái s un d Euploea . Zeitschr. wiss. Zool., 92: 210-290. FREUDENSTEIN, K . (1928 ) Da s Her z un d da s Circulationssyte m de r Honigbiene . Zeitschr. wiss. Zool., 132 : 404-475. FREW, J . G . H . (1923 ) On the morpholog y of the hea d capsul e an d mout h part s of Chlorops taeniopu s (Diptera) . Journ. Linn. Soc. London, Zool., 35: 399-410. FRIELE, ALWINE . (1930 ) Di e postembryonal e Entwicklungsgeschicht e de r rnann lichen Geschlechtsorgan e un d Ausführungsweg e von Psychoda alternata . Zeitschr. Morph. Ókol. Tiere, 18: 249-288. FULMEK, L . (1906 ) Beitrág e zu r Kenntni s de s Herzen s de r Mallophagen . Zool. Am., 29: 619-621. FULTON, B . B . (1933 ) Note s o n Habrocytu s cerealellae , parasit e o f the Angoumoi s grain moth . Ann. Ent. Soc. America, 26: 536-552. VAN GEHUCHTEN , A . (1890 ) Recherche s histologique s su r Fapparei l digesti f d e l a larve d e l a Ptychopter a contaminata . I . Étud e d u revétemen t épithélial e e t recherches sur la secretion . L a Cellule, 6: 185-289. GERBIG, F . (1913 ) Übe r Tipulidenlarve n mi t besondere r Berücksichtigun g de r Respirationsorgane. Zool. Jahrb., Syst., 3 5 :127—184. GEROULD, J. H. (1929 ) Periodic reversal of the heart action in the silkworm moth an d pupa. Journ. Morph., 48 : 385-429. GEYER, K . (1913 ) Untersuchunge n übe r di e chemisch e Zusammensetzun g de r Insektenhamolymphe und ihre Bedeutung fur die geschlechtliche Differenzierung . Zeitschr. wiss. Zool., 105: 349-499. GILMER, P . M . (1925 ) A comparative stud y o f the poiso n apparatu s o f certain lepi dopterous larvae . Ann. Ent. Soc. America, 18: 203-239. GLASER, R . W . (1912 ) A contribution t o ou r knowledg e of the functio n of the oeno cytes of insects. Biol. Bull, 23: 213-224. ———— . (1925 ) Hydroge n io n concentratio n i n th e bloo d o f insects . Journ. Gen. Physiol, 7 : 599-602. GLASGOW, H . (1914 ) Th e gastri c caec a an d th e caeca l bacteri a o f th e Hemiptera . Biol. Bull, 26: 101-156. GONELL, H . W . (1926 ) Rontgenographisch e Studie n a n Chitin . Zeitschr. physiol. Chem., 152 : 18-30 . GONIN, J . (1894 ) Recherche s su r le s metamorphoses de s Lépidoptéres. Bull. Soc. Vaudoise Sci. Nat., 31: 87-139. VON GORKA , E . (1914 ) Experimentelle un d morphologisch e Beitrág e zur Physiologi e der Malpighische n Gefáss e de r Kafer , Zool. Jahrb., Zool. Physiol., 34: 233-338 ,

632

PRINCIPLES OF

INSECT MORPHOLOGY

GRAHAM-SMITH, G . S. (1930 ) Further observation s on the anatom y an d function s of the probosci s of the blow-fly . Parasitology, 2 2 : 47-115. GBASSI, B . (1886 ) I progen i tori degl i insett i e dei miriapodi. L'Japy x e la Com podea. Atti Accad. Gioenia Sci. Nat. Catania, ser. 3, 19: 1-83 . ————. (1887 ) Anatomí a comparat a de i Tisanur i e considerazion i general i sul Forganizzazione degli insetti. Atti de 'Lincei, Mem. Cl. Sci, Fis., ser. 4,4: 543-606. GRÜNBERG, K. (1903 ) Untersuchungen über die Keim- und Náhrzelle n in den Hoden und Ovarie n der Lepidopteren. Zeitschr. wiss. Zool., 74: 327-395. GÜNTHER, K . (1901 ) Übe r Nervendigunge n au f de m Schmetterlingsflügel . Zool. Jahrb., Anat., 14: 551-572. ————. (1912 ) Di e Sehorgan e de r Larv e un d Imag o vo n Dytiscu s marginalis . Zeitschr. wiss. ZooL, 100: 60-115. HAFFER, O . (1921 ) Ba u un d Funktio n de r Sternwarze n vo n Saturni a pyr i Schiff , und di e Haarentwicklun g de r Saturnidenraupen . Archiv Naturg., 87 , Abt . A , Heft 1 : 110-166. HAMILTON, M . A . (1931 ) Th e morpholog y o f th e water-scorpion , Nep a cinérea . Proc. Zool. Soc. London, 1931: 1067-1136. HANSEN, H . J . (1903 ) Th e mouth-part s o f Glossin a an d Stomoxys . I n E . E . Austen's A monograph o f th e tsetse-flies, pp . 105-120 . London . ————. (1930 ) Studie s o n Arthropoda , III . O n th e comparativ e morpholog y of th e appendage s in th e Arthropoda . 37 6 pp. Copenhagen . HANSTROM, B . (1925 ) Compariso n betwee n th e brai n o f th e caterpilla r an d th e imago in Pieris brassicae . Entom. Tidskr., 46: 43-52. ———— . (1926 ) Eine genetische Studi e übe r di e Augen und Sehzentre n vo n Turbel larien, Annelide n un d Arthropoden . Kungl. Svenska Vetensk. Akad. HandL, ser. 3 , 4 , No . 1 , 176 pp. ———— . (1927 ) Da s zentral e un d peripher e Nervensyste m de s Kopflappen s einige r Polycháten. Zeitschr. Morph. Okol. Tiere, 1: 543-596. ———— . (1928 ) Vergleichend e Anatomic de s Nervensystem s de r wirbellose n Tier e unter Bemcksichtigun g seiner Funktion . Berlin . ————. (1928a ) Di e Beziehunge n zwische n de m Gehir n de r Polycháte n un d de m der Arthropoden . Zeitschr. Morph. Ókol. Tiere, 11: 152-160. ———— . (1929 ) Weiter e Beitrág e zu r Kenntni s de s Gehirn s un d de r Sinnesorgan e der Polycháten . Ibid., 13: 329-358. ———— . (1930 ) Übe r da s Gehir n vo n Termopsi s nevadensi s un d PhyIliu m pulcri folium nebs t Beitráge n zu r Phylogeni e de r Corpor a pedunculat a de r Arthro poden. Ibid., 19:732-773 . HARRISON, L . (1914 ) A preliminar y accoun t o f th e structur e o f th e mouth-part s in the body-louse . Proc. Cambridge Phil. Soc., 13: 207-226 . HASEMAN, L . (1910 ) Th e structur e an d metamorphosi s o f th e alimentar y cana l of th e larv a o f Psychoda alternata . Ann. Ent. Soc. America, 3: 277-308. HEADLEE, T. J . (1906 ) Bloo d gills of Simulium pictipes . Amer. Nat., 40 : 875-885. HEGNER, R . W . (1914 ) Studie s o n ger m cells , I . Th e histor y o f th e ger m cell s in insect s wit h specia l referenc e t o th e Keimbahn-determinants . II . Th e origin an d significanc e o f th e Keimbahn-determinant s i n animals . Journ. Morph., 25:375-509 . HEIDER, K . (1925 ) Übe r Eunice . Systematisches , Kiefersack , Nervensystem . Zeitschr. wiss. Zool., 125: 55-90. HENNEGUY, F . (1906 ) Le s mode s d'insertio n de s muscle s su r l a cuticul e che z le s arthropodes. C . R. Assoc. Anat., 8th reunion (Bordeaux) : 133-140. HENSON, H . (1930 ) O n th e developmen t o f th e mid-gu t i n th e larva l stage s o f Vanessa urticae. Quart. Journ. Micr. Sci., 73: 87-105.

REFERENCES

633

-. (1931) The structure and post-embryonic development of Vanessa urticae. I. Th e larval alimentary canal . Ibid., 74 : 321-360. ————. (1932 ) Th e developmen t o f th e alimentar y cana l i n Pieri s brassica e an d the endoderma l origin of the Malpighia n tubule s o f insects. Ibid., 7 6 : 283-305. BERBER, E . C. , and SLIFER , E . H . (1928 ) The regularit y o f respiratory movement s of Locustidae . Physiol Zool., 1: 593-602. HESS, W . N . (1917 ) Th e chordotona l organ s an d pleura l disc s o f Cerambyci d larvae. Ann. Ent. Soc. America, 10 : 63-74. HESSE, R . (1901 ) Untersuchunge n übe r di e Organ e de r Lichtempfindun g be i niederen Thieren . VIII . Vo n de n Arthropoden-Augen . Zeitschr. wiss. Zool., 70: 347-473. HEYMONS, R . (1892 ) Di e Entwicklun g de r weibliche n Geschlechtsorgan e vo n Phyllodromia (Blatta ) germánica . Zeitschr. wiss. Zool., 53: 434-536. ———— . (1895 ) Di e Embryonalentwicklun g vo n Dermaptere n un d Orthopteren . 136 pp. Jena . ————— (1897) Entwicklungsgeschichtliche Untersuchungen an Lepisma saccharina. Zeitschr. wiss. Zool., 62 : 583-631. ———— . (1899 ) Beitrág e zu r Morphologie un d Entwicklungsgeschicht e de r Rhyn choten. Nova Acta. Leop-Carol. Deut. Akad., 74 : 349-456. ———— . (1899a ) Übe r bláschenformig e Organ e be i de n Gespenstheuschrecken . Ein Beitrag zu r Kenntni s de s Eingeweidenervensystem s be i de n Insekten . Sitzungsbr. K. Preuss. Akad. Wiss. Berlin, 1899 : 563-575. ———— . (1901 ) Di e Entwicklungsgeschicht e de r Scolopender . Zoológica. OrigAbh. Gesamtg. Zool., 33: 244 pp. HICKERNELL, L . M . (1920 ) Th e digestiv e syste m o f th e periodica l cicada , Tibice n septendecim. Ann. Ent. Soc. America, 13: 223-242 . HILTON, W . A . (1902 ) The bod y sens e hair s o f lepidopterou s larvae . Amer. Nat., 36: 561-578. HOCHREUTHER, R . (1912 ) Di e Hautsumesorgan e vo n Dytiscu s margirialis , L. , ih r Bau und ihr e Verbreitun g a m Korper . Zeitschr. wiss. Zool., 103 : 1-114. HOFER, B . (1887 ) Untersuchunge n übe r de n Ba u de r Speicheldrüse n un d de s dazu gehorenden Nervenapparat s vo n Blatta . Nova Acta Leop-Carol. Deut. Akad., 51: 345-395. HOFFMANN, R . W . (1905 ) Übe r de n Ventraltubu s vo n Tomoceru s plumbeu s un d seine Beziehung zu den grossen unteren Kopfdrüsen. Zool. Anz., 28: 87-116. ————. (1911 ) Zu r Kenntni s de r Entwicklungsgeschicht e de r Collembolen . Ibid., 37: 353-377. HOLLANDE, A . C . (1909 ) Contributio n a F etude d u san g de s Coleópteros . Arch. Zool. Exp. Gen., ser. 5 , 2: 271-294. ————. (1911 ) Étude histologique comparé e du sang des insectes a hémorrhée et des insectes san s hémorrhée . Ibid., 6 : 283-323. ———— . (1914 ) Formation s endogéne s de s cristalloide s albuminoide s e t de s urate s des cellule s adipeuse s de s chenilles d e Vanessa i o et Vaness a urticae . Ibid., 53 : 559-578. ———— . (1914a ) Les cérodécytes ou oenocytes de s insectes. Arch. Anat. Micr., 16 : 1-66. ———— . (1922 ) La cellule péricardiale de s insectes. Ibid., 18 : 85-307. HOLMGREN, E . (1896 ) Di e tracheale n Endverzweigunge n be i de n Spinndrüse n de r Lepidopterenlarven. Anat. Anz., 11: 340—346. ————. (1896a ) Übe r da s respir a to rische Epithe l der Tracheen be i Raupen. Zool. Studies Festsk. W . Lilljeborg, pp . 79-96 . Upsala . ————

634

PRINCIPLES OF INSECT MORPHOLOGY

HOLMGREN, N . (1896 ) Zu r Kenntni s de r Hautnervensystem s de r Arthropoden . Anat. Anz., 12: 449-457. ————. (1902 ) Übe r di e morphologisch e Bedeutun g de s Chitin s be i de n Insekten . Ibid., 21 : 373-378. ————. (1916 ) Zur vergleichenden Anatomie des Gehirns vo n Polychaeten, Onycho phoren, Xiphosuren , Arachniden , Grustaceen , Myriopoden , un d Insecten . Kungl. Svenska Vetensk. Akad. HandL, 56 , No. 1 , 303 pp . HOLSTE, G . (1910) Da s Nervensysten i vo n Dytiscu s marginalis . Zeitschr. wiss. ZooL, 96: 419-476. ———— . (1923 ) Das Gehir n von Dytiscu s marginalis . Ibid., 120 : 251-280. HOOP, M . (1933 ) Háutungshistologi e einige r Insekten . ZooL Jahrb., Anat., 57 : 433-464. HOVENER, MARIA . (1930 ) De r Darmtractu s von Psychod a alternata Say , un d sein e Anhangsdrüsen. Zeitschr. Morph. Ókol. Tiere, 18: 74—113. HUETTNER, A . F . (1923 ) Th e origi n o f th e ger m cell s i n Drosophil a melanogaster . Journ. Morph., 37 : 385-419. HuGHES-ScHRADER, SALLY . (1930 ) Contributions t o th e lif e histor y o f th e iceryin e coccids, wit h specia l referenc e t o parthenogenesi s an d hermaphroditism . Ann. Ent. Soc. America, 23: 359-380. HUNGERFORD, H. B. (1919 ) The biolog y and ecolog y of aquati c and semiaquati c Hemiptera. Kansas Univ. Sci. Bull., 11: 1-328. IMMS, A . D . (1907 ) O n th e larva l an d pupa l stage s o f Anophele s maculipennis . Journ. Hygiene, 7: 291-318. ———— . (1931) Recent advances in entomology. Philadelphia. ———— . (1931a ) Recen t researc h o n th e wing-venatio n o f insects . Ent. Monthly Mag., 67 : 145-148. ———— . (1934 ) A general textbook o f entomology, 3 d ed. Londo n an d Ne w York . INGENITZSKY, J. (1894 ) Zur Kenntnis der Begattungsorgane de r Libelluliden. ZooL Anz., 16: 405-407. ISHIMORI, N . (1924 ) Distribution of the Malpighia n vessel s i n the wall of the rectu m of lepidopterou s larvae . Ann. Ent. Soc. America, 17 : 75-84. ISSIKI, S . T . (1931 ) O n th e morpholog y an d systematic s o f Micropterygida e o f Japan an d Formosa . Proc. ZooL Soc. London, 1931: 999-1039. ———— . (1933 ) Morphologica l studie s o n th e Panorpida e o f Japa n an d adjoinin g countries. Japanese Journ. ZooL, 4: 315-416. ITO, H . (1918 ) O n th e glandula r natur e o f th e corpor a allat a o f th e Lepidoptera . Bull. Imp. Tokyo Sericultural College, 1 , No. 4 : 63-103. JAHN, LYDI A A . (1930 ) Th e interna l anatom y o f th e m y das fly . Ohio Journ. Sci., 30:85-94. JANET, C . (1894 ) Su r l e systém e glandulair e de s fourmis. C . R. Acad. Sci. Paris, 118: 989-992. ———— . (1898 ) Systém e glandulair e tégumentair e d e la Myrmic a ruba : observation s diverses su r les fourmis. 30pp . Paris . ————• . (1898o) Sur les limites morphologiques des anneaux du tegument et sur la situation des membranes articulaires chez les Hyménoptéres arrive s a état d'imago. C. R. Acad. Sci. Paris, 126 : 435-438. ————— . (1902) Anatomie du gaster de la Myrmica rubra. 68 pp. Paris. ———— . (1906 ) Su r un orga n no n décri t d u thora x de s fourmis ailées. C. R. Acad. Sci. Paris, 14 3 : 522-523. ———— . (1907 ) Anatomi e d u corsele t e t histolys e de s muscle s vibrateurs , apré s l e vol nuptial, che z l a rein e de la fourmi . 14 9 pp. Limoges ,

