Dragonfly Nymphs of North America: An Identification Guide 331997775X, 9783319977751

Table of contents :
Front Matter ....Pages i-xiv
Front Matter ....Pages 1-1
Introduction (Kenneth J. Tennessen)....Pages 3-6
Nymph Anatomy and Instar Determination (Kenneth J. Tennessen)....Pages 7-29
Using the Keys (Kenneth J. Tennessen)....Pages 31-32
List of Species Treated (Kenneth J. Tennessen)....Pages 33-56
Front Matter ....Pages 57-57
Key to the Families (Kenneth J. Tennessen)....Pages 59-72
Aeshnidae (Kenneth J. Tennessen)....Pages 73-159
Gomphidae (Kenneth J. Tennessen)....Pages 161-295
Petaluridae (Kenneth J. Tennessen)....Pages 297-306
Cordulegastridae (Kenneth J. Tennessen)....Pages 307-328
Macromiidae (Kenneth J. Tennessen)....Pages 329-344
Corduliidae (Kenneth J. Tennessen)....Pages 345-406
Libellulidae (Kenneth J. Tennessen)....Pages 407-576
Front Matter ....Pages 577-577
Methods for Collecting, Rearing and Preserving Dragonflies (Kenneth J. Tennessen)....Pages 579-590
Future Research on Dragonfly Nymphs (Kenneth J. Tennessen)....Pages 591-600
Back Matter ....Pages 601-620

Citation preview

Kenneth J. Tennessen

Dragonfly Nymphs of North America An Identification Guide

Dragonfly Nymphs of North America

Kenneth J. Tennessen

Dragonfly Nymphs of North America An Identification Guide

Kenneth J. Tennessen Florida State Collection of Arthropods Gainesville, FL, USA Wautoma, WI, USA

ISBN 978-3-319-97775-1    ISBN 978-3-319-97776-8 (eBook) https://doi.org/10.1007/978-3-319-97776-8 Library of Congress Control Number: 2019933404 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Dedicated to all lifelong students who keep asking questions and learning

Preface

A comprehensive systematic treatment devoted solely to the nymphs of North American Anisoptera has not materialized previously. For many years, the keys in Needham and Heywood (1929) and Needham and Westfall (1955) were the foundation for identification of the dragonfly nymphs of North America north of Mexico. Recent revisions of “The Manual” (Needham et al. 2000, 2014) improved some of the keys to the nymphs, but the emphasis was on updating adult identification. The need for a more detailed treatise on Anisoptera nymphs of this region, including amply illustrated keys and diagnoses for all genera and species, has long been recognized. The numerous previous contributions of many workers have served as the basis and inspiration to produce a more thorough identification guide. My vision for an in-depth taxonomic work on the Anisoptera of North America began in the 1980s. It became apparent early that problems with existing keys stemmed from a lack of well-­ preserved, mature specimens and not enough attention paid to intraspecific variation. To this end, I borrowed specimens from institutions and private collections and asked numerous collectors to rear certain species. I also traveled throughout the United States and Canada in search of taxa poorly represented in collections and nymphs of previously unknown species. The result has been an accumulation of sufficient material to study both little-known species and supposedly well-known species. Data on intraspecific variation allowed for a better evaluation of character differences. The identification part of this book is intended mainly for use in the laboratory with aid of a microscope, but with experience, field identification of most genera, and some species at least, is possible. My main intention was to provide a moderately technical reference that is useful to beginners and experienced workers. An underlying motive was to introduce general naturalists and novices to identification of dragonfly nymphs, hopefully helping to make field excursions pleasurable and memorable. I hope that the book will not only provide a means to identify dragonfly nymphs of North America, but that it will also serve as a platform for further research on the taxonomy of dragonfly nymphs and ultimately research on their biology. Taxonomy is foundational for all biological discovery (Fleck et al. 2006). Identification of the organism(s) under study is the first step for nearly every kind of biological inquiry, including evolution, phylogeny, ecology, behavior, conservation, and for assessing habitat disturbance and the possible effects of climate change. Knowing the name of the organism in question allows comparison of results with previous and ongoing studies, facilitates discussion, and leads to further understanding and opportunity to add to the body of biological knowledge. Preparation of this book has taken many a turn, in libraries and museums, on winding back roads and under bridges, treks through endless swamps, fens and streams, and countless hours peering through microscopes and at computer screens. Although I have discovered much new information on dragonfly nymphs, I feel there is much more detail to be explored. This book will not be the final word on the nymphal taxonomy of North American Anisoptera, nor was it intended to be such. Hopefully, it will serve as a springboard for future discoveries on the Anisoptera nymphs of North America. There are many groups for which our knowledge of species distinctions and variation is woefully incomplete. I encourage those with the inclination to collect, rear, and study nymphs to continue to do so, especially in areas close to where they live. The effort of local observers and collectors, especially when enriched by careful vii

viii

notes on life cycles and microhabitat, is the route by which we will further our knowledge of these fascinating insects. I have been influenced and helped in my studies on dragonfly nymphs by many educators, colleagues, and friends. Early in my pursuit of entomological study, Dr. William L. Hilsenhoff, at the University of Wisconsin, encouraged me and provided support to further my interests in aquatic insects and especially Odonata. Dr. Minter J. Westfall, Jr., University of Florida, made possible further formal education and taught me invaluable lessons on how to study dragonflies and the importance of exploring all stages of their life cycle. I especially want to thank John C. Abbott, Robert B. DuBois, Marla C. Garrison, Jim T. Johnson, Steve Krotzer, and Rodolfo Novelo-Gutiérrez for going out of their way on numerous occasions to collect nymphs of under-collected species, many at my request and at their own expense. They have also tested some of the keys, reviewed certain sections of the draft manuscript, and offered data and helpful advice on certain taxa. I extend special thanks to Marla Garrison for her inquisitiveness, gifts of specimens, photographs, testing keys, proof-reading the entire manuscript, support, and field logistics; her editorial skills improved consistency of terms and syntax. I also want to specially thank Jim Johnson, one of my closest friends, for his keen editorial skills and ability to find elusive  mistakes in the manuscript; his diligent proof-reading  greatly improved the quality of the book. I am deeply indebted to my colleague and friend Dr. Boris Kondratieff (Colorado State University) for reading the entire draft manuscript and finding many errors, omissions, and inconsistencies; his comments, questions, and diligent and detailed critique resulted in major improvement of the manuscript. I also thank Dr. Paul McKenzie (US Fish and Wildlife Service) for his general review of the manuscript and many helpful suggestions. The reared specimens sent to me by Dennis R.  Paulson proved invaluable in my study of numerous hard-to-get species, and the information, advice, and encouragement he provided made completion of some difficult genera possible; my sincere appreciation is offered to him. Special thanks to Bob DuBois for specimens, data, long discussions, and memorable field trips, and for letting me badger him on countless issues. I thank my long-time friend Steve Valley for photographs, valuable discussions, and a lifetime of companionship, frivolity, and encouragement. The following individuals have helped me in one or more ways during work on this book by loaning specimens, taking measurements, providing photographs and SEM images, testing draft keys, and providing literature, field companionship, inspiration, discussions, and proof-­ reading: Ian Baird, Michael Blust, Mike Bolton, Cornelio Andrés Bota Sierra, Ethan Bright, Paul Brunelle, Rob Cannings, Tim Cashatt, Margi Chriscinske, Carl Cook, Jerrell J. Daigle, Carel de Haseth, Jürg De Marmels, Dana Denson, Nick Donnelly, Sid Dunkle, Chad Edgar, Gunther Fleck, Rosser Garrison, Bob Glotzhober, Enrique Gonzalez-Soriano, Dave Halstead, Kevin Hemeon, John Hudson, Pat Hudson, Pamela Hunt, Dan Jackson, Colin Jones, Edwin Keppner, Cary Kerst, Scott King, Brett Landwer, Linda “Stick” LaPan, Ellis Laudermilk, Dave Lenat, Bill Mauffray, Mike May, Francois Meurgey, Alan Myrup, Rodolfo Novelo-Gutiérrez, Mark O’Brien, Bronco Quick, Bryan Pfeiffer, Alonso Ramirez, Mike Reese, Steve Roble, Robert Rutter, Kurt Schmude, Martin Schorr, Clark Shiffer, Bill Smith, Malisa Spring, Wayne Steffens, Gunther Theischinger, Mike Turner, John Thurman, Hidenori Ubukata, Sandy Upson, Steve Valley, Freda van den Broek, Michael Veit, Tim Vogt, Natalia von Ellenrieder, Amy Wales, Jane Walker, Jessica Ware, Bill West, Hal White. To all these colleagues, I offer my gratitude, with the recognition that errors that remain are solely my responsibility. Institutions (and contacts) that lent support are: Florida State Collection of Arthropods, Gainesville, Florida (Dr. Paul Skelly, Head Curator; Bill Mauffray, Resident Research Associate); Smithsonian Institution, Washington, DC (Dr. Oliver Flint); Ohio Historical Society, Columbus, OH (Robert C. Glotzhober); Spencer Entomological Museum, University of British Columbia (Robert Cannings); Natural History Museum of Utah, Salt Lake City (Christy Bills); Cornell University Insect Collection, Ithaca (Dr. Jason J.  Dombroskie, Collection Manager).

Preface

Preface

ix

And most warmly, I thank my wife Sandi, who has supported me in my research pursuits on insects throughout our married life. It is not always easy living with an entomologist. Her unwavering love and understanding gave me added inspiration and time to complete this work. “What makes nature so perfect are its intricate imperfections.”

Wautoma, WI, USA

Kenneth J. Tennessen

References Fleck G, Brenk M, Misof B (2006) DNA taxonomy and the identification of immature insect stages: the true larva of Tauriphila argo (Hagen 1869) (Odonata: Anisoptera: Libellulidae). Ann Soc entomol Fr 42 (1):91–98 Needham JG, Heywood HB (1929) A handbook of the dragonflies of North America. Charles C. Thomas, Springfield, IL. 378 pp Needham JG, Westfall MJ, Jr. (1955) A manual of the dragonflies of North America (Anisoptera). University of California, Berkeley. 615 pp Needham JG, Westfall MJ, Jr., May ML (2000) Dragonflies of North America. Revised ­edition. Scientific Publishers, Gainesville. 940 pp Needham JG, Westfall MJ, Jr., May ML (2014) Dragonflies of North America: the Odonata (Anisoptera) fauna of Canada, the continental United States, northern Mexico and the Greater Antilles. 3rd edition. Scientific Publishers, Gainesville, FL. 657 pp

Contents

Part I Preliminary Material 1 Introduction���������������������������������������������������������������������������������������������������������������    3 References�������������������������������������������������������������������������������������������������������������������    6 2 Nymph Anatomy and Instar Determination�����������������������������������������������������������    7 2.1 The Head�����������������������������������������������������������������������������������������������������������    10 2.2 The Thorax���������������������������������������������������������������������������������������������������������    19 2.3 The Abdomen�����������������������������������������������������������������������������������������������������    20 2.4 External Morphological Outgrowths�����������������������������������������������������������������    24 2.4.1 Determining Gender������������������������������������������������������������������������������   24 2.5 Growth���������������������������������������������������������������������������������������������������������������    26 2.5.1 Determining Instar���������������������������������������������������������������������������������   27 References�������������������������������������������������������������������������������������������������������������������    29 3 Using the Keys�����������������������������������������������������������������������������������������������������������   31 3.1 Specimen Preparation ���������������������������������������������������������������������������������������    32 References�������������������������������������������������������������������������������������������������������������������    32 4 List of Species Treated ���������������������������������������������������������������������������������������������   33 4.1 Odonata Diversity in the Americas �������������������������������������������������������������������    33 4.2 The Anisoptera Fauna of North America�����������������������������������������������������������    33 4.3 State and Province Abbreviations ���������������������������������������������������������������������    34 4.4 Species List with Nymph References ���������������������������������������������������������������    35 References�������������������������������������������������������������������������������������������������������������������    52 Part II Systematics 5 Key to the Families���������������������������������������������������������������������������������������������������   59 5.1 Key to Anisoptera Families of North America, F-0 to F-5 �������������������������������    61 5.1.1 Family Diagnoses�����������������������������������������������������������������������������������   67 References�������������������������������������������������������������������������������������������������������������������    71 6 Aeshnidae�������������������������������������������������������������������������������������������������������������������   73 6.1 Aeshna Fabricius, 1775�������������������������������������������������������������������������������������    85 6.2 Anax Leach, 1815�����������������������������������������������������������������������������������������������   100 6.3 Hemianax Selys, 1883��������������������������������������������������������������������������������������   108 6.4 Basiaeschna Selys, 1883�����������������������������������������������������������������������������������   110 6.5 Boyeria McLachlan, 1896���������������������������������������������������������������������������������   113 6.6 Coryphaeschna Williamson, 1903���������������������������������������������������������������������   117 6.7 Epiaeschna Hagen, 1877�����������������������������������������������������������������������������������   123 6.8 Gomphaeschna Selys, 1871�������������������������������������������������������������������������������   126 6.9 Gynacantha Rambur, 1842 �������������������������������������������������������������������������������   131

