The Complete Insect: Anatomy, Physiology, Evolution, and Ecology 9780691243108, 9780691243115

Table of contents :
Cover
Contents
Foreword: Phil DeVries and Carla Penz
Editor’s Preface: The World of Insects
Chapter 1. Introduction
Chapter 2. Structure and Function
Chapter 3. Wings and Flight
Chapter 4. Development, Metamorphosis, and Growth
Chapter 5. Natural History
Chapter 6. Impacts on Humans and the Environment
Glossary
Further Reading
Index
Picture Credits
Acknowledgments

Citation preview

THE COMPLETE

INSECT

THE COMPLETE

INSECT Anatomy, Physiology, Evolution, and Ecology CONSULTANT EDITOR

David A. Grimaldi

Princ eto n U niversity Press Pr inc eto n • Ox fo rd

Published in 2023 by Princeton University Press 41 William Street, Princeton, New Jersey 08540 99 Banbury Road, Oxford OX2 6JX press.princeton.edu Copyright © 2023 by Quarto Publishing plc All Rights Reserved Princeton University Press is committed to the protection of copyright and the intellectual property our authors entrust to us. Copyright promotes the progress and integrity of knowledge. Thank you for supporting free speech and the global exchange of ideas by purchasing an authorized edition of this book. If you wish to reproduce or distribute any part of it in any form, please obtain permission. Requests for permission to reproduce material from this work should be sent to [email protected] ISBN: 978-0-691-24310-8 Ebook ISBN: 978-0-691-24311-5 Library of Congress Control Number: 2022951319 This book has been composed in Klein and Sagona Printed on acid-free paper Conceived, designed, and produced by The Bright Press Part of the Quarto Group 1 Triptych Place, London, SE1 9SH Publisher: James Evans Art Director: James Lawrence Editorial Director: Isheeta Mustafi Managing Editor: Jacqui Sayers Project Editor: Abbie Sharman Project Manager: Anna Southgate Design: Tony Seddon Picture Research: Jenny Quiggin Illustrations: Robert Brandt Commissioned by Jacqui Sayers Jacket images: Front: Robby Fakhriannur / Shutterstock Back (from left to right): Loren Image / Dreamstime.com; Tomatito26 / Dreamstime.com; HWall / Shutterstock; Dedy Hutajulu / Dreamstime.com; Lukas Jonaitis / Shutterstock; Nancy Bauer / Shutterstock; Carly Autumn / Shutterstock; David Clode / Unsplash; Drew Rawcliffe / Shutterstock Jacket design: Wanda España Photograph previous page: Marek R. Swadzba / Shutterstock Printed in Malaysia 10 9 8 7 6 5 4 3 2 1

Contributors Preface, Chapters 1 and 6, and consultant editor Dr. David A. Grimaldi is an entomologist and curator at the American Museum of Natural History in New York, where he is also a professor in its Gilder Graduate School. His work focuses on flies (order Diptera) and the geological record of insects, especially their fossilization in amber. He is broadly interested in the evolution, ecology, and diversity of insects, and how they came to dominate land. He is the author of 280 publications, including the book written with M. S. Engel, Evolution of the Insects (Cambridge: 2005).

Chapter 2 Dr. Steven R. Davis is a research associate at the American Museum of Natural History in New York, visiting scientist at the Tokyo University of Agriculture, and currently a specially appointed assistant professor at Kanazawa University, Japan. He conducts research on various aspects of insect biology: the structure and development of the insect cuticle; the tracheal system; the occurrence of hemoglobins in terrestrial arthropods; the ultrastructure and functioning of the firefly lantern; weevil evolution; and rostrum development, among others.

Chapter 3 Dr. Jessica Fox is a professor in the biology department at Case Western Reserve University in Cleveland, Ohio. She is interested in the neural mechanisms of sensory information processing and flight control in insects. She has a BSc in entomology from Cornell University and a Ph.D. in neurobiology and behavior from the University of Washington.

Chapter 4 Dr. Isabelle M. Vea studied at The Sorbonne (M.Sc.), the American Museum of Natural History (Ph.D., Comparative Biology), and has done postdoctoral research in genetics and developmental biology at the University of Nagoya, Japan, and the University of Edinburgh, UK. She is now at the University of Illinois, Chicago. One of her research focuses is the genetics and hormonal regulation of extreme differences in the development of males and females in scale insects.

Chapter 5 Dr. Michael S. Engel is a world authority on bees and the diversity of living and extinct insects. Among his 900+ scientific reports, reviews, monographs, and other publications is the recently published book Innumerable Insects (Sterling: New York, 2018).

Contents Foreword: Phil DeVries and Carla Penz 8 Editor’s Preface: The World of Insects 10

1

2 Introduction 12

3 Structure and Function 40

Wings and Flight 130

4 5 Development, Metamorphosis, and Growth 170

6 Natural History 228

Glossary 354 Further Reading 357 Index 358 Impacts on Humans and the Environment 316

Picture Credits 367 Acknowledgments 368

Foreword The Complete Insect is a book that bridges two worlds. Overall, it is steeped in time-honored, classical natural history that makes entomology and insects themselves such a beguiling field of study. On the other hand, the book uses nascent technology that melds molecular- and chemical-level insights into anatomy, physiology, and evolution, and it incorporates recent advances in microscopy and macrophotography to illustrate the incredible beauty of insects. The relationships between humans and insects are complex. Newcomers to the insect universe will learn that members of this ancient and tremendously diverse group of organisms are key components of a complex engine that sustains the integrity and diversity of terrestrial communities. In many respects, insects maintain the fabric of life on Earth. Do a thought experiment: imagine how desolate our world would be without insect pollinators. Nevertheless, in modern urban societies, there is a widespread revulsion and fear of insects. This book will challenge those who, without real experience, tightly cling to such misguided notions. In contrast, those of us who have an appreciation for insects will pore over these pages as if perusing a family photo album—searching for familiar species and, simultaneously, making new acquaintances. Here, the sheer magnificence of the protagonists will

8

remind us of how many cultures around the globe independently use insects as adornments, portray them in art, keep them as pets or amulets, or cast them in precious metals and stones. The photographs alone speak to audiences of all ages, and constitute the gateway for the scholarly content that elucidates insect diversity, behaviors, and evolution. Every aspect of their lives is covered in detail: development from egg to adult, organ systems, form and function, biological interactions, their place in the tree of life, and the impacts insects have on ecosystems and humans. The Complete Insect is a symbiosis among five distinguished entomologists who have produced a book that will edify in a fundamental manner. It will bring the readers closer to these undeniably fascinating organisms that, quite literally, surround us. This is an important, inspirational work that young people should have access to early on in their lives. As Earth has now embarked on a human-induced mass extinction event, The Complete Insect will entice curiosity and wonder in people about the natural world, and provide a much-needed ray of hope to conserve biodiversity, and perhaps humanity. Phil DeVries, Ph.D. and Carla Penz, Ph.D.

Editor’s Preface: The World of Insects Among my memories as a boy are many in woods and fields, at streams and ponds. I caught frogs and snakes, collected rocks, owl pellets, the rare animal skull. One of my most wondrous discoveries, around age 10, was a dead insect about 1.5 in (4 cm) long, with bold yellowblack-brown markings, and three filaments at the tail end that curved over the slender abdomen, much longer than the body. What is that? Does it sting? Could it go through your hand? Sealed in a small jar, it nestled among my other natural history treasures, awaiting study. I learned that my insect freak was Megarhyssa, a large ichneumon wasp. I couldn’t know my discovery would be a biological epiphany but, 55 years later, viewing insects through a microscope is still transfixing, an endless parade of intricacy, complexity, and curiosity. For those drawn to nature, Planet Insect has inescapable gravitational pull. With 1.1 million named species, and an estimated 3.5 million total, they will provide thousands of lifetimes of exploration and original discoveries. Moreover, the living species are a fraction of what has been estimated to be more than 100 million insect species that have ever lived. Clearly, this is the champion class of organisms in the 3.5-billion-year history of life on Earth. But why are insects so incredibly diverse and abundant? The answer, as shown here in the The Complete Insect, has several parts. Chapter 1 considers the first: age. Being among the original animals to colonize land, perhaps 420 million years ago, insects had a head start on later competitors. We know they had a marine ancestor because DNA research reveals that insects are actually crustaceans. Being an early colonist does not guarantee success, though; it also requires great adaptability. Chapter 2 presents, in striking images, the myriad anatomical and physiological adaptations of insects. A cuticle allowed early insects to avoid desiccation and punishing radiation on land. Gills are useless on land; insects instead have an intricate network of fine tubules within the body (tracheae), connected to the exterior by tiny spiracles, through which oxygen and CO2 pass. Growing within an exoskeleton is a problem, and the process of molting into a larger body is a complex one. Of all the major adaptations of insects, wings are arguably singular. Finding mates, tracking food and other resources, escaping, and dispersing to better habitats are all much more efficient using flight. Little wonder that 10

flowering plants partnered with them as pollinators. As Chapter 3 discusses, insects evolved wings at least 100 million years before pterosaurs, and hundreds of millions of years before birds and bats. The structure, muscular control, and movements of wings vary enormously among insects, as does their flight. Judging from the fact that 85 percent of living insects have complete metamorphosis, holometaboly is considered another major adaptation of insects. As Chapter 4 shows, insect development is hardly binary between those with and without a larval and pupal stage. It is much more of a spectrum, with many detours. The ability to subdue metabolism, or diapause, allows many insects to withstand harsh seasons. Small size, short generation times, and fecundity produce high genetic diversity, the clay for the sculptor of natural selection and evolution in general. Chapter 5 beautifully portrays how insects occupy virtually every conceivable niche on land and in freshwater. They are the premier herbivores and pollinators. There are many trillions of individual insects, but their numbers vary greatly. Among the most abundant groups are eusocial insects, or ones with castes, especially ants and termites. Living in a society provides coordinated defense and foraging that is so efficient it outcompetes solitary species. Insects have triumphed through evolutionary time and mass extinctions, now meeting their match in the “Insect Apocalypse.” The tiny percentage feeding on humans, their livestock, and crops have been in the crosshairs of chemical warfare, which kills many innocuous and even beneficial species. Habitat loss and climate change also contribute to the apocalypse. Despite all their services and virtues— pollination, soils, keeping forests healthy, as models for lab research, their beauty—insects not only deserve much better, but most of them have a public relations problem. It all starts with understanding. Books like The Complete Insect didn’t exist when I was a boy in the 1960s. Macrophotography was not well developed, and it was far before digital imaging and design. If you cannot wander woods and streams as I did—and still do—or even if you can, this book should spark a fascination of insects, maybe even your own Megarrhysa moment.

↑ An ichneumon wasp in action. These wasps drill into wood, locate a hidden larva, and inject eggs using its

Naming Insects

absurd ovipositor. When the eggs hatch, the wasp’s larvae gradually devour their host.

“The first part of knowledge,” a Chinese proverb says, “is getting the names right.” With millions of species, names for insects can be challenging. For the common names used here the entomological tradition is applied, best said by the great morphologist R.E. Snodgrass: “If the insect is what the name implies, write the two words separately; otherwise run them together. Thus, we have such names as house fly, blow fly, and robber fly contrasted with dragonfly, caddisfly, and butterfly, because the latter are not flies, just as an aphislion is not a lion and a silverfish is not a fish. The honey bee is an insect and is preeminently a bee; ‘honeybee’ is equivalent to ‘Johnsmith’.”

EDITOR’S PREFACE

11

Chapter

1

Introduction Insects comprise the largest class of multicellular organisms in the history of life. Estimates of their numbers vary, but consensus centers on around 3.5 million species, roughly three times the number of named ones. How did insects become so successful, especially since they live only on land and in fresh water? Genomic research shows that insects are closely related to several obscure, marine crustacean groups. At least 420 million years ago, insects colonized land, and radiated into millions of species in every terrestrial niche, classified into 26 living orders. The orders are extremely disparate, some with only a few dozen or few hundred species; four orders with complete metamorphosis each have several hundred thousand species. Great reproductive capacity and short generations promote genetic diversity and adaptive change. Very effective adaptations, such as an external cuticle, small size, wings, and metamorphosis, make them more resistant to natural levels of extinction. ← A robberfly (family Asilidae), perched with its beetle prey impaled on its mouthparts.

13

What is an Insect? This question might seem to have an obvious answer, since everyone learns that insects have six legs and a pair of antennae. While true, there are special distinctions that entomologists make to distinguish insects from other arthropods and even other six-legged relatives. Insects belong to a group of arthropods called hexapods, meaning “six-footed.” More than 99 percent of hexapods are insects, the remaining ones are small, wingless, and often cryptic. Insects are distinguished from other hexapods by their exposed mandibles, large compound eyes and three ocelli (eyespots) at the head end; and, at the tail end of the abdomen, a pair of antenna-like cerci and, in females, an egg-laying appendage (the ovipositor).

Exoskeleton Like crustaceans, spiders, centipedes, and other arthropods, the insect exoskeleton has soft, jointed areas that permit bending and movement, like a suit of armor. The exoskeleton itself is made of plates of different shapes and sizes called sclerites; the sutures between them can be anything from immovable to very flexible. Arthropods move using muscles that are attached to ingrown sections of adjacent sclerites, causing the segments to bend when muscles contract. There are numerous small segments comprising the appendages (antennae, legs, and mouthparts), and larger segments comprising the head, trunk (or thorax in insects), and abdomen. The segmentation of arthropod bodies is also reflected internally, in the repetitive structure of the nervous system and (for insects) also in the respiratory system.

Hexapods An entomobryomorph collembolan (springtail). Springtails rarely exceed ⁄5 in (5 mm) in length.

1

14

INTRODUCTION

When arthropods colonized land perhaps 430 million years ago (mya), the exoskeleton is believed to have had great significance for early arachnids (spiders, mites, scorpions, others), myriapods (centipedes and millipedes), and insects. Besides preventing drying out, the exoskeleton in these early terrestrial arthropods must have protected them from punishing ultraviolet radiation. The atmospheric layer of ozone (O3) that developed helped protect against UV radiation, the few land plants at the time grew close to the ground, with little shade, and there was little soil into which insects could burrow.

Hexapods All hexapods have three main segments of the thorax (the pro-, meso-, and metathorax)—a distinguishing feature from other arthropods—to which attach the fore-, mid-, and hind legs, respectively. Hexapods further have each leg segmented into a coxa (which attaches to the thorax), a small trochanter, a stout femur, a slender tibia, a tarsus, and, at the tip, a minute pretarsus that bears specialized pads and a pair of claws. Unlike their crustacean relatives, which have two pairs of antennae, hexapods have just one pair. The basic number of abdominal segments is 11, though many insects have lost some of the segments. In the primitively wingless hexapods, some abdominal segments have a pair of tiny appendages, the styli, and small sacs, called eversible vesicles. The styli act like skids for the abdomen, and the vesicles absorb water from the substrate. Both of these structures are lost in all winged insects. Lastly, hexapods have a pair of sensory appendages on the last abdominal segment, called cerci; these can be long and multisegmented, like a hind pair of antennae, or a one-segmented stub. In some insects the cerci have been lost altogether.

A diversity of body shapes in relation to the basic body plan of an insect, with a stonefly (Plecoptera) at center.

Ant

(Hymenoptera)

Head

Beetle

(Coleoptera)

Antenna

Butterfly

Dragonfly

(Lepidoptera)

(Odonata)

Foreleg

Thorax

Midleg

Hind leg

Abdomen

Cercus

Cat flea

Mosquito

(Siphonaptera)

(Diptera)

Sucking louse (Anoplura)

WHAT IS AN INSECT?

15

Noninsect Hexapods

Diplurans: Diplurans are named for the pair of cerci,

Noninsect hexapods are almost all quite small to minute wingless creatures living in wet to moist habitats near shores, under dead leaves, logs, and rocks. Their mouthparts are partly recessed into a small pouch, giving them the class name Entognatha (“inner chewing”). The most commonly encountered entognaths are springtails, but diplurans and proturans also belong to this class.

which are particularly long in Campodeidae. There are two predatory subgroups within Diplura: japygids, in which the cerci are modified into a pair of large, powerful pincers that make them look like earwigs; and the smaller projapygids. A japygid grasps and crushes its prey with its pincers, and feeds by curling the long abdomen over its back and dangling the prey in front of its mouth. The cerci of projapygids bear a minute spigot at the tip of each, through which a sticky substance is squirted to immobilize tiny prey.

Springtails: Springtails (Collembola) are the most abundant and species rich of the three entognath groups, so called because of an appendage under the abdomen (the furcula) that, when suddenly released, flings the springtail into the air. Springtails are stout and have just a few eye facets. They sometimes occur on shores in enormous numbers, and even over the surface of the water. Drowning is prevented by the hydrophobic cuticle.

16

INTRODUCTION

Proturans: Proturans are minute and walk using just the mid and hind pairs of legs; the forelegs have sensory structures and are held in front, acting like antennae. Both Protura and Diplura are blind, completely lacking any remnant of eyes.

← A dipluran of the family Campodeidae, showing the typical long, multisegmented antennae and cerci, which are sensory. All diplurans lack eyes. ↑ The head of a particularly colorful sminthurid springtail or Collembolan. The dark areas behind the antennae are small, raspberry-like eyes. → A minute, blind proturan, shown here walking on its mid- and hind legs. Its forelegs, held up and forward, are used for sensing, like antennae.

True Insects

The Egg-Laying Appendage

True insects—hexapods exclusive of entognaths—include the bristletails (order Archaeognatha), silverfish (order Zygentoma), and all the winged insects (subclass Pterygota). Insects are distinguished from the entognaths not just by having their mandibles and other mouthparts exposed, but by having large compound eyes (comprised of individual small facets), and three ocelli (small, lenslike eyespots on the top of the head that are sensitive to polarized light). Also, true insects do not have muscles entirely within the antennae, so their antennae cannot curl around an object as for diplurans. The tarsus in true insects is subdivided into smaller segments of up to five tarsomeres, allowing them to grasp.

At the tail end of the abdomen, true insects have an ovipositor (“egg placer”), an appendage in the female adult through which eggs pass so that they can be inserted into the ground, into plants, or even into other insects. The ovipositor is comprised of two to three pairs of appendages coming from segments eight and nine, which rigidly join in the center. Some orders of insects, such as moths and butterflies, and true flies, have lost the ovipositor. In many crickets and katydids, the ovipositor is very large and swordlike, but it reaches spectacular lengths—several times the body length—in parasitoid wasps, which drill into wood to inject their eggs into a larva within the wood. In the stinging wasps, the ovipositor has developed into a sting, which delivers venom; the eggs are laid through an opening just in front of the sting.

↑ A facial view of a silverfish, showing the characteristic scales and small eyes.

18

INTRODUCTION

The Original Insects If you were to travel back around 420 million years to the Late Silurian, the ancestral insect—one before wings evolved—would probably look like a modern bristletail or silverfish (sometimes called apterygotes). These are usually covered with fine scales, and scuttle among stones, under fallen leaves, under loose bark, in basements, some even live as symbionts in ant and termite nests. Both groups have a long, fine, multisegmented structure, called the median caudal filament, that looks like a third cercus between the two true cerci. This filament is absent in winged insects. Apterygotes also molt as adults, whereas all winged insects except mayflies do not molt as winged forms. Bristletails have large eyes, are hunch-backed, and can flex the body suddenly to jump into the air. Silverfish are flattened and have smaller eyes (some are blind). Despite their many similarities to bristletails, silverfish are actually more closely related to winged insects, which is reflected in a very obscure but important feature: mandibles that have two joints instead of one. This feature apparently allowed insects better to chew, grind, and manipulate food.

↑ A female grasshopper, with a

↓ A silverfish, which is probably very

swordlike, egg-laying appendage,

similar to what the earliest wingless

the ovipositor.

insects were like.

WHAT IS AN INSECT?

19

Where are the Marine Insects? With several million species of insects, it is not too surprising that they have evolved to occupy every conceivable ecological niche on land and in fresh water. Why, then, are they essentially absent from the oceans? There are a few insects in oceans, but they live on, or near, the surface: Halobates, the ocean water strider; and Pontomyia, a midge where males skate over the surface using wing stubs for sails (females are larviform, feeding on algae on rocks or sea turtles). Various other insects feed in the intertidal zone, but there are no submarine or benthic insects.

millipedes, based on the antennae, mandibles, and intricate system of fine breathing tubes within the body, called tracheae. But DNA evidence reveals that these features evolved independently, and that hexapods should now be included within Crustacea (in a group called Pancrustacea). Their closest relatives appear to be several obscure marine crustacean groups: Cephalocarida and Remipedia.

Evolution

Insects are Crustaceans

Truly marine insects are lacking for the simple reason that insects evolved from marine crustaceans. Traditional evidence using morphology indicated that the closest relatives of insects and other hexapods are centipedes and

Both of these groups were discovered relatively recently, cephalocarids in 1955 and remipedes in 1981. They are minute to small (2–4 mm and ½–1½ in/1–4 cm, respectively), eyeless and blind, slender, with many segments and pairs of short, branched appendages (hexapods have unbranched appendages). The approximately one dozen species of cephalocarids live as detritivores in sediments from the intertidal to benthic zones. Remipedes are predators and scavengers, some two dozen species living in “blue holes” near the shores of islands in the Caribbean, Canary Islands, and Australia. These are underwater limestone “sinkholes” partially fed with fresh water. There must have been many transitional forms between the ancestral hexapods and their marine crustacean relatives, no doubt amphibious forms. The question now becomes: Why have insects not returned to the oceans? The many adaptations that allow them to live on land and in fresh water probably constrain against living in seawater. Also, as insects conquered land, the many marine niches were tightly filled with myriad other organisms such as mollusks, worms, and various other crustaceans. The identity of hexapods as crustaceans illustrates the importance of knowing relationships when understanding evolution.

D

A

← Scanning electron micrographs of small

B

myriapods (A: a symphylan; C: a pauropod)

C

E

and wingless hexapods (B: campodeid dipluran; D, E: Collembola). They live in soil, humus, under rocks and logs.

20

INTRODUCTION

Arthropod Phylogeny Hutchinsoniella (CEPHALOCARIDA)

PANCRUSTACEA

Copepods (COPEPODA)

Tadpole and brine shrimp, Daphnia, other BRANCHIOPODA

Barnacles (CIRRIPEDIA)

Speleonectes (REMIPEDIA)

Insects and other HEXAPODA Crabs, lobsters, shrimp, isopods, others (MALACOSTRACA)

Three pairs of legs, pair of cerci, and antennae

Spiders, scorpions, mites and other ARACHNIDA

Sea scorpions (EURYPTERIDA)

Seed shrimp (OSTRACODA)

Sea spiders (PYCNOGONIDA)

Centipedes, millipedes (MYRIAPODA)

Horseshoe crabs (XIPHOSURA) Chelicerae

Mandibles

TRILOBITES

Exoskeleton, jointed appendages: ARTHROPODA

Velvet worms (ONYCHOPHORA)

Key Antennae

Water bears (TARDIGRADA)

Marine

Brackish / Saline Bodies, some organ systems segmented

Land

↗ An evolutionary tree of the main groups of arthropods and their closest relatives, the velvet worms and tardigrades, with some defining features. Arthropods evolved in the oceans,

Legs with claws

Land / Marine

including the earliest ancestors of insects.

WHERE ARE THE MARINE INSECTS?

21

The Importance of Insect Relationships The study of evolutionary relationships, or phylogeny, is critical to know in a class of organisms as spectacularly diverse as insects. Phylogeny organizes an extraordinary amount of information on species and is needed to interpret many aspects of insect biology. Because insects are intricate and complex, anatomy and morphology are still important for understanding phylogeny (and essential for studying fossils), but most phylogenetic work on living insects now uses genetic data from DNA and RNA sequences.

Grouping Species Systematics is the field of biology that explores for species, defining and naming them as well as grouping species (genera, families, and so on). The groupings are based on phylogeny. Having the species cerana, dorsata, and mellifera, for example, in the same genus (in this case Apis, or honey bees) indicates that the three species are closely related and share various specialized features. These features can be genetic, morphological (for example, wing venation), or behavioral (for example, living in a colony that builds a vertical comb for storing honey and rearing larvae, and communicating with a waggle dance). Every group, whether two species or tens of thousands, is defined by specialized features. In practice, studying relationships among hundreds of species and using numerous morphological characters or tens of thousands of DNA base pairs requires computation, since there are always characters in conflict. Programs are used that optimize certain criteria or rules for building phylogenies, such as

→ A museum drawer of hover flies, separated into genera and species using anatomical features. These groupings can be tested using DNA from dried tissues in the specimens.

22

INTRODUCTION

parsimony (which minimizes the number of gains and losses of features) and maximum likelihood (which estimates probabilities about the structure of any one phylogenetic tree against those of other trees).

Keeping Specimens Preserving and studying insects on pins, though seemingly antiquated, remains an important practice. Protected from pests and degradation, dried specimens hundreds of years old can retain some DNA. Also, insect cuticle is very durable, and its complexity yields steady discovery of new aspects in segmentation; hairs, sensilla, and other sensory structures; exocrine glands, wing venation, soundproducing and auditory organs, and other features. Advances in light, laser, and electron microscopy, digital imaging, and micro-CT scanning are facilitating the study of cuticular morphology. For internal anatomy specimens usually must be preserved in fluids.

Insects: An Evolutionary Tree

Ants, wasps, bees (HYMENOPTERA)

True flies (DIPTERA)

Roaches, termites (BLATTODEA)

Fleas (SIPHONAPTERA)

Mantises (MANTODEA)

Webspinners (EMBIOPTERA)

Scorpionflies (MECOPTERA)

Stick and leaf insects (PHASMATODEA)

Caddisflies (TRICHOPTERA)

Moths and butterflies (LEPIDOPTERA)

Rock crawlers (MANTOPHASMATODEA)

Beetles (COLEOPTERA)

Ice crawlers (GRYLLOBLATTODEA)

Twisted wings (STREPSIPTERA)

Crickets, grasshoppers, kaytdids (ORTHOPTERA)

Lacewings, snakeflies, antlions (NEUROPTERIDA)

Stoneflies (PLECOPTERA)

HOLOMETABOLA

POLYNEOPTERA Larva, pupa

Earwigs (DERMAPTERA)

Thrips (THYSANOPTERA)

Zorapterans (ZORAPTERA)

Bugs, aphids, cicadas, others (HEMIPTERA) Sucking mouthparts

Bark lice

(PSOCODEA)

True lice

Wings folded over back Hindwings fanlike

Dragon/damselflies (ODONATA) Loss of subimaginal molt

Mayflies (EPHEMEROPTERA) Wings

Silverfish (ZYGENTOMA) Mandibles with two joints

ENTOGNATHA

Bristletails (ARCHAEOGNATHA)

Proturans (PROTURA)

Ocelli, large compound eyes

Springtails (COLLEMBOLA)

Ovipositor

Diplurans (DIPLURA) ↖ This diagram shows the evolutionary Six legs Mouthparts recessed

relationships of the orders and other major groups of Insecta and other hexapods, along with some defining features.

THE IMPORTANCE OF INSECT RELATIONSHIPS

23

Fossils Fossils are the naturally preserved remains of organisms of significant age (thousands to many millions of years). They are not necessarily mineralized, though many are. Because of the small size of insects, they have been fossilized myriad ways: in sedimentary rocks, as mineralized replicas, in amber, in tar pits, in fossilized plants, and even in coprolites, or fossil dung.

Significance of Fossils

1

2

3

4

5

6

7

Fossils provide the only direct evidence of life from the past. While they are not necessary for reconstructing relationships among living species, knowing the evolutionary history of insects (or any major group) requires fossils. For example, studying just modern birds, we would never be able to predict the existence of their extinct relatives, the theropod dinosaurs, including tyrannosaurids. Likewise, studying just modern ants, we could never predict the existence of a radiation of bizarre, predatory Cretaceous ants, the Haidomyrmecinae (“hell ants”), which existed from 105–75 mya. These ants had sickle-shaped mandibles (colored red in the diagram) that closed vertically (instead of laterally), many with bizarre head appendages (blue in the diagram). They were the tyrannosaurs among ants.

A hell ant (Haidomyrmecinae) timeline 120 Mya

110

Early Cretaceous

100

90

80

Late Cretaceous

Haidomyrmodes Haidomyrmex

1

Clypeal horn or node

Haidoterminus Protoceratomyrmex Linguamyrmex

3

Ceratomyrmex

4

Dhagnathos Chonidris

2

5

6

Clypeal furrowing + expansion Aquilomyrmex

24

INTRODUCTION

70

7

Insect Fossils Like all groups, the oldest insect fossils (in the Devonian, around 405 million years old/myo) are very few and fragmentary. Increasingly younger deposits contain more and generally better preserved fossils, since erosion, faulting, and other geophysical processes have not destroyed them. Most insect fossils are compressions or impressions in sedimentary rocks, especially the wings (which are very durable, and identifiable from the venation). Some mineralized insects are preserved in three-dimensional detail, which requires special conditions.

Amber The finest preservation is in amber, or fossilized resin, which preserves the external cuticle of encapsulated arthropods with microscopic fidelity, and sometimes even their internal tissues. Amber deposits occur around the world, varying in abundance, quality, and in age (from around 300 million to several hundred years). The largest, most famous amber deposits are in northern Myanmar (100 myo), the Baltic region (45 myo), and the Dominican Republic (17 myo). An unparalleled accuracy in measuring evolutionary change is provided by all the fine features preserved in insects in amber. They can also be preserved intact mating, as well as with prey, parasites, and in other relationships.

↑ Extinct hell ant in 100 mya

↑↑ A fossil dragonfly. Wings are very

amber from Myanmar, in the genus

durable and the intricate venation is

Linguamyrmex. Rare specimens of

commonly preserved in insect fossils.

hell ants have even been preserved with prey pinned between their sickle-like mandibles and the head.

FOSSILS

25

Orders of Extant Insects and their Relationships The 26 living orders* of insects vary in size from around 20 described, named species to more than 380,000 species. All but two orders are worldwide in distribution. With the exception of Archaeognatha, or those in which mandibles have two muscle-attachment points (condyles) at the base, all insect orders belong to a taxonomic group called the Dicondylia. Besides orders, entomologists commonly refer to families (a grouping within orders, with an -idae ending), of which there are more than 1,000.

Nonwinged Insects 01 Order Archaeognatha (500 species): Bristletails comprise the earliest branching order of true insects. Like silverfish (Zygentoma, below) they have styli and eversible vesicles (small, paired, saclike appendages on the abdomen), and are covered with scales. They have long antennae, cerci, and a median caudal filament.

02 Order Zygentoma (550 species): Silverfish superficially resemble bristletails, but they are more flattened (without the thoracic hump), they scuttle close to the ground instead of leaping, most lack ocelli, and they have smaller eyes that do not meet on top. Unlike winged insects, which mate by coupling, male bristletails and silverfish deposit a small packet of sperm for the female to pick up for fertilization.

01 Bristletail, order Archaeognatha

02 Silverfish, order Zygentoma

26

INTRODUCTION

03 Mayfly, order Ephemeroptera

* For ease of navigation, the orders are numbered 01–26 on the following pages.

Pterygota, the Winged Insects There are several subgroups within Pterygota, the winged insects (see box). The largest of these, the Neoptera, have wings that fold flat over the back of the abdomen, which allows an insect to crawl into tight spaces. All but the basal orders of winged insects (orders 03 and 04) belong to this group. In Neoptera, the folding is made possible by a small plate or sclerite at the base of the wing, which is attached to a muscle that, when contracting, folds the wing over the abdomen.

03 Order Ephemeroptera (3,000 species): The most basal group of winged insects, the mayflies retain the primitive feature of molting in an initial winged stage (the subimago), before becoming reproductively mature (the adult). The immatures (nymphs or naiads) always live in fresh water, grazing on algae and diatoms, though some are predaceous. The adults emerge in swarms, cannot feed, and have “ephemeral” lives lasting a day or only a few hours.

04 Dragonfly, order Odonata

04 Order Odonata (5,000 species): Immature dragonflies and damselflies are aquatic; all immatures and adults are predatory, however, and the adults longer lived than mayflies. Molecular evidence indicates that mayflies and odonates are closely related (in a group called the Paleoptera), yet other features indicate that odonates are more closely related to the rest of the winged insects.

Polyneoptera 05 Order Dermaptera (2,000 species): Earwigs are well known for the distinctive pair of forceps-like cerci in the adults; the early instars have typical segmented cerci. The forceps are used for defense and predation, but some earwigs are scavengers and even ectoparasites. Parents of some species tend the eggs. 05 Earwig, order Dermaptera

Neopteran Groups Polyneoptera (orders 05–12) fly principally, or

Paraneoptera (orders 15–17) include a diverse range

entirely, with their hind wings, which are folded like a

of insects in which the cerci are lost, the labial palps

pleated fan (although some orders in this group have

are reduced or lost, and that have a large clypeus to

members without wings).

which is internally attached a muscular suction pump for feeding on fluids (called the cibarium).

Dictyoptera (orders 13–14), a subgroup of Polyneoptera, the Dictyoptera comprise the mantises

Holometabola (orders 18–26) undergo complete

and roaches (including the termites); they have a highly

metamorphosis in which development includes a

reduced ovipositor and their eggs are laid in sacs

soft-bodied larva, a pupa (which is inactive in all but

called oothecae.

a few small groups), and the adult.

ORDERS OF EXTANT INSECTS AND THEIR RELATIONSHIPS

27

06 Order Zoraptera

06 Order Zoraptera (40 species): These very small insects live under bark; they have no common name. Both pairs of wings are narrow with simple venation; the cerci are small and one-segmented and the thick hind femora have spines. Winged and wingless adult forms occur in each species.

07 Stonefly, order Plecoptera

07 Order Plecoptera (3,500 species): Stoneflies have aquatic nymphs that live in fresh, cool, flowing water, commonly found clinging to stones; they are intolerant of stagnant and polluted water. Adults are flattened and occur near where they emerged. The adults of some groups do not feed.

08 Order Orthoptera (20,000 species): The “singing insects”—grasshoppers, katydids, crickets, mole crickets, and wetas—are generally defined by the large hind femora, which enable them to jump, though not all species have this feature. Songs are produced in various ways by different families: crickets trill by dragging a scraper on one wing over a file on another wing; grasshoppers rasp by rubbing small spines on a leg against the edge of the forewing. All sing to attract mates. Most are herbivorous but some species are predatory.

08 Grasshopper, order Orthoptera

09 Order Grylloblattodea (30 species; also called Notoptera): A small order of wingless insects, ice crawlers get their name for their tolerance of cold. They occur at high northern latitudes and altitudes throughout the northern hemisphere, often found on the edges of glaciers and snowfields, where they scavenge dead insects and other remains. They cannot live at warm temperatures.

09 Ice crawler, order Grylloblattodea

28

INTRODUCTION

10 Order Mantophasmatodea (20 species): Rock crawlers comprise the most recent order to be discovered (in the late 1990s) and are closely related to Grylloblattodea. Like ice crawlers, they are wingless, but rock crawlers are predatory on other insects and are restricted to southern Africa.

11 Order Embioptera (400 species): Webspinners get their name from the thick webs of silk they spin over the surfaces where they live, mostly lichen-covered rocks and tree trunks in the world tropics. Females are wingless; males have wings, which they inflate with hemolymph using special sinuses in the wing membrane. Adult males do not feed. Silk is produced by glands on an enlarged segment of the fore tarsus. 10 Rock crawler, order Mantophasmatodea

11 Webspinner, order Embioptera

12 Order Phasmatodea (3,000 species): These insects are famous for camouflaging themselves with stick- or leaflike bodies, even swaying with branches. They are all herbivorous, many are wingless or have small wings that do not allow flight; some are parthenogenetic (see Chapter 3). The longest insect is a species of Phryganistria from Southeast Asia, females of which are up to 25 in (64 cm) in length, including the legs.

12 Stick insect, order Phasmatodea

ORDERS OF EXTANT INSECTS AND THEIR RELATIONSHIPS

29

13 Praying mantis, order Mantodea

14 Termite, order Blattodea

Dictyoptera 13 Order Mantodea (2,400 species): All mantises prey on other insects using strong raptorial forelegs, and walk on the other four legs. They ambush prey by resembling the green or brownish colors of leaves or bark (while also camouflaging themselves from birds and other predators); some species resemble flowers and lie in wait for pollinating insects.

14 Order Blattodea (8,000 species): Termites (called

14 Cockroach, order Blattodea

Isoptera; 3,500 species) are now recognized as a lineage of social, cellulose-feeding roaches that have lost the typical features of solitary roaches (such as the tegmina, or leathery forewings with a very distinctive venation, and the egg case, or ootheca). Various roaches have become household pests but the great majority of species live in the tropics, as do most termites. All termites are highly social (eusocial), living in colonies having reproductives, workers, and, usually, soldiers.

Paraneoptera 15 Thrips, order Thysanoptera

15 Order Thysanoptera (6,000 species): Thrips are small, slender insects with a very distinctive fringe of long, fine hairs on the margins of the wings in adults; each foot has a small, bladder-like sac at the tip, which allows the insect to cling to surfaces. Thrips feed on plants (including flowers and pollen) or fungal mycelia, using small, piercing mouthparts tucked under the head.

16 Order Psocodea (10,000 species): This order includes the free-living bark lice and book lice (Psocoptera) and the true, parasitic lice (Phthiraptera). Parasitic lice feed by either chewing on feathers, hair, and skin, or they have mouthparts for piercing and sucking blood. Many species live on particular species or genera of mammals or birds. 30

INTRODUCTION

18 Wasp, order Hymenoptera

16 Bark louse, order Psocodea

16 Head louse, order Psocodea

Parasitic lice are most closely related to the free-living book lice, such as Liposcelis, which is commonly found amid damp paper and in carpets, as well as in nests.

17 Order Hemiptera (80,000 species): This is the largest order of non-holometabolous insects, and includes plant hoppers and cicadas (Auchenorrhyncha), aphids and scale insects (Sternorrhyncha), and the true bugs (Heteroptera). Virtually all hoppers and sternorrhynchans feed on plant fluids, as do many Heteroptera; some are very injurious to crops and forests, even transmitting pathogenic microbes of the plants. Some Heteroptera are predatory, killing their prey by injecting potent saliva into a victim and sucking up the liquid contents. In all Hemiptera, the mandibles and maxillae are modified into two pairs of thin, piercing stylets, encased in a tubelike labium.

17 Stink bug, order Hemiptera

Holometabola 18 Order Hymenoptera (150,000 species): The earliest lineage to diverge from the rest of the Holometabola, this order includes ants, bees, and other wasps. Hymenoptera have a distinctive wing venation with many cells and a well-developed ovipositor. The earliest lineages are the sawflies, which feed on foliage or mine plant stems and even wood. They then transition to a wide array of families and species that lay their eggs in other terrestrial arthropods, where their larvae develop, killing the host. The most recent lineage is the Aculeata, in which the ovipositor has developed into a sting, as in bees, paper wasps, and many ants. Sociality has evolved in more Hymenoptera than any other order: in all ants, some wasps, and certain bees, including bumble bees, stingless bees, and honey bees.

19 Lacewing, order Neuropterida

19 Order Neuropterida (7,000 species): This group is often split into three orders or suborders: lacewings (Neuroptera), snakeflies (Raphidioptera), and dobsonflies (Megaloptera). With very few exceptions the larvae and adults are predators; larvae are especially voracious, antlions being a good example.

ORDERS OF EXTANT INSECTS AND THEIR RELATIONSHIPS

31

20 Stag beetle, order Coleoptera

20 Order Coleoptera (380,000 species): Beetles comprise the largest order of insects. Adults have forewings that are hard or leathery (the elytra); they fly principally using the hind wings, which lie folded under the elytra when at rest. The group is ecologically very diverse, including many species that are predatory as larvae and/or adults (for example, ground beetles, fireflies, ladybugs), or that feed on plants (for example, weevils, leaf beetles) or dung (for example, scarabs), boring in wood (for example, bark beetles, longhorns, metallic wood borers), and even some that are parasitoids (for example, wedge-shaped beetles). Beetles are particularly well adapted to living in dry habitats, including some of the driest places on Earth.

21 Twisted-wing parasitoid, order Strepsiptera

21 Order Strepsiptera (600 species): Twisted-wing parasitoids are an enigmatic group the name of which comes from their irregular hind wings and their life history. The immatures of both sexes and the adult females of most species live inside insect hosts. Females are larviform. Males have distinctive, large eye facets, forewings reduced to small clubs, and they do not feed. Their relationships have been obscure but are now believed to be closely related to beetles.

22 Scorpionfly, order Mecoptera

22 Order Mecoptera (600 species): Scorpionflies, some species of which are relict groups, live in small regions isolated by extinction. It represents one of the oldest holometabolous orders, with fossils more than 260 myo. Larvae and adults are mostly scavengers. The common name comes from the Panorpidae, a family in which males have the abdomen curled over the back and a bulbous genitalic capsule that looks like a scorpion sting.

23 Order Siphonaptera (2,500 species): All fleas are ectoparasites of mammals and birds. Adults feed on blood with piercing mouthparts; they lack eyes and wings; they are laterally flattened, spiny, with a comb near the head, and have large hind legs for jumping. The larvae feed on detritus in nests. 23 Flea, order Siphonaptera

32

INTRODUCTION

24 Fly, order Diptera

24 Order Diptera (150,000 species): The pair of forewings in true flies powers their flight, the hind wings are reduced to small sensory clubs called halteres. All flies have the labial palps developed into a spongelike structure called the labellum, used to suck up liquid food. The group includes some notorious species that feed on blood and transmit diseases, especially certain species of mosquitoes but also certain black flies, no-see-ums, horse flies, tsetse flies, and others. The order is ecologically very diverse, with species that are ectoparasites, endoparasitoids, herbivores, detritivores, predators, and pollinators.

26 Butterfly, order Lepidoptera

25 Order Trichoptera (14,500 species): The larvae

26 Order Lepidoptera (160,000 species): Moths

of all caddisflies are aquatic; many of them create a snug case they carry around, fashioned from sand grains, tiny pebbles, or bits of vegetation. Others spin a silken underwater net. Most feed on detritus or films of algae and diatoms. The adults usually have dense, fine hairs on the wings, and are commonly found fluttering near streams and rivers.

and butterflies comprise the largest lineage of animals that feed on plants, although the caterpillars of some groups feed on detritus, fungi, animal remains, and a few are even predatory and parasitic. All except the earliestdiverging families have a straw-like haustellum (sucking proboscis) that uncurls to suck up nectar and sugary and salty fluids. The wings and body are covered with flattened, grooved hairs (scales), which easily detach. The color of the scales, whether pigmented or ones that refract certain light wavelengths, impart the color patterns on the wings and body. The distinction between moths and butterflies is an artificial one, butterflies being a lineage of about 18,000 species of day-flying moths that have a naked pupa (the chrysalis) that lacks a silken cocoon. Also, some moth groups are day-flying, a feature that is associated with the caterpillar feeding on a toxic plant and the adults are colorful in order to advertise their toxicity. Many mimicry complexes have evolved in Lepidoptera as a result.

25 Caddisfly, order Trichoptera

ORDERS OF EXTANT INSECTS AND THEIR RELATIONSHIPS

33

How Many Insect Species? Scientists have precisely measured Earth’s diameter as 7,926 miles (12,756 km) and that of the moon as 2,159 miles (3,475 km), but we still do not know how many species of insects there are within one or two million. The solution is not a straightforward physical measurement, as we are reminded by the continued discovery of even new species of mammals and birds. Several thousand new species of insects are named and described each year.

34

INTRODUCTION

Uncertainty Entomologists agree that the 1.1 million described insect species are not even close to the actual number. Estimates vary from 1.5 million species (clearly too low) to 5 million (probably too high), and a consensus is converging on approximately 3.5 million species—some three times the number of described species. The traditional method of estimating total numbers of insect species is to measure the rate of species descriptions over many decades. But this approach largely reflects the number and activity of taxonomic specialists, and for many hyper-diverse insect groups (generally ones with small, obscure species) there are too few taxonomic specialists. Another method involves intensive sampling in an area and estimating regional and even global richness. The view that millions of insect species exist became a major topic beginning in the 1980s, when entomologists fogged tropical trees with short-lived insecticide, and found striking diversity in the specimens raining from the forest canopy. Excessive estimates of 30 and even 80 million species were proposed in some cases.

↑ The familiar Japanese beetle, a member of the largest order of insects.

3.5 Million Surveying taxonomic specialists on a wide range of insect groups, who work with large collections (that house many undescribed species), has provided a more precise approach on which the current 3.5 million species estimate is based. The proportions of undescribed species vary widely depending on the insect group and regions, being very high in montane and lowland tropical forests, although new species are being uncovered even in large cities in Europe and the United States. Species discovery has been facilitated by DNA-barcoding, which uses DNA sequences from one or two gene fragments. If there are around 3.5 million species of insects, the question becomes: Why are they so extraordinarily diverse?

← Museum collections archive living

→ Spraying insecticide up into the

and extinct diversity, on which many

forest canopy. Hundreds of species

scientists depend for their research,

of insects fall into the plastic sheets

such as this entomologist with a

below, are preserved, sorted, and

selection of beetles.

studied by entomologists.

HOW MANY INSECT SPECIES?

35

Why So Many Insect Species? Species number is dictated by the rates of species formation (speciation) and extinction. This number increases when the speciation rate is higher than the extinction rate. Some insects have among the highest speciation rates known, judging from the nearly 600 species of drosophilid flies endemic to the Hawaiian Islands—an area of only 6,423 sq mi (16,636 km2; slightly smaller than Israel), where the main islands are between 300,000 to 5 million years young. This is an exceptional situation, since the Hawaiian biota has been evolving in one of the world’s most isolated archipelagos, with few predators and competitors.

Extinction rates are much more difficult to measure, particularly of steady, background extinction. Insects appear relatively resilient based on some of the major mass extinction events, such as the one at the end of the Cretaceous 66 mya. They were seriously affected, however, 251 mya, in the largest extinction event in the history of life, at the very end of the Permian. There are various features intrinsic to insects that foster high speciation and low extinction rates. High speciation is a result of generally small body sizes of insects, short generation times, and relatively high fecundity. Great reproductive capacity generates high rates of genetic recombination and mutation, the raw material for evolution.

PREVIOUS PAGES Diversity in one family alone is staggering. With roughly 7,000 species, the Pentatomidae include stink bugs, jewel bugs, shield bugs, and burrower bugs. Pictured are Eurydema ornate, a shield bug (page 36) and Chrysocoris stollii, a jewel bug (page 37). → Idiomyia picticornis, an intricately patterned species among the hundreds of endemic drosophilid fruit flies in the Hawaiian Islands. These flies are probably the largest radiation of a group of closely related organisms in such a small area.

38

INTRODUCTION

Relatively low extinction rates in insects are due to a spectrum of highly adaptive features. Their cuticular armor, for example, is effective against desiccation, radiation, predation, and injury. Flight allows highly efficient mate-finding, escape, and foraging (especially on plants). Holometaboly allows larvae to exploit diets and niches that adults cannot, and diapause allows low metabolism and quiescence during stressful periods of drought or cold. Added to the high speciation and low extinction rates in insects is their age. Being among the earliest land animals, and having evolved flight 100–200 million years before pterosaurs and birds (respectively), species of insects have accumulated for many millions of years.

Radiations and Extinctions NOW: ANTHROPOCENE EXTINCTION

0

CENOZOIC

• Global cooling

• Eocene hothouse world • Modern groups of ants, bees, higher flies, butterflies radiate • Earliest tropical rain forests

END: CRETACEOUS MASS EXTINCTIONS

66

C R ETAC E OU S

• Non-avian dinosaurs, ammonites extinct

• Earliest ant fossils • Radiation of flowering plants • Production begins of large amounts of amber • Earliest social insects (termites)

145

JURASSIC

Y E A RS

• Supercontinent Pangea separates • Earliest Lepidoptera

END: TRIASSIC EXTINCTIONS

201

• Earliest true mammal TRIASSIC

M I L L I O N S

O F

• Dry Earth • Dinosaurs radiate

• Earliest wasps and flies • Radiation of beetles • Earliest dinosaurs

END: PERMIAN MASS EXTINCTIONS PERMIAN

252

• Extinction of Paleozoic insect orders

CARBONIFEROUS

299

• Coal swamps

DEVONIAN

359

• Earliest fossil winged insects

419

• Earliest hexapod fossils

WHY SO MANY INSECT SPECIES?

39

Chapter

2

Structure and Function Insects display an unparalleled evolutionary success in terms of their diversity of species. Like all arthropods, their forms and functions are derived by a body of repetitive segments, jointed appendages, and hardened exoskeleton. This deceptively simple body plan masks a daunting complexity of tissues and organ systems that yield beautiful shapes, hallucinogenic colors, spectacular structures, and marvelous functions. While the functions performed by insects— feeding, digestion, locomotion, reproduction, respiration, and circulation—are familiar to all life, the ways these are accomplished are often curious and quite unlike vertebrates. What is perhaps more remarkable are the myriad examples of convergent evolution—unrelated lineages developing similar features. From single cells to whole organism, the wild diversity of insect form is truly unparalleled. ← Insects can have bizarre body modifications. Flies in the family Celyphidae have converged on a beetle-like appearance because of a hardened shell covering the abdomen and wings.

41

Insect Body Plan Due to the constraint of having an external skeleton, the insect body is composed of a series of repeated units or segments called metameres. Although each metamere can potentially be unique and vary from another, they have become organized into three main functional groups, or tagmata: the head, thorax, and abdomen.

Thorax Head

Abdomen

42

The Generalized Form

Hox Genes

Each tagma performs a different task and, for the most part, in each insect is formed from the same number of segments. The head is composed of six segments, the thorax of three (this becomes modified in some Hymenoptera), and the abdomen of eleven (although the eleventh is lost in most insects that have complete metamorphosis). Metameres often bear a pair of appendages and fuse within tagma for basic functions, such as the fused segments in the head bearing mouthparts for feeding, the fused segments in the thorax bearing legs and wings for walking and flying, and the apical segments in the abdomen bearing modified genital appendages for reproduction and egg laying. Great versatility is afforded because tagmata are comprised of several segments. Consequently, aside from participating in these basic functions, each segment may derive additional features, yielding astounding variation in both structure and function.

How does an organism made up of repeated units diversify to such fantastic forms? The answer lies partially in the role homeobox (Hox) genes play in the development of the segments. These genes are master regulators, specifying the identity of a segment and its appendages. Ancestrally there are 10 Hox genes organized into a single complex in which the chromosomal order of the genes reflects their spatial arrangement along the length of the body. However, the organization of Hox genes has been disrupted in several insect groups. For example, they are organized into two clusters in the fruit fly Drosophila melanogaster, namely the Antennapedia and Bithorax complexes.

STRUCTURE AND FUNCTION

Webspinner Earwig

Female Scale

(Isoptera)

(Dermaptera)

(Hemiptera)

Fly

(Diptera)

House cricket (Orthoptera)

Walkingstick

Ant

(Phasmatodea)

(Hymenoptera)

Sucking louse (Psocodea)

Jumping plant louse (Hemiptera)

Cat flea

(Siphonaptera)

Thrips

(Thysanoptera)

Stink bug

(Hemiptera)

The remainder of the answer lies in the downstream targets that Hox genes regulate, various appendage patterning genes, and other signaling proteins that interact in complex networks. These signaling pathways are genetic cascades that pattern the complex features of the insect body. Changes that occur in any step of the pathway or network lead to the staggering arrays of colors, textures, and forms that are visible in the insect radiation.

Scarab beetle (Coleoptera)

Mosquito (Diptera)

African monarch (Lepidoptera)

↑ The insect body plan is fairly conserved in having three body regions, yet a bewildering diversity of forms have evolved.

→ Although the fruit fly, Drosophila melanogaster, is itself fairly derived from the basic insect body plan, research on this species has led profound insight into the genetics of insect and animal development alike.

INSECT BODY PLAN

43

↑→ Both silverfish and cockroaches have fairly generalized and flattened bodies modified for scurrying quickly across the ground and fitting into tight spaces.

Modifications of Form While insects share a basic body plan of segmented regions with appendages, the segments and their associated appendages are modified in ways that reflect evolution as well as function. Because they descend from a common ancestor, silverfish (Zygentoma) resemble one another more than they do insects in other orders, as is the case for cockroaches (Blattodea). Despite being unrelated orders, silverfish and cockroaches actually have somewhat

similar body types (flattened). Primitive wood wasps and parasitoid wasps are more closely related (as they both belong to the Hymenoptera), yet their bodies are quite different. While segments and body features change substantially in such cases, it is not too challenging to identify features shared due to common ancestry among the groups.

←↑ Wood wasps have stout bodies for ovipositing into plants, whereas parasitoid wasps have slender, flexible bodies for ovipositing into mostly other insects.

44

STRUCTURE AND FUNCTION

Extremes In other cases, modifications can become so extreme that recognizing the shared body plan becomes difficult. Several evolutionary pressures may produce wildly derived forms, including sexual selection, mimicry, and modes of life history. Sexual selection can produce exaggerated phenotypes, such as modified appendages, horns, and color patterns. Mimicry produces surprising color forms and striking body-plan modifications in unrelated groups. Insect inquilines (species living with ants and termites) often have strangely adapted features for avoiding detection and feeding in ant and termite colonies, perhaps the best example involving host mimicry syndromes in rove beetles. Plant mimicry is also common, in which parts of the body come to resemble colors, textures, and shapes of stems, flowers, bark, and green or dead leaves. Lastly, the same life history can produce the same bizarre body forms but in unrelated taxa, such as enlarged pretarsal claws, reduced head appendages, loss of wings, and stiff bristles along the body, a phenotypic syndrome that develops repeatedly among insects that are ectoparasites of birds and mammals.

← The evolutionary modification

↑ Sexual selection in insects has led

of insect body shape and color has

to bizarre forms, such as in stalk-eyed

led to stunning forms of crypsis and

flies. While females may have short

plant mimicry, as demonstrated by

eye stalks, those of males in some

this well-camouflaged katydid.

species can reach absurd proportions, about as long as their bodies.

INSECT BODY PLAN

45

Developmental Changes in Form The development of an insect from egg to adult proceeds in several steps, but interestingly these steps are not equivalent in all insect groups.

Ametabolous Orders The most primitive orders, the bristletails (Archaeognatha) and silverfish (Zygentoma), are ametabolous—there is no metamorphosis. The immature stage (nymph) hatches from the egg as a miniature version of the adult. With the exception of possessing functional genitalia, the immature stages are all similar and experience minimal tissue change, only increasing in size through successive molts.

↑→ Ametabolous insects, including the most basal orders Archaeognatha and Zygentoma, undergo few developmental changes from immature stages to adult. As highlighted by the tracheal system in the immature (right) and adult (far right) silverfish shown here, there are essentially no changes in its architecture from immature to adult.

46

STRUCTURE AND FUNCTION

Hemimetabolous Orders Within the Pterygota (winged insects) the hemimetabolous orders undergo incomplete metamorphosis. This includes all orders between the ametabolous groups and Hymenoptera. In these groups, immature stages bear some resemblance to the adult but lack functional genitalia and wings. Adult features grow incrementally through successive molts. Developing wings occur as immobile wing pads and parts of terminalia, such as ovipositors, begin as short projections that increase in length. Aside from the slow growth of such adult features, little tissue change occurs between stages. Late-stage wing development

Early-stage wing development

↑→ Hemimetabolous insects, including many orders such as Odonata, Blattodea, and Orthoptera, undergo a range of developmental changes from immature stages to adult, although the immature (nymphal) stages typically resemble the adult. Most changes seen in these insects involve appendage growth (for example, in the wings, legs, and genitalia). As highlighted by the tracheal system in the house cricket shown here, the immature (right) is similar but shows some changes to that of the adult (far right), particularly in the thorax where the wings develop incrementally in the immature stages.