REFERENCES

635

. (1911) Su r ¡'existenc e d'u n organ e chordotona l e t d'un e vésicul e pulsatil e antennaire che z Fabeill e e t su r l a morphologi c d e l a tet e d e cett e espéce . U Apiculture, 55: 181-183. JOB LING, B. (1928 ) The structure of the head and mouth parts in Culicoides pulicaris. Bull Ent. Research, 18: 211-236. ———— . (1933 ) A revisio n o f th e structur e o f th e head , mouth-par t an d salivar y glands of Glossina palpalis . Parasitology, 24 : 449-490. JOHNAS, W . (1911 ) Da s Facettenaug e de r Lepidopteren . Zeitschr. wiss. ZooL, 97 : 218-261. JUNKER, H . (1923 ) Cytologisch e Untersuchunge n a n de n Geschlechtsorgane n de r halbzwitterigen Steinflieg e Perl a marginata . Arch. Zellf., 17 : 185-359. KAPZOV, S. (1911 ) Untersuchungen über den feineren Ba u der Cuticula bei Insekten . Zeitschr. wiss. ZooL, 98: 297-337. KEILIN, D . (1916 ) Su r l a viviparit é che z le s Diptére s e t su r les larves de Diptére s vivipares. Arch. ZooL Exp. Gen., 55: 393-415. ————. (1917 ) Recherche s su r le s Anthomyides á larves carnivores . Parasitology, 9: 325-450. ————. (1924 ) On the nephrocytes in the larva and pupa of Lonchaea chorea. Ann. Mag. Nat. Hist., ser. 9 , 13: 219-223. ———— — . (1924a) On the appearance of gas in the tracheae of insects. Proc. Cambridge Phil. Soc. (BioL), 1: 63-70. KEIM, W . (1915 ) Da s Nervensyste m vo n Astacu s fluviatilis . Zeitschr. wiss. ZooL, 113: 485-545. KEMNER, N . A. (1918 ) Vergleichende Studien über das analsegment und das Pygopodium einige r Koleopterenlarven . 10 4 pp. Upsala . KEMPER, H . (1932 ) Beitráge zu r Biologi e der Bettwanze . III. Übe r den Mechan ismus des Stech-Saugactes. Zeitschr. Morph. OkoL Tiere, 24: 491-518. KENNEDY, C . H . (1917 ) Note s o n th e pene s of damselflies . Ent. News, 28 : 9-14 , 289-294. ———— (1922) Th e morpholog y o f th e peni s i n th e genu s Libellula . Ibid., 33 : 33-40. ———— . (1922a ) Th e homologie s of the trachea l branche s in th e respirator y syste m of insects . Ohio Journ. Sci., 22 : 84—89 . ————. (1927 ) The exoskeleto n as a facto r i n limitin g an d directin g th e evolutio n of insects . Journ. Morph., 44 : 267-312. ———— —. (1927a ) Som e non-nervous factor s tha t conditio n sensitivit y o f insect s t o moisture, temperature , light an d odors. Ann. Ent. Soc. America, 20: 87-106. KERN, P . (1912 ) Übe r di e Fortpflanzun g un d Eibildun g be i einige n Caraben . ZooL Am., 40: 345-351. KERSHAW, J . G . C . (1913 ) Anatomica l note s o n a membracid . Ann. Soc. Entom. Belgique, 57: 191-201. KEUCHENIDS, P . E . (1913 ) Th e structur e o f th e interna l genitali a o f som e mal e Diptera. Zeitschr. wiss. ZooL, 105: 501-536. KNABEN, N . (1931 ) Spermatogenes e be i Tischeri a angusticolella . Zeitschr. Zellforsch. u . mik. AnaL, 13: 290-323. KNOWLTON, G . F . (1925 ) The digestiv e trac t o f Longistigma caryae . Ohio Journ. Sci., 25:244-250. KOCH, C . (1932 ) De r Nachwei s des Chitin s i n tierische n Skeletsubstanzen . Zeitschr. Morph. OkoL Tiere, 25: 730-756. KÓHLER, W . (1932 ) Di e Entwicklun g de r Flüge l be i de r Mehlmott e Ephesti a kühniella Zeller , mi t besondere r Berücksichtigung de s Zeichnungsmusters . Zeitschr. Morph. OkoL Tiere, 24: 582-681. ————

636

PRINCIPLES OF

INSECT MORPHOLOGY

KORSCHELT, E . (1924 ) Bearbeitun g Einheimische r Tiere . Dytiscu s marginalis . Leipzig. KOWALEVSKY, A . (1883 ) Embryogéni e d u Chito n polii . Ann. Mus. Hist. Nat. Marseille, Zool. , 1 , 46 pp . ———— . (1892 ) Sur le s organes excréteurs che z Je s Arthropodes terrestres. Congres Internal. Zool., 2d sess., Moscow , part 1 : 187-235. KROGH, A . (1913 ) O n th e compositio n o f th e ai r i n th e trachea l syste m o f som e insects. Skand. Arch. Physiol, 29 : 29-36. ———— . (1920 ) Studie n übe r Tracheenrespiration . II . Übe r Gasdift'usio n i n de n Tracheen. Pflügers Arch. ges. Physiol., 179 : 95-112. ————. (1920a ) III . Di e Kombination vo n mechanischer Ventilatio n mit Gasdiffu sion nach Versuchen an Dytiscuslarven . Ibid., 179 : 113—120 . KUHL, W . (1924 ) De r feinere Ba u des Circulationssystems von Dytiscus marginalis. Zool. Jahrb., Anat., 46: 75-198. KÜHNELT, W . (1928 ) Ei n Beitra g zu r Histochemi e de s Insektenskeletts . Zool. Anz., 75: 111-113. ———— (1928a) Über den Bau des Insektenskeletts. Zool. Jahrb., Anat, 50: 219-278. KUNICKE, G . (1926 ) Chitin und Chitinseide . Kunstseide, 8: 182-183. LAMEERE, A . (1922 ) Su r l a nervatio n alair e de s insectes . Bull. Sci., Acad. Roy. Belgique, ser. 5, 8: 138-149. LANDOIS, H . (1867 ) Di e Ton - un d Stimmapparat e de r Insekte n i n anatomisch physiologischer und akustische r Beziehung . Zeitschr. wiss. Zool., 17: 105-184. LANDOIS, H. , an d THELEN , W . (1867 ) De r Tracheenverschlus s be i de n Insekten . Zeitschr. wiss. Zool., 17 , 187-214. LATJTENSCHLAGER, F . (1932 ) Di e Embryonalentwicklun g de r weibliche n Keimdruse be i de r Psychid e Solenobi a triquetrella . Zool. Jahrb., Anat., 56 : 121-162. LEBEDEW, A. (1914 ) Über die ais Serie teñen functionierenden Malpighischen Gefásse der Phytonomus-Larven . Zool. Anz., 44 : 49-56. LEE, M . O . (1925 ) O n th e mechanis m of respiration i n certain Orthoptera . Journ. Exp. Zool, 41 : 125-154. ————. (1929 ) Respiration i n the insects. Quart. Rev. Biol., 4: 213-232. LEGER, L. , an d DUBOSCQ , O . (1899 ) Su r le s tube s d e Malpigh i de s grillons . C . R . Soc. Biol. Paris, ser. 11 , 1: 527-529 . LEHMANN, F . E . (1925 ) Über die Entwicklung des Tracheensystems vo n Carausiu s morosus nebst Beitráge n zur vergleichenden Morphologic des Insekten-Tracheen systems. 8 6 pp. Zoolog.-vergl . anat. Inst. Univ. Zurich. LEITCH, I . (1916 ) Th e functio n o f haemoglobin i n invertebrate s wit h specia l refer ence to Planorbi s an d Chironomu s larvae. Journ. Physiol., 50: 370-379. VON LENDENFELD , R . (1903 ) Beitra g zu m Studiu m de s Fluge s de r Insekte n mi t Hülfe de r Momentphotographie . Biol. Zentrlbl., 23: 227-232 . LOMEN, F . (1914 ) De r Hode n vo n Cule x pipiens . Jen. Zeitschr. Naturwiss., 52 : 567-628. LOWNE, B . T . (1890-1895 ) Th e anatomy , physiology , morphology , an d develop ment o f the blow-fly . London . LOZINSKI, P . (1908 ) Beitra g zu r Anatomi c und Histologi e de r Mundwerkzeug e der Myrmeleonidenlarven. Zool. Anz., 33: 473-484. ————. (1911 ) Übe r di e Malpighische n Gefáss e de r Myrmeleonidenlarve n al s Spinndrusen. Ibid., 38: 401-417. MACKAY, D . O . (1927 ) Respiration o f insects. Science, 65: 446.

REFERENCES

637

MADLE, H . (1934 ) Zu r Kenntni s de r Morphologic , Okologi e un d Physiologi e vo n Aphodius rufipe s un d einige n verwandten Arten . Zool. Jahrb., Anat., 58: 303 396. MALOTJF, N . S . R. (1932 ) The skeleta l moto r mechanism of the thora x of the "stin k bug," Nezara viridula. Bull. Soc. Roy. Entom. d'Égypte, 1932: 161-203. ————. (1933 ) Studie s o n the interna l anatom y o f the "stin k bug. " Ibid., 1933 : 96-119. MAMMEN, H . (1912 ) Übe r di e Morphologi c de r Heteropteren - un d Homopteren stigmen. Zool. Jahrb., Anat., 34: 121-178. MANSOUR, K. (1928 ) The developmen t o f the larva l an d adul t mid-gu t o f Calandr a oryzae. Quart. Journ. Micr. Sri., 71: 313-352. MARCU, O . (1929 ) Nervenendigunge n a n de n Muskelfaser n vo n Insekten . Anat. Am., 67: 369-380. MARCUS, B . A . (1930 ) Untersuchungen über di e Malpighische n Gefásse bei Kafern . Zeitschr. Morph. Ókol. Tiere, 19: 609-677. MAREY, E , J. (1869 ) Mémoire sur le vol des insectes et des oiseaux. Ann. Sci. Nat., ser. 5, Zool., 12 : 49-150. ———— •. (1869a ) Recherche s su r l a méchanism e d u vo l de s insectes . Journ. Anat. PhysioL, 6: 19-36, 337-348. ————. (1874 ) Anima l mechanism : a treatis e o n terrestria l an d aeria l locomotion . Internat. Sci. Series, New York. ———— . (1891 ) L e vol des insectes étudi é pa r l a photochronographie . C . R. Acad. Sci. Paris, 113: 15-18. MARSHALL, W . S . (1915 ) Th e formatio n of th e middl e membran e i n th e wing s of Platyphylax designatus . Ann. Ent. Soc. America, 8: 201-216. ————. (1927 ) The Malpighia n tubule s o f the larv a o f Heptageni a interpunctata . Ann. Ent. Soc. America, 20: 149—154 . ————. (1928 ) The developmen t o f the compoun d eye of the confuse d flou r beetle , Tribolium confusum . Trans. Wisconsin Acad. Sci., Arts, Let., 23: 611-630. MARTIN, J . F . (1916 ) The thoracic and cervica l sclerites o f insects. Ann. Ent. Soc. America, 9: 35-83. MARTYNOV, A . B . (1925 ) Über zwe i Grundtype n de r Flüge l be i de n Insekte n un d ihre Evolution . Zeitschr. Morph. Ókol. Tiere, 4: 465-501. MATULA, J. (1911 ) Untersuchunge n über die Funktio n des Zentralnervensystem s bei Insekten . Pflügers Arch. ges. PhysioL, 138: 388-456. MAYER, A . G . (1896 ) Th e developmen t o f th e win g scale s an d thei r pigmen t i n butterflies an d moths . Bull Mus. Comp. Zool., 29: 209-236. MCARTHUR, J . M . (1929 ) An experimenta l stud y o f th e function s of th e differen t spiracles in certain Orthoptera . Journ. Exp. Zool., 53: 117-128. McGovRAN, E. R . (1931 ) A method o f measuring tracheal ventilation i n insects an d some results obtained wit h grasshoppers. Ann. Ent. Soc. America, 24: 751—761. MclNDOO, N . E . (1914 ) Th e olfactor y sense o f the hone y bee . Journ. Exp. Zool., 16: 265-346. ————. (1915) The olfactory sense of Coleóptera. Biol. Bull, 28: 407-460. MEHTA, D . R . (1933 ) O n th e developmen t o f th e mal e genitali a an d th e efferen t genital duct s i n Lepidoptera. Quart. Journ. Micr. Sci., 76: 35-61. DE MEIJERE , J . C . H. (1901 ) Übe r das letze Glei d de r Bein e bei den Arthropoden. Zool. Jahrb., Anat., 14: 417-476. MELVIN, R . (1931 ) A quantitativ e stud y o f coppe r i n insects . Ann. Ent. Soc. America, 24: 485-488. MERCER, W. F. (1900) The development of the wings in the Lepidoptera. Journ. New York Ent. Soc., 8: 1-20 .

638

PRINCIPLES OF

INSECT MORPHOLOGY

METALNIKOFF, C . K . (1896 ) Su r le s organs excréteurs d e quelque s insectes . Bull. Acad. Imp. Sci. Saint-Pétersbourg, 4 : 57-72. METCALF, C . L . (1929 ) Th e rnouthpart s o f insects . Trans. Illinois State Acad., 21: 109-135 . METCALF, C . L. , an d FLINT , W . P . (1928 ) Destructiv e an d usefu l insects . Ne w York. ————. (1932 ) Fundamentals o f insect life . Ne w York an d London . MINCHIN, E . A . (1905 ) Repor t o n the anatom y o f the tsets e fly (Glossina palpalis) . Proc. Roy. Soc. London, ser. B , 76: 531-547. MOBUSZ, A . (1897 ) Übe r de n Darmkana l de r Anthrenuslarve . Archiv Naturg., 63, Bd. 1:88-128 . MOLLISON, T . (1904 ) Di e ernáhrend e Tátigkei t de s Follikelepithel s i m Ovariu m von Melolonth a vulgaris. Zeitschr. wiss. ZooL, 77: 529-545. MONTI, RIÑ A (1893 , 1894) . Richerch e microscopich e su l sistem a nervos a degl i insetti. Boll Scient. (Pavia) , 15 : 105-122; 16: 6-17 . MUNSCHEID, LILI . (1933 ) Di e Metamorphos e de s Labiums de r Odonata . Zeitschr. wiss. ZooL, 143: 201-240. MUTTKOWSKI, R . A. (1921 ) Studies o n the respiratio n o f insects. I . Th e gase s and respiratory protein s of insect blood . Ann. Ent. Soc. America, 14: 150-156. ————. (1921a ) Copper : it s occurrenc e an d rol e i n insect s an d othe r animals . Trans. Amer. Micr. Soc., 40: 144-157. ———— . (1923 ) Studie s o n th e blood o f insects. I . Th e compositio n o f the blood . Bull. Brooklyn Ent. Soc., 18: 127-136. ———— . (1924 ) II. Th e structural element s of the blood. Ibid., 19 : 4-19 . ———— •. (1924a ) III . Th e coagulatio n an d clottin g o f insec t blood . Ibid., 19 : 128-144. NABERT, A . (1913 ) Di e Corpor a allat a de r Insekten . Zeitschr. wiss. ZooL, 104 : 181-358. NEEDHAM, J . G . (1897 ) Th e digestiv e epitheliu m o f dragonfl y nymphs . ZooL Bull. I: 103-113. NELSON, J . A . (1915 ) Embryology of the hone y bee. Princeton , N . J . ———— (1924) Morpholog y o f th e honeybe e larva . Journ. Agrie. Research, 28 : 1167-1213. NEWTON, H . C . F . (1931 ) O n th e "so-called " olfactor y pore s i n th e hone y bee . Quart. Journ. Micr. Sci., 74: 647-668. NOEL, R. , and TAHIR , E. (1929 ) Étude cytologiqu e des prolongements dits ciliforme s des cellule s d e 1'epitheliu m de s tube s d e malphig i che z Bomby x mori . Arch. Anat. Micr., 25 : 587-596. NOWIKOFF, M . (1931 ) Untersuchunge n übe r di e Komplexauge n von Lepidoptere n nebst einigen Bemerkungen über die Rhabdome der Arthropoden im allgemeinen. Zeitschr. wiss. ZooL, 138: 1-67 . NUTTALL,G. H. F., and KEILIN,D . (1921 ) On the nephrocytes of Pediculus humanus. Parasitology, 13 : 184-192 . OBERLE, E . (1912 ) Das Blutgefasssyste m vo n Dytiscu s marginalis . Marburg . ORLOV, J. (1924 ) Die Innervation de s Darmes de r Insekten . (Larve n von Lamellicorniern.). Zeitschr. wiss. ZoaL, 122: 425-502. ———— . (1924a ) Über den histologischen Ba u der Ganglion des Mundmagennervensystems de r Insekten. Zeitschr. mikr.-anat. Forsch., Abt . A, 2: 39-110. PAILLOT, A . (1933 ) L'infection che z le s insectes , immunit é et symbiose . Trévoux . PANKRATH, O . (1890 ) Da s Aug e de r Raupe n un d Phryganidenlarven . Zeitschr. wiss. ZooL, 49: 690-708.