xi

xii

Contents

6.10 Nasiaeschna Selys, 1900�����������������������������������������������������������������������������������   135 6.11 Oplonaeschna Selys, 1883���������������������������������������������������������������������������������   139 6.12 Remartinia Navás, 1911�������������������������������������������������������������������������������������   143 6.13 Rhionaeschna Förster, 1909�������������������������������������������������������������������������������   147 6.14 Triacanthagyna Selys, 1883�������������������������������������������������������������������������������   153 References�������������������������������������������������������������������������������������������������������������������   157 7 Gomphidae����������������������������������������������������������������������������������������������������������������  161 7.1 Aphylla Selys, 1854�������������������������������������������������������������������������������������������   179 7.2 Arigomphus Needham, 1897�����������������������������������������������������������������������������   185 7.3 Dromogomphus Selys, 1854 �����������������������������������������������������������������������������   192 7.4 Erpetogomphus Selys, 1858�������������������������������������������������������������������������������   196 7.5 Gomphurus Needham, 1901 �����������������������������������������������������������������������������   204 7.6 Hagenius Selys, 1854�����������������������������������������������������������������������������������������   216 7.7 Hylogomphus Needham, Westfall and May, 2000���������������������������������������������   219 7.8 Lanthus Needham, 1897 �����������������������������������������������������������������������������������   226 7.9 Octogomphus Selys, 1873���������������������������������������������������������������������������������   231 7.10 Ophiogomphus Selys, 1854�������������������������������������������������������������������������������   233 7.11 Phanogomphus Carle, 1986�������������������������������������������������������������������������������   250 7.12 Phyllocycla Calvert, 1948���������������������������������������������������������������������������������   263 7.13 Phyllogomphoides Belle, 1970 �������������������������������������������������������������������������   266 7.14 Progomphus Selys, 1854�����������������������������������������������������������������������������������   269 7.15 Stenogomphurus Carle, 1996�����������������������������������������������������������������������������   275 7.16 Stylogomphus Fraser, 1897 �������������������������������������������������������������������������������   279 7.17 Stylurus Needham, 1897 �����������������������������������������������������������������������������������   282 References�������������������������������������������������������������������������������������������������������������������   294 8 Petaluridae�����������������������������������������������������������������������������������������������������������������  297 8.1 Tachopteryx Uhler in Selys, 1859 ���������������������������������������������������������������������   299 8.2 Tanypteryx Kennedy, 1917���������������������������������������������������������������������������������   302 References�������������������������������������������������������������������������������������������������������������������   306 9 Cordulegastridae�������������������������������������������������������������������������������������������������������  307 9.1 Cordulegaster Leach, 1815 ��������������������������������������������������������������������������������� 311 9.2 Zoraena Kirby, 1890 ������������������������������������������������������������������������������������������� 321 References��������������������������������������������������������������������������������������������������������������������� 328 10 Macromiidae�������������������������������������������������������������������������������������������������������������  329 10.1 Didymops Rambur, 1842���������������������������������������������������������������������������������   332 10.2 Macromia Rambur, 1842���������������������������������������������������������������������������������   336 References�������������������������������������������������������������������������������������������������������������������   344 11 Corduliidae ���������������������������������������������������������������������������������������������������������������  345 11.1 Cordulia Leach, 1815 �������������������������������������������������������������������������������������   352 11.2 Dorocordulia Needham, 1901�������������������������������������������������������������������������   354 11.3 Epitheca Charpentier, 1840�����������������������������������������������������������������������������   357 11.4 Helocordulia Needham, 1901 �������������������������������������������������������������������������   368 11.5 Neurocordulia Selys, 1871�������������������������������������������������������������������������������   372 11.6 Somatochlora Selys, 1871�������������������������������������������������������������������������������   380 11.7 Williamsonia Davis, 1913 �������������������������������������������������������������������������������   401 References�������������������������������������������������������������������������������������������������������������������   406

Contents

xiii

12 Libellulidae ���������������������������������������������������������������������������������������������������������������  407 12.1 Brachymesia Kirby, 1889���������������������������������������������������������������������������������   434 12.2 Brechmorhoga Kirby, 1894�����������������������������������������������������������������������������   439 12.3 Cannaphila Kirby, 1889�����������������������������������������������������������������������������������   444 12.4 Celithemis Hagen, 1861�����������������������������������������������������������������������������������   447 12.5 Crocothemis Brauer, 1868�������������������������������������������������������������������������������   453 12.6 Dythemis Hagen, 1861�������������������������������������������������������������������������������������   457 12.7 Erythemis Hagen, 1861�����������������������������������������������������������������������������������   461 12.8 Erythrodiplax Brauer, 1868�����������������������������������������������������������������������������   468 12.9 Idiataphe Cowley, 1934�����������������������������������������������������������������������������������   473 12.10 Ladona Needham, 1897�����������������������������������������������������������������������������������   476 12.11 Leucorrhinia Brittinger, 1850 �������������������������������������������������������������������������   481 12.12 Libellula Linnaeus, 1758���������������������������������������������������������������������������������   490 12.13 Macrodiplax Brauer, 1868�������������������������������������������������������������������������������   502 12.14 Macrothemis Hagen, 1868�������������������������������������������������������������������������������   506 12.15 Miathyria Kirby, 1889�������������������������������������������������������������������������������������   511 12.16 Micrathyria Kirby, 1889 ���������������������������������������������������������������������������������   515 12.17 Nannothemis Brauer, 1868�������������������������������������������������������������������������������   519 12.18 Orthemis Hagen, 1861�������������������������������������������������������������������������������������   522 12.19 Pachydiplax Brauer, 1868 �������������������������������������������������������������������������������   526 12.20 Paltothemis Karsch, 1890 �������������������������������������������������������������������������������   529 12.21 Pantala Hagen, 1861���������������������������������������������������������������������������������������   532 12.22 Perithemis Hagen, 1861�����������������������������������������������������������������������������������   537 12.23 Planiplax Muttkowski, 1910���������������������������������������������������������������������������   541 12.24 Plathemis Hagen, 1861 �����������������������������������������������������������������������������������   544 12.25 Pseudoleon Kirby, 1889�����������������������������������������������������������������������������������   549 12.26 Sympetrum Newman, 1833 �����������������������������������������������������������������������������   552 12.27 Tauriphila Kirby, 1889�������������������������������������������������������������������������������������   560 12.28 Tholymis Hagen, 1867�������������������������������������������������������������������������������������   564 12.29 Tramea Hagen, 1861���������������������������������������������������������������������������������������   567 References�������������������������������������������������������������������������������������������������������������������   574 Part III Further Considerations 13 Methods for Collecting, Rearing and Preserving Dragonflies �����������������������������  579 13.1 Collection of Nymphs �������������������������������������������������������������������������������������   579 13.2 Collection of Exuviae �������������������������������������������������������������������������������������   582 13.3 Rearing Dragonfly Nymphs�����������������������������������������������������������������������������   583 13.4 Collection of Adults�����������������������������������������������������������������������������������������   585 13.5 Preservation of Specimens�������������������������������������������������������������������������������   587 References�������������������������������������������������������������������������������������������������������������������   589 14 Future Research on Dragonfly Nymphs�����������������������������������������������������������������  591 14.1 Systematics �����������������������������������������������������������������������������������������������������   591 14.2 Morphology (Form and Function)�������������������������������������������������������������������   593 14.3 Life History�����������������������������������������������������������������������������������������������������   594 14.3.1 Development from Egg to Final Instar�����������������������������������������������  594 14.3.2 Voltinism �������������������������������������������������������������������������������������������  594

xiv

Contents

14.3.3 Feeding�����������������������������������������������������������������������������������������������  594 14.3.4 Habitat Parameters�����������������������������������������������������������������������������  595 14.3.5 Additional Life History Research Topics�������������������������������������������  595 14.4 Dragonflies, the Environment, and Water Quality�������������������������������������������  596 14.4.1 Environmental Effects Research �������������������������������������������������������  596 14.4.2 Water Quality Assessments ���������������������������������������������������������������  597 14.5 Conservation ���������������������������������������������������������������������������������������������������  597 References�������������������������������������������������������������������������������������������������������������������  599 Appendices�������������������������������������������������������������������������������������������������������������������������  601 Glossary�����������������������������������������������������������������������������������������������������������������������������  609 Index�����������������������������������������������������������������������������������������������������������������������������������  613

Part I Preliminary Material

1

Introduction

The living part is gone leaving this tiny grotesque, in its own way resembling those wooden Japanese warriors guarding the gates to a temple of waters. Just behind the head at the juncture of the thorax is a hole with dried white sinewy laces. And so, it is also like an old leather shoe whose foot has left it to go barefoot on the open meadows. —Scott King 1999

Abstract

Dragonfly nymphs are major components of aquatic ecosystems. Species identification is critical for studies on their biology and conservation. Of the 330 species of Anisoptera in North America, the nymphs of approximately 85% have been formally described. Illustrated keys and diagnoses for the seven families, 72 genera, and 325 species are provided. Five species are unknown in the nymph stage.

Dragonfly nymphs are important components of freshwater ecosystems, functioning as both predator and prey (Corbet 1999; Cordoba-Aguilar 2008; Kalkman et  al. 2008). Their involvement in food webs and energy transfer contributes to the structure and diversity of natural communities (Benke et  al. 1982; Stoks and Córdoba-Aguilar 2012). In some aquatic habitats, such as lakes and ponds that lack insectivorous fish, they can be the top predators (McPeek 1998; Corbet 1999), greatly influencing the presence and abundance of other species (Fig.  1.1). Ecologists are only beginning to understand the indirect effects of ontogenetic niche shifts on communities (Knight et al. 2005), and some data on the role

of dragonflies in this regard have come to light (Brown et al. 2000; Burkle et al. 2012). In addition, dragonfly nymphs are also great subjects in their own right for studies on competition, niche partitioning, behavior, evolution and phylogeny to name just a few research areas. All of these quests for knowledge, from ecosystem dynamics to inquiries into the biology of individual species, depend on the ability to identify the organisms involved, in all life stages; in fact, identification is a basic need that can hardly be overemphasized in pursuit of answers concerning most aspects of living organisms. Taxonomic knowledge on Odonata nymphs has lagged behind the body of work amassed on adults. This is not unusual for most insect groups. Descriptions of new insect species are made almost exclusively based on adult specimens, discovery of immature stages usually coming later. Through the last century and a half, entomologists have associated nymphs and adults of approximately 40% of the Odonata species in the New World, the largest percentage of these being from North America (Table  1.1). In America north of Mexico, the situation is more advanced, as nymphs of approximately 85% of the Anisoptera species and 77% of the Zygoptera species have been formally described (see Appendix II for a synopsis of the history of taxonomic stud-

© Springer Nature Switzerland AG 2019 K. J. Tennessen, Dragonfly Nymphs of North America, https://doi.org/10.1007/978-3-319-97776-8_1

3

4

1 Introduction

Fig. 1.1  Nymphs of Aeshnidae preying on aquatic insects: (a) Aeshna umbrosa stalking a mosquito larva; (b) Nasiaeschna pentacantha devouring an ephemerid mayfly nymph

Table 1.1  Summary of status of knowledge on the Odonata nymphs of the New World (demarcation of the continents of North America and South America at the Isthmus of Panama)

# species # nymphs described % nymphs known

North America Anisoptera 517 375 73%

Zygoptera 320 190 59%

South America Anisoptera 666 227 34%

Zygoptera 790 152 19%

New World Suborders combined 2113 850 40%

Data inclusive up to January 2018 (source for species numbers was Paulson 2018, source for number of nymphs described was based on scattered literature)

ies on the Anisoptera nymphs of Canada and the United States). Yet the practice of identifying nymphs of North American Anisoptera continues to present challenges. There are many reasons for identification problems, including such shortcomings as: (1) many descriptions are brief or inadequate for diagnoses; (2) some species were described based on only one or two specimens, obviously underestimating the amount of variation in key characters; and (3) illustrations were few or did not show diagnostic characters. Moreover, the nymphs of approximately 15% of North American Anisoptera (N of Mexico) have not been described albeit key characters for a little more than half of these undescribed species have been made known (note: descriptions of the 50 undescribed species are not provided in this work). In short, most taxonomic works covering the dragonfly nymphs of this region are outdated or have dubious and untested characters and omissions. The objective of this work is to provide better means to identify the Anisoptera nymphs of America north of Mexico. Toward this end, existing key characters were tested on multiple specimens and an intensive search for new characters based on associated specimens was undertaken. The result of this work culminated in illustrated keys and diagnoses for the seven families, 72 genera and 325 of the  330 species known in the region. Each

genus and all known species within a genus are further diagnosed. Applicability of key characters are also assessed for instars younger than full grown. References to previous descriptions and notes on the reliability of characters used in previous keys are provided. Distribution, which is a useful aid in identification, is shown as a range map for each genus and is described for each species, and general habitat notes are provided. It is easy to see why more attention has been paid to dragonfly adults than nymphs. Adults are large, colorful insects (Fig. 1.2) with unique and interesting behaviors, and many species are easy to identify on the wing with numerous photographic field guides now available. They are also great subjects for photography and inspiration for creative writing, which adds to their popularity. However, adults of many species are highly vagile and often appear at habitats where breeding does not occur (pers. obs). Finding nymphs and exuviae at a site confirms habitat suitability and also yields definite locality records and identification of habitat parameters (Raebel et al. 2010; DuBois 2016). The difference in inventory methods, i.e., adult versus nymph/exuviae surveying, has implications for questions concerning habitat effects, such as those surrounding pollution events. Usage of presence/absence of nymphs and/or exuviae versus adult surveys

1 Introduction

Fig. 1.2  Adult male of Rhionaeschna multicolor, hovering over the edge of a pond in search of a mate (Boulder County, Colorado)

depends on the circumstances involved in individual studies; most studies are probably better suited to nymph or exuviae collections. Furthermore, habitat requirements of the immature stages will be needed if conservation needs arise. For many odonate species, population estimates are more easily obtained by sampling nymphs and exuviae than by trying to count fast-flying adults. Nymphs are useful in assessing water quality and monitoring changes in habitat integrity as they are usually longer lived than adults, especially in the Nearctic region where they require a few months to several years to complete development. For the individual naturalist, learning about the immature stages leads to a more complete understanding and appreciation of odonate biology. But to gain this understanding, the need to be able to identify them at the specific level is paramount. Prior to the last few decades, the general view was that nymphs had little to offer in uncovering phylogenetic relationships. Corbet (1963) stated that the caudal lamellae of Zygoptera are “. . . highly adaptive organs, and therefore not reliable indicators of phylogenetic relationship.” Walker (1966) took a dim view of the state of nymph taxonomy when he referred to a “. . . well-known difficulty of finding reliable generic characters applicable to Odonata larvae . . .” and went so far as to state that “. . . little weight can be attached to the immature stages when assessing affinity.” We can no longer hold to such a myopic view. Prior to and during Walker’s career, nymph taxonomy was in its infancy, as most genera were incompletely known, and some were unknown, and no one had looked critically on a large scale for similarities within groups to see if nymph morphology held value in uncovering relationships.1 Our knowledge of nymphal characters has improved steadily to a point where enough species are known that intensive and comprehensive