INSECT BODY PLAN

47

Holometabolous Orders Insects that undergo complete metamorphosis—the holometabolous orders—include the remaining pterygotes, from Hymenoptera to Lepidoptera. An insect hatches as a larva, an immature stage distinct from the adult and bearing no resemblance. Transformation to the adult stage proceeds after several larval instars into a unique transitional stage, the pupa. Pupae range in degrees of motility but all have in common a remarkable and often rapid progression of tissue remodeling and growth in which the larval epidermis, including many other tissues and organs, transition to, and are replaced by, adult tissues. The amount of tissue turnover depends on the insect lineage, but the precursor cells of these imaginal tissues (in small sacs called imaginal discs, for some groups) were all determined early in the embryo. The regulation and evolution of such different developmental modes is covered in more detail in Chapter 4, but the vivid contrasts in tissue remodeling that occur in these three developmental categories need to be highlighted. Through visualization of the respiratory system in different developmental stages, we can begin to appreciate the dramatic variation of changes in form that take place in the various insect lineages.

↖ Following pupation, the adult holometabolous insect that emerges is often strikingly different from its former immature stages. Immense tissue remodeling, involving both cell growth and death, is responsible for this remarkable transformation. ← In contrast to ametabolous and most groups of hemimetabolous insects, in which the immature stages resemble the adult, the immature stages of holometabolous insects, such as this butterfly larva, bear little resemblance to their adult stage. → Holometabolous insects, including Diptera, Lepidoptera, Coleoptera, and Hymenoptera, undergo drastic developmental changes during metamorphosis from immature (larval) stages to adult. For example, the tracheal system of this weevil larva (top) significantly contrasts to that of the adult (bottom), while its transitional stage, the pupa (middle), shows some changes distinct from the larva. Nearly all regions of the head, body, and abdomen, show striking changes in tracheal architecture from larva to adult.

48

STRUCTURE AND FUNCTION

Larva

Pupa

Adult INSECT BODY PLAN

49

Antennae

Wings

Mouthparts

Hindleg

Midleg

Appendages We tend to think of appendages as those that allow for common movements, such as walking, running, swimming, and, in most cases, flying. Indeed, the appendages of insects and other arthropods perform these, and many other functions, through a broad range of forms. Ancestrally in hexapods, these appendages were largely composed of repetitive segments. Across orders and

Foreleg

families, the appendages change in quality by modification of the existing segments and the number of segments. Insect appendage types include those on the head (antennae and mouthparts), the thorax (wings and legs), and abdomen (terminalia and sometimes leg remnants). In many cases, appendages exhibit specialized forms that are suggestive of function, examples of which follow.

Hindlegs

Antennae

Wings

Mouthparts

Foreleg Ovipositor Midleg

50

STRUCTURE AND FUNCTION

On the head: Antennae may be short and spindly as in Hemiptera and brachyceran flies; or long, bead- or threadlike, to sawlike and highly branched and strongly sexually dimorphic as in many Lepidoptera and some beetles and parasitoid wasps. Mouthparts can vary from the more common and robust chewing type to those of mopping, piercing, and delicate, elongated sucking forms as in Hemiptera, various dipteran groups, and Lepidoptera. Both antennae and mouthparts are discussed in greater detail on pages 54–57.

Gills While the general appendage descriptions hold true to some degree for immature and adult stages of ametabolous and hemimetabolous insects, there are exceptions in groups of the latter, such as Odonata and Ephemeroptera. In these groups immatures have developed modified appendages in the form of gills for aquatic respiration.

On the thorax: The three ventral, or lower, appendages (legs) are mainly used for walking/running, swimming, digging, or predatory/other behavioral functions and are often adapted for such purposes. The dorsal, or upper, thoracic appendages are the wings. Although their primary function is for flight, they have been reduced and modified in numerous ways, often relating to differences in life histories. See page 58 for a wider discussion of ventral and dorsal appendages.

Wings

On the abdomen: The terminal segments have appendages involved in reproduction (the terminalia), including structures used for transfer of sperm in the male and eggs in the female, termed genitalia. Forms of terminalia can vary wildly and possess an array of extensions, spines, and cuticular projections. While the majority of adult insects do not have abdominal appendages aside from the terminalia, the most basal insect orders (Archaeognatha and Zygentoma) possess ventral appendage remnants in the form of styli and eversible sacs/vesicles. Abdominal appendages are discussed in greater on page 59.

Male genitalia

Hindleg

Antennae Foreleg

↑ Appendages are wildly different among orders of insects. The antennae, mouthparts, wings, and legs, and even the genitalia exhibit bewildering differences.

INSECT BODY PLAN

51

The Convergence of Horn-Like Appendages Enlarged, horn-like mandibles are well-known features in some groups of insects, most notably in male stag beetles (Lucanidae) and the dobsonfly family (Corydalidae). These enormous mandibles are a sexually dimorphic pair of appendages that males use for combat. Interestingly a very similar pair of appendages has converged on the mandible form in a different order of insects but at the opposite end of the body, the abdominal apex. Earwigs (Dermaptera) have an enlarged pair of modified cerci, which are filamentous and multisegmented in many insects, but in earwigs they resemble various types of mandibular horns in lucanids, even bearing different-sized teeth. Extinct earwigs retained elongated, filamentous, and multisegmented cerci, but extant ones only have these stout cerci, modified into forceps and used for prey capture, defense, and mating. It would not be too surprising if similar developmental modes are responsible for the formation of these two similar yet very different forms of appendicular horns. Stag beetle

Earwig

52

STRUCTURE AND FUNCTION

Appendage-Like Extensions There are many kinds of cuticular extensions, projections, and expansions, so it can be confusing to distinguish these from true appendages. One key difference is that appendages are segmented and/or jointed, and moveable. Just the way appendages can be extremely modified, the same is true for appendage-like features. Non-appendicular horns and large outgrowths without any articulations can form on any part of the body. These outgrowths not only take the form of giant horns, but may also appear as tusks, antlers, eye stalks, expanded winglike extensions, and thoracic and abdominal paranotal lobes. The latter are striking in many recent insect forms and could be very elaborate in extinct lineages such as the beaked, Palaeodictyopterida. These so-called winglets may be serially homologous to the two pairs of functional thoracic wings but were not jointed or movable and therefore cannot be considered true appendages. Appendage-like projections can also form from true appendages themselves, such as from wings and legs. While not segmented or jointed, it is fascinating that these appendage-like extensions likely form by co-opting or integrating parts of appendage-patterning genetic pathways. It has been demonstrated that this is true in a few of the above examples and it likely is true to some degree in the others.

↑ Although not true, jointed appendages, the spines and horns present on many beetles, such as this leaf-rolling weevil, and other insects share some developmental aspects to appendages. → Spines can be present on nearly every segment and body region, such as on the thoracic segments in this ant.

INSECT BODY PLAN

53

The Insect Head

↑ The heads of flies often bear robust

↑↑ Insect heads, reminiscent of those

bristles and large compound eyes, but

of their crustacean relatives, are like

the appendages can differ drastically.

Swiss Army knife toolkits, equipped

Some flies have long, filamentous

with several sets of appendages

antennae, while others very short

modified for diverse functions. The

and knob-like. Mouthparts can be

large scissor-like mandibles of this

modified for piercing various tissues

tiger beetle head illustrate a few

and sucking, others for essentially

features typical of active predators.

mopping liquids, as in this fruit fly.

54

STRUCTURE AND FUNCTION

The head is the anterior-most tagma of all insects. It is formed of six segments, four of these bearing appendages, and includes: the ocular, antennal, intercalary, mandibular, maxillary, and labial segments. The ocular segment bears the compound eyes and up to three simple eyes (ocelli), although neither of these are appendages. The eyes and antennae serve sensory functions and may have fantastic modifications to serve such purposes. Compound eyes are present in immature and adult stages of ametabolous and hemimetabolous insects and the adult stage of holometabolous insects (larvae bear another form of simple eye, stemmata). One exception to this is in scorpionflies, in which the larvae possess compound eyes that are more representative of larval eyes of primitive holometabolans. While ocelli generally have poor visual acuity, sensing mostly differences in light intensity, larval eyes are quite different and vary in their visual acuity. Altogether, changes in individual head segments produce extraordinary variation in head shape and structure. Even among unrelated lineages that appear to have similar head shapes, the same shape or form can be produced in nearly unlimited ways because any of the six segments can change.

Insect Heads and Mouthparts Labium

Mandible

Maxilla

Labrum Clypeus

Sub-mentum

Cardo

Mentum Mandible

Maxillary palpus

Stipes

Pre-mentum

Labrum

Labial palpus

Maxillary palpus

Maxilla (galea)

Galea

Labial palpus

Retractor muscle

Dorsal elevator muscle

Glossa

Elevator muscle of proboscis

Tentorium

Hymenoptera

Galea base Compound eyes Ocelli

Primary oblique muscles

Frons Gena

Lepidoptera Clypeus Mandible Labrum Clypeus

Maxillary palpus

Labrum

Labial palpus

Orthoptera

Maxillary palpus

Antenna Compound eye

Clypeus Labium

Maxillary palpus

Maxilla

Labrum Labium

Pseudotracheae

Plane of transverse section below

Labrum

Mandible Labellum

Diptera (mosquito)

Stylets protruding from proboscis

Labellum

Hypopharynx

Diptera (fly)

Variation in insect head shape and structure is vast such that it can be quite staggering to know they are produced from a homologous set of six segments and their associated appendages (when present).

Food canal

Maxillary stylet

Salivary canal

Mandibular stylet

Hemiptera

INSECT BODY PLAN

55

Antennae This pair of appendages ranges from being barely noticeable to several times the length of the body. They may be simple and filamentous or highly branched and feathery, the differences sometimes being attributable to sexual dimorphism. Antennae also have various sorts of sensilla (sense organs) to detect things from surface and airborne pheromones and other molecules to ones in food, but also humidity and vibrational movement.

Mouthparts The mandibles, maxillae, and labium may serve multiple functions, including sensory, chewing and ingestion of food, application of silk, grooming, fighting and defense, and carrying objects to organize and build domiciles, cocoons, and pupation chambers. Related to such adaptations, the mouthparts have evolved myriad forms, but as with so many hexapod features, convergence is a major theme. For example, while several groups of insects use elongated, tubelike mouthparts to ingest fluids (such as water, nectar, and blood), many of these forms have evolved independently and their unique structural variations reveal such differences in evolutionary history.

↑↑↑ Longhorn beetles have antennae

↑↑ The antennae of ground beetles

that often surpass the length of the

in the subfamily Paussinae have

body. Such long antennae are

expanded segments packed with

common in nocturnal insects, those

glands, the secretions of which allow

that are active primarily at night.

the beetles to interact with ants.

↖ Adult antlions, within the order

↑ In Lepidoptera, although mandibles

Neuroptera, have antennae that

are still present in a few of the most

often are held erect and can have

basal families, the vast majority

a small bulb at their apices.

possess an elongated proboscis formed by just a subsection of the maxillary appendage.

56

STRUCTURE AND FUNCTION

Insect Rostra: The Convergence of Snouts Among the many fascinating anatomical features in

(Mecoptera, see page 62) and a unique genus of sweat

insects that have evolved convergently is something

bees, Chlerogella. While all of these rostrate insect

best-known in the weevils—the snout, or rostrum—an

groups look superficially similar, they independently

elongation of the head where the mouthparts, typically

evolved such structures by modifying the head

unmodified, are situated at its apex. The rostrum in

segments in different ways and for various functions,

weevils (superfamily Curculionoidea) is formed by a

most related to feeding.

number of head segments and can be several times longer than the body, with tiny chewing mouthparts at the tip, which can be nearly absent. They can project from the head at various angles and be adorned with all sorts of projections, setae, and scales. Although weevils have capitalized the most on rostrum form, they are not the only insects to do so. Other beetle lineages, both extant and extinct, have members with rostra, including the families Salpingidae, Staphylinidae, Laemophloeidae, and Lycidae. These rostra are all present in the adult stage; however, one taxon possesses a rostrum in the larva—Metaxyphloeus (Laemophloeidae). Furthermore, other insects aside from beetles have evolved rostrate taxa, including scorpionflies

Nut weevil

Brain Pharynx

← A rostrum is formed by an elongation of the head in which the mouthparts, at the tip, remain relatively unmodified. In a weevil head, for example, the internal tissues (muscles,

Mandibular muscles and tendons

tendons, nerves, pharynx, and so on) become elongated and extend the length of the rostrum.

INSECT BODY PLAN

57

↑ In flies, hindwings have become reduced to small knob-like structures called halteres. Many large bristles, all mechanoreceptors, also project from the fly thorax.

The Thorax The thorax—a powerhouse for locomotion—is a tagma composed of three segments. These subdivisions are easily discernable in primitive insects such as silverfish (Zygentoma), but become more obscure and modified in the winged insects, the Pterygota. The thorax can become so modified in form that sometimes the best way to delineate its segments is by examining the attachment of its appendages, the wings and legs. The additional modification in the thorax of winged insects seems due to how the wings evolved and the formation of the pleuron or the side wall of the thorax.

58

STRUCTURE AND FUNCTION

These two events likely occurred together, as at least the base of the wing (the joint or hinge region) and the pleuron appear to be formed from an ancestral proximal leg segment. This hardened pleuron, between the dorsal tergum and ventral sternum, paved the way for other thoracic modifications. For example, in beetles the pleuron of the first thoracic segment (prothorax) becomes largely fused with the sternum and tergum to form a fortified segment. In a subset of Hymenoptera (Apocrita), while the thorax appears relatively normal, its last segment is actually the first abdominal segment that is fused to the metathorax (the third thoracic segment). Aside from changes in shape and structure, additional thoracic modifications can take the form of thoracic hearing organs (tympana), as in some moths, or soundproducing stridulatory or amplificatory structures, as in some extinct and extant grasshoppers and locusts.

The Abdomen Despite being a rather basic tagma with repetitive segments, the abdomen bears much of the visceral organ volume and is crucial to digestion, excretion, and reproduction. Excepting the basal two insect orders Archaeognatha and Zygentoma, all other orders generally lack appendages along the adult abdominal segments preceding the genitalia. Surprisingly, novel appendagederived features occur in the adults of a few other insects, such as in Cixiidae planthoppers and Sepsidae flies. Abdominal appendages are also present in the immature stages of many orders, such as the abdominal gills of mayflies (Ephemeroptera), lacewings (Neuroptera), dobsonflies (Megaloptera), and some beetles (Coleoptera), as well as the abdominal appendages (prolegs) of Lepidoptera, scorpionflies (Mecoptera), and some Hymenoptera.

↖ Silverfish, along with jumping bristletails, are the most basal two insect orders and the only to bear remnants of abdominal appendages. Various types of abdominal appendages have subsequently evolved in many insect groups. ← Appendage-like feet have evolved on the abdomen of immature stages of various insects, such as in Lepidoptera, Mecoptera, and some Hymenoptera. Other abdominal extensions may also be present, such as the whiplike abdominal filaments in this puss moth larva, Cerura vinula.

INSECT BODY PLAN

59

Insect Genitalia In all insects, the abdominal segments bearing

for sperm transfer. Again, excepting the apterygotes,

the genitals, namely eight and nine, have ventral

copulation is largely a mechanical process that

appendages that have been modified for copulation

occurs via a lock-and-key mechanism. Accordingly,

and oviposition. These structures are fairly simple

structural changes in the copulatory organs of one

in the apterygotes and become heavily modified in

sex of a species usually reflect analogous changes in

pterygote orders to the extent they are unrecognizable

the other. That said, the extraordinary complexity of

as appendages. Generally, the female appendages are

male genitalia in most insects leads us to wonder how

present in the form of two pairs that fit together as an

closely these copulatory features fit together and in

egg-laying structure enclosing the central egg canal, or

what fashion. This complexity is routinely used by

as a single pair in which the second pair is reduced or

entomologists to separate and define closely related

lost. The male appendages generally form a clasping

species of insects.

device for maneuvering and holding onto the female

Segment 9 Cercus

Gonapophysis

Epiproct

Penis

Paraproct Gonocoxite Ovipositor

Gonostyle Gonapophyses Gonocoxite (segment 9) Gonocoxite (segment 8)

Male genitalia

→ Pterygote insect orders show an amazing diversity of genitalic forms. For example, scorpionfly males bear enlarged, bulbous genitalia (resembling a scorpion sting) that are fit with clasping devices to maneuver the female copulatory organs.

60

STRUCTURE AND FUNCTION

Female genitalia

Needles for Appendages: The Convergence of Stylets The modificatiOn of insect appendages into stylet-like

Siphonaptera (extant and extinct families),

structures has not only occurred independently

and Diptera, as well as the extinct superorder

numerous times, it has done so on both the head and

Palaeodictyopterida. They are used to draw up

the abdomen. Stylet-like mouthparts have evolved in

liquid foods by piercing a variety of plant and

the adults of Thysanoptera, Hemiptera, Psocodea,

animal tissues, including those of other insects.

Lepidoptera, Mecoptera (extinct families),

Mouthparts: Stylets can be produced from any

Ovipositors: At the posterior end of female insects

of the mouthpart appendages (labium, maxilla,

in several groups, stylet-like appendages also occur

mandible, or labrum) and collectively may be referred

in the form of an egg-laying structure, the ovipositor.

to as a proboscis or haustellum (a sucking proboscis).

These stylets, termed gonapophyses, are modifications

Some groups possess relatively rigid stylets, such as in

of ventral abdominal appendages from segments eight

many Hemiptera and the dipteran families Tabanidae

and nine. Ovipositors are present in many insect

and Culicidae, while others have more flexible stylets,

orders in some form, but stylet-like ones are less

such as Thysanoptera and most Lepidoptera. A variety

common. Ovipositors can be remarkably flexible and

of proboscides have evolved that incorporate these

extremely different in length. Stylet-like ovipositors

pairs of mouthpart appendages in different

can be found in the orders Orthoptera, Hymenoptera,

combinations and to varying degrees. Some bear long

and Rhaphidioptera. In both Orthoptera and

stylets from all sets of mouthparts as in mosquitoes

Hymenoptera, ovipositor stylets can reach lengths

(Culicidae), some with stylets from the maxillae and

surpassing that of the body. While a stylet-like

labium as in the extinct scorpionfly lineage

ovipositor typically is used to insert eggs into

Pseudopolycentropodidae, others with stylets from the

compacted soil or woody plant tissues, in the parasitic

maxillae and mandibles as in Hemiptera and thrips

Hymenoptera it functions as a hypodermic needle to

(Thysanoptera), and still others with just one pair of

deliver eggs inside a host’s body. The hosts, however,

stylets from the maxillae as in most Lepidoptera.

can sometimes be deep within a plant or substrate.

Although thrips have stylets from both the maxillae

Some insect groups have solved such oviposition

and mandibles, the mandibular stylets are not paired

dilemmas without stylets by evolving elongated,

(one side having been lost).

telescoping genital segments. Ovipositor length roughly corresponds to the distance the eggs must travel from the insect to the host, penetrating the tissue or substrate in between.

INSECT BODY PLAN

61

The Integument Considering the near endless variation of shapes, colors, and sizes of insects, it is extraordinary to realize that the source of all of this diversity emanates from a simple, single-cell-layer-thick epithelium. This layer of epithelial cells is responsible for secretion (and digestion during molting) of the cuticle and its marvelous modifications. It not only forms the entire external surface of the insect from egg to adult, including the eyes and cuticular adornments such as horns, setae, and scales, but invaginates inward to form strengthening structures, tendons for muscle attachment, the entire respiratory system, as well as to line the fore- and hindguts.

The integument, consisting of both the living epidermis and non-living cuticle, serves multiple functions. It supports the body and provides attachment sites for skeletal muscles— that is, an exoskeleton. It also protects the body from physical harm, parasites, and reduces dehydration. Although the epidermis generally proliferates through developmental stages, from egg to adult, it also undergoes much reorganization and cell death. Particularly in the holometabolous orders, much of the larval epidermis is degraded and replaced by adult-specified epidermis from imaginal tissue or imaginal discs.

↑→ Cuticular types may not change drastically between immature and adult stages of hemimetabolous insects, such as these treehoppers, but can be quite different among stages of holometabolous insects.

62

STRUCTURE AND FUNCTION

Cuticle Structure Epicuticle Cement layer Exocuticle

Wax layer Superficial layer Outer epicuticle

Mesocuticle

Inner epicuticle Exocuticule

Pore canal

Procuticle Wax filament Endocuticle

Formation zone

Epidermis

Basement membrane

The Cuticle The cuticle is secreted by the epidermis as a fluid of complex composition that differs in its structure and components both spatially and temporally. It then goes through a process of polymerization with the addition essentially of chemical accelerants to form the exoskeleton. It is generally composed of three distinct layers: the innermost procuticle, the epicuticle, and the envelope. This last is the outermost, thinnest cuticular layer, composed of cuticulin. Slightly thicker is the epicuticle, composed of polyphenols. The procuticle is the thickest layer, except in tracheoles, the smallest diameter tracheae that lead to tissues, where it is absent. The procuticle is the only layer containing the polysaccharide chitin, and being the thickest, also bears distinct layers: the exo-, meso-, and endocuticle. Above the cuticle are wax and cement layers of varying thickness. Depending on the developmental stage and body location, these cuticular layers, in particular the

procuticle, can vary tremendously in thickness and composition. Segment regions (those bearing sclerites) generally have a thick cuticle, while a thinner cuticle (arthrodial membrane) exists between segments and sclerites to allow for movement. The procuticle is largely comprised of chitin and proteins, the latter of which more than 200 have been identified that are specific to the cuticle. Chitin is present in parallel fibers that are arranged in lamina within a matrix of proteins, inorganic elements, and water molecules (similar to reinforced concrete). The orientation of the fibers within each lamina changes, producing a helical pattern from the stacked layers of chitin and protein. The exocuticle is heavily cross-linked and insoluble, while the meso- and endocuticles are reduced in such properties. Due to the degree of cross-linking, protein composition, and ratio of chitin to proteins, insect cuticle displays a fantastic range of hardness, flexibility, elasticity, and durability.

THE INTEGUMENT

63

Secretion and Molting The process by which a single cell layer produces a compositionally and topologically complex suit of armor is fascinating. It is a well-orchestrated sequence of secretion events by the epidermis that is spatially and temporally regulated by individual cells. Depending on the developmental stage and region of the body, not all of the cuticular layers may be present. For example, the larval and pupal cuticles of holometabolous insects typically are much softer and elastic compared to that of the adult stage. Different suites of proteins may be secreted to form unique cuticles in egg, larva, pupa, and adult stages. And, as if the process of forming a suit of armor was not already amazing, insects must remove it and synthesize a new one several times during their lifetime to accommodate the development of organs and tissues and a general increase in body size. The process involving digestion of the old cuticle and preparation for secretion of a new one is that of molting.

64

STRUCTURE AND FUNCTION

When the insect emerges from the old cuticle, its newly formed epi- and exocuticles are still soft and appear lighter colored. Various chemical agents (quinones) are then secreted into these newly formed layers to facilitate sclerotization. Following sclerotization, although the insect has completed the molting process, the remaining mesoand endocuticles continue to be secreted for variable periods of time depending on the type of insect and may not complete until shortly before the next molting cycle. As a reminder, in addition to the cuticle on the exterior of the body, the cuticle of the foregut, hindgut, and tracheal system must be shed as well. How is this accomplished? As the old cuticle is detached from the epidermis and the insect slowly wriggles out, it too pulls out the old internal cuticle linings. Although the smallest tracheal branches are not shed, the remaining tracheal branches are pulled out of the body, a truly amazing feat to undergo not just once but several times in the insect’s life.

Cuticle Control of Molting Epicuticle Exocuticle

Endocuticle Dermal gland

Exuvial space

Oenocyte Epidermis

2. Apolysis As the epidermal cells divide and proliferate, they change shape, causing the cuticle to detach and a space (the exuvial space) to open between it and the epidermis.

1. Mature cuticle The mature cuticle is composed of an epicuticle and a fully differentiated procuticle.

Endocuticle (partially digested) Molting fluid (active) Ecdysial membrane New procuticle Molting fluid (inactive) New epicuticle

3. New cuticle produced The epidermal cells secrete a new envelope and epicuticle, which quickly become cross-linked or sclerotized for protection from later digestion. A fluid containing enzymes (at first, inactive), such as chitinase and protease, is secreted into the exuvial space.

4. Endocuticle digested The so-called molting fluid then digests the old endocuticle (and mesocuticle if present), leaving the old exocuticle and epicuticle intact due to their sclerotization. At the same time begins secretion of the bulky procuticle, beginning with the outer exocuticle and followed by the meso- and endocuticles.

Old exocuticle

Ecdysial membrane

Undifferentiated procuticle

5. Molting fluid resorbed The procuticle continues to be secreted, thickening, and the molting fluid and digested products are resorbed. Before shedding the remnants of the old cuticle, waxes are secreted to the surface of the outer epicuticle via ducts extending from the epidermal cells, called pore canals.

6 New cuticle after ecdysis (undifferentiated) The old cuticle then breaks along weakened points on the body as the insect goes through a period of peristaltic muscular contraction and the swallowing of air to increase body volume and escape the cuticle. To reach a mature, differentiated cuticle, the procuticle later undergoes sclerotization, although the basal-most layers may continue to be secreted for variable durations in the intermolt period.

THE INTEGUMENT

65

Cuticular Modifications The diversity of insect cuticular modifications and microsculpturing has no match, not even in the plumage of birds. Not only do cuticle forms contrast widely among the adults of different groups, they often uniquely vary among developmental stages, from egg to adult.

Egg Cuticle Referred to as the chorion, the egg cuticle has loosely comparable layers to that in adults but is different in organization and composition. While generally fairly thin, the egg chorion can be thicker than the adult cuticle in some insects. It is secreted by the follicular epithelium in the ovaries, also a single cell layer, except with the apical surface directed inward to the developing egg cell, or oocyte. As such, the cuticle is secreted inward instead of outward (as it occurs in all other developmental stages), much like that of the tracheae, except the follicular epithelium degenerates following secretion. The first layer to be secreted around the oocyte is the vitelline envelope, followed sometimes by a wax layer and then the chorion layers, which are akin to a procuticle. Within the layers of the chorion, the endochorion and exochorion are typically structured with labyrinth meshworks, canals, and chambers that function with systems of aeropyles, or pores, that function as a respiratory system for the egg, allowing for more efficient ventilation. Sometimes these aeropyles extend through elaborate arrangements of respiratory

66

STRUCTURE AND FUNCTION

↑ Lacewing eggs are positioned at the tips of silk stalks and have a rough microsculpturing to their outer cuticle (scale 40 μm).

horns that function as snorkels for eggs laid in moist environments or partially immersed in wet substrates. Eggs that are laid in aquatic habitats typically have extra cuticular meshworks (termed a plastron) in the exochorion that facilitate gas diffusion between the air-liquid interface. After the chorion forms, the follicular epithelium degenerates. Once fertilization occurs and following oviposition, yet another cuticle is secreted between the developing embryo and the chorion from the extraembryonic epithelium (the serosa).

Immature and Adult Cuticles Immature cuticles can highly resemble adult cuticles in ametabolous and hemimetabolous insects. They differ much more in holometabolans and a larva often has a very different cuticle to that of the adult. In these groups, larvae typically have soft and elastic cuticles, except for perhaps the heads, legs, and sometimes body sclerites. These more pliable cuticles have thin exocuticles and are mostly composed of a less sclerotized endocuticle with larger amounts of elastic proteins such as resilin. Adult cuticles range in thickness and composition depending on the body region and insect lineage. Some insects have very thin cuticles, composed of few layers and lamellae, which afford the insect much agility. Others have thick, robust cuticles that are rigid and render the insect a small tank. Regardless of cuticle thickness, the exo- and epicuticles can be fashioned with incredible assortments of spines, processes (such as hair-like microtrichia), textures, and microsculpturing. This structural diversity applies to cuticles of any developmental stage, egg to adult. ↑ Adult cuticles display an incredible range in form and material properties, from thin to thick, elastic to rigid, and smooth to sculptured. Adult weevil cuticle often is quite rigid and although it looks smooth to the unaided eye, it can be bizarrely microsculptured as shown in this scanning electron micrograph (scale 100 μm).

↑ Immature insects often have quite

→ Similar to mosquitoes, the

different cuticles from their adult

immature stages of mayflies are

stage, particularly in holometabolous

aquatic and the adult is terrestrial

insects in which the immature stages

with wings. Immature mayflies,

drastically contrast with the adult.

however, have legs and some are

Although mosquito adults are

adept swimmers, their agility arising

terrestrial and fly, the immature

from a combination of cuticular

larvae are aquatic and legless. Their

structure and sensilla, as shown

ability to swim is facilitated by various

here (scale 40 μm).

setae and cuticular features along the body, as shown here (scale 30 μm).

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67

Bristles and Scales

↑↗→ An astounding array of cuticular macrochaetae, setae, bristles, and scales adorn the exoskeleton of insects. These structures are formed by single epithelial cells (scale from top to bottom: 5, 10, and 200 μm).

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STRUCTURE AND FUNCTION

In addition to the vast range of surface extensions and textural modifications, the cuticular landscape is complemented by another swath of exoskeletal structures, a subset of sensory structures, or sensilla, termed macrochaetae (bristles and scales) and including hair sensilla. These are long cuticularized processes formed by specialized epithelial cells. They serve a basic function of mechanoreception, but fulfill an extensive list of roles, such as insulation and temperature regulation, chemical dispersal, sound absorption (important for bat or nocturnal predator avoidance), waterproofing, and various aspects related to flight (see Chapter 4). They can be colorful and therefore function in warning coloration, camouflage/ mimicry patterns, and species recognition. Scales that detach easily from the body, as in Lepidoptera, can also help as escape mechanisms, such as from spider webs. Scales are actually modified (flattened) bristles with distinct ventral and dorsal surfaces. Macrochaetae are produced by a single epithelial cell, anchored in a socket such that the shaft can pass through the cuticle, and associated with several other cells for stabilization and sensory purposes of the structure. In development, the bristle or scale cell extends its body via the elaboration of cytoskeletal elements (actin bundles and microtubules) to produce a framework for its final shape. As in other epithelial cells, the cell then secretes layers of cuticle that sclerotize, after which the cell recedes/ degrades—a trait only shared with egg follicular epithelium—leaving the hardened external structure. Aside from scales and various types of bristles, other types of sensilla, including hair sensilla, retain living cell matter in the sclerotized shaft. If present in multiple developmental stages, the bristles and scales must reform at each molting period along with the secretion of the rest of the insect cuticle.

Scale Diversity and Convergence

Scales are modified macrosetae that are flattened

with the cell membrane before cuticular secretion.

and have distinct surface polarity. The ventral

Internal structures, such as three-dimensional

surfaces have smooth cuticle and the dorsal surfaces

photonic crystals, are formed by the convolution of

have textured cuticle. Scale morphology varies

the smooth endoplasmic reticulum, also in association

tremendously in insects. Scales may not only be

with various binding proteins, with the plasma

diverse within families or orders of insects, but a

membrane. After secretion of the cuticle into this

species may bear several distinct types of scales on

convoluted framework, the cell degrades and the

a single individual. Their shapes vary from round,

lattice structure remains.

rectangular, and shingle-like to somewhat amorphous,

Despite the already many examples of

porous, frilly, and anemone-like. Scales can be

evolutionary convergence in insects, it may still be

relatively hollow, have internal cuticular structures,

surprising that scale structures have convergently

or secrete viscous liquids such as waxes or chemical

evolved in multiple lineages. Not only do scales in

volatiles. Just as in other macrochaetae, scales form

these different groups converge on general external

a single epithelial cell via extension of the cell body.

appearance, they develop similar internal structures

After a framework of the scale shape has been formed

that disperse light in similar fashion, such as through

by cytoskeletal elements, the cuticle is secreted, it

photonic crystals. Scales have evolved independently

becomes sclerotized, and the scale cell recedes or

in at least the orders Collembola, Archaeognatha,

degrades. Formation of surface features and textures

Zygentoma, Orthoptera, Hemiptera, Psocoptera,

(such as longitudinal ribs/ridges, windows, and

Hymenoptera, Coleoptera, and Diptera, but are

projections) are a result of the interaction of these

best known in the Lepidoptera. They also appear

cytoskeletal elements and various binding proteins

independently within most orders.

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69

↑ Insect coloration can be formed by various chemical compounds/ pigments or cuticular structural features; sometimes both are involved. Many fulgorids have vivid color patterns attributed to different pigments deposited in the cuticle.

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STRUCTURE AND FUNCTION

Coloration Beyond Belief Insect coloration is generally produced in two ways— through pigmentation or physical interference (structural coloration). Either way, perceived colors are a result of the absorption, reflection, and transmission of the wavelengths in white light. If all wavelengths are reflected equally, the appearance is white, and if all are absorbed, the appearance is black. While very white and black coloration occurs in some insect groups, most appear with various amounts of other colors. Coloration can be relatively uniform along the body or appear in a wild assortment of patterns. In some cases, the structure of cuticular layers is organized is such a way as to reflect certain wavelengths while others are transmitted. In other cases, pigments are present, which may also reflect certain layers and absorb others. In other cases still, both structural and pigmentary colors may be present.

Pigmentary Colors Most pigments are compounds synthesized by the insect (melanin, ommochromes, pterins, quinones, papiliochromes, tetrapyrroles) and some can only be acquired through their diet (carotenoids, flavonoids). These pigments, due to their molecular structures, reflect certain wavelengths of light while absorbing the remainder. For example, dark brown or black is often produced by melanin deposited as granules in the cuticle. Pterins may be found as granules in the cuticle or epithelial cells and produce various white, yellow, and red colors. Some types of tetrapyrroles (bilins, usually blue) may be associated with other pigments (such as carotenoids, some yellow) to produce a green coloration or it may only be produced by bilin. While carotenoids are mostly yellow, orange, and red colors, they may be blue when bound to proteins.

Although not a pigment, the waste product uric acid is often deposited in various tissues instead of being entirely excreted, and is responsible for the white coloration in many insects. In general, pigments are a result of necessary metabolic processes and the removal of waste products from those processes. Due to their broad range of derivation, they are found in multiple tissues, such as hemolymph, fat body, cuticle, epithelium, muscle, and gut. It is intriguing to consider how the type and distribution of pigments in a given insect are related to its diet, life history, and lineage evolution.

↓ Insects with scales covering their bodies, such as this butterfly, have pigments in the cuticle of their scales.

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71

↗ Iridescence and metallic

→ Pigmented coloration, such as in

coloration in insect cuticles are

this female Cairns birdwing butterfly,

widespread, from jewel beetles and

can form warning patterns, typically

jewel wasps, to metallic-colored flies,

incorporating red, white, yellow, and

mantises, bees, and this clown weevil.

black, advertising unpalatability.

Structural Colors Colors such as some blues, whites, and all metallic, iridescent, and opalescent colors are produced by the scattering, interference, and/or diffraction of light by surface structure. For example, granules in the cuticle or epidermis can scatter light, reflecting single colors if the distance between granules is similar to the wavelength of the particular color. Stacked surfaces may create interference as light is reflected from the different layers. Depending on the layer spacing, if they are regularly spaced, certain wavelengths will be reflected in phase, and are reinforced, while the others are reflected out of

Color Changes While most insect coloration is constant between developmental stages, some can change color and do so fairly quickly. Some tortoise beetles, for instance, are able to alternate colors within minutes by changing the amount of blood (hemolymph) in cuticular layers. This modification of layer hydration alters the optical properties of the layers and causes light of different wavelengths to be reflected.

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STRUCTURE AND FUNCTION

phase, and cancel out, resulting in a uniform visible color. Colors change between different insects or even among different regions on the same insect when the layer spacing and/or refractive indices of the layers (due to differences in cuticle composition) changes. If layers in a stack are not spaced uniformly, metallic gold and silver are the result. Photonic crystals are structures that can produce colors through light interference or scattering. They range from one-dimensional photonic crystals, such as the vanes on some butterfly scales, to three-dimensional, such as the internal lattice network found within some insect scales and cuticles.

Muscles There are two general types of muscles in insects, skeletal and visceral. The main differences between the two involve structural features and how they function. Skeletal muscles attach to the inside of the exoskeleton at each end of the muscle (their points of origin and insertion) with one end sometimes connected to the cuticle via a tendon, or apodeme. The typical insect has more than 100 skeletal muscles, which move appendages and the abdomen, among other functions. Visceral muscles do not attach to the cuticle, or may occasionally attach at one end, and are involved in the functioning of visceral tissues.

Skeletal Muscles There are several forms of skeletal muscle that differ in structure and function. Depending on the location in the body and the forces experienced in those locations, skeletal muscle bundles may contain one or more of these types. Slow-contracting fibers are comprised of longer sarcomeres (the units bearing the contractile proteins), fast-contracting fibers have shorter sarcomeres, and there are also intermediate types. Among these various muscle forms, they are further divided into synchronous and asynchronous (or fibrillar) muscles. These types are separated based on their modes of nervous system activation. Lateral

Synchronous Muscles These muscles contract synchronously with motor neuron excitement. Because a single nerve impulse elicits a single muscle contraction, there is an upper limit to synchronous muscle contraction rates. Between impulses, the neuron membrane potential must be reset, and this can only occur so quickly. In insect orders with synchronous flight muscles, this results in an upper contraction frequency limit of ca. 500 Hz.

Dorsal

PREVIOUS PAGES Iridescence is

→ Skeletal muscles attach to the

common in beetles and is thought

inside of the exoskeleton, sometimes

to help camouflage the insects

connecting via cuticular tendons

in sun-dappled vegetation. The

(such as the elongated mouthpart

ever-changing colors may also act

tendons in the weevil rostrum). The

as a deterrent to predatory birds.

flight muscles are the large bundles in the thorax, extending anterior to posterior and dorsal to ventral.

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STRUCTURE AND FUNCTION

Ventral

↓ Arrays of skeletal muscles (yellow) can be seen connecting

Asynchronous Muscles In asynchronous muscles, a single nerve impulse elicits many contractions, the result being much higher wing beat frequencies. This is accomplished because such muscles are stretch activated, in which the nerve impulse makes the muscle sensitive to mechanical stretching. Successive impulses then sustain this contractile sensitivity. Asynchronous skeletal muscles have larger fibers and have a few additional proteins and protein forms that are not found in synchronous muscles. It is interesting to note that since the muscles are activated by tension, it is not the nerve impulse rate that affects the contractile frequency but the weight or load of

← Lacewing larvae are active

the abdominal segments in this

predators and can operate their large

termite. The abdomen has been

sickle-like mandibles (which actually

opened to better visualize these

siphon the liquified tissues from their

muscles. Also visible (and in yellow)

prey) via large skeletal muscles in the

are the abdominal ganglia running

head (shown in blue).

along the middle of the image.

the attached structures. Therefore, lighter wings result in faster contractions, with higher contraction frequencies in insects reaching 1,000 Hz. These different contractile properties are afforded by slight alterations in composition and structure. Most insect orders have synchronous flight muscles, while several orders appear to have independently acquired asynchronous muscles. In addition to the skeletal muscles involved in flight, there are others in the appendages used for a variety of activities such as feeding, walking, running, digging, swimming, mating, and a sort of body undulation in reference to the soft-bodied larvae of groups such as Lepidoptera, Hymenoptera, and many Coleoptera.

MUSCLES

77

Visceral Muscles

Aiding digestion: The alimentary canal is lined with sets

Visceral muscles do not connect to the exoskeleton (intrinsic) but may connect at one end to the body wall cuticle (extrinsic). Different from the smooth visceral muscles of vertebrates, insect visceral muscle is striated due to the regular filament alignment in the sarcomeres. It is similar to the skeletal muscle in this respect, but differs in other aspects such as filament arrangement, innervation, and number of nuclei per fiber.

of visceral muscles to help macerate and move food and excretory products through the system. Beginning in the head and the foregut, the pharynx is lined with extrinsic dilator muscles attached to the inner walls of the head and intrinsic circular muscles that continue around the esophagus. Fluid-feeding insects have enlarged pharyngeal dilator muscles that form a sucking pump-like organ, or cibarium in the head. Thin circular and longitudinal muscles may be present along the crop that then become more robust on the proventriculus (a gizzard-like structure) depending on the development of this structure. The intrinsic muscles along the midgut are usually thin and consist of circular and longitudinal sets, similar to the crop. These muscle sets usually become more robust again in the hindgut, particularly near the pyloric valve that separates the midgut from the hindgut, and the rectum. These areas also may have various extrinsic muscles.

Aiding circulation: The dorsal aorta and heart possess sets of intrinsic longitudinal and circular muscles surrounding the dorsal vessel and extrinsic alary muscles. Although the alary muscles and various areas along the dorsal vessel have some nerves to assist in regulating heart contractions, cardiomyocytes also are present that initiate contractions without nerve impulses. In some insect groups, the dorsal vessel lacks innervation and the heartbeat, therefore, is regulated entirely by the cardiomyocytes.

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STRUCTURE AND FUNCTION

← Visceral muscles expand across

↑ Visceral muscles are present

glandular tissues to expel various

in the circulatory, digestive, and

secretions. For example, a dense

reproductive systems. Shown here

wrapping of muscles covers the

in green, crossing strands of visceral

surface of the accessory glands

muscles line the male accessory

in a male firefly, the contents of

glands (the thicker muscles) and

which are delivered with sperm

Malpighian tubules (the more

during copulation.

slender muscles) in a firefly.

Aiding reproduction: The genitalia typically possess many sets of skeletal muscles for a wide range of copulatory motions and behaviors (as they are modified segmental appendages). Associated with these typically hard copulatory structures are the soft reproductive organs of the male and female. In both sexes, several structures possess intrinsic visceral muscles to help move sperm, eggs, chemical compounds and adhesives, and even venoms through various ducts.

↑ The alimentary canal contains several distinct regions, often with various sets of circular and longitudinal visceral muscles. In cross-section, the rectum sometimes appears hexagonal in shape and usually contains robust sets of muscles, as seen here in this immature dragonfly rectum in red. The rectal gills project in the space within the muscle ring and some tracheae appear outside of the muscles (all in green/blue). ← Visceral muscles (in yellow) cover the surface of helical accessory glands (in blue) in a male firefly.

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79

Nervous and Sensory Systems The sensory systems of insects are vast. The central nervous system controls and integrates a wealth of sensory information from an assortment of visual, olfactory (smell), gustatory (taste), mechanosensory (both touch and hearing), and other tissues, glands, sensilla, and receptors from the peripheral and visceral nervous systems.

The Central Nervous System

Protocerebrum: The protocerebrum is composed of

The central nervous system (CNS) consists of the brain/ cerebrum and the ventral nerve cord. Collectively this system is responsible for the integration of visual and sensory organs of the head, as well as sensory information relayed via the ganglia from the nerves of the peripheral and visceral nervous systems. Interneurons and motor neurons are aggregated to form ganglia, with each body segment primitively containing one ganglion. Within the ganglia, the neuron cell bodies, or somata, are generally grouped peripherally, while the central region contains the mass of fibers and branchings of these various neurons, termed the neuropil.

several regions and neuropils, including the optic lobes (connecting the compound eyes), mushroom bodies (corpora pedunculata), and central complex. Collectively these areas are involved in visual and olfactory sensing, flight control, and pattern recognition through the processing of these associated inputs including those from the ocelli (if present). Neuropils of the central complex and the relative size of the mushroom bodies are shown to be associated with complexity of behavior and memory, being particularly large in social insects.

Deutocerebrum: The deutocerebrum contains the antennal lobes and is mainly responsible for olfaction.

The Insect Brain Sometimes aggregately termed the supraesophageal ganglion, the insect brain is divided into three main regions/ganglia: the protocerebrum, the deutocerebrum, and the tritocerebrum. The brains of large-sized insects have, on average, around one million neurons, while some of the smallest insects (less than 0.5mm in length) may only have a few hundred. The laboratory fruit fly has about 100,000 neurons in its brain, which have been mapped. In contrast to the ganglia of the ventral nerve cord, where each have a single paired neuropil, the brain contains many pairs of neuropils. Functions can be associated with certain brain regions, although the processing that occurs in one neuropil may result from input and information that has been processed in another.

Tritocerebrum: The tritocerebrum connects to the ventral nerve cord and connects the CNS to the visceral nervous system. It also receives neurons from the labrum.

The ganglia: The first ganglion in the ventral nerve cord, and the only one in the head, is the subesophageal ganglion, a compound fusion of three ganglia that serve the mandibular, maxillary, and labial segments on the head. There are three thoracic ganglia, which may be fused in various ways, and eight abdominal ganglia that may also be fused. It is thought that, primitively, there were eleven abdominal ganglia, in which the last visible one (the eighth) is actually a fusion of eight to eleven. These ganglia of the ventral nerve cord are joined by lateral connectives and possess the somata of motor neurons and interneurons responsible for the activities of their respective segment.

← Interneurons and motor neurons (innervating muscles) are aggregated into ganglia that run along the ventral region of the insect. In many weevils, as shown here, there are three thoracic ganglia, whereas most of the abdominal ganglia are fused into one or two ganglion bodies.

80

Central Nervous System

Ocellar nerves Antennal nerve

Brain

Optic lobe

Endocrine

Stomatogastric nervous system

Corpora cardiaca

Frontal ganglion

Corpora allata

Hypocerebral ganglion Subesophageal ganglion

Mushroom bodies

Lobula Medulla

Antennal lobe Subesophageal ganglion

Honey bee brain: surface reconstruction The insect brain is quite complex. So far, studies have identified 47 distinct neuropils and many smaller subregions. Here, some of the major neuropils can be seen as well as the subesophageal ganglion. This complexity is also appreciable in viewing the neurons that comprise and traverse these neuropils such as some of those associated with the antennal lobes (highlighted here).

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81

Insect Eyes The organs for photoreception in insects are various but consist mainly of simple and compound eyes. Ocelli, found in adult insects, and stemmata, found in larval holometabolous insects, are classified as simple eyes.

Compound Eye Structure

Compound Eyes: Composed of facets called ommatidia, compound eyes are present in all but a few adult insects as well as in immature hemimetabolous insects. Although there are several differences between these eye types, common components include a transparent corneal lens, pigment cells—which prevent the focused light from traveling outside of the eye—and retinula, or photoreceptor, cells that contain the visual pigment rhodopsin. Retinula cells possess areas of dense microvilli called rhabdomeres. When the rhabdomeres of separate retinula cells approximate one another, they form a functional unit referred to as a rhabdom. Compound eyes may contain two to five visual pigments, but these pigments may be distributed differently in the ommatidia across the compound eye, depending on the function of the eye in relation to the insect’s biology. Visual acuity and resolution can also vary across the compound eye through changes in interommatidial angle, lens thickness, and rhabdom diameter.

Corneal lens

Crystalline cone

Primary pigment cell Secondary pigment cell

Rhabdom Retinula cell

Secondary pigment cell

Basement membrane Axon

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STRUCTURE AND FUNCTION

Ocelli Structure Pigment in pigment cells

Corneal lens

Ocelli: Ocelli may be found on the dorsal surface of the Vitreous layer

Retinula cells Retinula axons Interneurons

Ocellar ganglion

Ocellar nerve

Simple eyes of nymphs and adults

head in a set of three and arranged at the corners of an inverse triangle, although there can be two, one, or none. They represent the least complex of visual structures in insects. A typical ocellus consists of a corneal lens, lateral pigment cells, and many closely packed retinula cells— usually several hundred. Although each retinula cell gives rise to an axon, these synapse repeatedly with many other retinula cells. Due to this extensive convergence of retinula cells, and the apparent lack of image focusing on the retina—the lens focuses light to an area behind the ocellus—image perception by the ocelli is likely crude. Instead, the structure of ocelli suggests they are sensitive to light intensity and function in concert with compound eyes to detect movement during locomotion.

→ Ocelli are smaller, simple eyes that can be present on the dorsal side of the head between the compound eyes. They vary in size and may be present as one, two, three, or absent.

←← Various insect groups display fascinating colors and patterns even on their compound eyes, such as the fly family Tabanidae. ← Just like tabanid flies, damselflies have large compound eyes. Also similar to tabanids, dragonfly and damselfly eyes can display an assortment of colors and patterns.

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83

Stemmata: Simple lateral eyes, similar to ocelli in function but more like to ommatidia in structure, stemmata may be present as a single pair or a small cluster of around six in holometabolous larvae. They likely provide coarse images compared to those of compound eyes. The stemmata are comprised of a corneal lens, crystalline cone, which helps focus light, lateral pigment cells, and just a few to several thousand retinula cells. One to three different rhodopsins may be present that are sensitive to different light wavelengths, including UV light.

Stemmata Structure Corneagenous cell

Corneal lens

Crystalline lens

Rhabdom

Retinula cell

Pigment cell

↑ In this moth larva the stemmata are near the lower region of the head and arranged in a line.

Exception to the Rule Larvae of the holometabolous order Mecoptera have large, faceted eyes that are in fact compound eyes and not stemmata. They essentially represent an evolutionary intermediate from true compound eyes in hemimetabolous immature nymphs to stemmata in holometabolous larvae. The difference in Mecoptera is that the larval eyes are replaced during metamorphosis just as in the remainder of Holometabola. Furthermore, larval stemmata may not be completely eliminated during metamorphosis. There is evidence that elements of these stemmata remain associated with the optic lobe of adults and play a role in circadian rhythm maintenance.

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STRUCTURE AND FUNCTION

Simple eyes of larva

Compound Eye Types

A

Insect compound eyes are of two general forms, although much variation exists. These forms include apposition and superposition eyes, differing mainly in how (and how much) light is focused on the cell rhabdoms.

A

c

c

cc

cc p p

f

cz rh

rh

l

l

Apposition

Ommatidia The hexagonal facets seen on the outside of a compound eye mark each ommatidia, which are deep, narrow structures reaching well below the

f

A c cc cz f

diameter of the aperture corneal facet lens crystalline cone clear zone focal length (in superposition eyes, this is measured from the eye’s center of curvature, which is not shown here) l rhabdom length p screening pigment rh rhabdom.

Superposition

Apposition eyes: In apposition eyes, often seen in daytime insects such as bees, the retinula cell rhabdoms are long and abut the lenses, in which the image is inverted in the lens and light intensity may vary among the ommatidia. As each ommatidium generally receives a small amount of light, this structure causes the ommatidia to collectively receive a mosaic of spots that form an image.

surface. A compound eye may contain several hundred to several thousand ommatidia, and in

Superposition eyes: In superposition eyes, more likely

some cases where eyes become reduced, just a

associated with nighttime insects such as moths and beetles, the rhabdoms are distant from the lenses, in which the light is focused within a clear zone and forms a single upright image on the retinula cell rhabdoms. Adaptations for seeing in various amount of light, including in the dark, involve variations of pigment cell position, either reducing light collection through obstruction above the rhabdom or increasing it by allowing penetration between ommatidia. Changes in retinula cell structure and the presence of tracheal tubes can also increase light reflectance within the rhabdom in nocturnal insects.

single ommatidium. In the cases of a higher number of ommatidia, the eyes may be contiguous medially and encompass nearly the entire head. Incredibly, many insects possess areas of different ommatidial size and structure on each compound eye, affording variation in sensitivity and function across a single surface. They may also occur in an astounding assortment of colors—such as in the eyes of horse flies—due to various setae between the ommatidia and the cuticular structure of the corneal lens. While there is substantial diversity among the types, compound eyes occur in two general forms, apposition and superposition eyes.

NERVOUS AND SENSORY SYSTEMS

85

Peripheral Nervous System

Mechanoreceptors

The peripheral nervous system, also called the sensory nervous system, represents the vast network of nerves that radiate from the central nervous system. These include the motor neurons that innervate muscles and the nerves that subtend the numerous types of sensilla. These sensilla bestow incredibly sensitive perception from an unassuming body of rigid sclerotized plates. The sensilla span a diverse array that allow for mechanoreception and proprioception (involved in touch, sound, and vibrational sensing), stretch reception, visible and infrared light reception, chemoreception, thermoreception, and hygroreception (moisture, humidity). They also originate from the same ectodermal tissues (proneural cells) that give rise to the remaining nervous system. The basic structure of a cuticular sensillum includes a sensory neuron such as a chemoreceptor or mechanoreceptor cell; a trichogen cell, which creates the body or shaft of the seta; a tormogen cell, which creates a socket for the shaft; and a thecogen cell, which protects the axons and nourishes the neurons. Sensilla are commonly categorized based on their structure, but may also be separated based on their function. The former categorization defines such sensillar groups as trichoid, placoid, coeloconic, campaniform, and basiconic, although form does not always reflect function.

The most common and visible on an insect’s body, most mechanoreceptors are trichoid, or hair, sensilla and form setae or robust bristles. Specialized elongate trichoid sensilla that can detect airborne vibrations are called trichobothria. Less noticeable, but not necessarily less common, mechanoreceptors are campaniform sensilla. Instead of a shaft to which the neuron connects, this sensillum has a cuticular dome with an area of softer cuticle surrounding it. Therefore, campaniform sensilla are sensitive to changes in cuticular stress. They are commonly present in areas that may receive different amounts of bending and twisting, for example, near joints and along extended cuticular regions susceptible to warping, such as along the appendages (legs, wings, antennae) and body. If found near joints, these mechanoreceptors function as proprioceptors, sensing body movement. They can determine relative body/ appendage position in response to adjacent cuticular deformation caused by gravity, flexion, air or water movement, or vibrations, and can even determine directionality depending on orientation of the sensilla. Stretch receptors also contribute to proprioception. These represent neurons found throughout visceral organs and muscles, which sense distention or movement.

Peripheral Nervous System Sensilla Machrochaetes (mechanoreceptors) Microchaetes (mechanoreceptors) Campaniform sensilla (mechanoreceptors) Olfactory chemoreceptors Contact chemoreceptors Central nervous system

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STRUCTURE AND FUNCTION

Mechanoreceptors Cuticle Epidermis

Common types of mechanoreceptors include trichoid sensilla, campaniform sensilla, and scolopidia. These sensilla are involved in various aspects of touch (trichoid), vibration (scolopidia), and cuticular stress (campaniform) sensing.

Cap cell Hair (or seta) Scolopale cap

Cilium

Scolopale Scolopale cell

Cuticle

Root apparatus

Dendrite of sensory neuron

Root of cilium

Receptor lymph cavity Epidermal cell Trichogen cell

Trichoid sensillum

Dendrite

Sensory neuron (nerve cell)

Sheath cell

Dome

Cuticle Receptor lymph cavity

Nucleus Sensory neuron (nerve cell)

Epidermal cell

Schwann cells

Sensory neuron

Axon

Tormogen cell Axon

Trichogen cell

Campaniform sensillum

Scolopidium

← In addition to the motor neurons that innervate muscles, the peripheral nervous system includes a vast network of nerves that extend their dendrites across the surface of the insect body to subtend each and every sensilla. A couple of different types of these nerves, classified based on their branching patterns, are visible here, tiling the surface of a Drosophila larva.

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87

Chemoreceptors

Olfactory sensillum

Pore

Cuticle

Dendrites

Other sensilla include carbon dioxide receptors, such as those found in mosquitoes and other blood-feeding flies. Various chemoreceptors, such as olfactory (smell) and gustatory (taste) sensilla, are found on the antennae, the mouthparts, and even other appendages like the legs. Olfactory sensilla contain fluid-filled (sensillum lymph) spaces that bathe the dendritic branches of the neurons. When stimulant molecules land on surface pores located on the sensillum, they enter into the lymph fluid, bind to odorant-binding proteins and then to the odorant-binding receptors.

Thermo- and hygroreceptors Sensillum lymph

Sensory neuron

Axon Thecogen cell

Tormogen cell Trichogen cell

→ The hindwings of flies are reduced to small rods that extend just behind the forewings. Termed halteres, these structures bear several dense patches of specialized campaniform sensilla and gyrate during flight. The sensilla sense bending motions of the halteres and contribute to the amazing versatility and maneuverability of dipteran flight.