REFERENCES

639

PARKER, H . L . (1934 ) Note s o n th e anatom y o f tenthredinid larvae , wit h specia l reference t o th e head . Boll. Lab. Zool. R . Inst. agr. Portici, 28: 159-191 . PATTEN, W . S. , an d EVANS , A . M . (1929 ) Insects , ticks , mites , an d venomou s animals. Croydon . PAULCKE, W . (1901 ) Übe r di e Differenzierun g de r Zellelement e i m Ovariu m de r Bienenkonigen. Zool. Jahrb., Anat., 14: 177-202. PAWLOWA, M . (1895 ) Übe r ampullenarteg e Blutcirculationsorgan e i m Kopf e ver schiedener Orthopteren . Zool. Anz., 18 : 7-13 . PAWLOWSKY, E . (1906 ) Übe r de n Stech - un d Saugappara t de r Pediculiden . Zeitschr. wiss. Insektenbiol., 2: 156-162, 198-204 . PEACOCK, A . D. (1918 ) Th e structur e o f the mout h parts and mechanis m of feeding in Pediculu s humanus. Parasitology, 11 : 98—117. • PEACOCK, A . D. , an d GRESSON , A . R . (1928 ) Th e role s o f th e nurse-cells , oocyte s and follicle-cell s in Tenthredinid oogénesis . Quart. Journ. Micr. Sci., 71 : 541561. PÉREZ, C . (1910 ) Recherche s histologique s su r l a metamorphos e de s Muscides . Arch. ZooL Exp. Gen., ser. 5, 4: 1-274. PETERSON, A . (1915 ) Morphologica l studie s o f th e hea d an d mout h part s o f th e Thysanoptera. Ann. Ent. Soc. America, 8: 22-57. ————. (1916 ) Th e head-capsul e an d th e mouth-part s o f Diptera . Illinois Biol. Monographs, 3 , No . 2 , 11 2 pp . PETRUNKEVITCH, A . (1900 ) Di e Verdauungsorgan e von Periplanet a orientali s un d Blatta germánica. Zool. Jahrb., Anat., 13: 171-190. ————. (1901 ) Di e Richtungskorpe r un d ih r Schicksa l i m befruchteten un d unbe fruchteten Bienenei . Ibid., 14 : 573-608. ————. (1903 ) Da s Schicksa l de r Richtungskorpe r i m Drohnenei . Ibid., 17 : 481 516. PFLUGSTAEDT, H . (1912 ) Di e Haltere n de r Dipteren . Zeitschr. wiss. Zool., 100 : 1-59. PHILIPTSCHENKO, J . (1907 ) Anatomisch e Studie n übe r Collembola . Zeitschr. wiss. Zool., 85: 270-304. ———— . (1912 ) Beitrág e zu r Kenntni s de r Apterygoten . III . Di e Embryonal entwicklung von Isotoma cinérea . Ibid., 103 : 519-660. PICKLES, A . (1931 ) O n th e metamorphosi s o f th e alimentar y cana l i n certai n Ephemeroptera. Trans. Ent. Soc. London, 79: 263-274. PLATEAU, F . (1884 ) Recherche s experimentales sur les mouvements respiratoires de s insectes. Mém. Acad. Roy. Belgique, 45: 219 pp. PLOTNIKOW, W . (1904 ) Über di e Háutun g un d übe r einig e Element e de r Hau t be i den Insekten . Zeitschr. wiss. Zool., 76: 333-366. POPOVICI-BAZNOSANU, A . (1905 ) Beitráge zu r Kenntni s de s Zirculationssystem s de r Insekten. Jen. Zeitschr. Naturwiss., 40 : 667—696 . POWELL, P . B . (1904 , 1905) Th e developmen t o f wings of certain beetle s an d som e studies o n th e origi n o f th e wing s of insects . Journ. Ne w York Ent. Soc., 12 : 237-243; 13:5-22. POYARKOFF, E . (1914 ) Essai d'une théorie de la nymphe des insectes holométaboles . Arch. Zool Exp. Gen., 54: 221-265. PRELL, H . (1912 ) Gliederun g un d einig e Muskulatu r de r Bein e vo n Acere n tomón und Eosentomon . Zool. Anz., 40: 33-50. ———— •. (1913 ) Da s Chitinskelet t vo n Eosentomon . Zoológica. Orig.-Abh. Gesamtg. Zool., 25, 58 pp. PROCHNOW, 0. (1924, 1925) Mechanik des Insektenfluges. In Schroder's Handbuch de r Entomologie, 1: 534-569.

640

PRINCIPLES OF INSECT MORPHOLOGY

RAMME, W . (1913 ) Di e Bedeutun g de s Proventriculus bei Coleoptere n und Orthop teren. Zool. Jahrb., Anat., 35: 419-456. VON RATH , O . (1888 ) Übe r di e Hautsinnesorgan e de r Insekten . Zeitschr. wiss. Zool., 46:413-454. RENGEL, C. (1898 ) Über die periodische Abstossung und Neubildun g des gesammten Mitteldarmepithels be i Hydrophilus , Hydra s un d Hydrobius . Zeitschr. wiss. ZooL, 63: 440-455. ———— . (1903 ) Übe r de n Zusammenhan g vo n Mitteldar m un d Enddar m be i de n Larven der aculeaten Hymenopteren. Ibid., 75 : 221-232. RETHFELDT, C . (1924 ) Di e Viviparita t be i Chrysomel a varian s Schaller . Zool. Jahrb., Anat., 46: 245-302. REYNE, A . (1927 ) Untersuchunge n über di e Mundteil e de r Thysanopteren . Zool. Jahrb.] Anat., 49: 391-500. RILEY, W . A . (1904 ) The embryologica l development of the hea d of Blatta. Am-er. Nat., 38:777-810. RIPPER, W . (1931 ) Versuc h eine r Kriti k de r Homologiefrag e de r Arthropoden tracheen. Zeitschr. wiss. ZooL, 138: 303-369. RITTER, W . (1911 ) Th e flyin g apparatu s o f the blow-fly . Smithsonian Misc. Coll., 56: No . 12 , 76pp. ROBERTS, A . W. R. (1921 ) On the lif e history o f "wireworms" of the genu s Agriotes, with som e note s o n tha t o f Athou s haemorrhoidalis . Ann. Applied Biol., 8 : 193-215. ROGOSINA, MARIE . (1928 ) Übe r da s peripher e Nervensyste m de r Aeschna^Larve . Zeitschr. Zellforsch., 6 : 732-758. ROHLER, E . (1906 ) Beitrág e zu r Kenntni s de r Sinnesorgan e de r Insekten . Zool. Jahrb., Anat., 22: 225-288. RUCKES, H. (1919) Notes on the male genital system in certain Lepidoptera. Ann.a Ent. Soc. America, 12: 192-209. RUL.AND, F. (1888 ) Beitrág e zur Kenntnis der antennalen Sinnesorgan e der Insekten . Zeitschr. wiss. ZooL, 46: 602-628. RUNGITJS, H . (1911 ) De r Darmkana l (de r Imag o un d Larve ) vo n Dytiscu s mar ginalis. Zeitschr. wiss. ZooL, 98: 179-287. Russ, E . (1907 ) Übe r di e postembryonal e Entwicklun g de s Mitteldárm s be i de n Trichopteren. ZooL Anz., 31 : 708-710. SAMTLEBEN, B . (1929 ) Zu r Kenntni s de r Histologi e un d Metamorphos e des Mittel dárms de r Steckmückenlarven. Zool. Anz., 81: 97-109. SANFORD, E. W , (1918 ) Experiments o n the physiolog y of digestion i n the Blattidae . Journ. Exp. ZooL, 25: 355-411. SAYCE, O . A . (1899 ) O n th e structur e o f th e alimentar y syste m o f Gryllotalp a australis, wit h som e physiologica l notes . Proc. Roy. Soc. Victoria, n.s. , 11 : 113-129. SCHLÜTER, C . (1912 ) Beitrág e zu r Physiologi c un d Morphologi c de s Verdauungs apparates de r Insekten. Zeitschr. allg. PhysioL, 13: 155-200. SCHMIDT, E . (1916 ) Vergleichend e Morphologic de s 2 . un d 3 . Abdominalsegments bei mánnlichen Libellen . Zool. Jahrb., Anat., 39: 87-200. SCHMIDT, W . J. (1930 ) Submikroskopischer Bau und Fárbun g de s Chitins. 3 . Wander~ver'sammlung Deutscher Entomologen in Giessen, 1929 : 100—103. SCHMIEDER, R . G . (1928 ) Observation s o n th e fat-bod y i n Hymenoptera . Journ. Morph., 45 : 121-184. SCHNEIDER, H . (1923 ) Di e Haar e un d sonstige n Chitingebild e de r Kohlraup e (Pieris brassicae) . ZooL Anz., 56: 155-160.

REFERENCES

641

SCHNEIDER, K . (1917 ) Di e Entwicklun g de s Eierstocke s un d Eie s vo n Deilephil a euphorbiae. Archiv. Zellf., 14 : 79-143. SCHON, A . (1911 ) Ba u un d Entwicklun g de s tibiale n Chordotonalorgan e be i de r Honigbiene und bei Ameisen. Zool. Jahrb., Anat., 31: 439-472 . ScHOENEMUND, E . (1912 ) Zur Biologi e und Morphologi c einige r Perlaarten . Zool. Jahrb., Anat., 34: 1-56 . SCHRODER, C . (1928 ) Handbuc h de r Entomologie . Jena . SCHWABE, J . (1906 ) Beitrág e zu r Morphologi c un d Histologi e de r tympanale n Sinnesapparate der Orthopteren . 8 6 pp. Stuttgart . SCH WANG ART, F. (1904 ) Studien zur Entodermfrage bei den Lepidopteren. Zeitschr. wiss. Zool., 76: 167-212. SEIDEL, F . (1924 ) Di e Geschlechtsorgan e in de r embryonale n Entwicklun g vo n Pyrrhocoris apterus. Zeitschr. Morph. Okol. Tiere, 1: 429-506. SETJRAT, L . G . (1899 ) Contribution s a F étude de r Hyménoptére s entomophages . Ann. Sci. Nat., Zool, 10 : 1-159. SHARP, D. , an d MUIR , F. (1912 ) The comparativ e anatomy o f the mal e genital tube in Coleóptera . Trans. Ent. Soc. London, 1912: 477-642. SHINJI, G . O . (1919 ) Embryolog y of coccids , with especia l referenc e t o th e forma tion o f the ovary , origin and differentiatio n o f th e ger m cells, germ layers, rudi ments of the midgut , and the intracellular symbiotic organisms. Journ. Morph., 33: 73-126. SIHLER, H. (1924 ) Die Sinnesorgane an der Cerci der Insketen. Zool. Jahrb., Anat. t 45: 519-580. SIKES, ENI D K., and WIGGLESWORTH , V. B. (1931 ) Th e hatchin g o f insects fro m th e egg, an d th e appearanc e of air in the trachea l system. Quart. Journ. Micr. Sci., 74: 165-192 . SIKORA, H . (1916 ) Beitrág e zu r Anatomie , Physiologi c un d Biologi e de r Kleider laus. Archiv Schiffs- Tropenhygiene, 20 , Beiheft 1 : 2-76 . SILVESTRI, F. (1905) Über die Projapygidae und einige Japyx-Arten. Zool. Anz., 28:638-643. ———— — . (1933) Sulle appendici del capo degli "Japigidae" (Thysanura entotropha) e rispettiv o confront o co n quell e de i Chilopodi , de i Diplopod i e de i Crostacei . Ve Cong. Internal. Entom., 1932: 329-343. SINGH PRUTHI , H. (1924 ) O n the postembryoni c development and homologie s of th e male genital organs of Tenebrio molitor. Proc. Zool. Soc. London, 1924: 857-868. ————. (1925 ) The morpholog y of the mal e genitali a i n Rhynchota . Trans. Ent. Soc. London, 1925: 127-267 . SMITH, R . C . (1922 ) Hatchin g i n thre e specie s o f Neuroptera . Ann. Ent. Soc. America, 15: 169-176. SMRECZYNSKI, S . (1932 ) Embryologisch e Untersuchunge n übe r di e Zusammenset zung des Kopfes vo n Silpha obscura . Zool. Jahrb., Anat., 55: 233-314. SNODGRASS, R . E . (1921 ) The mout h parts of the cicada . Proc. Ent. Soc. Washington, 23 : 1-15 . ———— . (1922 ) Mandibl e substitutes i n the Dolichopodidae . Ibid., 24: 148-152. ) Anatom y an d metamorphosi s o f th e appl e maggot . Journ. Agrie. ————. (1924 Research, 28: 1-36 . ————. (1926 ) Th e morpholog y o f insec t sens e organ s an d th e sensor y nervou s system. Smithsonian Misc. Coll., 77: No. 8 , 80 pp. ———— . (1927 ) Morphology and mechanis m of the insec t thorax . Ibid., 80 , No. 1, 108 pp. ————. (1928 ) Morpholog y and evolutio n o f th e insec t hea d an d it s appendages . Ibid., 81 , No. 3 , 158 pp.

642

PRINCIPLES OF

INSECT MORPHOLO GY

. (1929 ) Th e thoraci c mechanis m o f a grasshopper , an d it s antecedents . Ibid:, 82, No. 2 , 11 1 pp. ———— . (1930 ) How insects fly . Smithsonian Rept., 1929 : 383-421. ———— . (1931 ) Morpholog y of the insec t abdomen . Par t I . Genera l Structur e of the abdome n and its appendages . Smithsonian Misc. Coll., 85, No. 6, 128 pp. ————. (1932) Evolution of the insect head and the organs of feeding. Smithsonian Rept., 1931 : 443-489. ————. (1933 ) Morpholog y o f the insec t abdomen . Par t II . Th e genita l duct s and th e ovipositor . Smithsonian Misc. Coll., 89 , No . 8 , 14 8 pp . ———— —. (1935 ) Th e abdomina l mechanism s of a grasshopper. Ibid., 94 . SPBYER, W . (1922 ) Di e Muskulatu r de r Larv e vo n Dytiscus marginalis . Ei n Beitrag zur Morphologic der Insektenkorpers. Zeitschr. wiss. Zool., 119: 423492. STAHN, I . (1928 ) Übe r di e Atmungstregulation , bebesonder s di e Kohlensaure regulation, be i Dixippus morosus und Aeschna grandis. Ei n Beitrag zu r Atmung der Insekten . Zool. Jahrb., Zool., 46: 1-86 . STEINER, L . F . (1929 ) Homologie s o f trachea l branche s i n th e nymp h o f Ana x junius based o n their correlatio n wit h th e muscle s they supply . Ann. Ent. Soc. America, 22: 297-308. STEINKE, G . (1919 ) Di e Stigme n de r Káferlarven . Arch. Naturg., 85 , Abt . A. , Heft 7 : 1-56 . STELLWAAG, F . (1916 ) Wi e steuer n di e Insekte n wáhren d de s Fluges ? Biol. Zentralbl, 36: 30-44. STORMER, L . (1933 ) Ar e the trilobite s relate d t o th e arachnids ? American Journ. Sci., 26 : 147-157 . ———— . (1934 ) Merostomat a fro m th e Downtonia n sandston e o f Ringerike , Nor way. Skr. Vid.-Akad. Oslo, I. Mat.-Natur. KL, 1933 , No . 10 , 125 pp. STRINDBERG, H . (1913 ) Embryologische Studie n a n Insekten . Zeitschr. wiss. Zool., 106: 1-227. STROHM, K . (1910 ) Di e zusammengesetzte n Auge n de r Mannche n vo n Xeno s rossii. Zool. Anz.j 36 : 156-159. STUHLMANN, F . (1907 ) Beitrág e zu r Kenntni s der Tsetseflieg e (Glossin a fusca , un d Gl. tachinoides). Arbeiten Kaiserl. Gesundheitamte, 26 , Heft 1 : 301-383. SULC, K . (1927 ) Da s Tracheensyste m vo n Lepism a (Thysanura ) un d Phylogeni e der Pterygogenea . Acta Soc. Sci. Nat. Moravicae, 4 , Fas. 7 , Sig. F 39 : 227-344 . SUSLOV, S . (1912 ) Übe r di e Kopfdrüse n einige r niedere n Orthopteren . Zool. Jahrb., Anat., 34: 96-120. SWINGLE, H . S . (1925 ) Digestiv e enzyme s o f a n insect . Ohio Journ. Sci., 25 : 209-218. TANAKA, T . (1926 ) Homologie s o f th e win g vein s o f th e Hemiptera , Annotat. Zool. Japonenses, 2: 33-54. THOMPSON, CAROLIN E B . (1913 ) A comparative stud y o f th e brain s o f three gener a of ants , wit h specia l referenc e to th e mushroo m bodies. Journ. Comp. Nenrol., 23:515-571. THOMPSON, W . R. (1921 ) Recherche s sur le s Diptéres parasites . 15 1 pp. Paris . THOMSEN, M . (1927 ) Som e observations o n th e biolog y an d anatom y o f a cocoon spinning chalci d larva , Euplectru s bicolor . Saert. Vid. Medd. Dansk. Naturh. Foren., 84: 73-89. THORPE, W . H . (1930 ) Th e biology , post-embryoni c development , an d economi c importance o f Cryptochaetu m icerya e parasiti c o n Icery a purchasi . Proc. Zool. Soc. London, 1930: 929-971. ————