James G. Needham was among the first North Americans to look at nymphs for help in distinguishing genera; for example, in his 1901 paper, he recognized that the nymph of Ladona deplanata did not belong in Libellula as many other workers at the time had claimed. 1 

5

studies can be conducted. There are impressive examples of nymph characters being used to clarify generic boundaries when analysis of adult characters yielded inconclusive results (Westfall 1976, 1988; Ramirez and Novelo-Gutiérrez 1999; Fleck 2004). These successes are reason for future researchers to look more critically at odonate nymphs in both a taxonomic and a phylogenetic sense as advances in anisopteran phylogeny are being made (Blanke et al. 2013; Carle et al. 2015; Kohli et al. 2016; Letsch et al. 2016). Fleck et al. (2008) analyzed relationships of a number of Libellulidae genera based on adult and nymph characters and concluded that “. . . larval characters preserve more of the phylogenetic signal relevant for deep-level systematics within Libellulidae and potentially in other families as well.” Furthermore, Pilgrim and von Dohlen (2008) found that many features of adult wing venation in the family Libellulidae are homplasious and therefore are not useful in inferring relationships among subfamilies. Although phylogeny of the Anisoptera was not the focus of this work, I have summarized my observations made in the process of evaluating family-level characters and attempted to put them in a rough phylogenetic construct. The data and conclusions are presented in Chap. 5 preceding the key to families. More work is needed on the utility of immature characters in odonate phylogeny, especially determining the degree of homoplasy and the polarity of characters under consideration. Regarding disagreement among entomologists over the terms available for the immature stages of insects (larva, nymph, naiad), I offer the following explanation for my choice of “nymph” for the Odonata. The term “naiad” was coined by Comstock (1918) for the juvenile stages of three orders (Odonata, Ephemeroptera, Plecoptera) recognizing that they are aquatic, and immatures do not resemble adults in form (see Bybee et  al. 2015 for further discussion). However, some families of other orders, such as Hemiptera and Orthoptera (both hemimetabolous) are aquatic and/or semiaquatic. There is no basic difference in the life cycles of these orders from the aforementioned orders; they undergo incomplete metamorphosis, i.e. egg, immature, adult. Furthermore, resemblance of immatures and adults is a matter of degree and any judgement on similarity is subjective. The term naiad has been used relatively infrequently in recent aquatic insect literature. For these reasons I refrain from using the term naiad. The term larva is used for all immature insects by some authors whereas others restrict its use to the holometabolous orders (insects undergoing complete metamorphosis, i.e., egg, larva, pupa, adult). The term is also used by ichthyologists for fish hatchlings, and it has been used also in other animal groups (various worms, newts, etc.). With such wide usage, clearly the term larva is not based on homology despite arguments for such adherence in terminology (Rédei and Štys 2016). In being applied to widely diverse animal groups, “larva” conveys little information. Furthermore, Truman and Riddiford (1999) found that

6

hormonal control of the immature stages differs between Hemimetabola and Holometabola. In essence, the larval form that hatches from the egg of Holometabola is homologous to the embryonic stages within the egg of Hemimetabola, and the pupa of Holometabola is homologous with the nymphal instars of the Hemimetabola; development is controlled by differing titers of juvenile hormone. Thus there appears to be a fundamental difference in the growth and life cycles of these two major groups. The term “nymph” has been used for the Hemimetabola by a vast majority of major entomological works in North America (e.g. Triplehorn and Johnson 2005; Merritt et  al. 2008; Resh and Cardé 2009). Therefore, I have applied the term nymph for Odonata immatures on these grounds. The geographic region covered in this work is the North American continent north of Mexico, i.e., Canada and the lower 48 states of the United States plus Alaska. Extralimital species that occur south of the U.S. border but close to our region are diagnosed (if known) in a few genera, in event of future discovery in North America. Introductory Chaps. 2, 3 and 4 cover dragonfly nymph anatomy, advice on using the keys, and a list of the species treated. Chapter 5 contains the key to the families. The families are covered in depth in Chaps. 6, 7, 8, 9, 10, 11 and 12, including keys to genera and species; diagnostic notes are included in the Remarks section following the keys. Chapter 13 addresses field and laboratory practices for studying dragonfly nymphs. Chapter 14 presents topics in need of research, including taxonomic shortcomings, nymph biology, life history and growth, function of certain morphological features of nymphs, conservation, and water quality indices using dragonfly nymphs. Though this book is not intended to be a field guide, the habitus drawings in Appendix I provide a means for spot-identifying most nymphs and exuviae to genus.

References Benke AC, Crowley PH, Johnson DM (1982) Interactions among coexisting larval Odonata: an in situ experiment using small enclosures. Hydrobiologia 94:121–130 Blanke A, Greve C, Mokso R, Beckman F, Misof B (2013) An updated phylogeny of Anisoptera including formal convergence analysis of morphological characters. Syst Entomol 38:474–490 Brown JM, McPeek MA, May ML (2000) A phylogenetic perspective on habitat shifts and diversity in the North American Enallagma damselflies. Syst Biol 49:697–712 Burkle LA, Milhajevic JR, Smith KG (2012) Effects of an invasive plant transcend ecosystem boundaries through a dragonfly-­mediated trophic pathway. Oecologia 170:1045–1052 Bybee SM, Hansen Q, Büsse S, Cahill Wightman HM, Branham MA (2015) For consistency’s sake: the precise use of larva, nymph and naiad within Insecta. Syst Entomol 40:667–670 Carle F, Kjer KM, May ML (2015) A molecular phylogeny and classification of Anisoptera (Odonata). Arthropod Syst Phylogeny 73:281–301 Comstock JH (1918) Nymphs, naiads, and larvae. Ann Entomol Soc Am 11(2):222–224

1 Introduction Corbet PS (1963) A biology of dragonflies. Quadrangle Books, Chicago, 247 pp Corbet PS (1999) Dragonflies. Behavior, ecology of Odonata. Comstock Publishing Associates, Ithaca/New York, 829 pp Cordoba-Aguilar A (2008) Dragonflies and damselflies. Model organisms for ecological and evolutionary Research. Oxford University Press, Oxford, 290 pp DuBois RB (2016) Detection probabilities and sampling rates for Anisoptera exuviae along river banks: influences of bank vegetation type, prior precipitation, and exuviae size. Int J  Odonatol 18:205–215 Fleck G (2004) Contribution à la connaissance des Odonates de Guyane française. Les larves de Macrothemis pumila Karsh, 1889, et de Brechmorhoga praedatrix Calvert, 1909. Notes biologiques et conséquences taxonomiques (Anisoptera: Libellulidae). Annales de la Société entomologique de France (NS) 40(2):177–184 Fleck G, Brenk M, Misof B (2008) Larval and molecular characters help to solve phylogenetic puzzles in the highly diverse dragonfly family Libellulidae (Insecta: Odonata: Anisoptera): The Tetrathemistinae are a polyphyletic group. Org Divers Evol 8:1–16 Kalkman VJ, Clausnitzer V, Dijkstra K-DB, Orr AG, Paulson DR, van Tol J (2008) Global diversity of dragonflies (Odonata) in freshwater. Hydrobiologia 595(1):351–363 Knight TM, McCoy MW, Chase JM, McCoy KA, Holt RD (2005) Trophic cascades across ecosystems. Nature 437:880–883 Kohli MK, Ware JL, Bechly G (2016) How to date a dragonfly: Fossil calibrations for odonates. Palaeontol Electron 19(1.1FC):1–14 Letsch H, Gottsberger B, Ware JL (2016) Not going with the flow: a comprehensive time-calibrated phylogeny of dragonflies (Anisoptera: Odonata: Insecta) provides evidence for the role of lentic habitats on diversification. Mol Ecol 25:1340–1353 McPeek MA (1998) The consequences of changing the top predator in a food web: a comparative experimental approach. Ecol Monogr 68(1):1–23 Merritt RW, Stewart KW, Berg MB (eds) (2008) An Introduction to the aquatic insects of North America. Kendall/Hunt, Dubuque, 1158 pp Paulson D.  Dragonflies. https://www.pugetsound.edu/academics/ academic-resources/slater-museum/biodiversity-resources/dragonflies/. Accessed 2 Jan 2018 Pilgrim EM, von Dohlen CD (2008) Phylogeny of the Sympetrinae (Odonata: Libellulidae): further evidence of the homoplasious nature of wing venation. Syst Entomol 33:159–174 Raebel EM, Mercls T, Riordan P, McDonald DW, Thompson DJ (2010) The dragonfly delusion: why it is essential to sample exuviae to avoid biased surveys. J Insect Conserv 14:523–533 Ramirez A, Novelo-Gutiérrez R (1999) The Neotropical dragonfly genus Macrothemis: new larval descriptions and an evaluation of its generic status based on larval stages (Odonata: Libellulidae). J N Am Benthol Soc 18(1):67–73 Rédei R, Štys P (2016) Larva, nymph and naiad – for accuracy’s sake. Syst Entomol 41:505–510 Resh VH, Cardé RT (eds) (2009) Encyclopedia of insects. Academic, Elsevier, 1132 pp Stoks R, Córdoba-Aguilar A (2012) Evolutionary ecology of Odonata: a complex life cycle perspective. Annu Rev Entomol 57:249–265 Triplehorn CA, Johnson NF (2005) Borror and DeLong’s introduction to the study of insects. Thomson Brooks/Cole, Belmont, 864 pp Truman JW, Riddiford LM (1999) The origins of insect metamorphosis. Nature 401:447–452 Walker EM (1966) On the generic status of Tetragoneuria and Epicordulia (Odonata: Corduliidae). Can Entomol 98(9):897–902 Westfall MJ Jr (1976) Taxonomic relationships of Diceratobasis macrogaster (Selys) and Phylolestes ethelae Christiansen of the West Indies as revealed by their larvae (Zygoptera: Coenagrionidae, Synlestidae). Odonatologica 5(1):65–76 Westfall MJ Jr (1988) Elasmothemis gen. nov., a new genus related to Dythemis (Anisoptera: Libellulidae). Odonatologica 17(4):419–428

2

Nymph Anatomy and Instar Determination

Emerging dragonfly— climbing out of itself headfirst Issa (Reproduced with permission from Dragonfly Haiku (2016). Copyright Red Dragonfly Press.)

Abstract

Knowledge of the external anatomy of Odonata nymphs is necessary for learning how to use the keys for identification. Basic morphology and terminology of all structures crucial for identification of Anisoptera nymphs to family, genus and species are described and illustrated in detail beginning with orientation of the dragonfly nymph body and the basic body parts. Data on growth rate and a method for determining the last five instars are included.

This treatment of the external anatomy of Anisoptera nymphs is limited mainly to structures that provide useful taxonomic characters and is not intended to be an exhaustive treatise on the morphology of immature Odonata. Basic morphology is discussed to better orient the reader and lead to further

understanding of structures that are useful in identification. Entomology, as with all fields of science, is replete with jargon, due in large part to the diversity and myriad specializations of insects but due also to some degree in the diversity in entomologists’ opinions. For the most part, terminology presented here is based on classic works of literature such as Tillyard (1917), Snodgrass (1935, 1954) and Asahina (1954), and it conforms to homologies in other aquatic insect orders to the extent possible. In hopes of maintaining harmony with terminology used in other insect groups, I have attempted to provide general entomological terms rather than specialized terms that were coined by previous workers on Odonata. However, a few morphological structures that provide useful taxonomic characters for Odonata nymphs have not been homologized, such as the “movable hook” of the palpus. Terminology for a few

© Springer Nature Switzerland AG 2019 K. J. Tennessen, Dragonfly Nymphs of North America, https://doi.org/10.1007/978-3-319-97776-8_2

7

8

structures differs from that in other aquatic insect orders. For example, the shoulder-like dorsolateral margins of the pronotum, prominent in Petaluridae and Cordulegastridae nymphs, are termed “epaulets”; in stonefly nymphs, these produced margins are termed “marginal pronotal flanges,” whereas in mayfly nymphs they are not developed. I think it is prudent to retain the specialized odonate terms for the sake of continuity until a more thorough homology is established. Nymphs of the seven Anisoptera families that occur in America north of Mexico vary markedly in their habitus, or body form. Some groups are elongate and cylindrical, e.g., Aeshnidae and Cordulegastridae; others are dorsoventrally flattened, e.g., Gomphidae. Such body forms are adaptations to specialized nymphal habitats. For example, aeshnids are primarily climbers that blend in with their environment and stalk their prey by moving, and a sleek, long body is an advantage in this type of strategy. Gomphids burrow into aquatic substrates to hide and to ambush their prey, and a flat body with short, powerful legs is advantageous. Within each family, adaptations to specialized habitats have led to further variation in their habitus. This is especially true in stenoecious species such as the genus Aphylla which has an extremely long terminal abdominal segment for inhabiting soft, deep mud. Another example is the nymph of Hagenius, which inhabits sunken leaf packs and woody debris; it is a dead-leaf/bark mimic. To identify dragonfly nymphs, one must find the body parts upon which key characters are based. This anatomical navigating begins with the three dimensions: (1) longitudinal (long axis from head to tip of abdomen), (2) vertical (height from top to bottom), and (3) horizontal (width from side to side). The following terms that describe where characters are located depend mainly on these three axes. In the longitudinal axis, the head end is the anterior end, and the abdominal tip is the posterior end (Fig.  2.1). The terms anterior and posterior also are used to refer to positions of certain characters. In the example shown, the posterior margins of the metathoracic wing sheaths overlie the anterior end of abdominal segment 4 (S4). In the vertical axis, a character located on or near the upper surface of the body is referred to as dorsal; a character positioned on or near the underside of the nymph is referred to as ventral (Fig. 2.2). In the horizontal axis, characters at or near the side of the body or outer aspect of an appendage or segment are lateral; those at or near the middle of the body are medial (Fig. 2.1). Any character positioned near the median plane but slightly lateral to it is admedial; addorsal means near the middle of the dorsum. If a specimen is rotated in a single plane, the above terms are used in combination. For example, if the view is halfway between dorsal and lateral (a 45° angle from above, rotated on its longitudinal axis as on a spit), it is a dorsolateral view; if the view is halfway between ventral and lateral, it is a ventrolateral view. Another example is an anterodorsal view;