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STRUCTURE AND FUNCTION

These sense temperature and humidity, typically through basiconic sensilla. These sensilla comprise a small cuticular peg within a pit, in which deformations of the peg due to changes in temperature and humidity excite the neurons. Somewhat similar to thermo- and hygroreceptors, infrared receptors comprise a cuticular bulb instead of a peg. Deformation of the cuticular bulb due to infrared radiation emitted from forest fires excites the underlying neuronal dendrites. Amazingly, light reception also occurs outside of the primary photoreceptive organs, the simple and compound eyes. The pervasiveness of this phenomenon in insects in unclear, but at least in Drosophila melanogaster larvae there is a specific class of highly branched, lightsensitive neurons over the body wall. Excitation of these neurons by light enables the larva to move away from potentially dangerous situations, such as excessive heat or dryness.

↑ The tarsi of insects have many types of setae and sensilla, which facilitate walking and attaching to various substrates and surfaces. Some aquatic insects, such as adult male predaceous diving beetles (family Dytiscidae), have suction cups (formed from modified patches of setae) for adhering to the female while mating. ← Chemoreceptors, such as the cactus-like gustatory sensilla near the tip of this moth proboscis, are diverse in form and are commonly found on the mouthparts. They also can be found on other areas of the body, such as the antennae and legs, and may include other types of sensilla such as olfactory and carbon dioxide receptors.

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Hearing and Sound Production Whether it is the deafening shrill of cicadas, the evening choir of orthopteran katydids and crickets, or the unheard stridulatory squeaks of countless smaller insects, insect melodies are well known across various landscapes. How and why are these sounds produced? Can insects actually hear all of these sounds? And if so, how?

sound production and hearing. Their

Hearing

hearing mechanism is astonishingly

The basis of insect hearing resides in a specialized type of mechanoreceptor, the scolopidium. These sensilla are comprised of four cell types: one or more neurons, a glial cell that protects the sensory cells, an attachment cell, and a scolopale. They are subcuticular and may attach at one or both ends, sensing subtle cuticular distortions or vibrations.

Seta Johnston’s organ

Base of flagellum

Scolopidia of outer ring

Pedicel

Scolopidia of inner ring

Basal plate

Single scolopidia

Scape

Nerve

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STRUCTURE AND FUNCTION

↘ Katydids are well-equipped for

↓ Chordotonal organs are specialized

similar to that of vertebrates yet

mechanoreceptors that form the

distinct, utilizing features such as

basis of an insect’s hearing. They can

tympana and fluid-filled chambers in

be present in many different body

addition to tracheae to communicate

regions depending on the insect

with a broad range of frequencies,

group and do not always accompany

varying from around 2 to 150 kHz.

a tympanum. A common chordotonal

In this image, the oval structures

organ, Johnston’s organ, is found at

on the swollen areas beneath the

the base of an insect’s antenna.

“knees” are the hearing tympana.

Chordotonal organs: Found along various regions

Subgenual organs: Another type of chordotonal

of the body in larvae and adults, often serving as proprioceptors in addition to the other mechanoreceptor types, chordotonal organs consist of groupings of scolopidia. The other major function of chordotonal organs is hearing. Hearing organs have evolved more than 20 times throughout insects. They can be found on the wings, legs, antennae, thorax, and abdomen, and individuals can have multiple areas/sets of chordotonal organs for hearing not just airborne sounds, but also substrate-borne vibrations. A specific chordotonal organ, called Johnston’s organ, is present in the pedicel (second antennal segment) of many, perhaps all, insect orders. It functions to sense antennal movement and has become quite developed in groups such as mosquitoes and midges, where it can sense distinct sound frequencies caused by the wing vibration of mates.

organ, subgenual organs are found in the tibiae of many, perhaps all, insect orders. In these organs, the scolopidia are attached to the leg cuticle at one end and to tracheae at the other. Subgenual organs are most sensitive to substrate-borne vibrations but may also have some sensitivity to airborne sounds.

Tympanal organs: The main chordotonal specialization for airborne sound detection, tympanal organs consist of a chordotonal organ attached to a thin cuticular membrane, the tympanum, which is also often, perhaps always, backed by an enlarged trachea or air sac. Many species and families of moths have tympana that are sensitive to the high-pitched sonar of bats, using evasive dives and maneuvers when they hear the bat.

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91

Plectra

Sound Production In insects, sounds are generally produced in one of three ways: percussion (vibrations produced by the impact of the body against another surface), stridulation, or tymbals.

Percussion: Percussion sounds are often produced

File

by drumming or striking a part of the body/appendage against a substrate. Some moths produce percussion sounds by striking parts of their wings together during flight; stoneflies and green lacewings use their abdomens to drum on a branch or leaves.

Stridulation: Stridulation is achieved using many body

↑ The most common method of sound production in insects is stridulation. It is accomplished by scraping cuticular plectra (ridges or knobs) against a file (toothed ridge), resulting in a wide assortment of songs, chirps, and squeaks. In this weevil, the plectra are located at the tip of the abdomen on the dorsal surface and the files are located at the tips of the elytra (the hardened forewings) on the ventral surfaces. To produce sound by stridulation, the weevil rapidly moves the tip of the abdomen against the elytra.

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STRUCTURE AND FUNCTION

regions. It is done by scraping cuticular plectra (ridges/ knobs) across a file (toothed ridge). Depending on the structure of the plectra and file, the sounds produced can vary in frequency and pattern. Grasshoppers tend to use their hind leg against the forewing, crickets and katydids stridulate by scissoring their forewings, Hemiptera may have stridulatory structures on their legs, wings, or dorsal body surface, assassin bugs rasp the tip of their proboscis against a file between the forelegs, beetles may use parts of their thorax, legs, abdomen, and elytra, and the list continues.

Tymbals: Sound production via tymbals occurs in cicadas and some moths. It involves a middle area of thin cuticle surrounded by a rigid frame in which the middle area buckles rapidly due to muscle activity. Methods for sound amplification have also evolved and involve structures such as the mirrors on the forewings of some Orthoptera as well as air sacs of some arctiine moths and cicadas.

← The delicate, translucent membrane of the cicada tympana becomes visible when viewed from the front of the abdomen. ↓ The loudest insects are cicadas, in which their sound is produced by a dorsal pair of large tymbal organs at the base of the abdomen (shown here as viewed from the side). Curiously, their hearing organs (tympana) are just adjacent to the tymbals, located ventrally at the base of the abdomen. Although somewhat

Tympana

similar in appearance, the tymbals are composed of a more rigid, rubbery cuticle, while the tympana are an extremely delicate, elastic, and membranous cuticle.

Visceral Nervous System Similar to the autonomic nervous system of vertebrates, the visceral nervous system of insects senses visceral organs and maintains internal functions. It forms a network of small peripheral ganglia and innervates the digestive system, endocrine glands, and dorsal heart. In regulating muscles of the digestive system, nerves of the visceral nervous system assist in food ingestion and maceration, digestion, and excretion of waste (see pages 104–110). A subset of the visceral nervous system, the stomatogastric nervous system innervates the foreand midgut and contains the frontal, hypocerebral, and ventricular ganglia. It connects to the tritocerebrum and also innervates important endocrine glands, the corpora allata and corpora cardiaca, along with the esophageal nerves. These endocrine glands are responsible for regulating metamorphosis and releasing various hormones into the hemolymph, respectively (see Chapter 4). Along with the stomatogastric nervous system, nerves from the CNS, particularly the subesophageal ganglion and specialized secretory neurons, innervate these endocrine glands. In addition to the glands, these secretory neurons are part of the endocrine system and are found in all ganglia of the CNS.

Thorax

Tymbal

Tympanum

Abdomen

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93

Respiratory System Respiration in insects is a fascinating but complex topic because of the many habitats and different adaptations for thriving in them. Adding to this complexity is the metamorphosis of immature to adult tracheal systems, particularly in the Holometabola, and contrasting respiratory requirements of these different developmental stages. For example, an insect may be a slow-moving larva that transforms into a rapidly flying adult, or an aquatic naiad that becomes a terrestrial adult.

Tracheal System The tracheal system forms via invagination of the epithelium to form a tubular network inside the body, often very intricate and branching. Its primary function is to deliver oxygen to the living tissues and allow carbon dioxide and other byproduct gases to escape the body. Once this network is complete, the cells secrete the tracheal cuticle inward in which the apical layer forms fortifying rings called taenidia. The basic structure of the tracheal system begins at the spiracle, an opening on an insect’s external body, and ends at the smallest tubes that interface with cells and tissues, called tracheoles. Aside from being very small, tracheoles are distinguished from larger tracheae in that their cuticle is not shed. In some insects, the immature stages lack functional spiracles (they are apneustic) and apparently respire through their cuticle.

Gas Exchange The tracheal system in insects is complex and may function quite differently among insect orders. In most insects breathing occurs via one or a combination of mechanisms that include spiracular opening and closing and ventilatory movements. The latter can involve muscle contractions in various body regions to produce rhythmic thoracic or abdominal compression or extension movements, as well as diaphragm-like motions when the muscles are situated adjacent to air sacs. Directionality of airflow occurs via coordination of spiracle opening and closings, ventilatory movements, and the presence of unidirectional flow valves, all of which require further investigation. In relation to the active ventilatory control mechanisms in insects, they may undergo a range of gas-exchange patterns depending on metabolic needs and may be partly due to developmental constraints. Resting to highly active insects may breathe continuously, cyclically, or discontinuously. These patterns could also be influenced by environmental conditions, in which spiracular opening is minimized to reduce water vapor loss through the tracheal system.

← The insect respiratory system begins at the interface between tracheae in the body and the atmosphere, the spiracle. Spiracles are the entrance and exit not only of respiratory gases, but also water vapor. This weevil spiracle shows dense cuticular branches at the spiracular opening, which act as a filter, keeping particulate matter from entering the tracheal system.

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STRUCTURE AND FUNCTION

Aquatic Systems Most insects that have aquatic immature stages bear gills that allow gases to diffuse through a thin cuticle and into dense tracheal mats: mayflies, damselflies, stoneflies, dobsonflies, caddisflies, among others. These gills are located at various sites along the body and may be flattened and paddle-shaped or tubular and filamentous. Other insects that have aquatic stages bear a unique cuticular structure, termed a plastron, that allows gaseous diffusion into a heavily porous, spongy cuticular space that connects to the spiracles. Plastrons can also form from dense coverings of hydrofuge setae or microtrichia that retain a thin film of air around certain parts of the body surface that include some spiracles. Eggs, immatures, and adults of various insect groups can all possess plastrons. Still other aquatic insects, again of various developmental stages, do not acquire air from the water at all and instead breathe through snorkel-like extensions from the spiracles or trap air bubbles underneath the wings (in adults) as a sort of scuba apparatus.

← The immature stages of mayflies are aquatic and respire through paddle-shaped abdominal gills, the tracheae which can be seen inside. → While immature mayflies are aquatic and breathe via abdominal gills, the adult is terrestrial, bears pairs of spiracles along the body, and has enlarged tracheal tufts in the thorax that penetrate the flight muscles.

RESPIRATORY SYSTEM

95

Tracheal System Evolution in Arthropods

96

Given the elaborate structure of the tracheal system

evolved tracheae; however, the spiracles in this group

and its many specializations in insects and other

have no closing apparatus as in arthropods. While the

hexapods, this system is not unique to these

tracheal systems in these groups have specializations

arthropods. Perhaps one of the more striking instances

that reflect their independent acquisitions, some are

of convergence is the evolution of tracheal systems in

quite similar. For example, the tracheal systems of

arthropods. Within the phylum Arthropoda, tracheae

some myriapods are indeed so similar to those in

also occur in other terrestrial groups: in myriapods

some insects that these two groups were formerly

(centipedes, millipedes) and several groups of

considered related on this basis. After sufficient

arachnids (at least some spiders, mites, harvestmen,

comparative data revealed that the two groups are not

sun spiders, whip spiders, and hooded tick spiders).

in a related clade—a relatively recent finding—it was

Additionally, velvet worms (Onychophora), a group

realized that their tracheal systems were yet another

closely related to arthropods, also independently

example of impressive evolutionary convergence.

STRUCTURE AND FUNCTION

← Myriapods are a separate

↖ The chelicerates are another

↑ Mites are a very diverse and

subphylum from insects and include

subphylum of arthropods, containing

abundant group of arachnids.

centipedes and millipedes. Although

groups such as arachnids and

Despite their typically tiny size,

only distantly related to insects, the

horseshoe crabs. Arachnids have

they also have a tracheal system.

myriapods are terrestrial and have

convergently evolved a tracheal

Similar to harvestmen, mites also

independently evolved a tracheal

system perhaps in several groups,

have only one pair of spiracles.

system, as shown in this common

such as in the well-known harvestmen

brown centipede.

or daddy longlegs.

← Tracheal system architecture can be quite different between the immature and adult stages of holometabolous insects, such as in the larva and adult of soldier flies (family Stratiomyidae). They can also be different due to changes in biology and ecology. For example, the larval soldier (far left) fly is legless and feeds on decaying organic matter on the ground, whereas the adult (left) feeds on liquids, bears the normal set of appendages, and flies. → In ametabolous insects, such as this jumping bristletail (order Archaeognatha), the immature stages do not differ much from the adult. Their tracheal systems are also quite simple due to the fact that their lineages evolved before wings.

Tracheal System Architecture and Metamorphosis Tracheal system form and elaboration is related to general modes of life history, physiological, and environmental demands. The degree of change in tracheal architecture generally corresponds to developmental changes in metamorphosis—for example, whether an insect goes through incomplete or complete metamorphosis. In ametabolous or hemimetabolous insects, the tracheal system typically does not change much from one instar (stage) to the next. In holometabolous insects, tracheal networks typically undergo dramatic rearrangement from larva to adult. This trend is complicated through divergence among species with different life histories, such as the presence of terrestrial or aquatic stages, parasitic or

unusual associations, and so on. In these cases, even the hemimetabolous Odonata show striking tracheal system modifications due to the different respiratory modes and requirements of the aquatic immature stages and the flying adult, which has greater metabolic demands. Flies with maggot-type larvae have a single posterior pair of spiracles and another anterior pair, and a relatively simple tracheal system; this system transforms into a full set of spiracular pairs along the body in the agile, fast-flying adults. Meanwhile, a first-instar jumping bristletail (Archaeognatha) hatches as a miniature version of the adult, mostly becoming larger through each successive molt and later acquiring functional genitalia and reproductive organs. They otherwise are not much different as adults as they are as younger nymphs.

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97

Specialized Modifications

While the primary function of tracheae is respiration, these structures have been co-opted to function in weight relief, hearing, and even bioluminescence, among other modifications. Good examples of this are the tracheal bushes in the posterior part of the abdomen of many caterpillars, dense networks that form in the thoracic flight muscles of pterygotes, and various forms of air sacs in adults.

Tracheal tufts: Perhaps to aid in moving gases more quickly into and out of an elongated body, various caterpillars have enlarged tracheal tufts branching out from the last pair of spiracles in the abdomen. This area of dense tracheation is accompanied by specialized muscles that function in a similar way to aortic valves. In concert with the dorsal aorta, it forms a lunglike structure that oxygenates nearby blood cells, which are then circulated anteriorly and throughout the body. This “lung” is a dramatic contrast to the other spiracles of the larva.

Flight muscles: Dense bushes of tracheae are also formed in the flight muscles of various flying insects. Going from an apterygote insect lineage such as a silverfish to one with wings, such as a dragonfly, fly, or beetle, the appearance of the tracheal networks feeding the massive oxygen-demanding flight muscles ramify and often form air sacs that surround the muscles.

Air sacs, tubes, and bulbs: Small and large air sacs can be found in all parts of the body, including appendages and appendage-like outgrowths. Air sacs in some groups may work as compressible lungs, those in others appear to have evolved to reduce weight. For instance, many beetle families have body regions filled with small air sacs, perhaps for weight relief in bulky bodies burdened with fairly thick cuticles. Tracheal tubes, bulbs, and/or air sacs are also nearly always adjacent to hearing organs such as subgenual organs and tympana, in order to amplify sounds and tune frequencies, and also to protect the delicate internal organs from sonication by the extremely loud whines made by cicadas.

↑ The evolution of wings in insects was accompanied by the development of unique, robust flight musculature and voluminous tracheal networks to feed these metabolically active tissues. In this milkweed bug, the enlarged, dense tracheal tufts that feed the flight muscles in the thorax are highly contrasting to the simpler tracheal networks in the head and abdomen. ← The tracheal system of caterpillars is fairly simple, consisting of repetitive

Luminescence: Another striking modification of the tracheal system is its development with the luminescent organs (the lantern) of fireflies. Similar to flight muscles, because of the high oxygen demand of the light reaction involving luciferin and luciferase, an elaborate tracheal brush has evolved in the abdominal light organ tissue of flashing fireflies. 98

STRUCTURE AND FUNCTION

units along their elongated bodies. In many lepidopteran groups, the last pair of spiracles on the larva have a particularly dense tuft of tracheae. This region, in concert with the heart and specialized muscles, acts like a lung-like structure that delivers oxygen to nearby tracheocytes and circulates them in the hemolymph.

Trachael “horn”

↑ Enlarged air tubes and sacs accompany the hearing organs in many insects. In katydids, sound can be transmitted through an enlarged, curved tracheal “horn” that leads from the first spiracular pair on the thorax into the forelegs, acting as an auditory amplifier that helps deliver sound to the tympana. ← Modification of the typical tube-like tracheae into air bulbs and sacs occurs in many insect orders and serves various functions. In different beetle groups, such as this cetoniine scarab beetle, small, dense air bulbs are found throughout the body and perhaps help to reduce body weight for flight, among other functions.

↑ The light organs of fireflies are

→ A dense tracheal network

highly metabolically active tissues and

penetrates the light organs of fireflies

require the rapid movement of

to feed the oxygen-demanding

respiratory gases in large volumes.

bioluminescent reaction involving

Due to these demands, the tracheal

luciferin and luciferase. The tracheae

network that enters the light organ

end at specialized photogenic cells

tissue of fireflies has developed into

where they form tracheoles, the

dense tracheal brushes.

smallest tubes of the tracheal system that interface with cells and tissues.

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99

Circulatory System As in all arthropods, the insect circulatory system is open, meaning there is no distinction between blood and interstitial fluid, or lymph, as in vertebrates. The blood circulates freely throughout the body cavity. Therefore, the blood is called hemolymph and contains a liquid component, plasma, and cells, hemocytes.

Structure For circulatory purposes, the insect body is generally divided into three regions: the dorsal pericardial sinus, the medial perivisceral sinus, and the ventral perineural sinus. These regions are created by the dorsal and ventral diaphragms. The dorsal vessel and accessory muscles work together to regulate hemolymph flow into and out from appendages while circulating it around the body. In addition to these muscles, skeletal muscles generally exert pressure on different regions of the body during locomotion and flight to assist in circulation, as do air sacs. Contraction of the accessory organs do not necessarily correspond with heartbeat and, interestingly, the heartbeat experiences periodic reversals depending on circulatory needs.

←↑ The heart is a tube along the dorsal side of an insect. Helical muscle fibers wrap around it in addition to lateral pairs of alary muscles that hold it in place and assist in pumping, such as in this firefly heart (magnified left).

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STRUCTURE AND FUNCTION

Circulatory System: The Basics Ovipositer heart Diaphragm

Cercus heart

Pumping muscle

Vessel

Wing hearts

Pumping muscle Ostium Alary muscle

Dorsal diaphragm Cercal vessel Caudal chamber Dorsal vessel (bi-directional flow)

Antenna heart Vessel Ampulla Pumping muscle

Intracardiac valve Leg heart

Ostium

Diaphragm Pumping muscle

Circumoesophageal vessel ring Antennal vessel

Dorsal diaphragm

The dorsal diaphragm is formed by a membrane and by the alary muscles that connect to the dorsal vessel. It typically mostly spans the abdomen and reaches into the thorax to varying degrees.

Ventral diaphragm

The ventral diaphragm, though not present in all insects, is often a membrane just above the ventral nerve cord that may also contain sheets of muscle.

Dorsal vessel

This muscular tube runs nearly the full length of the insect body, from the head to near the end of the abdomen. In many adults, and perhaps most immature insects, it is a straight tube, although in several orders such as Hymenoptera and Lepidoptera, it loops between the longitudinal flight muscles in adults. The dorsal vessel is divided into two parts, the anterior aorta and posterior heart.

Alary muscle

The heart bears alary muscle connections to assist in pumping and contains tiny ostia (paired openings) that have unidirectional valves.

Aorta

The aorta is the anterior portion of the dorsal vessel that does not have ostia and is slightly narrower than the heart.

Heart

The heart is the posterior portion of the dorsal vessel that bears alary muscles and ostia. In a few orders, such as Blattodea and Mantodea, the heart does not contain any excurrent ostia but bears segmental vessels in the thorax and abdomen through which the hemolymph exits.

Accessory diaphragms

Most insect legs have some form of medial septum or diaphragm that facilitates bidirectional flow with the help of the pulsatile organ.

Accessory pulsatile organs

In addition to the dorsal vessel, the body may contain a number of accessory hearts or pulsatile organs. These extra muscular ampullae, diaphragms, and dilators form in areas where consistent blood flow is difficult to maintain, such as at the bases of appendages (the antennae, wings, legs, and caudal filaments) and within the legs.

Ostia

The number of ostial pairs varies depending on the insect order, but typically is between two and twelve. The direction that the valves open to permit hemolymph flow also varies between orders and they are referred to as being incurrent or excurrent ostia.

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101

Function Hemolymph may occupy 15–75 percent of insect body volume. It is lowest in volume during intermolt periods and highest before each molt. Increased hydrostatic pressure is needed for an insect to break through and wriggle out of the old cuticle. Hydrostatic pressure is also important for the locomotion of holometabolous larvae, which are generally soft-bodied; their hemolymph typically contains more water (around 50 percent) than that of adult insects (around 20–25 percent). Hemolymph is far from being a static solution. It is an extremely dynamic tissue that is constantly changing with the physiology of the insect. It also shows wide variation in dissolved compounds, often differing between and within orders, but most importantly between insects with

different diets. Major inorganic solutes include sodium, potassium, calcium, magnesium, chloride, and a few metals. Amino acids, carbohydrates, defensive compounds such as terpenoids and cantharidin, and diverse proteins are other major hemolymph constituents. The types and levels of various proteins heavily depend on the developmental stage and environmental conditions, but include storage proteins, lipid transport proteins, and enzymes. Among the other components of hemolymph are several types of hemocytes. Wound healing typically involves coagulation and melanization of the wound site, incorporating granulocytes and clotting proteins produced by the plasmatocytes. Phenoloxidases, one of the many enzyme components in the hemolymph, harden the clot and the epidermis regenerates.

Immune System One function of the hemolymph involves a basic form

by deposition of melanin to harden the enclosed

of innate immunity, in which an insect can essentially

surface. Complementing the action of hemocytes,

recognize self from nonself. Various proteins on the

humoral immunity occurs in which the adipocytes

hemocyte membrane with receptors will recognize

(fat body cells) synthesize and release a mix of

nonself/foreign molecules and initiate various

antimicrobial peptides upon microbial infection.

immune responses depending on the foreign object.

Surprisingly, the insect immune system also

For small foreign objects, phagocytosis (ingestion by

demonstrates immunological memory and more

the hemocytes) may occur. The recognition of larger

robust immune responses can be primed by first

foreign objects, such as a parasite, parasitoid,

introducing less virulent exposures, not unlike the

bacterium, or fungi, may involve encapsulation, in

vaccination process.

which specialized cells envelope the object, followed

Crystal cell

Melanization

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STRUCTURE AND FUNCTION

Plasmatocyte

Phagocytosis

Lamellocyte

Nodulation

Encapsulation

Cold and Freeze Tolerance An important function of hemolymph components in insects that live in temperate regions is cold and freeze tolerance. Increased levels of hemolymph solutes like carbohydrates and cryoprotectants, such as glycerol and sorbitol, help to lower the freezing point of the hemolymph. Antifreeze proteins also can assist in lowering the freezing

↑ All insects that live in temperate regions must survive changes in temperature (both hot and cold), whether as an egg, immature stage, or adult. Those that must endure cold temperatures, such as this woolly bear caterpillar that overwinters as a final-instar larva, do so through several means involving various solutes, cryoprotectants, and antifreeze proteins in the hemolymph.

point as well as bind to ice crystals to prevent them from growing.

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Digestion and Excretion Insects are able to consume virtually all types of organic matter, leading to significant diversity and modification of not only the mouthparts used to acquire and ingest these food resources, but also the alimentary canal that is tasked with decomposition, digestion, nutrient and water absorption and balance, defense against pathogenic microorganisms and growth of symbiotic ones, and excretion of waste products.

Alimentary Canal

Foregut

Though greatly modified depending on life history and diet, the insect alimentary canal is generally divided into three main regions: the foregut, midgut, and hindgut, often a continuous tube. Exceptions include many Hemiptera that feed exclusively on liquids, larval Neuroptera, adult Heteroptera that extra-orally digest their prey, and the larvae of social Hymenoptera that excrete waste only upon pupation, all of which have unique modifications.

The foregut is usually differentiated into the pharynx, esophagus, crop, and proventriculus. As food is masticated with the mouthparts or siphoned up, the salivary glands (formed by epidermal cells of the labial segment) secrete saliva, usually a mixture of water and digestive enzymes, into the oral cavity to help lubricate and dissolve the food. In many insect groups, including larvae and adults, the salivary glands can be so large

Alimentary Canal: The Basics Foregut

Midgut

Gastric caecum Denticle

Hindgut

Stomodeal valve

Peritrophic membrane

Ventriculus

Proventriculus Crop

To opposite salivary gland and reservoir Esophagus

Pharynx

Rectum

Mouth

Pyloric valve Ileum Salivary reservoir Salivary gland Salivary duct

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STRUCTURE AND FUNCTION

Malpighian tubule

that they extend to the end of the abdomen. Food is ingested and moved backward through the pharynx with strong muscles. Many insects that feed on solid foods have spines lining the pharynx, which point backward to assist in moving food. The esophagus often bears an expanded crop for storing food as it passes to the posterior part of the foregut, the proventriculus. This structure is typically surrounded by strong muscles and can be armed internally with a fantastic array of spines, teeth, and cuticular projections for further food mastication. At its simplest, the proventriculus is merely a sphincter that regulates food passage into the midgut.

Midgut Involved in enzyme production and secretion, as well as nutrient absorption, the midgut has at least four types of cells in its epithelial layer: regenerative cells, endocrine cells, goblet cells, and columnar cells (principal cells). The last are the most numerous, each containing thousands of microvilli on their luminal surface for nutrient absorption. Goblet cells are important in regulating ion movement and maintaining proper gut pH. Along the midgut, usually along the anterior or posterior regions, can be found elongated diverticula or gastric caeca. These tubular pockets can provide extra surface area for

↑ The foregut ends in the proventriculus, which often is armed with arrays of spines and teeth for helping to masticate and filter ingested food and liquids. The proventriculus shown here is from a weevil and has been spread open to reveal the internal armature.

absorption of nutrients and water, and they often house various communities of symbiotic microorganisms that serve important digestive roles. Because only the foregut and hindgut have thickened cuticular linings that are shed at each molt, the midgut epithelium is susceptible to damage from the passage of food. Therefore, a delicate lining of chitin fibers and matrix form a protective barrier between the lumen and epithelial cells, called the peritrophic matrix (PM), or envelope. The PM may also contain various layers. In addition to protection from food abrasion, the PM serves as a barrier to pathogens and to harmful chemicals to some degree, although these can penetrate and damage the underlying epithelium. In insects that consume large amounts of fluid, various forms of filter chambers have evolved in which portions of the posterior midgut approximate the anterior midgut, thereby allowing most water to pass quickly for excretion and not be absorbed to dilute the hemolymph.

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105

Hindgut: General Function

Nitrogenous wastes, water, amino acids, sugars, salts

Resorption of water, amino acids, sugars, salts

Midgut

Anus

Nitrogenous wastes, other excreted substances

Ileum and colon

Rectal pads (Proximal)

Malpighian tubule

(Distal)

Hindgut

The ileum: This often narrow tube can be expanded and

The insect hindgut serves for osmoregulation—that is, maintaining the balance of water and solutes in the hemolymph—and excretion. It is generally differentiated into the pylorus (bearing a valve or sphincter separating midgut from hindgut), the ileum, and rectum.

lined with cuticular spines and projections to house various microorganisms that further assist in digestion, common in termites and some cockroaches. In larval scarab beetles, this region is called a fermentation chamber and is where a large quantity of bacteria thrives to assist in decomposition of the large bolus of food products that accumulate.

Malpighian tubules: Long Malpighian tubules stem from the pylorus, or slightly before, and are the primary excretory organs besides the rectum. They are critical in hemolymph filtration and ion balance. While metabolism is occurring in the midgut and water and nutrient absorption occurring in the rectum, these components and products are constantly being moved into the hemolymph. The Malpighian tubules then resorb these contents and metabolic wastes to pass them back through the hindgut as primary urine. The primary cell types of the Malpighian tubules include principal cells and stellate cells that both contribute to absorbing and transporting ions and fluids. 106

STRUCTURE AND FUNCTION

The rectum: This enlarged region has lobes of highly permeable epithelium with many principal cells, termed rectal pads. As waste products and primary urine continue to pass through for excretion through the anus, these pads further absorb liquid and water from newly introduced contents and the primary urine. The rectal pads are particularly important for insects living in dry environments like deserts. In these insects, the pads absorb as much fluid content as possible that may have passed through the midgut. Additionally, such insects (various beetles, for example) may have a cryptonephridial system in which the

Rectal Gills A curious modification of the hindgut in immature dragonfly larvae is the presence of gills in the rectum, as shown here. A fairly typical rectum can be seen toward the top of the image, with the visceral muscles (in red) wrapping around it. The rectum then enlarges into a wide chamber lined internally with numerous cuticular lamellae that are the rectal gills. Tracheae, only partially visible here along the side of the chamber, approximate the gills for gas exchange. Many noodle-like Malpighian tubules can be seen at the top of the image.

↓ The Malpighian tubules function to filter and resorb water, various solutes, and metabolic wastes from the hemolymph and pass them back through the hindgut as primary waste. Spheric crystals of uric acid salts can be seen packed into this broken Malpighian tubule (as well as a few small tracheal tubes following on the outside).

distal ends of the Malpighian tubules are fused to the hindgut, enabling them to resorb water more efficiently. With such hindgut modifications, some insects can even obtain sufficient amounts of water just from air moisture. Following nutrient/water resorption from primary urine and continued uric acid precipitation, secondary urine is produced and excreted through the anus along with solid waste products. An interesting modification of the hindgut occurs in immature dragonfly naiads, which possess tracheal gills for respiration inside the rectum. These lamellae also contain specialized cells (chloride cells) for filtering ions from the water that is pumped in and out of the rectal chamber.

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Excretion and Waste One of the primary forms of metabolic wastes is ammonia. Because it is toxic in this form, ammonia is typically converted to fairly inert substances such as uric acid and allantoic acid or combined with other ions to form various urate salts. Uric acid is synthesized in the adipocytes (fat body cells) and although much of it is excreted, a fair amount can also be retained in the fat body for many purposes. Some uric acid may be stored, and actually metabolized later, when nitrogen resources are low. It is unclear how many insects may be able to achieve this feat and it may require certain endogenous communities of bacterial symbionts. At least some cockroaches incorporate uric acid into spermatophores, which is then metabolized by the female after mating to help nurture egg development. Since uric acid and urate salts are generally white in color, other uses when sequestered include incorporation into or near the cuticle to form white or reflective colors. Some caterpillars, for example, concentrate uric acid in the cuticle to form white areas, which camouflage the larvae as a bird dropping. Urate salts are also sequestered in fat bodies and deposited in the light organs of fireflies, forming a distinct and dense layer behind the photogenic (light-producing) layer. This functions to reflect light outward that is produced during the light reaction in the photogenic tissue. Additionally, it has been shown that urates may be incorporated into the bulb of some fly halteres—sensory organs used to guide steering during flight. This may serve to provide extra weight in the bulb and allow for increased sensitivity in the campaniform sensillar arrays on the stalk while the halteres rapidly move up and down during flight.

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STRUCTURE AND FUNCTION

↑ Fireflies sequester uric acid in the

↓ Some lepidopteran larvae, such as

fat bodies behind the photogenic

this swallowtail butterfly caterpillar,

tissue, which functions as a reflective

incorporate uric acid into the cuticle

layer to direct the light produced

to form white areas, in this case to

by bioluminescence.

mimic a bird dropping.

← Fireflies retain uric acid and deposit it in the form of urate salts in the abdominal segments bearing the light organs. The whitish to yellowish color of the light organ area is due to the presence of urate salts in the tissue. By dissolving the tissues, the urate deposits can be easily visualized as shown in this firefly abdomen.

Frass The final representation of what has been eaten, digested, and excreted, frass is the product that is left behind by an insect. Frass varies tremendously in composition and form due to an insect’s diet. It may be pure liquid, or “fly specks,” as in many Hemiptera and Diptera, somewhat amorphous and soft, or geometrically shaped, symmetric, and unassumingly intriguing. Particular shapes of frass, or the lack of a defined shape, are due to the structure of the rectum, the size and shape of the rectal pads, and the form of musculature along the rectum that is responsible for contracting and squeezing out waste.

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Microbiotic Interactions Symbiotic microbial interactions are critical in digestion. It is thought that these mutualisms may have begun as parasitic associations. As more research is conducted, the ways in which these microbiotic interactions will be found to assist and affect insect development and survival will only expand. Nitrogen is a limiting resource for insects, plants, and other life, and it is becoming more evident that microsymbionts contribute the production of it in their hosts, including the cellulose-digesting protists of termites and bacterial gut communities. Bacterial endosymbionts also occur, for example, in many hemipterans, such as aphids, and are important for adding nitrogen and other nutrients to a diet of plant phloem that is typically rich in sugars but poor in essential elements. Many plants are well known for being able to convert atmospheric nitrogen into biologically available nitrogenous compounds (ammonium) via specialized bacterial associations with the roots, but various invertebrates and insects can do so as well. Indeed, several insect groups, such as termites, some flies, and

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STRUCTURE AND FUNCTION

mostly wood-feeding beetle larvae, are capable of nitrogen fixation. Although it has not been studied in all of these cases, many types of gut-associated nitrogen-fixing bacteria are responsible and this phenomenon may be more prevalent among hexapods than we currently know. It has been discovered that at least some beetles, particularly weevils, harbor bacterial endosymbionts near the anterior midgut, which are needed for proper cuticular development. These proteobacteria specifically aid the synthesis of the amino acid tyrosine, which is an integral precursor for hardened cuticles. In experiments where these bacteria are depleted, the resulting cuticle is discolored, soft, and malformed.

↓ It is well known that nitrogen-fixing bacteria form symbiotic relationships

→ Termites, such as these

with plant roots, allowing for the

Nasutitermes, possess symbiotic

conversion of atmospheric nitrogen

bacteria that digest the cellulose

to compounds metabolically usable

in the wood and function in

by the plant. Insects, particularly ones

nitrogen fixation, converting

that live in leaf litter or feed on wood,

atmospheric nitrogen into a

also have such bacterial relationships.

biologically usable nutrient.

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Glands From hormones and signaling molecules secreted by endocrine glands, to digestive fluids, toxins, communication chemicals, waxes, and silk from exocrine glands, the diversity of insect secretions is truly astounding.

Exocrine Glands What insects do not say with sound, they communicate via a plethora of chemicals and compounds. Exocrine glands develop from specialized epithelial cells and produce compounds for external secretion. They are very diverse in insects. Substances produced by these glands range from digestive fluids, waxes, silks, pheromones, venoms, as well as compounds for defense, alarm, dispersal, trail following, and appeasement. Chemicals that function within a species, or intraspecifically, are typically referred to as pheromones, while those with an interspecific function— that is, between species—are called allelochemicals.

close-range pheromones that promote copulation, aggregation pheromones to attract individuals of both sexes for mating as well as feeding, marking pheromones that mark where a female has oviposited, and trail pheromones (used by ants and other social insects) to demarcate paths between food sources and the nest. Alarm pheromones typically alert others of danger and cause individuals to disperse or flee, or in the case of social wasps, to attack. The pheromones of social insects communicate duties to individuals to maintain colony order and structure.

Allelochemicals Pheromones Typically, pheromones are a mixture of many compounds that are derived from precursors in the insect’s food or diet, such as from plants. They may act as sex attractants to lure mates of the opposite sex, mating/aphrodisiac/

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STRUCTURE AND FUNCTION

Production of allelochemicals generally occurs from exocrine glands, which can be eversible (as soft lobes that are protruded and withdrawn back into the body), or non-eversible. Examples of eversible glands include the osmeterium, a tubular structure that swallowtail butterfly

caterpillars use when disturbed; and paired eversible sacs along the sides of chrysomelid and other beetle larvae. Non-eversible exocrine glands often have a reservoir under the cuticle surface to produce large quantities of the compound. Examples of such glands include the frontal gland of nasute soldier termites. They have a long spigot (nasus) on their head from which a defensive chemical is squirted. Some carabid beetles possess pygidial glands at the tip of the abdomen that can eject a hot, caustic spray of quinones. Metathoracic stink glands are typical of many Hemiptera, and romaleid grasshoppers can emit a frothy defensive chemical from paired glands with long ducts near the metathoracic spiracle. Venoms can be produced by glands at the base of hollow, stinging hairs, such as with urticating setae in many caterpillars, or they can be administered by a sting (a modification of the female ovipositor) in aculeate (stinging) wasps.

Toxicity While many defensive compounds are expelled from the body, other insects sequester them in the hemolymph, cuticle, or fat body (similar to uric acid sequestration). These chemicals can be quite toxic and include cyanogenic compounds, cardiac glycosides, cantharidin, pederin, and alkaloids. Toxic insects usually advertise themselves with bold warning coloration.

↖ Aphids secrete an array of defensive and alarm pheromones from curious, elongate spigot-like structures on the dorsal side of their abdomen, termed cornicles. ↗ Some adult male tiger moths use large balloon-like coremata that are everted from the tip of the abdomen to disperse long-range pheromones to attract mates. → Stink glands are present in many different insects, appearing on areas of the thorax and abdomen. In many Hemiptera, such as this assassin bug (family Reduviidae), they are present laterally on the metathorax. Some assassin bugs also have glands along the tibiae of raptorial forelegs that exude sticky substances and assist in catching prey.

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113

114

Plant Galls

Salivary gland secretion: Gall-inducing compounds

Another fascinating phenomenon of convergence in insects is gall formation. Galls are insect-induced areas of excessive growth on plants in which the insect progeny live, providing food and shelter.Various groups of insects are able to induce gall formation, including within Thysanoptera, Hemiptera, Diptera, Hymenoptera, Lepidoptera, and Coleoptera (as well as some mites and even nematodes). While much remains unknown regarding gall induction, some interesting things are being discovered. For example, in gall midges (Cecidomyiidae), galls are induced to form via proteins secreted by the larvae that suppress host defenses and modulate growth activities. In gall-inducing sawflies, the insect larva synthesizes and secretes compounds that mimic phytohormones (growth inducers).

are often secreted from the salivary gland. Typically, insect saliva is mostly water mixed with various digestive enzymes and solutes. The glands can be extremely large and elongated, producing copious amounts of saliva, depending on the function. In gall-inducing insects, additional compounds and proteins are secreted in the saliva that interfere with a plant’s defense response and hijack growth pathways within the localized tissue. After the female has oviposited into the plant tissue and the insect larva has hatched, the larva secretes saliva laced with these compounds, causing gall tissue to form as long as the larva feeds. It is also likely that microbial symbionts are present in the saliva that are involved in gall development, as many of the microbial associates found in these insects are able to synthesize phytohormones.

STRUCTURE AND FUNCTION

Venom gland secretion: In addition to induction of galls through salivary gland secretions, it is also possible that gall wasps possess gall-inducing compounds produced from another type of exocrine gland, the venom gland. These glands are associated with the ovipositor and stinger in Hymenoptera. Interestingly, in gall wasps, gall formation is initiated after the female oviposits into the plant tissue and before the larva hatches. After the larva hatches, it is likely that salivary gland secretions from the feeding larva maintain gall growth. Aside from the defensive venoms produced by stinging Hymenoptera, venoms produced in parasitoid wasps are diverse and typically function to suppress the host’s immune response, such as through the unique polydnaviruses found in the venoms of ichneumonoid parasitoid wasps.

← Silk button galls are produced by a gall wasp and illustrate one of the many types of galls formed on oak trees. Many gall wasps oviposit into leaves, thereby producing galls from the leaf surface. ↗ Various types of plant tissue can be found inside oak galls, including small egg-like structures that contain the developing gall wasp larva. → Gall wasps induce the formation of a spectacular array of gall shapes and colors, such as these large spherical galls, sometimes called oak apples, that are initially green but take on color as they develop, much like a ripening fruit.

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115

↑ Some insects, such as this iceryine scale insect, produce wax onto the cuticular surface in copious amounts.

Wax Glands Wax glands are found in most, if not all, insects and have become greatly modified in many groups. In general, insects secrete waxes via ducts through the cuticle from epithelial exocrine glands. These waxes are exuded onto the surface of the cuticle and serve to protect against dessication, mechanical damage, and predation. They also serve as pheromone and intraspecific communication signals among many insects, particularly in social insects. Some insects, such as nesting bees, produce comb wax or waxes that are used for larval cells. They secrete these waxes from large glands on the ventral surfaces of abdominal segments four to seven.

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STRUCTURE AND FUNCTION

While many insects do not produce much wax, some produce copious amounts that appear as blooms or cottony tufts decorating the insect. Larval sawflies (Hymenoptera) and many families in Hemiptera have large numbers of wax glands for such high production (wax is what puts the “mealy” in mealybugs, a group of scale insects). They utilize these large wax deposits for protection from predators, parasitoids, and as a cryptic defense. Although all of these secretions are called wax, they show a broad and complex diversity in composition. Epicuticular waxes often contain high amounts of hydrocarbons (around 90 percent) of different lengths, while other waxes may contain varying amounts of alcohols, esters, sterols, fatty acids, resins, and true waxes. Beeswax is composed of around 70 percent esters and the remainder lipoidal substances. Various waxes in Hemiptera can contain large amounts of true wax to little wax, or more resin such as in shellacs from scale insects.

← Similar to larval lepidopterans, adult bark lice (order Psocoptera) produce silk from their labial glands. Different from Lepidoptera, however, bark lice cover and secure their eggs with strands of silk.

produce silk from glands in the basal tarsomere of the forelegs and use it to line their tunnel domiciles. Lacewings spin silk for egg stalks from a set of accessory exocrine glands in the abdomen, the colleterial glands.

Endocrine Glands

Silk Although most insect groups produce silk and silk compounds via exocrine glands, a few, such as Thysanoptera, some groups of beetles, and lacewings, produce these from the Malpighian tubules and are exuded through the anus. Silks have diverse protein compositions but are largely composed of different fibroins, which are keratins. Similar to some components in silks, different types of proteinaceous glues are also produced from the salivary glands and typically used to affix eggs to various surfaces. Salivary glands are commonly specialized for secreting silk (then they are also termed labial glands), various caterpillars being best known for this. Other silk producers include various Hymenoptera (bees, ants, and wasps), which use silk to construct nests. Several other hymenopteran groups and fleas use silk to spin cocoons. Psocopterans also use their salivary glands to spin silk over their eggs and nests. Webspinners (Embioptera)

The endocrine system of insects is concerned primarily with the production of hormones and other signaling molecules, and their regulation by two primary means, endocrine glands and secretory neurons. These hormones are of fundamental importance, forming cascades of molecular signals that regulate other hormones and gene expression. They are released into the hemolymph and influence physiological, developmental, and behavioral activities, such as molting and metamorphosis, yolk synthesis, embryonic cuticle development, diuresis, the mobilization of energy for flight, aspects regarding overwintering (diapause), migration, and polyphenism (morphological variation/castes induced by hormones and environmental stimulation). Most endocrine glands are present in the head and thorax (see Chapter 4). Corpora cardiaca are known to produce prothoracicotropic hormone, which regulates the synthesis of ecdysteroids. Endocrine cells also have been identified scattered throughout the midgut epithelium, which produce various peptide hormones. Although their roles are uncertain, they are likely involved in the management of digestive enzymes and absorption. Epitracheal (peritracheal) glands are small glands located near each spiracle and attached to a trachea. They produce ecdysis triggering hormone. While these glands have only been identified in Lepidoptera, they are likely present in other insects. There are a multitude of other hormones, as well as various vertebrate-like hormones, such as insulin-like peptides.

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117

Reproduction Along with digestion and waste excretion, the other main function of the abdomen is reproduction and there are several specialized tissues and structures, both in males and females, that accomplish this process. Insect reproductive systems and genitalia are fascinatingly complex.

Male Reproductive System The male reproductive system primarily consists internally of the testes, or sperm-producing organs, connecting with paired seminal vesicles (storage organs) and a median ejaculatory duct. The external organs are the genitalia.

Testicular tube Vas deferens

Accessory gland Seminal vesicle

Ejaculatory duct Intromittent organ Gonopore

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STRUCTURE AND FUNCTION

↓ Many male insects produce various excretions in addition to delivering sperm during copulation. In many Orthoptera, a large, protein-rich spermatophore is transferred to the female, on which the female feeds to help nourish the developing eggs.

Male genitalia typically are adapted for grasping the female and delivering (as well as sometimes removing) sperm. Clasping structures arise from different segments on a genital capsule and the aedeagus fits through it. The genitalia of an adult male carpenter moth (family Cossidae) are shown here with the aedeagus removed and placed alongside the genital capsule.

Internal Organs

External Organs

The testes often consist of a series of tubes or follicles. Sometimes the follicles are not completely separated or are undivided sacs. Follicle numbers in each testis can range from a single follicle to more than one hundred. In each follicle, sperm production begins at the distal ends in an area called the germarium. As the germ cells progress down the follicles, they continue to divide, grow, and mature, passing through different stages until they reach final mature spermatozoa. The vas deferens connects from the testes and leads to an enlarged area, the seminal vesicles, where sperm are stored before transfer to the female. In many cases the seminal vesicles are separate tubes extending from the ejaculatory duct. The ejaculatory duct is the median duct to which the vas deferens and seminal vesicles join. This duct often is a simple tube, but in insects that produce larger and complex spermatophores, the duct may be of varying shape and structure. Other exocrine glands, termed accessory glands, open into the ejaculatory duct and may produce an assortment of fluids (seminal fluid) and proteins that become mixed with the sperm or incorporated into spermatophores.

Male genitalia in insects are extraordinarily variable, to the extent that they are routinely used to separate and even diagnose closely related species. They generally consist of the intromittent organ (phallus or aedeagus) through which passes a membranous endophallus and the ejaculatory duct. Lateral parameres (derived from the gonocoxae) may form clasping structures. From this generalized form, the genitalia take on all forms, with bizarre spines, projections, hooks, and cuticular extensions. Such complexity is likely due to sexual selection of copulatory mechanisms, involving strong selective pressures toward structures that are effective at holding/ grasping mates, delivering sperm into differently shaped female genitalia, stimulation, and in some cases using different methods to remove or displace sperm packets deposited by other males.

REPRODUCTION

119

Copulation

↓ Many groups of flies, such as these robber flies, must mate in different orientations and/or rotate the tips of their abdomens during mating because the male genitalia often have

120

rotated during development, from

→ Lepidopterans, such as these tiger

around 45 to a full 360 degrees.

moths, mate end-to-end.

STRUCTURE AND FUNCTION

Due to changes in the orientation of male and female genitalia, as well as internal structural changes in the female copulatory opening, copulation occurs in various positions. The male may mount dorsally on the female or vice versa, or the two may pair end to end with both oriented normally or with one upside down, usually due to the genitalia being rotated in development. Males may hold on to the females with their legs, mandibles, and even antennae. Odonate copulation is exceptional: he transfers his sperm to secondary (accessory) genitalia located on his second abdominal segment. The female, therefore, must extend the apex of her abdomen forward and underneath her body to receive the sperm, while the male holds her with claspers just behind her head. Other forms of insemination include the male aedeagus puncturing the cuticle of the female in, or near, the copulatory opening, or even further away and injecting sperm into the hemolymph, as in bed bugs, in which the sperm migrate toward the ovarioles.

Paired ovaries Terminal filaments Germarium

Ovariole Mature oocyte

Vitellarium Pedicel

Calyx

Spermathecal gland

Lateral oviduct Common oviduct

Spermatheca

Accessory gland

Gonopore

Genital chamber Vulva

→ Insect eggs come in marvelous shapes, colors, and textures. Depending on the insect, the eggs also may be laid in clusters, as these moth eggs, or singularly.

Female Reproductive System The female reproductive system consists internally of the ovaries—the egg-producing organs—which connect to a central median oviduct via lateral oviducts. The median oviduct opens into the genital chamber, which may possess a narrow tube, the vagina, or is often enlarged to form a bursa copulatrix to receive the male aedeagus. The bursa copulatrix then leads to the gonopore, or copulatory opening. Often, connecting to the bursa are additional ducts leading to the spermatheca, for sperm storage, and accessory exocrine glands.

Internal Organs The ovaries are paired structures, each consisting of a number of egg tubes called ovarioles, comparable to the follicles of the male testes. Ovariole number in each ovary can range from just a couple to a couple of thousand. Also, similar to the male follicles, egg cell (oocyte) production begins at the distal tip of the ovarioles in the germarium. The oocytes then mature as they pass through the vitellarium, where the follicular epithelium surrounding the oocyte secretes the egg cuticle (chorion) and the eggs move into the oviduct. There are types of oocytes, differences being mainly in their production and development within the ovarioles, as well as differences in nutritive tissue that will be available for the developing

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STRUCTURE AND FUNCTION

embryo. The spermatheca can take on various shapes, sizes, and number, ranging from one to three. The accessory glands typically produce various types of sticky proteinaceous glues, sometimes with silk protein components, for adhering eggs to a range of surfaces. In mantids and cockroaches, these glands produce a specialized coating on a group of eggs that hardens for protection, termed an ootheca.

External Organs Female genitalia generally are not under sexual selection. Therefore, their structure and function is more adaptive, guided by natural selection. The gonopore, the external opening through which the eggs pass, typically is on segment eight. Some Lepidoptera and flies are unique in that females have no specialized egg-laying structures and simply drop/lay eggs onto the substrate. Many insect groups have a simple ovipositor, formed by the terminal abdominal segments telescoping outward to reach into crevices, that can be inserted into plants, soil, or under rocks. Ovipositor structure becomes more specialized in insect groups with an appendicular ovipositor, formed from appendages of abdominal segments eight and nine. These ovipositors can be remarkably long and flexible, capable of placing eggs deep within plant tissues, soil, or on hosts that are deep within a substrate (see also, Chapter 4).

Female genitalia are often adapted for function; therefore, their structure reflects their primary function, oviposition. The ovipositor of an adult female carpenter moth (family Cossidae) is telescopic and extensible via the two pairs of stick-like apodemes to which skeletal muscles attach.

Fertilization Egg fertilization occurs when a sperm enters the egg through a tiny opening, the micropyle. Following fertilization, most insects proceed to lay the eggs (oviparity). However, some females may retain the eggs in the genital tract without providing additional nourishment, and lay eggs that are just about to hatch (ovoviviparity) or lay first-instar larvae that had just hatched within the female (larviparity). Still a few others will not only retain the fertilized eggs, but nourish the developing embryos in a modified vagina, termed the uterus, and later give birth to the mature larvae (viviparity).

→ Specialized protective egg cases are produced by a few insect groups, such as cockroaches and mantises (pictured). These cases are an elaborate exoskeleton for the eggs, composed of various proteins, chitin, and other cuticular components.

REPRODUCTION

123

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STRUCTURE AND FUNCTION

↑ Due to their non-functioning mouthparts and

→ Goliath beetles are among the heaviest of

inability to feed, mayfly adults have some of the

insects, the larger individuals weighing close

shortest life spans, only lasting for one day.

to 3½ oz (100 g).

Biological Limitations In all of their astounding varieties of form and function, insects push the limits on various fronts of biological evolution. Aside from the topics highlighted here, and elsewhere in this chapter, jumping ability, flight speed and maneuverability, and countless more exist that are equally as fascinating.

Life Span

Body Size

Average insect life spans are extremely variable, including the development times of life history stages. For example, many adult mayflies live for only one day because they do not feed, but each immature stage may have lasted for more than a week. Cockroach life spans average many months to more than a year; termites are a couple of years (queens can survive for decades); and cicadas, including development time of the subterranean nymphs, may live a couple of years to nearly two decades.

Body sizes of living and extinct insects vary profoundly. Extant species range in size from the extremely minute fairy wasps (Mymaridae), which can be slightly less than 0.2 mm in length, to some very large species: ones that are very long (some stick insects up to 24 in/60 cm in length), very wide (atlas moths with wingspans close to 12 in/30 cm), or very heavy (hercules, goliath, and titan beetles, giant wetas up to 22 oz/70 g). While extinct insect sizes are difficult to estimate, they seem to have been similar in size to living

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125

↑ Hercules beetles are among the largest beetles. While the cuticle of the horns and prothorax are strong and fairly thick, that of the elytra (modified forewings) is thinner, since the beetle still needs to fly.

species except for a few of the largest insects to have ever lived, in the Palaeodictyopterida, and Meganeuropsis in the dragonfly-like Protodonata. Meganeuropsis attained wingspans of around 30 in (71 cm). Explanations for these extinct insect giants center on higher levels of oxygen in the atmosphere at the time. However, the explanation is likely more complex, perhaps also involving a lack of aerial predators, different tracheal system architecture, differences in respiratory proteins (such as oxygen-binding affinities), and in-flight behavior and metabolic needs.

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STRUCTURE AND FUNCTION

Miniature insect sizes appear to be limited by sizes of the central nervous system and the reproductive system and eggs, as well as surface and capillary forces required for feeding.

Cuticular Strengths The extreme variation in cuticle types as biopolymers is one of the most fascinating aspects of insect and arthropod biology. Cuticles can be transparent, form myriad shapes, structures, and designs, and have astounding variations in thickness, flexibility, and hardness. Insect cuticles range from thin, delicate membranes to impenetrable cuticles in some beetles like weevils. The cross-sectional strength of the insect exoskeleton works extremely well at smaller body sizes, allowing an ant, for example, to lift something 40 times its body weight. At larger body sizes, though, this design becomes too bulky and weak.

Temperature Tolerance Upper temperature tolerances of insects approach ca. 113oF (45oC) and most insects cannot tolerate such levels of heat, at least not for extended periods. A few insects can tolerate temperatures as high as 122oF (50oC). Lower tolerances tend to be quite variable depending upon certain adaptations. These include supercooling (lowering the freezing point of the hemolymph, often to around 14oF/–10oC or even up to –40oF/–40oC) and the presence of cryoprotectants (substances conferring protection from freeze damage and ice crystal formation) in the hemolymph. Cryoprotectants can also function as supercooling compounds, including various sugar alcohols, sorbitol, glycogen, and other dissolved solutes. Various peptides and proteins, such as ice-nucleating and antifreeze proteins, are also often present that either help prevent ice formation or contain the spread of ice by

↑ Insects that can overwinter and endure freezing temperatures include woolly bear caterpillars, the larvae of some moths in the family Erebidae.

binding to the ice crystals, allowing some insects to withstand temperatures below –58oF (–50oC) and even partially freeze. Few insects can withstand temperatures beyond these extremes, and those that can do so through cryptobiosis in which they partially dehydrate their bodies.

NEXT PAGE Asbolus verrucosus, the desert ironclad beetle is known for its ability to withstand extremely high temperatures. It has warty, fused elytra that help to protect the insect against desiccation.

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127

Chapter

3

Wings and Flight Insects were the first animals to take to the skies, at some point during the Devonian, and there are currently more than one million species of winged insects. Wings have allowed insects to live and thrive in new habitats, and to change their location quickly when their current habitat becomes less desirable. By exploring how and why insects gained their wings, and what they do with them, we are better able to understand the ways in which wings have shaped insect evolution, diversity, and behavior.