REFERENCES

643

TILLYARD, R . J . (1915 ) O n th e physiolog y o f th e recta l gill s i n th e larva e o f ani sopterid dragonflies . Proc. Linn. Soc. N. S . W., 40 : 422-437 . ————. (1917 ) Th e biolog y of dragonflies. Cambridge . ————. (1919 ) Th e Panorpi d complex , par t 3 , Th e wing-venation . Proc . Linn. Soc. N . S, W., 44: 533-718. ————. (1922 ) O n th e larv a an d pup a o f the genu s Sabatinca . Trans. Ent. Soc. London, 1922 : 437-453. ————. (1923 ) On the mouth-parts o f the Micropterygoidea . Ibid., 1923: 181-206. ————. (1928 ) Kansa s Permia n insects , par t 10 . American Journ. Sci., 5t h ser. , 16:185-220. ———— . (1930 ) The evolutio n of the clas s Insecta. Roy. Soc. Tasmania, Papers and Proc., 1930, 8 9 pp . TIMÓN-DAVID, J. (1930) Recherches sur les matiéres grasses des insectes. Ann. Fac. Sci. Marseille, ser. 2, 4: 29-207. TIRELLI, M. (1929) Sbocco d i tubi malpighiani nel mesointestino. Atti R. Accad. Lincei, ser. 6, 10: 278-281. TONKOV, VERA . (1923 ) Zu r mikroscopische n Anatomi c de r Rectaldrüse n be i de n Insekten. Rev. Russ. d.'Entom., 18 : 69-80. ———— — . (1925) Über den Bau der Rectaldrüsen bei Insekten. Zeitschr. Morph. Okol. Tiere, 4: 416-429 . TONNER, F . (1933 ) Ei n Beitra g zu r Anatomi e un d Physiologi e de s periphere n Nervensystems von Astacu s fluviatilis . Zool. Jahrb., ZooL, 53: 101—152 . TOWER, D . G . (1914 ) The mechanis m of the mout h part s o f the squas h bug , Anasa tristis. Psyche, 21: 99-108. TOWER, W . L . (1903 ) Th e origi n an d developmen t o f th e wing s o f Coleóptera . Zool. Jahrb., Anat., 17: 517-572. ————. (1906 ) Observation s o n th e change s i n th e hypodermi s an d cuticul a o f Coleóptera durin g ecdysis. Biol. Bull, 10: 176-192. TRAPPMANN, W . (1923 ) Di e Malpighische n Gefáss e vo n Apis mellifica . Archiv Bienenkunde, 5, Heft 6 : 1-23 . ———— . (1923a ) Di e Rectaldrüse n vo n Api s mellifica . Archiv Bienenkunde, 5 : 213-220. TSCHUPROFF, HELENE . (1904 ) Übe r di e Entwicklun g de r Keimblatte r be i de n Libellen. Zool. Anz., 27 : 29-34. TULLOCH, G . S . (1929 ) Th e prope r us e o f th e term s parapside s an d parapsida l furrows. Psyche, 36 : 376-382. ULRICH, W . (1924 ) Di e Mundwerkzeug e der Spheciden . Beitra g zur Kenntnis der insektenmundwerkzeuge. Zeitschr. Morph. Ókol. Tiere, 1: 539-636. UVAROV, B . P . (1928 ) Insec t nutritio n an d metabolism . Trans. Ent. Soc. London, 1928:255-343. UZEL, H . (1897 ) Beitrág e zu r Entwicklungsgeschicht e vo n Campode a staphylinus . Zool. Anz., 20: 232-237 . VENEZIANI, A . (1905 ) Valor e morphologic o e fisiológic o de i tub i Malpighiani . Redia, 2: 177-230. VERHEIN, A. (1921 ) Die Eibildung de r Musciden . Zool. Jahrb., Anat., 42 :149-212. VERSON, E . (1890 ) D i un a seri e d i nuov i organ i escretor i scopert i ne l filugello . R. Sta. Bacologica, Padova, Spec . V, 30 pp. VIALLANES, H . (1882 ) Not e su r le s terminaison s nerveuse s sensitive s de s insectes . Bull. Soc. Philomath, Paris, ser. 7, 6: 94-98. VOGEL, R. (1911 ) Übe r die Innervierun g der Schmetterlingsflüge l und Übe r den Bau un d di e Verbreitun g de r Sinnesorgaii e au f dejq,§e,lb§n , fiqiteGhr , wi$$.. Zool^

98;6§-m,

644

PRINCIPLES OF

INSECT MORPHOLO GY

-. (1920 ) Zur Anatomi c des Stechrüssels von Glossin a fusca . Zool. Am., 51: 269^279. ————-— . (1921) Kritische und ergánzende Mitteilungen zur Anatomie des Stechapparats de r Culiciden und Tabaniden . ZooL Jahrb., Anat., 42: 259-282. ———— . (1921a ) Zu r Kenntni s de s Baue s un d de r Funktio n de s Stachel s un d de s Vorderdarmes der Kleiderlaus. Ibid., 4 2 :229-258. ———— • . (1923) Zur Kenntnis des feineren Baues der Geruchsorgane der Wespen und Bienen. Zeitschr. wiss. ZooL, 120: 281-342. Voss, F . (1913 , 1914 ) Vergleichend e Untersuehungen übe r di e Flügwerkzeug e de r Insekten. Verh. Deut. ZooL GeselL, 23: 118-142; 24: 59-90. WACHTER, SIBY L (1930 ) The moultin g of the silkwor m and a histological study o f th e moulting gland . Ann. Ent. Soc. America, 23: 381-389. WALKER, E . M . (1919 , 1922 ) Th e termina l abdomina l structure s o f orthopteroi d insects: a phylogenetic study. Ann. Ent. Soc. America, 12: 267-316; 15: 1-76 . ———— . (1931 ) O n th e clypeu s an d labiu m o f primitiv e insects . Canadian Ent., 63:75-81. ————. (1932 ) Prognathism an d hypognathism in insects. Ibid., 64 : 223-229 . WALLENGBEN, H . (1913 ) Physiologische-biologisch e Studie n übe r di e Atmun g be i den Arthropoden . I . Di e Atmun g de r gehirnlose n Aeschna-Larven . Lunds Univ. Arsskr., N.F. , Afd . 2, 9, 30 pp . ———— . (1914 ) II . Di e mechanik de r Atembewegunge n be i Aeschnalarven . Ibid., 10: 24pp. WALLING, EULALI A V. (1906 ) The influence s of gases and temperatur e on the cardia c and respiratory movement s in the grasshopper. Journ. Exp. Zool., 3: 621-629. WEBER, H. (1924 ) Das Grundschema des Pterygotenthorax. Zool. Anz., 60 : 17-37, 57-83. Anat. u. EntwickL, ————. (1924a ) Das Thorakalskelett der Lepidopteren. Zeitschr. 73:277-331. ————. (1925 ) Der Thorax de r Hornisse. Ei n Beitrag zur vergleichenden Morphologic des Insektenthorax. ZooL Jahrb., Anat., 47: 1—100 . ————. (1928 ) Zu r vergleichende n Physiologi c de r Saugorgan e de r Hemipteren . Zeitschr. vergl. PhysioL, 8: 145-186. ———— — . (1928a) Skelett, Musculatur und Darm der schwarzen Blattlaus, Aphis fabae. Zoológica. Orig.-Abh. Gesamtg. ZooL, 28, Hef t 76 , 12 0 pp . ———— . (1929 ) Kopf un d Thorax von Psylla mali. Zeitschr. Morph. OkoL Tiere, 14: 59-165. ————. (1930 ) Biologi c der Hemipteren. Berlin . ————. (1933 ) Lehrbuch der Entomologie. Jena . WEISMANN, A . (1864 ) Di e nachembryonal e Entwicklun g de r Muscide n nac h Beo bachtungen an Musca vomitoria un d Sarcophag a carnaria. Zeitschr. wiss. ZooL, 14:187-336. WELCH, P . S . (1922 ) Th e respirator y mechanis m i n certai n aquati c Lepidoptera . Trans. Amer. Micr. Soc., 41: 29-50. WESTER, D . H. (1910 ) Übe r die Verbreitung und Lokalizatio n de s Chitins im Tierreiche. ZooL Jahrb., Syst., 28 : 531-558. WETTINGER, O . (1927 ) Das Circulationssyste m de r Tipulidenlarve n mi t besondere r Berücksichtigung vo n Tipul a selene. Zeitschr. wiss. ZooL, 129: 453—482. WETZEL, A . (1934 ) Chordotonalorgan e be i Krebstieren (Caprell a de n tata). ZooL Anz., 105: 125-132 . WHEELER, W . M . (1889 ) Th e embryolog y o f Blatt a germánic a an d Doryphor a decimlineata. Journ, Morph., 3: 291-386. ————

REFERENCES

645

. (1891 ) Neuroblast s in the arthropo d embryo . Ibid., 4: 337-343. . (1892 ) Concernin g th e "bloo d tissue " o f insects ; Psyche, 6 : 216-220 , 233-236 253-258 . ———— • —. (1893) A contribution to insect embryology. Journ. Morph., 8: 1-160. ———— . (1893a) The primitive number of Malpighian tubules in insects. Psyche, 6: 457-460, 485-486, 497-498 , 509-510, 539-541 , 545-547 , 561-564 . WHITE, G . F. (1918 ) A note on the muscula r coa t of the honey bee. Proc. Ent. Soc. Washington, 20 : 152-154. WIESMANN, R . (1926 ) Zu r Kenntni s de r Anatomi e un d Entwicklungsgeschicht e der Stabheuschrecke Carausiu s morosus . III . Entwicklun g un d Organogenes e der Colomblásen. 205pp . Zool-vergl . anat. Inst. Univ. Zurich. WIGGLESWORTH, V . B . (1928 ) Digestio n i n th e cockroach . III . Th e digestio n o f proteins an d fats . Biochem. Journ., 22 : 150—161 . ———— . (1929 ) Digestio n i n the tsetse-fly : a study o f structure an d function. Parasitology, 21 : 288-321. ———— . (1930 ) Th e formatio n of the peritrophi c membran e in insects , wit h specia l reference t o th e larva e o f mosquitoes. Quart. Journ. Micr. Sci., 73: 593-616. ———— (1930a) A theory o f tracheal respiratio n i n insects. Proc. Roy. Soc. London, B, 106 : 229-250 . ) Th e respiration o f insects. Biol. Rev., 6: 181-220. ————. (1931 ———— . (1931a ) Th e physiolog y o f excretio n i n a blood-suckin g insect, Rhodniu s prolixus (Reduviidae) . I . Compositio n o f th e urine . Journ. Exp. Biol., 8 : 411-427. ———— . (19316 ) II . Anatom y an d histolog y o f th e excretor y system . Ibid., 8: 428-442. ————. (1931c ) III . Th e mechanism of-uri c aci d excretion. Ibid., 8 : 443-451. ———— . (1932 ) O n th e functio n o f th e so-calle d 'recta l glands' o f insects. Quart. Journ. Micr. Sci., 75: 131-150. ————• . (1933) The effect of salts on the anal gills of the mosquito larva. Journ. Exp. Biol, 10: 1-15 . ———— —. (1933a ) Th e functio n o f th e ana l gill s o f th e mosquit o larva . Ibid., 10 : 16-26. ———— . (19336 ) Th e physiolog y o f the cuticl e an d o f ecdysi s in Rhodnius prolixus; with special referenc e t o th e functio n o f the oenocyte s and o f the derma l glands. Quart. Journ. Micr. Sci., 76: 269-318. ———— . (1934 ) Factor s controllin g moultin g an d "metamorphosis " i n a n insect . Nature, 133 : 725-726 . ———— —. (1934a ) Th e physiolog y o f ecdysi s i n Rhodniu s prolixu s (Hemiptera) . II. Factor s controllin g moultin g an d 'metamorphosis. ' Quart. Journ. Micr. Sci., 77: 191-221 . WILLEM, V . (1900 ) Recherche s su r les Collemboles et les Thysanures. Mem. Cour. et Mém. Sav. Étrang., Acad. Roy. Belgique, 58 : 14 4 pp. WILLEM, V. , an d SABBE , H. (1897 ) Le tub e ventra l e t le s glande s cephalique s de s Sminthures. Ann. Soc. Entom. Belgique, 41: 130-132. VAN WISSELINGH , C . (1898 ) Microchemisch e Untersuchungen übe r di e Zellwand e der Fungi . Jahrb. wiss. Bot., 31: 619-687. VON WISTINGHAUSEN , C . (1890 ) Übe r Tracheenendigunge n i n de n Sericterie n der Raupen . Zeitschr. wiss. Zool., 49: 565-582. WOLFF, B . (1922 ) Schlammsinnesorgan e (pelotaktisch e Organe ) be i Limnobiien larven. Jen. Zeitschr. Naturw., 58: 77-144. WOODS, W. C. (1929) The integument of the larva of the alder flea beetle. Bull, Brooklyn Ent. Soc., 24: 116-123 . ————

————

646

PRINCIPLES OF

INSECT MORPHOLOGY

WREDE, F . (1926 ) Beitrág e zu r Atmun g de r Insekten . I . Übe r di e Tracheen atmung bei Raupen. Pflügers Arch. ges. PhysioL, 211: 228-243. YEAGER, J . F . (1931 ) Observation s o n cro p and gizzar d movement s in Periplanet a fuliginosa. Ann. Ent. Soc. America, 24: 739-745. YEAGER, J . F. , an d HENDRICKSON , G . O . (1934 ) Circulatio n o f th e bloo d i n wings and win g pads of Periplaneta americana . Ann. Ent. Soc. America, 27: 257-272 . YEAGER, J . F. , an d KNIGHT , H . H . (1933 ) Microscopi c observation s on . blood coagulation i n severa l differen t specie s o f insects. Ann. Ent. Soc. America, 26: 591-602. YEAGER, J . F., SHULL, W. E., an d FARRAR , M. D. (1932 ) On the coagulatio n of blood from Periplanet a orientalis , wit h specia l referenc e to bloo d smears . Iowa State College Journ. Sci., 6: 325-339. YUNG-TAI, TSCHANG . (1929 ) L'histogenése e t l'histophysiologi e d e Pépithéliu m d e l'intestin moye n che z u n lépidoptér e (Gallerí a mellonella) . Supplement 12 , Bull. Biol. France e t Belgique, 14 4 pp . ———— •. (1929a ) Sur Forigine d e l a membran e péritrophique dans Pintestin e moye n des chenilles d e Lépidoptéres. Bull. Soc. Zool. France, 64: 255-263. ZANDER, E . (1900 ) Beitrág e zu r Morphologi c de r mánnliche n Geschlechtsanhánge der Hymenopteren . Zeitschr. wiss. Zool., 67: 461-489. ———— . (1901 ) Beitrág e zu r Morphologi c de r mánnliche n Geschlechtsanháng e der Trichopteren. Ibid., 70 : 192-235 . ———— . (1903 ) Beitrág e zu r Morphologi c de r mánnliche n Geschlechtsanháng e der Lepidopteren. Ibid., 74 : 557-615. ZAWARZIN, A . (1911 ) Histologisch e Studie n übe r Insekten . I . Da s Her z de r Aeschnalarven. Zeitschr. wiss. Zool., 97: 481-510. ———— . (1912 ) II . Da s sensibl e Nervensyste m de r Aeschnalarven . Ibid., 100 : 245-289. ———— . (1912a ) III . Übe r da s sensible Nervensyste m de r Larven vo n Melolonth a vulgaris. Ibid., 100 : 447-458 . ———— . (1914 ) IV . Di e optische n Ganglie n de r Aeschna-Larven . Ibid., 108 : 175-257. ———— -. (1916 ) Quelques données sur la structure d u systéme nerveux intestinal des insectes. Revue Zool. Russe, 1: 176-180. ———— . (1924 ) Histologisch e Studie n übe r Insekten . V . Übe r di e histologisch e Beschaífenheit de s unpaaren ventra l Nerv s de r Insekten . Zeitschr. wiss. Zool., 122:97-115. ———— . (1924a ) VI . Da s Bauchmar k de r Insekten . Zeitschr. wiss. Zool., 122 : 323-424. ZICK, K . (1911 ) Beitrág e zu r Kenntni s de r postembryonale n Entwicklungsge schichte der Genitalorgan e bei Lepidopteren. Zeitschr, wiss, Zool., 98: 430-477.