2  Nymph Anatomy and Instar Determination

Fig. 2.1  Dorsal view of a typical Anisoptera (Aeshnidae) nymph showing horizontal (a) and longitudinal (b) axes

Fig. 2.2  Left lateral view of a typical Anisoptera (Aeshnidae) nymph showing vertical axis

here the specimen is viewed from the front with the anterior end tilted downward. Oblique views, based on rotation in more than one plane, are more complex. For example, if a specimen in ventrolateral view is turned so that the anterior end of the nymph is slightly oriented toward the viewer (rotation on horizontal axis), it is in an anteriorly oblique ventrolateral view. The above terms apply mainly to the orientation of the viewer but can also be applied to the position of a specific character. For example, a dark stripe extending down the middle of the abdomen is termed a middorsal stripe. Other terms also apply to character position, especially those characters that are limited to a certain body part. For example, a group of setae on the base of the metathoracic femur is a proximal setal cluster (at a point near the body). The contrasting condi-

2  Nymph Anatomy and Instar Determination

tion is called distal, meaning the character is at a point farther away from the body. The term mesal refers to the inner margin of a paired body part, such as a tooth on the mesal margin of the palpus. A posterolateral character is along the side and toward the rear. Many combinations of such terms are possible. Two other terms used occasionally, ental and ectal, refer respectively to the mesal (inner) and lateral (outer) surfaces of a body part, especially mouthparts. Dragonfly nymphs have the same basic body plan as most insects, i.e. three divisions, a distinct head, thorax and abdomen (Fig. 2.3). Dragonflies are mandibulate insects, as compared to sucking, or haustellate insects such as the Hemiptera (true bugs, cicadas, leafhoppers, aphids, etc.). Dragonfly nymphs have been described as hypognathous (Walker 1932), a condition in which the mouthparts are directed ventrally. However, in some groups the head and labium are directed more anteriorly, resembling more a prognathous condition (Popham and Bevans 1979). With either type of head orientation, the long, specialized labium is thrust forward to catch prey and bring it to the mandibles and maxillae that then do the ingesting. The thorax, which is the leg- and wing-bearing division of the body, is similar to the thorax of the adult. The first thoracic segment, or prothorax, is readily distinguished from the second, or mesothorax. The latter segment is joined with the metathorax to form the pterothorax. The sides of the pterothorax extend dorsally and Fig. 2.3  Nymph of Erythemis (Libellulidae) illustrating the three major body divisions (labeled on left) and general structures (labeled on right)

9

slant rearward, similar to the adult but not as askew. The oblique posture is evidenced by the lateral sutures extending from the anteroventral portion of the pterothorax above the legs to the posterodorsal portion at the base of the wings (Figs.  2.2, 2.25). The three pairs of legs of the nymph are configured for locomotion, whereas adult legs are bunched forward to a greater degree and are not well-designed for walking. The abdomen, while considerably variable in shape, is usually elongate but much wider and shorter than in the adult. The abdomen is composed of ten segments terminating in five tapered anal appendages. In addition to recognizing the general body parts, knowledge of segmentation and associated terminology is necessary to recognize structural characters of dragonfly nymphs. As with all insects, dragonfly nymphal bodies are composed of segments, annular subdivisions of the body. Segmentation gives flexibility to a body with an otherwise rigid exoskeleton. In its simplest form, such as in some insect larvae, a segment is composed of a hardened ring and a soft, flexible intersegmental constriction. However, most insects, including Odonata, have evolved secondary segmentation in which the functional intersegmental membrane that divides succeeding segments has become a posterior ring (see Snodgrass 1935 for hypothetical illustrations). The flexible intersegmental membrane of the abdominal segment of a dragonfly nymph is folded under-

10

2  Nymph Anatomy and Instar Determination

Fig. 2.4 Longitudinal section, lateral view, of abdominal segments 6–8 of Aeshna sp. (Aeshnidae) illustrating secondary segmentation

neath the posterior hardened portion (Fig.  2.4). This arrangement allows a succeeding segment to retract partly within the preceding segment (called telescoping), and it also allows movement of the abdomen in all directions. To demonstrate this flexibility, for example, hold a live large aeshnid nymph in your fingers and see how it twists to attempt to jab you with its sharp-tipped anal appendages and posterolateral spines. The dorsal portion of the segment is the tergum, the ventral portion is the sternum; these hardened, or sclerotized, plates are separated by the lateral pleural regions, more membranous areas that also may bear sclerotized plates (sclerites). Whereas juxtaposition of the tergum and sternum has been retained in the abdominal segments, this basic segmental arrangement no longer is applicable to the dragonfly nymph head and thorax because of fusing and also extension and reduction of certain structures through evolution.

2.1

Fig. 2.5  Head of Aeshna canadensis (Aeshnidae), lateral view; an example of a dorsoventrally compressed head capsule found in prognathous groups

The Head

The head or cranium is an oval or spherical capsule, varying in shape from slightly compressed in its vertical axis to more spherical (Figs. 2.5 and 2.6). The lateral and posterior margins of the head capsule may be curved or relatively straight in either dorsal or lateral view; tubercles and setae of taxonomic importance may be present.

The dorsal part of the head, or epicranium, bears the compound eyes (composed of many facets called ommatidia) and a pair of segmented antennae. Dorsally, the ­anterior portion is made up mostly of the frons, and the posterior portion is comprised of the occiput (Fig. 2.7). In most Anisoptera nymphs, there is an anterior, transverse crest on the frons called the frontoclypeal ridge which varies from prominent to barely distinct. The posterior limit of the frons is marked by a faint

2.1  The Head

Fig. 2.6  Head of Libellula pulchella (Libellulidae), lateral view, an example of a nearly spherical head capsule found in hypognathous groups

line that is broadly sort of Y-shaped (resembling the outline of a goblet) that extends from the anterolateral margin of the compound eyes to the posteromedial margin of the head (Fig. 2.7). I have termed this line the epicranial fracture line, as it does not appear to be a true suture or sulcus; it could also be termed an ecdysial line as it breaks open during emergence to the adult stage in order for the head to be withdrawn from the exuvia. The branches of the line are somewhat difficult to

Fig. 2.7  Head of Aphylla williamsoni (Gomphidae), dorsal view, showing terminology of the epicranium

11

detect on the compound eyes of nymphs, but they are very evident in exuviae due to the split in the cuticle. The frons bears the ocelli (in the adult, the ocelli are set on a prominence called the vertex). The portion of the occiput behind the compound eye is the occipital corner, which sometimes protrudes posteriorly; it has been referred to also as the postocular lobe (Tillyard 1917). The ventrolateral portion of the head capsule on each side of the head, immediately ventrad of the compound eyes, is the gena (plural genae). The elongate, narrow posterior portion of the head, or postocciput, is attached to the prothorax by the membranous cervix. The mouthparts include the labrum (analogous to an upper lip), which is located anteriorly, paired mandibles, paired maxillae, the hypopharynx located anteroventrally, the mouth opening behind the hypopharynx, and the labium (analogous to a lower lip) which extends along the entire venter (underside) of the head (also for a variable distance between the thoracic legs when retracted). The labrum is a relatively large, sclerotized, movable flap at the front of the head (Fig.  2.8). The distal margin usually bears a fringe of setae and other sensory receptors. The proximal margin of the labrum articulates with the clypeus, an undifferentiated, lightly sclerotized area with smooth exocuticle. The base of the clypeus connects to the more central part of the epicranium called the frons. The orientation of the labrum and clypeus depends on the gnathous condition of the mouthparts. In prognathous families (e.g. Aeshnidae), they are nearly in the same longitudinal plane as the epicranium (Fig. 2.5); in families closer to a hypognathous condition (e.g. Libellulidae), their orientation is closer to the vertical plane (Fig. 2.6).

12

2  Nymph Anatomy and Instar Determination

Fig. 2.8  Head of Phanogomphus spicatus (Gomphidae), dorsolateral view; left antenna and most setae, except on labrum, omitted Fig. 2.9  Head of Phanogomphus spicatus (Gomphidae), ventral view; prementum and left maxilla removed

Fig. 2.10  Mandibles of Boyeria vinosa (Aeshnidae), ental view: L = left, R = right; incisors numbered 1–4; outer teeth of molar crest lettered a and b, m designates molar teeth between a and b (m1,2,3,4,5 in

this example; if inner molar teeth are absent, m is omitted from the formula); y  =  additional tooth between incisors and molar crest, its absence designated with a 0

The mandibles, maxillae and hypopharynx are ventral to the labrum and clypeus (Fig. 2.9). Mandibles articulate dorsally with the clypeus and ventrally with the cranial margin on the underside of the head. They are short, stout opposing jaws, with two sets of heavily sclerotized teeth, the incisors and the molars. Left and right mandibles of most species usu-

ally have slightly different configurations of teeth. Watson (1956) standardized notation of a mandibular formula for the seven families of Odonata in the region of coverage. The incisors are numbered whereas the molars are given letter designations (Fig. 2.10); in this example, the mandibular formula is designated as L 1 2 3 4 0 a (m1,2,3,4,5) b R 1 2 3 4 y a (m1,2,3,4,5) b

2.1  The Head

(L = left mandible, R = right mandible). In rare instances, the incisor teeth and/or the molar teeth are fused; in such cases, the formula contains plus signs, such as 1 + 2 + 3 + 4 for fused incisors and a +  b for fused molars. Some families lack a molar crest and the molars are positioned on the medial surface of the mandible. Mandibular morphology has not often been used to separate nymphs in treating North American odonates (Wright and Peterson 1944, Walker 1958, Needham and Westfall 1955, Walker and Corbet 1975, Westfall 1987, Needham et al. 2000, 2014). While differences in mandibles exist at family and sometimes at generic levels, interspecific differences appear to be slight or nonexistent. Moreover, mandibles are not as easily observed as other external structures because they are hidden by other mouthparts and must be extracted for a clear view of their structure (it is easier to extract and view mandibles in exuviae). Due to difficulty in viewing the mandibles, I have not included mandibular characters in the identification keys, although for each genus the mandibles are illustrated, and the mandibular formula is included in the generic description. Although the maxillae and hypopharynx do not offer clear taxonomic characters beyond family and are not considered further in this treatment, these structures are discussed to help in understanding their structure and function regarding food ingestion. The maxillae have fork-like teeth on the lacinia that pull prey held by the palpi into the mouth (Fig.  2.11). Snodgrass (1954, p.  10, Fig.  4) illustrated the hypopharynx of Anax junius. The hypopharynx apparently acts somewhat like a tongue to help ingest food (Tillyard 1917) and to prevent food from slipping back past the oral opening. The salivary duct opens behind the hypopharynx. The large, extendable, prey-catching labium (Figs.  2.5 and 2.6) is the most unique and diagnostic anatomical feature of Odonata nymphs. It offers a wealth of taxonomic characters and has been relied upon greatly in all existing keys. The present-day odonate labium is greatly modified compared to the generalized insect labium (Fig.  2.12) as interpreted by Snodgrass (1935). Corbet (1953) and Snodgrass (1954) homologized the various parts of the odonate labium, making possible comparative study. Before this breakthrough, odonate labial terminology was highly specialized and varied from author to author (see Table 2.1). In the early evolution of the Odonata, the glossae and paraglossae (appendages of the generalized ligula) were greatly reduced or lost, the labial palpi became highly specialized and diversified, the prementum enlarged along with a closing of the medial cleft, and the subdivisions of the postmentum (mentum and submentum) merged. Since the labium offers valuable taxonomic characters, familiarity with details of its morphology are necessary for nymph identification. The labium does not articulate directly with any sclerotized portion of the cranium but rather is connected ventrally

13

Fig. 2.11  Right maxilla of Phanogomphus spicatus (Gomphidae), ventral view

Fig. 2.12  Hypothetical generalized insect labium. (Concepts based mainly on Snodgrass 1954 and Chapman 1971)

to the membranous portion of the head posterior to the hypopharynx and maxillae. The labium is comprised of two parts, the proximal postmentum and the distal prementum (Fig. 2.13). The postmentum is a tube that supports the prementum and gives the labium flexibility and greater extendibility. Few taxonomic characters are found on the postmentum, although its length determines how far the labium extends posteriorly between the bases of the legs on the underside of the thorax, a character which varies in some

2  Nymph Anatomy and Instar Determination

14

Table 2.1  Terms applied to various structures on the Odonata labium in classic Odonata literature. A dash indicates the structure was not addressed Present work Postmentum Prementum Premental setae Palpus Palpal setae Movable hook Ligula Ligula cleft End tooth

Snodgrass (1954) Postmentum Prementum Premental setae Palpus Palpal setae Sharp hook Ligula – –

Needham and Westfall (1955) – Mentum Mental setae Lateral lobe Lateral setae Movable hook Median lobe Labial cleft End hook

Asahina (1954) Submentum Prementum – Lateral lobe – Movable hook Median lobe Median cleft –

Tillyard (1917) Submentum Mentum Mental setae Lateral lobe, palp Lateral setae Movable hook Median lobe Median cleft Apical hook

Fig. 2.13  Labium of Phanogomphus spicatus (Gomphidae): (a) left lateral view and (b) ventral view

genera. The prementum articulates with the postmentum at an elbow-like joint, somewhat analogous to a door that is hinged to a door jamb. I prefer the term “labial hinge”, or simply hinge (Fig. 2.13), instead of the term “elbow region of labium” used by Snodgrass (1954). The hinge is an important landmark in interpreting several characters involving the labium. The prementum possesses more taxonomic characters than any other mouthpart. It varies in shape from nearly flat or spatulate (Fig.  2.13) to slightly or moderately cup- or scoop-shaped (Fig. 2.14). Length and width are also highly variable, as are any setae that are present. The most conspicuous setae are on the ental (dorsal) surface of the prementum, although these setae are not present in all families. In the Cordulegastridae, Macromiidae, Corduliidae and Libellulidae, they are set in socket-like bases (usually conspicuous) and are usually long, relatively stout though flexible, and movable at the base; the term raptorial setae has been applied to them (Needham 1901, Needham et  al. 2014). It seems doubtful that these setae are capable of seizing prey, thin and wispy as they are, although they may help retain a struggling organism. Another function may be to flush debris from the mouthparts after prey has been consumed, as they are flicked in unison numerous times after

an organism has been consumed (pers. obs.). These setae may serve more than one function; therefore, I prefer to call them premental setae rather than raptorial setae. When present, the premental setae comprise a row on each side of the prementum, and each row is usually made up of a number of larger more lateral or distal setae and a greater or lesser number of shorter, more medial setae (Fig.  2.14). Tillyard (1917, p. 79) introduced two appropriate terms for these series: (1) primary setae for the lateral series of larger setae, and (2) secondary setae for the medial series of smaller setae (Fig.  2.15). The larger, more lateral premental setae may be separated by a gap from the smaller, more medial setae and in such cases the two groups, primary and secondary, are distinguishable. However, in some genera, there is no gap between the larger and smaller setae and there may also be a gradual decrease in length from lateral to medial; in such cases primaries and secondaries cannot be readily distinguished, and the entire series is better referred to as “prominent” premental setae. Premental setal counts typically are given for one side of the prementum. Small spiniform setae are present on the ental surface of the prementum of some species. Various types and sizes of setae and spinules may be present along the lateral margins of the prementum. The edge, or dorsolateral portion of