← Kallima inachus, the Indian leaf butterfly has wings that mimic richly veined dried leaves—a camouflage it uses to dupe would-be predators.

131

Wing Evolution and Structure The first insects to evolve from a six-legged ancestor did not possess wings, and their descendants are still around today in the form of apterous (wingless) insects like silverfish and bristletails. Though it is unknown which specific environmental forces prompted the evolution of wings, the earliest wings or winglets must have had some other purpose besides lift generation.

The first wings may have been protective appendages for legs or spiracles (the respiratory openings on an insect’s body), a means of changing an insect’s appearance to make it more appealing to mates or less conspicuous to predators, or a way for an insect to escape a predator by gliding away. Though the specific evolutionary pressures that resulted in insect wings remain unclear, we do know, based on shared morphology and genetic similarity, that

wings evolved only once in the insect class. When the first insects evolved wings, they set the stage for one of the most massive radiations in animal history. The advantages insects gained from wings allowed them to become the most speciose and successful group of organisms on the planet.

Wing Origins and Development A cursory glance at the skeleton of a bird or a bat hints at the origins of its wings: the long bones inside bat wings, for example, look like nothing other than a set of spindly fingers in a handlike wing, and the bones connecting bird wings to the body look like forelimbs or arms (see box on page 134). For insects, however, the transition from nonflying to flying is much less clear. The wings of insects did not arise from other appendages: the first winged descendants of the apterous hexapods still had all six legs. The nonflying ancestral insects did not have wings, and flying insects suddenly did. Where did these wings come from? How did the insect body adapt to create a liftgenerating surface? Even hundreds of years ago, entomologists were able to draw educated and ultimately strongly supported conclusions about the origins and development of some parts of the insect body, like abdominal segments and legs, for the simple reason that the parts bear a strong morphological resemblance to each other. The antennae and mouthparts of many beetles and grasshoppers resemble insect legs, because they essentially are legs: the same developmental signaling pathways that result in legs on the thorax are modified slightly to produce the leg-like appendages on the head.

132

WINGS AND FLIGHT

← Insect wings evolved only once,

→ The owl butterfly has huge

during the Devonian period. By the

eyespots that resemble owls’ eyes on

time this Cretaceous fossil was

its wings. The patterning is a form

formed, insects had already been

of mimicry that enables the butterfly

flying for 300 million years.

to avoid being eaten.

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133

For the wings, however, there is no readily visible homologous segment. The wings consist of hard veins with flexible membrane in between, and immature insects do not possess wings. In many of the hemimetabolous and all of the holometabolous insects, wings appear only at the last molt to adulthood. In holometabolous insects especially, the change is profound: an immature insect may have leg-like

appendages for crawling and mouthparts for eating, but it is astounding to see a wormlike final instar become a pupa and then emerge with a complete set of wings that seem to have derived from thin air. In holometabolous larvae, blobs of undifferentiated cells ultimately develop into the appendages. What becomes of these cells when the insect pupates and takes on its adult form?

Comparing Wings in Vertebrates and Insects

134

Vertebrate wings are modified forelimbs. A human hand shows

In insects, the wingless silverfish does not have any appendages

many of the same structures as a bat wing and a bird wing.

that resemble the wings of dragonflies or house flies.

WINGS AND FLIGHT

← Insects with complete metamorphosis (holometaboly) fully develop wings during the pupal (cocoon) stage.

Current Hypotheses The insect thorax is made up of three segments, and the wings appear on the second and third segments (the mesothorax and the metathorax, respectively). There are three viable hypotheses for the developmental origin of wings. First, they may be specializations of the notum, the dorsal part of the thorax. Second, they could be projections of the pleuron, the piece of cuticle that forms the side of the thorax. Finally, both hypotheses could be accurate, with the wings formed from the merger of projections from both body parts.

Currently, evidence points toward the third hypothesis. All cells in the body come from a single cell that then divides many times, changing whichever of its genes are expressed in each division as those cells take on their identities as muscle cells, nerve cells, gut cells, and so on. By comparing proteins expressed in cells in the wings to proteins expressed in other parts of an insect’s body, we can determine which tissues might have shared a common origin and thus may be related. Tissues in both the notum and the pleuron contain many of the molecular markers seen in the wings, suggesting that the wing is derived from

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135

Wing: dorsal surface

Notum

Wing: ventral surface

Pleuron

↑ A paper wasp, Polistes metricus,

↓ A relative of lacewings, the

native to North America.

mantisfly, Mantispa styriaca, sheds its pupal skin to emerge as an adult insect. It will be an hour or so before it can use its wings.

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both of these parts. The wing is thus a two-layered structure consisting of cells projecting from the top of the thorax on the dorsal (upper) surface, and cells from the side of the thorax forming the ventral (lower) surface. Though the pronotum typically does not have a wing attached to it, the cells of the pronotum, the dorsal surface (notum) of the first thoracic segment, contain all of the molecular “instructions” for forming a wing. One gene suppresses the formation of wings in the pronotum, and when this gene’s translation is blocked, a wing develops. This ectopic wing contains parts from both the notum and the pleuron, and takes the form of thin membranes supported by hardened veins. This result demonstrates that wings are derived from both the top and side of the thorax, and that the instructions for making wings are present in each segment of the insect body. Taken together, these manipulations of the genes and proteins that underlie wing structures demonstrate how insects develop their wings and show us where the wings of the first winged insects may have originated.

Wing Expansion A brand-new adult insect crawls out of its pupal case compressed and wet. To fly away, it must expand its wings and dry itself off. An insect emerging from its pupa or final nymph stage will instinctively crawl upward to find a dry place; many crawl up plant stems or walls. In its chosen location, the insect swallows air and pumps its abdomen to circulate blood through the wing veins, stiffening the veins and blowing up the wings like inflatable life rafts. A small set of neurons in the insect’s thoracic and abdominal nervous systems releases hormones that control its behavior to make sure each step is executed in the correct order; insects that fail to complete these behaviors before their nervous systems release bursicon, the hormone that hardens the adult cuticle, are left with permanently shriveled wings. Following eclosion and wing expansion, the neurons in the thorax and abdomen that control the behavior and the release of hormones undergo programmed cell death, having reached the end of their useful life. At the same time, epidermal tissue in between the top and bottom wing surfaces also dies and is sucked out by the circulatory system, leaving a thin layer of cells to form the wing surface. Once the wings are inflated, dried, and hardened, the insect can flap them and fly away mere hours after it emerged from its pupa with brand-new appendages.

Wing Veins In between the thin membranes, the wing is supported by a set of wing veins. These contain tracheae (air tubes), blood vessels, and nerve cells. The veins can be species specific and are often one of the characteristics that allow taxonomists to tell closely related species apart. The veins are crucial to the biomechanical properties of the wings and can help protect the wings from damage. Typically, wings have a thick vein on the anterior edge that provides significant spanwise stiffness, and a series of smaller and more flexible veins that branch from the base, providing structure to the wing but allowing for chordwise bending and twisting. The nerve cells in the veins are connected to sensors that help the insect detect wing bends and twists.

Gaining and Losing Wings By evolving wings, insects gained access to a third dimension. Some insects were able to increase their speeds as well, with insects like hover flies attaining speeds of nearly 20 mph (32 km/h) in the air. Suddenly, this new mode of behavior opened up changes in lifestyles for many insects, providing several advantages over their ground-based relatives. Though there are many adaptive advantages to flight, numerous species in many insect orders have lost or severely reduced the size of their wings. Others still have lost their wings, only to re-evolve them at a later date. What did insects gain with their wings, and why would they give them up?

Advantages of Wings When insects flew, they gained a new means of evading predators. Takeoffs in some flies can be extremely rapid, taking the fly from standing to fully airborne in a handful of milliseconds. A jumping escape from a predator on the surface might suffice (and, indeed, insects can make a directed jump away from a looming predator in tens of milliseconds), but the evolution of wings permitted insects to move farther away from attackers, and in an unpredictable way: gravity forces jumping prey to return to the ground, but flying prey can choose a new path by steering their wings. Many insects were thus able to avoid 138

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predation by taking to the air. In turn, flying prey insects forced their insect predators to fly as well if they wanted to continue to use winged insects as a food source. For hunting insects like dragonflies and robber flies, flight allowed them to chase and consume small flying insects that would be unavailable to wingless predators. Flight also expanded the foraging range for herbivorous insects, giving them larger areas in which to search for food and a means to do so more efficiently. Even for insects with smaller ranges, wings can provide important advantages. A wingless insect foraging for fruit in a tree, crawling to a distal branch to feed, would need to crawl

↙ Robber flies hunt prey in the

↓ Wings enable honey bees to gather

air, returning to a roosting site

nectar and pollen from flowers up to

to consume it.

several miles away from their hives.

back to the trunk before exploring another branch. The wingless insect could fall to a lower branch to explore it, but only at considerable risk of falling all the way to the ground. A flying insect could easily flit from one fruit to another, collecting more energy and nutrients in a shorter period of time. By increasing the physical size of their range, insects also opened new niches for themselves, allowing them to specialize on specific food sources or live in places that would be inaccessible to crawling insects. They were able to avoid competing with nonflying insects and other animals, and by dispersing more widely, avoided depleting resources in their habitat.

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139

Loss of Wings The evolution of wings occurred once, but loss of wings has occurred thousands of times. In some groups, the wings remain, but the energetically costly flight muscles are absent, rendering the insect winged but flightless. Still other groups show sexual dimorphism in wings and flight: in the order Strepsiptera (twisted-wing parasites), only males make the short mating flights to find the wingless females in their wasp hosts, and in some parasitic wasps in the family Braconidae, one sex has lost wings while the other retains them. Why would insects give up the power of flight? There are multiple forces that lead to wing loss. In extreme habitats, wings can come with high costs. For the midges that populate Antarctica, severe winds ↓ Some parasitic flies, like this nycteribiid, have lost their wings and remain attached to their mammalian hosts, rather than flying.

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make flying impossible, and involuntary sailing inevitable. Simultaneously, the wings provide a large surface for water loss, and the Antarctic midge needs to retain as much water as possible in its dry climate. For insects, wing losses are likelier when their habitat is more stable, even if it is a hostile one like Antarctica. A stable climate is one from which future generations of insects would not need to disperse, once they are adapted to its harsh conditions. For other insects, the wings are simply not used for life under bark or rocks: many species of earwigs, beetles, and crickets have lost their wings. For example, some species of carrion beetles are wingless, and feed exclusively on decaying animals on the ground. Some parasitic insects, including lice and some flies, have lost wings to stay attached to their hosts. In social insects like ants and termites, wing losses occur when flight is no longer a part of the insect’s job description, and allows them to pack more tightly in their nest. For many insects, despite flight’s advantages, their specific habitats drive them to give it up.

Re-Evolving the Wings Stick insects (order Phasmatodea) had a common ancestor with wings, but a majority of the 3,000 species of stick insects have lost their wings, and some have regained them. How do wing losses, and reversal of this loss in re-evolution of the wings, occur? Re-evolving wings after a loss seems unlikely at first. Once a wing is lost, the genes responsible for patterning the wings would accumulate random mutations over generations, and because they are unused, there would be no natural selection against these random mutations. Thus, the genes that correctly pattern the wings could be lost or degraded, and the population would only have individuals with mutated genes, unable to develop the wings properly. However, we now know that wings are not a singular appendage, but derive from two structures of the thorax, the notum and the pleuron. Even when the wings are absent, insects experience significant natural selection on these structures—a mutated notum or pleuron could prevent an insect from walking, eating, or mating correctly. The selection on these body parts would eliminate random mutations from spreading, even when the wings are absent. Some animal structures, like eyes, are patterned by a “master control gene” that determines the location of the structure and directs the expression of other genes. It is possible that such a gene controls the development of wings, and could explain why re-evolving wings is so easy that the stick insects alone have done so at least four times.

↑ Stick insects have lost and re-evolved wings multiple times. ← A wingless stick insect, Phobaeticus serratipes.

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141

↑ Pronotal structures on treehoppers

Dorsal ornaments: Treehoppers (family

are made up of wing-like organs, but

Membracidae) show a large diversity of dorsal ornaments on the first segment of the thorax, the pronotum. Microscopy reveals that the ornaments are actually connected to the notum by a stiff joint, much like a wing, and the genes expressed in the ornamental tissue as it develops are similar to the genes expressed in developing wings. Thus, treehoppers took advantage of an existing developmental pathway, used in the other segments to make wings, and evolved novel structures on their first thoracic segment. These then took on numerous shapes, in part because they were free from the selective pressure that acts on wings to maintain their functionality.

are not used for flight. They can take on a wide variety of shapes.

Modifying the Wings Insect bodies are arranged in segments. As the insect develops, those segments take on their specific identities by turning controller genes on or off to express different proteins and thus develop different shapes. Though the insect has the same genes in every cell, cells in the head will express different proteins from cells in the thorax, allowing each segment to do its specific task. Controller genes called homeobox genes are responsible for turning genes on and off to create the identity of each segment. The three segments of the thorax are each capable of developing a pair of wings and a pair of legs, but over the course of evolution, different insect groups have modified these appendages for their specific lifestyles. 142

WINGS AND FLIGHT

Elytra: In some of the most successful insects, beetles, the mesothoracic (middle) wings are thickened and hardened into a pair of elytra, and the membranous flying wings on the third thoracic segment are folded underneath them. Some of the change in these wings is mediated by a homeobox gene called ultrabithorax, which controls the expression of other genes and can be turned on and off

during development. Mutating the ultrabithorax gene in the flour beetle Tribolium results in a second pair of elytra on the metathorax, replacing the membranous wing.

Halteres: In another group of successful insects, flies, the metathoracic wings are modified into a pair of clubshaped organs known as halteres. The halteres do not generate any significant lift, but rather they experience inertial forces and are used as sensory organs to steer a fly’s wings and head. Essentially, halteres are made from condensed wing veins without any interspersed membrane. Modifying the ultrabithorax gene in the fruit fly Drosophila results in a second pair of membranous wings. Other genetic evidence shows that ultrabithorax’s role in development of the halteres is to suppress genes that form the wings and activate specific haltere-forming genes. In this way, the flies changed the developmental program that typically results in a membranous wing and created a haltere on their third thoracic segment instead.

↑↑ Hard elytra provide protection for

↑ The rear wing of a true fly is

beetles, helping them to become one

reduced to a sensory organ called

of the most successful groups of

a haltere, visible here as the light

insects on the planet.

yellow, paddle-shaped rod.

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143

Flight Once an adult insect emerges from its final molt, it cannot change the shape of its wings. And unlike a bat or a bird, it cannot use muscles in its wings to adjust their shape or movement because insect wings lack intrinsic muscles. To fly, the insect can only control some aspects of the movement of the wings, using muscles of its thorax: the wingbeat frequency, the stroke amplitude, and the angle of attack of the wing blade. All insect flight maneuvers stem from a combination of these three variables.

Flight Styles and Movement For hovering flight, which is defined as flight without any translational velocity, all of the necessary forces for remaining aloft and stable must be generated by the wings. This works best in smaller insects, because the ratio of wing area to body mass is generally greater, and this may explain some of the diversity of tiny flies and wasps. Hovering flight has also shaped the coevolution of insects and plants, as many pollinating insects can inspect flowers while hovering and visit higher numbers of flowers, improving their efficiency as pollinators.

→ A monarch butterfly. Gliding uses less energy than flapping and is a more efficient mode of flight.

Flight Power Muscles in Odonata and Diptera Dragonflies and damselflies (Odonata) flap and steer their wings using direct flight muscles.

Depressor muscles pull the wings down. Elevator muscles pull the wings up.

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WINGS AND FLIGHT

Gliding flight may have been a means for early winged insects to “fly” with or without primitive wings. Indeed, gliding behavior is observed in wingless insects like bristletails (order Archeognatha), which use their filamentous appendages to land on the trunk when they fall out of trees. Few extant insects retain gliding behavior, however. Gliding does not require much energy, but it is relatively slow and uncontrolled. Those insects that do glide are often unpalatable to predators, like some species of butterflies, including monarchs, or have unique flight muscles that allow them to rapidly stop gliding and switch to a faster escape, like dragonflies.

↑ Insect wings can flap hundreds of times per second.

Flight Muscles Birds and bats flap their wings by contracting muscles that extend from the thorax into the wing itself, the same way that humans can extend and wave their arms. In basal insects, muscles that power flight do attach to structures at the wing’s base, but in many derived insects, including some of the best fliers, muscles deform the thorax rather than moving the wing itself.

Direct and Indirect Flight Muscles In most insects, two sets of large muscles fill the majority of the thoracic cavity. The dorsal longitudinal muscles connect the anterior and posterior parts of the thorax (front to back), and the dorsal ventral muscles connect the notum to the interior notum (top to bottom). By alternately contracting and relaxing these large muscles, known as the indirect flight muscles, the insect can squeeze and stretch its thorax, pulling the wings up and pushing them back down. This arrangement of muscles and thorax allows insects to flap the wings rapidly, but it limits their movements somewhat. The two forewings always flap together, and in flies, the halteres (modified hindwings) always flap antiphase with the wings. When scientists mechanically lifted one wing of a dead fly, the wing on the opposite side also lifted, and the halteres (the reduced hindwings found in flies) moved down. This experiment showed that the passive physical properties of the fly’s thorax, and not specific patterns of neural or muscle activity, coordinate much of the movement of the flight appendages. Whereas the indirect flight muscles power the wing’s flapping, a set of direct flight muscles, attached to structures at the wing base, provide aerodynamic control.

Other insects, including the fly (Diptera) shown here, use

Dorsal-longitudinal muscles

indirect flight muscles to oscillate the thorax to flap their

contract to push the notum up,

wings and a small set of direct muscles to steer them.

lowering the wings.

Dorsal-ventral muscles contract to pull the notum down, which raises the wings.

FLIGHT

145

↑ Dragonflies can steer all four wings independently, a unique characteristic of their order (Odonata).

Steering The thorax motions driven by the indirect flight muscles provide only the large flapping motion that generates the insect’s lift. To steer, the insect needs to adjust the amplitude of the wing, as well as its angle of attack. Insects accomplish this with a small set of muscles attached to sclerites, tiny pieces of cuticle near the wing hinge. These muscles tug on the sclerites to change the biomechanics of the wing hinge and adjust the angle of attack, and the firing phase of these muscles relative to each other and relative to the wing’s sensory inputs determines how the wings are steered. The rapid, precise steering movements in hovering flies like Drosophila are accomplished with only 12 pairs of these muscles. Because insect flight is inherently unstable, these muscles constantly work to accomplish both straight flight and complex turns.

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Independent Control of the Wings The only group showing an exception to the muscle arrangement described above is the Odonata, dragonflies and damselflies. Dragonflies are superb aerial predators, with a 97 percent accuracy rate when catching flying prey. In these insects, each wing is independently controlled by a set of direct flight muscles, allowing them to move the wings separately. This flexibility gives dragonflies unique control over their flight, permitting both hovering and gliding, along with rapid turning. Moving the wings out of phase with each other allows the dragonfly to hover with the minimal amount of power, and moving them in phase provides acceleration necessary for takeoff.

Activating the Flight Muscles In most animals, a firing event in a motor neuron (a neuron that connects to, and stimulates, a muscle) causes a single contraction in the muscle. This one-to-one activation method makes it easy for the nervous system to finely control muscle contractions, and a simple circuit of neurons firing in an alternating pattern can form a rhythmic motor output that can drive flight in, for example, a locust. Though sensory information from wing stretch receptors can modulate the wing frequency, the rhythmic pattern can continue on its own without any input from sensors or the rest of the nervous system.

For some insects, however, generating sufficient lift requires the wings to flap faster than the nervous system can properly generate action potentials. The muscles that contract to deform the thorax must do so very rapidly, sometimes more than 400 times per second. These insects with faster wingbeats power their flight with “asynchronous” muscles. In asynchronous muscles, the stretching of the muscle itself activates the next contraction without further impulses from the nerve, allowing the muscle to oscillate and produce rhythmic activity. Asynchronous muscle is an adaptation that is only found in insects, but it occurs in about three-quarters of known insect species. Though it is almost always used for flight, asynchronous muscle can power other behaviors. In cicadas, an asynchronous muscle bends a springlike piece of cuticle, which rebounds and stretches the muscle to start another contraction much more rapidly than a neuron could.

↓ While some insects can take off in a split second, it can take a ladybug time to get off the ground. It lifts up its elytra to allow the wings beneath to unfold.

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147

Energetics of Flight Flying is roughly 50 times more energetically costly than other forms of locomotion. What fuels the highenergy flight of insects? The ratio of carbon dioxide released relative to the oxygen taken in indicates that most insects are burning carbohydrates, and in particular an insect-specific sugar molecule called trehalose, to power their flight.

during flight than at rest, a ratio higher than necessary for any other form of arthropod locomotion and higher than the ratio required for vertebrate flight. In bats and birds, oxygen consumption changes as forward flight speed increases, but for bumble bees, oxygen needs remain constant over a range of flight speeds.

Flight and Migration

↑ Locust swarms are damaging and dangerous to human populations. The metabolism of locusts has been intensively studied.

Insects that fly for long periods, such as migratory locusts, switch from burning carbohydrate to burning lipids from the fat body after about 30 minutes, and many insects with slower wingbeats, such as butterflies and moths, also burn fat in flight. Specific hormones that are released during flight, such as octopamine (the insects’ equivalent of norepinephrine), help to regulate the energy supply. Flying also takes a significant amount of oxygen, which is supplied by the network of tracheae and tracheoles, the large and small tubes that connect to the spiracles and allow an insect to exchange gases. Because insect hemolymph does not carry oxygen, each cell in the insect’s body must be within diffusion distance of a tracheole, which limits insect body size. Large fossil dragonflies with wingspans of more than 12 in (30 cm) date from parts of the Paleozoic era, when oxygen concentrations were higher. Flying insects require 50 to 200 times more oxygen

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Migration is an extreme form of self-protection in which animals can permanently escape from specific predators or adverse conditions, like a drought or an oncoming winter. In doing so, they can change their habitat to better suit their survival. For insects, the only way to migrate effectively is to fly. Migration is observed in many insect groups, including dragonflies, butterflies, beetles, and flies, and the migrations of large groups can have significant ecological effects for both good and ill: migrating hover flies, for example, provide pollination and biocontrol as their predatory larvae eat agricultural pests, but migratory locusts devastate crops and cause massive economic damage. Some insects live without wings for most of their lives and develop wings only for migration. Aphids are parthenogenic and thus are genetically identical, but wingless females can produce winged offspring when the host plant becomes overcrowded. Simulating overcrowding by putting female insects in a confined space also makes them produce winged offspring, suggesting that touch signals on the aphid’s body are the cue for this reproductive change. In addition to the wings, the migratory offspring also possess a full set of thoracic muscles for flight as well as more developed sensory organs (ocelli, larger compound eyes, and specialized antennae), all of which help them to disperse. In locusts, the flight muscles function differently when the locust is in its migratory state, allowing them to sustain flight for longer than their solitary counterparts.

Monarch Butterfly Migration Monarchs can migrate more than 2,500 miles

larger wings are correlated with increased migration

(4,000 km). In monarch butterflies, migration is

distance. Monarchs store lipids before they migrate,

an ancestral trait, but some populations of monarchs

but they mostly use the lipids on overwintering, not

do not migrate. Their genomes show variations in

on the flight itself. Butterflies glide on southerly winds

many genes associated with flight musculature and

when they are available and often rest when they are

muscle efficiency. Within migratory populations,

not, rather than flapping the entire way. CANADA

KY

C RO

Pacific Ocean

S

N TAI

UN MO

WINTER

Atlantic Ocean

Monarch Butterfly: Fall & Spring Migrations Fall migration Spring migration

MEXICO

Gulf of Mexico

Summer breeding areas Spring breeding areas

NON-MIGRATORY POPULATION

Overwintering areas Non-migratory population High monarch production WINTER

← Aphids can develop into wingless and winged forms depending on their environment.

NEXT PAGE Migrating monarchs. It takes three to four generations of butterflies to complete the journey.

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149

Beyond Flight Insect wings have evolved to serve numerous other purposes beyond flight, and insects use them to sense their environment, hide from predators, and impress potential mates. By changing the wings’ color or shape, or by moving them in specific ways, insects can use their wings to entice mates or scare and confuse their enemies.

Visual Wing Displays for Courtship Many insects use bright colors or high-contrast patterns on their wings to attract mates. In addition to attracting attention, colorful patterns can help closely related species tell each other apart and avoid mating with the wrong species. Butterfly patterns are made from scales, which are nonliving secretions of epithelial cells on the wing surface. These scales are pigmented according to the expression of specific genes, which direct the production of pigments and other compounds. Some of these patterns are created by scales that reflect only ultraviolet light, making them invisible to humans but entirely visible to other butterflies. Indeed, many butterflies that appear identical to humans show unique patterns that are only revealed when the ultraviolet (UV) reflections are visible. In an example, two related species of sulfur butterflies, Colias eurytheme and Colias philodice, use UV patterns and other mechanisms to avoid heterospecific mating. In some species of the

butterfly genus Heliconius, the wings reflect patterns of polarized light, also not visible by humans. These patterns allow two closely related species, H. cydno and H. melpomene malleti, to discriminate between conspecifics and others. While butterflies and moths are known for their colorful wings, other insects with patterned wings create kinetic visual displays. These wing movements are distinct from flight in their amplitudes, directions, and frequencies, and most of them occur at a much lower frequency than that used for flight. Flies in multiple families (including Sepsidae, Tephritidae, Ulidiidae, Dolichopodidae, and Drosophilidae) have patterned wings and wave them in the presence of conspecifics, presumably as a prelude to mating. Though these behaviors are conspicuous and occur almost exclusively in flies with colored patterns on their wings, it remains unclear exactly what the movements are intended to do: both males and females wave their wings, and sometimes the flies make the same wing-movement displays to the same sex, perhaps in a show of aggression. Further, it is unknown whether these displays make mating more likely or successful. However, wing-waving remains rare in species without patterned wings, and even species with very sparse patterns, like a single dark spot, still wave their wings. This suggests that the combination of wing patterns and movement are important to the flies’ life history, and also indicates that the purpose of the wing movements is visual signaling and not dispersing scents or creating mechanical vibrations. ← Ultraviolet patterns used by Colias eurytheme (top) and Colias philodice (bottom) to signal their species and sex to potential mates. These patterns are essential for the male’s mating success.

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↑↙ Flies with patterns on their wings will wave their wings at other flies, potentially as a courtship signal. From top to bottom: a picture-winged fly of the Tephritidae family; a Urophora cardui, or thistle gall fly, also of the Tephritidae family; and a long-legged fly of the Dolichopodidae family.

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153

← Mosquito antennae are exquisitely sensitive to tiny vibrations caused by the wingbeats of other mosquitoes.

periods between bursts, and the frequency, are species specific, and females detect them using mechanosensors in their antennae. In species of Drosophila other than the laboratory model D. melanogaster, the song can be more variable and can evolve rapidly. Though some species of insects perform specialized wing movements to impress the opposite sex, others take advantage of their prospective mates’ wing movements to locate them. The antennae of male mosquitoes are biomechanically tuned to vibrate at the same frequency as the female mosquito’s wingbeat, allowing them to locate and navigate toward potential mates. Though it is typically males that search for females and not vice-versa, the antennae of both sexes are exquisitely sensitive to winginduced air movements at nanometer scale.

Mechanical Wing Signals for Mating Insects without pigmentation in their wings may also wave their wings for courtship purposes, but they are creating mechanical, and not visual, signals. In many fly species, including the model organism Drosophila melanogaster, males initiate courtship with females by extending their wings and vibrating them at low amplitude and high frequency. The specific patterns of wing movements, including the length of the bursts of movement, the

Camouflage, Crypsis, and Mimicry Many insects effectively use the wings to trick predators into falsely identifying them. They can do this in two different ways: they can camouflage themselves, by blending into the background or patterning themselves after a non-threatening object like a leaf; or they can mimic a dangerous animal, including the predator itself.

Fruit Fly Courtship Song Olfactory/visual cues

Mechano-/ chemosensory cues

→

A

C

Mechano-/ chemosensory cues

→

E

→

B

G F

D B: Orientation

C: Tapping

D: Wing extension and courtship song

Drosophila courtship is a complex behavior controlled by a specific set of neurons. The specific amplitude and frequency of the wing movements are species specific.

E: Licking

F: Attempted copulation

G: Copulation/ sperm transfer

Camouflage The object-mimicking insects are from diverse taxa, including the katydids and grasshoppers, the stick insects, the butterflies and moths, and the mantises. While the former taxa mimic the stems and foliage of the plant to hide from predators, the mantises also mimic flowers and prey on pollinators of that flower, a tactic known as aggressive mimicry. In many of these mimics, the insect is not attempting to be invisible, but is rather conspicuously pretending to be something it is not. Some insect wings contain features of remarkably realistic leaves, including chew marks, brown spots, and leaf veins. Some Lepidoptera, including many larvae, but also the adults of the genus Eudryas, resemble the droppings of birds. In other insects, invisibility comes from patterning the wing to look like the background. Moths even use patterns of dark and light markings to create three-dimensional camouflage, disrupting their predators’ ability to detect edges and distinguish objects and making themselves very difficult to find.

↑ Insects such as this katydid mimic leaves as a means to evade detection by predators. → Other insects, such as this grasshopper, camouflage themselves to remain hidden against a natural background.

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155

↑ Bright circles on a moth’s wings can aid in predator evasion.

Intimidation Though crypsis (invisibility) is a reliable strategy, many insects take an opposite approach and attempt to intimidate their predators. In some insects, most conspicuously in butterflies and moths, the wings possess large, high-contrast circles, often surrounded by a highcontrast ring. These spots bear a dramatic resemblance to the large eyes of vertebrate predators, like owls. There are three hypotheses about why insects might possess these eyespots. First, the spots may trick predators into reacting to the insect as they would to a large vertebrate, scaring them off. Second, the spots may simply be a surprise, startling the predator away without evoking any resemblance to vertebrate eyes. Third, the spots may confuse the predator about the insect’s orientation: a bird 156

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attempting to grab the insect’s head may lunge at its wing instead, perhaps damaging the wing but allowing the insect to survive the encounter. Evidence supporting and refuting all of these hypotheses exists, and it is not clear that predators on these insects perceive the spots as eyes the same way humans do. Nevertheless, the sudden display of a pair of “eyes” from an otherwise drab moth is an exciting phenomenon, and buys the moth a few seconds to escape.

Predator Mimicry In an even more specific display of mimicry, some insects attempt to mimic their predator. Predator mimicry is relatively rare, with a few examples among birds (owls with feather tufts that mimic mammalian predators) and fish, but several insects use their wings to engage in it. There are diverse species, including planthoppers (Derbidae), metalmark moths (Choreutidae), and fruit flies (Tephritidae), that take this approach to mimicry, and they all mimic the same predators: jumping spiders in the family Salticidae. Each group has patterns on the wing that allow the insects to resemble a jumping spider, and

evidence suggests that jumping spiders are less likely to prey on these insects. Indeed, in an experiment that replaced a fruit fly’s patterned wings with the plain wings of a donor house fly, jumping spiders were significantly more likely to attack the plain-winged flies than their patterned conspecifics. Jumping spiders have keen vision, as sharp as a domestic cat’s, and they perform ritualized behaviors for mating and defending territory. An insect that successfully mimics a jumping spider could be met with a complicated territorial or mating display, rather than a rapid attack. In this way, the intricate patterns on the wings help the insects survive their interaction with these sharp-eyed predators.

↓ Many tephritid flies have two to four dark “jumping spider eyes” on the thorax. Together with bold wing markings, they resemble the legs of a crouching spider when viewed in certain directions.

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Wings as Protection

Alarm Bells and Armor

In the most successful order of insects, the beetles (Coleoptera), the forewings are hardened into structures known as elytra. Similar protective adaptations of the forewings are found in some members of other orders, such as true bugs, locusts, and earwigs. The elytra are able to protect beetles from predation in multiple ways. As described above, they can camouflage the beetle against being seen by predators. They can warn predators of a distasteful beetle with bright colors, and they can physically block the predator from biting the beetle’s squishy abdomen.

Lady beetles (family Coccinelidae) contain chemicals in their bodies that are poisonous to potential predators, and they color their elytra red in a warning. Other beetle species, including the weevil Pachyrhynchus and the burying beetle Nicrophorous possess bright spots on dark elytra, advertising their bad taste to potential predators. By hardening their wings, beetles defend themselves from attacks by predators. For example, wolf spiders are able to attack Tribolium beetles with their elytra intact, but are far more likely to kill the beetles when scientists remove their elytra, demonstrating that they provide protection from predators. The strength of the elytra is even further increased in flightless beetles with thickened, fused elytra. In an extreme example, the ironclad beetle Phloeodes diabolicus and its relatives have elytra so strong that they can survive being run over by cars. Though insects evolved wings to allow them to take to the skies, these beetles adapted their wings to take to the ground.

↓ Ironclad beetles can withstand large crushing forces.

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Environmental Challenges Elytra enable insects to insulate themselves from cold shock and prevent wing damage. They also prevent water loss, allowing some beetles to inhabit desert environments that are unavailable to other insects. Because an object’s surface area varies as the inverse exponent of its volume, a small insect has a much larger surface area relative to its volume than, for example, a large elephant. This makes insects especially susceptible to water loss, which can evaporate from the spiracles on the thorax and abdomen. Many beetles with thickened elytra are able to cover their spiracles with them and thus protect themselves from desiccation. In an extreme example, a flightless beetle with fused elytra uses its specialized forewings as a means of collecting water. Stenocara sp., a beetle living in the Namib desert, uses the specialized microstructure of its elytral surface to capture extremely small droplets of water on its elytra in the morning fog and collect them into a larger droplet that rolls toward its mouth. By protecting the beetles from the elements, the elytra have let them thrive under a broad variety of conditions. ↑↑ Desert beetles can use their elytra

↑ Many beetles, including ladybugs,

to collect water from the air.

have brightly colored elytra, warning predators of their bad taste.

BEYOND FLIGHT

159

Wings and the Nervous System Insect wings interact with the nervous system in two important ways. First, they act as sensory organs, sending signals about the air and about the insect’s body movements to the brain. Second, their movements are controlled by muscles that are driven by motor neurons. The wings are thus highly responsive structures that actively interact with the environment.

Wings as Sensory Structures

Campaniform Sensilla

Though the wings themselves contain no muscles, they are not passive airfoils. They are covered with sensory structures that monitor mechanical touch, wing deformation, and even tastes and smells in the air. These include sensory hairs, which are attached to neurons under the cuticle and stimulated by mechanical deflections, and campaniform sensilla, which are bell-shaped structures in the cuticle that detect forces as the wing bends or twists. Using the sensors on the wing, the insect can rapidly monitor environmental conditions during flight.

On the wings, campaniform sensilla occur occasionally along the wing veins and in several large clusters at the wing base. Sensory input to these structures occurs every time the wings flap and is used to control the movement of other appendages, including the ruddering abdomen —in experiments where the wings of a flying moth were mechanically stimulated, the abdomen moved in response to the stimulus. Input from wing campaniform sensilla is also used to structure the timing of contraction of flight and steering muscles. Campaniform sensilla appear in numerous locations on the insect body beyond the wings, including the legs and the head, but recordings of the neurons attached to them suggest that they can encode nearly any kind of force that bends them, and can do so extremely rapidly. Insects have evolved to develop campaniform sensilla in locations on the body where they need to monitor various forces, typically on moving parts or at joints. The campaniform sensilla on the wings can thus be used to detect flapping, bending, and twisting of the wing as the insect flies.

Sensory Hairs Sensory hairs on the wings provide at least two functions: mechanosensation and chemosensation. Though mechanosensory hairs are common on the wings of most insect species, little is known about which specific forces they may detect, because recording their activity is challenging. In the fruit fly Drosophila melanogaster, the mechanosensory hairs are interspersed with chemosensory hairs. These hairs are connected to neurons that express taste receptor proteins, and though their exact function is unknown, their presence suggests that flies may “sniff” the air with their wings.

Moth Wing Sensors

Dorsal Each yellow circle indicates the position of a campaniform sensillum (shown magnified in images shown here).

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Ventral

Dragonfly Wing Sensors

The wings of insects are not passive airfoils but possess many sensory structures. Dragonfly wings have sensory hairs and other structures, and the wings of other insects possess campaniform sensilla that allow them to sense inertial and aerodynamic forces during flight.

Chordotonal Organ At the base of the wing, a specialized structure underneath the cuticle called a chordotonal organ detects vibrations and stretching, allowing insects to monitor their wing movements. These organs appear in insects with synchronous flight muscles, and feedback from the chordotonal organ can be used to sense wing movements and guide future ones. (In insects with asynchronous muscles, the muscles contract in response to stretch, without feedback from wing proprioceptors). In some moths, the cuticle above the organ is thin and an air-filled trachea tube is nearby, making the chordotonal

organ sensitive to small vibrations in the air. These “ears” respond strongly to sounds in the night that are common and loud, but are inaudible to humans—the ultrasonic calls of echolocating bats, for example. When the chordotonal organ is stimulated by a bat call, a moth rapidly changes course or drops from the sky. Some moths even make their own noises in return, confusing the bat. By adapting a wing mechanosensory organ into a sensitive auditory apparatus, moths keep themselves safe from aerial predators.

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161

Fly Halteres and their Associated Sensory Structures

The halteres (hindwings of true flies) have several fields of campaniform sensilla at their bases. These allow flies to sense forces resulting from body movements. The arrangement of sensora varies according to family, as seen on the right.

162

The Role Halteres Play

Stability in Flight

Because the wings can act as sensors, they do two jobs simultaneously: passing sensory information to the nervous system, and providing lift and thrust. In one of the most successful orders of insects, the hindwings are relieved of the force-producing task, and are shrunken to a tiny club-shaped sensory structure that does not generate any lift. These are known as the halteres, and they are found almost exclusively, and nearly universally, in flies (Diptera). The only flies that do not possess halteres are those that have lost flight completely, like the mammalian parasites in the families Braulidae and Streblidae.

In the flying flies, the number of campaniform sensilla found at the base of the modified hindwing is greatly increased compared to those at the base of the front wing, the homologous structure. The campaniform sensilla at the base occur in distinct fields, and there can be up to about 300 campaniform sensilla on the haltere in addition to a large chordotonal organ under the haltere base. The haltere is oscillated just like the wing and provides wingbeat-synchronous sensory input to the nervous system. Without the halteres, flies are able to take off, but they cannot maintain stable flight and they invariably crash. What kind of information is transmitted by the large arrays of campaniform sensilla at the haltere base? There are two important functions for the haltere: it can act as a metronome, providing precise timing information about its oscillations, and it can act as a gyroscope, monitoring

WINGS AND FLIGHT

rotations of the body. Fly flight is inherently unstable, requiring flies to constantly correct for pitch rotations with each wing stroke, and random gusts of wind can knock flies off their straight flight course. When the fly is flying in a straight line, the arrays of campaniform sensilla are stimulated in a particular pattern. A sudden body rotation changes this pattern: the inertia of the oscillating haltere combines with the force of the rotation to result in a Coriolis force at the haltere’s base. This force deforms the surfaces of the campaniform sensilla array in a different pattern, stimulating different neurons at different times. The halteres send their signals to the motor neurons of the wings, and a change in the timing of their activity changes the tension on a muscle that steers the wings. Using the halteres, then, flies are able to react to body rotations in a few milliseconds, much faster than if they used vision to fly on a straight course. Flies also send haltere input to neck motor neurons, allowing them to stabilize their gaze as their bodies rotate.

↓ In males of the order Strepsiptera, the twisted-wing parasites, the forewings are modified into haltere-like structures that stabilize flight in the same way that true flies use their hindwing halteres.

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163

Insects Lacking Halteres The extreme success of the flies in terms of both their flight ability and the sheer number of unique species prompts a question: why do other insects lack halteres? If gyroscopic input is essential to stabilizing flight, how do other insects do it? First, the flight of insects like butterflies and moths is often slower and more stable, making it easier for vision and mechanoreception to control it. Second, Coriolis force information could come from any sensory organ that is oscillating in its own inertial frame, including legs, antennae, or the wings themselves. Indeed, the large hawk moth Manduca sexta uses mechanosensory input from its antenna to stabilize flight, and other insects are likely using the campaniform sensilla on their wings to provide a haltere-like function. In many wasps and bees and in some of the Lepidoptera, the hindwings are significantly smaller than the forewings, suggesting

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that these wings play a smaller lift-generating role, and possibly a larger sensory role, in the insect’s flight. In one interesting group of insects, the forewings have been modified into a haltere-like structure. The males of the twisted-wing parasites (order Strepsiptera) have tiny club-shaped forewings (females are wingless) with a cluster of campaniform sensilla at the base, and their removal prevents them from stabilizing their gaze when their bodies are rotated in tethered flight. Evidence suggests that the Strepsiptera are more closely related to beetles than to flies, indicating convergent evolution of the haltere-like wing shrinkage. Though there are a handful of other insects with very reduced wings, including some braconid wasps and beetles with shortened elytra, there are no current data about whether these wings may be used as stabilizing sensory structures.

←← In many wasps and bees, the hindwing is much smaller than the forewing, suggesting that its role in generating lift is also reduced. ← The antennae of the hawk moth Manduca sexta provide mechanosensory information that stabilizes the moth’s flight, much like a haltere does for a fly.

Speedy Takeoff A subset of recently derived flies, the Calyptratae (including some very successful human-associated flies like house flies, blow flies, and flesh flies) use their halteres to stabilize rapid takeoffs, potentially protecting them from fast predators. When the halteres are removed from these flies, their takeoffs are slower and less stable. Similarly, the same flies also use the halteres to sense gravity falls while standing—flies normally adjust their bodies or take off when the surface on which they are standing begins to fall, but flies with their halteres removed do not.

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165

How the Brain Steers the Wings Guiding the wings as they flap and steer is a challenge for the insect nervous system. Many insects flap their wings at high frequencies, leaving only a handful of milliseconds for the insect to take in sensory input from the environment, send it along its neurons, and adjust the tension on the wing-steering muscles to change the wing’s angle of attack. What are the mechanisms by which insects control their wings during flight? Steering the wings is a complex process that requires sensory neurons to process information from the environment, motor neurons to change the tension on the muscles, and, in most circuits, interneurons to connect the sensory neurons with the motor neurons. Insects will change their wing-steering in response to many different kinds of stimuli, including attractive smells (fruit flies steer strongly toward a current of air scented with apple cider vinegar) or threatening visual signals (for example, locusts and other insects turn strongly away from a small dot that rapidly grows in size, because this is the signal that would result from an impending collision with an object).

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Insects will steer their wings when they experience optic flow, the movement of the entire visual surrounding, as would occur during the insect’s own movements through the world. Like humans, insects possess a strong optomotor response to optic flow. Wide-field visual motion, as occurs during self-motion or when looking through the window of a moving train, causes eye and head movements that minimize motion blur and stabilize the visual scene. In insects, wide-field motion of the surroundings leads to steering motions of the wing that attempt to stabilize it. As the visual scene moves to the left, a tethered fly increases the flapping amplitude of the right wing and decreases the amplitude of the left wing in an attempt to turn left and follow the visual movement. These wing-steering behaviors are controlled in part by a circuit of neurons that lead from the fly’s retina all the way to motor neurons that control the tension on the wing-steering muscles. These descending neurons were explored first in dragonflies and blow flies, and recently have been identified at the genetic level in fruit flies.

These cells receive input from cells in the fly’s brain called lobula plate tangential cells, which are visual neurons sensitive to wide-field motion, and send their output to motor centers near the fly’s wings. By activating different combinations of these neurons, different visual inputs result in different wing motions. In dragonflies, eight pairs of neurons, known as the target-selective descending neurons, each respond to small objects moving in different directions and in different parts of the fly’s visual field. Spiking activity in each of these neurons results in specific wing movements. By sending visual information to the wings, the dragonfly can steer its wings toward moving prey, potential mates, or invaders of its territory.

← Wide-field visual motion of the surroundings allows an insect to make wing movements that keep it stable as it steers to avoid small obstacles.

← Insects rely on a combination of sensory and motor neurons to help steer them toward a stimulus, such as food, and away from danger. → Flying insects constantly change their steering in response to the movement of the air around them.

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167

↑ The model organism Drosophila melanogaster has been instrumental to understanding how neurons drive specific wing movements.

The Neural Basis of Wing Movements for Courtship In addition to their role in flight, the wings of many insects are used for other behaviors, including the courtship and anti-predator displays discussed above. In fruit flies, males oscillate their wings in a pattern unique to their species. The insect nervous system consists of a brain in the head capsule that connects via a ventral nerve cord to a processing center called a ganglion in each body segment. Neurons project from each ganglion to bring in sensory information and send motor signals to appendages of that segment. For the brain to move an appendage, stimulating activity passes through cells of the ganglion. Evidence suggests that for crickets and fruit flies, commands for courtship songs originate in the brain, but specific movements of the wings are dictated by cells in the thoracic ganglion. A circuit of neurons in the thoracic ganglion acts as a central pattern generator. When

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WINGS AND FLIGHT

commanded by the neurons in the brain, the ganglion neurons work together to produce rhythmic output, and they do this without receiving rhythmic input. The rhythmic neural activity leads to rhythmic motions of the wings. Using the model organism Drosophila melanogaster, scientists have learned much about the ways in which neurons drive specific wing movements. Genetic tools allow researchers to activate or inactivate specific groups of neurons, and the resulting behaviors show which parts of the song require those groups of neurons. In other animals, surgically removing or electrically stimulating groups of neurons is invasive and imprecise, but the genetic tools developed in the fruit fly permit specific neural activations or inactivations and still allow the fly to behave, demonstrating how the manipulation of the neural circuit influences its behavior. These experiments still require careful interpretation: some manipulations have no effect, and others completely abolish the behavior. These do not necessarily mean that those neurons are unimportant or crucial, respectively; they may mean that those neurons influence the behavior in different contexts, or are essential for some other precondition of the behavior. However, genetically manipulating specific sets of neurons has permitted a mechanistic model of how the brain drives a complex behavior like courtship (see box).

← Crickets use a specific set of neurons to control the movement of their wings to produce courtship songs.

Fruitless Gene Expression In male fruit flies, neurons expressing the gene fruitless are needed for courtship behavior. If fruitless is not present, male flies can no longer vibrate their wings in song. This discovery led scientists to determine the roles of smaller sets of fruitless-expressing neurons by manipulating each neuron or group of neurons and observing whether the fly sang, how its song was structured, and Noncourting male fly

whether it successfully copulated.

↑ Inactivating these neurons renders the fly unable to sing.

Three kinds of neurons in the thoracic

↑

ganglion, a collection of neurons outside

Two kinds of neurons in the

of the brain, structure the song.

brain trigger the song.

↓

↓

Changing the activity of these neurons

Activating these neurons

changes the pattern of wing movement,

stimulates singing.

altering how many pulses per minute or cycles per pulse are present in the song. ↓ Courting male fly

WINGS AND THE NERVOUS SYSTEM

169

Chapter

4

Development, Metamorphosis, and Growth An insect spends the majority of its life developing inside an egg and growing into a short-lived adult whose primary goal is reproduction. Because it is constricted by an external skeleton, insect growth requires a different process than vertebrate progressive growth; like all other arthropods, insects need to molt in order to grow in size. Metamorphosis arose as a clever solution to integrate wings into the molting process, leading to an explosion of diversity, by allowing insects to conquer myriad habitats through the decoupling of juveniles and adults. From simple molting events to more complex variations of metamorphosis, insects not only evolved an incredible morphological diversity, but also developed a wide range of life history adaptations that allow them to be present almost everywhere on the planet.

← To form a chrysalis a caterpillar hangs upside down from a structure and sheds it outer layer of skin. Beneath lies its chrysalis, which hardens to protect the caterpillar as it metamorphoses, a process that takes 10 to 14 days.

171

An Insect Life Cycle Although we are most familiar with the appearance of its adult form, an insect spends most of its life going through developmental changes and very little time as an adult. For example, a cicada can spend 17 years as a juvenile buried under trees before emerging as an adult that lives for only a few days.

The life cycle of an insect comprises all of the developmental changes it undergoes, as well as how it reproduces during the adult stage. Depending on the species, life cycles can vary dramatically and are usually associated with the insect ecological niche. Therefore, they are extremely diverse in terms of length, types of life stages, reproductive strategies, and so on. This chapter considers the general aspects of an insect life cycle and presents some examples of insects with unique life histories.

Chromosomes and sex determination: In insects, chromosome types and number show great variation. This has an impact on how sex is determined and how individuals reproduce in different species. Males and females are determined by specific genetic systems and mechanisms involving sex chromosomes or other types of sex determination systems (see pages 174–179). Reproduction: While the majority of insects use sexual reproduction to produce offspring, some of them reproduce asexually. For example, some insects reproduce using parthenogenesis. In insects reproducing sexually, various mating strategies have evolved as a result of sexual selection (see pages 180–184). Egg laying: An insect’s survival depends on where and how it lays its eggs. Because of the various habitats in which insects live, egg-laying strategies are quite diverse, ranging from laying large numbers of eggs to retaining the eggs inside the mother until hatching (see pages 190–192).

Developing egg: Once fertilized, an egg is laid and the embryo inside needs to survive until hatching. The structure of an insect egg is adapted to the various environments it is subjected to (see pages 193–196). Embryos undergo various stages of development that are specific to insects (see pages 188–189).

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Juvenile stage: Insects spend the majority of their life cycles in the juvenile stage, as this is the feeding and growing stage. Depending on the type of metamorphosis, insects have either nymphal stages or larval stages (see pages 204–209).

Growth: Growth is a crucial process in an insect life cycle. However, because they possess an external skeleton, insects do not grow organs in the same way that vertebrates do. Overall body size is defined during the juvenile stage, and in holometabolous insects, adult traits are already present inside larvae as imaginal discs (see pages 222–223). Metamorphosis: In all insects, postembryonic development and metamorphosis are major parts of the life cycle. Some insects develop through successive molts with progressive wing growth, while other insects have juvenile stages that are completely different from the adult, and therefore they need to undergo drastic changes during metamorphosis. In a few insect groups, metamorphosis evolved as peculiar variations, largely due to their specific life histories (see pages 196–203).

Life cycle adaptation: An insect life cycle is highly adapted to its ecological niche. In some of these habitats, the environmental conditions can change unfavorably, such as changes in seasons. A slowdown or complete stop at any stage of the insect life is therefore necessary to increase survival (see pages 226–227).

Life Cycle of the Monarch Butterfly

Adult Emerging adult

Mating adults

Chrysalis (pupa)

Egg laying

Mature Caterpillar

Developing egg

Young Caterpillar

AN INSECT LIFE CYCLE

173

Chromosomes Most insects are diploid. Just like humans, the offspring inherits two sets of chromosomes, one from the mother and one from the father. The type and number of chromosomes, however, varies quite a lot across insect lineages, and this has consequences on the way they determine sex and reproduce, and how cell division takes place.

Types of Chromosomes During cell division, chromosomes are in pairs of chromatids (chromosomes first duplicate at the beginning of cell division, creating two identical chromatids). Until they are separated, both chromatids are usually attached together by a constriction called the centromere. The centromere contains proteins, the kinetochores, and this is where chromatids are pulled away from each other at the end of cell division. Having different types of chromosomes

carries consequences on how cells divide (mitosis) and produce gametes (meiosis) and therefore in their modes of reproduction and sex determination. Some insects carry holocentric chromosomes, where chromatids are tied together through their entire length. During cell division, the chromatids are separated at multiple points, allowing the chromosomes to tolerate better fragmentation under conditions such as radiations. In chromosomes with one centromere, in the presence of radiation, fragments are

Chromosome Structure in Insects

Centromere

Holocentromere

Monocentric

Acrocentric

Metacentric

Monocentric chromosomes

Holocentric chromosomes

Chromosomes with one centromere, as found in

Chromosomes do not always have a centromere. In the

humans, are called monocentric chromosomes.

absence of this constriction, chromatids are attached

If the centromere is right in the middle of the

by kinetochores spread throughout their length. These

chromosome length, it is a metacentric chromosome.

chromosomes are holocentric. Several insect orders

If it is located closer to one of the tips, the chromosome

carry holocentric chromosomes, including Hemiptera,

is acrocentric. Monocentric chromosomes are found

Odonata, Thysanoptera, Trichoptera, Ephemeroptera,

in major insect groups such as Blattodea, Diptera,

and Lepidoptera.

Orthoptera, Coleoptera, and Hymenoptera.

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Holocentric

DEVELOPMENT, METAMORPHOSIS, AND GROWTH

What is Inverted Meiosis? Metaphase

Anaphase

lost during cell division, while in holocentric chromosomes, fragmented chromosomes can still be separated and passed down to daughter cells. This ability in holocentric chromosomes may have allowed the evolution of more complex karyotypes.

Chromosome Number The average diploid chromosome number (2n) across insect orders varies quite a lot. For example, Lepidoptera has 30 chromosomes on average, while Diptera has 11. Additionally, the range of chromosome number within an insect order can vary. While chromosome number in flies varies from 6 to 26, Hemiptera has species carrying 4 to 192 chromosomes. This variation in number is likely explained by the presence of holocentric chromosomes in these insect orders.

Standard meiosis

Inverted meiosis

Insects with holocentric chromosomes undergo a nonconventional type of meiosis to create gametes: inverted meiosis. In normal meiosis, to create haploid gametes, two successive divisions are required. The first division separates chromosome homologs (one of each parent), and the second division separates sister chromatids (the identical copies from chromosome duplication). In inverted meiosis, the reverse process takes place: the first division separates sister chromatids, while the second division separates homologs. Inverted meiosis has been studied extensively in hemipteran species.

← Like all Hemiptera, the horned

↑ In stick insects (Phasmatodea) and

treehopper (Leptocentrus sp.) carries

related orders such as grasshoppers,

holocentric chromosomes.

praying mantises, and cockroaches, chromosomes are monocentric.

CHROMOSOMES

175

Sex Determination The way sex is determined is connected to chromosomes. In insects, the sex determination mechanism relies on genetic systems—the individual’s karyotype defines which sex they belong to. This is called genotypic sex determination. Most insect lineages use sex chromosomes to determine sex, among which is the XY system, similar to that of humans. However, there is a larger diversity of sex chromosomes in insects, as well as sex determination relying on other systems.

↑ Palpares sp., is an antlion of the order Neuroptera. Its sex is determined via male heterogamety. → Complex sex chromosones have evolved in different subspecies of the Ailanthus moth, Samia cynthia, which are called neo-chromosomes.

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Sex Chromosomes The majority of insect lineages rely on sex chromosomes as a genetic system. With this system, individuals carry two types of chromosomes: the autosomes, which are the numbered pairs of chromosomes, and the sex chromosomes, which is usually one pair of chromosomes that is assigned a letter. In sex chromosome systems, one of the sexes has two different letters (heteromorphic chromosome), and the other sex has the same letter (homomorphic chromosome).

Male heterogamety: In some insect species, the male carries heteromorphic chromosomes. Similar to humans, sex chromosomes are XX for females and XY for males. Male heterogamety is found in the majority of winged insects, especially in holometabolous insects, including Diptera, Coleoptera, and other smaller insect orders such as Strepsiptera and Neuroptera.

Female heterogamety: Heterogamety can also occur in females, with the female carrying heteromorphic chromosomes, although it is rare. In this case, insects carry Z and W sex chromosomes, where the female is defined by ZW and the male ZZ. Female heterogamy has evolved once in Lepidoptera and Trichoptera.