INDEX Air sacs , 448 defined, 46 1 Abdomen, 41 , 246 Alary muscle s (muscle s of dorsal diaphragm), 404 appendages. 267-26 9 defined, 42 0 cerci, 255 Alimentary canal , 347 characteristics o f segments, 25 1 anterior intestine , 38 0 eleventh segment , 255 buccal cavity , 35 1 general structur e o f segments, 24 7 cardiac valve, 35 9 genital segments , 252 crop (ingluvies) , 353 musculature, 257-267 defined, 38 7 pleurites, 250 degeneration an d regeneratio n o f digestiv e pygopods, 253 cells, 371 development of , 347 sclerotization, 24 8 sterna, 250 digestive cells , 363 , 370, 371 tenth segment , 25 3 embryonic, 371 filter chamber , 383 terga, 24 9 function of , 2 9 twelfth segment , 25 6 gastric caeca , 361 visceral segments , 25 2 general structur e of , 348 Abdominal appendages , 267 Abdominal appendage s of , coleopterou s larvae , Malpighian tubules , 378 , 417 272, 27 9 mesenteron, 359 , 360, 363 oesophagus, 552 Collembola, 26 8 peritrophic membrane , 366 ephemerid larvae, 273 pharynx, 352 lepidopterous larvae , 276 , 277 posterior intestine , 38 0 Protura, 268 proctodaeum, 374 pterygote larvae , 272 proventriculus, 35 4 sialid larvae , 272-27 4 pyloric valve, 37 7 tenthredinid larvae , 278 , 279 pylorus, 37 6 trichopterous larvae , 27 5 rectal organ s ("glands") , 381 Thysanura, 27 0 rectum, 38 0 Abdominal muscles , classified , 26 0 regenerative cell s o f ventriculus, 36 3 of grasshopper , 26 4 Abdominal musculature , comple x type, 266 replacement o f ventricular epithelium , 37 2 secretion an d absorptio n i n ventriculus , 370 general pla n of , 258 stomodaeum, 34 9 Abdominal pleurites, 25 0 ventricular caeca , 361 Abdominal segments , characteristic s of , 251 ventriculus, 359 , 360 , 363 Abdominal sterna, 25 0 Alinotum, 17 4 Abdominal terga, 249 defined, 19 0 Acarida, leg of , 92 Alveolus o f seta, 57 Accessory genita l glands , defined , 27 8 defined, 6 8 female, 552 , 566 Alula, 225, 227 male, 573 defined, 24 3 Accessory lobe s o f brain, 48 2 Amnion, 33 Acrosternite, 7 8 defined, 4 4 defined, 8 1 Amniotic cavity , defined , 4 4 Acrotergite, 76 Amphipneustic respiration , defined , 46 1 defined, 8 1 Anal fold of wing (see Plica vannalis) Acrotrophic eg g tube, 557 Anal glands , 38 3 defined, 57 8 defined, 38 7 Aedeagal apódeme , defined , 62 0 Anal veins , 22 3 Aedeagus, 589 defined, 24 3 defined, 62 0 Anapleurite, 16 3 Afferent nerves , 46 7 defined, 19 0 defined, 50 7 647 A

648

PRINCIPLES OF

Anellus, 602 defined, 62 0 Animal behavior , 47 0 Annelida, 2 brain, 47 2 corpus pedunculatum , 486 parapodial muscles , 85 Anoplura, feedin g mechanism , 344 Anteclypeus, 11 1 defined, 12 7 Antecosta, 7 6 defined, 8 1 Antecostal suture , 76 defined, 8 1 Antenna, 130 , 13 1 defined, 15 5 muscles of , 13 2 Antennafer, 13 2 Antennal glands , 153 Antennal sclerite , 11 2 defined, 12 7 Antennal socket , 132 Antennal suture , 109 , 13 2 defined, 12 7 Anterior intestine, 375, 38 0 defined, 38 7 Anterior mesentero n rudiment , 26 , 29 defined, 4 4 Anterior nota l wing process, 183 defined, 19 0 Anterior pharynx , 285 , 352 defined, 38 7 Anus, defined , 387 Aorta, 398 , 402 defined, 42 0 Apical cel l o f sperm tube , 56 9 defined, 57 8 Apical plate, 472 Apneustic respiration , defined , 46 1 Apodemes, 49 , 54 defined, 6 8 Apophysis, defined , 68 Appendages, basa l mechanism of , 83 general structure of, 83 lobes of , 8 7 segmental, 37 segmentation of , 84 segments of , 86 Appendages of , abdomen, 26 7 Annelida, 8 5 Arthropoda, 8 3 head, 13 0 Onychophora, 8 5 thorax, 19 3 Apterygota, invaginatio n o f embryo, 33 mandibles, 13 6 pretarsus, 199 spiracles, 42 8 Arachnida, 8 chelicerae, 13 3 legs, 92 , 93 Arborizations o f nerve fiber , 46 7 defined, 50 7 Archenteron, defined , 44 Archicephalon, 40

INSECT MORPHOLOGY Archicerebrum, 31, 472 defined, 44 , 50 7 Arcuate vein (ven a arcuata) , 223 defined, 24 5 Areas o f cranium , 11 1 of thoraci c alinotum , 17 8 Arolium, 19 9 defined, 98 , 209 Arthropoda, 4 Articular corium , 19 3 Articular membrane , 5 4 Articulation o f wings, 218 Articulations, 49 , 54 defined, 6 8 dicondylic, 5 5 extrinsic, 5 5 intrinsic, 5 5 monocondylic, 5 5 Association neurone , 468 defined, 50 7 Associative memory, 47 1 Atrium o f spiracle, 46 1 defined, 46 1 Auxiliae, 19 9 defined, 20 9 Axillary cord , 218 defined, 24 3 Axillary muscles , 231 Axillary regio n o f wing, 225, 22 6 Axillary sclerites , 218 , 243 Axon (neurite) , 467 defined, 50 7 B Basal apódem e o f phallus, 58 9

defined, 62 0 Basal fol d o f win g (see Plica basalis ) Basal mechanis m o f a primitiv e appendage, 8 3 Basal plates o f phallus, 58 9 defined, 62 0 Basalar muscles , 188 , 231 Basalares, 18 4 defined, 19 0 Basement membrane , 49 , 52 defined, 6 8 of eye , 533 Basic structur e o f thoracic sterna , 167 Basicosta, 19 3 defined, 20 9 Basicostal suture , 193 defined, 20 9 Basicoxite, 19 3 defined, 20 9 Basipodite, defined, 9 8 Basisternum, 17 0 defined, 19 0 Basitarsus, 90 defined, 90 , 209 Basivalvula, defined, 622 Beak o f Hemiptera, 33 2 Behavior, animal, 47 0 conscious, 47 2 mechanistic, 47 1

INDEX Biforous spiracles , 44 4 defined, 46 1 Blastocoele, 1 7 defined, 4 4 Blastoderm, 17 , 21 defined, 4 4 Blastomeres, 1 7 defined, 4 4 Blastopore, 1 7 defined, 4 5 Blástula, 1 7 defined, 4 5 Blood, 389 clotting of , 396 corpuscles, 389 course i n circulation , 40 7 defined, 42 0 during metamorphosis , 395 plasma, 389 Blood gills , 423, 424 defined, 46 1 Body cavity , defined , 4 5 definitive, 3 8 Body form , 4 0 Body ganglion , general structure, 494 Body regions , sclerites, segmentation , 70 Body tracheation , general plan , 429 Body wall , 48 completion o f in embryo , 37 defined, 6 8 and its derivatives , 4 8 external processe s of , 55 structure of , 49 Brain, 469 , 473, 477 defined, 50 7 fiber tracts of , 493 general structur e of , 477 nerves of , 477, 479 theories o f segmentation, 47 4 Branchia, defined , 46 1 Branchiopneustic respiration , defined , 46 1 Buccal cavity , 285 , 350, 351 defined, 38 7 Bursa copulatrix , 563 defined, 57 8 C

Caeca o f ventriculus, 36 1 Calypter, 225 , 227 defined, 24 3 Calyx, o f corpus pedunculatum, 48 4 of oviduct , 56 2 defined, 57 8 Campaniform sens e organs , 521 Cap cel l of sense organ , 516 defined, 54 8 Cardia, 36 0 defined, 38 7 Cardiac sinus , 39 , 389, 397 defined, 45 , 420 Cardiac (stomodaeal ) valve , 349, 359 Cardioblasts, 3 9 defined, 45 , 420

Cardo, 134 , 142 defined, 15 5 Carpopodite, 8 6 defined, 9 8 Cells o f wing, defined, 24 3 Cellular element s o f testicular tube , 56 9 Central nervou s system , 467 , 469, 472 defined, 50 7 Cephalic lobes o f embryo, defined, 4 5 Cephalic stomodaeum, 284 Cerodecytes (see Oenocytes ) Cervical sclerites , 160 defined, 19 0 Cervix, 157 , 15 8 defined, 19 0 Chelicerae, 133 defined, 15 5 Chelicerata, 6 Chemoreceptor, defined , 548 Chiasma, defined, 507 Chilopoda, 10 antennae, 132 head, 115 , 13 2 legs, 96 mandibles, 134 , 135 preantennae o f embryo, 130 , 13 1 spiracles, 42 8 tarsus, 97 trochanters, 96 Chitin, 5 0 defined, 6 8 distribution i n animals , 51 formula of , 5 0 Chordotonal organs , 526 defined, 54 8 Chorion, 18 defined, 45 , 57 8 formation of , 560 Cibarium, 114 , 281 , 282 defined, 12 7 Circulation o f blood, 40 7 organs of , 39 7 Circumoesophageal connectives , 478 defined, 50 7 Classification o f sense organs , 512 Clavóla, 13 2 Cleavage, 16 of egg , 19 defined, 4 5 holoblastic, 1 9 meroblastic, 1 9 Cleavage cells, defined, 45 Closing apparatus o f spiracles, 43 9 defined, 46 1 Clotting o f blood, 39 6 Clypeus, 111 defined, 12 7 of Díptera , 316, 32 2 Coelenterata, 17 Coelome, defined , 4 5 Coelomic sacs, 3 5 defined, 4 5 Coleóptera, abdome n o f larva, 25 1 alimentary canal , 361 biforous spiracle s o f larva, 44 4

649

650

PRINCIPLES OF INSECT MORPHOLOGY

Coleóptera, brain , 484 eyes o f Dytiscus larva , 53 8 feeding mechanism , 286 head, 120 , 124 , 125 , 294 labium, of adult , 293 of larva , 29 0 male genitalia, 596 mandibles, 286 maxillae, 289 mouth part s o f larva, 290 thoracic tergum , 18 1 urogomphi o f larva, 27 9 wing venation, 22 8 Collateral o f nerve, 467 defined, 50 8 Collembola, 1 2 abdominal appendages , 26 8 alimentary canal , 349 cleavage o f egg , 19 development, 19 , 20, 34 endoderm, 26, 27 eyes, 54 0 invagination o f embryo, 33 mesoderm o f Anurida, 25 pleural sclerites , 164 spiracles o f Sminthurus, 428 superlinguae, 14 0 thoracic pleurites , 164 Colleterial glands , 567 defined, 57 8 Collophore, 268, 269 Colon,.375, 380 defined, 38 7 Commissure (nerve) , 473 defined, 50 8 Completion o f body wal l of embryo, 37 Complex type s o f abdominal musculature, 266 Compound eye , 105 , 529 , 542 acone type , 54 4 appositional eyes , 547 corneagenous cells , 543 crystalline cone , 544 defined, 54 8 eucone type, 544 lens, 54 3 ommatidia, 543 pigment cells , 544, 546 pseudocone type, 544 retinula, 543 , 545 superpositional eyes , 547 tapetum, 54 6 Conditional reflex , 47 1 Conductivity o f nerve, 46 5 defined, 50 8 Conjunctiva, 7 3 defined, 8 1 Connective (nerve) , 473 defined, 50 8 Consciousness, 472 Copulation, organ s of , 581 Corium, 5,4 , 19 3 Cornea, 531 defined, 54 8

Corneagenous cells , 531 defined, 54 8 Coronal suture , 10 7 defined, 12 7 Corpora allata , 60, 61, 411 defined, 42 0 óptica, 482 , 487 pedunculata, 482 , 483 ventralia, 482 , 486 Corpus céntrale , 482 , 483 Corpus luteum , 56 1 defined, 57 8 Cortical cytoplasm , 1 8 defined, 4 5 Costa (win g vein), 221 defined, 24 3 Course o f blood i n circulation , 407 Coxa, 86 , 193 , 19 4 articulation of , 19 5 defined, 98 , 209 Coxal corium , defined, 209 Coxal suture, 195 Coxomarginale, 193 defined, 20 9 Coxopleurite, 163 defined, 19 0 Coxopodite, 86 , 87 defined, 9 8 Coxosternite, 25 1 Coxosternum, 251 Cranium, area s of , 111 defined, 12 7 sutures of , 10 6 Crop (ingluvies) , 285 , 349 , 353 defined, 38 7 function of , 35 3 Cross-veins, 22 3 defined, 24 3 Crustacea, 9 compound ey e o f Astacus, 541 development o f mesenteron, 2 3 exopodite, 95 head o f Eubranchipus, 102 , 10 4 legs, 94 , 95 mandibles, 135 , 13 6 maxilliped o f Apus, 95 paragnatha, 14 0 protocephalon, 100 second antenna , 13 3 Crystalline bod y o f eye, 531 defined, 54 8 Crystalline con e of compound eye, 544 defined, 54 8 Cubitus, 22 2 defined, 24 3 Cuticula, 48, 49, 51, 52 defined, 6 8 Cystocytes, defined , 57 8 of eg g tubes , 55 5 of sper m tubes , 57 1 Cyton (see Neurocyte) Cytoplasm, cortical , 18 perinuclear, 1 9

INDEX D

Dactylopodite, 86 , 198 defined, 9 8 Definitive bod y cavity , 38 Definitive bod y form , 4 0 Definitive insect head , 103 Dendrons (dendrites) , 467 defined, 50 8 Dermaptera, mal e genitalia, 583 Deutocerebrum, 474, 478, 492 defined, 50 8 Deutoplasm, 16 defined, 4 5 Development, 1 4 alimentary canal , 29, 347 blastoderm, 17 , 19 cleavage, 19 completion o f body wall , 37 definitive bod y cavity , 38 definitive bod y form , 4 0 dorsal bloo d vessel, 39, 397 dorsal ocellus , 534 early stage s of , 10 ectoderm, 1 7 embryonic coverings , 32 endoderm, 2 5 fat body , 39 gastrulation, 17 , 28 germ band, 2 1 germ cells , 22 inner ger m layers, 23 median nerve , 501 mesoderm, 17 , 25 mesodermal organs , 38 nervous system , 30 , 472 organs o f reproduction, 39 , 57 4 segmental appendages , 3 7 segmentation, 3 5 stomodaeal nervou s system, 501 tracheal system , 40, 426 wings, 21 4 Diaphragm, defined , 42 0 dorsal, 397 , 404 ventral, 398 , 405 Dicondylic joint, 55 , 194 defined, 20 9 Diffusion tracheae , 448 defined, 46 2 Digestive cell s of ventriculus, 363, 370, 371 defined, 38 7 Dilator muscle s o f alimentary canal , defined, 38 7 proctodaeal, 37 5 stomodaeal, 285 , 351 Dioptric apparatus , 53 0 defined, 54 8 Diplopoda, 1 0 legs, 95 mandibles, 13 4 Diplura (Thysanur a entotrophica) , 12 head, 136 hypopharynx, 115 mandibles. 115 , 14 3 thoracic spiracles, 428

651

Diptera, alimentar y canal , 360 eyes o f larvae, 537 feeding mechanism , 311, 31 5 germ cells , 22 male genitalia, 605 methods o f feeding o f Calliphora, 32 4 mouth parts o f adult hors e fly, 316 of adul t muscoi d types, 320 , 324 of larva , 31 1 of muscoi d larvae, 31 3 of orthorrhaphou s larvae, 312 optic lobe, 490 peritrophic membrane, 368 photoreceptors o f larva, 53 0 piercing, type o f mouth parts, 32 4 ptilinal suture, 109 pretarsus, 20 0 spiracles o f larva, 44 6 sponging type o f mouth parts, 320 thoracic pleura, 185 thoracic sterna, 17 1 thoracic tergum, 181 tracheation o f larva, 437 , 43 8 wing venation, 22 8 Discharge o f eg g fro m ovariole , 560 Distal proces s o f sensory nerv e cell , defined, 508 , 548 Dorsal abdomina l muscles, 261, 264 Dorsal blastoderm , defined , 4 5 Dorsal bloo d vessel, 398 defined, 42 0 development of , 39, 39 7 Dorsal diaphragm , 397, 404 defined, 42 0 Dorsal muscle s o f abdomen, 26 1 of thorax , 187 , 229 , 230 Dorsal ocelli , 529, 533 Dorsal orga n of embryo, defined, 4 5 Dorsal sinus , 398, 405 defined, 42 0 Dorsal trachea , 429 defined, 46 2 Dorsal trachea l trunk , defined , 462 Dorso-pleural line , 71 defined, 8 1 Dorsum, 70 defined, 8 1 Ductus ejaculatorius , 567, 572 defined, 579 , 620 E

Ecdysis, 64 , 65 defined, 6 8 Ectoderm, 1 7 defined, 45 , 68 Ectodermal glands , 60 Ectophallus, 58 9 defined, 62 0 Effector, defined , 50 8 Efferent nerves , 467 defined, 50 8 Ejaculatory duc t (see Ductus ejaculatorius ) Egg, o f Collembola , 1 9 general for m an d structure , 18 , 19

652

PRINCIPLES OF INSECT MORPHOLOGY

Egg chambe r o f ovariole, 555 defined, 57 9 Egg guide , defined, 62 2 Egg-laying organs , 281 Egg tub e of ovariole, 554 defined, 57 9 Eleventh abdomina l segment, 255, 256 Elimination, 413 organs of , 413 Embryo, completio n of body wal l of , 3 7 coverings of , 32, 4 4 invagination of , 33 involution of , 33 Embryonic coverings , 32 Embryonic membranes, 34 Empodium, 199 defined, 20 9 End chambe r o f gonadial tube (see Germarium ) Endite, 87 , 89 defined, 9 8 Endocuticula, 4 9 defined, 6 8 Endoderm, 17 , 25 of Anurida, 27 defined, 4 5 of Lepisma, 2 7 of Pieris, 27 Endophallus, 58 9 defined, 62 1 Endopodite, 89 defined, 9 8 Endopterygota, 1 3 Endoskeleton, 4 9 defined, 6 8 Endotheca, 58 9 defined, 62 1 Enteric epithelium , 348 Enveloping cel l o f sense organ, 51 6 defined, 54 8 Ephemerida, 12 abdominal appendage s o f larva, 27 3 hypopharynx, 140 male genitalia , 583 ocelli, 535 wing base , 220 wing venation, 22 2 Epicranial suture, 10 7 defined, 12 7 Epicranium, defined , 12 7 Epicuticula, 49 , 51 defined, 6 9 Epidermis, 49 , 52 defined, 6 9 Epididymis, 567 defined, 57 9 Epimere, 590 defined, 62 1 Epimeron, 165 defined, 19 0 Epineural sinus, 38, 389 defined, 45 , 420 Epiopticon, 488 Epiphallus, 593 defined, 62 1