2.1  The Head

15

Fig. 2.14  Labium of Libellula pulchella (Libellulidae), dorsolateral view, showing cup-shaped prementum

Fig. 2.15  Prementum of Cordulegaster maculata (Cordulegastridae), ental view, showing primary and secondary series of premental setae

the prementum, is formed into a stout dorsolateral rim that often possesses fine and/or short stout setae (Figs. 2.14, 2.15). The ligula composes the distal margin of the prementum (Fig.  2.16), and it is of taxonomic importance in many groups. It is a much-reduced structure compared to that found in the adult (Snodgrass 1954, p. 18). The ligula varies from concave to convex, cleft or entire, and with or without teeth; it usually has a fringe of setae (Fig.  2.17) which varies in density and stature. The margin can be crenate or crenulate, and the median portion can be rounded or produced into a median lobe. The degree that the ligula arcs forward can be of taxonomic value, but it is difficult to quantify. The method for defining the ligular curvature is to draw an imaginary line (chord) from one corner of the ligula across to the other corner and then measure the distance from the center of that line to the distal-most point of the ligula (Fig. 2.17e). The measure is not precise and therefore

I have used it cautiously as a character in the identification keys. The labial palpi (also called palps; singular palpus) are paired, highly modified lobes that articulate at the distolateral margins of the prementum; the raised points at the distolateral margin of the prementum appear to be vestigial palpigers (Fig. 2.16). The palpi are the most variable mouthparts of the Anisoptera. They are prey-grasping organs that vary in their movement based on the plane in which the base is oriented: 1. if the bases of the two palpi articulate with the prementum in a horizontal plane, the lobes move in the same plane in which the prementum lies, i.e., the edges of the flat palpi move toward the anterior edge of the prementum (Fig. 2.18). The narrow, flat palpi do not cover the labrum when the labium is retracted (Fig.  2.5); nymphs of this type are prognathous (Aeshnidae and Gomphidae).

16

2  Nymph Anatomy and Instar Determination

2. if the palpal bases are rotated dorsally so that the lobes are oriented in a more vertical plane, the broad sides of the lobes move toward the anterior margin of the prementum in an upright position (Fig. 2.19). The widened, incurved palpi completely cover the labrum when the labium is retracted (Fig.  2.6); nymphs of this type approach a hypognathous condition (Cordulegastridae, Macromiidae, Corduliidae, and Libellulidae).

Fig. 2.16  Labium of Aeshna canadensis (Aeshnidae), ental (dorsal) view

The two types of palpal movement can be visualized by imagining two wide, thin boards: if the boards are laid flat on a table and rotated, the edges approach each other (Type 1); if the boards are held upright, the wide surfaces approach each other (Type 2). When the palpi are retracted, the movement is termed adoral (toward the mouth) and when they are extended, the movement is termed aboral (away from the mouth). The Petaluridae are somewhat intermediate between these two conditions. The palpi are upright and slightly curved, covering only the lower part of the labrum when the labium

Fig. 2.17  Variation in shape of the ligula in Anisoptera: (a) Petaluridae, (b) Aeshnidae, (c) Cordulegastridae, (d) Libellulidae, (e) defining curvature of ligula by drawing an imaginary chord (dashed line)

Fig. 2.18  Flat palpi of Aeshna canadensis (Aeshnidae), anterodorsal view; arrows indicate direction of movement toward ligula upon retraction

is retracted; however, head orientation in Petaluridae is intermediate between prognathous and hypognathous, and the palpi are not greatly widened. Therefore, petalurids do not conform more to one type than the other. The main body of the palpus (in some references called the palpal lobe) varies from a narrow, flat blade (in the families Aeshnidae and Gomphidae I refer to it as the palpal blade) to a wide, incurved flap (in the other five families I refer to it simply as the palpus). The palpus bears a long, stout to needle-like projection called the movable hook which is slightly curved and undoubtedly used to help grasp prey (Fig.  2.20). The movable hook articulates preapically on the dorsolateral rim of the palpus and differs in stature depending on family. In Aeshnidae and most Gomphidae, the movable hooks are large and stout, capable of holding and

2.1  The Head

17

Fig. 2.19  Movement of wide, curved palpi of Libellula pulchella (Libellulidae), anterodorsal view

Fig. 2.21  Right palpus of Celithemis elisa (Libellulidae), dorsal view, showing slender movable hook

Fig. 2.20  Right palpus of Arigomphus sp. (Gomphidae)

possibly stabbing large struggling prey. In the Libelluloidea, the movable hooks are usually smaller and flimsier than in other families (Fig.  2.21); in many libellulid genera, the movable hook is only slightly more robust than the large palpal setae on the palpal dorsal rim. It is not likely that libellulids pierce large prey with the movable hooks, but rather that the hooks are adapted for retaining smaller prey. The only family in which setae are present on the movable hook is the Aeshnidae, varying from small and inconspicuous to fairly long. The palpus often has a series of projections on the mesal (inner) margin. In families with a series composed of regular tooth-like projections that are relatively small and either

rounded, triangular or rectangular (sometimes sharply pointed) these projections are called palpal teeth (Fig. 2.20). The most distal tooth in the series may be enlarged, in which case it is called the end tooth or end hook (not to be confused with the movable hook). In other families the mesal margin is slightly indented to smooth and may have small projecting setae (Fig.  2.22a). In these families, the distal margin of the palpus has projections that are more lobe-like and curved (scalloped), in which case they are known as palpal crenations. Crenations may be small and regular, as in Corduliidae and Libellulidae (Fig.  2.22a) or they may be large, pointed and irregular as in Cordulegastridae (Fig.  2.22b), although the latter type occurs much less frequently. Various types of setae are present on the palpus of most Anisoptera. Palpal setae (although absent in some groups) on the mesal side of the dorsal rim are aligned in a row and vary greatly in number and robustness (Fig.  2.22). These setae, like the long premental setae, also have been termed raptorial setae (Needham et  al. 2014). However, because these setae do not fit the classical definition of prey grasping structures (Grimaldi and Engel 2005), they are referred to in this work simply as palpal setae. The number of palpal setae is an important character; if the most proximal seta is slighter in stature (shorter and thinner) than the more distal setae, it should still be counted. As for premental setae, palpal setal counts in the generic descriptions are given for one side. Setae can be broken off but if the setal bases are evident, they should be included in the setal count. As nymphs increase in size, the number of palpal setae increases; setae are added at the proximal end of the series or less often within the series. Such precursory setae are smaller than those that developed

18

2  Nymph Anatomy and Instar Determination

Fig. 2.22  Right palpus, ental view: (a) Libellulidae (Celithemis elisa); (b) Cordulegastridae (Cordulegaster erronea)

Fig. 2.23  Variation in Anisoptera antennae, right antenna, dorsal view: (a) Aeshnidae; (b) Libellulidae; (c) Petaluridae; and (d) Gomphidae. Scale = 1 mm divided into tenths; as = antennal socket

in previous instars. There are often other types of setae present on the palpi. In families with an enlarged, vertically ­oriented palpus, the lateral margin of the dorsal rim often bears one or more types of smaller setae, sometimes barely detectable (Fig.  2.22a) but often including piliform setae (Fig. 2.22b) and/or short, stout setae; short, spiniform setae are usually present entally near the basal hinge. Stout spiniform setae are usually present on the distal crenations and along the ventral margin (Fig. 2.22a). Numerous genera of Corduliidae and Libellulidae have dark spots or blotches on the lateral surface of the palpus and prementum. The compound eyes are usually located anterolaterally on the cranium (Figs. 2.3, 2.5 and 2.6) and are usually relatively large. The eyes may follow the contour of the lateral margin of the head or project laterally or dorsally. Their size, position, and shape constitute important taxonomic characters. The diagonal mesal line that demarcates the mesal margin of the compound eye can be obscured in nymphs by the black tissue underlying the integument; it is much more easily seen in final instar exuviae. In final instars that are nearing maturation and ultimately metamorphosis, the developing adult eye enlarges and further obscures the true outline of the

nymph eye. Three dorsal ocelli are often evident, appearing as transparent, small oval spots on the frons (Fig. 2.7); the anterior, or median ocellus, is approximately level with the contour of the frons, whereas the more posterior paired ocelli may be slightly raised. Whether ocelli are functional in nymphs was questioned by Tillyard (1917, p.  74). Adult ocelli might be important in detecting light intensity (Chapman 1971), possibly aiding the compound eyes in adjusting to different light levels; apparently this has not been studied in dragonfly nymphs. Nymph antennae are well-developed and more obvious as compared to those of adults and are set in a forward position on the epicranium (Fig. 2.3), presumably an adaptation to a predatory lifestyle. Odonate antennae are equipped with numerous sensillae of various types and are extremely important in detecting stimuli such as movement, texture, and possibly chemical. The number of segments, or antennomeres, ranges from 4 to 7  in our fauna (Fig.  2.23), but some Anisoptera in other parts of the world have more than 7 segments (Theischinger and Endersby 2009). Typically, with insects, the first antennal segment, counting from the base of the antenna, is called the scape; the second segment is called

2.2  The Thorax

the pedicel and the remaining segments make up the flagellum. To simplify terminology for odonate nymphs, all the antennal segments are referred to as antennomeres, abbreviated antm (e.g., antm1 = the first antennal segment). The base of antm1 articulates in a membranous antennal socket, which in turn is set in an immovable, tubercle-like ridge on the frons (Fig. 2.23a). I have not found taxonomic characters based on this tubercle but point it out only because it could be mistaken for antm1. Antennae provide taxonomic characters at the family level and in some cases will serve to distinguish genera; interspecific differences based on antennomere length exist in some groups, but relative antennomere length is subject to a degree of variation yet to be quantified. In the literature on Odonata, the antennomere ratio (AR) is based on the relative lengths of the individual antennomeres, with antm1 set at a value of 10. For example, with a 7-segmented antenna, if the actual antennomere measurements from antm1 to antm7 were 0.25, 0.28, 0.42, 0.25, 0.28, 0.33 and 0.33 mm, the AR would be 10:11:17:10:11:13:13. In most species that I studied, variation in antennomere length was too high for comparative purposes, and therefore I did not include antennomere ratios in the generic descriptions.

2.2

The Thorax

The odonate thorax consists of a small but distinct, movable prothorax and a large, fused pterothorax (Fig.  2.3). The dorsal surface of the prothorax, or pronotum, is raised and usually beset with various types of setae, especially laterally. The lateral portion of the pronotum is usually distinctively produced as a shoulder-like protuberance termed the epaulet (Fig. 2.24). Ventral to each epaulet and dorsal to the coxa (in the propleural region), there may be a projection known as the supracoxal process (Fig. 2.24); it may be entire or bifid with sharp or dull point(s) directed laterally, anterolaterally, or less often anteriorly. The prothoracic legs are usually relatively short. The physical configuration of the pterothorax was altered so extremely early in Odonata evolution that the orientation of the segmental plates no longer conforms to basic secondary segmentation as discussed on p. 9 and presented in

Fig. 2.24  Pronotum of Aeshna canadensis (Aeshnidae), dorsal view

19

Fig. 2.4. The lateral and dorsal aspects of the pterothorax are formed by pleurites, predominantly the episternum and epimeron of both the meso- and metathorax which in other insects usually form only the lateral wall. The notal sclerites (plates that form the thoracic tergum in most insects) of Odonata are greatly reduced and tucked between the wings; in nymphs they are not exposed and are not important as taxonomic structures. The dragonfly nymph thorax has been modified thusly: the mesepisternum makes up the bulk of the anterodorsal and anterolateral portions of the pterothorax; from each side of the pterothorax, the mesepisternal sclerites approximate each other at the middorsal line anterior to the bases of the wing sheaths (Fig. 2.25, indicated by arrow), although they are slightly separated at the midline. The mesepimeron and metepisternum on each side are fused and comprise the middle portion, and the metepimeron comprises the posterior portion on each side. The mesothoracic spiracle, which is well-developed, is situated dorsolaterally between the prothorax and the mesepisternum. In some groups, these spiracles appear to be open and therefore functional but in other groups are apparently closed. Posterior to the spiracles lays the membranous mesopleural region, in some groups split into two pale, flat triangles with an oval-shaped, centrally located acrotergite, best seen in dorsal view (Fig. 2.26). The second or metathoracic spiracle, located on the metepisternum above the metinfraepisternum, is scarcely noticeable and apparently closed and nonfunctional. The pterothorax bears the meso- and metathoracic legs and two pairs of wing sheaths (also called wing pads or wing cases) in which form the fore (mesothoracic) wings and hind (metathoracic) wings of the adult. The wing sheaths (Figs. 2.3 and 2.25) overlie the anterior abdominal segments and are either parallel or divergent. The mesal pair are the mesothoracic wing sheaths, the lateral pair the metathoracic wing sheaths; note that the exposed surface of the metathoracic wing sheaths is the ventral side. The legs, variously adapted for walking, digging and holding the nymph in position, are comparatively large and widely separated in contrast to those of the adult. Leg segments consist of coxa, trochanter, femur, tibia, and tarsus (Fig.  2.27a). The coxa is short, broad and tapers distally. The trochanter appears to be composed of two fused segments that do not articulate with each other, although Snodgrass (1954) considered it to be a single segment with a constriction. The femur is the largest and stoutest leg segment and usually has numerous long and/or short setae along its length. The tibia is usually long and narrow and usually bears long setae. The tarsus consists of either 2 or 3 segments called tarsomeres (Fig.  2.27b); the pretarsal claws, usually called tarsal claws, are well-­ developed in Odonata. The thoracic venter (underside) is flattened though usually concave medially where the postmentum rests when the