XY, ZW, and XO genetic systems

XY

M X

Complex sex chromosomes: A derivation of sex chromosome systems is sex determination based on complex sex chromosomes. Here, individuals carry more than two sex chromosomes and can be found in both XY and ZW systems. A simple explanation for the occurrence of a system like X1X2Y would be the fission of a single X. Fusion of sex chromosomes with autosomes creates other combinations. For example, when an ancestral X chromosome merges with one autosome, the resulting individual carries two Ys and one X. Complex sex chromosomes are mainly found in Lepidoptera.

F X

XX

XY

XY

Y

M

XX

XX

Male heterogamety

ZZ

M

F Z

ZW

M

Z

F W

ZZ ZZ

ZW

ZW

Female heterogamety

The XO–XX system: In the XO–XX system, males are determined with one X chromosome while females carry two Xs. In this instance, females produce one type of gamete, with one X, while males produce two types of gametes, one carrying one X and the other not carrying any sex chromosomes, but only autosomes. The XO–XX system is found in hemimetabolous insects mostly, such as crickets and grasshoppers (Orthoptera) or cockroaches (Blattodea).

F

XO

M

F

X

XO

XX

XX

X

XX

M

F X

XO–XX system Key M = meiosis / F = fertilization / Blue = male / Red = female

SEX DETERMINATION

177

XY Sex Determination Systems The fruit fly Drosophila melanogaster carries XY

Fruit fly

Humans

sex chromosomes. Although it appears similar to the human XY system, the genetic mechanism downstream that controls sex differentiation is completely different. While in humans, a gene

XX

XX

XY

factor on the Y chromosome regulates male-specific phenotype, in fruit flies, these genes are regulated not by the presence of the Y chromosome, but by the

Female

ratio of the X chromosomes compared to all the other XY

autosomes. For example, in females there are two X chromosomes, and as flies are diploid, the ratio of

Female

autosomes to X is 2X:2A (autosome). In males, however, trigger a cascade of genes that progresses to female differentiation. In contrast, the 1X:2A ratio will not

Male

Male

the ratio is 1X:2A. In females, having this ratio will

XXY

XXY

XO

set off this gene cascade and male differentiation will take place. This type of sex determination works like an on-off gene switch. Although the fruit

Female

fly sex determination mechanism has been studied extensively, new mechanisms are being discovered

XO

in other insect groups, showing again the multilevel diversity of insects.

Male Male

Haplodiploidy in Hymenoptera

nn

M

n

n

n

F

nn nn

n

Key M = meiosis / F = fertilization Blue = male / Red = female

→ Honey bees are among the best-known haplodiploid insects. While the queen and female workers are diploid and come from fertilized eggs, the drones (males) develop from unfertilized eggs.

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

n

Female

Other Systems Although most insect groups use sex chromosomes as their sex determination system, a few insect lineages have evolved other types of genetic systems, some of which rely on the number of chromosome sets, on chromosome condensation state, or even on additional fragments of chromosome acting like parasitic elements.

Haplodiploidy in Hymenoptera The majority of insects are diploid. They possess two sets of chromosomes, each from a male and a female. In these cases, males and females are usually determined from the sex chromosome system, similar to humans. In Hymenoptera (wasps, bees, ants) and Thysanoptera (thrips), sex is determined with the haplodiploid system: the number of sets of chromosomes determines which sex an individual will be. On one hand, females develop from fertilized eggs so they are diploid. On the other hand, males come from unfertilized eggs and are therefore haploid, as they only carry the set of maternal chromosomes.

Paternal-Sex-Ratio In some haplodiploid wasp species, even fertilized diploid females can change to haploid males. The paternal-sex-ratio (PSR) chromosomes are small fragments of chromosomes that can be found in haplodiploid arthropod populations. These chromosomes are transmitted through the sperm and, early in development, eliminate the paternal chromosomes of a fertilized egg destined to become a female. Therefore, an embryo coming from a fertilized egg but carrying PSR becomes haploid and male. PSR is a selfish parasitic fragment of DNA considered to be responsible for changes in sex ratio in insect populations. It has been found and largely studied in the jewel wasp Nasonia vitripennis.

Female

Male

x

Unfertilized (~20%)

Fertilized (~80%)

n

Wild type

n

Male

Female

n

PSR+

n

Paternal Genome Elimination In a number of scale insect species (Hemiptera), sex is determined by a variation of haplodiploidy. Instead of being haploid (only carrying one set of chromosomes), scale insect males are diploid, but functionally haploid. During embryogenesis, the paternal genome of a future male becomes highly condensed and all the genes are therefore inactivated, while in future females, both paternal and maternal genes stay active. This system was coined paternal genome elimination since, later on, males are only able to produce sperm containing the maternal genome. In fact, the paternal chromosomes that were condensed during their embryonic life are eliminated during spermatogenesis.

n

Male

n

Endosymbiont Influence Feminization of males can also happen with the presence of endosymbionts. This phenomenon happens when bacteria are transmitted by the mother. In fact, since females are necessary for these bacteria to survive, producing males threatens their survival. This is why some bacteria living in insects have the capacity to change their sex. Wolbachia is a bacteria genus commonly found in insects. In the butterfly species Eurema mandarina, with a ZW sex chromosome system (ZW = female and ZZ = male), female-looking individuals were found to carry ZZ chromosomes instead of ZW. These females carry a strain of Wolbachia that feminizes genotypic males during larval development. Another example was found in the leafhopper Zyginidia pullula with an XO sex chromosome system. When infected with Wolbachia, females produce exclusively female offspring. However, not all of them carry XX as it should be. Some females are XO.

n

Male

← Nasonia vitripennis embryos carrying the PSR chromosome fragment, although fertilized, will become haploid by eliminating the paternal DNA.

Paternal Genome Elimination

nn

n

M

F

n

n

F

nn

nn

nn

PGE

n

n

Key M = meiosis / F = fertilization / Blue = male / Red = female

← In addition to displaying sexual dimorphism, mealybugs like Pseudococcus longispinus determine sex through paternal genome elimination.

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Modes of Reproduction How insects reproduce varies according to their life history and the environment in which they live. Although the majority of insects use sexual reproduction to produce offspring, a number of species have evolved several times the ability to use asexual reproduction, or a combination of both.

Sexual Reproduction Most insects reproduce sexually. A male gamete (spermatozoon) has to fertilize a female gamete (oocyte) to produce an embryo. In insects, fertilization of the oocyte almost always takes place inside the female and various copulation strategies have evolved for males to transfer sperm into the female. After mating, sperm is generally transferred in the female reproductive tract via a spermatophore, a sac containing the spermatozoa. Spermatozoa can also be stored by the female in an organ near her ovaries, called the spermatheca, for later fertilization. For example, in honey bees, the sperm stays alive in the queen’s spermatheca and eggs are fertilized as needed.

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Asexual Reproduction by Parthenogenesis Parthenogenesis, or the production of offspring without the need for oocyte fertilization, and therefore by only using the genome of females, has evolved several times in insects and can be separated broadly into two types:

Arrhenotoky: Both sexes are present, but not all individuals come from asexual reproduction. Males develop from unfertilized eggs, while females come from fertilized eggs. Arrhenotokous parthenogenesis is found in haplodiploid insects, including all Hymenoptera species.

→ The European blackcurrant aphid, Cryptomyzus galeopsidis, gives birth to live juveniles when reproducing through parthenogenesis. Live birth occurs in good environmental conditions and can be observed in greenhouses. ← Male katydids or bush crickets from the family Tettigoniidae produce a spermatophore containing a spermatophylax, a nutritious gift that will be eaten by the female after mating.

Thelytoky: Females are produced from unfertilized eggs, and males are absent. This type of parthenogenesis is not common and is generally found in insects that live in stable habitats with an abundance of resources. Thelytoky allows populations to grow faster than sexually reproducing species. The downside, however, lies in the lack of genetic variability, which can be detrimental if the environmental conditions are to change suddenly.

Alternating Between Sexual and Asexual Reproduction Although reproducing asexually is easier, as it does not require the involvement of males, the lack of genetic variation can become a problem when environmental conditions change. Aphids have found a way to circumvent this problem: they use both sexual and asexual reproduction. The switch between these two modes of reproduction is controlled by environmental conditions, and in aphids by temperature and the amount of daylight. During the summer, when conditions are optimal, aphids produce offspring via parthenogenesis. When winter comes, a switch to sexual reproduction occurs. With temperatures falling and daylight decreasing, winged males are produced to mate with females. Genetic variation can be maintained for adverse or unpredictable conditions and therefore better adaptation.

Hermaphrodism Hermaphrodism is extremely rare in insects. To date, only three insect species belonging to the same Hemiptera tribe were observed to be hermaphrodites. Among them is the cottony cushion scale insect, Icerya purchasi. Anatomically, the female-looking individuals (wingless and juvenile-like) possess an organ called an ovitestis, which produces both eggs and sperm, and a spermatheca that allows sperm storage. The sperm that is produced by the hermaphrodites fertilizes eggs inside the same individual. In this species, however, winged males are also present, and despite their rarity occurring, they can also mate with the hermaphrodite, although it is not its primary mode of reproduction.

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181

Mating Most insects use internal fertilization as a mode of reproduction. Mating between a male and a female is therefore an essential part of the life cycle. Pre-fertilization strategies can be divided into four steps: mate searching or attraction, courtship, copulation, and post-copulation.

Finding a Mate Insects use different types of signals to attract mates. They can be visual, auditory, or olfactory. Insects can smell, but instead of using a nose, they use their antennae to capture chemical signals such as pheromones. Females secrete sex pheromones that are detected by males with antennae bearing extremely developed sensory organs called sensilla. Moths are a beautiful example of highly specialized antennae that look like feathers. Different sex pheromones have been identified in moths but also in flies, bees, beetles, and other insects. Visual cues are also used by insects to attract mates. The best-known examples are found in fireflies. At dusk during the mating season, males flying in search of a female to mate with flash light from their abdomens. The light is created using a compound called luciferin. Intermittent

light flashing creates patterns while the male flies around. These patterns are unique to each species so that females can recognize the conspecific males. Once a female on the ground identifies a male that she is interested in, she flashes back at him to reveal her location. But visual signals can take other forms than light; they can be color patterns on the wings of butterflies. Finally, and probably the most conspicuous signal to humans are sound cues made by insects such as Orthoptera, Hemiptera, or Coleoptera. Various ways have evolved in insects to create patterned sound signals. Grasshoppers generate sound by stridulation (they rub a row of pegs on their hind legs to the edge of the forewings). A cicada emits sound by vibrating an organ on the side of its abdomen, called a tymbal. These sounds are in intricate patterns and highly specific to each species.

Firefly Light Patterns

Photinus pyralis

Photinus marginellus

Photinus consimilis

Photinus collustrans

Photinus carolinus

In the night, fireflies lighting while flying may look like a chaotic picture ↖, but they are actually precise patterns that allow females to recognize the males of their own species ↑.

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Courtship After finding a partner, some insects go through a session of courtship. Like other animals, different methods have evolved. Dancing, gifting, touching, or even using aphrodisiac emissions are among the different courtship strategies. For example, the males of various fly species use wing fluttering and flapping while approaching females. The males of the dance fly, Empis snoddyi, create gifts by weaving sacks of empty silk bubbles as offerings to females. The size of these sacks defines how fit a male is, allowing a female to choose. Scorpionflies also offer gifts in the form of dead insects (which they steal from spiderwebs) or masses of saliva.

Copulation As in most terrestrial animals, insect fertilization happens inside a female, and so copulation allows males to transfer their sperm by inserting their copulatory organ into the female. Some species have developed various copulation strategies. A spectacular type of copulation occurs in dragonflies and damselflies: the male first transfers sperm from his primary genitalia at the tip of his abdomen, to secondary genitalia under his second abdominal segment. He then grabs the female at the back of her neck using his claspers, also at the tip of the abdomen. The female then brings her genitalia to

↑ Dance flies, Empis sp., mating while carrying the nuptial gift that the male weaved for the female. She chose this male for the size of his gift. ← The white-legged damselflies, Platycnemis sp., create a heartshaped wheel while mating. The blue male grasps the yellow female’s neck, while she attaches her genitalia to his thorax.

MATING

183

↑ The black and red bug, Lygaeus equestris stays attached for hours. This is a post-copulation strategy of the male to reduce competition from other males, by preventing them to mate with the female.

the male’s secondary genitalia for sperm transfer. The way they attach to each other creates a distinctive heartshaped wheel. Bed bug copulation is also unusual. A male uses its copulatory organ to pierce the female’s abdomen and transfer sperm through the wound created, which goes directly into the hemolymph. Finally, primitively wingless hexapods such as springtails (Collembola) do not technically mate. Instead, the male produces a spermatophore, a delicate stem with a small sphere at 184

DEVELOPMENT, METAMORPHOSIS, AND GROWTH

the top, that he places near a female, or she may be guided by silk strands. The female then approaches the spermatophore and places her genital opening on top of the sphere and stores the sperm until fertilization.

Post-Copulation Sexual selection strategies have also evolved at the postcopulatory stage. Among them, blocking access to other males to mate is quite common. For example, the males of the true bug Lygaeus equestris (Hemiptera) stay attached to the females for up to 24 hours after sperm transfer. Mating plugs are another way to prevent other males to transfer sperm into a female. Plugs are used in a lot of species such as Heliconius butterflies, stingless bees, or Drosophila species. In mosquitoes, the spermatophore also acts as a plug that blocks the reproductive tract and prevents other males to mate with the female.

Oogenesis and Spermatogenesis Gametes are produced through oogenesis in females and spermatogenesis in males. Insect females usually produce a large number of oocytes in uniquely shaped ovaries, while the structure of sperm varies a lot across insect orders.

Oogenesis Female gametes are formed through the process of oogenesis in the ovaries. Insect ovaries comprise subunits called ovarioles, in which an oocyte undergoes different stages of oogenesis. During its development and maturation into an egg, the oocyte travels through the tubelike ovariole, which is divided into two main sections. First, the oogonia, or primary oocyte, develops in the germarium, then progresses to the vitellarium where it grows in size by accumulating yolk and developing the protective shield of the egg, the chorion. At the end of the vitellarium the egg is ready to be ejected into the oviduct where it will encounter the spermatozoa for fertilization. There are two types of ovaries, defined by the types of cells forming in the follicle and therefore the type of oogenesis. Panoistic ovaries are found in insect groups that include cockroaches, true bugs, and grasshoppers and consist of a follicle containing only the future oocyte and a layer of follicular cells. In meroistic ovaries, the follicle is formed by the future oocyte and additional cells, called nurse or trophic cells, which provide the resources necessary for oocyte formation. Meroistic ovaries are found in holometabolous insects, including flies, butterflies, beetles, wasps, and bees.

Types of Ovarioles in Insects

Panoistic Nurse cells

Meroistic

→ Scientists can look at insect ovaries using different imaging tools. These ovarioles, branching out from the middle, were imaged using confocal microscopy to access layers of the ovaries in Drosophila melanogaster.

OOGENESIS AND SPERMATOGENESIS

185

Spermatogenesis Insect male gametes are formed inside a pair of testes. Similar to the ovaries, one testis is divided into sections where the cells undergo meiosis and spermatogenesis. The first half of the testis is composed of the germarium and the zone of growth. There, the germ cells (that will become gametes) are diploid and undergo mitosis for cell multiplication. In the second half of the testis are the zones of maturation and transformation in which the cells undergo meiosis to create haploid cells that mature into sperm bundles. In the zone of transformation, the gamete elongates and becomes a spermatozoon.

Sperm Structure Sperm structure varies across insect orders, but in a lot of cases is quite conserved within an order. Insect sperm is generally composed of an acrosome, the nucleus,

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

a centriole, and a flagellum that includes mitochondria as the powerhouse to make the sperm mobile. However, in numbers of orders, sperm evolved into unique and some very unusual forms. For example, in Blattodea, some Hemiptera, and Hymenoptera, sperm consists of a bundle of spermatozoa held together by a glycoprotein matrix. In some species of orders, including Ephemeroptera, Protura, Trichoptera, or Diptera, most of the structures except for the nucleus have disappeared and the sperm is immobile. Finally, in Lepidoptera, sperm is dimorphic within each individual: Eusperm is larger, contains the nucleus and fertilizes the oocyte, while parasperm is smaller and does not possess the ability of fertilization as it lacks a nucleus. In some butterflies and moths, the parasperm accounts for the majority of produced sperm. The parasperm contributes to successful fertilization of the eusperm by helping the latter to move inside the female reproductive organ.

Structure of Drosophila melanogaster spermatozoon

Axoneme

Major mitochondrial derivative

Minor mitochondrial derivative

Paracrystalline material

← Confocal image of a fruit fly testis. At the top we see the dividing cells

Nucleus

that progress into spermatogenesis, and, the bottom left, spermatozoa forming like bundles of filaments.

The Longest Sperm on Earth The fruit fly Drosophila bifurca is considered to have the longest spermatozoa in the animal kingdom. When uncoiled the sperm cells measure 21/4 in (5.8 cm), or more than 20 times the body length of a male. This impressive size is the result of sexual selection at the post-copulatory level. In fact, having such long spermatozoa allows males to compete with each other. The longest sperm takes over the female reproductive tract and displaces smaller sperm.

OOGENESIS AND SPERMATOGENESIS

187

Embryogenesis Embryogenesis in insects was first extensively studied in the fruit fly Drosophila melanogaster, which undergoes complete metamorphosis. After oocyte formation and maturation, the unfertilized egg enters the oviduct where it is ready to welcome a spermatozoon.

Early Embryogenesis A zygote is formed once a spermatozoon enters the mature unfertilized egg through the micropyle, an opening at the anterior part of the egg. Immediately after fertilization, a wave of calcium similar to that in vertebrates blocks the entry of other spermatozoa, and meiosis achieves its last stage while embryogenesis can start. Unlike vertebrates, it is not cells that start dividing but nuclei only. The embryo proceeds to 13 rounds of fast division resulting in about 6,000 nuclei sharing the same cytoplasm. At about round 10 of division, the nuclei migrate to the surface of the embryo to form the syncytial blastoderm. The embryo now has nuclei in the center and also surrounding its edge, but it is still only one cell. Only after all nuclei have found their place, cellularization, or the creation of cell membrane around the nuclei takes place.

Maternal morphogen gradient

Nanos Bicoid

After cellularization is achieved, the cells start moving to form the ventral furrow, an invagination where cells go inside the embryo. This stage is called gastrulation. The cells that are invaginated become the future mesoderm (a precursor of all internal organs). Other insects undergo the same stages of early embryogenesis, but the subsequent stages vary depending on the insect lineage.

Genes Involved in Embryogenesis In the fruit fly, an array of genes controls embryogenesis. There are three types: the maternal effect genes, the segmentation genes, and the homeotic genes.

Maternal effect genes: These play a role at the beginning of embryogenesis. During oocyte formation, maternal mRNAs are left in the oocyte. These genes define the anterior and posterior part of the embryo, by forming a gradient along the anterior posterior axis. The genes bicoid and hunchback regulate the anterior part, while nanos and caudal regulate the posterior part.

Segmentation genes: These are responsible for segment formation in the embryo. To create segment identity, some genes are expressed to determine the boundary of each segment, while others determine which side of the segment is anterior and which side is posterior.

Kruppel

Homeotic genes: These control how the body is

Posterior

Anterior

Genes expressed by the zygote

patterned and, in particular, which identity each segment will have. Remarkably, the cassette of genes involved in body patterning are found in the same region on the chromosome, in the same sequential order as that of their action along the fly body.

Even-skipped ← mRNA coming from the mother is distributed in a defined gradient inside the egg and determines the

Effects on maternal gradient on zygotic genes in an embryo 188

DEVELOPMENT, METAMORPHOSIS, AND GROWTH

orientation of the embryo.

Homeotic Genes in Drosophila

Antennapedia complex Labial

Proboscipedia

Deformed

Sex combs reduced Antennapedia

Bithorax complex Ultrabithorax

Abdominal-A

Abdominal-B

Types of Segmentation During Embryogenesis An insect is created from multiple segments, groups

Short germ-band

Long germ-band

of which specialize into the head, thorax, and abdomen. Embryo segmentation determination happens by the blastoderm stage. Insects are divided into two main categories related to segmentation. The long germ band insects, including Diptera, Lepidoptera, and Hymenoptera have embryos that create the head, thorax, and abdomen segments at the same time. The embryo fills up the egg completely and does not move a lot. In contrast, in short germ band insects, the embryo only fills a small portion of the egg, and head, thorax, and abdomen segments are specified in a progressive manner, from the head to the abdomen. At the blastoderm stage, only the head segment is defined. Short germ band insects include Blattodea, Orthoptera, and Hemiptera.

EMBRYOGENESIS

189

Egg Laying Generally, insects are oviparous animals—they lay eggs. Also called oviposition, the action of egg laying is quite complex and varies among species. For instance, female parasitoid wasps (Hymenoptera) have elaborate egg-laying organs called ovipositors to allow them to inject their eggs inside another host. Other species, such as many true bugs (Hemiptera), simply oviposit egg clusters on substrates, such as leaves, but in intricate patterns. Finally, there are some insects that give birth to live juveniles.

retained by the mother in the oviduct for a longer period after fertilization and is more advanced in its development at the time of oviposition. This is the case with some mealybugs (Hemiptera), where, at the time of oviposition, the embryo has undergone half of its development. Retaining eggs longer before oviposition allows for better survival of the offspring, especially in unfavorable environmental conditions or where there is a higher chance of predation.

Larviparity ↑ The female bush cricket, Decticus albifrons, uses her long ovipositor to lay eggs in the soil.

Oviparity After oocyte maturation, fertilization takes place inside the female (see pages 180–181 for other modes of reproduction) before they are oviposited. Since egg laying occurs in different conditions and habitats depending on the biology of the species, females possess very diverse ovipositors. The ichneumonid wasp, Megarhyssa macrurus, has the longest ovipositor, up to 4 in (10 cm) long, which the female uses to lay eggs inside a larval insect host (a species of horntail wasp) that lives inside wood tunnels. Depending on the insect and its habitat, a number of species lay eggs at different stages of embryonic development. For instance, in the fruit fly Drosophila melanogaster, females lay eggs immediately after fertilization of the oocyte and the embryo develops completely externally. In other cases, the egg can be

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

In more extreme cases, the embryo can develop entirely and hatch inside the female. She then gives birth to live juveniles. This strategy is found in flesh flies (Sarcophagidae) and various other flies, which lay early instar larvae. Other flies, such as louse flies and tsetse flies, have taken the most extreme approach, by undergoing most of their juvenile development inside the female (nurtured by a uterus-like structure), and emerge as fully mature larvae that immediately pupate.

Viviparity In rare instances, some species have evolved viviparity, similar to mammals. In these insects, the embryos develop inside the female. She provides nutrients and gas exchange takes place between the mother and the embryos. Viviparity is found in a few earwig species such as Arixenia esau (Dermaptera). In the cockroach Diploptera punctata, females give birth to juveniles and feed them an equivalent of milk. The main difference between true viviparity and oviparous insects giving birth to juveniles is that nutrition is provided by the mother directly for the former, or by the egg for the latter. Viviparity is very rare in insects.

↑ In ichneumon parasitoid wasps, Megarhyssa sp., females have an ovipositor several times longer than their own length to reach woodboring insects in which they lay eggs. ← In the earwig Arixenia esau, larvae develop inside the mother’s reproductive system where gas exchange and respiration happen.

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191

Egg Protection To increase chances of offspring survival, some insects still lay eggs, but use different protective methods. For example, cockroaches that lay early embryos produce a protective case called an ootheca that can contain dozens of eggs. Sedentary female scale insects have evolved a panoply of ovisacs made of waxy secretions or even using their own bodies as shields to shelter their eggs.

→ Ensign scale insects, like Orthezia urticae, lay their eggs inside an ovisac that they secrete using specialized pores. The eggs are carried around inside the ovisac until they hatch.

Telescoping Generations in Aphids Some aphids (Hemiptera) do not lay eggs but

hatch very early within her, and her developing

have evolved an extraordinary type of embryonic

daughters already have developing embryos in their

development. This not only involves giving birth

ovaries, very similar to Russian dolls! This type of

to live juveniles, but development using telescoping

viviparity allows the aphid population to grow rapidly

generations. In species that are parthenogenetic,

by shortening the life cycle time and therefore quickly

a female carries unfertilized eggs that develop and

exploit ideal environmental conditions.

Daughter embryo Granddaughter embryo

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Egg structure, Size, and Shape Once fertilized, most insect eggs are laid on land and, in most species, are left alone throughout embryonic development until hatching. Therefore, insect eggs have to achieve their function in an environment where adverse conditions and desiccation are a threat. Moreover, in addition to strategies such as laying a very large number of eggs to ensure survival of a fraction of offspring, the structure of insect eggs is adapted to protect itself from predators or to avoid them. Insect eggs are therefore extremely diverse in shape, size, and the structures they carry at the surface, making them unique in each species.

Gas Exchange and Water Loss Once laid, an egg needs to protect the embryo from water loss at the same time as allowing gas exchange in and out of the egg. This creates a dilemma since, as soon as oxygen and carbon dioxide travel in and out of the egg, water loss is inevitable. The egg is composed of different layers: the periplasm is the innermost layer; the vitelline membrane is a layer of proteins that forms during oogenesis, that is permeable to water and ions but becomes impermeable to sperm, water,

and other larger molecules after fertilization; and the chorion is the outermost layer of the egg and in direct contact with the outside world. This envelop is both very resistant and allows air exchange and minimal water loss. It is composed of two sublayers called the endochorion and the exochorion. The exochorion is sculptured with microgrooves and ridges and possesses microscopic holes called aeropyles, which allow oxygen and carbon dioxide to be exchanged.

Insect Egg Structure Exochorionic jelly (some species) Micropylar atrium

Exochorion Endochorion Vitelline membrane

10u Outer network

Atrial opening

Aeropyle Middle layer

Inner network

EGG STRUCTURE, SIZE, AND SHAPE

193

Imperial hairstreak butterfly

Hawaiian fly

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Two-spotted cricket

Milkweed bug

Size and Shape In addition to being extremely resistant, insect eggs show an incredible diversity of size and shape across species. For example, with regard to egg volume, the smallest known egg belongs to the parasitoid wasp Platygaster vernalis (Hymenoptera), while the largest egg is laid by the earth-boring beetle Bolboleaus hiaticollis (Coleoptera). The latter is 800 million times larger than the former. Egg size depends on the environment in which the eggs are laid. In terms of shape, insect eggs can be completely spherical, flat, elongated to sticklike, or crescent shaped. Some ideas were developed as to explain a relationship between egg size and shape. For example, larger eggs would be more elongated and so more easily oviposited, or larger eggs belong to larger insects. However, these hypotheses were refuted to the benefit of an ecological hypothesis: larger eggs tend to be oviposited in soil or under leaf litter while smaller eggs that are rounder tend to be laid in water. Finally, the small asymmetrical ones are laid in other animal hosts.

Mimicry in Eggs Stick insects are herbivorous insects living in plant habitats and have evolved a high degree of mimicry. They are known to melt into the landscape among plant branches or leaves. What is less known is the mimicry they also evolved with their eggs. Phasmid eggs have an incredible diversity of shape that is specific to each species, and many of them look like plant seeds. Interestingly, seeds are not inconspicuous, so why do stick insects, with their capacity to hide, have eggs shaped as something that can bring attention? These seed-shaped eggs also have a modified structure at the tip called a capitula. This appendage actually mimics a similar structure in seeds that attract ants because it is filled with fat. Ants find seeds and bring them back to the underground colony as a food source. With stick insect eggs, ants are tricked to bring these eggs too, providing them protection against predators and especially parasitic wasps.

EGG STRUCTURE, SIZE, AND SHAPE

195

Insect Metamorphosis Insect metamorphosis is one of the most striking aspects of insect biology. Because they possess an external skeleton, insects cannot steadily grow in the same continuous manner as vertebrates. Like other arthropods, they need to shed their previous cuticle layer to grow in size—an event called molting. By doing so, an insect grows by approximately 1.4 times its prior size each time it molts. However, insect development went beyond simple molting events and evolved metamorphosis. This type of postembryonic development enabled greater evolutionary advantage by shortening the period of juvenile stage, but also allowing them to develop a diversity of features and conquer multitudes of ecological niches.

What is Metamorphosis? In insects, metamorphosis occurs after the end of the embryonic stage, or when the egg has hatched. Between this event and the emergence of the insect adult (or imago), the juvenile undergoes progressive changes through metamorphosis. Metamorphosis is a combination of molting events, where the insect grows in size and develops features such as wings or wing

Evolutionary Impact of Metamorphosis An adult swallowtail, Papilio machaon, emerges from its pupa after undergoing complete metamorphosis. The evolution of metamorphosis had an extraordinary impact on insect evolution by driving the diversification of insect groups and allowing the development of incredible morphological features. Metamorphosis played a critical role in the evolution of the insect wing, and through different life stages allowed insects to exploit multiple habitats. Complete metamorphosis (holometaboly) has evolved once and the larvae and pupa are considered a key innovation, as the evolution of these life stages has driven holometabolous insects to diversify extensively. Holometabolous insects constitute 40 to 60 percent of all known species on Earth.

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

buds/pads. The final transition between the last juvenile stage and the adult stage can show more or less drastic changes but is most striking in butterflies, beetles, wasps, and flies. In the most extreme cases, the insect undergoes a complete and complex reorganizing of its organs and tissues, especially during the pupal stage of insects undergoing complete metamorphosis.

Diptera

Mecoptera

Siphonaptera

Lepidoptera

Trichoptera

Coleoptera

Strepsiptera

Polyneoptera

Neuroptera

Megaloptera

Rhaphidioptera

Hymenoptera

Psocodea

Hemiptera

Thysanoptera

Isoptera

Paleoptera

Blattodea

Mantodea

Phasmatodea

Embioptera

Grylloblattodea

Mantophasmatodea

Apterygota

Orthoptera

Plecoptera

Dermaptera

Zoraptera

Ephemeroptera

Entognatha

Odonata

Zygentoma

Archaeognatha

Diplura

Protura

Collembola

Evolution of Metamorphosis Across Insect Orders

Condylognatha Endopterygota

0 Ma

100 Ma

200 Ma

300 Ma

400 Ma

500 Ma

Key

Ametaboly

Hemimetaboly

Holometaboly

INSECT METAMORPHOSIS

197

Three Major Types of Metamorphosis

Ametabolous development Pronymph

Adult

Hemimetabolous development Pronymph

Holometabolous development

Larva (instar 1)

Nymph (instar 1)

Nymph (instar 2)

Adult

Larva (instar 2)

Pupa Metamorphic (pupal) molt

Adult

Postembryonic Development

Winged Insects

Three major types of postembryonic development are found in insects and can be separated into unwinged and winged insects.

Insects evolved the ability to fly at the adult stage, allowing them to disperse farther and find mates and food resources. The necessity to develop wings had enormous consequences for their life cycles and this is how metamorphosis originated. Today, there are two major types of metamorphosis.

Unwinged Insects The oldest insect groups that did not evolve wings develop through a finite number of successive moltings. Eggs hatch into a miniature version of the adult, and successive moltings allow them to grow progressively in size. Following sexual maturation, where reproductive organs are functional, they continue to molt frequently. This type of postembryonic development is not really considered as metamorphosis and is called ametaboly. It is found in two insect lineages: the Archeognatha (jumping bristletails) and Zygentoma (silverfish).

↑ The silverfish, Lepisma saccharina, is a wingless insect that develops through ametaboly, only by molting.

198

Imaginal molt

DEVELOPMENT, METAMORPHOSIS, AND GROWTH

→ Hemimetaboly (incomplete metamorphosis): Eggs hatch as juveniles called nymphs that are very similar to the final adult (or imago). The major changes occurring in nymphs are progressive growth through molting and the development of wing pads (developing but not functional wings). When the nymph is ready to transition to the adult stage, the last molting event allows the maturation of reproductive organs, and fully formed, functional wings. The only exception to this are mayflies (Ephemeroptera): a winged, pre-adult stage called the subimago will undergo another molt before it becomes an adult. An insect adult, in fact, is defined as the reproductive stage. Insects undergoing incomplete metamorphosis commonly live in the same habitat regardless of the life stage, although many have aquatic nymphs.

↑ Cicadas develop through hemimetaboly, emerging as winged adults after the last nymph.

Looking Inside a Chrysalis in Real Time The pupa is a remarkable stage of complete metamorphosis. Until recently, we knew very little about what exactly happens, or at least could not observe directly the changes during the pupal stage. However, using imaging technology such as CT scanning, scientists have been able to obtain direct observations of changes taking place inside a butterfly pupa, or chrysalis. As previously thought, a pupa is not a case in which all larval tissues are destroyed into a liquid and completely replaced with adult organs. In fact, not all organs

↑ Beetles undergo complete metamorphosis, emerging as winged adults from a pupal stage.

are destroyed. For example, in butterflies, the tracheal system stays almost intact, while the muscles are digested by enzymes into small groups

→ Holometaboly (complete metamorphosis): This involves more dramatic changes between the juvenile and adult stages, where the invention of the pupal stage was an essential step to allow the transition between juvenile to imago. In these insects, eggs hatch as larvae—in butterflies these are caterpillars, in higher flies, maggots. The larval stage allows the insect to feed and grow exclusively at each molt. At the end of the last stage, the larva wanders to find a location to start its metamorphosis. It then molts into a nonfeeding pupa that almost always is immobile (an exception being the pupae of snakeflies [Raphidioptera], which can walk). During the pupal stage, differentiation and reorganization of the body takes place: most of the larval organs and tissues are replaced with the future adult tissue and organs. From this transformation, a drastically different adult insect, with completely remodeled body, emerges from its case. Pupae vary a lot across insects with complete metamorphosis. For example, since this stage is immobile, it needs to protect itself from predation. Many pupae that arise from larvae feeding on plants have pupae that mimic plant matter, such as the chrysalis of butterflies, or which are concealed under dead leaves and logs.

of cells that are later repurposed into adult tissues. Additionally, as previously known, larvae carry small groups of cells called the imaginal discs that are the basis of adult legs, antennae, or wings.

Day 1

Day 4

Day 7

Day 10

Day 13

INSECT METAMORPHOSIS

199

Atypical Metamorphosis Although the majority of insect lineages undergo either ametaboly, hemimetaboly, or holometaboly, a few groups have evolved unique variations of metamorphosis. These are rarer situations, but they are intriguing examples as to how metamorphosis might have evolved or instances of specialized biology, such as parasitism.

Prometaboly In mayflies (Ephemeroptera), the nymphs are aquatic and do not develop wings throughout their juvenile life. However, the last nymphal stage molts into a preadult stage with fully developed wings—this stage is called a

↓ Mayflies are the only insects with a fully winged, pre-adult stage. This individual needs to molt one last time to become sexually mature.

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

subimago or dun. This stage resembles an adult but is sexually immature, the legs and eyes are not fully developed and the body is less colorful. An additional molt is necessary to produce the adult stage. This molt takes place after a few minutes to a couple of days. Through prometaboly, mayflies are the only winged insects to undergo an additional molt with fully developed wings. A winged molt is also considered by some to be a primitive feature retained from the ability of adult silverfish and bristletails to molt.

Neometaboly in Thrips (Thysanoptera)

Egg (inside plant tissue) 2–4 days

Adult 30–45 days lays 150–200 eggs

Pupa 1–3 days

1st instar nymph 1–2 days

2nd instar nymph 2–4 days

Propupa 1–2 days

1 mm

Neometaboly Among hemimetabolous insects, a couple of lineages have evolved an alternative type of metamorphosis called neometaboly. In these groups, the nymphs undergo successive molts as they feed and grow in size, but do not develop wing pads. During the last couple of molts, the stages stop feeding and become quiescent. These stages are reminiscent of the pupal stages of holometabolous development and therefore were named prepupa (or propupa) and pupa, even though they are completely different. During the pre(pro)pupal stages, wing buds

appear and elongate at the next molt. Legs and antennae are present and undergo the same molting process as hemimetabolous insects. At the final molt, an adult emerges with fully developed wings. Neometaboly is known in thrips (Thysanoptera), and the males of scale insects (Sternorrhyncha) and has evolved independently in these two lineages. However, the prepupa in scale insects and propupa in thrips show some differences. For example, male scale insects produce a cocoon made of filamentous secretion to cover themselves as a protective cover since they become vulnerable as they stop moving.

ATYPICAL METAMORPHOSIS

201

flower after mating. After hatching, the first larva, called a triungulin, either waits for a bee to come to the flower or crawls in the ground looking for ground bee nests. Once it finds a nest, the larva molts into the second type of larva, completely different and less mobile, since it has to ingest an abundance of food in the nest. After ample feeding, it molts into a third type of larva that lacks appendages and undergoes diapause (see page 226). After molting to another larva with more mobility again, the beetle becomes a pupa where the rest of metamorphosis takes place. An adult emerges from the pupa and leaves the bee nest. Hypermetamorphosis is found in some families of beetles (Coleoptera), flies (Diptera), mantis flies (Neuroptera), twisted-wing insects (Strepsiptera), and Hymenoptera.

Hypermetamorphosis Hypermetamorphosis arose as a variation of complete metamorphosis in several lineages independently. While in holometaboly, all larval stages are the same but progressively larger after each molting event, the larvae of hypermetabolous insects are different. The first instar, or planidium, is mobile and looks for food. From the second instar, the larvae changes in appearance and behavior, and is the stage where growth takes place. Then at the last larval stage, molting gives rise to the pupal stage. The majority of insects undergoing hypermetamorphosis have highly specialized life histories—many of these insects are adapted to parasitism or are predators during the larval stages. For example, the blister beetle undergoes hypermetamorphosis as it needs to find its source of food, bees’ nests. The female lays eggs in the ground or on a

Hypermetamorphosis in the Blister Beetle

Pupa

Egg

Adult

Triungulin

VII instar larva

II-V instar larva Coarctate larva

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Origin of Complete Metamorphosis It is not an understatement to say that metamorphosis,

The pupa is therefore a merged version of all nymphal

and especially holometaboly, had an incredible

stages, where wings develop. Since nymphs develop

impact on today’s insect diversity. In fact, the advent

wing buds externally, the invention of a pupa would

of complete metamorphosis allowed different

allow juvenile stages (here larvae) to develop wings

developmental stages in a single species to exploit

internally (as imaginal discs) and therefore allowing

more than one habitat and reduce competition and

larvae to burrow into food substrate. In this case,

exposure to pathogens and predators. For example,

holometabolous larvae are not equivalent to

a caterpillar chews plant matter, while the adult

hemimetabolous nymphs.

butterfly has a proboscis and sucks flower nectar. The fossil record suggests complete metamorphosis originated during the Permian (around 300 mya). Since Aristotle, scientists have tried to

2. The “Pupa Comes From a Nymph” Hypothesis The second category of hypotheses focuses on the end

understand how complete metamorphosis originated.

of metamorphosis, and attempts to understand the

Where exactly did the larva and pupa come from?

origin of the pupal stage. The main suggestion is that

Phylogenetically, complete metamorphosis originated

the pupa arose from the last nymphal or a pre-adult

once from incomplete metamorphosis. A few

stage of hemimetabolous insects. In this scenario,

hypotheses trying to explain how it originated have

hemimetabolous nymphs are equivalent to

been proposed, all of which fall into two categories:

holometabolous larvae. This hypothesis was later

1. “De-Embryonization” Hypothesis

supported by paleontological evidence, phylogenetic studies and endocrinology. In fact, study of juvenile

The first category of hypotheses centers on

hormone function in both hemimetabolous and

understanding the origin of the larva. Scientists have

holometabolous insects has shown that juvenile

proposed that larvae are embryos that hatched very

hormone—which controls the juvenile status during

early, called a pronymph. This means that all larval

metamorphosis—prevents adult metamorphosis,

stages should be considered the equivalent of an

supporting the suggestion that larvae and nymphs

extended embryonic stage of hemimetabolous insects.

are equivalent stages.

Hypotheses for the origin of insect metamorphosis

1.

PN

2.

E1

E2

Juveniles

Pupa

Adult

ATYPICAL METAMORPHOSIS

203

Juveniles In insects, juveniles are an important part of the life cycle, since they represent the stage that feeds the most and accumulates resources for growth and to allow metamorphosis. Two main types of juveniles can be distinguished in insects: in holometabolous insects, the juveniles are called larvae, whereas, in other groups, they are called nymphs. In many insects, juveniles and adults do not live in the same habitat and this is particularly true for the larvae of holometabolous insects or aquatic nymphs.

Nymphs All insects that do not undergo complete metamorphosis develop through several nymphal stages. Generally, nymphs are recognizable as they are usually similar to the adults, except they are smaller and do not have completely developed wings. In fact, nymphal stages grow by progressively developing wing pads that will only be fully functional wings after the adult emerges. Terrestrial species commonly have the same diet as the adults. Nymphs typically have three pairs of legs, more or less separated

head, thorax, and abdomen, and they may have patterns of colors that are different from the adult stage. The number of stages varies across insect groups. For example, cockroaches grow through six nymphs, while head lice only have three. In unwinged insects, such as the silverfish, there are four nymphs. The lifespan of nymphs also varies across insects. An extreme example is the cicada, where the nymph of some species stays underground for years before emerging as an adult that lives for a few days.

Cockroach

Cicada Planthopper

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Stick insect

Assassin Bug

Grasshopper

JUVENILES

205

01 Caterpillar

Larvae In holometabolous insects, the juvenile stage is drastically different from the adult. Called larvae, they have a general “larviform” shape, with legs, antennae, and eyes reduced or absent and the body usually soft with a great deal of exposed membrane. They can be classified into five different categories:

01 Eruciform: Includes caterpillars; they have a cylindrical body with a head capsule, and legs can be found on the thorax (true legs) and sometimes on the abdomen as well (soft prolegs).

02 Scarabeiform: Found in the Coleoptera superfamily Scarabeoidea; they have true legs on the thorax but lack prolegs on the abdomen. They are also called grubs and are usually slow. 02 Scarab

03 Campodeiform: Less larviform, these are active predators. Therefore they possess well-developed legs and antennae, as well as mouthparts that face forward. These larvae can be found in some Coleoptera and Neuroptera, typically predatory species, as well as the aquatic Trichoptera (caddisfly) larvae.

04 Elateriform: Heavily sclerotized and elongated, these are found in a family of Coleoptera, the Elateridae. 03 Lacewing

04 Elateriade beetle

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Aquatic Nymphs

05 Vermiform: Includes maggots; they are elongated larviform bodies and do not possess legs. In maggots (the larvae of the higher, or cyclorrhaphan, flies) the head capsule (normally sclerotized in larval insects) is highly reduced and hidden inside the body. These larvae are found in the majority of holometabolous insects such as higher Diptera, most Hymenoptera, some Coleoptera, and a few Lepidoptera.

For some insect groups, although the adults are terrestrial and winged, the juvenile stages develop exclusively in fresh water. These aquatic nymphs are found in dragonflies and damselflies (Odonata), mayflies (Ephemeroptera), and stoneflies (Plecoptera). These nymphs are adapted to the aquatic life and possess gills, which are lost as adults. The gills are bush-like, external extensions of the tracheae. They also develop wing pads during the successive molts while living in water. Before the last molt, they crawl out of the water or near to the surface to emerge as the terrestrial adult form.

05 Hover fly

Mosquito Larvae are Aquatic Mosquito development takes place in water, except for the adult form. Females lay eggs at the surface of the water and, after hatching, the larvae develop near the surface, breathing air through specialized spiracles that stick through the surface (these allow mosquitoes to breed in stagnant water). The pupae develop in water and float toward the surface so the adult can emerge.

NEXT PAGE Mosquito larvae molt several times before reaching their pupal stage. A pupa does not feed, but stays in the water until the adult mosquito emerges.

JUVENILES

207

Juvenile-Like Insects Not all insects look like a typical winged and legged insect. In some lineages, the females look nothing like their male counterparts, and skip traditional metamorphosis. In these cases, they usually look like juveniles, except they have functional reproductive organs and can mate.

210

Termites

Scale Insects

Termites are social insects such as ants or bees and live in colonies with different castes. Each caste accomplishes a distinct role. In general, in a single colony there is only one reproductive female. However, a special caste called neotenics, or secondary reproductives, has evolved specifically in termites. Neotenics have the external appearance of a juvenile, but the reproductive organs are mature. Their role is to provide a replacement if the main reproductive female dies. Therefore, they are generated when necessary, and derive from nymphs or even older stages. To become a neotenic, molting is necessary and the differentiation is regulated by the juvenile hormone. Since termites are hemimetabolous insects, the way castes are produced is very different from ants or bees that undergo complete metamorphosis.

Another hemimetabolous group, scale insects are plant parasites feeding on sap, with reproductively mature females that look like the juvenile stage. The juveniles grow through a few molts, although the nymphs never develop wings. After the last molt, a female emerges with the same appearance as the previous stage except she can mate with males and becomes much larger. A lot of female scale insects even have extremely reduced legs and antennae and become living egg sacs. A female finally lays hundreds of eggs either in a protective ovisac or a wax cover that she secretes and then dies. Scale insect juvenile appearance in females seems to be linked with a shorter life cycle and the production of as many offspring as possible. Since females are sedentary on the plant, they do not invest in developing wings.

DEVELOPMENT, METAMORPHOSIS, AND GROWTH

→ This male twisted-winged insect (Strepsiptera) is looking for a larviform female to mate with. She is hidden inside the abdomen of a wasp.

Twisted-Winged Insects The Strepsiptera or twisted-winged insects are called obligate endoparasites since their life cycle relies entirely on parasitizing and feeding inside wasps or other insects. Their development is unique, with males and females going through two different types of metamorphosis. The males resemble a typical insect with wings, antennae, and legs and arise from a pupa. However, the female does not have such features and after becoming sexually mature, she looks like a larva with only a small portion of her body (the cephalothorax) having hardened cuticle. In all but the most primitive family of Strepsipterans the female does not need to leave the host she has parasitized; she only needs to stick the cephalothorax out of the host parasite so males can find her and mate. After hatching into a mobile first larva, the female life cycle is quite short and she directly molts into a larviform larva, and finally molts into the sexually mature juvenile-like adult with her cephalothorax.

Why Females? In almost all insects described here, the juvenilelike adult is specific to one sex: the female. Why is only the adult female juvenile-like? These insects have unique life histories; they are either plant parasites, parasites of other insects, or are eusocial. In these situations, the insect does need to disperse as much as other insects to find a mate. On the other hand, increasing reproductive success becomes more important. Therefore, females divest in dispersal features such as wings and legs, to invest in reproductive organs and production of eggs. Another explanation is that the JH that controls the juvenile status during metamorphosis also regulates reproductive functions. A shift in timing of JH levels may allow early reproductive maturation in juvenile stages. The evolutionary

←↑ Termites are organized with a caste system just like

and developmental explanations are not exclusive.

ants. Among the castes, neotenics (the light-brown individuals opposite) can develop as back-up reproductive members in case the main reproductive female dies.

JUVENILE-LIKE INSECTS

211

Developmental Plasticity Sometimes, insects within a species can look very different to one another. These different adult forms are either sexual differences (a male and a female with different features) or the result of developmental plasticity. The latter occurs when individuals with the same genes are exposed to different environmental conditions during their development. Developmental plasticity can provide an evolutionary advantage by helping the species adapt to frequent variations in environmental conditions. Depending on the species, the environmental cues triggering developmental plasticity can be changes in temperature, amount of daylight, humidity, nutrition, or density of juveniles.

Density and Polyphenism One of the most studied examples of developmental plasticity is found in the locusts Schistocerca. These locusts are best known for their devastating effects in agriculture when they appear as large swarms, traveling long distances and ravaging crops. Locusts can develop into one of two phases, the gregarious or the solitary phase. This type of developmental plasticity is called locust phase polyphenism. The gregarious phase produces individuals with bright colors and forms swarms that travel together

in large numbers. In the solitary phase, the locust is usually inconspicuous in color and avoids other locusts. These two phases can come from the same egg pool, except that density of nymphs determines the developmental trajectory: when living in dense populations, nymphs stay together and develop into the gregarious, bright-colored form, while low density triggers the solitary phase.

↑ The desert locust can develop into either of the two forms: a less colorful adult if the juveniles grow in the solitary phase (top), and a brightly colored adult if the juveniles grow in the gregarious phase (bottom).

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Temperature and Pigmentation Developmental plasticity can also affect colors on insects. The harlequin bug, Murgantia histrionica, is known as an important pest of crops such as cauliflowers, broccoli, and turnips. It also has the ability to change pigmentation, more specifically the degree of melanization at the adult stage, after being exposed to varying temperatures during its postembryonic development. When nymphs grow under low temperatures, the resulting adults have darker areas on their backs than nymphs that grew in higher temperatures. This type of developmental plasticity allows an adult to adapt to lower temperatures, as it can absorb heat faster and is therefore more active than the lighter bugs coming from warmer conditions.

← This harlequin bug, Murgantia histrionica, developed under low temperatures. As a result its back is almost entirely black.

DEVELOPMENTAL PLASTICITY

213

Nutrition and Size

↓ A dung beetle rolls a brood ball. An egg has been laid inside it. After hatching, the larva will feed on the dung. If the ball is larger than those of the siblings, the larva will become the largest beetle of the brood.

Nutrition is an important environmental factor in generating developmental plasticity. This is well known in horned dung beetles, Onthophagus. These beetles roll brood balls made of dung inside which the female lays eggs so that the larvae can feed on the dung on hatching. The males possess elaborate, large horns, the females are hornless. Horn size varies drastically among males and is determined by both aspects of dung beetle biology: males compete with each other to mate and use the horns to fight, and females compete with each other for resources to build the brood ball. Since brood balls vary in size and quality of dung, and that each brood ball contains only one feeding larva that stays inside until pupation, this results in nutritional developmental plasticity. Male larvae that feed on higher-quality brood balls become larger and develop larger horns than male larvae growing in lower-quality brood balls. This improves the fitness, or breeding success, of the larger males. A better diet for the immatures of all insects generally leads to a larger adult.

Juvenile Hormone and Castes

Soldier differentiation

JHII titer PE

Alate

Molt

Molt Sensitive period

12 days –8 days –5 days –1 day

Presoldier

Soldier

Soldier

Soldier differentiation

Alate differentiation Molt

JHII titer

Alate differentiation

Sensitive period

PE

Nymph

Soldier

–5 days –1 day

Alate

Presoldier

JHII titer

Stationary molt

Stationary molt

Pseudergate

Molt Sensitive period

Pseudergate

–5 days –1 day

Pseudergate

Regulation of Termite Caste

termite species. In the damp-wood termite

The endocrine role of developmental plasticity has

Hodotermopsis sjostedti, after hatching, juveniles molt

been studied extensively in termites, eusocial insects

seven times before attaining a stationary molt called

that have a caste system. Termites are hemimetabolous

pseudergate, which is the worker caste. From this

insects and develop through successive unwinged

stage, the juvenile can either continue molting and

nymphs; only the adult stage has wings and mature

stay a pseudergate, molt and differentiate into an alate

reproductive organs. Therefore, workers and soldiers,

reproductive form, or undergo soldier differentiation.

which are not reproductive castes, are nymphal stages.

The decision relies on JH titers at the time of molting.

The caste system relies on high plasticity during

For soldier differentiation, JH titers are high before

development, and caste fate is generally determined

and at the molting event. To undertake alate

after egg hatching by environmental conditions such

differentiation, JH titers are low before and at

as temperature, humidity, or interaction among

the molting event, and finally, to stay a pseudergate,

members of a colony. Moreover, the developmental

JH titers are low before and high during molting.

trajectory for each of the castes depends on the

DEVELOPMENTAL PLASTICITY

215

Endocrine Regulation of Metamorphosis Metamorphosis is a complex process that requires timely changes in a precise order during development. For example, an insect knows when to molt to another juvenile stage or when it is time to metamorphose into an adult. But how does it know when and what to do?

Hormones play a key role in regulating metamorphosis, controlling the different events that lead to an adult insect. Just like vertebrates, insects need a system to communicate information to the body and to coordinate physiological changes. They also use hormones to deliver signals and information to multiple organs, and so the endocrine system is the ideal system for this role. Hormones produced and secreted from different endocrine organs in the insect head are delivered through the hemolymph.

Wigglesworth Experiment In the 1930s, Vincent B. Wigglesworth discovered both molting and juvenile hormones, although they were named later, by doing Frankensteinesque experiments with the kissing bug, Rhodnius prolixus (a bug that feeds on blood and is an important vector of Chagas disease). The molting hormone was discovered using a decapitation method. Kissing bug hemimetabolous nymphs undergo several successive molts before the last molt produces an adult. These molts take place precisely six days after a blood meal. When a nymph was decapitated

Wigglesworth Experiment

Normal development

Feed

Molt (6 days)

Decapitation experiment Decapitate

No molt

Feed

Decapitate (3rd day after feeding)

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Molt

immediately after being fed, it never molted to the next stage. However, decapitation three days after feeding resulted in a molt to the next stage. Wigglesworth concluded that there was a factor that was released from the brain after feeding and triggered molting. The second experiment was to attach two bugs at different nymphal stages, one with the brain still attached and the other decapitated. When attached together, the bugs shared their circulating hemolymph. When the final nymphal stage was decapitated and attached to the head and body of a younger nymph, the following molt resulted in an additional nymph instead of an adult. In this experiment, Wigglesworth showed that in the head of the younger nymph, something was produced and transferred to the last nymphal stage and prevented it from metamorphosing into an adult. For this reason, this factor was later called the “juvenile hormone (JH)”. The molting hormone is called ecdysone.

Hormone Titers During Metamorphosis Ecdysone and JH orchestrate how postembryonic development takes place, and this works by the release timing of these hormones in the hemolymph. For example, in the fruit fly Drosophila melanogaster, which undergoes complete metamorphosis, ecdysone levels or titers peak at molting events during the larval stage, allowing the larva to increase in size progressively. At the same time, JH levels are high during the molting events and direct the larvae to molt into a next larva. Toward the end of the last larval stage, the JH titer decreases at the same time as the occurrence of a larger peak of ecdysone; this is when the larva is ready to wander to find a spot to become a pupa and undergo complete metamorphosis. The last larger peak of ecdysone with the absence of juvenile hormone coincides with changes in the pupa that will create the adult features.

Juvenile Hormone and Ecdysone Titers During Drosophila melanogaster Development

1st molt

2nd molt

1

2

3

Pupariation

Eclosion

JH and 20E titers in development

Hatching

0

4

5

6

7

8

9

days

ENDOCRINE REGULATION OF METAMORPHOSIS

217

Molting and “Status Quo” Hormones Insect metamorphosis is controlled by two important hormones. The molting hormone ecdysone dictates when the insect should molt and the “status quo” hormone JH defines what type of developmental stage should be after molting. Where do these hormones come from and what are the differences between the control of incomplete and complete metamorphosis?

Ecdysone Ecdysone is a steroid hormone that is synthesized from cholesterol. Ecdysone synthesis is initiated by the production of a neuropeptide produced by the brain, the prothoracicotropic hormone (PTTH). PTTH then stimulates the prothoracic glands (PG), which are endocrine glands located near the brain. Before acting on different tissues, ecdysone is converted into its active form, called 20-hydroxyecdysone or 20E in the hemolymph. This active form directly regulates molting and metamorphosis. Ecdysone regulates insect metamorphosis by inducing molting events and adult metamorphosis. Peaks of ecdysone at each molting event is necessary and the titer is the highest during adult metamorphosis. Because it is a steroid hormone, ecdysone can directly enter the cell and binds to a receptor in the nucleus called the ecdysone receptor. This directly activates different genes that will produce a new cuticle and promote other physiological changes necessary for an insect to molt.

↑ This cicada is freshly emerged from the last molt, triggered by a peak of ecdysone in the hemolymph.

Where Do JH and Ecdysone Come From?

Brain Ring gland

Central ganglion Brain

The ring gland contains the prothoracic glands (PG) and corpora allata (CA).

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

JH III Bisepoxide O

O

O JH III bisepoxide

O ↖ Recently discovered in Heteroptera, JH III bisepoxide is just one example of naturally occuring juvenile hormone.