Epipharynx, 106 , 113 defined, 12 7 Epipleurites, 184 defined, 19 0 Epipodite, 89 defined, 9 9 Epiproct, 25 5 Episternum, 165 defined, 19 0 Epistoma, 11 2 Epistomal suture, 108 defined, 12 7 Epithelial sheath o f ovariole, 55 3 of testicula r tube , 56 8 Eulabium, 146 Euplantulae, 19 8 defined, 20 9 Eupleuron (see Anapleurite ) Eurypterida, 7 Eusternum, 16 7 defined, 19 0 Eutrochantin (see Coxopleurite) Excretion, 413 Excretory organs , 413 alimentary canal , 415 integument, 41 4 Malpighian tubules, 378 , 417 nephrocytes, 415 pericardial cells , 416 urate cells , 410 Exite, 87 , 89 defined, 9 9 Exocuticula, 49 defined, 6 9 Exopodite, 89 , 95 defined, 9 9 Exopterygota, 13 Exoskeleton, 49 defined, 6 9 External genitalia , 550 , 581 female, 581 , 60 7 male, 581 , 58 2 External processes o f body wall , 55 multicellular, 56 noncellular, 56 unicellular, 5 7 External respiration, 42 2 defined, 46 2 Exteroceptors 47 0 Extraoral cavity (see Preoral cavity) Exuviae, defined , 69 Exuvial glands , 65-6 7 defined, 6 9 Eye, defined , 54 8 Eyes, 52 8 compound eyes , 529, 542 dorsal ocelli , 529,.533 general structure of , 530 lateral ocelli , 529, 536 simple lateral eye s o f adult insects , 540 F

Fastigium, 11 1 Fat body , 407

INDEX Fat body , defined , 420 development of , 39 Fat cell , defined , 420 Fatigue in nerve , 465 Feeding mechanis m of , Anoplura , 344-346 Diptera, 311 , 32 5 Hemiptera, 328-343 Hymenoptera, 295-30 2 Lepidoptera, 302-31 1 Neuroptera an d Coleóptera , 286-29 5 Siphonaptera, 325-326 Thysanoptera, 326-32 8 Female accessor y glands , 566 Female externa l genitalia , 607 Female interna l reproductiv e organs , 552 (See also Reproductiv e organs ) Femur, 86 , 193 , 197 defined, 99 , 20 9 Fiber tract s o f brain, 49 3 Filter apparatu s o f spiracle, 439 defined, 46 2 Filter chamber , 383 defined, 38 7 First axillary , 218 defined, 24 3 First maxillae , defined, 15 5 (See also Maxillae) First thoraci c spiracle, o f caterpillar, 44 3 defined, 19 1 of Dissosteira, 440 First trochanter , 86 defined, 9 9 First valvifers , 61 1 First valvulae , 610 Flagellum o f antenna, 13 2 defined, 15 5 Flight o f insects, 240-24 3 hovering, 243 speed of , 24 2 steering, 242 wing motio n of , 23 5 Follicle (see Egg chambe r o f ovariole ) Follicle cell s o f ovariole , 55 5 defined, 57 9 Food canal , 338 Food meatus , 281 Foramen magnum , 105 defined, 12 7 Fourth axillary , 219 defined, 24 3 Frenulum, defined , 24 3 Frons, 11 1 defined, 12 7 Frontal ganglion , 501 defined, 50 8 Frontal ganglio n connectives, 481 defined, 50 8 Frontal sutures, 107 defined, 12 7 Frontoclypeal region , 111 modifications of , 11 9 Frontoclypeal sutur e (see Epistomal suture ) Fulturae, 11 5 defined, 12 7

653

Furca, 169 , 171 defined, 19 1 Furcasternum, 170 , 17 1 defined, 19 1 Furcula, 268 , 270

G Galea, 142 defined, 15 5 Ganglia, 32, 468 abdominal, 477 cephalic, 473-475 thoracic, 476 Ganglion, defined , 50 8 general structur e of , 494 internal structur e of , 49 5 suboesophageal, 494 tracheation of , 49 8 Gastrocoele, 17 defined, 4 5 Gastrula, 1 7 defined, 4 5 Gastrulation, defined , 4 5 generalized, 16 resume o f in insects , 2 8 Gena, 111 defined, 12 7 General organizatio n and development , 14 Genital chamber , 552 defined, 57 9 of female , defined , 62 2 and it s derivatives , 56 3 of male , defined , 62 1 Genital ridge , 574 defined, 57 9 Genitalia, external , 550 , 581 of female , 60 7 of male , 58 2 internal, 550 of female , 55 2 of male , 56 7 Germ band , 22 defined, 4 5 Germ cells , 14 , 21 defined, 45 , 57 9 and th e soma , 14 Germ layers, 23 Germ tract, 22 defined, 4 5 Germarium, 575 defined, 57 9 of eg g tube 55 5 of sper m tube , 56 9 Gills, blood , 424 defined, 46 2 proctodaeal, 425 , 451 tracheal, 45 0 Glands, anal , 383 antennal, 15 3 colleterial, 56 7 ectodermal, 60 exuvial, 65 female accessory , 566 of hea d appendages , 15 3

654

PRINCIPLES OF INSECT MORPHOLOGY

Glands, labial , 154 male accessory, 567, 573 mandibular, 15 3 maxillary, 154 multicellular, 61 of Philippi , 30 5 of poiso n setae, 58 , 5 9 preputial, 573 of stin g o f bee , 61 9 unicellular, 6 1 Glia cells , 468 defined, 50 8 Globuli cells , 469 , 482 defined, 50 8 Glomerulus, 468 defined, 50 8 Glossae, 14 8 defined, 15 5 Gnathal region of body, 4 1 Gnathal segments , 100 defined, 4 5 Gnathocephalon, 101 , 10 2 defined, 12 7 Gonad, defined , 57 9 Gonapophyses, 609 female, defined , 62 2 male, defined , 621 Gonopore, defined , 622 female, 57 9 male, 62 1 Gonosomite, defined , 62 1 Gonostyli, defined , 62 1 Gula, 122 , 123 of Coleóptera , 292, 293 defined,127 of Neuroptera , 29 0 Guiar sutures , 12 5 defined, 12 7 H

Haemocoele, 38 , 389 defined, 45 , 42 0 Haemocyanin, 392 Haemocytes, 389 , 390, 393 defined, 42 0 Haemoglobin, 392 Haemolymph, 389, 390 composition of , 39 0 defined, 42 0 hydrogen-ion concentratio n of, 391 Hair sens e organ , 515 Harpagones, 592 defined, 62 1 Hatching, 4 3 Hatching membrane , 44 defined, 4 5 Head, 10 0 appendages of , 13 0 areas of , 11 1 definitive, 42 , 10 3 definitive structur e of , 104 endoskeleton of , 10 4 general externa l structur e of , 105 general morpholog y of, 10 0

Head, gnathocephalon , 102 hypognathous type of , 105 modification of , 11 8 modifications i n frontoclypea l region, 119 in posterio r ventra l region, 121 opisthognathous typ e of , 10 5 procephalon, 101 prognathous typ e of, 105 sutures of , 10 6 Head of , Anoplura, 344 Coleóptera, adult , 124 , 125, 294 larva, 120 , 124 , 294 Díptera, adult , 323 larva, 31 3 Hemiptera, 329-33 2 Hymenoptera, adult , 120 , 122 , 300 larva, 120 , 122 , 296 Isoptera, 126 Lepidoptera, adult , 307 , 308, 310 larva, 12 1 Neuroptera, 125 , 147 Orthoptera, 109 , 110 Thysanoptera, 14 7 Thysanura, 32 6 Heart, 39 9 defined, 42 0 reversal o f beating of , 40 2 Heart chambers , 399, 400 defined, 42 0 Hemimetabola, 1 3 Hemipneustic respiration , defined , 46 2 Hemiptera, ai r chambe r of cicada, 449 beak, 332 feeding mechanism , 328 filter chambe r o f Homoptera, 38 3 gastric caec a o f Heteroptera, 362 head, 329-33 2 hypopharynx, 332 labium, 338 male genitalia, 59 7 mandibular an d maxillar y bristles, 336-33 8 mouth, 333 ovipositor, 61 5 piercing mechanism , 339-343 salivary syringe , 335 sucking pump, 334 thoracic muscles , 230 thoracic sterna, 17 1 wing venation, 22 8 Hepatic cell s (see Nephrocytes) Hermaphroditism i n insects , 550 , 551 Hexapoda, 1 legs of , 9 7 Holoblastic cleavage , 19 defined, 4 5 Holometabola, 1 3 Holopneustic respiration, defined , 46 2 Humeral plate, 21 8 defined, 24 3 Hymenoptera, divide d femur , 19 7 eyes o f tenthredinid larvae , 53 8 feeding mechanis m of adult, 29 7 head, o f adult, 120 , 122, 300 of larva , 120 , 122 , 296 larval abdomina l appendages, 278

INDEX Hymenoptera, mal e genitalia , 602 mouth part s o f larva, 295 ocelli o f ant, 534 , 53 5 ovipositor, 61 6 thoracic tergum, 181 Hypandrium, defined , 62 1 Hyperpneustic respiration , defined , 462 Hypocerebral ganglia , 502 Hypodermis (see Epidermis ) Hypognathous type o f head, 10 5 Hypomere, 590 defined, 62 1 Hypopharyngeal suspensorium , 115 Hypopharynx, 105 , 113 , 14 0 defined, 12 7 Hypoproct, 25 5 Hypostoma, 11 2 defined, 12 7 Hypostomal bridge , 112 , 122 Hypostomal sutures, 10 8 defined, 12 7 I

Ileum, 375 , 380 defined 38 7 Ingestión, organ s of , 280 Ingluvies (crop) , 285, 349 , 353 defined, 38 7 Instar, 64 Intercalary appendages , 13 3 Intermediate mesenteron rudiment , 26, 29 defined, 4 5 Internal genitalia , 550 Internal respiration, 422 defined, 46 2 Internuncial neurones , 468 Interoceptors, 47 0 Intersegmental membrane , defined , 8 1 Intersternite, 76 defined, 81 , 19 1 Intervalvulae, 611 defined, 62 2 Intestine, 375 anterior, 375 , 380 defined, 38 7 posterior, 375 , 380 Invagination o f embryo, 33 defined, 4 6 Involution o f embryo, 3 3 ' defined , 46 Iris, 534 defined, 54 8 Iris pigment cells , 546 defined, 54 8 Ischiopodite. 86 defined, 9 9 Isoptera, endoderm , 26 head, 126 labium, 149 J

Johnstonian sense organ , 527 defined, 54 8

655

Joints, 54 of legs , 19 3 Jugal fol d o f wing (see Plic a jugali s Jugal region of wing, 225, 227 defined, 24 4 Jugal veins, 223 Jugum, 22 5 L

Labial glands, 150 , 154 defined, 15 0 Labial musculature , 15 1 Labial suture , 146 defined, 15 5 Labi estípites, 147 defined,155 Labium, 130 , 14 5 defined, 15 5 generalized structur e of , 146 lígula, 146 , 148 mentum, 15 0 musculature of , 15 1 palpi, 146 , 148 palpigers, 147 postlabium, 14 8 postmentum, 14 9 prelabium, 146 prementum, 14 6 submentum, 15 0 Labium of , Coleóptera, adult , 29 3 larva, 290 Díptera, 319 Hemiptera, 33 8 Isoptera, 14 9 Lepidoptera, 310 Neuroptera, 147 , 289 Odonata, 14 9 Orthoptera, 149 , 150 Thysanoptera, 32 7 Thysanura, 149 , 150 Labral muscles , 113 Labrum, 11 3 defined, 12 7 muscles of , 11 3 Lacinia, 142 defined, 15 5 Lamina ganglionaris , 487 supra-analis, 257 Laminae infra-anales, 25 7 Lancets, defined , 62 2 Lateral abdomina l muscles , 262, 266 Lateral nerve cords , defined , 4 6 Lateral ocelli, 491, 529 Lateral oviduct (see Oviductus lateralis) Lateral plate o f embryo, 2 5 defined, 4 5 Lateral trachea l trunk , defined , 462 Laterosternite, 167 , 250 defined, 19 1 Laterotergite, 72, 250 defined, 8 1 Legs, 19 3 articulation o f segments, 19 4 basal mechanis m of, 201

656

PRINCIPLES OF

Legs, basa l muscle s of , 201-203 joints of , 19 3 mechanism of , 200, 201 muscles of , 200, 203-209 segmentation of , 84 segments of , 193-20 0 structure of , 193 thoracic, 19 3 Legs of , Arachnida , 92 caterpillar, 97 Chilopoda, 96 Crustacea, 9 4 Diplopoda, 9 5 Dissosteira, 203 Hexapoda, 9 7 Pauropoda, 9 6 Protura, 9 7 Pycnogonida, 9 3 Symphyla, 95 Trilobita, 9 0 Xiphosura, 91 Lens o f eye, 53 1 defined, 54 8 Lepidoptera, eyes , o f adult, 546 , 54 7 feeding mechanism , 302 of adult , 307 , 30 8 generalized mout h parts , 307 genital openings of female, 563 head, o f adult, 307 , 308 , 31 0 of larva , 12 1 larva abdomina l appendages , 275-27 8 alimentary cana l 358 , 37 7 brain, 481 eyes, 53 9 feeding an d spinnin g organs , 303 feeding organs , 303

leg 97 , 19 4 median nerve , 499 nervous system , 47 6 spinning organs , 303 , 304 stomodaeum, 306 thoracic muscles , 267 tracheation, 432-43 6 male genitalia, 60 1 mesenteron, 2 8 middle plat e cell s o f embryo, 27 , 2 8 optic lobe, 489 reproductive organ s o f female, 56 1 thoracic tergum, 181 transformation o f alimentary canal , 35 9 Leucocytes (see Haemocytes) Ligula, 146 , 14 8 defined, 15 5 Limb basis, 86 , 87 defined. 81 , 99 Limb segmen t defined , 8 5 Lingua, 11 4 Lobes o f appendages, 8 7 M

Male accessor y glands , 567 Male genita l chamber. , 587 defined, 62 1

INSECT MORPHOLOGY Male genita l segment , 58 6 defined, 62 1 Male genitali a (external) , 582 characteristics o f in pterygot e orders , 59 2 genital segment , 586 periphallic organs , 591 phallic organs . 587 Male genitali a of , Coleóptera, 59 6 Dermaptera, 583 Diptera, 60 5 Ephemerida, 58 3 Hemiptera, 59 7 Hexapoda wit h paire d gonopores , 583 Hymenoptera, 60 2 Lepidoptera, 60 1 Mecoptera, 59 9 Odonata, 59 3 Orthoptera, 594 Protura, 58 3 Pterygota, 586 Thysanura, 58 5 Trichoptera, 60 1 Male gonopore , 567 defined, 62 1 Male organ s o f reproduction (internal) , 567 (See also Reproductiv e Organs ) Malpighian tubules , 378 , 413, 41 7 defined, 42 0 function o f 41 9 histology of , 417 muscles of , 41 8 number of , 379 Mandibles, 130 , 13 3 defined, 15 5 glands of , 15 3 morphology of , 13 7 Mandibles of , apterygot e insects , 13 6 caterpillar, 30 3 Chilopoda, 13 5 cicada, 336 Coleóptera, 286 Crustacea, 13 5 Diplopoda, 13 4 horse fly, 317 Lepidoptera, 30 9 Neuroptera, 28 6 pterygote insects , 13 7 Thysanoptera, 32 7 Mandibular glands , 15 3 defined, 15 5 of Machilis, 15 3 of Mantis, 153 Mandibulata, 8 Maxillae, 130 , 141 defined, 15 5 general structur e of, 141 glands of , 15 4 muscles of , 14 2 structural variations of , 143 Maxillae of , Apterygota , 14 4 cicada, 336, 33 7 cockroach, 144 Coleóptera, 289 horse fly , 31 7

Lepidoptera, 307-30 9

INDEX Maxillae of , Neuroptera , 289 Thysanoptera, 32 7 Maxillary glands , 154 defined, 15 5 Maxillipeds, defined , 15 6 Maxillulae, defined , 15 6 Mechanism o f respiration i n tracheoles , 459 Mechanistic theory o f behavior, 47 1 Mecoptera, mal e genitalia, 599 thorax, 17 9 wing venation, 22 4 Media, 222 defined, 24 4 Median nerve , 495, 498 defined, 46 , 50 8 origin of , 3 2 Median oviduc t (see Oviductu s communis ) Median plate s o f wing base, 218 , 219 defined, 243 , 244 Medulla extern a (o f optic lobe ) 48 8 interna, 48 9 Medullary tissue , 46 8 defined, 50 8 Membrana fenestrata , 533 Mentum, 150 defined, 15 6 Meroblastic cleavage , 19 defined, 4 6 Meroistic typ e of egg tube, 55 7 defined, 57 9 Meron, 196 defined, 20 9 of Diptera , 19 6 Meropodite, 8 6 defined, 9 9 Mesenchyme, 17 defined, 4 6 Mesenteron, 348, 359 defined, 46 , 387 development of , 23, 26-29 Mesenteron rudiments , 24 , 26, 29 defined, 4 6 Mesoblast, 1 7 defined, 4 6 Mpsoderm, 17 , 25 defined, 4 6 Mesodermal organs , developmen t of , 38 Mesothorax, 15 7 defined, 19 1 Metamere, 3 5 defined, 46 , 8 1 Metapneustic respiration , defined, 462 Metathorax, 157 defined, 19 1 Metopic sutur e (see Coronal suture) Micropyle, 18 , 560 defined, 4 6 Middle plate of embryo, 25 defined 4 6 Monocondylic joint, 55, 194 defined, 20 9 Mórula, 17 defined, 4 6 Motor nerv e endings , 507 Motor nerves , 646