20

Fig. 2.25  Pterothorax of Aeshna canadensis (Aeshnidae), left lateral view. f.w. = fore wing (mesothoracic) sheath; h.w. = hind wing (metathoracic) sheath; mspt  =  mesepisternum; mspm  =  mesepimeron;

2  Nymph Anatomy and Instar Determination

mtpt  =  metepisternum; mtpm  =  metepimeron; msf  =  mesinfraepisternum; mtf = metinfraepisternum

is produced anteriorly in some groups. The shallow depressions on each side of the sternum are the furcal pits, joined by a transverse suture. The pits mark the location of the true sternal plate. In some groups (Aeshnoidea, Petaluroidea, Gomphoidea and Cordulegastroidea) there is a median, longitudinal suture on the metasternum termed the discriminal line (Asahina 1954) that branches posteriorly from the transverse suture. The sclerite between the metinfraepimeron and first abdominal segment is the poststernite. In the Libelluloidea, the discriminal line is no longer existent and the transverse suture that joins the furcal pits is now the same as the suture forming the anterior border of the poststernite. Morphology of the ventral sternites, their sutures, and the distances between the coxae help distinguish some families and genera. Fig. 2.26  Pterothorax of Somatochlora kennedyi (Corduliidae), dorsal view of medial portion. Act = acrotergite; msp = mesothoracic spiracle; mmr = membranous mesopleural region

labium is retracted. A few characters based on configuration of the thoracic venter are useful in identification, and I believe that additional characters will be found under more thorough investigation. The following brief treatment will suffice for the characters I have utilized. The prosternum is independent and movable, connected to the mesothoracic sternum by an intersegmental membrane. The ventral surface of the pterothorax is formed by fusion of the mesinfraepimeron and metinfraepimeron with the sternal plates (Fig. 2.28). The anterolateral corner of the mesinfraepimeron

2.3

The Abdomen

The abdomen is composed of 10 segments. I have designated each segment with the letter “S” and a number (S1  =  segment 1, S2 = segment 2, etc.). The easiest way to enumerate segments is to start at S10 and count backwards. Unlike the thorax, where the pleurites have migrated upward to form the bulk of the dorsum, the abdomen has retained a more normal juxtaposition of tergites and sternites. The dorsal tergites are arched upward to a varying degree; sternites vary from slightly convex (arched ventrally) to slightly concave. Abdominal shape varies within the Anisoptera, from nearly cylindrical and elongate to wide, depressed and short; its length/width ratio offers useful taxonomic characters in

2.3  The Abdomen

21

Fig. 2.27  Leg morphology: (a) metathoracic leg of Libellulidae; (b) mesothoracic tarsus of Aeshnidae, showing numbering of tarsomeres

Fig. 2.28  Thoracic venter of Aeshna canadensis (Aeshnidae); stippling indicates membranous exocuticle. Msfpm = mesinfraepimeron, mtfpm = metinfraepimeron, prst = prosternum, ptst = fused pterothoracic sternum

some groups. It is usually widest at the middle segments. The smallest segment is S10, which is often partly to nearly completely retracted into the end of S9. S10 is much narrower than the preceding segments and lacks ventral sutures. Valuable taxonomic characters on the abdomen include (1) degree of taper posteriorly (gradual versus abrupt); (2) length:width ratio of certain abdominal segments; (3) presence or absence of middorsal hooks and posterolateral spines (Fig. 2.29), and (4) relative size and shape of the latter processes. Middorsal hooks [usually referred to as “dorsal

hooks” in the literature, except Cashatt and Vogt (2001) termed them “middorsal hooks”] range from low, arched protuberances (Fig.  2.30) to sharply pointed, elongate, curved or straight processes. The contour of the array of middorsal hooks (in lateral view) sometimes has been termed the “skyline” but herein is referred to as the abdominal profile. Along the side of each segment, there is a lateral carina where the tergum meets the sternum (Fig. 2.31a, b). On segments with a posterolateral spine, the lateral carina terminates in the spine. Ventral to the lateral carina is a very

22

2  Nymph Anatomy and Instar Determination

narrow longitudinal membranous area with a small, anterior laterosternite; the laterosternite flexes during respiratory movements of the abdomen (Fig. 2.31a). This small sternite is also called the anterolateral sclerite; its presence or absence on certain segments sometimes offers helpful taxonomic characters. Major characters on the abdominal venter include (1) length/width ratio of individual segments; (2) shape and dimensions of the sternal sclerites as delineated by sutures, especially on S7–9 (Fig. 2.32); and (3) nature and density of

Fig. 2.29  Abdomen of Ophiogomphus rupinsulensis (Gomphidae), dorsal view. S = segment

Fig. 2.30  Abdomen of Ophiogomphus rupinsulensis (Gomphidae), lateral view

Fig. 2.32  Abdomen of Ophiogomphus rupinsulensis (Gomphidae), ventral view

Fig. 2.31  Lateral carina of S7 of Aeshna canadensis (Aeshnidae), (left side): (a) slight ventrolateral view; (b) cross section at mid-length. Alst = laterosternite; light stippling indicates pleural membrane. To scale

2.3  The Abdomen

23

Fig. 2.33  Middorsal third of S10 of Ophiogomphus rupinsulensis (Gomphidae), enlarged to show granular appearance of cuticle

setae. In some groups, setal bases (cuticular area that protects the setal membrane or alveolus) are raised. If these roughly circular basal areas are conspicuous and numerous, they can give the body surface a granular appearance both dorsally and ventrally (Fig.  2.33); this is especially true if they differ in color from the surrounding flat surface of the cuticle. Small closed spiracles on the venter of S1–8 can be seen in some taxa, although in this work no taxonomic characters are based on them. For the most part, characters based on length and width of segments and posterolateral spines require accurate measurements. The keys in this work usually specify that such measurements be taken in ventral view where a suture can be used to mark a definite beginning point to place the zero of an ocular micrometer or other measuring device. Exact methods for measurement are given at the beginning of the keys. This technique allows greater precision and repeatability of measurements than the dorsal side where there are no sutures to mark the base (point to begin) of the measurement. In some genera, posterolateral spine length is dependent on habitat type (e.g., Flenner et al. 2009), resulting in variability in fish versus fishless habitats; discovery of this phenomenon has cast doubt on the reliability of spines as characters to separate certain species. Further details of problems that have arisen are discussed in the treatment of genera where applicable. Sharp spinules and various setae may be present on the lateral margins of the segments. The abdominal surface may appear smooth or granulate (maculose); a granulate appearance is usually caused by dark, raised setal bases. Puncta (sing. punctum), which are oval or circular impressions in the integument, are often apparent on the dorsum of the abdomen, especially if darker than the surrounding integument; puncta are seldom useful for species identification. The posterior margins of the segments, both dorsal and ven-

Fig. 2.34  Anal appendages of Aeshna canadensis (Aeshnidae) female, an example of an emarginated epiproct; setae omitted

tral, are often beset with sharp spiniform setae and/or long fine setae, especially on the more apical segments. Five caudal appendages make up the anal appendages (sometimes lesser-used terms are applied such as anal valves, anal pyramid or anal triangle, e.g. Chelmick 2001). These appendages consist of the dorsomedial epiproct, a pair of dorsolateral cerci (singular cercus), and a pair of ventral paraprocts (Fig. 2.34). This terminology follows Snodgrass (1954); in early treatments of nymphs, the terms used were “superior appendage” for epiproct, “lateral appendages” for the cerci, and “inferior appendages” for the paraprocts (Walker 1933). Their apices are usually sharply tapered, although the tip of the epiproct may be acute, rounded, square or emarginate. When held together, the epiproct and para-

24

2  Nymph Anatomy and Instar Determination

procts cover the anal opening; they are important aids in respiration and jet propulsion. The shape and relative lengths of the anal appendages provide a number of useful characters. Color pattern is evident in some taxa, but it can be highly variable and difficult to ascertain. Nymphs of a particular species may be dark or pale, a trait that is probably largely dependent on the background of the habitat in which the nymphs spent most of their developmental time. Dark blotches, longitudinal striping and horizontal banding can be useful in some genera, but for the most part color pattern is of rather limited use as a taxonomic character. Markings that are longitudinal are called stripes, those that are horizontal are called bands. Both kinds of markings are seen on the venter of the abdomen in the genus Leucorrhinia (Libellulidae).

2.4

External Morphological Outgrowths

According to Snodgrass (1935), there are two basic types of external processes (outgrowths) on insect cuticle, i.e., noncellular and cellular. Noncellular projections are minute cuticular points or nodules, such as spicules and small ridges; these structures are seldom used in odonate taxonomy. Cellular outgrowths include setae, spines, spinules, spurs and tubercles, all of which are highly useful in Odonata taxonomy and defined as follows: seta—a single-celled process (receptor), highly variable in shape spine—multi-celled, immovable (not separated from the cuticle by a joint) process of the body wall spinule—a small spine spur—multi-celled, movable (connected to the body wall by a joint), spine-like process of the body wall tubercle—multi-celled, round or oval, immovable raised area of the integument Numerous types of setae are found on dragonfly nymphs, some of which are important in taxonomy. Long, stiff (sometimes called raptorial) setae on the labial palpi and prementum have already been discussed. There is a great variety of setae on other parts of the body, some of which are useful for identification purposes. The following treatment categorizes Anisoptera setae based solely on shape. The basic categories are: (1) piliform, or hair-like; (2) spiniform (stout or spine-­ like); (3) flattened; (4) claviform (club-shaped), peg-like, or papilliform. Piliform setae are found in all families; length and stature are highly variable (Figs. 2.35a–d), from nearly straight to curving, weak and silk-like (curvature easily influenced) to somewhat stiff (flexible but curves retain their shape after bending, similar to a cat whisker or an eyelash). Spiniform setae vary in thickness and length but are usually

inflexible with a sharp apex, often resembling small spines; they can be scattered on the surface of the integument or aligned along a margin of an abdominal segment (Figs. 2.35e– f). A special variation of the spiniform seta type is the trifid setae often found on the apices of the tibiae and sometimes on the tarsomeres (Fig. 2.35j). Flattened setae may be wider than long or vice versa; their edges may be parallel or slightly divergent (Fig.  2.35g); they are sometimes blade-like. Claviform setae are variable in their club-like form, from elongate with an abruptly enlarged tip to short with a gradually widened tip. Papilliform setae are pale and slender at base with a somewhat more bulbous apex (Fig. 2.35h). Peg setae are very small, pale rounded cones (Fig.  2.35i). The majority of setal types presented here probably serve a tactile function, innervating a dendrite of a sensory neuron. Setae that are sparsely situated probably function to receive outside stimuli; when in clusters near other body parts or joints they are probably proprioceptive. Spines and spinules, immovable outgrowths of the body wall (Fig. 2.36), are typically found on margins of the head, thorax and abdominal segments. Spurs are seldom found on dragonfly nymphs but do occur on the tibiae in the Petaluridae (Fig. 8.4). Tubercles are found on various parts of the body such as the epicranium, thorax and epiproct. External gills are unknown in Anisoptera; respiration takes place through the integument and the internal rectal, or branchial, chamber (Snodgrass 1954). Sensillae on the body of Anisoptera nymphs have been grossly under-utilized as taxonomic characters. In the keys, I have introduced numerous new characters based on setae and spines which are relatively easy to examine on nymphs although magnification is necessary; on exuviae, unfortunately, setae are often matted down or lost.

2.4.1 Determining Gender Gender can be determined in F-0 and F-1 nymphs by several abdominal characters, although these differ somewhat in the seven families in North America. Males have slightly raised, delineated areas located medially on the venter of S2 and S3 (Fig.  2.37); these markings are smooth, usually lack setae, and are often pigmented slightly differently than the surrounding integument. The markings indicate the area where the adult male accessory genitalia develop internally. Another uniquely male characteristic is the presence of a quasi-round tubercle or pair of small tubercles on the dorsal aspect of the epiproct (Fig.  2.38). In most Libelluloidea (Macromiidae, Corduliidae, Libellulidae), both of these male indicators are often marginally developed and faint, making them difficult to see. Females lack definable structural delineations on the venter of S2 and S3 of males; instead, the medial portion resem-

2.4  External Morphological Outgrowths

Fig. 2.35  Various types of setae on Anisoptera nymphs: (a) piliform setae on lateral margin of abdominal segment 6 of Stylurus amnicola; (b) recurved setae on antennomere 2 of Gomphurus fraternus; (c) short setae on dorsal rim of palpus of Somatochlora kennedyi; (d) curved stiff seta on anterolateral corner of head of Stylogomphus albistylus; (e) i  =  small spiniform setae with raised bases on abdominal segment 7 middorsal hook of Ophiogomphus rupinsulensis; ii = sharp, spiniform

Fig. 2.36  Posterolateral spine (Psp) and spinules (spn) on posterolateral margin of abdominal segment 9 of Gomphurus fraternus; setae omitted

bles the rest of the integument. Females also lack epiproctal tubercles. Females may be positively recognized by outgrowths that represent the internally-developing (incipient) egg-laying apparatus. Female aeshnids, petalurids and cordulegastrids are easily recognized by the developing ovipositor which projects from the ventroposterior margin of S8 and continues distally along the underside of S9 (Fig. 2.39a). In Gomphidae, a pair of small finger-like lobes (the vulvar

25

setae on posterior margin of abdominal segment 7 of Paltothemis lineatipes; (f) sharp stout setae on abdominal segment 9 lateral margin of Libellula pulchella; (g) flat blade-like setae on frontal ridge of Cordulegaster erronea; (h) pale papilliform setae on antennomere 3 of Lanthus vernalis; (i) peg setae on antennomere 3 of Stylogomphus albistylus; (j) trifid seta (enlarged) on apex of prothoracic tibia of an aeshnid. Not to scale; scale for j = 0.1 mm

l­ aminae) projects from the posterior margin of S8 (Fig. 2.39b). The basal portion of these mediobasal projections is often hidden under the posterior margin of S8, and sometimes the entire structure is concealed. In the three libelluloid families, the vulvar laminae are vestigial and very difficult or impossible to discern; vestigial vulvar plates can sometimes be found by pressing down on the medioposterior margin of S8 with blunt forceps and examining the membranous area between it and S9. Another sexual character that will assist in separating males and females of Libelluloidea is the developing adult gonads: in males, on the venter of S9, a small gonopore is located anteromedially (Fig.  2.40a); it varies from dark to quite faint and it usually has a slightly darkened, horizontal line, or shelf, over its anterior point. In females, a pair of obscure small medial depressions is present (Fig. 2.40b); these pore-like depressions are separated by a distance greater than the width of either one. Most of these sexual distinctions discussed above are not yet developed in nymphs younger than F-1, although the developing ovipositor is present in F-2 and F-3 Aeshnidae, Petaluridae and Cordulegastridae, albeit on a relatively smaller scale. Several characters unique to gender are useful for taxonomy. In male nymphs, the epiproct may be swollen dorsally, consisting of either a basal raised elongation or a pair of anteapical tubercles; the position and size of these protuberances may vary among species. In females, rudiments of the gonapophyses or vulvar laminae (depending on family) on

26

2  Nymph Anatomy and Instar Determination

Fig. 2.37 Abdominal segments 2 and 3, ventral view: (a) Ophiogomphus carolus (Gomphidae); (b) Libellula luctuosa (Libellulidae), showing raised areas indicative of male accessory genitalia

Fig. 2.38 Anal appendages of male Hylogomphus abbreviatus (Gomphidae), dorsal view; arrows indicate tubercles on epiproct (e)

the venter of S8–9 help differentiate species in some genera. As stated above, small rudiments are often hidden beneath the intersegmental membrane of S8.