Juvenile Hormone The juvenile hormone (JH) is a sesquiterpenoid hormone that is synthesized and secreted by endocrine glands near the brain called the corpora allata (CA). There are several types of JH in insects. Even though JH III is the predominant JH, other types exist in butterflies, flies, and true bugs. JH regulates metamorphosis and other biological processes such as reproduction and polyphenism. Its presence during insect postembryonic development dictates the next developmental stage. For example, a larva molting in the presence of JH will stay a larva. A drop of titer in JH signals the end of juvenile development and adult metamorphosis takes place. Although JH was named for its role in maintaining the juvenile state during metamorphosis, not long after its discovery, scientists also found that this hormone has a major role in insect reproduction. The action of JH does not stop after the insect becomes an adult. In fact, JH is still produced in females to control the maturation of eggs inside her, by promoting the accumulation of yolk, for example. This is an important aspect of JH control, as any variation can have consequences in both the type of metamorphosis it undergoes and reproductive maturation.

→ This damselfly ended its aquatic life as a juvenile and molted into the adult form, or imago. The decision to molt into another juvenile or an adult is made by JH.

MOLTING AND “STATUS QUO” HORMONES

219

Hormonal Regulation: Differences Between Hemimetaboly and Holometaboly Hemimetaboly Final larval

Adult

Relative titer

Penultimate larval

Holometaboly Pupal

Final larval

Adult

Relative titer

Penultimate larval

Relative level of JH in hemolymph

Relative level of ecdysone in hemolymph

Hemimetaboly vs. Holometaboly Both ecdysone and JH need to act in concert for metamorphosis to take place normally. For example, for each juvenile stage to molt to the next, both hormones need to be present in the hemolymph. There are, however, differences in hormone titers between complete and incomplete metamorphosis, especially toward the end of postembryonic development. Ecdysone titer peaks at each molt in hemimetabolous insects, with a final peak before the imaginal molt. In holometabolous insects, a small peak in the middle of the last larval stage signals the larva to stop feeding and

220

DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Molting event

Period of commitment to next molt

wander to commit to molt into the pupal stage. Another ecdysone peak at the end of the larval stage allows it to molt into a pupa. Finally, during the pupal stage, the largest peak of ecdysone titer allows the insect to prepare for the adult molt. JH titers also differ in incomplete and complete metamorphosis. While in hemimetaboly, JH is present at each molt throughout the nymphal stages, except for the last molt into the adult stage, in holometaboly, JH titer decreases at the beginning of the last larval stage, but the hormone appears again before the pupal molt.

Pest Control Insecticides have been used to control pests. Some, called insect growth regulators,

O

were specifically developed to

O

target different juvenile stages by mimicking the action of hormones controlling metamorphosis. These growth regulators disrupt a normal insect life cycle by binding to the receptors of these hormones. For example, a JH analog applied to insect juveniles binds to the JH receptor and prevents its adult metamorphosis.

O

O

Pyriproxyfen

↑ Pyriproxyfen is one of the insect growth regulators that affects metamorphosis in insects, as it mimics the action of JH.

The Right Study System? Research in biochemistry and genetics was delayed because the action of JH was obscure, even though JH was discovered around the same time as ecdysone. The status quo hormone was initially discovered in an hemimetabolous insect using traditional physiology experiments. With the advent of genetic tools, especially the development of the fruit fly Drosophila melanogaster as a major genetic model system, scientists attempted to understand the molecular action of JH, but in Drosophila could not find a strong effect of JH during its metamorphosis. Research on the genetics of JH was therefore delayed until genetic tools could be developed in other insects such as the flour beetle, Tribolium castaneum, or hemimetabolous

Drosophila melanogaster

Tribolium castaneum

insects such as the true bug, Pyrrhocoris apterus, and the German cockroach, Blattella germanica. This example shows how important it is to choose the right insect when seeking the answers to biological questions.

Pyrrhocoris apterus

Blattella germanica

MOLTING AND “STATUS QUO” HORMONES

221

Insect Growth In insects, growth takes place exclusively during their juvenile life. Because insects are restricted by an exoskeleton, molting events are coordinated with growth. In complete metamorphosis, the juvenile stage is morphologically and physiologically dedicated to feeding and growing in size.

How Do Insects Grow?

Endocrine Growth Control

In most insects, growth takes place during the juvenile stage. It is even more dramatic in complete metamorphosis, since the juvenile or larval stage has the sole function to grow in size before the pupal stage where differentiation to the adult morphology takes place. Therefore, in these insects, the final size is reached at the end of larval development when larvae stop feeding and start their wandering behavior. When does an insect decide it has grown enough? Three aspects of growth determine an insect’s final size. The first is called the critical size: the size at which a larva starts initiating metamorphosis physiologically. However, when critical size is attained, there is still a lag until growth completely stops. The second aspect of growth is therefore the length of that lag, or period, that is called the terminal growth period (TGP). Finally, the last aspect determining final size is the rate at which growth occurs during the terminal growth period.

Two main factors define how insects grow and attain a certain size: growth rate, the speed at which growth is achieved, and growth period, how long growth takes. These two parameters are physiological controlled by the insulin pathway and ecdysone. Insulin pathway in insects uses insulin-like peptides and their action to promote growth at different levels. In general, the insulin pathway controls growth rate as a response to nutrition (and other environmental factors) during the larval stage. Ecdysone, which signals molting events, also controls growth duration.

Imaginal Discs In holometabolous insects, adult features develop fully only at the end of postembryonic development during the larva. During embryogenesis, some cells from the ectoderm become clusters of epithelial cells that are called imaginal discs. These clusters undergo moderate growth during the larval stages and their fate is already established. However, they do not develop to their defined structures until the pupal stage. Different imaginal discs are found inside the larva, around their final location. At the front, we find pairs of imaginal discs for head structures such as the antennae and eyes; around the future thorax are the leg and wing discs, while the posterior end carries the genital discs. During the pupal stage, the cells of imaginal discs undergo rapid proliferation and the discs evaginate so that the center of the disc becomes the tip, while the periphery becomes the base of the structure. For example, each leg segment is already defined with the future claws corresponding to the most central cells of the imaginal discs, and the coxa corresponds to the cells at the periphery.

← In Drosophila, growth during the last larval instar will determine the adult’s final size.

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Drosophila Larval Growth and Indication of Critical Size and TGP Embryo

L1

L2

L3

Body size

Body’s TGP

Pupa

Final body size

Critical size

Imaginal Discs in Drosophila Larva

Eye / antennae

Leg

Haltere

Wing

Genital

INSECT GROWTH

223

Exaggerated Traits in Insects In general, body parts are in proportion to body size, regardless of how big or small an insect is. In a lot of species, however, some features can become disproportionate to their overall body. We call these parts exaggerated traits. These traits can be elongated appendages or an oversized head. Entomologists are fascinated by these exaggerated traits and are trying to understand why they exist and how they grow.

Big-headed ant

Insects Possessing Exaggerated Traits In general, exaggerated traits are found in various insect orders but the majority in Coleoptera, Diptera, Hemiptera, and Hymenoptera. Among the most recognizable traits are the enlarged horns of stag beetles, the hindlegs of leaffooted bugs, the raptorial forelegs of praying mantis, the enlarged jumping legs of grasshoppers and crickets, stalked eyes in various flies, and the enormous heads of termite or ant soldiers.

Why Do They Exist? There are three explanations for the presence of exaggerated traits in these insects. The first, and most studied, is sexual selection. In many cases, the enlarged appendages are found in males and are used as weapons to compete (in battle using these appendages or to secure territories or resources) for female access. When not used for fighting, these larger ornaments are considered favorable by females looking for a mate. The enlarged horns of the male stag beetle are a well-known example. Leaf-footed bug

224

DEVELOPMENT, METAMORPHOSIS, AND GROWTH

The second explanation is the use of exaggerated traits to move around or as weapons for prey capture. In these cases, developing enlarged appendages can have negative consequences. Having disproportionately large forelimbs to capture prey may prevent fast movement, for example. However, these insects usually find strategies to circumvent such challenges. For example, the praying mantis has enlarged forelimbs and instead of running after a prey, this insect ambushes them.

The third explanation for exaggerated traits is the evolution of eusociality. In social insects, soldiers usually have enlarged traits to defend the colony. For example, termites can have larger bodies and limbs, or overly large and sclerotized heads compared to the workers. The fact that labor is divided among different castes allowed specific castes to evolve these exaggerated traits.

Grasshopper

Praying mantis

How Do They Grow? The mechanisms controlling the growth of exaggerated traits have been studied almost exclusively in the context of sexual selection. For example, rhinoceros beetle horn size varies extensively within the species. This is because this trait is developmentally more sensitive to the mechanisms controlling growth through nutrition; their growth responds more sensitively to the insulin pathway. Big males have disproportionately large horns. Moreover, a gene from the sex determination pathway, as well as JH, is responsible for the sex-specific nature of enlarged horns. Females do not have the same response to nutrition on their horn growth and final size.

EXAGGERATED TRAITS IN INSECTS

225

Diapause For many insects, the life cycle can be punctuated by an arrest in development and activity when external conditions would not allow them to survive. This type of developmental arrest occurs in the majority of insect species, especially those living in regions with strong seasonal fluctuations. Diapause is an important part of an insect’s life as it improves its survival chances and requires very complex environmental sensing and internal regulation to enter and exit diapause.

↑ The cinnabar moth, Tyria jacobaeae, needs to go through obligatory diapause in its pupal stage during the winter to finish its development. → The Arctic woolly bear caterpillar, Gynaephora groelandica, can live up to 14 years. It spends most of each year frozen, likely preparing itself physiologically each time by first diapausing.

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DEVELOPMENT, METAMORPHOSIS, AND GROWTH

Why and When? Diapause allows an insect to complete its life cycle by avoiding periods of unfavorable environmental conditions that could otherwise kill the insect. In the majority of species undergoing diapause, the insect stops its development and its metabolism is suppressed after detecting specific environmental cues (facultative diapause). In rarer cases, to complete its life cycle, an insect is required to undergo diapause, regardless of the environmental cues. This arrest is called obligatory diapause and often happens in species that have one generation per year. Depending on the species, insects can undergo diapause at different developmental stages of their life cycle. For example, diapause during the egg stage or embryo is commonly found in Lepidoptera and Hemiptera. The most common time for diapause in larvae is the last larval stage, and usually occurs in Lepidoptera. Pupal diapause is found in Lepidoptera and Diptera, and, finally, diapause at the adult stage is very common and found in a large range of insect groups, including Coleoptera, Hemiptera, Hymenoptera, Orthoptera, Neuroptera, Lepidoptera, and Diptera. Diapause rarely occurs more than once during an insect life cycle.

Extreme Dessication A fly of the family Chironomidae, or nonbiting midges, Polypedilum vanderplanki has taken diapause to another level. This species, found in West and East Africa, is known for its capability to desiccate at the larval stage so that only a small percentage of water content is left. This form of desiccated larvae can withstand extreme temperature fluctuations and survive drought for more than a decade. Obviously, the larvae have completely stopped their development and metabolism, waiting for rainfall to revive them.

Environmental Cues for Diapause In insects with facultative diapause, detecting environmental changes is critical both before entering and exiting diapause. Among the environmental changes that can trigger diapause, temperature (especially cold winter temperature) is the major factor. With winter, change in daylight is also detected by insects diapausing during this season. However, other less consistent factors can trigger diapause such as dry environments or food shortage. Although triggered by environmental cues, diapause is still a biological process that is regulated genetically, meaning that it is a physiological adaptation of an insect. Therefore, drastic, rapid, and permanent environmental changes such as climate change may not give enough time for the insect to adapt.

DIAPAUSE

227

Chapter

5

Natural History When we think of insects, we conjure images of bees and butterflies drifting amid flowers, of beetles and earwigs crawling on rocks and bark, of grasshoppers and caterpillars chewing on plants. Invariably, such images focus on insects within a terrestrial realm. Insects are dominant on land and in the air, and although they secondarily invaded streams and ponds, and have become successful and abundant in these freshwater environments, their diversity in water continues to pale by comparison to their terrestrial counterparts. While there are more than a million described species of insects, not quite 8 percent of these are classified as aquatic. And even among these numbers, most are more accurately amphibiotic—living in water for only a portion of the life cycle—and comparatively few are wholly aquatic. Thus, the story of insects is largely one of adventures on land, with side steps into puddles, creeks, rivers, lakes, and marshes. Some even dip their pretarsus into the seas. ← Leaf-cutter ants have specific roles within their colony: the ants that cut the leaves are different to those that carry them back home to the nest.

229

Aquatic vs. Terrestrial Life began in the oceans, and the initial diversifications that would separate plants from animals, among others, took place in the seas. Similarly, the evolutionary divergences of the early precursors of all animal phyla were oceanic events. By the start of the Paleozoic, our oceans were already rich in life and during the Cambrian, the first paleontological clues appear to how varied animal life, and particularly early arthropod life, was becoming. By the subsequent Ordovician, plants made their way ashore, and by the Silurian, animals had joined them.

The Transition to Land Although we lack direct paleontological evidence of the ancient lineages that made the auspicious leap to land, we do have robust working hypotheses as to the events and timing for this major evolutionary transition. Ancient chelicerates (arachnids and their relatives), myriapods, and some crustaceans simultaneously made their way onto the shores in the Silurian, and this included the common ancestor of the insects and entognaths (springtails, proturans, and diplurans). Eventually, these ancient hexapods became fully terrestrial and then diverged into the first insects and the first Entognatha. What we do know is that by the end of the Silurian and dawn of the Devonian, insects and their entognathan sisters were already resident on land. In fact, the earliest

hexapodan fossils, preserved as inclusions in chert from Rhynie, Scotland, are ascribable to groups that already exhibit those characteristic traits that define their lineages. In other words, these earliest of fossils are not representative of some terrestrial lineage predating hexapods. For example, springtails (Collembola) from Rhynie are of the primitive collembolan order Poduromorpha, which persists to this day. Similarly, a highly fragmentary fossil from the same deposits preserves what is perhaps the head capsule of a winged insect, indicating that flight had already evolved among insects and that considerable diversification had taken place such that not only had the originally wingless insects diversified, but that the earliest of those hexapods with gossamer wings had already taken to the skies. What these examples point to is that the invasion of land by the earliest of hexapods was already history by the start of the Devonian, and so it is in the Silurian, if not earlier, that we need to explore for our most ancient of insects and their earliest ancestor who ventured onto land. Regardless, once on land, insects found a world of niches that had never before been exploited by animals, providing a diverse evolutionary landscape in which to evolve. Today’s insect diversity is at least in part a result of their longevity, having been among the first to exploit terrestrial habitats before other animal phyla followed them.

↗ A globular springtail. These hexapods continue to bear many specializations that have been identified in the earliest fossils.

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NATURAL HISTORY

← A sample of rock from the Rhynie

→ A fossil Collembola from the upper

chert fossil beds in Scotland, a site

Eocene, 45 to 36 million years ago.

of extraordinary fossil record from

Clearly visible is the abdominal lock

the Early Devonian, around 412

and spring that allowed the hexapod

to 410 million years ago. 

to propel itself through the air.

AQUATIC VS. TERRESTRIAL

231

Aquatic Insects Every freshwater ecosystem is awash with insect life and most insect groups have invaded aquatic systems one or more times, and at different phases, during the long history of insects. In the Paleozoic, nymphs of some species in the extinct order Palaeodictyoptera, a group that was among the most dominant of insects at the time, lived in the moist litter layer of riparian zones and were likely somewhat amphibious, moving in and out of water. Ultimately, some of these may have become truly aquatic for a portion of their life. Regardless, these semiaquatic immatures would reemerge to molt to adults that were entirely terrestrial. Eventually, this same transition would take place independently in the Odonata (dragonflies and damselflies) and Ephemeroptera (mayflies), although early Paleozoic ancestors of each of these lineages were either terrestrial or semiaquatic before becoming wholly aquatic as immatures (the adults were, and remain to this day, entirely terrestrial).

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NATURAL HISTORY

Either at the end of the Paleozoic or in the Triassic, the immatures of Plecoptera (stoneflies) similarly became aquatic, and following the cataclysmic End Permian Event that decimated life on Earth, various groups of bugs, beetles, and flies also invaded freshwater ecosystems in the Triassic. Later, in the Mesozoic, aquatic beetles, flies, and bugs would be followed by caddisflies, dobsonflies, and alderflies, some Neuroptera (spongillaflies, osmylid and nevrorthid lacewings), and in rare cases even parasitoid wasps. Freshwater environments clearly represented a diversity of inviting niches for insects, and ones in which they could thrive and diversify. The biology of aquatic insects is as varied as the kinds of insects inhabiting these waters.

Dragonflies and Damselflies Representing the order Odonata, dragonflies and damselflies are perhaps the most famous of amphibiotic insects. The adults, with their sometimes large sizes, flashy wings, and garish colors are easily observed during their aerial acrobatics over ponds and streams. The approximately 6,000 modern species of Odonata are adept aerial predators, capable of snatching prey out of the air or coming to an abrupt stop to hover and survey their territory. Before taking to the skies, however, their nymphs (typically referred to as naiads), reside within streams, ponds, or lakes. Gravid females lay their eggs on the water, or often on vegetation overhanging water, with the emerging immature dropping to the water. The naiads develop in the water and are effective predators of a wide range of aquatic invertebrates, or sometimes larger species will even prey on small fish or amphibians. Many dragonfly naiads have evolved a remarkable propulsion system, whereby they pull water into the rectum as they breathe, and then expel the water with great force. The result of the water’s forceful expulsion is to shoot the naiad toward its unsuspecting prey or as a means of quickly evading a predator of its own.

↓ A damselfly nymph. The three leaflike appendages at the tip of the ↙ An osmylid lacewing, one of the

abdomen are gills. In dragonflies, the

many semiaquatic species associated

gills are concealed within the tip of

with the Mesozoic.

the abdomen.

AQUATIC INSECTS

233

↑ A mayfly naiad, or nymph. For

↓ Stonefly naiads spend much of

nourishment, some species scavenge

their time adhering to submerged

algae from stone surfaces, whereas

substrates such as rock surfaces.

others are active predators of small invertebrates in the water.

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NATURAL HISTORY

Mayflies and Stoneflies The biology of mayflies (Ephemeroptera) and stoneflies (Plecoptera) is quite similar to the amphibiotic life cycle of Odonata, although each group evolved this mode of life quite independent of the others. As adults, mayflies are short-lived and have vestigial mouthparts, or these structures may be lacking entirely. Thus, all nourishment is consumed while as an aquatic naiad. Given the short duration of the adult stage, most of a mayfly’s life is spent in the water as a naiad. Stoneflies can certainly feed as adults, but also have comparatively short lives and so spend much of their time seeking mates rather than feeding. The naiads of stoneflies, like those of mayflies, lack anything analogous to the jet-propulsion system of dragonflies, and are often poor swimmers. Nonetheless, long-bodied stonefly naiads have evolved a unique suite of abdominal muscles that allow them to produce undulations analogous to the movements of fish. Most feed on aquatic vegetation or can be omnivorous scavengers, or more rarely they can be active predators.

Water Beetles Water beetles have evolved in many groups, and many have unique ways in which they live out the aquatic portion of their lives. At least eight families of beetles have evolved to live out their entire lives—from egg to adult—in water, while many others live amphibiotically. Among the more conspicuous are those ellipsoid species of the family Gyrinidae, or whirligig beetles, so-called owing to the rapid whirling gyrations adults make while swimming on the water surface. These lay their eggs under water and

the larvae, like their parents, are active predators, feeding on a range of aquatic invertebrates. More diverse than the gyrinids, are the predaceous diving beetles, or family Dytiscidae, which comprise the majority of aquatic beetle species, currently numbering more than 4,000 species.

↓ The larva of a great diving beetle feeds on a tadpole.

AQUATIC INSECTS

235

↑ A dance fly. Adults favor wet

→ Adept architects, caddisfly larvae

habitats, with their larvae occurring

build themselves protective retreats.

in leaf litter, soil, or water.

In some cases, they affix these cases to a surface; others are detached and portable.

Flies As in beetles, flies have evolved into aquatic systems numerous times independently, and at different periods, throughout the long history of the order, stretching back into the Triassic. Many fly families have aquatic larvae, including the exceptionally species-rich families of Chironomidae (nonbiting midges) and Ceratopogonidae (biting midges). Watery habitats are varied, ranging from seeps to fast-moving rivers, and all imaginable kinds of these have been colonized by specialized flies, including the shores of the seas. Significant families, including aquatic to semiaquatic flies, are Ephydridae (shore flies), Dolichopodidae (long-legged flies), Tipulidae (crane flies), Empididae (dance flies), Muscidae (stable flies), Athericidae (ibis flies, or water snipe flies), Psychodidae (sewer flies and moth flies), and Chaoboridae (phantom midges and glassworms).

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NATURAL HISTORY

Mosquitoes Perhaps the most infamous of aquatic flies are those species of the family Culicidae, commonly named mosquitoes. A female mosquito, such as the widespread Culex pipiens, will lay her eggs in clusters directly onto the water’s surface, and these will hatch into larvae that suspend themselves downward into the water (see page 239). Eventually they form a pupa that is also suspended from the surface of the water and which splits along its back to allow the new adult to emerge into the air.

Caddisflies Caddisflies (Trichoptera) live as aquatic larvae, and often as pupae, before emerging as delicate, densely hairy mothlike adults. The larvae spin protective cases using silk secreted from specialized labial glands and incorporating a variety of materials ranging from small pebbles to pieces of bark, and with diagnostic patterns that can be indicative of particular caddisfly lineages. Some caddisflies also spin small nets outside of their cases, from which they can filter for food. Others build safety lines, silken threads that are pinned to the substrate and allow them to drag into the water column to capture small prey, before “reeling” themselves back to shelter.

No Place to Hide Several parasitoid wasps have evolved mechanisms

identified, the wasp plunges beneath the water,

to take advantage of hosts living beneath the water’s

carrying with her a bubble of air trapped beneath her

surface. One such creature is Trichopria columbiana,

wings. She swims to her host, injects numerous eggs

a tiny parasitoid wasp of the family Diapriidae, which

into the pupa’s hemolymph, and then returns to the

at its greatest extent reaches about 2 mm in length.

surface to dry

These wasps parasitize the larvae of shore flies

and seek out another host. The wasp’s eggs develop

(Ephydridae) in the genus Hydriella. Species of Hydriella

within the pupal fly, eventually killing it, and what

feed as larvae on aquatic plants and eventually form

was at first numerous parasitoid larvae is typically

a pupa in shallow water. The adult female of

whittled down to a single survivor, which chews its

Trichopria columbiana will land near, or even on, the

way out of the fly to pupate, remaining underwater

water and insert her antennae to detect chemical

and within the host’s puparium. Once the wasp

signals given off from either a damaged plant upon

emerges as an adult, it co-opts the air from the host’s

which a larva was feeding or the pupa itself. Once she

puparium, entangles the air around hairs on its

receives suitable signals that a pupal host has been

abdomen, and floats to the surface.

AQUATIC INSECTS

237

Freshwater Specializations

Primary Respiratory Systems

A shift to living in water necessitates a whole suite of physiological, morphological, and anatomical specializations for obtaining sufficient oxygen. Given the vast number of independent evolutions of aquatic life from terrestrial insect ancestors, as well as reversals to land, and even secondary reversals back to water, it is unsurprising that the specializations for breathing and thriving in aquatic environments vary considerably. In terrestrial insects, oxygen transport is largely achieved via passive movement through the tracheae. The same is true of those insects living underwater, where oxygen is moved via diffusion. The difference, however, is that aquatic insects have to maintain airflow into their respiratory system while simultaneously holding back the waters that would otherwise drown them. These challenges are faced at all stages, albeit somewhat differently for eggs, nymphs, larvae, pupae, or adults. Accordingly, different mechanisms are employed at different life stages.

Aquatic respiratory systems can be organized at first into those that are open versus closed. In other words, the tracheal system either retains some direct opening to the environment, or is entirely closed, lacking an external connection and requiring some other mechanism by which air can enter. Whether open or closed, some insects can enhance their breathing by undulating the body, generating actions that serve to move air through the body and aid diffusion at the points of gas exchange.

Direct Absorption The simplest system for obtaining oxygen is through direct absorption down a gradient and through the cuticle. An insect’s cuticle is impermeable to oxygen, but in early life stages it may be exceptionally thin and less sclerotized, thereby permitting some movement of air. Eggs in water absorb oxygen directly through the chorion, and the earliest stages of immatures can sometimes use diffusion through the cuticle to obtain oxygen. Beyond these stages, however, the cuticle becomes thicker and more impermeable, necessitating other, and more rapid, mechanisms for oxygen exchange.

Closed Systems: In insects that have closed tracheal, or apneustic, systems, the typical mechanism by which respiration occurs is via gills. Gills are cuticular extensions of the body through which tracheae run. Although it is seldom thought of, respiratory pigments such as hemoglobin are found throughout insects, and these can serve to enhance oxygen exchange with the tracheae of the gills. Gills of these sort are often found extending from the abdominal segments, typically laterally, but they may also arise dorsally, ventrally, or caudally. They can also be located at the base of legs, around the mouthparts, or within the rectum.

Open Systems: Open tracheal systems are more varied, as numerous different mechanisms have evolved by which oxygen may move through the spiracular openings and into the body. In some immature flies, such as mosquito larvae, there is a terminal siphon, a single opening at the apex of the abdomen and placed at the terminus of an extended tube. More prevalent in open tracheal systems is the entrapment of air around a portion of the body. Temporary air reserves are achieved by trapping a bubble of air at the surface of the water. The air adheres to the surface of the body, typically the abdomen, and surrounds the open spiracles. This bubble creates a microscopic atmosphere in which the insect can breathe and as oxygen is depleted through metabolic activity, diffusion will bring more oxygen in from the surrounding water. A plastron is an elaboration on the air bubble system and results in a more permanent store of air. The air bubble is larger and is produced by holding water away from the abdomen by a cuticular mesh or, more often, hydrofuge hairs. This creates a permanent air space around the spiracles with an oxygen gradient established between the water and plastron, and plastron to body. ← A mayfly naiad with leaflike gills on its back. The cuticle of gills is often thin and of a shape to maximize the surface area available for gas exchange.

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NATURAL HISTORY

↑ Dozens of Culex pipiens hang beneath the water’s surface. They are suspended from their terminal siphons, which open to the air and allow the larvae to breathe. ← A diving beetle with an air bubble. The oxygen can be replenished several times from the surrounding water before the beetle must return to the water’s surface to ensnare a new bubble.

AQUATIC INSECTS

239

Predation It is a bug-eat-bug world out there. Literally. One fundamental truth about life is that it consumes—be it base minerals and compounds or other forms of life. The act of feeding on other animals is predation. The relationship of predator and prey is a complex dance involving both participants, albeit antagonistically. The securing of prey takes the form of many modes among insects.

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NATURAL HISTORY

The Ambush While this may seem to be a simple strategy in which a predator merely has to place itself in the path of unsuspecting prey, ambushing is far from simplistic. Sit-and-wait predators must locate an ideal location in which to await their prey, and although many insects lack the visual acuity to easily pick out a motionless predator, nonetheless both predator and prey must ultimately be in sufficient proximity to allow for capture as to allow even myopic victims to discern their attackers. Further complicating the matter, insects are excellent at detecting chemical cues, including those of a potential predator.

Mimicry Many sit-and-wait predators employ a variety of forms of mimicry and camouflage. For example, a number of species of mantises in the family Hymenopodidae mimic flowers, sometimes quite elaborately, so that they may sit amid blooms and await pollinating insects or other floral visitors, capturing them upon their arrival. This kind of aggressive mimicry, in which it is the predator that mimics a harmless model, is also found in systems whereby the predator actively engages in some movement amid its prey. Larval green lacewings (Chrysopidae), rather than mimicking the surroundings, build elaborate packets of debris upon their backs, ultimately camouflaging themselves. Using

exogenous materials from the immediate surrounding of their prey, typically aphids, these larvae can approach their prey gradually, evading detection and permitting them to even sit unnoticed among a herd of feeding aphids. These sheep-in-wolves’ clothing predators may even use the carcasses of their prey to build the packet of camouflage, adding a further layer to their mask in that they take on the scent and tactile feel of their prey.

Speed and Agility Some ambush predators rely on the speed of their attack or build traps to subdue their victims. Such ambushers conceal themselves in high traffic areas, maximizing the chances of encounter while simultaneously minimizing their energy investment into the hunting effort. Dragonfly naiads conceal themselves amid aquatic vegetation and with a push from their rectal propulsion system can “fly” onto passing prey, snaring them with their elaborate labial mask. Antlions, the larvae of myrmeleontid lacewings, dig backward into the ground, such as sandy soil, ultimately constructing a steep pit. When an ant or other small arthropod passes and slips down the slope, the larva snaps its jaws around the prey, typically pulling it entirely or partially beneath the surface to restrict the victim’s ability to defend itself or escape.

← The larvae of owlflies are ambush

↑ A lacewing’s camouflage serves

↑ An antlion larva lies in wait inside its pit

predators. They lie in wait on

as a form of aggressive mimicry,

with its elongate and spinose jaws pulled

vegetation, with their large,

and also has a defensive function

back to their maximal extent.

toothed mandibles at the ready.

as it also renders the larva more challenging to find by its own predators and parasites.

PREDATION

241

Active Hunting Like ambushers, more active foragers must seek out suitable hunting grounds where they can increase their changes for encountering food.

Random Foraging Some predators proceed along a search path, pausing momentarily to survey the immediate surroundings for prey. If none are found, the searching “walk” continues. If the predator encounters a source of food, it feeds and then concentrates its search within the immediate area for further prey. By this mechanism a predator may, for example, stumble across a herd of feeding aphids or scale insects and predate upon the concentration of prey for a while before resuming the more wandering search pattern. This kind of search mechanism explains the construction of galleries of many subterranean or wood-boring insects.

Targeted Hunting In targeted hunting strategies, a predator uses cues for locating prey. At its simplest, hunters can have proscribed hunting grounds that they survey for prey. Examples include dragonflies, which may be either perchers or hawkers. The former sit on bordering foliage and fly in rapid zigzag patterns through vegetation, even dense forests, and swoop from their perches to snag a passerby.

The latter actively survey their hunting ground and seek their prey from the air, diving rapidly to snatch their victims on the wing. Similar behavioral mechanisms are employed for any territorial insects, even those that are not predatory, with some monitoring the surroundings from specific positions and others constantly patrolling their perimeters.

Visual Cues Visual signals between conspecifics are often co-opted by predators and parasites as a means of locating their prey. Perhaps the most famous insects utilizing visual signals are fireflies, whose characteristic flashes are a communication system between mating pairs. Parasitic flies use the flashes of mating fireflies as a means of honing the location of their victim. More deceptive are species of the firefly genus Photuris, which actively mimic the flashing pattern of females of other firefly genera. Males of these other genera think they are moving toward the luminescent display of a receptive female, only to be confronted instead by an awaiting Photuris, which then consumes the hapless suitor.

Auditory and Chemical Cues To aid prey detection, active hunters often rely on auditory and/or chemical cues. Mating calls, for example, can be tracked or even co-opted by predators. Ormiine tachinid flies “listen” in for the mating calls of crickets and then lay live larvae, rather than eggs, near to the singer. The larvae then bore into the vocalist. Other predators may even use the wing-beat frequency of their prey as a means of locating their meal, and many parasites of wood-boring insects use the vibrations generated by the boring larva as a mechanism for localizing their prey. In these same fashions, chemical signals are a rich source of information for hunters as they seek their food. In fact, chemical communication predominates among insect signal-receiver systems, meaning that these are often the primary way in which predators track their prey. It is not uncommon for predators and parasites to “sniff” out prey by following the latter’s sex attractant signals, or sometimes even the odiferous emanations of the prey’s frass.

← Photuris lighting up at dusk. In this

→ Having snagged its prey from the

species, bioluminescence is used to

air, a percher-type dragonfly will

attract prey, rather than as a means

return to its perch or settle on the

of conspecific communication or a

ground to feed.

warning to predators.

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NATURAL HISTORY

Influence of Predation on Prey While predation may be lethal for a given individual, for the population and species as a whole, the influence of predators can lead to the selection of defensive specializations among the prey.

Concealment Any number of insects have evolved to build retreats in which to hide. For example, the cases of caddisfly larvae look like inedible objects, such as clusters of sand, tiny stones, or pieces of bark. One system by which an insect can enhance its ability to hide, even while allowing the prey to continue its own feeding, seeking of a mate, or reproduction, is crypsis—when an insect resembles its general surroundings or other objects, even the smell, behavior, or sound of other objects. At its simplest this may result from an overall cryptic coloration (camouflage), rendering the insect difficult to distinguish against the

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NATURAL HISTORY

background. In these cases, the insect is not necessarily otherwise physically modified such that it is the color and pattern alone that aids the ability to hide, although camouflage is also combined with morphological specializations. In mimicry, a specialized form of crypsis, the camouflage is extended to include augmentations of the insect’s morphology to resemble a specific object. The truly iconic mimics among insects are the many species of stick and leaf insects (order Phasmatodea). Through exaggerations of their morphology, as well as cryptic coloration, these insects have an uncanny resemblance to the foliage on which they feed.

Physical Defenses Several insects have evolved physical defenses such as an abundance of spines. Thorny stick insects, like the endangered Lord Howe Island stick insect, are large and richly beset with stout, sharp spines throughout the body. Alternatively, some stick insects, if captured, will simply shed a snagged limb through autotomy, allowing the insect to drop and escape. In some autotomized stick insects and mantises these legs can be regrown as the process triggers an unusual post-adulthood molt. If the autotomy takes place as a nymph, then the leg is regrown at the next molt in the usual process of growth.

Chemical Defenses Chemical defenses are rife among insects. The aforementioned stick insects have a characteristic pugnacious gland in the prothorax that will spray a noxious chemical for discouraging predators. Likewise, chemicals sprayed from the cornicles of aphids are effective at defending against predators. ↑ Two Lord Howe Island stick insects. ← Insects have evolved forms of

Any predator grasping these insects is

mimicry in nearly every lineage—from

confronted with a stabbing pain that

treehoppers resembling thorns on

will likely trigger the release of the

stems (top left) and katydids with

potential prey.

wings shaped and even mottled and veined like leaves (bottom left) to

→ It is likely that aphids with the

moths virtually indistinguishable

longest cornicles use them to

from bark or broken twigs (right).

scent-mark colony intruders. 

INFLUENCE OF PREDATION ON PREY

245

Top Predators We live in a world of predators and prey (even herbivores can be thought of “predating” on plants). In a literal sense, life is about hunting or being hunted and evolution has resulted in a myriad of melodies to which this dance is played.

246

Assassin Bugs and Mantises

Vampiric Lacewings

Assassin bugs (the family Reduviidae of the order Hemiptera) and mantises (order Mantodea) are ideal examples of sit-and-wait predators. In both, predators situate themselves in locations where prey are known to traffic, such as on flowers for those feeding on anthophilous insects. Many have a cryptic coloration that allows them to blend into the setting, and they remain motionless until prey arrives. While assassin bugs can grasp their prey, their primary means of subduing a victim comes from quickly stabbing it with their stout proboscis and injecting saliva that begins to predigest the internal fluids of the prey. Mantises, with their characteristically long, spiny forelegs, can rapidly snatch prey. Spines on their forelegs aid in partially crushing the prey, permitting the mantis to commence its feeding.

As adults, lacewings are delicate insects, typically flitting amid flowers and feeding on pollen and nectar. As larvae, lacewings are literally vampires feeding on the blood and other digested tissues of their prey. The larvae have specialized, typically elongate jaws composed of united mandibles and maxillae. These form a feeding channel between them through which the larva can secrete salivary enzymes that extraorally digest prey, and also through which the larva can then suck the fluids of its victims.

↑ Assassin bugs use their front legs to

↑ Mimicking an orchid, this mantis’s four

hold their prey still while they feed on

walking legs resemble petals. The mantis

them using a proboscis.

uses it front pair of legs to grasp prey.

NATURAL HISTORY

Gift-Giving Scorpionflies and True Flies Predatory flies and scorpionflies are much like most predators, but some are noteworthy in that they may capture prey as an offering to their mates. Nuptial gifts

← A green lacewing larva subdues an aphid. These larvae also prey on mites, thrips, and small caterpillars.

have evolved in many insect groups, but two of the best examples are among the scorpionfly Bittacidae, more widely referred to as hangingflies, and the true fly family Empididae, or balloon flies. In both, there are species in which the males capture small arthropod prey that they in turn offer to the female as a nuptial gift. If the female accepts the gift, she typically commences feeding while the male inseminates her. Balloon flies wrap their gift within a silken case, and in more elaborate cases males build an empty silken balloon, offering it to the female as a gift and obtaining access to her without actually including any prey within.

Wasps Wasps are famous predators. Many are generalist, although just as many are known to specialize on particular groups. For example, among stinging wasps there are those that prefer spiders, or leafhoppers, or various families of beetles, among others. In each case the predator has evolved special handling techniques for subduing their prey, and in each they will bring the prey back to a burrow, often built in advance and then provisioned, although sometimes the prey is killed first and the wasp builds a burrow at or near the site of the kill, thus avoiding the need to transport a potentially heavy prey item back to the nest. Often the wasp lays a single egg on the prey before beginning a new brood chamber, but in some species multiple eggs are laid. Either way, the larva(e) feed and develop on the prey.

← A blue spider wasp takes on a spider at least its own size. Spider wasps have spines on their front legs for burrowing into the ground.

TOP PREDATORS

247

Parasites and Parasitoids Parasitism is a special form of predation in which the predator lives upon or within its host, consuming only enough to permit its own life to proceed but without leaving the prey so harmed that it cannot complete its own life cycle. Many insects do not simply live at the expense of their host, but go so far as to kill their host during the process. Such insects are parasitoids. Both parasitism and parasitoidism have evolved numerous times independently throughout insects, and with their victims restricted to other invertebrates.

Blood-Feeding Parasites The consumption of vertebrate blood (hematophagy) by insects is a form of parasitism that in many cases is harmless to the host. For example, the bite of a black fly (Simuliidae) can be painful or that of a mosquito can produce an irritating itch, but in the absence of a pathogenic nematode, protozoan, virus, or other infectious agent, these temporary episodes of feeding do not do permanent damage to the host.

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NATURAL HISTORY

Many hematophagous insects do not live on their host, arriving only during episodes of feeding. Thus, they are a kind of bridge between true predators and more permanently affixed parasites. They feed as parasites but engage in many of the same hunting mechanisms as those of predators. In many cases the evolution of blood-feeding has taken place in insect lineages where the mouthparts were already specialized for piercing-sucking forms of feeding. Thus, many of our most notorious blood-feeders

had their origins among lineages of phytophagous insects feeding on plant fluids, before supplementing their diets with blood. In other cases, blood-feeding insects arose from insects that were already predators of other arthropods, piercing the exoskeletons of their prey and sucking internal fluids, including extraorally digested tissues and hemolymph. For those parasites that are obligate hematophages— that is, they feed exclusively on blood—it is typical to find such species living in close association or upon their hosts. Two such groups are bed bugs (Cimicidae) and kissing bugs (Reduviidae: Triatominae), both in the order Hemiptera. These live in the roost or nest of their host, spending much of the active period of their hosts, which are often diurnal, in sheltered peripheral areas of the nest or neighboring environment. When the hosts rest, the hematophages emerge from their refuges, climb on to the host, and commence their feeding. Once full, the parasites return to their refuges. Feeding on a prey that is left alive is a challenge, well beyond simply locating a suitable host. Prey fight back. Most hematophages have evolved mechanisms to feed quickly and undetected, departing before the host has realized they have been fed upon. For example, mosquitoes secrete saglin with their saliva. This protein circumvents coagulation and vasoconstriction by the host, while also initiating inflammation and potentially reducing immune responses. The result is a cascade of physiological responses that aid the parasite’s feeding, allowing it to consume effectively and rapidly.

↑ A bed bug. Like mosquitoes, bed bugs can locate a host by their exhalations of breath. ← A kissing bug may secrete lipocalins and other compounds into a wound to subdue its prey long enough for it to feed. → Perhaps the most familiar blood-feeding insects are the many lineages of flies. They are often nothing more than a botheration, but some can transmit disease.

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249

Non-Obligate Blood-Feeders Many parasites, including black fly and mosquitoes (pictured), are not obligate blood-feeders and continue to feed on nectar or pollen as did their phytophagous ancestors. Indeed, male black flies and mosquitoes do so exclusively and are not known to feed on blood.

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NATURAL HISTORY

Ectoparasites and Ectoparasitoids Many parasites live externally on their host. Called ectoparasites, they face the even greater challenge of not only feeding on the host, but evading detection while completing their life cycle from egg to adult. They are often cryptically colored so as to blend into the integument of the host. Beyond this, such parasites lose their ability to fly as the wings are no longer used for movement and would be in the way while crawling on hosts that typically have some form of integumental covering (feathers or hair). An abililty to compress the body renders ectoparasites more challenging to dislodge, grasp, and/or detect. Parasitoid larvae may also live externally on their host. Unlike ectoparasites, which are often adult insects, the adults of ectoparasitoids only interact with the host in order to lay eggs, and do not feed on the host themselves (most ectoparasitoids feed on nectar or pollen as adults). It is the larva that feeds externally on the host, killing it as it reaches the completion of its development.

↑ Typically, ectoparasites are minute relative to the size of their hosts; this helps them to avoid easy detection, such as this flea. ← Ectoparasites have evolved structures for grasping their host, so as to not be dislodged during the regular movement of the host as well as to make them harder to remove if discovered. Ticks (actually arachnids, not insects), such as this, can be notoriously difficult to dislodge once they have begun to feed.

PARASITES AND PARASITOIDS

251

Parasites: Lice and Fleas

↓ A louse fly, Crataerina hirundinis, an ectoparastic fly that favors the house martin.

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NATURAL HISTORY

Typical of ectoparasites, lice and fleas have lost their wings and have evolved short yet elaborate grasping legs for holding onto the hair or feathers of their hosts. Both are compressed, lice dorso-ventrally and fleas laterally. While lice rely on host-to-host contact, fleas have evolved characteristic jumping hind legs, allowing for them to rapidly leap from one host to another. Like all parasites, both feed on the host, but do not do permanent damage unless allowed to proliferate to unsustainable numbers for both the host and the parasite population.

Lice

Fleas

Lice are a parasitic lineage of the free-living bark lice, and together comprise the group Psocodea. While bark lice are scavengers or phytophages, lice are obligate hematophagous ectoparasites. Historically, they have been organized into two groupings based on their mouthparts and mechanism of feeding. Sucking lice, as their name implies, have a short proboscis through which they feed on the fluids of exocrine glands, such as hair follicles. By contrast, chewing lice feed on skin fragments or feathers, or other debris that may accumulate on the bodies of their hosts. Lice have been about since at least the Cretaceous, likely feeding on feathered dinosaurs and early mammals, and then diversifying more extensively as mammals radiated in the Cenozoic. Interestingly, on a given host, lice species often specialize onto specific body areas. Human lice are an excellent example of this specialization, with head lice, body lice, and pubic lice. This can also be true for bird lice, with different species found, for example, only on the head or tail.

Fleas (Siphonaptera) are a parasitic lineage of scorpionflies (Mecoptera), having diverged from their other scorpionfly relatives at least by the Late Jurassic, with particularly large, nearly 2-in-long (5 cm) fleas present at that time and already exhibiting those features consistent with a transition to living upon a host. These Jurassic fleas likely fed on feathered dinosaurs and early mammals, perhaps multituberculates. Adults have piercing-sucking mouthparts for consuming the blood of their hosts, while their larvae have chewing mouthparts and feed on skin or other debris, but also on the fecal material of adult fleas, which includes dried blood from the host.

↓ A closeup of the bird louse Dennyus hirundinis on a feather.

PARASITES AND PARASITOIDS

253

Endoparasites and Endoparasitoids

Superparasites

Parasites that live and feed within their host are called endoparasites, and those that kill the host are endoparasitoids. In these systems, whether as parasites or parasitoids, the infecting agent is the larva, and the adults are free-living. In the simplest of systems the adult female delivers the egg to the host directly, while in more complex systems, such as that of the bot fly, eggs are delivered through an intermediary (see page 331). While this seems an evolutionary favorable strategy meant to avoid easy removal by the host, such a transition comes with its own challenges, most important of which is the evasion of the host’s immune response. The deployment of proteins that inhibit the immune response are analogous to those used by temporary hematophages, such as bed bugs.

This is the phenomenon whereby an adult female injects numerous eggs that develop simultaneously within the host. Many endoparasitoid larvae kill competitors within the host, but some are tolerant of conspecifics. Naturally, in these cases, the parasitoids are exceptionally smaller than the host, so that there is sufficient tissue for all of the developing larvae. Parasitoids are not immune from falling victim to yet other parasitoids, the latter called hyperparasitoids. In such cases the hyperparasitoid is called a secondary parasitoid, and secondary parasitoids may themselves be victimized by tertiary parasitoids.

→ A mouse bot fly. These flies lay their eggs close to rodent dens at the edges of forests. Their larvae burrow into white-footed mice and other hosts, through any orifice or open wound.

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NATURAL HISTORY

Avoiding the Immune System A critical challenge for the success of any

place in the wasp glands, seemingly initiated in the

endoparasitoid is overcoming the host’s immune

wasp’s pupal stage. Regardless, genes expressed while

defenses. Endoparasitoids can be loosely grouped

inside the host produce proteins that interfere with

into host conformers versus host regulators,

the host’s immune response. These include a reduction

depending on how they respond to, and interact

of the host’s specifically targeted immune response

with, the host’s endocrine system. Host conformers

against the parasitoid egg and larva, along with an

are presumably influenced by the host’s hormones,

overall general diminution in the host immune system.

with the result that the timing of molts, diapause,

Some may also reduce the number and/or block the

and other developmental and physiological responses

spread of the host’s hemocytes, augment host feeding

are effectively synchronous. In host regulators, the

and development, and elevate dopamine levels, among

parasitoid secretes chemicals that assume control

a suite of other physiological cascades.

over the host’s development. Some parasitoid species secrete juvenile hormone (JH) to slow the development of their host or may release toxins that completely arrest molting in the host. Either way, a critical step for the parasitoid is to prevent encapsulation, whereby the host’s immune system surrounds the foreign body by hemocytes, ultimately forming a tight covering, which surrounds the parasitoid and by phagocytosis and isolation from nutrients kill the invader. Some endoparasitoids cover themselves with host tissues or secrete compounds that mimic the host’s own proteins so as to “hide” within the body. More disruptive means include consuming the host rapidly, such that in its more weakened state the immune response is unable to overwhelm the parasitoid. An alternative is to suppress the host’s immune response, which is usually achieved via viruses associated with the parasitoid. Parasitoid wasps of the families Ichneumonidae and Braconidae deploy polydnaviruses, released from glands associated with the ovaries or oviducts. The viruses do not express their genes within the wasp but once inside the host they become expressed, although the viruses do not replicate. Replication only takes

→ Sometimes known as scorpionwasps, ichneumon wasps typically have long, slender, curved bodies with tiny waists.

PARASITES AND PARASITOIDS

255

→ A female cuckoo bee, Nomada goodeniana. She lays her egg in the nest of an Andrena species host bee and her larva will feed on its pollen after hatching.

256

Manipulating Hosts

Cleptoparasites

Along with the chemical manipulation of the host’s immune system, physiological influences on the host can also result in the parasitoid stimulating the host to do things counter to its own biology but instead in favor of the parasitoid’s ultimate goals. Some Strepsiptera parasitizing ants influence their hosts behaviorally to position themselves at the tips of foliage to increase the chances of males locating the host and exposed female, as well as to aid the dispersal of emerging adult males and larvae. An ichnuemonid parasitoid of web-spinning spiders actually induces the spider to spin a completely different kind of web as it nears the completion of its life cycle within the spider host. Ultimately, the spider spins a web that isolates the spider at the end of a long, sturdy, nonsticky silken structure in which the wasp is eventually able to emerge and form a cocoon within, safe from its own potential predators and parasites.

A specialized kind of parasitism, cleptoparasitism has evolved numerous times in insects. Here, rather than feeding on its host, a parasite consumes resources collected by the host. These are known as cuckoos, for the birds that lay their eggs in the nests of others, which then feed the cuckoo fledglings. This behavior appears among flies, beetles, bugs, wasps, and other arthropod lineages. In bees, cuckoos account for nearly 13 percent of bee diversity, significantly outnumbering social species, which comprise only about 5 percent of global bee species. Cuckoo bees, as their name implies, sneak into the nests of solitary bees, lay an egg in the brood chamber of the host, and then depart. The cuckoo bee larva emerges, and usually exhibits a series of dramatic modifications to the head capsule necessary for dispatching the host egg, before molting into an otherwise typical bee larva that then consumes the pollen and nectar within the chamber. Other cleptoparasitic bees raid the nests of other bees to steal their resources, while others still take over an entire existing colony, forcing the workers of the host to raise the new queen’s brood.

NATURAL HISTORY

Surprising Parasites While insects like fleas and lice are well-known

lay their eggs on vegetation overhanging water and

parasites, there are numerous other lineages and

upon hatching the specialized larvae drop into the

forms of parasitism that largely go unnoticed.

water where they seek out freshwater sponges. This host specialization gives them their common name

01 Earwigs: The family Arixeniidae includes five

as spongillaflies. The larvae use piercing mouthparts

species of rather robust, blind, and wingless earwigs

to suck nutrients from the host’s cells. Once it

that live as ectoparasites on bats in Southeast Asia.

completes its various larval stages, a developing

In much the same fashion, the family Hemimeridae

spongillafly crawls onto land and forms a pupa,

comprises 12 species of similarly robust, blind,

hidden in crevices amid bark or rocks, that waits

wingless, and forceps-less earwigs, although they

out winter before emerging as an adult.

are typically smaller than arixeniids. Hemimeridae are ectoparasites of African rodents, and are unique

03 Butterflies: Complex forms of ectoparasitism

among earwigs for giving birth to live young rather

occur in the Lepidoptera, among them the Eurasian

than laying eggs. Species of both Arixeniidae and

butterfly Maculinea arion, which is a parasite of the

Hemimeridae live in the roosts and nests of their hosts,

ant Myrmica sabuleti. The adult butterfly lays its eggs

crawling onto the host and amid its fur to scrape dead

on thyme. The emerging caterpillar has specialized

skin. The amount of time they spend on the host varies,

chemicals that help it mimic the scent of ant larvae,

with some only crawling about for brief periods to

and soon worker ants of M. sabuleti bring the

feed, while species of Hemimerus, for example,

caterpillar back to their nest and begin feeding it.

scarcely ever depart from the host.

Sometimes the caterpillar shifts to feeding directly on the ant larvae, rather than begging for food from

02 Lacewings: Among lacewings, one lineage, the

the worker ants, although parasitic caterpillars far

family Sisyridae, has evolved into parasites with a

outnumber those that choose to become predators.

rather surprising host choice. Species of Sisyridae

01 Arixenia esau earwigs.

02 A Sisyra fuscata spongillafly larva.

03 A Myrmica species ant tends to a large blue butterfly larva.

PARASITES AND PARASITOIDS

257

Parasitoids: Wasps Wasp parasitoids are diverse because they are ancient and because they have specialized on virtually all other groups of insects, and even other arthropods such as spiders. Thus, as other insects diversified for varied reasons of their own, the parasitoids were in tow, radiating to take advantage of new insect lineages as they appeared. The first Hymenoptera were phytophagous, feeding as adults and larvae on a range of floral hosts. By the end of the Triassic, some of these had evolved specialized wood-boring larvae,

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NATURAL HISTORY

feeding on the tissue as it is broken down by a symbiotic fungus. The adult female carries the fungus at the base of her ovipositor and inoculates the wood at the same time she deposits an egg. Likely by the end of the Triassic, there had been a shift in which some wood wasps became parasitic on other wood wasps, feeding as ectoparasitoid larvae on the larvae of their nonparasitoid relatives. From there these parasitoids spread to other wood-boring insects, principally beetles. Parasitoid wood wasps persist to present and are represented by the family Orussidae.

The Wasp Waist By the Early Jurassic, the ancient wasp parasitoids evolved a constriction between the first and second abdominal segments, giving the abdomen considerable flexibility while the first segment fused into the back of the wasp thorax. The result of this new waist was greater control of movement for the ovipositor, meaning eggs could be placed with greater accuracy and within a much broader range of substrates—from piercing through thick wood to reach a beetle larva within to stabbing through the side of an exposed caterpillar. With this innovation parasitoids exploded in diversity, such that today Hymenoptera, dominated by parasitoids, is one of the most species-rich orders of insects; some postulate ← A female ichneumon wasp. Among the most diverse of groups within the Hymenoptera, there are currently as many as 40,000 known species of

that the as-of-yet still largely unexplored diversity of microscopic wasps (microHymenoptera) will ultimately surpass that of beetles, making Hymenoptera the most diverse group of all animal life.

ichneumon wasps.

PARASITES AND PARASITOIDS

259

Parasitoids: Flies and Beetles After Hymenoptera, the second largest number of larval parasitoids are found among the flies, with beetles in third place. While parasitoidism evolved once in Hymenoptera, parasitoids evolved independently many times among flies and beetles.

Tachinid Flies The most diverse group of parasitoid flies are the many species of the family Tachinidae, which develop as endoparasitoids of moth caterpillars (and the dayflying moths known as butterflies) and sawfly larvae, although their complete range of hosts includes crickets, bugs, and even beetle larvae. Many tachinids oviposit eggs directly into their host’s body, the females having evolved well-developed ovipositors to perform these injections. Other tachinids lay eggs or even live larval young in the environment of their host and the mobile planidium larva seek out the host. In some rare cases females lay live larval young directly onto the host.

Wedge-Shaped Beetles Parasitoids have evolved multiple times in beetles, the most famous of which are the many species of the family Ripiphoridae, commonly called wedge-shaped beetles. As larvae, ripiphorids are parasitoids of wood-boring beetles (subfamily Pelecotominae), aculeate Hymenoptera, such as bees and wasps (subfamily Ripiphorinae), or roaches (Ripidiinae). Taking the last as an example, the larviform adult females of Ripidiinae lay eggs in habitats occupied by roaches. The first instar, a triungulin, locates a roach nymph, burrows its head into an area of intersegmental membrane, and commences feeding. After a few weeks of external feeding, the larva molts to a more wormlike second instar and moves fully within the roach as an endoparasitoid where it, at first, sets up a quiescent existence within the roach’s abdomen. Once the roach has grown to a sufficient size, the beetle again molts to a third-instar larva that has long legs, and eventually to a similar fourth-instar larva, which when ready to pupate, bursts from the roach. In rare cases the roach reaches adulthood before it is dispatched, but in most instances the beetle terminates its host while still a large nymph.

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NATURAL HISTORY

← Metoecus paradoxus, also known

↖ A birch shield bug with the eggs

as the wasp-nest beetle, a parasitoid

of the tachinid fly Subclytia

of wasps.

rotundiventris around its head.

Unexpected Parasitoids As with the parasites, there are numerous other lineages and forms of parasitoidism that largely go unnoticed.

Mantispid Lacewings Adult mantispid lacewings are active hunters of a wide range of small arthropod prey, but as larvae they live quite different lives. The larvae of the subfamily Mantispinae, for example, parasitize spiders, feeding on their eggs; larvae of the subfamily Symphrasinae parasitize hosts as diverse as moths, scarab beetles, sawflies, wasps, and bees, laying eggs within their brood chambers. A larva feeds externally on its host until it is ready to pupate, by which time it has killed its host. This is certainly an unexpected start to the life of what will otherwise appear as a comparatively delicate, albeit raptorial, lacewing as an adult.