657

Motor nervou s system , 3 0 Motor neurone , defined, 50 8 Motor an d sensor y nerves , 466 Moulting, 64 , 65 defined, 6 9 Moulting fluid , 6 7 Moulting glands , 65 , 66, 67 defined, 6 9 Moulting hormone , 67, 68 Mouth, 106 , 281 defined, 128 , 38 7 Mouth cavity , 106 , 281, 282 Mouth part s (see Organs o f ingestión) general structure of , 133-15 3 Movements o f wings , 233 , 23 6 Muscles, attachmen t of , on body wall , 62 , 68 development of , 3 8 in metamorphosis , 6 8 Musculature of , abdomen, 257-26 7 antennae, 13 2 cerci, 25 5 cibarium, 282 hypopharynx, 11 4 labium, 151 labrum, 113 legs, 189 , 200 , 203-209 Malpighian tubules, 41 8 mandibles, 135-13 9 maxillae, 142 ovipositor, 61 1 parapodia, 8 4 phallic organs , 58 8 primitive appendages , 8 3 proctodaeum, 376 salivarium, 283 stomodaeum, 284 , 351 thorax, 18 6 *ventriculus, 370 wings, 228-233 Myotome, defined , 8 1 Myriapoda, 1 0 N

Neala, 227 Nebenlappen (o f brain), 482 Neck (see Cervix) Neopterygota, 1 2 Nephrocytes, 415 defined, 42 0 Nerve, defined , 50 8 Nerve commissures , 32 Nerve connectives , 32 Nerve fiber , defined , 50 8 Nerve trunks , 467 defined, 50 8 Nerves, o f brain, 477 , 479 of ganglion , 49 5 motor ending s of , 507 sensory ending s of , 504 Nervous system, 46 4 animal behavior , 470 brain an d it s nerves , 477 central system, 469 , 472 deutocerebrum, 492

658

PRINCIPLES OF INSECT MORPHOLOGY

Nervous system , developmen t an d organizatio n of, 30 , 46 6 fiber tract s o f brain, 49 3 functions of , 46 5 general structur e of , 465, 473 integumentary, 50 5 internal structure o f brain, 482 median nerve , 498 motor nerv e endings , 507 nerves o f brain, 47 9 ocellar centers , 49 2 optic centers , 487-49 1 optic lobes , 478 organization of , 466 origin of , 3 0 peripheral, 469 , 503 protocerebrum, 48 2 sense organs , 59 , 470, 510 sensory neurones , 503 stomodaeal system , 50 1 structure o f body ganglion , 494 suboesophageal ganglion , 494 sympathetic syste m (see Stomodaea l system) tritocerebrum, 49 2 ventral nerv e cord , 494 Nervus antennalis , 48 0 ganglii occipitalis , 480 labrofrontalis, 48 1 lateralis, 48 0 ocellarius, 479 opticus, 47 9 postantennalis, 482 subpharyngealis, 48 1 tegumentalis, 48 0 Neural groov e o f embryo, 31 defined, 4 6 Neural ridge s o f embryo, 3 1 defined, 4 6 Neurilemma, 467 defined, 50 8 Neurite (see Axon) Neuroblasts, 3 0 defined, 4 6 Neurocyte (cyton) , 467 defined, 50 8 Neurone, 46 7 defined, 50 8 Neuropile, defined , 508 Neuroptera, abdomina l appendage s o f larva , 272-274 feeding mechanism , 286 head o f adult, 125 , 14 7 of larva , 12 5 ocelli o f larva o f Myrmeleon, 53 8 Nidus, 365 defined, 38 7 Notaulices, 180 defined, 19 1 Notum, 71 defined, 81 , 19 1 Noncellular processe s o f body wall , 56 Nutritive (nurse ) cell s o f gonads, defined , 57 9 of ovarioles , 555 , 557 , 55 8 of testicula r tubes , 570 , 57 1

O Occipital arch , 111 , 11 2 defined,128 Occipital condyle , 112 defined, 12 8 Occipital forame n (see Foramen magnum) Occipital ganglion , 502 defined, 50 9 Occipital suture , 108 defined, 12 8 Occiput, 112 defined, 12 8 Ocellar centers , 482 , 487, 492 Ocellar pedicels , 478 Ocelli, 10 5 development of , 53 4 dorsal, 533 lateral, 536 Ocellus, defined , 54 8 Ocular sclerite , 11 2 defined, 12 8 Ocular suture , 10 9 defined, 12 8 Odonata, 1 2 development o f mesenteron, 2 7 gills o f zygopterous larvae, 25 5 male genitalia , 593 optic lob e o f larva, 49 0 structure o f body ganglion , 496 terminal segments , 257 trochanters 97 , 193 wing base , 220 , 221 wing venation , 222 Oenocytes, 60 , 61 , 410 defined, 42 0 Oesophagus, 285 , 349 , 352 defined, 38 7 Ommatidium, 543 defined, 54 8 Onychophora, 3 early development , 1 8 leg muscles , 8 5 Oócytes, 555 defined, 57 9 Oógonia, 55 5 defined, 57 9 Oótheca, deáned , 4 6 Opisthognathous type o f head, 105 Optic centers , 48 7 of Aeschna larva , 49 0 of blo w fly , 49 1 of Deilephila, 48 9 Optic lobes, 478 Optic nerves , 487 Optic organs , genera l structure , 530 Opticon, 488 Organ o f Johnston, 527 , 528 defined, 54 8 Organization, general , 14 Organs of , alimentation, 34 7 circulation, 39 7 copulation an d egg-laying , 581 distribution, conservation , and elimination, 389 elimination, 378, 41 3

INDEX Organs of , ingestión , 280 reproduction, 550 respiration, 422 Orthoptera, abdominal muscles of Dissosteira, 264 alimentary cana l o f Dissosteira, 357 antennal gland s o f Periplaneta, 15 3 brain o f Dissosteira, 478, 479 coxa o f Dissosteira, 196 female reproductiv e organ s o f Dissosteira 55 9 head o f Blatta, 109 of Dissosteira, 139 of Gryilus, 11 0 labium, 149 , 152 male genitalia, 594 mandibles o f Dissosteira, 139 maxilla o f Blatta, 14 4 muscles o f abdomen, of Dissosteira, 264-266 of leg , 203-209 nervous syste m o f Dissosteira, 476 ovipositor, 612 pharynx o f Blatta, 352 preantennae o f Dixippus, 13 1 preoral cavity , 11 4 pretarsus, 200 proventriculus o f Blatta, 35 6 salivarium, 114 , 152 stomodaeal muscle s o f Dissosteira, 354 tentorium, 11 6 thoracic pleura, 185 thoracic spiracle s o f Dissosteira, 440 thoracic sterna o f Blatta, 170 thoracic tergum, 181 wing muscle s o f Dissosteira, 230 wing venation, 22 4 of Blatta, 22 2 of Dissosteira, 230 Ostia o f heart, 40 1 defined, 42 0 Ostium bursae , defined , 622 Ovarial ligament, 579 Ovariole, 552 defined, 57 9 origin an d relatio n o f cells of , 55 7 structure of , 553 Ovariole pedicel , 556 defined, 57 9 Ovary, 552 , 574 defined, 57 9 development of , 574 Oviductus communis , 552, 562 defined, 57 9 Oviductus lateralis , 552, 561 defined, 57 9 Ovipositor, 60 7 defined, 62 2 general structure o f in pterygot e insects , 610 Ovipositor of , Hemiptera , 61 5 Hymenoptera, 61 6 Orthoptera, 612 Pterygota, 61 0 Thysanura, 60 9 Oviporus, defined , 62 2 of Lepidoptera , 56 3 Ovum, defined, 57 9

659 P

Palpifer, 14 2 defined, 15 6 Palpiger, defined , 15 6 Palpus, defined , 15 6 (See also Telopodite ) Panoistic type of egg tube, 557 defined, 57 9 Pantapoda (see Pycnogonida) Paraglossae, 14 8 defined, 15 6 Paragnatha, 13 0 defined, 15 6 of isopod , 14 0 Parameres, 59 0 defined, 62 1 Paranotal lobes , 158 , 21 2 defined, 24 4 Paraprocts, 25 5 Parapsidal furrows , 18 1 defined, 19 1 Parapsides, 18 1 defined, 19 1 Paraptera (see Epipleurites ) Paratergite (see Laterotergite ) Parietals, 11 1 defined, 12 8 Pars, intercerebralis, 482 Patella, 86 defined, 9 9 Pauropoda, 1 0 legs of , 9 6 Pedicel o f antenna, 13 1 of ocellus , 478 , 49 2 of ovariole , 55 6 Pedipalp, 13 3 defined,156 Penis, 56 7 defined, 62 1 Pericardial cavity , 398 Periopticon, 48 8 Periphallic organs , 586, 591 defined, 62 1 Peripheral nervous system , 467 , 469, 503 defined, 50 9 Peripneustic respiration , defined , 462 Periproct, 35 , 70 defined, 4 6 Peristome, 11 2 defined, 12 8 Peritoneal sheath of , ovary, 552 testis, 568 Peritreme, 43 9 defined, 46 2 Peritrophic membrane , 366 defined, 38 7 Peri visceral sinus , 398 defined, 42 1 Phagocytes, 393 , 396 defined, 42 1 Phallic organs , 586, 587 Phallobase, 58 9 defined, 62 1

660

PRINCIPLES OF INSECT MORPHOLOGY

Phallocrypt, 589 defined, 62 1 Phallomeres, 587 defined, 62 1 Phallotheca, 589 defined, 62 1 Phallotreme, 58 9 defined, 62 1 Phallus, 56 7 defined, 62 1 Pharynx, 285, 349, 352 anterior, 352 defined, 38 8 posterior, 352 Phonoreceptor, defined, 54 8 Photoreceptors (see Eyes ) of muscoi d maggot , 530 Phragmanotum (see Postnotum ) Phragmata, 77 , 161 defined, 19 1 Pigment cell s of eyes, 533 Planta, 199 defined, 20 9 Plate sense organs, 523 Plecoptera, pleuro n o f prothorax, 164 wing venation, 224 Pleura, thoracic , 161 Pleural apophysis , 165 defined, 19 1 Pleural region , 70 defined, 8 1 Pleural ridge, 165 defined, 19 1 Pleural sclerites, 71 , 78, 162 Pleural suture, 165 defined, 19 1 Pleural win g process, 175 defined, 19 1 Pleurite, defined , 8 1 Pleurites, 71, 79 abdominal, 250 of Apterygota , 16 3 of Diptera , 185 , 18 6 of winge d segment, 18 4 Pleuron, 79 , 88 of Apterygota , 16 3 defined, 81 , 19 1 of Pterygota , 163 , 18 3 Pleurosternite, 167 , 251 Pleurosternum, 251 Pleurostoma, 112 defined, 12 8 Pleurostomal suture , 108 defined, 12 8 PI euro-ventral line , 71 defined, 8 1 Plica basalis , 226 defined, 244 Plica jugalis , 226 defined, 24 4 Plica vannalis , 225 defined, 24 4 Podial region, 70 Podites (podomeres) , 37, 85 defined, 9 9

Poison seta, 5 8 Polytrophic type of egg tube, 55 defined, 57 9 Postalar bridg e (postalare) , 174 defined, 19 1 Postantennal appendages , 130, 133 defined, 15 6 Postclypeus, 111 defined, 12 7 Postcoxal bridge (postcoxale) , 165 , 184 defined, 19 1 Postcubitus, 22 3 defined, 24 4 Posterior intestin e (rectum) , 375, 380 defined, 38 8 Posterior mesentero n rudiment, 26, 29 defined, 4 6 Posterior notal win g process , 183 defined, 19 1 Posterior pharynx , 285 defined, 38 8 Postfrontal sutures , 107 defined, 12 8 Postgenae, 112 defined, 12 8 Postlabium, 146 , 148 Postmentum, 14 6 defined, 15 6 Postnotum (phragmanotum) , 77 defined, 8 1 Postoccipital suture , 103 , 108 defined, 12 8 Postocciput, 111 , 11 2 defined, 12 8 Poststernite, 80 defined, 8 1 Posttergite, 80 defined, 8 1 Prealar bridg e (prealare) , 174, 183 defined, 19 1 Preantennae, 13 0 defined, 15 6 Precosta, 217 defined, 24 4 Precoxal bridg e (precoxale) , 165 , 184 defined, 19 1 Prefemur, 8 6 Pregenital segments of abdomen, 247, 252 Prelabium, 14 6 defined, 15 6 Premandibular appendage s (see Postantenna l appendages) Prementum, 146 defined,156 Preoral cavity , 106 , 281 Prepectus, 184 defined, 19 2 Preputial glands , 573 Preputial sac, 590 Prescutal suture, 180 defined, 19 2 Prescutum, 18 0 defined, 19 2 Presternal suture, 170

INDEX Presternum, 170 defined, 19 2 Prestomum, 32 2 Pretarsus, 86, 98 , 193 , 198 , 199 defined, 99 , 21 0 Primary cutícul a (exocuticula) , 49 Primary segmentatio n 7 4 defined, 8 1 Primary sense cells, 504 Proboscis of , Anoplura, 344 Diptera, 320 Lepidoptera, 30 9 Procephalon, 101 defined, 12 8 Proctodaeum, 348 , 374 anal glands , 38 3 anterior intestine , 38 0 defined, 46 , 38 8 development of , 3 0 histology of , 37 6 Malpighian tubules, 378 , 417 muscular sheat h of , 381 posterior intestine , 375 , 380 pyloric valve , 377 pylorus, 375 , 376 rectal organ s ("glands") , 381 rectal valve , 380 subdivisions of , 37 5 Prognathous typ e o f head, 10 5 Propneustic respiration , defined , 462 Propodeum, 252 defined, 19 2 Propodite, 8 6 defined, 9 9 Proprioceptors, 47 0 Prostomium, 36 , 70 defined, 4 6 Prothorax, 157 , 17 2 defined, 19 2 Protocephalon, 40, 100 defined, 4 6 Protocerebral lobes , 482 Protocerebrum, 47 3 478 , 482 defined, 50 9 Protopodite, 9 4 defined, 9 9 Protura, 1 1 abdominal appendages , 2 6 abdominal segments , 24 6 leg o f Eosentomon, 97 male genitalia , 58 3 reproductive organs , 576 thoracic pleurites , 163 , 164 Proventriculus, 349 , 354 defined, 38 8 functions of , 356 Pseudotracheae, 31 9 Pteralia, 218 defined, 24 4 Pterothorax, 160 , 17 3 defined, 19 2 general structur e of , 173 Pterygota, 12 Ptilinal suture , 10 9 defined,128

661

Ptilinum, 109 defined, 12 8 Pulsating membranes , 404 , 406 defined, 42 1 Pulvilli, 19 9 defined, 21 0 tarsal, 19 8 Pycnogonida, 7 legs of , 93 , 9 4 Pygopods, 253 , 274, 275 (See also Socii ) Pyloric valve , 349 , 377 defined, 38 8 Pylorus, 375 , 376 defined, 38 8 R

Radius, 222 defined, 24 4 Rami valvularum, defined , 622 Receptaculum semini s (spermatheca) , 552 , 565 defined, 58 0 Receptive apparatus , defined , 54 8 of opti c organs , 53 0 Receptors (sensory) , defined , 50 9 external an d internal , 470 Rectal caecum , 375 Rectal organ s ("glands") , 381 defined, 38 8 Rectal sac, 375 defined, 38 8 Rectal valve , 380 defined, 38 8 Rectum, 375 , 380 defined, 38 8 Recurrent nerve , 501 defined, 50 9 Reflexes, 471 Regenerative cell s o f ventriculus, 363 , 36 5 defined, 38 8 Regenerative crypt s o f ventriculus, 366 defined, 38 8 Remigium, 225 , 226 defined, 24 4 Reproductive organs (internal) , 550 , 552 cellular element s o f an eg g tube, 55 9 of a testicula r tube , 56 9 chorion formation, 56 0 development of , 39, 574 ductus ejaculatorius , 572 epididymis, 567 , 572 female accessor y glands, 566 female organs , 552 general morpholog y of, 57 3 genital chambe r o f female, 56 3 of male , 58 7 lateral oviducts , 561 male accessory glands , 567 , 573 male organs, 567 ovaries, 55 2 oviductus communis , 562 spermatheca (receptaculu m seminis), 565 structure o f an ovariole , 553 of a sper m tube , 56 8

662

PRINCIPLES OF INSECT MORPHOLOGY

Reproductive organ s (.internal), testes , 567 trophic functio n o f the eg g tubes, 55 6 uterus, 56 4 vagina, 56 3 vas efferens , 56 8 vasa deferentia , 572 Respiration, 42 2 air sacs , functio n of, 457 course o f air i n th e tra'cheae , 455 defined, 46 2 external, 422 gas diffusio n i n tracheae , 452 gas transportation i n blood , 392 , 423 gills, functio n of , 42 5 integumentary, 42 3 internal, 42 2 respiratory movements , 453 tracheal ventilation , 45 3 tracheole mechanism , 459 Respiratory nerv e centers , 459 Respiratory proteins , 392 Respiratory stimuli , 457 Respiratory system , 422 biforous spiracles , 44 4 blood gills , 424 course o f air in th e tracheae , 45 5 defined, 46 2 development o f tracheae, 42 6 dorsal spiracle s o f dipterous larvae , 44 6 function o f ai r sacs , 45 7 general mechanis m o f tracheal respiration , 452 general pla n o f body tracheation , 42 9 integument a s a respirator y organ , 423 mechanism o f respiration i n tracheoles , 459 modifications o f tracheal system , 43 7 movements in , 453 number o f spiracles, 42 6 organization o f tracheal system , 42 9 respiration b y ga s diffusio n i n tracheae , 452 by trachea l ventilation , 453 spiracles wit h externa l closin g apparatus, 441 with internal closin g apparatus, 440 stimuli to , 457 structure o f spiracles, 43 8 of tracheae , 44 7 tracheal ai r sacs , 448 tracheal gills , 450 tracheal system , 425 tracheation o f abdomen, 43 5 of head , 431 of thorax , 43 3 of wings , 435 tracheoles, 449 Retina, 532 defined, 54 8 Retina cells , defined, 54 8 Retinal pigmen t cells , 546 defined, 54 8 Retinula, 543 defined, 54 8 Retractile vesicle, 268 , 272 , 274, 276 Rhabdom, 533 defined, 54 8