2.5

Growth

Several terms are basic in any discussion of insect growth. The process by which an insect sheds its exocuticle (outer skin) in order for its body size to increase is called ecdysis. Exoskeletons limit growth by not being expandable; insects can grow only by successively shedding their skin. Before ecdysis occurs, there is a separation of the exocuticle from the endocuticle and a new layer of soft cuticle is laid down; this process is known as apolysis (Chapman 1971, Riddiford 2009). Apolysis involves resorption of the endocuticle and formation of procuticle (still underneath the exocuticle) before molting can occur. The new skin is not revealed until the old outer layer is shed, after which the new pliable cuticle can be expanded; later the upper portion of procuticle becomes the new exocuticle. The actual shedding of the old exocuticle (for instars prior to metamorphosis to the adult stage) takes about 10 min or less (Corbet 1999); body expansion occurs at this time, after which hardening of the exocuticle is initiated (a complex chemical process called sclerotization; see Hopkins and Kramer 1992). However, not all parts of the exocuticle harden; intersegmental membranes remain pliable to give the insect flexibility (to demonstrate dragonfly nymph flexibility, try holding a live Anax nymph

with your fingers  – a trapped Anax nymph can twist and swing its abdomen forward in an attempt to poke its attacker with its sharp anal appendages). Hardening of the exocuticle is temperature-dependent; in Odonata nymphs it probably takes from 1 to 3 h, the duration depending mainly on temperature (Corbet 1962, p. 83). Two terms have been applied to the visible insect form between ecdyses, resulting in a division regarding usage within the odonatological community. The controversy involves the traditional term instar versus a more recent switch that favored the term stadium. Instar and stadium were considered interchangeable terms by definition according to Torre-Bueno (1937). Hinton (1973, 1976) and Jones (1978) broke from tradition and applied the term instar to the form between apolyses (as explained above, apolysis cannot be seen because it occurs inside the old exocuticle) and the term stadium to the observable form between ecdyses. They restricted the term molting to the retraction of the old exocuticle from the newly formed procuticle (i.e., molting occurs during apolysis). Corbet (1999) followed this reasoning and applied it to the Odonata; thus stadium has been used predominantly in the Odonata literature for more than a decade. However, the term instar can be traced back to Linnaean usage (Whitten 1976); it has been used for the insect form between ecdyses by the clear majority of entomologists for over 250  years. Moreover, apolysis does not take place simultaneously in all parts of the developing immature insect’s body (Wigglesworth 1973) and is therefore not a discrete event. I think that the term stadium, if it is to be used, be restricted to the interval between molts (apolyses) and that the proper term for the odonate nymph between ecdyses is instar. Because instar is the actual nymph, it is redundant to say, for example, seventh instar nymph; it is the seventh instar. By quantifying the change in size of a specific body part from one ecdysis to the next, a growth ratio (GR) can be calculated. The method is also known as Dyar’s rule, which was initially based on geometric increases in head width after successive ecdyses of Lepidoptera larvae (Dyar 1890) and was later extrapolated to other insects. For example, if head width of a particular instar is 3.0  mm, that value is divided by head width of the previous instar, say 2.4 mm, to yield a ratio of 1.25. In other words, the insect increased 25%

2.5 Growth

27

Fig. 2.39  Developing female genitalic structures on venter of abdominal segment 9: (a) developing ovipositor of Anax junius (Aeshnidae); (b) developing vulvar laminae of Hylogomphus abbreviatus (Gomphidae)

Fig. 2.40  Venter of abdominal segment 9, Libellula luctuosa (Libellulidae): (a) male gonopore; (b) female depressions

in size after ecdysis; any such ratio should specify the body part that was measured, in this case head width. For the majority of hemimetabolous insects, growth ratios are usually slightly less than 1.3 (Cole 1980). Leggott and Pritchard (1985) reported a mean GR of 1.24 for Argia vivida (Coenagrionidae), and Painter et al. (1996) reported a mean GR of 1.3 for Erythemis simplicicollis (Libellulidae). Although head width has been commonly used as a metric, metathoracic femur length is also a convenient character to measure. I calculated the GR of four instar progressions (F-4 to F-3, F-3 to F-2, F-2 to F-1, and F-1 to F-0) representing the last five instars of 12 species of North American Anisoptera; measurements were head width and metathoracic wing sheath length. Median GR based on head width was 1.26– 1.29, significantly lower than that for metathoracic wing length, which was 1.82–2.13 (Fig.  2.41). Precise values of GR can be obtained by rearing individual nymphs of a species from hatching to the final instar, preserving all the exuviae, and then measuring head width and/or metathoracic femur length of each instar. At one time there was hope that this approach would give researchers the ability to determine how many instars a particular species went through, and also the ability to determine the instar of a particular individual specimen. However, research has shown that not all individuals of a species go through an equal number of molts. Leggott and Pritchard (1985) reported that of 18 reared Argia vivida, the number of instars varied; the majority had 12 instars (n = 12), although a few had 13 (n = 5) and one nymph had 14. Moreover, growth ratios vary not only between individuals but also between combinations of successive ecdyses for any one individual. This variability is disconcerting, considering that rearing is intensive and time-consuming work.

Fig. 2.41  Average growth ratio (GR) of last four instars based on head width (solid line) and metathoracic wing sheath length (dashed line) of 12 species of Anisoptera

It also means that determining the total number of instars from field-collected specimens is not feasible.

2.5.1 Determining Instar For purposes of identification, and for many biological studies, it is desirable to be able to determine the instar of the specimen in question. In fact, one of the most perplexing problems in studying Odonata nymphs is “What age nymph am I looking at?” The question then becomes, “How does one go about determining the various instars?” The solution is in development of the wing sheaths. Designation of instars follows Lutz (1968), where F-0 is the final instar, F-1 the penultimate instar, and so on. The following discussion is based on Assis et al. (2000) and Tennessen (2017).

28

As with most identification keys to aquatic insect immatures, the characters in this work are based mainly on final instar nymphs, although some earlier instars in certain groups can also be identified. I would advise, however, as a general rule that identification of any instar earlier than F-3 should not be attempted. The method described below to determine instar depends on measurements of the head and metathoracic wing sheath. The ratio of metathoracic wing sheath length (HwL) to head width (HW) usually allows determination of instars from F-0 back to F-3, and sometimes F-4. Generalizations presented here for each family are based on only a few species, as I have not established an HwL/HW ratio for each species in North America. Head width is measured as the maximum distance in dorsal view, which is usually across the compound eyes. Metathoracic wing sheath length is measured in dorsal view also, from the base of the metathoracic wing sheath at its medial juncture with the metepimeron to the apex of the wing sheath (Fig. 2.42). The ratio HwL/HW can then be calculated. HwL/HW ratios (Table 2.2) for final instars ranged from 0.99 to 1.35. The data indicate that for F-0, metathoracic wing sheaths are usually longer than the width of the head in all seven families; the ratio is rarely slightly less than 1.0 (mean = 1.16). For F-1, the ratios in all seven families were from about 3/5 to nearly 9/10 (mean  =  0.70); for F-2 the ratios were from about 2/5 to 3/5 (mean = 0.46). Instar F-2

2  Nymph Anatomy and Instar Determination

overlapped slightly with F-1 at values between 0.57 and 0.60. The ratio for F-3 overlapped slightly with F-2 at values between 0.38 and 0.42, and F-4 overlapped with F-3 between 0.26 and 0.32. Although average ratios do not necessarily reflect the value for any particular species, they do offer general guidelines for determining instar: HwL/HW ratio for F-4 is about 1/4, for F-3 about 1/3, for F-2 about 1/2, for F-1 about 2/3, and for F-0 greater than 1.0. However, it should be kept in mind that species differ quite widely in their ratios from instar to instar, and more data are needed to estimate variation and Table 2.2  Mean ratio of metathoracic wing sheath length/head width in the five latest instars of 14 species of Anisoptera Species Tachopteryx thoreyi Boyeria vinosa Anax junius Nasiaeschna pentacantha Erpetogomphus designatus Phanogomphus lividus Ophiogomphus colubrinus Cordulegaster maculata Didymops transversa Macromia illinoiensis Neurocordulia molesta Somatochlora cingulata Erythemis simplicicollis Plathemis lydia Overall mean: Low range: High range:

F-4 – 0.15 – 0.18 – 0.23 0.24 0.26 0.29 0.24 0.18 0.24 0.19 0.32 0.23 0.15 0.32

F-3 0.27 0.26 0.36 0.32 0.35 0.32 0.33 0.35 0.39 0.32 0.26 0.32 0.28 0.42 0.32 0.26 0.42

F-2 0.42 0.39 0.42 0.43 0.50 0.49 0.49 0.49 0.50 0.45 0.38 0.47 0.43 0.60 0.46 0.38 0.60

F-1 0.69 0.57 0.68 0.65 0.78 0.71 0.74 0.63 0.82 0.68 0.60 0.70 0.67 0.87 0.70 0.57 0.87

F-0 1.24 1.00 1.11 1.15 1.22 1.18 1.17 1.14 1.27 1.12 0.99 1.12 1.17 1.35 1.16 0.99 1.35

Dash in F-4 column indicates specimens unavailable

Fig. 2.42  Ophiogomphus colubrinus (Gomphidae) F-5 instar, dorsal view, showing measurement of head width and metathoracic wing sheath

Fig. 2.43  Growth of Plathemis lydia (Libellulidae) based on head width (diamonds) and metathoracic wing sheath length (circles)

References

limits within each family. One of the few studies that reported HwL/HW data was by Cannings and Cannings (1985). Measurements on which Table 2.2 was based indicate that wing sheaths grow at a greater rate than do other body parts. When wing buds first appear, usually at the sixth instar, they are markedly smaller than the head, but they grow rapidly so that by the final instar their length surpasses head width (Fig. 2.43). For example, after the molt from F-1 to F-0 in Plathemis lydia, growth ratios were 1.29 for head width and 2.00 for metathoracic wing sheath length. Wing sheaths are a prime example of allometric growth (growth rate of a particular body part is different from another part), a field of study that has received little study regarding Odonata.

References Asahina S (1954) A morphological study of a relic dragonfly Epiophlebia superstes Selys (Odonata, Anisozygoptera). The Japan Society for the Promotion of Science, Tokyo, 153 pp Assis JCF, Carvalho AL, Dorville LFM (2000) Aspects of larval development of Limnetron debile (Karsch), in a mountain stream of Rio de Janeiro state, Brazil (Anisoptera: Aeshnidae). Odonatologica 29(2):151–155 Cannings SG, Cannings RA (1985) The larva of Somatochlora sahlbergi Trybom, with notes on the species in the Yukon territory. Can Odonatol 14(4):319–330 Cashatt ED, Vogt TE (2001) Description of the larva of Somatochlora hineana with a key to the larvae of the North American species of Somatochlora (Odonata: Corduliidae). Int J Odonatol 4(2):93–105 Chapman RF (1971) The insects. Structure and function. American Elsevier Publishing Company, Inc, New York, 819 pp Chelmick DG (2001) Larvae of the genus Aeshna Fabricius in Africa south of the Sahara (Anisoptera: Aeshnidae). Odonatologica 30(1):39–47 Cole BJ (1980) Growth ratios in holometabolous and hemimetabolous insects. Ann Entomol Soc Am 73:489–491 Corbet PS (1953) A terminology for the labium of larval Odonata. Entomologist 86:191–196 Corbet PS (1962) A biology of dragonflies. Quadrangle Books, Chicago, 247 pp Corbet PS (1999) Dragonflies. Behaviour and ecology of Odonata. Comstock Publishing Associates, Cornell University Press, Ithaca/ New York, 829 pp Dyar HG (1890) The number of molts of lepidopterous larvae. Psyche 5:420–422 Flenner I, Olne K, Suhling F, Sahlen G (2009) Predator-induced spine length and exocuticle thickness in Leucorrhinia dubia (Insecta: Odonata): a simple physiological trade-off? Ecol Entomol 34:735–740 Grimaldi D, Engel MS (2005) Evolution of the insects. Cambridge University Press, New York, 755 pp Hinton HE (1973) Neglected phases in metamorphosis: a reply to V. B. Wigglesworth. J Entomol (A) 48(1):57–68 Hinton HE (1976) Notes on neglected phases in metamorphosis, and a reply to J. M. Witten. Ann Entomol Soc Am 69(3):560–566 Hopkins TL, Kramer KJ (1992) Insect cuticle sclerotization. Annu Rev Entomol 37:273–302