Caddisflies As larvae the Australian Orthotrichia muscari feeds externally on the pupae of other caddisflies, specifically those in the family Hydropsychidae, before ultimately killing their host. Other species of Orthotrichia are not known to be parasitic and instead feed on algae, although the biology of most species from Southeast Asia, New Guinea, and elsewhere in Australia remains undiscovered. Thus, there could be an entire diversity of caddisfly parasitoids awaiting discovery in the margins of rivers, ponds, lakes, and streams throughout the region.

Moths Perhaps the most unexpected ectoparasitoids are those among moths. Species of the crambid moth genus Chalcoela invade the nests of eusocial paper wasps (Polistinae). The moth larvae feed on the larval wasps within their brood chambers, ultimately killing their host and spinning a cocoon of webbing, but which is effectively ignored by the adult wasps. ↑↑ Mantispid lacewings superficially

↑ Crambid moths of the genus

resemble praying mantises, with

Chalcoela overwinter in cocoons that

well-developed raptorial forelegs

they spin inside wasps’ nests before

positioned forward on an elongate

emerging as adults in the spring.

prothoracic segment, and large compound eyes on a moveable head.

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261

Sociality Insects are diverse, shockingly so, in fact. They not only account for the greatest number of species in our world, but they also represent tremendous proportions of our global animal biomass, with an estimated 10 quintillion (1 with 19 zeros) individual insects alive at any given moment. And yet, if we look carefully at the organization of this abundance and ecological heft, it is readily apparent that it is not distributed equally across all lineages. Indeed, in terms of biomass ants and termites together are the real behemoths, dominating insects and other animals. A common feature of these two ecological titans is their sociality. Both live in large, perennial colonies and can leverage their abundance to reshape entire landscapes. Ants and termites are often referred to as ecosystem engineers as the sheer number of their individuals can denude and recycle woodlands and seed the formation of new forests.

Parental Care The simplest social interactions are those between mother and offspring. In many insects, the mother and her offspring never meet. Her investment in their success ends by providing the egg the best chance for survival in the form of nutrients and a placement in which to develop unharmed. This is, perhaps, for the best, as in some predatory species, the hatching offspring may quickly devour the mother and fight with siblings as the most their

→ Ants and termites can alter landscapes at such a tremendous scale as to be most appreciated when viewed from orbiting satellites.

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NATURAL HISTORY

relatives can offer is an easy meal. Innumerable times throughout the insects, there have been notable deviations from this, with mothers and offspring overlapping peaceably, and maternal investment can carry forward through a sharing of resources and added protection.

Doting Mothers Maternal care can be found virtually everywhere, from treehoppers to leaf beetles, and in some unexpected groups such as earwigs, roaches, zorapterans, web spinners, and bark lice. In some of the simplest systems, a mother will cover her eggs or larvae with her body. In more elaborate systems, the female digs a small burrow into the soil into which she lays her eggs and will eventually provide them with food, all while she guards the entrance. Some females may feed their young unfertilized eggs before foraging for more proper food sources, and in species like

carrion beetles, the female may even start by feeding her larvae regurgitated food before they begin feeding directly themselves. The regurgitation of food from the mother to her young offers not only the transfer of nutrients, but also elements of the gut microbiota, which may be critical to the nutritive success of the youngling throughout its life. Maternal care typically involves defense from predators and parasites, provisioning of food, grooming, and sometimes sharing of some kind of protective burrow. Grooming is an important function of mother–brood interactions, or even between siblings or any individuals in a communal arrangement. For example, zorapterans living in small subcortical spaces feed on fungal spores and small arthropods living in the decayed wood. Naturally, as their log undergoes further and further stages of decay, individuals can become covered with hyphae and these may include at times pathogenic fungal species. The gregarious individuals in a zorapteran colony regularly groom each other in order to remove these hyphae, keeping their siblings or colony mates healthy.

Caring Fathers While it is true that most cases of parental care involve a mother tending her brood, in some species males do participate. Male dung beetles sometimes assist the female in building the dung ball and burying it for their young, and males of giant water bugs will actually carry the eggs on their backs, even those fertilized by other males.

→ A giant water beetle, Abedus ↓ A female stink bug shields her eggs

indentatus, carries a clutch of eggs

with her body, much like a bird sitting

on his back as he floats on the

on her eggs.

water’s surface.

SOCIALITY

263

The Spectrum of Societies Several types of social arrangements exist. Common to all of them is the existence of a nest, or some kind of area in which the interacting individuals congregate and live. The space is used as a collective shelter and mothers will defend their brood crowded within.

Communal Nesting Some simple levels of social organization are tolerant of unrelated conspecifics living within close proximity, forming an aggregation of individuals living solitary lives but alongside others of their species. Such tolerance of conspecifics may even be formed as a communal society in which individuals continue to provision for their own

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NATURAL HISTORY

brood, but share a common burrow off which each female works in an otherwise solitary manner. In such systems, the individuals benefit from shared defense as, at any given moment, the shared burrow may have a female within to defend the nest from any invading predator or parasite. Communal nests may initially form from daughters remaining and building their own burrows within the natal nest when suitable nesting locations may be scarce in the environment. Naturally, communal nesting may be easily gamed by selfish individuals that attempt to take over nesting space and provisioned resources. Thus, while shared defense against predators and parasites is a plus, an individual may need to invest an equal amount of time into defending against any potential usurpation.

← A sweat bee peeks out from the entrance of its communal nest. This is one of several species of bees that exhibit the full range of societies. → Tent caterpillars are subsocial as larvae, building a shared nest of silk (the tent). If the population grows large, then the insects can be quite a pest. ↘ Webspinners, such as this Oligotoma nigra, live gregariously within a communal silken gallery. Each webspinner rears her own brood independently, but they share a common gallery for defense.

Subsocial Societies A subsocial society may be formed as a gregarious association whereby related or unrelated individuals live together with overlapping generations, but without any further specialization in terms of roles within the society (that is, there are no workers, queens, or kings and all individuals undertake their own reproduction). Zoraptera are highly gregarious insects, living in small colonies of up to 125 individuals. Individuals are tolerant of conspecifics, even those that are distantly related, meaning that nearby colonies can be encouraged to merge without much difficulty as the zorapterans welcome others into their gregarious community. Individuals groom one another and benefit from living within a shared subcortical space, and the protection their numbers and colonial chamber provides. SOCIALITY

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Eusocial Societies Beyond communal nests and subsocial societies, all other societies have a common feature: a reproductive division of labor. This basically results in a society in which there are some individuals that forego their own reproduction in order to assist a sibling or parent achieve their reproduction. At its most extreme level of expression, this reproductive division of labor is fixed, developmentally and morphologically, resulting in reproductive individuals (queens and kings) opposite a larger number of sterile individuals that cover most all other functions of societal life, including caring for brood that is not their own, although still closely related. In addition, there is an overlap of generations, with offspring assisting their parents. Such societies are called eusocial (literally meaning, “truly social”). Eusocial societies can be described as either “dynamic” or “anchored” when referencing the degree of fixity between workers and queens. In dynamic systems, workers are still reproductively capable, either behaviorally

← Young worker honey bees are called nurse bees. Their role in the nest is to feed the larvae, including a growing queen, with a mixture of honey and pollen, together with a substance they secrete called royal jelly.

↑ A small hole in the ground is the only evidence of a termite nest below the surface. A few workers wait at the entrance.

or physiologically suppressed by the queen, but often exhibiting a range of sizes such that there is little distinction between a major worker and the queen, other than dominance and reproductive output. If the queen dies, a worker can mate and assume control over the nest as a new queen. In anchored systems, the workers are sterile, or effectively so, and there is a distinct morphological distinction between workers and queens.

Quasisocial and Semisocial Societies There is a wide range of social systems that have been wrongly lumped under the term “presocial,” which implies that they are not yet quite social even though there is, in fact, considerable social organization and cooperation. The basic difference in these societies relative to a eusocial colony is the lack of overlapping generations. Quasisocial societies are those in which individuals of the same generation live communally, cooperate in brood care, and are all reproductively viable and lay eggs. Semisocial systems are a twist on this in that they add to this a reproductive division of labor, but lack the overlap of generations that would otherwise make them a eusocial society. SOCIALITY

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Communication

Recognition Signals

Regardless of whether a colony is quasisocial, semisocial, or eusocial, it is necessary that individuals have some means of communicating with one another. Whether it is identifying family members versus unrelated individuals or broadcasting messages relating to defense, tending brood, foraging, or reproduction, no society can operate without signal-receiver mechanisms.

Guards need to recognize nestmates, admitting entry to the nest rather than sounding an alarm. This is usually achieved via an “odor” of cuticular hydrocarbons, that allow individuals to identify close relatives versus unrelated intruders. In fact, not only do they allow for recognition, but individuals of nearly every social insect lineage will orient toward such odors, serving as an attractant to the nest and nestmates. Chemical cues are also emitted by queens in some social insects to control workers, signaling them to tend to the queen and relinquish their reproductive activities. The famed “queen substance” of honey bees is one such cue, secreted from the queen’s mandibular glands and acting to control workers, suppressing the development of their ovaries and initiating grooming of the queen by workers.

Alarm Signals The nest of a social insect represents a tantalizing concentration of resources and individuals, attractive to any predator. In every kind of society there are alarm chemicals or behaviors that tell nestmates either to flee the source of attack or to join the defense. Alarm chemicals are rife throughout insects. In termites, alarm signals tell workers to retreat but bring soldiers to the site of the conflict. In addition, some species vibrate against the walls of the nest to create an audible noise and detectable surface vibration, all of which also signals to nestmates that the colony is in distress.

↓ With the trail pheromones

→ Ants use a range of volatile

produced in their abdominal glands,

chemicals and stridulation to

worker ants leave trails that connect

raise the alarm.

food sources to the nest entrance.

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Relaying Information Other forms of communication include the laying of foraging trails by ants or recruitment cues that bring workers to a nesting site or valuable food resource. Both termites and ants employ abdominal glands to lay down trail pheromones, typically to food sources, but also in termites to bring workers to sites of nest damage (such as breaches by predators) to begin sealing the opening while soldiers engage combatants. Social wasps and some bees also use a trail pheromone, but rather than depositing it as a semicontinuous signal along the ground, these insects place the chemical signal at the vegetation at various points along a flight path, much like a series of signal fires spread over a distance.

The Waggle Dance Perhaps the most remarkable form of communication

in all but three Asiatic species of honey bees performs

in social insects is the famous waggle dance of honey

her actions in the dark of an enclosed hive and on

bees. A worker returning to the nest after locating a

the surface of a vertical comb, and so the information

food source recruits additional bees to the location

about the sun’s position is abstractly represented by

by conveying information via odor cues from the

a straight upward dance. Thus, if a dancer wishes to

site as well as by dancing in a figure-eight fashion.

convey a location situated 30 degrees to the left from

The middle straight portion of the dance consists

the position of the sun in the sky, she dances 30 degrees

of a vigorous waggling of the abdomen, while the

to the left of straight up on the surface of the comb. All

alternating return dances lack waggling. The waggle

seven species of honey bees communicate via a dance,

portion of the dance conveys both direction and

although in the dwarf and giant honey bees nests are

distance information. The frequency of the waggle

built in the open rather than within cavities and so

indicates the distance from the hive, and is detected

there are variations to the dance in relation to their

by potential recruits that place their antennae near

orientation toward the sun and whether they are

the dancer’s abdomen, detecting the fine frequency

dancing on the horizontal top of the nest.

of the vibrations. The orientation of this waggle portion indicates the direction to the resource in relation to the position of the sun. A dancing bee

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Polyethism Division of labor goes beyond just reproductive capacity in insect societies. The castes perform different duties such as foraging, cleaning, grooming, nest construction, and defense, among others. This range of behaviors is referred to as polyethism, and not only do the behaviors differ between the castes, but even within a caste, behaviors may shift throughout the course of an individual’s life. For example, young workers often remain within the nest tending to brood and queen, grooming, and cleaning, and then shift toward more activities outside of the nest, such as foraging, as they age. Some castes lack much in the way of polyethism, such as termite soldiers, which are effectively confined to their one role as they lack morphological traits allowing them to do anything else, including feeding themselves.

Next page A queen of Azteca protects her brood inside her domatium within Cordia nodosa. The plant produces domatia—small chambers formed of specialized leaves to shelter the ants. In turn, the ants actively defend the plant from herbivores. Together the Azteca ants and their host plants ↓ Inside their nest, worker bees are

are one of the most remarkable of

busy cleaning the honeycomb.

insect-plant mutualisms.

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The First Societies: Termites Long before any other societies appeared, there were termites (Isoptera). Their societies were the first on Earth and appeared during a time in which the planet was populated by pterosaurs, early theropods and feathered dinosaurs, mammaliaform groups like Multituberculata, and the earliest of therian mammals, all while plesiosaurs roamed the oceans. In these environments, some ancient form of wood roach began living in small societies within rotting logs, and consuming the wood with the aid of their intestinal protozoan symbionts. From so humble a beginning, in the shadow of dinosaurs, sociality was brought to the world by the first termites.

The Pathway to Sociality The pathway to sociality in termites is the focus of considerable research and is not without contention, but there is a strong argument that the evolution of termite sociality is diet related. Living gregariously within a single piece of decomposing wood, the ancestor of wood roaches and termites would have been, like its modern descendants, faced with considerable challenges in terms of nutrition, particularly in relation to nitrogen.

Feeding on wood necessitated an association with cellulolytic protozoans capable of breaking down one of the toughest materials on Earth, lignocellulose. In addition, gut bacteria were also needed in order to aid this by further nutrient recycling within the intestines. This association may have first come about from early feeding on decomposing wood that had already been broken down by free-living protozoans, that over time became more and more tightly associated with their hosts, ultimately living within the termite gut. Given that the gut is shed during molting, the symbionts would be lost. In order to inoculate new individuals or reinoculate molted individuals with these endosymbionts, wood roaches and termites to this day feed each other portions of their waste through a process called proctodeal trophollaxis. This likely first evolved as a result of feeding on the comparatively nutrient-rich waste of others within the gregarious association, trying to make use of all the resources within their nutrient-poor habitat and diet. Once established, these symbiotic associations made the gregarious association of individuals more critical than ever and formed the fundamental requirement for termite sociality.

← Termites are exclusively eusocial, wood-feeding roaches, and have been present since at least the Late Jurassic. → Termites are amazing architects, with mounds sometimes extending to the height of trees. In Africa, massive mounds can be taller than elephants and used by these mammals as “scratching posts.”

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Alloparental Care

Diversification

Wood roaches and termites are altricial. That is, their young require care, most critically in the inoculation of symbionts as well as other food early in life. The result of their gregariousness and interdependence was an overlapping of generations, and ultimately older nestmates provided proctodeal feeding to their younger siblings, freeing the mother (queen) to focus more exclusively on reproduction over that of brood care. This is called alloparental care. Ultimately, developmentally arrested juveniles took over brood care responsibilities, becoming pseudergates. Pseudergates are nymphs that undertake the tasks of workers within a colony, but remain capable of furthering their development to a fully reproductive individual. This is in contrast to true workers, which are fixed in their role developmentally and cannot become a reproductive. True workers would appear later in termite evolution as a specialization of a system otherwise predominated by pseudergates. In the first societies, however, pseudergates represented the first workers.

Although termites diversified into many varieties and different lineages, including the first caste of individuals specialized solely on defense—soldiers—they did not experience a real explosion in diversity for nearly 100 million years after their origin. It came about following a shift or refinement in their diet and the emergence of endosymbiotic biota. Sociality, caste specializations, and reliance on a unique nutritional niche were also critical steps. In some subterranean termites, particularly the genera Coptotermes, Heterotermes, and Reticulitermes, their diet of wood became supplemented with microbially and organically rich soil. These genera are related to the family Termitidae, the most diverse group of all termites, which encompasses more than 75 percent of the approximately 3,100 world’s extant species and which initially began its diversification 55–60 mya. The common ancestor of termitids lost their reliance on intestinal protozoans, and bacteria assumed an increased role in the digestion and breakdown of matter within the gut.

NATURAL HISTORY

In addition, more and more termitids became either obligate or facultative soil feeders, or in the case of the two earliest diverging subfamilies, they evolved agriculture—growing gardens of fungi or bacteria. The appearance of soldiers in the Early Cretaceous was certainly driven by the increased likelihood of colonies to survive attacks if individuals in the nest were more and more adept at defense. A termite colony would certainly represent a concentration of resources and nutrients tempting to any predator. In fact, one of the most prominent competitors and predators of termites today are ants, a social group that was coming into its own at the same time, and it is perhaps among the ants that we could find the initial predatory pressures that helped tilt the landscape of survival in favor of colonies with soldiers.

↖ Termites achieved the first societies

↑ Once termites began to shift

through the nutritional-dependent

from eating cellulose to eating soil,

model developed by the earliest

the group expanded into new dietary

termite communities.

niches, fueling a considerable rise in diversity.

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Termite Soldiers Ancient among termites, soldiers are distinguished

Termite soldiers are adept at chemical warfare and

from workers by a large number of morphological and

a large gland to the front of the head has evolved to

physiological specializations distinct from other

produce a variety of defensive chemicals—from toxic

castes and solely for colony defense. The heads of

substances brushed on attackers to sticky materials

soldiers are typically larger, more heavily sclerotized,

sprayed at attackers and miring them on the field of

and often armed with defensive specializations. In the

battle. In some of the most iconic termite soldiers,

earlier-diverging termite lineages, the primary mode

the head has been modified into a nozzle through

of defense is via crushing with long, robust mandibles

which the frontal gland secretions are sprayed

with impressive teeth. These mandibles typically

during combat. Yet other soldiers have evolved

overlap apically so as to produce the maximum

snapping mandibles. These are long mandibles that

damage when brought together with a combatant

use built-up elastic energy to produce a powerful

between. The legs of soldiers are typically more stout,

strike. These are efficient means of defense, and a

for holding their own against an attacker, and the

single snap can split an attacker in half or crush them

bodies generally tougher. During the course of termite

against the gallery walls. Soldierless species are not

evolution, the variety of soldiers and mechanisms of

without defense and workers can take on effective

defense proliferated greatly, even into three instances

means of protecting a colony. The most remarkable

of losses of soldiers among the soil-feeding lineages.

are the exploding workers of some soil-feeding

Beyond crushing, soldier mandibles evolved into long

termites. In these there is a rupture of the body and

falcate slicing structures, which can easily cut through

expulsion of toxic chemicals onto nearby attackers.

an invader, or even a human finger with quite a bit of pain and damaging effect. → Soldiers are a defining feature of all termites, although some have subsequently lost their soldiers. Nonetheless, soldier diversity is breathtaking among termites and they are lethally effective against invaders.

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Ants Ants (Hymenoptera: Formicidae) may have been second to the sociality game, but with more than 14,700 extant species, they are three times as diverse as eusocial wasps and have 10 times the diversity of eusocial bees.

Eusociality Ants belong to the vespoid wasp family Formicidae and are effectively specialized eusocial wasps, forming colonies of one or more queens supported by a vast force of workers, which are largely sisters and tend to the queen(s) as well as the large number of eggs, larvae, and pupae. The workers are also responsible for colony defense, although some ants have dimorphic workers in which larger individuals are specialized for defense, as soldiers. In some ants, workers can be phragmotic, as in some termites, using their large, heavily reinforced, and sometimes flattened heads to block nest entrances, or they may have such modifications on the abdomen and similarly use them to prevent entry into the colony. The coordination of individuals within the colony is achieved through a long list of semiochemicals, making ants true masters of chemistry.

↓ Larger workers in leaf-cutter ants

↑ A queen ant inside her nest,

are more effective at cutting and

surrounded by worker ants tending

transporting leaf fragments back to

to the colony’s larvae.

the nest. Smaller workers serve as “soldiers” by riding the leaf fragments and fending off parasitoid flies that attempt to attack the major workers.

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↑ Trap-jaw ants are specialized predators in which the elongate mandibles may be retracted to an angle greater than 180 degrees and when a potential prey is encountered, they can be snapped with incredible speed and force. → Atta laevigata ants are among those that tend fungus gardens, maintaining them and meticulously cleaning workers entering so as to avoid introducing any pathogens that could cause harm.

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Ant Biology Reflected in their large number of species is a stunning breadth of biology, encompassing life histories as varied as army ants, dracula ants, trap-jaw ants, seed ants, leaf-cutting/fungus-growing ants, exploding ants, domatiainhabiting ants, aphid-tending ants, seed-harvesting ants, and socially parasitic ants, among many others. In these ways, ant biology is more varied than any other lineage of social arthropods. Their colonies can also range, like termites, from small associations to massive societies with millions of individuals. The large neotropical genus Azteca includes arboreal species, some of which live mutualistically with Cecropia, and in domatia, specialized galls grown by the plant rather than induced by a foreign agent. The plant provides the ants with a residence, and gains protection from herbivores by what are rightly described as highly defensive ants. Species of Azteca have also evolved animal husbandry, tending mealybugs and scale insects for food. Yet other ants have evolved agriculture by building and tending gardens of fungi, grown from leaf fragments harvested from the surroundings, which they ultimate harvest as a source of food. Dracula ants are, as their name indicates, vampiric, but on a host that one might least suspect. Adult ants are actually ectoparasitic on their own larvae, feeding on the hemolymph.

Inclusive Fitness A major factor in the evolution of sociality within Hymenoptera has been considered to be the haplodiploid sex determination system of this order. In this system, rather than sex being determined by specific sex chromosomes, females are diploid, while males are haploid—that is, resulting from unfertilized eggs. A peculiar byproduct of this system is that an individual female will be more closely related to her sisters than she is to her own offspring, sharing 75 percent of her genome with a sibling versus 50 percent with her children. Females within a society therefore can pass more of their genes on to the next generation by assisting their mother to produce more offspring, rather than raising their own brood. This is referred to as inclusive fitness, where the total fitness of a given individual has to be considered in a more inclusive manner than simply evaluating one’s direct contribution to the next generation. While some see a eusocial colony in a haplodiploid system as a reproductive individual controlling generations of their offspring to rear more brood, others consider it a system in which

Army ant syndrome: Having evolved more than once

workers enslave their mother as a machine for

among ants, notably in the Old World genera Dorylus and Aenictus and separately in a group of five New World genera, “army ant syndrome” encompasses a frequent relocation of the nest, formation of massive uncoordinated foraging swarms, and specialized queen morphology. When on the move, these ants can form massive columns of millions of individuals, and can devour virtually any animals that stumble into their path. Old World army ants relocate their colony periodically and new colonies are formed via budding, while New World army ants alternate between reproductive and foraging episodes. In the reproductive phase, the queen becomes physogastric and so the colony remains in place. When the queen ceases laying eggs, the foraging episode commences and the colony relocates to a new site almost daily, called the nomadic phase.

producing more and more siblings to their own long-term genetic benefit.

→ Like some species of Azteca, Acropyga ants have evolved to tend mealybugs or aphids, feeding on the nutrient-rich secretions of their “herd.”

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Stinging Wasps Most wasps (Hymenoptera: Vespidae, Crabronidae) are solitary predators, parasites, or parasitoids. Eusociality has evolved twice among them: once within a subset of the family Vespidae (yellow jackets, hornets, paper wasps, and their relatives) and once in the family Crabronidae, a relative of the bees.

Vespids In the family Vespidae, the related subfamilies Vespinae (yellow jackets and hornets) and Polistinae (paper wasps) are exclusively eusocial of the anchored eusocial grade. The subfamily Stenogastrinae includes species that are facultatively eusocial, while all other groups of vespids, such as potter wasps or pollen wasps, are solitary. All social wasps are predatory and bring premasticated prey back to their brood. The larvae, in response, produce a high amino acid content fluid upon which the adults feed, as well as feeding on nectar and pollen while on hunting excursions. Colonies can sometimes be small in some species, but more often are rather large, such as those of hornets. The record holders, however, are some neotropical and Australian paper wasps, which have been discovered to build massive nests within caves and estimated to support millions of workers. These kinds of nests, which are rare, are likely the result of closely related colonies that were initially built in proximity of each other and that fused over time. Nonetheless, their sizes rival the massive nests of some ants and termites.

Crabronids

↑ A crabonid wasp, Bicyrtes quadrifasciatus, named for the four white bands on either side of the female’s abdomen; males have five such bands. ← A wasp nest of the crabronid Microstigmus, suspended from a filament on the underside of a leaf.

A single species of the neotropical genus Microstigmus is the only known eusocial Crabronidae, and falls within the dynamic behavioral grade. There are other multifemale associations in other crabronid genera and even in other species of Microstigmus, but none of these are yet known to have a reproductive division of labor and are therefore communal or subsocial, at best. Microstigmus comes as a notable exception, with a reproductive division of labor and cooperative brood care by overlapping generations of individuals. Castes are based on size dominance, with the queen being the largest female within the colony, sometimes nearly twice as large as minor workers. Nests are built at the end of thin, threaded stalks such that they dangle from overhanging vegetation or rocky outcrops, making them difficult to access by predators such as predatory ants. There are many species within the genus and for which the biology remains unstudied, meaning it is possible that further eusocial species will eventually be identified.

↑ Europe’s largest eusocial wasp, the European hornet, Vespa crabro. ← A paper wasp nest. Nests of social vespids are quite varied, reflecting the evolution of the individual lineages and sometimes giving clues as to the evolutionary relatedness of particular social wasp genera.

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← The corbicula, or pollen basket, into which the bee places its pollen for carrying back to the nest. → The hairy-footed flower bee, Anthophora plumipes, is a solitary bee that nests in soft vertical faces, such as soft mortar joints.

Bees Although we often think of bees (Hymenoptera: Anthophila) in relation to sociality owing to our familiarity with honey bees and bumble bees, the reality is that only about 6–7 percent of our world’s species of bees are social. Most are solitary, and almost 13 percent are cleptoparasites. Thus, in terms of sheer number of species, sociality is not the underlying theme in bee evolution. Nonetheless, sociality has evolved (and been lost) multiple times among bees. Most of these societies are subsocial or dynamically eusocial, some facultatively so. Dynamically eusocial bees can be found among the sweat bees (Halictidae) and allodapine bees, a group related to carpenter bees and occurring throughout Madagascar and Africa, eastward across southern Asia, and through the Indomalayan and Papuasian realms into Australia. Aggregations and communal behavior can be found in varied bee groups, while subsocial, semisocial, and quasisocial systems can be found in groups like small carpenter bees, but are most prevalent across the Halictinae. In fact, the subfamily Halictinae is often the focus of research regarding the evolution and maintenance of social behavior in bees given the number of independent origins within the group, the considerable

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variation of social life among species within different genera, and the presence of facultatively eusocial species. In this last group, environmental factors lead to females forming small primitively eusocial colonies or nesting solitarily. This is even the case in some of the nocturnal halictines, which live in facultative societies and forage during matinal and crepuscular periods, quite different from their day-flying counterparts.

Corbiculate Bees The iconic social bees are those species of corbiculate bees, a group that includes orchid bees, bumble bees, stingless bees, and honey bees, the last three of which are eusocial to varying degrees. Corbiculate bees are so named for the presence of a corbicula in nonparasitic females. The corbicula is a specialized pollen-carrying structure that is formed of a broadened, polished, and concave outer surface on the hind tibia, variously fringed by overarching hairs that help to define a space into which wetted pollen can be placed. Excluding parasitic and robber species, the three social groups of corbiculate bees are exclusively eusocial—bumble bees of the dynamically eusocial grade, and stingless and honey bees of the anchored eusocial grade.

→ Bumble bees like to nest close to the ground, often in leaf piles, tree hollows, or mossy undergrowth. →→ Few insects are as woven into the fabric of our own societies as are honey bees, which are infused into our cultures, mythologies, and religions.

Bumble Bees Encompassing about 250 species, bumble bees live in dynamically eusocial colonies comprised of 20–1,700 individuals, although most colonies fall within a range of 50–350 workers. Nests in temperate habitats are on an annual cycle, with queens overwintering before founding a new nest in spring. From spring through fall, the queen produces multiple broods, thereby building up a retinue of workers that aid her in ultimately producing new queens and drones by the end of the season, and which mate before the next generation of queens settles in for the winter months. Bumble bees nest in subterranean cavities, such as abandoned rodent burrows, or sheltered amid dense vegetation, while brood and colony stores are placed in clusters of vertical waxen pots.

Stingless Bees

↑ Members of the Brazilian stingless bee, Scaptotrigona xanthotricha, defend the entrance to their nest.

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The most species-rich group of social bees, stingless bees number more than 550 species found throughout the tropics. Unlike a colony of bumble bees, some species of stingless bees can produce massive colonies of up to 80,000 workers. Colonies are perennial and the castes, typical of the anchored eusocial grade, are more fixed

between queens and workers. There is no real soldier caste as in termites, although in some stingless bees there are workers that are specialized for defense and take on the behavioral role of a soldier. As their name indicates, stingless bees have lost the use of a functional sting, although some African species still possess a modestly developed apparatus. Although stingless, they are certainly capable of defending themselves by other means. Usually, their nests are constructed in locations or of materials that give them rather effective protection. Species of Oxytrigona have well-developed mandibular glands that secrete formic acid and the bees can inflict third-degree chemical burns on those that choose to tangle with nests. Some species are quite defensive and will bite with mandibles bearing sharp teeth or pitch resinous materials into the eyes of attackers.

Honey Bees Perhaps the quintessential social insect is the European honey bee, Apis mellifera. Honey bees have been the focus of human interest since before written history, and this association likely extends back deep into hominid evolution as earlier human species likely “preyed” on nests for honey and the consumption of the brood along with it. It is not uncommon for a colony to have 40,000–80,000 workers, which work collectively to build an iconic nest of waxen combs, with horizontally oriented hexagonal cells for brood and honey, within an existing cavity, usually a tree hollow. As in stingless bees, the colonies are perennial and the castes are fixed. Aside from A. mellifera, there are a further six species of honey bees, all restricted to Asia, but increasingly invasive in the Middle East and Africa.

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Other Social Insects Eusociality has evolved in many other groups of insects. The lesser appreciated eusocial insects include some aphids, thrips, and even beetles.

Eusocial Aphids Sociality in aphids is quite unique and is effectively clonal. Females of some species of gall-inducing aphids in the subfamilies Hormaphidinae and Pemphiginae reproduce parthenogenetically, thus producing clones as offspring. Some of these offspring include sterile nymphs, which possess heavily sclerotized frontal horns and robust forelegs and are aggressive to unrelated individuals and other insects. These sterile nymphs serve as soldiers, defending good feeding sites and protecting the entrances to the colony, which itself is formed within a gall. Some have argued that the social aphids do not meet the criteria for eusociality and therefore should be considered a rather elaborate example of a subsocial system. A reproductive division of labor does exist and while the overlap of generations is minimal, it could be considered as sufficient given that nymphal offspring remain with their mother either as soldiers or to feed as normal colony inhabitants. Unlike other social insects, the immature aphids are quite capable of caring for themselves and so the absence of

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cooperative brood care is perhaps not surprising. Yet, given that the gall itself is a form of “care” by providing a critical shelter for the colony and its many inhabitants, one might argue that the simple action of the many nymphs feeding and stimulating gall formation is in itself a form of cooperative care. If so, then in their own unique fashion, these aphid societies could also be classified as eusocial.

Eusocial Thrips Much like the galling aphids, gall-inducing thrips in the suborder Tubulifera have similarly evolved societies that some choose to treat as merely subsocial. Nonetheless, good arguments can be made for some of them constituting truly eusocial societies. Thrips sociality includes a broad spectrum of societal types, from simply communal species to those species in which individuals live communally but also cooperate in foraging and brood care, although each individual ultimately reproduces and not all remain to care for their young through to adulthood. There is variation among females, with some having reduced wings along with enlarged, stout forelegs for defense, while others have fully developed wings and normal forelegs. In addition, some females can become remarkably physogastric, like a termite queen, and produce a large number of eggs, in contrast to others that scarcely reproduce.

Eusocial Beetles Despite the overwhelming number of beetles in this world, only one species has been confirmed as eusocial. Austroplatypus incompertus is a species of ambrosia beetle occurring in southeastern Australia. Like other ambrosia beetles, A. incompertus lives symbiotically with ambrosia fungi, a fungal pathogen of the trees in which the beetles live. The beetles maintain a garden of the fungus, upon which they feed. Females have specialized pouches on their bodies, called mycangia, in which they store the fungus during dispersal and for use in establishing gardens in new colonies. Remarkable for a beetle, these small colonies consist of a single, reproductive female, which is supported by a modest retinue of unfertilized beetles and which perform all other tasks of colonial life. It is perhaps not surprising that sociality among beetles was to be found in a wood-boring species, as beetles living in wood breed in large aggregations, brought together by both the habitat as well as attractants from the wood itself, which tends to congregate such beetles. Much as was the case for the origins of termite sociality, dietary constraints may have played an integral role, along with the subcortical environment, in promoting social interactions among these ambrosia beetles.

↑ Austroplatypus incompertus builds elaborate galleries bored through their host trees, species of Eucalyptus. ← A spiral gall of the aphid Pemphigus spyrothecae on a leaf petiole of a black poplar tree. The inside of a spiral gall is shown on the far left.

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Ecological and Evolutionary Significance of Sociality The sheer abundance of social insects attests to their ecological significance. Ants and termites outweigh vertebrates in most tropical ecosystems. Termites on their own have a global biomass equivalent to humans, while ants globally are perhaps the most abundant of all animals in terms of numbers of individuals. It is almost impossible to sample a terrestrial ecosystem without being confronted by social insects. Not only are they abundant, but social insects wield a disproportionate influence on the ecology and evolution of other organisms. Predators of all kinds rely on social insect colonies for their survival, from apes to spiders, the considerable concentration of resources into a single location sustains a vast community of animals, microbes, fungi, and plants that feed directly on the colonies or the wastes they produce. Indeed, refuse piles from ant colonies support their own unique ecosystem of specialists, which would not persist were it not for the dead bodies, wastes, and debris accumulated outside of the colony. Abandoned ant and termite mounds can actually be sites of nucleation for the formation of forests, and the subterranean excavations of these same societies till soils, bring nutrients to the soil surface, and generally make the environment more suitable for the support of other plant and animal life. Aerial views across South America and Africa reveal vast landscapes dotted with the mounds of living and deceased ants and termites, and concentrations of vegetative growth around those that are abandoned given the concentration of nutrients. In this respect, ants and termites can be thought of as ecosystem engineers. Beyond this, social insect colonies have triggered the evolution of a whole array of inquilines and parasites, living within the nest. Rove beetles are one group of insects in which termitophily and myrmecophily has evolved numerous times, and exhibiting a range of chemical and morphological changes that allow them to evade the colony’s usual defenses and means of recognizing alien residents. Inquilines include mites, caterpillars, spiders, millipedes, bees, and any number of other arthropods. Likewise, thousands of species, of insects and others like fungi, have evolved as specialist parasites and parasitoids of social insects. An insect society supports far more than the prescribed members of the community, revealing an ecological and evolutionary heft almost unrivaled in the history of life.

↑ Rove beetles exhibit a range of

→ Blueberry plants grow on an

chemical and morphological changes

abandoned anthill in Finland.

that allow them to evade the colony’s usual defenses and means of recognizing alien residents.

Next page A microcosm of biological interactions. As a hawk moth attempts to visit its host plant for nectar, and to potentially pollinate the flower, it is forced to face off with a camouflaged assassin bug hoping to make a meal of the floral visitor. Pollinating a flower is often risky and insect pollinators face many dangers as they go about performing this vital service.

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NATURAL HISTORY

Plant–Insect Interactions As soon as there were insects, there were insects exploiting plants for both food and shelter. The earliest of insects scraped algae and lichen, as well as feeding on spores at the tips of sporangia-bearing branches. Thus, the world’s dominant herbivores—the insects—and plants have a storied history to their evolutionary interplay, extending back more than 400 million years.

During the intervening millennia, plants have evolved mechanisms to defend themselves from the damages of insect feeding, and insects have concomitantly evolved their own variations to feeding as well as a means of avoiding or rendering ineffective plant defenses. Ultimately, insects evolved to feed on every imaginable type of plant tissue; from leaves and shoots, roots, and stems, to flowers and fruits; and at every stage of life and decay.

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NATURAL HISTORY

Insects also use these same plants as shelters, and in some species the insects alter the plant’s development to form galls of benefit to themselves. Over time, the antagonistic association between insects and plants has evolved into complex mutualisms, to the benefit of both. No relationship has been as critical to the shaping of life on Earth as has been the intimate association between plants and insects.

Phytophagy When it comes to feeding on plants, most insects are not too choosy. Many of the world’s herbivores feed on a variety of plants and tissues, although that variety is typically confined to the species of a single plant family or genus, and these insects are therefore oligophagous. Less commonly, an insect species may be monophagous, specializing to feed on a single plant species. At the other end of the spectrum are polyphagous herbivores, which feed generally on a wide diversity of plant lineages. Gall wasps are good examples of monophagous herbivores, inducing galls in specific species of oaks, or, in one lineage of these wasps, only in roses. Monarch butterflies are oligophages in that they feed exclusively on milkweeds, but are less discriminating as to what particular floral species. Spongy moths are excellent polyphages, feeding on nearly any broadleaf tree, but also on some conifers.

← A group of migratory locusts,

↑ A cluster of spongy moth

Schistocerca gregaria, devour some

caterpillars feeds on the leaves

maize. A single locust needs to eat its

of a hornbeam tree.

weight in vegetation every day.

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295

↑ A hawk moth caterpillar, Theretra oldenlandiae, is considered a pest on many a garden flowering plant, including fuchsias and arum lilies. ← Caterpillars eat their way through a mulberry leaf, literally skeletonizing it. ↗ Fruit borers, such as bean weevils, are particularly damaging to plants and therefore are an agricultural pest.

296

NATURAL HISTORY

Chewing The basic plan of insect mouthparts, from the start of insect life, are those of a biting and chewing construction. Not surprisingly, biting and chewing plant tissues is therefore a generalized mode of herbivory. In fact, margin feeding on leaves is a great example of chewing herbivory, with characteristic cuts into the edges of leaves, although some insects may specialize to feed on the upper or lower surfaces of leaves. Stout mandibles, with strong teeth and powered by strong muscles, are used to cut through plant tissues, such as leaves. Some chewing insects devour the entire leaf with the exception of the veins, effectively skeletonizing the leaf. Other species feed generally on many different plant tissues, or specialize to bite and chew through petals, stems, roots, and so on. Some larvae that chew plant tissues may mine into the layers of the plant, eating their way through the parenchyma while leaving the epidermis of both the upper and lower leaf untouched. The result is

a characteristic mine with a hole at the terminus where the grown larva emerged from within the leaf. Remarkably, the pattern of the mining is generally consistent within a particular group of miners, meaning that from the damage caused by the mine one can often identify the offending insect even in the absence of the insect itself.

Boring An analogous kind of feeding, but on stems and roots, are those insects that bore. Borers, like miners, feed internally within the plant and in those that bore through wood, they may have closely associated fungal, bacterial, or protozoan symbionts allowing for them to break down the tough plant tissues and/or produce nutrients in a form suitable for the insect’s absorption. Any chewing insect obviously does damage to the plant and therefore can reduce the plant’s ability to survive or produce viable seeds, particularly if the herbivores are in high numbers.

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297

Sucking On multiple occasions in the history of insects there have been independent specializations of mouthpart structures for piercing and sucking fluids. The first insects to evolve piercing mouthparts were the many species of the extinct superorder Palaeodictyopterida, and which collectively represented the first major diversification of specialized herbivores. Palaeodictyopterida thrived during the Carboniferous and Permian, disappearing by the End Permian Extinction Event. Modern analogs of the Palaeodictyopterida are the Hemiptera, another radiation of largely herbivorous insects with piercing and sucking mouthparts. Hemiptera use their stylets to pierce stems, leaves, and bark, extending within the plant to locate the vascular bundles or phloem. Accessory glands and saliva are mixed to produce a lubricant on the outside of the stylets, aiding their sometimes meandering track through the plant and between fibrous tissues. The internal pressure of the plant along with muscular pumps within the insect’s head capsule serve to bring fluids up the channels among the mouthpart stylets and into the alimentary tract of the insect. The insects also inject their saliva, which begins the process of digestion, often before the fluid has even finalized its journey to the mouth.

298

NATURAL HISTORY

As is the case for many herbivores, a vast amount of material must be digested in order to obtain sufficient nutrients, and this is as true for fluid feeders as it is for those chewing on leaves. Phloem-feeding insects, such as aphids or scale insects, pass large volumes of fluid through their systems, passing the concentrated sugary fluid from the anus as a substance called honeydew. Honeydew, in turn, may be used by other insects, such as ants that have evolved to tend certain aphids. Piercing mouthparts often do not leave easily visible damage to the plant, although under microscopic examination, the site of feeding and the course of the stylets through the plant can be identified. If the feeding is extensive and drains sufficient fluid from the plant, then wilting can occur, while other insects may infect the plant with viruses that initiate infections causing more noticeable and significant disease within the plant. Minute thrips, which like Hemiptera have piercing and sucking mouthparts, would on the surface appear to be of little harm to a plant, yet they often pierce individual cells, draining the fluid and causing entire plant tissues to wilt and potentially die.

Galling A gall is an abnormal growth of plant tissue that results from hyperplasia and/or hypertrophy induced by the action of another organism, such as a virus, bacterium, fungus, nematode, mite, or insect. This habit likely evolved from among ancestral plant miners and borers that triggered galls in their host plants. Many insects induce galls in plants, and galls can form in virtually any tissue of the plant, although galling insects typically specialize in attacking a particular plant organ system. More than 12,500 species of insects produce galls and they are found among the Hemiptera, Hymenoptera, Coleoptera, Diptera, and Lepidoptera, although most cecidozoans (gall-inducing animals) are clustered among the Diptera. This induction differs among insect lineages and the process is not entirely understood, although in Hemiptera, salivary gland products appear to be key to influencing the activity of plant growth hormones critical to the formation of the gall. In gall wasps there also seem to be symbiotic viruses introduced to the plant by the wasp, and that these viral particles are also involved in the redirection of the plant growth substances. The cecidozoan lives and feeds within the gall, gaining both a feeding site with a consistent source of resources as well as a shelter from predators and parasites.

↖← The variety of fluid-feeding mouthparts are considerable, ranging from the coiled proboscis of moths (above) and butterflies to the comparatively stiff, piercing, needlelike mouthparts of true bugs (below). → The rose bedeguar gall of a Diplolepis rosae gall wasp on a wild rosebush.

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299

The Fossil Record of Phytophagy Feeding leaves excellent traces of its action and there is a rich record of insect herbivory preserved in the paleobotanical record.

a Carboniferous Records → The range of damage types observable on fossil plants is considerable, and even traces of palaeodictyopteridan stylets are preserved within fossilized vascular plant tissues. In some of the more remarkable examples, broken stylets are preserved still in place, the Devonian fossil tree Archaeopteris hibernica

fluid-feeder either having been picked off by a predator or dying while in the act of feeding.

o Devonian Records → The earliest evidence of oviposition → As early as the Middle Devonian,

damage appears at this time, with

there is evidence of feeding damage

insects injecting their eggs into plants,

on ancient liverworts and land

often ancient seed ferns, and from

plants, ranging from margin feeding

which their nymphs would emerge to

to piercing and sucking, and

feed on the plants.

demonstrating that considerable diversification in mouthpart

→ Ovispositional scars were

specializations for feeding on

likely produced by species of

plants had taken place by this time.

Palaeodicytopterida, or any of the other major herbivorous groups,

300

→ The piercing damage on Devonian

albeit with chewing mouthparts,

→ The earliest plant-feeding insects

land plants indicates that the

among the ancient relatives of the

were likely stem feeders, detritivores,

superorder Palaeodictyopterida,

Polyneoptera such as Caloneurodea

or sporophagous, feeding on decaying

or their earliest ancestors, were

or species of the “Protorthoptera,”

plant tissues, scraping lichens,

in existence and were becoming

a group of insects of uncertain

chewing on stems, or consuming the

specialized fluid-feeding herbivores,

relationship but including early

nutritious sporangia of early vascular

alongside a range of scraping and

relatives of the Orthoptera,

plants in the Latest Silurian and

chewing insects, which would have

Dermaptera, and Dictyoptera and

Early Devonian.

predated the fluid feeders.

with many chewing herbivores.

NATURAL HISTORY

Triassic Through Cretaceous → The end of the Paleozoic was marked by the single most extreme cataclysm in the history of life Eocene fossil leaf with feeding damage

on Earth, with estimates of up to 96 percent of all life becoming extinct. Life, including insects, rebounded from the middle Triassic.

Carboniferous fossil plant Neuropteris heterophylla

o Paleocene Through Eocene

→ By the Late Triassic, a number

→ Herbivory proliferated

of new floral lineages proliferated

considerably during global thermal

and, with them, evidence of insect

maxima of the Paleocene-Eocene

feeding rebounds considerably.

Thermal Maximum and the Early

Herbivores such as Palaeodictyoptera

Eocene Climatic Optimum. During

and those among the Protorthoptera

these periods, the variety and

were replaced predominantly by

abundance of damage produced

those of the Orthoptera, Hemiptera,

by herbivorous insects increased

Coleoptera, and early Hymenoptera

considerably.

(the phytophagous wood wasps and sawflies).

→ A decreased nutritional value in plants due to a lower nitrogen content,

Primitive Permian plant-sucking insect Dunbaria fasciipennis

o Permian Records → There is evidence of insect feeding

→ By the end of the Triassic and

a result of increased CO2, suggests

Early Jurassic, early moths appeared,

insects may have literally needed to

representing a new lineage of

feed more in order to get necessary

expanding of herbivores, first with

levels of nutrition from the plants.

larvae and adults chewing on mosses, liverworts, and other early plant

→ Examples from the fossil record

lineages, and likely consuming pollen.

demonstrate the considerable

expanding away from wet habitats

influence of average global

and into more arid floras. During this

→ Adults of Lepidoptera became

temperatures and atmospheric

same period there is evidence of more

specialized external fluid-feeders,

concentrations of CO2 on insect

specialized herbivory including the

and their larvae one of the dominant

herbivory, a cautionary tale as we

consumption of pollen and siphoning

groups of herbivores, by the end of

move through our own era of global

of pollen drops from early seed plants.

the Jurassic and Early Cretaceous.

climate change.

PLANT–INSECT INTERACTIONS

301

→ Plant hairs, or trichomes, can inhibit an insect’s ability to get a foothold on a plant for long enough to feed or oviposit its eggs.

Plant Defenses Plants have evolved both mechanical and chemical defenses in order to deter or minimize being fed on by herbivores.

effective protection against an herbivore inflicting the damage. Some internal-feeding insects may even be crushed by the increase in cell growth within a wound.

Resins Physical Defenses These include systems by which a plant makes difficult an insect’s ability to feed or reproduce. At the simplest, some plants evolve thickened cell walls, rendering them exceptionally tough to cut or chew, and/or leaf shapes and colors that make them difficult to detect. Toughness is also sometimes achieved by the incorporation of silica into the plant epidermis, making the plant difficult to chew as well as seriously abrading the mandibles of insects attempting to do so. The surface of a plant may be covered by minute hairs, called trichomes, and although they also aid water retention and other physiological functions, these inhibit insect oviposition, digestion, and feeding on the plant. Waxy surfaces, found in most vascular plants, evolved initially as a water-retention mechanism, but these same hydrophobic compounds reduce insect feeding. A plant’s own physiological response to a wound may also be an 302

NATURAL HISTORY

Plant resins are both a form of physical and chemical defense. These are chemicals produced by the plant in response to stress or damage, and can easily ensnare herbivores. Resins are the natural source for fossilized resin—that is, amber. Simultaneously, the resin is a chemical defense in that it can, among other things, serve as an attractant to parasitoids, which will arrive to parasitize the herbivores confounding the plant.

Phytochemicals Perhaps the most important defenses among plants are the many phytochemicals (secondary plant compounds) that have evolved in response to insect feeding. Toxins of every variety have evolved repeatedly throughout plants, from alkaloids (of which nicotine is a famous example) to terpenoids (such as the pyrethrines of Chrysanthemum). These chemicals are noxious to herbivores and therefore

Entrapment A plant’s ultimate defense is simply to consume an insect. Pitcher plants, sundews, and fly traps use chemical cues to attract insects, which are ultimately drowned or are physically entrapped before being digested by enzymes from the plant.

naturally deter feeding or oviposition. Or, even if they do not kill the insect outright, they can so significantly reduce the digestibility of the plant (such as many tanins) as to hinder herbivores from trying to consume them. Yet other plants have evolved to produce compounds that interfere with insect development (phytoecdysones). These chemicals arrest the development of an insect before it can reach maturity, ultimately killing the herbivore; those that reach maturity may be left sterile. Another strategy is the evolution of nectar-producing organs not associated with flowers and pollination in order to attract predatory insects that serve to deter potential herbivores. For example, passion vines have evolved extrafloral nectaries that attract ants, which in turn may prey on herbivorous caterpillars.

→ Tree resin. The number of insects trapped within amber attests to the effectiveness of this physical defense.

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303

Insect Responses

While plant defenses are impressive, insects have

and sometimes the phytochemical deters most

evolved means to evade their deleterious impacts. Even

herbivores but will end up being an attractant to those

a carnivorous plant like a pitcher plant has midges that

that have evolved a workaround for the chemical. For

have evolved to be impervious to the digestive enzymes

example, cucumber beetles are actually attracted to

of the trap. Larvae of the midge Metriocnemus knabi

the triterpenoid cucurbitacins that otherwise deter

and the mosquito Wyeomyia smithii live within the

most herbivores of Cucurbitaceae (squash and their

fluid of some pitcher plants, feeding on captured prey

relatives). Monarch butterflies famously sequester the

alongside the plant. Some ants, undeterred by the

deadly cardiac glycosides of milkweed, using these in

slippery sides and downward-directed trichomes that

defense against predators.

otherwise keep potential prey from escaping the pit, are adept at climbing within and pulling out prey that the plant had intended for itself. And there are species of moths that have actually specialized to feed solely on the tissues of pitcher plants. For nearly every chemical evolved by plants, there is some insect that has conversely evolved means to sequester or detoxify the toxin by breaking it down,

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NATURAL HISTORY

A larva, possibly Wyeomyia sp., in the liquid of a purple pitcher plant, Sarracenia purpurea.

Pollination

Plant–Herbivore Mutualism

Pollination is the process by which pollen from a male anther is transferred to the female stigma, either within the same flower (called perfect flowers) or between sexually differentiated flowers (called imperfect flowers, which are either male or female). Plants with imperfect flowers of both sexes are monoecious, while those with only male or female imperfect flowers are dioecious. The process of pollination may be mediated by abiotic factors, such as wind-borne pollen (anemophily), but dioecious plants are more often pollinated by animalmediated transfer (in the context of insects referred to as entomophily). For prime pollinators, see pages 311–313.

Pollination is a process by which plants and their dominant herbivores form a mutualism, benefitting both insect and flower alike. It is considered one of the major factors leading to the evolutionary diversification and proliferation of both since the mid-Cretaceous. Flowers provide to their insect mutualists food in the form of pollen and sometimes nectar or other substances, as well as shelter. Pollen is rich in protein, sugars, starch, and fat, all necessary nutrients for the insect, and nectar is a concentration of sucrose, glucose, and fructose. Dominant insect pollinators are flies (myophily), beetles (cantharophily), wasps (sphecophily), bees (melittophily), thrips (thripidophily), moths (phalaenophily), and butterflies (psychophily). The insects are attracted to the flowers by the pollen and nectar rewards, but may also be encouraged by attractants such as fragrances (although this name belies the fact that some of these odors are quite pungent, smelling of corpses rather than anything we might consider “fragrant”), colors, and patterns, or may even falsely mimic the form, texture, and smell of a female insect to bring forth witless males that attempt to mate (pseudocopulation), and in their failure they transfer pollen between the plants.

Fertilization Pollination must not be confused with fertilization, as many plants may be pollinated but not all will achieve successful fertilization. Fertilization comes from the pollen grains successfully growing a pollen tube through the style of the female pistil to the ovary and ultimately delivers gametes to the ovules. Once this happens, the flower ultimately forms a fruit, which is the ripened ovary containing seeds. Some of these fruits are familiar to us, in the form of edible fruits such as pomegranates and raspberries. Others are similarly familiar, although in common parlance we do not refer to them as fruits, such as bean pods, nuts, or grains, and yet these are the fruiting bodies of their respective plants. Currently, there are just about 300,000 species of flowering plants, and 80 percent or more of these rely on animals to vector their pollen between the anther and stigma. These numbers alone emphasize the critical importance of animal pollinators for maintaining the function of every ecosystem on Earth. From the perspective of economics and human food security, animal pollinators of our food crops add 217 billion dollars to the global economy annually and are responsible for one-third of world food production. What is even more impressive is that of the 200,000 species of animals that vector pollen for plants, 99.5 percent of them are insects. Without insects, our world would wither and die.

→ A bee dives into a pomegranate flower. Most pomegranates are self-fruitful, which means the flowers on one tree can be cross-pollinated.

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305

Arctic Exchange Some arctic flowers act as solar reflectors, concentrating warmth in which their pollinating insects bask. In return, the flowers gain increased pollination efficiency, particularly in those environments where anemophilic pollination would be ineffective. The increased efficiency of pollination also means that less of the total pollen produced is wasted, given that the vectors target the transfer directly to the next flower.

306

NATURAL HISTORY

Fig Pollination

One of the best-known examples of insect pollination

she squeezes through the tiny space. Bear in mind that

is the association between figs and their specialized

the wasps themselves are already quite minute, with

fig wasps (Agaonidae). Figs (Ficus) are a richly diverse

some scarcely larger than the head of a pin. Given that

genus of flowering plants, and in most tropical

fig wasps are born within mature figs, the female is

ecosystems their massive trees and fruits support

carrying with her pollen from her birth fig and upon

a whole host of animal life, representing a key floral

arriving within the new immature fig this pollen

lineage in many major food webs. Figs, by the way,

fertilizes the fruit. At the same time, the female wasp

are inflorescences, but ones where the reproductive

will lay her own eggs before dying as a prisoner within

parts grow inside; the fig flower is turned outside-in.

the maturing fig. Her brood emerge as larvae that feed

Naturally occurring figs have evolved a species-

within the fig, gaining sustenance as well as protection

specific mutualism with wasps of the family

from most predators and parasites. Once mature, the

Agaonidae. Each mature fig fruit is the result of at

brood will consist of new females as well as males,

least one wasp, and sometimes multiple wasps. An

which are wormlike. The males mate with the females

immature fig has a small opening through which a

and then sacrifice themselves by chewing an exit hole

mated female fig wasp will enter, crawling deep within

to the surface of the fruit, through which the newly

and often having her wings ripped from her back as

mated female wasps emerge.

PLANT–INSECT INTERACTIONS

307

The Fossil Record of Pollination When we think of pollination, we tend to imagine an insect, perhaps a bee or butterfly, visiting a flower. Yet, the conveyance of pollen from one plant to the next by insects predates the first fossil flowers by more than 100 million years.

Cross-section through a Jurassic-aged cone

o Jurassic Records → Following the global decimation at the End Permian Event, the insect groups that were visiting Permian plant lineages disappeared.

→ By the Jurassic, proper pollination systems had evolved,

Permian Records

involving gymnosperms and siphonate insects, apparently feeding at pollen drops from tube-like ovulate organs and

→ Insects fed on pollen from wind-pollinated plants

likely also feeding from the pollinate organ.

and there are insects known with mouthparts modified as siphons, for feeding on external plant fluids such as

→ During feeding, pollen could be incidentally

pollen drops. While these alone do not indicate active

transferred between the reproductive organs of early

pollination and instead simply demonstrate various types of

gymnosperm lineages such as bennettitaleans, cycads,

pollinivory, the size of some Paleozoic pollen grains indicates

and ancient conifers.

that they would be poorly dispersed by wind and Permian

308

plants have been discovered with ovulate and pollinate

→ Mandibulate insects that today still pollinate cycads, such

reproductive organs, indicative of some form of insect-

as the beetle family Boganiidae, have been discovered from

mediated pollen transfer.