Rhabdomeres, 533 defined, 54 8 Rhynchota (see Hemiptera ) S

Sagittae, 604 defined, 62 1 Salivarium, 114 , 155 , 281 , 28 3 defined, 12 8 Salivary canal , 338 Salivary (labial ) glands , 154 defined, 15 6 Salivary meatus , 28Í , 336 Salivary syringe , 284 of bees , 30 1 of cicada , 333 , 33 5 of hors e fly , 31 8 Scales, 5 8 defined, 6 9 sensory, 519 Scape o f antenna, 13 1 defined, 15 6 Sclerites, 4 8 defined, 69 , 8 1 Scleromata, 7 3 defined, 8 1 Sclerotization, 51 , 53 abdominal, 248 defined, 6 9 Scolopophorous sense organs , 525 Scolops (scolopale) , 525, 526 defined, 54 8 Scutellum, 179 defined,192 Scutoscutellar suture , 179 defined, 19 2 Scutum, 17 9 defined, 19 2 Second antennae , 133 defined, 15 6 Second axillar y sclerite , 21 8 defined, 24 4 Second maxillae , 145 defined, 15 6 Second spiracle , defined, 192 of Dissosteira, 44 0 Second trochanter , 86 , 90 defined, 9 9 Second valvifers , 61 1 Second valvulae , 610 Secondary cuticul a (endocuticula) , 49 Secondary segment , 7 6 Secondary segmentation , 7 4 defined, 8 2 Secondary sens e cells, 504 Segment, o f appendage, 84 , 8 6 defined, 4 6 of body , 35 , 70 , 73 , 7 6 defined, 46 , 8 2 Segmental appendages, 37, 83 defined, 4 7 Segmentation, 73 of appendages , 8 4 of body , 35 , 7 0

INDEX Segmentation, embryonic , 35 primary, 7 4 secondary, 7 4 Segments of , abdomen, 24 7 head, 100-10 3 legs, 86 , 193 , 194-20 0 thorax, 157 , 16 0 Sense cell, defined , 509 , 54 9 Sense cells , primary , 504 secondary, 50 4 of Typ e I , 470 , 503,504,513 of Typ e II , 470 , 503, 504 Sense organs , 59 , 470, 510 campaniform organs , 52 1 classification, 512 , 514 compound eyes , 542 defined, 69 , 509 dorsal ocelli , 533 eyes, 528 , 53 0 general structur e of, 512 hair organs , 51 5 lateral ocell i of holometabolous larvae , 53 6 organ o f Johnston, 52 7 photoreceptors o f muscoid larvae , 53 0 plate organs , 523 scolopophorous organs , 52 5 simple lateral eye s o f adult insects , 54 0 Sensilla ampullacea , 514 , 519 basiconica, 514 , 519 campaniformia, 514 , 52 1 chaetica, 514 , 51 8 coeloconica, 514 , 519 óptica, 515 , 52 8 placodea, 514 , 523 scolopophora, 515 , 52 5 squamiformia, 514 , 51 9 trichodea, 514 , 51 7 Sensillum, 51 4 defined, 54 9 Sensitivity, 46 5 defined, 50 9 Sensory innervatio n o f alimentary canal , 506 of integument , 504 , 50 5 of muscles , 50 6 Sensory nerves , 467 , 503 Sensory nervou s system , 3 0 Serosa, 3 3 defined, 4 7 Seta, defined , 6 9 poison, 5 8 structure of , 57 Setal alveolus , 57 Setal membrane , 5 7 defined, 6 9 Silk pres s o f caterpillar, 284 , 30 5 Simple lateral eyes o f adult insects , 54 0 Sinus, cardiac , 3 9 dorsal, 398 , 40 5 epineural, 3 8 peri visceral, 398 ventral, 39 8 Siphonaptera, eyes , 542 mouth parts , 325 Socii, 601, 602 , 605 defined, 62 1

663

Soma, 1 4 defined, 4 7 Somatic cells , 1 4 defined, 4 7 Somatopleure (somati c laye r o f mesoderm), 3 8 defined, 4 7 Somite, 3 5 defined, 47 , 82 Spatha, 60 4 defined, 62 2 Sperm cyst , 571 defined, 58 0 Sperm tube , 567 defined, 58 0 structure of , 568 Spermatheca, 552 , 565 defined, 58 0 Spermatid, defined , 58 0 Spermatocyte, defined , 58 0 Spermatogonium, defined , 580 Spermatozoon, defined , 47, 580 Spina, 78 , 167 defined, 19 2 Spinasternum, 78 , 167 defined, 19 2 Spines, 5 6 defined, 6 9 Spinneret, 28 4 Spiracles, 426 , 438 abdominal, 429 biforous, 44 4 cephalic o f embryo, 42 6 defined, 46 2 external closin g apparatus of , 440 filter apparatu s of , 439 internal closin g apparatus of , 441 number of , 426 position of , 426 prothoracic o f embryo, 42 7 structure of , 438 thoracic, 161 , 42 7 Spiracles of , Alaus larva, 444 caterpillar, 44 3 dipterous larvae , 446 Dissosleira, 440 Donada larva , 446 Heterojapyx, 42 8 Lithobius, 428 Sminthurus, 428 Spiracular trachea , 429 defined, 46 2 Splanchnopleure (splanchni c layer o f mesoderm) , 38 defined, 4 7 Spurs, 5 6 defined, 6 9 Squama, 604 defined, 62 2 Stemmata (see Latera l ocelli ) Sterna, abdominal , 250 thoracic, 166 Sternacosta, 17 0 defined, 19 2 Sternacostal suture , 170 defined, 19 2

664

PRINCIPLES OF INSECT MORPHOLO GY

Sternal apophysis , 169 defined, 16 9 Sternellum, 170 defined, 19 2 Sternites, 71 defined, 8 2 Sternopleurite, 163 defined, 19 2 Sternum, 71 , 77 defined, 82 , 19 2 Stimuli o f sense organs , 51 1 Stimulus, 46 5 defined, 50 9 Sting o f honey bee , 61 9 Stipes, 134 , 142 defined, 15 6 Stomach (see Ventriculus ) Stomach mouth , 358 defined, 38 8 Stomatogastric nervou s system, 469, 501 Stomodaeal nervou s system , 469 , 501 defined, 50 9 Stomodaeal (cardiac ) valve , 349 defined, 38 8 Stomodaeum, 348 , 349 cephalic, 248 defined, 47 , 388 development of , 30 histology of , 350 muscles of , 285, 286 Strepsiptera, ey e of , 542 Striated borde r o f ventricular epithelium , 363 defined, 38 8 Stylet o f sting, 61 9 defined, 622 Stylus o f appendages, 270 , 271, 592 Subalar muscles , 189 , 23 2 Subalares, 18 4 defined, 23 2 Subantennal suture , 10 9 Subcosta, 22 1 defined, 24 4 Subcoxa, 88, 16 3 defined, 82 , 99 Subgalea, 14 5 defined, 15 6 Subgenal areas , 11 2 defined,128 Subgenal sutures , 10 8 defined,128 Subgenital plate, 252 of female , defined , 62 2 of male , defined , 62 2 Submentum, 150 defined, 15 6 Subocular suture, 109 Suboesophageal (substomodaeal ) commissure , 474, 478, 493 defined, 50 9 Suboesophageal ganglion , 32, 476, 494 defined, 50 9 Sucking pump of , cicada, 334 horse fly, 320 Lepidoptera, 31 0 Thysanoptera, 32 8

Superlinguae, 113 , 130 , 139 , 140 , 162 defined, 15 6 Surface region s of trunk, 7 0 Suspensorium o f hypopharynx, 115 defined, 12 8 Suspensory ligamen t o f ovary, 55 2 Suspensory muscles of alimentary canal (see Dilator muscles ) Sutures, 5 3 antecostal, 76 , 161 , 179 , 249 antennal, 10 9 basi costal, 194 coronal, 107 coxal, 19 5 cranial, 106 defined, 6 9 epicranial, 10 7 epistomal, 10 8 frontal, 10 7 frontoclypeal (epistomal) , 107 guiar, 125 hypostomal, 108 labial, 146 metopic (coronal) , 107 notaulices, 180 occipital, 10 8 ocular, 109 parapsidal, 18 1 pleural, 165 , 17 5 pleurostomal, 10 8 postfrontal, 10 7 postoccipital, 108 prescutal, 18 0 préster nal, 170 ptilinal, 10 9 reversed notal , 179 scutoscutellar, 17 9 sternacostal, 170 subantennal, 10 9 subgenal, 109 subocular, 109 transscutal, 182 transscutellar, 18 2 Sympathetic nervou s syste m (see Stomodaea l system) Symphyla, 10 legs, 95 , 96 Synapse (nervous) , 60 , 468 defined, 50 9 Syncephalon, defined , 1 7 T

Taenidium, 447 defined, 46 2 Tagma, defined , 8 2 Tagmata, 7 0 Tagmosis, 8 0 Tangoreceptor, defined , 54. 9 Tapetum, 533 defined, 54 9 Tarsal pulvilli , 198 Tarsomeres (tarsites) , 87 , 198 defined, 9 9

INDEX Tarsus, 86 , 90, 19 3 19 8 defined, 99 , 210 Tegmen, 597 Tegula, 218 defined, 24 4 Tegumen, 602 Telopodite, 86 , 89 defined 9 9 joints of , 90 segments of , 90 Telotarsus, 9 0 defined, 9 9 Telson, 35 , 70 Tenaculum, 268, 270 Tenth abdomina l segmen t 253 , 255 Tentorial arms , 11 6 Tentorial maculae , 110 Tentorial pits , 109 , 11 0 defined,128 Tentorium, 104 , 11 6 defined, 12 9 Terga, abdominal , 249 thoracic, 160 Tergites, 71 defined, 8 2 Tergum, 71 , 76 defined, 8 2 Terminal filamen t o f ovariole, 553 , 554 , 576 defined, 58 0 Testis, 567 defined, 58 0 general structur e of , 568 Third axillar y sclerite , 219 defined, 24 4 Third valvulae , 611 Thoracic ganglia , 476 Thoracic legs , 19 3 Thoracic muscles , 186 Thoracic pleura , 16 1 Thoracic sterna , 16 6 basic structur e of , 167 reversed overlappin g of , 168 Thoracic sterna of , Apterygota, 168 cicada, 17 1 cockroach, 170, 171 Diptera, 172

Pterygota, 169 Thoracic terga , 16 0 Thorax, 41 , 157 evolution, 15 7 general structure , 16 0 muscles, 18 6 transposition o f ventral muscles , 168 Thysanoptera, feedin g mechanism , 326 Thysanura, 1 2 abdominal appendages , 27 0 ectotrophica, 1 2 endoderm, 27 entotrophica, 12 (See also Diplura ) eye, o f Lepisma, 54 0 of Machilis, 53 4 head, 136 , 14 7 hypopharynx, 140 labium, 14 9

665

Thysanura, mal e genitalia , 58 5 mandibles, 13 6 mesoderm, 25, 27 ovipositor, 60 9 postantennae, 13 3 Tibia, 86 , 193 , 19 8 defined, 99 , 210 Titillators, 590 defined, 62 2 Tonofibrillae, 62 , 63 defined, 6 9 Tormae, 11 3 Tormogen, 57 defined, 6 9 Tracheae, 42 3 defined, 46 2 development of , 426 structure of , 447 Tracheal ai r sacs , 448 Tracheal gills , 450 defined, 46 2 Tracheal orifice , defined , 46 2 Tracheal respiration , 452 Tracheal system , 42 5 defined, 46 2 development of , 40 general pla n of , 429 modifications of , 43 7 organization of , 429 Tracheation of , abdomen, 435 abdominal ganglion , 498 head, 431 thorax, 433 wings, 43 5 Tracheoles, 44 9 defined, 46 3 respiration in , 46 3 Transverse muscles, 258, 260, 263 Trichogen, 57 defined, 6 9 Trichopore, 5 7 defined, 6 9 Trichoptera, abdomina l appendage s o f larva, 27 5 male genitalia , 60 1 Trilobita, 5 legs, 90 structure o f appendages, 8 7 Tritocerebrum, 474, 478, 492 defined, 50 9 Trochanter, 86 , 193 , 197 defined, 99 , 210 Trochanters o f Odonata, 97 , 19 7 "Trochantin, 164 , 166 defined, 19 2 Trophic functio n o f egg tubes, 55 6 of testicula r tubes , 569 , 57 0 Trophocyte, defined , 421, 580 Trophocytes o f egg tube, 55 5 of fa t body , 40 9 Tropisms, 47 1 Trunk,70 defined, 8 2 Tunica propri a o f ovariole, 55 3 Twelfth abdomina l segment , 25 6 Typical earl y stage s o f development, 16

666

PRINCIPLES OF INSECT MORPHOLOGY U

Uncus, 602 defined, 62 2 Ungues, 98, 199 defined, 99 , 210 Unguifer, 19 9 defined, 21 0 Unguitractor plate , 199 defined, 98 , 210 Urate cells , 409 , 410 defined, 42 1 Urogomphi, 279 Uterus, 56 4 defined, 58 0 V

V-shaped nota l ridge, 179 defined, 19 2 Vagina, 552 , 56 3 defined, 58 0 Valves o f heart, 40 0 defined, 42 1 Valvifers o f ovipositor , 61 1 defined, 62 2 Valvulae o f ovipositor, 610 , 61 1 defined, 62 2 Vannal fol d (see Plic a vannalis ) Vannal veins , 223 defined, 24 4 Vannus, 225, 227 defined, 24 4 Vas deferens , defined, 580 Vas efferens , 56 8 defined, 58 0 Vasa deferentia , 567 , 572 Veins o f wings, 214-217, 221-225 convex an d concave , 224 defined, 24 4 development of , 214 Vena arcuata, 223 defined, 24 4 Vena cardinalis , 223 defined, 24 4 Vena dividens , 223, 227 defined, 24 4 Venation o f wings, 215, 216 , 22 1 Venter, 7 0 defined, 8 2 Ventilation tracheae , 44 8 defined, 46 3 Ventral diaphragm , 398 , 405 defined, 42 1 Ventral muscles o f thorax, 18 9 of abdomen , 26 1 Ventral nerve cord , 30 , 469, 473, 494 defined, 50 9 Ventral sinus , 398 defined, 42 1 Ventral trachea , 426 defined, 46 3 Ventral trachea l trunk , 43 0 defined, 46 3

Ventriculus, 359 activities o f epithelium, 370 basement membrane , 366 caeca, 361 defined, 47 , 388 degeneration and regeneratio n of digestive cells, 371 delamination an d replacemen t o f epithelium , 372 digestive cells , 364 epithelium, 363 excretion in , 371 general for m of , 360 histology of , 36 3 muscularis, 370 peritrophic membrane , 366 regenerative cells , 365 regenerative crypts , 366 secretion an d absorption , 370 Versonian cell , 569 defined, 58 0 Versonian glands, 6 6 Vertex, 111 defined, 12 9 Vesica, 590 defined, 62 2 Vesicula seminalis , 567 defined, 58 0 Vestibulum defined , 62 2 Vinculum, 602 defined, 62 2 Virga, defined , 622 Visceral segments o f abdomen, 247 , 252 Visceral trachea, 430 defined, 46 3 Visceral tracheal trunk , 430 defined, 46 3 Vitellarium o f ovariole, 55 5 defined, 58 0 Vitelline membrane, 1 8 defined, 47 Vitellophags, 21, 28, 29 defined, 4 7 Vitreous body , 531 defined, 54 9 Viviparity, 55 1 Volsellae, 604 defined, 62 2 Vulva, 563 defined, 62 2 W

Wing base, defined , 244 of Ephemerida , 22 0 of Odonata , 22 1 of wing-flexin g insects, 21 8 Wing-bearing segments , genera l structur e of , 17 3 Wing movements , 23 3 anteroposterior, 23 4 downstroke, 234 extension, 240 flexion, 237-240 and extension , 233 , 236 flight, 23 3

INDEX Wing movements , rat e of vibration, 23 6 rotary, 23 4 upstroke, 23 3 Wing muscles , 228, 232 Wing muscle s o f heart (see Alary muscles) Wing regions, 215 , 225 axillary, 225 defined, 24 4 jugum, 225, 227 remigium, 225 vannus, 22 5 Wings, 158 , 21 1 articulation t o body , 215, 218 development, 214 flight, 24 0 motions in flight, 235 movements, 233 muscles, 22 8 origin an d evolution , 21 1 regions, 240

667

Wings, structure , 21 5 veins, 215, 216 , 221 X

Xiphosura, 7 legs, 9 1 Y

Yolk (deutoplasm) , 16 , 19 defined, 45 , 4 7 Yolk cells , 21 defined, 4 7 Yolk cleavage , defined, 4 7 Z

Zygote, defined , 4 7