29 Jones JC (1978) A note on the use of the terms instar and stage. Ann Entomol Soc Am 71(4):491–492 Leggott M, Pritchard G (1985) The life cycle of Argia vivida Hagen: developmental types, growth ratios and instar identification (Zygoptera: Coenagrionidae). Odonatologica 14(3):201–210 Lutz PE (1968) Life-history studies on Lestes eurinus say (Odonata). Ecology 49:576–579 Needham JG (1901) Odonata. In aquatic insects in the adirondacks. Bull New York State Mus 47:381–612 Needham JG, Westfall MJ Jr (1955) A manual of the dragonflies of North America (Anisoptera), including the greater Antilles and the provinces of the Mexican border. University of California Press, Berkeley, 615 pp Needham JG, Westfall MJ Jr, May ML (2000) Dragonflies of North America. Scientific publishers, Gainesville, 939 pp Needham JG, Westfall MJ Jr, May ML (2014) Dragonflies of North America. Scientific Publishers, Gainesville, 657 pp Painter MK, Tennessen KJ, Richardson TD (1996) Effects of repeated applications of Bacillus thuringiensis israelensis on the mosquito predator Erythemis simplicicollis (Odonata: Libellulidae) from hatching to final instar. Environ Entomol 25:184–191 Popham EJ, Bevans E (1979) Functional morphology of the feeding apparatus in larval and adult Aeshna juncea (L.) (Anisoptera: Aeshnidae). Odonatologica 8(4):301–318 Riddiford LM (2009) Molting. In: Resh VH, Cardé RT (eds) Encyclopedia of insects, 2nd edn. Academic, Burlington, pp 649–654 Snodgrass RE (1935) Principles of insect morphology. McGraw Hill, New York, 665 pp Snodgrass RE (1954) The dragonfly larva. Smithson Misc Collections 123(2):1–38 Tennessen K (2017) A method for determining stadium number of late stage dragonfly nymphs (Odonata: Anisoptera). Entomol News 126(6):299–306 Theischinger G, Endersby I (2009) Identification guide to the Australian Odonata. Department of Environment, Climate Change and Water, Sydney South, 283 pp Tillyard RJ (1917) The biology of dragonflies (Odonata or Paraneuroptera). University Press, Cambridge, 396 pp Torre-Bueno JR (1937) A glossary of entomology. Brooklyn Entomological Society, Brooklyn/New York, 336 pp Walker EM (1932) Prognathism and hypognathism in insects. Can Entomol 54:223–229 Walker EM (1933) The nymphs of the Canadian species of Ophiogomphus Odonata, Gomphidae. Can Entomol 65:217–229 Walker EM (1958) The Odonata of Canada and Alaska, vol. 2. Part III: the Anisoptera, four families. University of Toronto Press, Toronto, 318 pp Walker EM, Corbet PS (1975) The Odonata of Canada and Alaska, vol 3. University of Toronto Press, Toronto, 307 pp Watson MC (1956) The utilization of mandibular armature in taxonomic studies of anisopterous nymphs. Trans Am Entomol Soc 81:155–202 Westfall MJ Jr (1987) Order Odonata. In: Stehr FW (ed) Immature insects, vol 1. Kendall/Hunt, Dubuque, pp 95–117 Whitten JM (1976) Definition of insect instars in terms of ‘apolysis’ or ‘ecdysis’. Ann Entomol Soc Am 69(3):556–559 Wigglesworth VB (1973) The significance of “apolysis” in the moulting of insects. J Entomol (A) 47(2):141–149 Wright M, Peterson A (1944) A key to the genera of anisopterous dragonfly nymphs of the United States and Canada (Odonata, suborder Anisoptera). Ohio J Sci 44:151–166

3

Using the Keys

Abstract

Advice on how to proceed in using the taxonomic keys to the nymphs is provided. Emphasis is placed on arriving at correct generic determinations and checking the diagnoses and illustrations to confirm identification. Variation in characters states and pitfalls of certain types of characters are explained. The importance of preparing specimens and cleaning certain specimens (both nymphs and exuviae) for examination is detailed.

While constructing the keys to families, genera, and species, I tried to adhere to several guidelines. First, taxonomic keys should be designed so that users who are unfamiliar with the group can arrive relatively easily at a determination, especially at the family and genus level; precise wording of each character is paramount. Second, the keys should be amply illustrated, with illustrations arranged on the same page as the couplet being considered. Third, the keys should use at least two of the most easily observed characters. Fourth, the most obvious or reliable diagnostic character should come first. And last, the key should take into consideration variation of all taxa included. These guidelines usually result in keys that are not necessarily indicative of phylogenetic relationships. A final goal was to provide additional diagnostic information to assist in verifying determinations. A taxonomic key is often more a means of eliminating possibilities than an accrual of characters. I think of the keying process as “calculatus eliminatus.” Each couplet requires a choice: does the specimen being examined display this character state or the other? There are times one cannot unequivocally determine a character state but rather can observe that the specimen at hand does not possess the contrasting character state. For example, the first part of a couplet may state “lateral spines on segments 6–9” and the opposing part of the couplet “lateral spines only on segments 8 and 9.” If a lateral spine is present on segment 7 but you are not certain that there is a spine on segment 6, the choice still would be the first statement. The seven Anisoptera families are keyed in Chap. 5. Each family is then treated individually (Chaps. 6, 7, 8, 9, 10, 11 and 12). In each chapter, a key to the genera is provided first;

in the keys to genus, characters are based mainly on the species that occur in the region covered in this book, i.e., Canada and the United States. This scope of treatment will undoubtedly limit the effectiveness of attempting to use the keys for specimens from outside this region. Generic determinations should be verified by referring to the detailed descriptions, diagnoses, illustrations and range maps provided for each genus. The genera are treated individually in alphabetical order within each family. For each genus, a detailed generic description is provided, based on all species known in the nymphal stage and predominantly on the species that occur in North America. A key to the species known in the nymphal stage is provided if sufficient characters were found. Nymphs are usually more difficult to determine to species than adults. Whereas adults of closely related species can be distinguished using color pattern and sexual characters, nymphs rarely differ in such characters, often necessitating use of more difficult species characters. The species are further diagnosed following each key. As with most keys, some specimens may be indeterminable because of extreme variation or deformed or missing structures. Furthermore, not all specimens will match exactly the illustrations given for structural and color pattern characteristics. Although I tried to study specimens from various areas of the geographic range of a given species, this was not always possible. Therefore, some key and diagnostic characters might be more variable than indicated. I am confident that students following this work will find more variation in size and other characters than reported here. The dragonflies are not going to be reading this book and therefore will not adhere to everything I have presented. To begin, read the choices in the first couplet, and carefully compare your specimen with the associated figures for each dichotomy. Once you have made a choice as to which part of the couplet best fits your specimen (or does not), the couplet will indicate whether you have arrived at a determination or if you need to proceed further to another couplet. If a name is given, go to the treatment for that particular taxon and verify your determination by checking (1) diagnostic remarks and the description, (2) any other figures provided, and (3) information on distribution. As a last note here, after

© Springer Nature Switzerland AG 2019 K. J. Tennessen, Dragonfly Nymphs of North America, https://doi.org/10.1007/978-3-319-97776-8_3

31

32

you have looked at certain characters, look again, for sometimes our eyes deceive us. In the key to the families, the tarsal formula is the number of tarsal segments on the prothoracic, mesothoracic, and metathoracic legs. For example, a tarsal formula of 2–2-3 for the Gomphidae conveys that the tarsi of the prothoracic and mesothoracic legs have only two segments and those on the metathoracic legs have three segments. Other abbreviations are: antm = antennomere (e.g., antm3 = third antennomere; L  =  length; W  =  width; L:W  =  ratio of length to width; S = abdominal segment (e.g., S5 = segment 5). The generic range maps were generated by combining the species distributions as they appeared on the OdonataCentral website (Abbott 2006–2019). I suggest that once you determine a specimen to genus level, you go to the treatment of that genus and check to see if the locality of the specimen being examined is within the range as shown on the map for the genus. For example, if a specimen from Virginia is keyed to Octogomphus, note that the range map for that genus will clearly indicate that that genus is restricted to western North America. Returning to the key to gomphid genera, it is likely the specimen will key to one of two similar genera, either Lanthus or Stylogomphus. The distribution ranges of genera are overall indicators of where to expect the genus and importantly not all areas within the range will support the genus. The maps are based on locality records collected over many years, including areas where the genus may be extirpated at present; on the other hand, the maps might not include areas where the species exists but has not yet been reported. Species ranges are subject to change due to survey efforts and various environmental factors, including global climate change (Hassall and Thompson 2008). In the generic treatments, the size range of final instar (F-0) nymphs of each species is given in a table. Six characters were measured: total length (TL), head width (HW), prementum maximum width (PrementW), abdomen length (AbdL), abdomen width (AbdW), and epiproct length (EpiL). The data in these tables are supplemental to the keys and are not necessarily cited in the diagnoses or remarks sections. Total length is highly variable, depending on both time since molting and state of preservation. Nymphs collected shortly after molting may shrink or expand depending on the type and dilution of alcohol in which they are preserved and whether or not they were par-boiled (see Chap. 13); such specimens may have a total length less than or greater than more mature nymphs. Moreover, poor preservation of nymphs affects telescoping of the abdominal segments and results in overestimates. Exuviae easily become distorted and as such are usually less reliable than nymphs for total length and abdomen measurements. I included mainly well-­preserved nymphs and some exuviae that appeared to have little or no telescoping of the abdominal segments for measurement of total length and abdomen length, and as a result the data presented here do not always coincide with previous literature. Some descriptions in the literature

3  Using the Keys

give the relative lengths of the antennomeres. However, antennomere lengths are highly variable even within a species; instead of calculating antennomere ratios for each genus, I indicate which antennomeres are relatively longer.

3.1

Specimen Preparation

For many specimens, cleaning is necessary in order to see certain structures, especially at higher magnifications. Needham (1901) recognized the problem when he stated (referring to the genus Libellula), “… the hairs serving to hold an ambuscade of silt about the body.” This statement applies to nymphs of most Anisoptera taxa. A soft artist brush is useful to remove mud, silt, sand and flocculent materials, but care must be taken not to break off antennae and setae, especially on exuviae. Sometimes nymphs are encrusted with hard mud or coated with salts or even minerals. Carefully scrape the obscured surface with a fine needle and then use a soft brush to further clean the specimen; invariably setae will be lost during this endeavor. The type(s) of sediments on the body may hold clues for microhabitat studies, although we must keep in mind with exuviae that the final instar may crawl on or through substrates different than the microhabitat in which the nymph developed, and in doing so, other types of materials may adhere to its body. Dried exuviae are difficult to study without breaking off certain structures, such as antennae, labium and legs. If detailed structures need to be examined, especially the labial palpi and prementum, the exuviae can be placed in alcohol for several hours before study, and then the structures should be manipulated very slowly and carefully to avoid breakage. I transfer preserved nymphs from vials to a Petri dish of alcohol to prevent desiccation during study under the microscope. Specimens tend to drift when making measurements, drawings or photographs. Ware and Louton (2009) used a commercial hand cleaner in the dish to help keep specimens from moving and to position legs and other structures into more life-like poses for photographic purposes. However, the hand cleaner contains glycerin, which can coat specimens and mat down setae; rinsing in 80% ethanol is recommended to remove the glycerin. Methods for preserving nymph specimens are presented in Chap. 13.

References Abbott JC (2006–2017) OdonataCentral: an online resource for the distribution and identification of Odonata. Available at http://odonatacentral.org. Accessed Oct 2017 Hassall C, Thompson DJ (2008) The effects of environmental warming on Odonata: a review. Int J Odonatol 11(2):121–153 Needham JG (1901) Aquatic insects in the Adirondacks. Bull New York State Mus 47:384–612 Ware JL, Louton J (2009) A larva worth a thousand words: imaging preserved dragonfly nymphs using a digital camera. Argia 21(2):10–12

4

List of Species Treated

Abstract

The diversity of Anisoptera in America north of Mexico (herein referred to as North America) is about half that of South America (330 species compared to about 650). Only about 12% of the species are shared by these two large continents. In the seven families represented in North America , the majority (66%) of species belong to two families, the Gomphidae and Libellulidae. Five species are currently unknown in the nymph stage and an additional 43 species are inadequately described or poorly known. A table listing the 330 species is provided, including literature references for each species in which descriptions appeared.

4.1

Odonata Diversity in the Americas

The number of species of Anisoptera known to occur in northern North America (Canada and the continental United States plus Alaska) is 330 (main sources: Paulson 2017a, Paulson 2018). The region has an area of approximately 19.3 × 106  km2 (7.5 × 106 mi2). In South America, a slightly smaller region with an area of 17.9 × 106 km2, nearly twice as many species (647) of Anisoptera have been recorded (Paulson 2017b), and many more new species await description (Garrison et  al. 2006). Although species diversity is lower in our region of coverage, the number of Anisoptera families (7) is only one less and the number of genera (72 versus 88) is fairly comparable between the two large regions. The other suborder of Odonata, the Zygoptera or damselflies, is much more diverse in South America than in the region of North America covered in this book (13 families with over 790 species compared to four families with 136 species). Adding the suborders together, the number of Odonata species in South America is nearly twice that of North America (the land mass north of the Isthmus of

Panama). By comparison, the region of Mexico and Central America combined (Middle America not including the West Indies), with about 1/8 the land area (2.5 × 106 km2) of northern North America, has about 525 species of Odonata (Paulson 2017c). The entire North American continent (north of the Isthmus of Panama) shares a very low percentage of odonate species with South America (about 12% of the Anisoptera and 0.25 and cercus L:paraproct L 0.50–1.20.............................................................................................................................. Corduliidae Prementum without a ventromedial groove (Fig. 5.17b); palpus usually without pale, piliform setae on lateral edge of dorsal rim (Fig. 18b), if present then ratio of palpal crenation #3 or 4 H:L usually