Cretaceous deposits, complete with their cycad pollen in tow.

NATURAL HISTORY

a Cretaceous Records → Flowering plants first appeared at the start of the Cretaceous. By the end of the Cretaceous, the angiosperms had diversified and achieved hegemony over the botanical realm, with a dwindled diversity of gymnosperms.

→ The first flowers visited by insects were assuredly pollinated by various lineages of late Jurassic and Early Cretaceous flies and beetles, and likely moths and perhaps even some predaceous or parasitoid wasps that fed on pollen or drank nectar as adults.

→ Early and mid-Cretaceous deposits throughout the world preserve an abundance of fly and beetle lineages either in association with pollen, or with specializations known to be used for pollen consumption or transfer. Cretaceous fossil gymnosperm (cycad fir)

→ The flower fossils from the Early Cretaceous themselves also implicate flies as early pollinators given the presence of

Post-Mesozoic Records

structures called fly traps.

→ Following the mass extinction at the end of the Mesozoic, → As flowers began their dramatic rise in diversity by the

pollination systems continued to proliferate, with the first

mid-Cretaceous, various new pollinating insect lineages

appearance of bilaterally symmetrical flowers and better-

appeared and were undergoing their own initial

developed nectaries.

diversifications at the same time, such as glossate moths, higher thrips, and bees.

→ In more recent eons, some of the familiar specialized pollination systems came about, such as higo chumbo cacti

→ The efficacy of animal-mediated pollination grew

and their hawk moths, yucca and their yucca moths, figs and

considerably, and flowering plants continued their

their wasps, and a great variety of oligolectic bees, including

ecological and evolutionary rise through the Late

in the New World tropics the diversification of orchid bees as

Cretaceous, as did their growing variety of pollinators.

the specialized pollinators of many Orchidaceae.

Floral variety, with morphological specializations specific to their insect pollinators, increased considerably in the

→ Elsewhere in the world, orchids evolved pseudocopulatory

Late Cretaceous, such as flowers with structures specific

mechanisms to coerce male wasps and some bees and flies to

to bee-pollination systems.

pollinate them.

PLANT–INSECT INTERACTIONS

309

Pollinating Flies

Pollinating Beetles

The first pollinators of flowers were flies and beetles. Among modern flies, the Syrphidae, or hover flies, are perhaps the most familiar of myophilous Diptera. Hover flies are frequent at flowers and many superficially resemble bees or wasps, owing to a mimicry in body coloration and size, and they are often seen alongside bees at the same flower. Unlike bees, hover flies continue to visit flowers during adverse weather and they may spend more time at flowers than most other pollinators. Another group of pollinating flies that often mimic bees are the aptly named bee flies (Bombyliidae), large, robust, and typically densely hairy flies, much like bumble bees. While these beelike flies are certainly conspicuous, pollinating flies occur in many families. Noteworthy is the fact that while the adults may be pollinators, the larvae of these flies can be parasites or predators.

Beetles are also effective pollinators, although in many such pollination systems the ovary of the flower is greatly thickened as a defense against the beetles, which may attempt to chew into the ovule. While they include some of the earliest pollinators of flower plants, alongside the flies, today they are perhaps in third or fourth place, behind Hymenoptera, Diptera, and likely Lepidoptera. Nonetheless, for those plants on which they have specialized, they are obviously most important. Notable beetle families that are, or include, pollinators are the Cantharidae (soldier beetles), Cerambycidae (longhorn beetles), Cleridae (checkered beetles), Buprestidae (jewel beetles), Meloidae (blister beetles), Mordellidae (tumbling flower beetles), Nitidulidae (sap beetles), and Scarabaeidae (scarabs).

← Calliphoridae (blow flies, pictured), Tabanidae (horse flies), Tephritidae (fruit flies), Bombyliidae (beeflies), Syrphidae (flower flies), Tachinidae, Chironomidae (nonbiting midges), and even Culicidae (mosquitoes) are among the many flies that pollinate flowers. → The spotted longhorn beetle Rutpela maculata feeds on various flowers, including Apiaceae, hogweed and cow parsley, Rubus, and thistles.

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311

Pollinating Bees Bees and moths came later to the pollination game than did flies and beetles. Nonetheless, today bees are the preeminent pollinators, with more than 20,500 species known, all but a few of which are exceptionally efficient at vectoring pollen between flowers. Bees are nothing more than vegetarian stinging wasps, evolving to provision their brood with pollen and nectar rather than insect prey. All bees, whether parasitic or not, visit flowers for nectar. Females of nonparasitic species also visit flowers for pollen, and sometimes also for floral oils and fragrances. Given this reliance on pollen and nectar, bees have become vital pollinators throughout the world and in habitats as varied as the tundra to hot deserts to wet and humid tropical forests. Some key examples of host specialization among bees are the squash bees, which visit species of Cucurbitaceae or those genera that visit sundry genera or a single genus of Solanaceae (the nightshade family). Most of the remaining bees are polylectic, generalists that feed from a wide array of plant families, such as honey bees and bumble bees. Strictly monolectic species, in which one bee species visits only one floral species, are rare.

312

NATURAL HISTORY

Bees have numerous morphological specializations that allow them to access pollen, many of which are formed of combs of modified hairs that serve to collect pollen from the anthers or are used to manipulate the pollen to specific areas of the body for transport back to the nest. The most famous of these structures is the corbicula, or pollen basket, of honey bees, stingless bees, and bumble bees (also present in the largely nonsocial orchid bees). Other bees may transport the pollen within the crop, or more typically in pouches of specialized hairs on the hind femora or tibiae, on the sides of the posterior of the thorax, or even on the underside of the abdomen.

Pollinating Moths Just as bees are vegetarian wasps, butterflies are nothing more than colorful, day-flying moths. Thus, collectively, moths and butterflies comprise a single major group of pollinating insects. Almost 98 percent of all species of Lepidoptera belong to a massive group called Ditrysia, which began its diversification alongside flowering plants in the mid-Cretaceous. Most of the key pollinators among moths (and therein butterflies) are found in the families

Sphingidae (hawk moths), Noctuidae (owl moths), Geometridae (inchworms), Hesperiidae (skipper butterflies), Nymphalidae (brush-footed butterflies, which includes the beloved monarchs), and Papilionidae (common butterflies). Excellent pollinators do exist in other families, such as the yucca moths (Paradoxidae), other butterfly families, and countless other groups of moths. Plants, of course, should have a love-hate relationship with these moths given that as larvae they are voracious herbivores, before metamorphosing into adults that feed on nectar and pollen, and act as a conveyance for pollen transport.

Pollinating Wasps and Thrips The adults of many wasp families are efficient pollinators owing to their frequent feeding on nectar and/or pollen. Many species of the parasitoid wasp family Ichneumonidae are efficient pollinators, and many groups of predatory stinging wasps are likewise. Thrips, which are usually a millimeter or less in length, are perhaps the most ignored in terms of their role as pollinators. Yet, pollen-feeding species of these Lilliputian insects are excellent pollinators, sometimes the obligate pollinators for certain floral species, and some are even critical in crop ecosystems, such as the growing of chili peppers.

←← Around 60 percent of all bees are pollen specialists to one degree or another, being oligolectic and visiting a suite of floral species within a given family or even genus of plants. ← A white-lined sphinx moth, Hyles lineata, hovers over a flower, feeding with its proboscis. ↓ A female ichneumon wasp feeds on cow parsley flowers.

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Plant–Insect Evolution Evolution is a probabilistic process, one in which the source of change (mutation) is stochastic and the probability of survival at any given instant shifts in accordance with a seemingly infinite number of biotic and abiotic variables. The result is that depending on circumstances, both extrinsic and intrinsic, the survival of an individual is a game of chance. Simultaneously, the copying of our genomes is not error-proof, providing an incessant source of variations among individuals in a population, and those errors that do not outright result in termination of life, and that can be inherited, alter the probability of survival at any given moment. At first a change is likely neutral, neither lowering nor improving a chance of survival. But nothing remains the same, and when the local environment changes, a once neutral change may give those individuals with the alteration an increased chance of surviving and producing offspring. Eventually these, along with many other such changes, result in sufficient differences, over many iterative

generations, as to make one population now specifically distinct from those other populations that, in the distant past, were once all parts of a contiguous species. The statistical law of truly large numbers in probability theory indicates that with sufficiently large enough independent samples, any unlikely event that has a probability greater than 0 is likely to be observed. A classic example of this in biology is human reproduction. The chance of a single sperm cell to encounter the egg is exceedingly unlikely, but with millions of individual sperm released in a single event, fertilization becomes a statistical likelihood. Coincidences are only coincidental in a single sample, but with millions, billions, or trillions of repeats, nothing is coincidental. This does not mean that anything and everything will happen, merely that if there is a greater than 0 chance for one random mutational variant over another, then, if all things remain equal, and the population size is sufficiently large in each generation along with the number of generations, the change will take place. And at

Speciation: Where One Becomes Two All species have variations appearing and disappearing at any given moment, and across nearly all traits—from genetics and physiology to morphology and behavior. Speciation is the process whereby one ancestral species ends up splitting to produce two descendant species.

1. One ancestral species.

2. A new variant of a given species arises. 3. The new variant increases in proportion. 4. A change in climate or geography occurs. 5. The two populations become isolated, experiencing different changing environments. 6. Once-neutral traits become beneficial or detrimental in different ways. 7. One population fixes on one suite of variants and the other on a different suite. 8. Even if the populations come back into contact, they do so as two new species.

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← A constant evolutionary back-and-forth between yucca plants and yucca moths has given rise to one of the most intimate insect–floral associations, one that is truly mutualistic and that was ultimately produced from stochastically arising variations in each generation. The moth’s mouthparts are uniquely modified to insert pollen into the yucca flowers.

the root of such change is the simple chance of whether a given nucleotide will be faithfully reproduced during meiotic cell divisions. With this in mind, and considering the short lifespans of most insects, their vast biomass in each generation, the innumerable number of generations that can occur within an exceedingly brief period of geological time, the error rate of DNA replication, and the law of truly large numbers, it is easy to see how, via mere probability alone, 400 million years’ worth of insect generations and abundance would generate the infinite variety of insects and their plant associations, regardless of how seemingly complex. What would be unlikely in any given generation, becomes highly likely. At the simplest, imagine a univoltine insect herbivore—that is, an insect species that has only a single generation per year, with a global abundance of a few billion individuals. That means that there are potentially 3 billion reproductive events each year. Within a century there will be 300 billion reproductive events. One study estimated the average life span of species to be about 5 million years, with mammals on the lower end of the scale at about 1 million years, barring any catastrophic event. Assuming the average species life span for our herbivore, there could be up to 1.5 quintillion reproductive events in which variants can alter the probability of survival

within the context of a given environment. Even if 80 percent of all individuals died (which would leave alive 300 quadrillion), the numbers here are sufficiently large as to give a leg up to any nonzero chance of survival. With these truly large numbers for both the insects and their plants, the evolutionary back and forth is an inevitability. Imperfect replication will give rise to variants among the insects, some of which will survive better on a plant than others. By probability alone this will lead to a broad change in the herbivores, so much so that after generations of differential survivorship, the ending population will be a separate species with new defenses or methods of feeding. Conversely, the same process in the plants will give rise to a plant better able to defend itself, leading to sufficient changes as to produce new floral species with new defenses. This process will continue generation after generation, for quintillions of individuals. Change is inevitable even for low differences in the probability of survivorship given the vast numbers involved, and even if the ultimate source of variation is truly stochastic. Plant–insect evolution is impressive, as is the inordinate variety of other insect life—from dragonflies propelling themselves with jets of water to bees pollinating orchids.

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Chapter

6

Impacts on Humans and the Environment By virtue alone of their sheer biomass, insects are the ecologically premier group of land animals. They pollinate at least 80 percent of the world’s flowering plants. Millions of species feed on plants, from weevils, caterpillars, and leaf beetles to scale insects, grasshoppers, and more. A small proportion are destructive to crops and forests. Parasitoid and predatory insects, in turn, are critical to regulating the populations of herbivorous insects and use in biocontrol. Insects provide critical pollination services for crops; also silk, honey, dyes, sustainable protein, and among the experimental insect species, one—Drosophila melanogaster, the fruit fly—has been utterly transformative for science and society. Nonetheless, insects have long been targets of the most extensive, deliberate poisoning on Earth along with weeds. For this and various other reasons their numbers are dropping; thousands of species are in decline or endangered; many have become extinct.

← The volume of pollinatordependent fruit and vegetable crops has increased threefold over the past 50 years, making humans increasingly dependent on pollination.

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Ecosystem Services Overall, the ecosystem services provided by insects, including their roles in pollination, herbivory, predation, and soil formation have been transformative for life on land.

Pollination

Predation and Parasitoidism

The transfer of male gametes (pollen) to the female stigma of other flowers, called pollination, insures outcrossing of plants and genetic variation. Of the 295,000 species of angiosperms, 80–95 percent of them are pollinated by insects. These include plants from the most primitive living lineages to the spectacular radiation of orchids, which have elaborate mechanisms for luring insects and attaching pollen to them. Abundant fossil evidence shows that angiosperms partnered with insect pollinators deep into the Cretaceous and probably ever since the origin of the flower. Insect pollination also allows distant plants to pollinate each other, making them better able to exploit patchy and limiting resources like moisture, forest light gaps, and soil nutrients.

Birds and bats are important predators of insects, but the main enemies of insects are other insects. Ground and surface predators include ants; various beetles like carabids and staphylinids; the adults and larvae of lacewings, antlions, and other neuropterans; mantises, as well as assassin and other predatory bugs. Aerial predators include dragonflies and damselflies; robber flies and empidoid flies; and many aculeate (stinging) wasps. Parasitoid insects have even more impact on populations of various insects. These include more than 80,000 species in 50 families of generally small parasitoid wasps, which lay their eggs in the immature stages (generally larvae and nymphs) of other insects; some very tiny species oviposit into the eggs of other insects, such as Mymaridae and Trichogrammatidae. Diptera are also important parasitoids, such as scuttle flies (Phoridae), bee flies (Bombyliidae), assorted acalyptrates, and especially the Tachinidae, which are important regulators of caterpillar populations.

Herbivory Just the way that predators maintain healthy populations of prey species, herbivorous (or phytophagous) insects cull weak and dying plants and help sustain plant populations. It is difficult to assess how much vegetation (leaves, fruits, flowers, and so on) is eaten by insects, but since approximately 40 percent of the world insect species feed on plants, they must consume enormous quantities. Most quietly nibble away on plants; in some cases, whole landscapes are defoliated by insect herbivores. In the United States, eastern tent caterpillars (Malacosoma americanum) defoliate cherry trees. The Lepidoptera, in fact, comprise the largest lineage of plant-feeding animals. Second to this are the phytophagan beetles: weevils (Curculionidae), leaf beetles (Chrysomelidae), and longhorn beetles (Cerambycidae), with about 100,000 species, but also many scarabs and some others.

→ Tent caterpillars congregate on their web, from which they emerge to forage. Insects and plants have steered each other’s evolution over hundreds of millions of years.

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Soils For plants to optimally absorb nitrogen and key nutrients, organic material from dead and decaying plants must be decomposed. In this role insects are superb, one of the best examples being termites. In primarily tropical and subtropical regions of the world, termites consume the most abundant biomolecule on land, lignocellulose. They do this with assistance from symbiotic protists and bacteria in their gut, which metabolize this durable substance. Besides consuming up to 50 percent of the dead wood in tropical forests, they also consume humus and even soil. On savannas, termite galleries radiate out from towering mounds, through which workers forage for grass and ungulate dung. In East Africa a geological formation 10,000–100,000 years old is formed of a stratum 16 ft (5 m) thick and covers 3,400 sq mi (8,800 km2), all formed from the eroded mounds of the living termite, Macrotermes falciger. Ants are also landscape architects: in Florida, a colony of Pogonomyrmex ants is known to excavate 44 lb (20 kg) of sand in just five days, through 33 ft (10 m) of tunnels.

↑↑ A hornworm caterpillar with

↑ The ant biologist Walter Tschinkel

parasitoid wasp cocoons attached.

standing next to a cast of a harvester

The larvae feed on, and eventually kill,

ant nest. Ants are master excavators:

their hosts, critical to regulating

the colony took slightly longer than a

populations of many other insects.

month to construct it.

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↑ A Ficus tree with its harvest. Figs are inflorescences turned outside-in. Each species is pollinated inside the fig by a specific species of wasp.

Overall Ecological Impact Ecologists have known for years that biological communities with rich arrays of species are more resilient and resistant to catastrophic change. Herbivory, predation, and other interactions impose selection pressures that keep certain species, even highly competitive ones, from overwhelming a community. Measuring the cumulative effects of insects in nature is extremely difficult, but some indications are given by the effects they have had on the evolution of other groups.

Plant–Insect Interactions Plants define the world’s terrestrial biomes, and the fact that some 80 percent of flowering plants rely on insects for pollination reveals the pervasive impact of insects. Pollinator relationships can be extremely specific, such as in orchids, but also in other groups, like tiny fig wasps and their huge Ficus tree hosts in the tropics, with a 1:1 species interdependence. Also, the thousands of secondary → A female fig wasp piercing a fig with her ovipositor, which she uses to lay her eggs.

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compounds produced by plants (including many toxins) probably defend against insect herbivory, although some insects have very effectively broken through the defenses and even rely on the compounds for their own defense. Myrmecophytes are plants (in 40 families) that have specialized structures for housing ant colonies. The ants, in turn, provide protection to the plant as well as vital nitrogen from the colony waste.

Social Insects Colonies are very efficient and competitively superior to solitary species, and have had profound effects on other arthropods. Ants, for example, are models for more than 2,000 species of arthropods that mimic them (called myrmecomorphs). Their colonies, furthermore, host thousands of specialized arthropods (called myrmecophiles) that live as parasites, inquilines, and predators. Colonies of the Central American army ant, Eciton burchellii, harbor more than 300 species of myrmecophiles.

Insect–Vertebrate Interactions On the scale of vertebrate populations, insects have had no less evolutionary impact. More than 5,000 species of birds and mammals are specialized for feeding on insects: nighthawks, swifts and swallows, woodpeckers, flycatchers, and numerous other passerines; also, most of the world’s bats, as well as aardvarks, anteaters, pangolins, shrews, tenrecs, and moles. In turn, birds and mammals host about 4,000 species of parasitic lice, fleas, and flies and contribute to the regulation of their populations. Take away all the mammals of the world and nature may not look very different. Take away all the insects and ecosystems on land would collapse.

↑↑ ”Termi-henge”: A grassland in

↑ One measure of the ecological

Queensland, Australia, studded with

impact of social insects is the number

the mounds of termites. The mound

of other animals specialized to feed

functions as a chimney for the colony,

on them, like aardvarks, pangolins, or

which resides just below the base.

this giant anteater in South America.

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Crops Insects that are destructive to crops all belong to species-rich groups of herbivorous insects— Lepidoptera, Curculionidae (weevils), Chrysomelidae (leaf beetles), Hemiptera, and others. They chew leaves and stems, bore into stems or fruits, mine leaves and stems, feed on fruits and roots, and suck the vascular fluids of the plant. Damage can also be caused by the transmission of bacteria and viruses pathogenic to the plants, such as citrus greening disease. This bacterial disease is causing serious damage in Florida and is transmitted by a psyllid from Asia, Diaphorina citri.

Destructive species are commonly invasives, but some are native species that transitioned to cultivated hosts similar to their natural ones, like the Colorado potato beetle, Leptinotarsa decemlineata. It is native to montane regions of the western United States and northern Mexico, where it feeds on several wild species in the nightshade family, genus Solanum, but quickly spread in the 1800s throughout North America and Eurasia on Solanum tuberosum, the potato.

The Big Four Global trade and industrial-scale agriculture created optimal conditions for the spread of insects that damage crops. Several thousand species of insects feed on crops;

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several hundred species are considered major pests. Globally, the main crops are grains (wheat, rice, corn, and sorghum); more wheat is grown by tonnage than all other crops combined, rice and corn being second and third. Among the “big four” crops are several dozen insects that are truly damaging: certain weevils, leaf beetles, and several genera of moth caterpillars (“cutworms”), in the family Noctuidae (Agrotis, Helicoverpa, Spodoptera) and Pyralidae (Ostrinia). Cutworms get their name by eating through a stem, killing the plant. They generally feed at night; by day these caterpillars take refuge in the soil. Along with certain caterpillars, weevils and leaf beetles, such as the Colorado potato beetle, are damaging to various crops. The larvae of a beetle native to Central

← Insects that are most impactful on agriculture are commonly certain species in the large, herbivorous families, such as this Colorado potato beetle, a member of the very diverse family of leaf beetles, Chrysomelidae. → A caterpillar of the European cornborer, Ostrinia, eating its way through not just the kernels, but also the cob. ↘ The cotton bollworm, caterpillar of the Helicoverpa armigera moth, feeds on various crops besides cotton: maize, beans, sorghum, and tobacco.

America, Anthonomus grandis (the boll weevil), is a notorious historical pest of cotton, and still is. Another weevil, Hypothenemus hampei, threatens the well-being of Western societies by boring into the seeds of a critical crop, coffee. Another group of great agricultural importance are sternorrhynchan insects: aphids, whiteflies, and scale insects, at least one species of which occurs on each major crop. One of the most notorious is the grape phylloxera, a psyllid native to North America that devastates European grapes. North American grape species are partially to wholly resistant to phylloxera, so these are hybridized with European varietals or serve as root stock onto which branches of European varietals are grafted.

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Medical Significance It has been estimated that, over the entire existence of Homo sapiens, 108 billion people have ever lived. It has also been estimated that more than half of them died from insect-borne diseases, primarily malaria, but also viruses, typhus, plague, sleeping sickness, and others. Insect-borne pathogens have changed the outcomes of wars, defined the boundaries and fortunes of empires, and effects of the diseases have even been encoded into our DNA.

Blood and Disease All of the most devastating diseases are transmitted into blood. Blood feeding (hematophagy) has evolved independently dozens of times among insects: in Hemiptera (bed bugs, Triatoma assassin bugs), fleas, and lice, but no order is nearly as significant as Diptera (flies). Mosquitoes alone (family Culicidae) make flies the most medically important group, but there are also black flies (Simuliidae), no-see-ums (Ceratopogonidae), sand flies (Psychodidae: Phlebotominae), horse flies (Tabanidae),

↓ The legs and claws of this human head louse are extremely effective for grasping individual hairs. In past wars lice were serious vectors of typhus.

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and certain calyptrates like stable and other muscid flies, as well as tsetse flies (Glossinidae) and their close relatives, the bird and bat flies (Hippoboscidae, Streblidae, Nycteribiidae). (For more on blood-feeding insects, see pages 248–253.) While there are many species of insects that can be bothersome or annoying, only a small proportion are important in transmitting diseases to humans and their animals. There are, for example, 3,500 species of mosquitoes, but only about 100 are vectors for human diseases and only a few dozen of these are seriously significant, like Aedes aegypti and A. albopictus. For anyone unfortunate enough to spend a sleepless night fending off tiny, flightless hemipterans that feed on blood with irritating “bites,” it may not be much consolation to

know that bed bugs (Cimex spp.: Cimicidae) are unknown to transmit diseases. They hide in creases, folds, and cracks during the day, coming out to feed at night. To be an important vector, an insect must be able to live near or in human settlements. In some cases, such as yellow fever, humans are the primary reservoir of the pathogen. In others, a disease can be spread via domestic animals (for example, ducks for West Nile virus/WNV), rats, rabbits, and other animals living close by. An efficient vector should be fairly long-lived, disperse well, and it must feed multiple times (to become infected and then to transmit the pathogens). Lastly, the pathogens must be able to grow within the body of the vector, called vector competence. Among the hundreds of pathogens transmitted by insects, the most significant ones are summarized on the following pages.

Why flies? Presumably it relates to their agile flight, ancestral diets, and how readily they evolve mouthparts that can draw blood. Adult flies need a liquid diet, and mosquitoes (pictured), midges, and horse flies have each evolved stylet-like mouthparts for piercing skin to either siphon out the blood or feed as it wells to the surface. Calyptrate flies, on the other hand, have fine, sharp scales on the labellum of their proboscis, used to scrape away skin to expose blood.

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Arboviruses This is a term derived from arthropod-borne viruses, referring to any kind of virus transmitted by an arthropod. A notorious association exists between Flavivirus and certain species of Aedes mosquitoes. Various species in this genus of virus cause yellow fever, dengue, WNV, and Zika, the last three of which are serious emerging diseases. Yellow fever, dengue, and Zika are transmitted by certain species of Aedes, especially A. aegypti and A. albopictus, which live in close association with humans and feed multiple times. WNV (which readily sickens and kills horses) and various encephalitis viruses are transmitted by Culex mosquitoes. For most of these viruses, humans and sometimes their domestic animals are the primary reservoirs, but natural reservoirs include primates (yellow fever, dengue) and birds (WNV).

Proteobacteria

326

↑ A female Aedes aegypti feeding

↓ A female phlebotomine sand fly

with her hypodermic-like proboscis,

engorged on blood. Certain species

the blood meal visible through the

of these delicate flies are carriers of

pleural membrane of her abdomen.

Oroyo fever and leishmaniasis.

IMPACTS ON HUMANS AND THE ENVIRONMENT

Called rickettsias, these bacteria-like microbes have a genome very similar to that of mitochondria. The most virulent rickettsial disease has been epidemic typhus, which is spread in crowded conditions by human lice (Pediculus and Phthirus) and brings body aches, rashes, high fever, and delirium. It killed tens of millions of people throughout

history, especially during wars. Murine typhus is transmitted by certain fleas, bartonellosis (Oroya fever) is spread by sand flies, and Rocky Mountain spotted fever (RMSF) by ticks.

Bacteria An infamous arthropod-borne bacterial disease is plague, caused by Yersinia pestis and transmitted by several species of fleas in the genus Xenopsylla. Urban reservoirs for the disease are rats; domesticated animals are susceptible to it. The infected bites from fleas cause dark swellings (“bubos,” hence bubonic plague), although Yersinia can also be spread among humans by coughing (pneumonic plague). The bacterium propagates in the lymph system, it interferes with antibody production, and eventually infects the lungs and causes septic shock. Even though plague is easily treated with antibiotics, it is not eradicated; it occurs naturally in ground squirrels and some other rodents; several thousand new human cases occur every year around the world. The D32 mutation of the CCR5 gene is believed to have conferred resistance of some people to plague; it also confers resistance to HIV, but the mutation has other health consequences. The most common disease transmitted by arthropods in the United States is Lyme disease, or borreliosis, caused by spiral-shaped bacteria in the genus Borrelia and transmitted by Ixodes ticks. Recent infections of Lyme are easily treated with antibiotics, but latent infections cause long-term neurological and rheumatic problems. The disease was localized in New England but has spread throughout North America (some 500,000 people are diagnosed each year). Ironically, a vaccine was developed in 1998 but is used only for dogs and horses because of vaccine misinformation.

↑ The black rat flea, Xenopsylla, vector for plague. Cities hundreds of years ago—crowded, unhygienic, filled with rats—had ideal conditions for these fleas to breed and spread the bacterium.

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Protozoans The insect-borne disease that has impacted humans more than any other is malaria, caused by Plasmodium. Humans are susceptible to five species of Plasmodium (P. falciparum is the most virulent), but there are dozens of species found naturally in other mammals, in birds, and in reptiles. Human malaria is transmitted by Anopheles mosquitoes; in Africa, species in the A. gambiae complex are the main vectors. Up to the early 20th century, human malaria was global, occurring as far north as southern Canada and across Siberia; it is now eradicated in North America and Europe but kills approximately 400,000 people yearly in tropical areas, most of them young children. Mosquito control and the use of bed nets have helped control malaria, but a vaccine has been very difficult because Plasmodium quickly develops immunity. One is now available. Sickle-cell anemia is a serious and often deadly condition of red blood cells

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← A female mosquito laying her raft of eggs. The larvae (also called wrigglers) breathe air from the surface, allowing them to thrive in stagnant water. ↙ Bed nets, which can be sprayed with insect repellent, are very effective at reducing exposure to mosquitoes that carry malaria and other diseases. → An assassin bug of the genus Panstrongylus, found in Brazil and Peru. Like Triatoma and Rhodnius, it is known to transmit Chagas disease.

that can cause organ failure; mutations responsible for it are believed to have spread because they conferred partial resistance to the most lethal form of malaria, falciparum. Trypanosomiasis is caused by protozoans in the genus Trypanosoma. In Africa the trypanosomes cause sleeping sickness, transmitted by several species of tsetse (genus Glossina); in South and Central America they cause Chagas disease, transmitted by blood-feeding assassin bugs of the genera Triatoma and Rhodnius. These are very debilitating diseases that affect the blood and lymph systems, and eventually the central nervous system. Millions of people are infected. Leishmaniasis, caused by protists in the genus Leshmania, is transmitted by sand flies, which look superficially like small, pale mosquitoes. Cutaneous forms of the disease can cause scarring sores; visceral forms of the disease (called kala-azar) can be lethal if untreated.

Nematodes (filariases) Various species of parasitic nematode worms are transmitted by flies to humans and their domesticated animals, in the form of minute, young stages of the worms, called microfilariae. The most impactful disease is onchocerciasis, or river blindness, which is not lethal but debilitating. It is transmitted by Simulium black flies (which breed in rivers), and when an infection has built up in the host, the microfilariae lodge in finer capillaries, including those of the retina, causing blindness. Elephantiasis is caused by two other species of nematodes, transmitted by Culex mosquitoes; the worms invade the lymph system and can cause grotesque swellings of the lower limbs. Dog heartworm is the most widespread nematode of domesticated animals, transmitted by various common genera of mosquitoes. Fortunately, nematode infections are easily treated with common and inexpensive drugs.

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Venoms Venoms are toxic molecules injected by one animal into another for prey capture or defense. Though best known in scorpions and certain snakes, they are also injected by aculeate (“stinging”) wasps (including ants and bees) through their stings and by certain caterpillars through specialized spines and hairs on their bodies. There is even a venomous longhorn beetle in South America, Onychocerus albitaris, which has a sting at the tip of each antenna; it feels like that of a honey bee. The sting of an aculeate is formed from the ovipositor, modified to deliver venom (the eggs pass through an opening at the base of the sting). In many aculeates the sting paralyzes prey to feed their larvae, and for defense (exclusively so for bees). While most aculeate venoms consist of various polypeptides and enzymes, there is great variation in their potency. American entomologist Justin Schmidt compared the pain levels of various aculeate stings, and found the most painful are velvet ants (Mutillidae), Pepsis wasps (Pepsidae), and Paraponera ants (so-called “bullet ants” because their sting feels like a bullet wound). In general, the venom from a single wasp or caterpillar is not potent enough to kill a person, except for those who have severe allergic reactions to stings (they should be quickly treated with epinephrine). Fewer than 100 people per year die in the United States from stings. There are occasional, lethal encounters with large hives of aggressive hornets and Africanized honey bees.

Even more specialized are the bot flies, a group of fewer than 100 species of stout, fast, beelike flies, the adults of which do not feed. A female will quickly dart onto a host and glue eggs to the hair; when first instars hatch, the host will either lick and ingest them (in the case of stomach bots, or Gasterophilinae) or the larvae will burrow into the skin (in the case of skin bots, or Cuterebrinae). Others live in the nasal passages of their hosts. Ungulates, rodents, rabbits, and elephants are common hosts, kangaroos and humans also. A female of the human skin bot, Dermatobia hominis, will capture a mosquito or other fly on which she lays some eggs; when the mosquito feeds, the first instar larva crawls down and burrows into the puncture site.

Venomous Caterpillars Assorted caterpillars are adorned with sharp spines and irritating hairs that deliver venom. These occur in some species of wild silkworm moths (Saturniidae), saddleback caterpillars (Limacodidae, pictured), and flannel moths (Megalopygidae). As in aculeate venoms, potency of these venoms varies greatly. Some cause just minor rashes, others produce fiery welts and severe allergic reactions. In Brazil, the caterpillars of Lonomia, called taturana, can be abundant, and when they molt en masse, the shed skins falling from trees cause extreme reactions.

Myiasis This refers to an infection by fly maggots in living tissue, either in muscle, under skin, in the intestinal tract, or other area. It occasionally results from accidental ingestion of fly eggs, but is usually caused by flies that are specialized vertebrate parasites. Screwworms are the larvae of several blow flies (family Calliphoridae), which infect sores and wounds in livestock and even in people. Cochliomyia hominivorax in the New World and Chrysomya bezziana in the Old World are the two most impactful species.

← A pepsid, or spider hawk wasp, advertising itself in striking colors. The stings of these wasps are extremely painful.

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Agricultural and Medical Control In a group like insects with millions of small species that are adaptable and disperse so well, there will inevitably be ones that feed on humans, their crops, and domesticated animals. A broad array of tools is available to contend with these species, some of which are precise, others that unfortunately kill nontarget species.

Insecticides These are compounds specifically manufactured for killing insects. They tend to be inexpensive to produce and apply. Some are plant derived, such as pyrethrins (derived from the flowers of chrysanthemum-like plants). Others are entirely synthetic, like organophosphates (for example, malathion) and DDT. A class of synthetic insecticides that has become popular is the neonicotinoids, functioning (as the name implies) like hyperconcentrated nicotine. They are several thousand times more toxic to insects than DDT. Mutations for insecticide resistance are genetically relatively simple, so with populations of most pest species

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in the billions, the origin and spread of these mutations is inevitable. Widespread use of insecticides thus becomes an arms race requiring increasing amounts of more lethal forms—a global experiment that is unintentionally selecting for superbugs. Insecticides cause collateral damage to sensitive, nontarget species (which most insects are) and to other animals in the environment. It must be stressed, however, that in developing countries insecticides may be the only economical means for food production. DDT has been banned in the United States since 1972, and neonicotinoids are banned in the European Union.

Juvenoids O

These are molecules that mimic juvenile hormone (JH) and prevent immature stages from developing into reproducing adults. They are, thus, not effective for adult insects,

O

JH II

OCH3

Lepidoptera

and, like insecticides, will also affect

O

nontarget species. OCH3

Methoprene (synthetic)

Biological Control This involves the use of natural predators, parasites, parasitoids, and pathogens to control populations of the damaging insect, or, conversely, use of an insect to control an invasive weed. Predators can be used, such as ladybug beetles (family Coccinellidae) and green lacewings (Chrysopidae), but which may not limit their feeding to a pest species. More commonly, parasitoid wasps and flies are preferred, since the larvae of some species feed very specifically on the eggs or larvae of certain pest species. Tiny wasps in the Aphelinidae and Encyrtidae, for example, specialize on aphids, scales, and other sternorrhynchans. It is essential, though, to know the biology of the insects, because an introduced parasite/parasitoid could wreak havoc on nontarget species. Native moths in Hawaii, for example, were found to be attacked by several parasitoid wasps that were introduced to control caterpillars that were damaging crops. There are also success stories, such as the use of the predatory ladybug Rodolia cardinalis, to control the cottony cushion scale (Icerya purchasi), a species that almost ruined the citrus industry in California when it was introduced there. A tiny weevil, Cyrtobagous salviniae, was introduced

O

into Brazil and very successfully controls a floating weed, Salvinia molesta, which had been choking dams, rivers, and reservoirs. Increasingly, microbial pathogens—viruses, bacteria, fungi, and nematode worms—are being used because these can be very specific to certain insects. The bacterium Bacillus thuringiensis (Bt) is one in most use; it produces a toxin that kills the larvae of Lepidoptera, black flies, and some beetles. It has been so effective that 85 percent of the corn grown in the United States is genetically engineered to produce Bt toxin. Even though about 80 strains of Bt have been developed, some insects have still developed resistance to it, such as the diamondback moth (Plutella xylostella).

→ Larvae of Rodolia ladybug beetles are effective predators of the cottony cushion scale.

AGRICULTURAL AND MEDICAL CONTROL

333

↑ Ever since the spongy moth, Lymantria dispar, was introduced into North America in 1860, its caterpillars have been one of the most serious forest defoliators. This is a male; the plump females can hardly fly. → Scoltyines, like this European spruce bark beetle, invade trees that are commonly weakened by drought and fires. Their galleries under the bark kill the trees, greatly increasing the risks of forest fires. PREVIOUS PAGES A swarm of migratory locusts, near Isalo National Park, Madagascar, in 2013.

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Pheromones and Trapping The chemical communication system that many insects use to attract mates, using pheromones, has been very successfully exploited to control, for example, spongy moth (Lymantria dispar) and tephritid fruit flies. Traps baited with the aggregation pheromones of bark beetles (Curculionidae: Scolytinae) are also used. This method can very specifically target certain species.

Best Practices Effective vaccines have been developed for some arthropod-borne diseases, such as yellow fever, but others remain intractable, malaria having been notorious. However, the use of bed nets in malarial areas between the years 2000 and 2015 caused the number of new infections to drop by 40 percent. In agriculture, breeding for fruits and vegetables that maximize flavor, size, yield, and storage ability always comes at a genetic cost, one being that the natural hardiness of the plants is compromised, including the ability to withstand insect attacks. Crop rotation is an additional strategy.

Genetic Manipulation and Engineering A very effective but labor-intensive method of control is the Sterile Insect Technique (SIT). A destructive species of insect is mass-reared, then irradiated to sterilize the adults. These are then released into the wild; they will mate but reproduction is halted, especially in species that mate just once. SIT has allowed the eradication of screwworm, Cochliomyia, from North and Central America. It has also been successful in reducing populations of pest tephritid fruit flies around the world and tsetse in Africa. Most recent is the use of engineered genetic incompatibility, used in May 2021 to control the introduced mosquito Aedes aegypti in the Florida Keys. The male (which does

↑ A female tsetse fly that has just

not feed on blood) carries a gene that is passed to

given birth to a mature larva, which

female offspring that die as larvae. Males do not die but the population declines as more females die. Despite the controversy, this technique holds great promise in the control of invasive species. Both SIT

will quickly pupate. The larva is nourished within her abdomen by special “milk glands.” Sterilized flies grown in the lab and released into the wild are very effective at control.

and genetic incompatibility do not cause collateral damage in nontarget insect species.

AGRICULTURAL AND MEDICAL CONTROL

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Model Experimental Organisms Despite the notoriety imposed upon insects by disease vectors, the use of insects in biomedical research is responsible for profound breakthroughs in genetics, biochemistry, cell biology, physiology, and behavior— all essential to the health of humans, their animals, and crops. Insects are ideal for lab studies: easy and inexpensive to rear, fecund, with short generations, small and convenient to experiment with, and plenty of replicates. Ironically, species that are human pests or annoyances turn out to be particularly useful, since they are so easy to raise in abundance. Many dozens of insect species are used in lab research; the following are a few of the most significant ones.

biochemistry, cell biology, development, and neurobiology, particularly when genetics is needed. In 2018, for example, scientists mapped all neurons of the fruit fly brain, around 100,000 of them, allowing better genetic dissection of behaviors such as learning and olfaction. Stock centers with hundreds of mutant strains and species of Drosophila are provided to researchers around the world.

Bombyx mori

↑ The workhorse of experimental biology, Drosophila melanogaster (female shown here). The short life cycle, four chromosomes, and fecundity make it ideal for genetics.

Drosophila melanogaster The eukaryote that has had the largest impact in biology is arguably the fruit fly, native to central Africa but now common throughout the world. It has a life cycle of about 10 days (30 generations per year). Its original use was in genetics, spearheaded by the famous T.H. Morgan “fly group” at Columbia University from 1905 to 1935. They discovered fundamental genetic features, such as the arrangement of genes on chromosomes, sex linkage, duplications, inversions, translocations, and crossing over. Large, banded polytene chromosomes (found in the salivary glands of lots of flies) facilitated the study of chromosome mapping. The Morgan lab generously shared mutant strains with new labs, from which the field of genetics was born. Now Drosophila is used in physiology, 338

IMPACTS ON HUMANS AND THE ENVIRONMENT

Silkworm moths have been cultivated in China for more than 5,000 years, for the copious silk the caterpillar spins for its cocoon. As a result of its domestication, the domestic silk moth has lost pigmentation on its body and wings and the ability to fly; it cannot live in the wild. Recent research shows that Bombyx mori was derived from the wild silkworm Bombyx mandarina, native to China. Like many bombycoid moths the adult does not feed. The life cycle takes six to eight weeks. Traditionally, silkworm caterpillars were fed on the leaves of their natural host, mulberry, but a lab diet is now also used. Hundreds of mutant strains were developed for silk production, and the caterpillars have been important for the study of hormones and development. Its genome has been sequenced, and silkworms are being genetically engineered to produce drugs, proteins, antibiotics, and even synthetic skin from the silk.

Apis mellifera There has always been substantial scientific interest in honey bees, because of their importance in the pollination of crops and honey production. But as a highly intelligent, eusocial species the European honey bee is the focus of nonapplied research on behavior (for example, learning and communication among nestmates; vision, taste, orientation ability, and sensory systems; and brain structure

← The thick, silky cocoons of the domesticated silkworm are still harvested in large quantities, but now there is also active biological research on these moths. ↓ The European honey bee, Apis mellifera, provides more than honey and pollination services. It is also a model organism for studying the biological bases of social behavior.

and neurobiology), as well as sociality (for example, how castes develop and how colonies regulate themselves). There is a great deal of applied research on their foraging behavior, pollination biology, viruses, gut microbiomes, parasites, and toxicology. Research on honey bees, for example, led to discoveries on the toxicity for pollinating insects of certain pesticides like neonicotinoids. The European honey bee is native to southern Europe and northern Africa.

Tribolium castaneum This species, the red flour beetle, is exceedingly easy to culture: just add flour, or whatever grain you have handy. To collect the beetles and their larvae, sift the flour. Originating from Asia, it is one of the most impactful pests of stored grains in the world. The red flour beetle has a rather long life cycle of 40 to 90 days from egg to adult, and adults can live for as long as three years. The simplicity of culturing them has made them popular for research on population growth and regulation, but also in studies of reproduction, development, and insecticide resistance. Stock centers house more than 100 mutant strains; its genome has been sequenced.

→ The red flour beetle, Tribolium, makes up for a relatively slow life cycle by being probably the easiest experimental insect to raise in the lab.

MODEL EXPERIMENTAL ORGANISMS

339

Oncopeltus fasciatus The large milkweed bug is a native of North America and northern Central America and belongs to the family of seed bugs (Lygaeidae). It is easy to rear on milkweed seeds; the life cycle is approximately 30 days. Like all insects feeding on milkweeds, Oncopeltus has bold, aposematic coloration derived from the toxins it sequesters from the milkweeds. It was a favorite hemimetabolous subject of the famed insect physiologist Sir Vincent Wigglesworth (1889–1994), who pioneered work on the hormones involved in insect development. Its nymphal development and ease of use makes it ideal for studies on both molting and development.

Periplaneta americana Those who live in city apartments and in the tropics will find it unusual that some scientists actually cultivate roaches. For a living. They are popular in the lab because they are so hardy, easy to feed, and are quite large. The American cockroach (in fact a native of northern Africa and the Middle East) is used extensively for experiments in behavior and locomotion, such as sensory perception, neurobiology, mechanics, and even as models for robotics. The smaller Blattella germanica, or German cockroach

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IMPACTS ON HUMANS AND THE ENVIRONMENT

(likely native to Southeast Asia), is also popular and has a much shorter life cyle of about 60 days. Both have very large genomes. Other useful roaches are the speckled cockroach (Naupheta cinerea), and the giant cave cockroach (Blaberus giganteus).

Calliphora and Lucilia Besides their veterinary, medical, and forensic importance, several species of these blow flies are used in research on the mechanics of flight and muscular and neural control, for which they are ideal. They are very fecund and complete a life cycle in just three weeks; their flight is quite powerful and maneuvered; the large size makes it easy to tether individuals and experiment with them.

Chironomus tentans The larvae of chironomid midges are called bloodworms because their hemolymph has high concentrations of dissolved hemoglobins (that is, not within special cells), used to absorb the little oxygen in stagnant or very deep water. The larvae contain multiple forms of hemoglobins, making them excellent models for studying how hemoglobins function, are synthesized, and how the genes duplicate and evolve.

← Insects that feed on milkweeds

↑ Usually associated with carrion,

have bold colors to advertise their

blow flies will also feed on nectar and

toxicity. Milkweed bugs (Oncopeltus)

pollen. Their short life cycle and

are hardy and easily kept by feeding

larger size make them very useful for

them milkweed seeds.

studies in flight and physiology.

MODEL EXPERIMENTAL ORGANISMS

341

Evolutionary Studies The species and adaptive diversity of insects, and their accessibility, make them compelling for lab and field studies in evolutionary biology and ecology. As the major group of organisms feeding on plants, insects have been a focus of research on how, for example, they detoxify plant toxins, or even sequester them for defense. This diet is a major reason why there has been the repeated origin of aposematism, or warning coloration, in Lepidoptera. Be it toxic or noxious insects, or wasps and ants that sting, aposematism has spawned the origin of thousands of species that mimic them.

Coevolution

→ A Heliconius butterfly caterpillar feeding on a passion flower vine. The leaves of many species of Passiflora contain toxic molecules (alkaloids, coumarins, glycosides), which the caterpillars sequester for defense and retain even as adults.

Studies on the close, widespread relationship between insects and plants led to the concept of coevolution, in which two unrelated groups evolve or speciate in parallel. The classic paper on coevolution involved butterflies and their caterpillar host plants. Certain genera of butterflies, for example, specialize on toxic plants like pipevine (Aristolochia), passionflower (Passiflora), and milkweeds (Asclepias). Subsequent studies on various insect herbivores, however, failed to find phylogenies that mirrored that of their host plants. Yet studies of lice have shown better evidence for coevolution, probably because these insects are quite specific to their bird or mammal hosts and spend their entire life cycle on the host. For lice, host use probably promotes speciation; for phytophagous insects the association is more adaptive than anything else.

Mimicry Mimicry was made famous through studies of

from the American tropics. Widespread species such as

Henry Walter Bates (1825–92) (for whom Batesian

Heliconius erato and H. melpomene each have dozens of

mimicry is named) and Fritz Müller (1822–97)

mimic forms in different parts of their distribution,

(ibid., Müllerian mimicry), based on their studies of

which we now know can evolve rapidly because of

butterflies and other insects. In Batesian mimicry

relatively simple genetic bases for the wing patterns.

an innocuous mimic resembles a toxic or stinging

While mimicry between poisonous and nonpoisonous

model species; in Müllerian mimicry several toxic

species occurs, for example, in some snakes, animal

or stinging species mimic each other, reinforcing

mimicry is largely an arthropod phenomenon.

the warning signal. Darwin wrote to Bates how mimicry exemplified natural selection in action: “In my opinion [your paper on mimicry] is one of the most remarkable and admirable papers I ever read in my life.” The most impressive examples are the mimicry complexes of Heliconius butterflies

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IMPACTS ON HUMANS AND THE ENVIRONMENT

→ The Müllerian mimics Heliconius erato (top one in each pair) and H. melpomene (bottom one in each pair) converge on the same pattern where they co-occur.

Venezuela

Brazil

Ecuador

Peru

EVOLUTIONARY STUDIES

343

Genetic Variation Because evolutionary change is based on inheritance, it was a natural step that evolutionary studies would focus on Drosophila, but not just D. melanogaster. Some of the earliest studies on genetic variation in natural populations were made by Theodosius Dobzhansky (1900–75), who with his students examined variation in the large, banded, larval polytene chromosomes, particularly inversions (these are segments of the chromosomes that are flipped around). Their favorite species was Drosophila pseudoobscura. Inversions were even used to produce phylogenies in Drosophila, well before the use of current molecular sequencing. The finding of so much genetic variation in

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IMPACTS ON HUMANS AND THE ENVIRONMENT

Drosophila was a major contribution to what became known as the “New Synthesis” in evolutionary biology. A natural laboratory for the study of evolution involves the Hawaiian Islands. Here, thousands of insect and plant species have evolved in the most isolated archipelago in the world, free of competitors and predators on continents. The largest radiation in Hawaii involves drosophilid fruit flies, many of which have small distributions restricted to particular islands or to forests on certain volcanoes and peaks. Studies on Hawaiian radiations of crickets, moths, and other flies are also providing unique insight into evolution and how species can form so readily.

← A Hawaiian fruit fly. To date, there are

↓ Two variants of the peppered moth, Biston

some 570 named species of drosophilid

betularia, on birch bark. An increase in the

fruit flies in Hawaii, with additional species

darker morph during industrialization became

as yet unnamed.

a famous example of selection in the wild.

Natural Selection Insects have provided striking examples of natural selection, or more appropriately “seminatural” selection, over time scales of years rather than millions of years. The most famous of these was the study in the United Kingdom of the peppered moth, Biston betularia, and the case of industrial melanism. The moths rest on tree trunks when at rest, against which they are camouflaged. Prior to the Industrial Revolution in the mid-1800s, the moths were all light; during and after this period, dark, or melanic, forms predominated. The dark morphs were better camouflaged on bark coated with industrial soot, although subsequent work was unable to confirm that predation by birds differed between the two morphs. The most dramatic examples of selection in action involve the evolution of insecticide resistance and tolerance in hundreds of species of insects and mites. Mutations that

provide immunity to the poisons will spread quickly through the population, conferring a special ability to excrete or detoxify the poisons with enzymes, or the development of less sensitive receptor sites or other cellular targets. Frequently, insects will evolve resistance to a variety of insecticides, seen commonly in phytophagous species that feed on a wide variety of plants. The caterpillars of the diamondback moth, for example, are the most damaging species on all kinds of crucifer crops worldwide, and have developed resistance to nearly all insecticides as well as Bt. Mosquitoes have also quickly developed insecticide resistance, first seen in the 1940s in populations of certain species of Anopheles mosquitoes after the use of DDT, continuing to the present with Anopheles, Aedes aegypti, and other mosquitoes with all sorts of insecticides such as pyrethroids.

EVOLUTIONARY STUDIES

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Eusociality Mimicry and silk are largely or entirely arthropod inventions, and so is advanced sociality, or eusociality (the only eusocial vertebrate is the naked mole rat). Colonies of bees and ants had been seen for centuries as highly organized, cooperative societies, but to Darwin they posed a major problem for his theory of natural selection. Why would workers forsake their own reproduction (in evolutionary terms, their “fitness”) to live in a colony? The answer came in the 1960s when W.D. Hamilton proposed “inclusive fitness,” in which an individual could propagate his/her genes by caring for close relatives. Because of haplodiploid sex determination in Hymenoptera (see page 281), the workers, which are sisters, are more closely related to each other than they would be to their offspring, which is why eusociality has evolved in so many bees, ants, and vespid wasps. Groundbreaking studies by Karl von Frisch (1886–1982) on the sophisticated communication among honey bee nestmates, and by E.O. Wilson (1929–2021) and Bert Holldobler on pheromones in ants, further revealed how integrated and coordinated insect colonies can be. These and thousands of other studies led to the concept of insect societies as “super-organisms,” and to the formation of a field of biology devoted to social organisms, sociobiology.

Systematics The field of systematics, which deals with the diversity, relationships, and classification of species, was profoundly influenced by the German entomologist Willi Hennig (1913–76). Hennig was broadly interested in insects, but his research focused on the Diptera: adult morphology, immature stages, fossils, new species, and relationships. His goal was to make reconstructions of evolutionary relationships more empirical and scientific, so he developed phylogenetic systematics (also called cladistics; see page 22), which is essentially in universal use today. Given the importance of phylogeny for understanding evolution and all aspects of comparative biology, Hennig’s influence in biology has been unique.

← A queen ant with her workers and larvae. Ants comprise the most diverse group of social organisms; their study attacts enormous attention from biologists. → Bees have the ability to determine the gender of their offspring and create many more female bees than males. In a hive, more than 90 percent of the bees will be female.

EVOLUTIONARY STUDIES

347

Sustainable Food The efficiency with which insects convert their food into protein makes them an extremely sustainable food source. The amount of food it takes to produce 31/2 oz (100 g) of crickets is 1/6 the amount it takes to produce 31/2 oz (100 g) of a cow. The protein in insects varies, but is generally and sometimes significantly higher than that in beef or pork (as well as being leaner), comparable to that in soybean; it is also rich in iron and other minerals.

More than 1,000 species of insects are used as food by hundreds of ethnic groups around the world, including such staples as beetles (especially the larvae) and caterpillars; grasshoppers, locusts, and crickets; cicadas; termites, ants, and others. John the Baptist of the New Testament is mentioned as sustaining himself in ancient Palestine on locusts and honey (Matthew 3:4). Insects are even cultivated for mass production and commercial marketing as food in Western countries. Favorite species include mealworms, house crickets, and locusts. Because insects are crustaceans, those with shellfish allergies are probably also allergic to insects, even if they do not taste like shrimp or lobster.

↑ Insects are one of the most sustainable and efficient sources of animal protein and fats, such as these deep-fried caterpillars. → A dish of deep-fried grasshoppers.

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IMPACTS ON HUMANS AND THE ENVIRONMENT

Insect Declines In 2017, several studies alerted the world with reports that insect populations in Germany declined over the past 20 years by 70–80 percent. This was quickly named “the insect apocalypse.” Similar declines have been reported from other countries, even Greenland, and more studies are underway, quantifying what had been observed for decades: where are all the insects? In the meadows, at porch lights, on the car windshield, anywhere? There are myriad reasons for the declining populations.

Biocides As of 2021, the world applied approximately 5.6 billion pounds (2.5 billion kilograms) per year of insecticides, herbicides, and other pesticides to farms, lawns, dwellings, roadsides, and other human habitats, to control weeds, insects, and fungi. One-fifth of those biocides were used in the United States alone. The environment is laced with biocides, including residues in our food. Natural areas are not immune from drifting pesticides applied in nearby agricultural areas, or that seep into streams and rivers.

Habitat Loss The world is losing its natural areas at an alarming rate. Among the hardest hit are coastal habitats (which are very desirable for luxury homes, but that have species unique to

these areas), land which is agriculturally useful (even if suboptimal), and forests, which have the highest biodiversity. In the past 20 years the world has lost 12 percent of its forests, the worst being in the southern areas of the Amazon basin in Brazil; the Atlantic and Gulf Coastal Plain forests in the southeastern United States; and the rain forests of Sumatra, Java, Borneo, and other islands in Indonesia.

↓ Deforestation is the most serious threat to rain forests, such as this area in Guyana in South America. Rain forests contain the greatest diversity of the world’s insects.

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Climate Change

Commercial Harvesting

No longer controversial or ambiguous, climate change is causing longer and more intense droughts along with wildfires and intensifying storms. As temperatures climb, the ranges of some species of insects are shifting northward, or higher in altitude, or shrinking. In 1985, entomologists discovered the first Asian tiger mosquito (Aedes albopictus) in Texas, concerned because this a notorious multiple-biter and vector of several diseases. At the time it was believed that a northward spread of this largely tropical species would be checked by freezing temperatures. It is now known in New York and Chicago.

Butterfly farms in tropical countries are responsibly raising various species in captivity to sell to butterfly houses and vivariums around the world. But there exists a very lucrative and illegal trade in rare and endangered butterflies, beetles, and other insects, harvested from natural populations to sell to private collectors. This has caused the decline of some of the most striking species, such as the sabertooth longhorn beetle (Macrodontia cervicornis), one of the largest beetles in the world. It is long lived: in the Amazon Rainforest, the larvae feed for several years under the bark of certain trees, so populations are quickly depleted. A particularly large specimen can sell for several thousand US dollars.

↑ The striking, large longhorn beetle, Macrodontia, which lives in South American rain forests. Populations are declining because these long-lived insects are intensively harvested to sell to private collectors.

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IMPACTS ON HUMANS AND THE ENVIRONMENT

Invasive Species Populations of native insects are just as susceptible to introduced species (insects and otherwise) as are people. Populations of the drone fly, Eristalis brousii, and the nine-spotted lady beetle (Coccinella novemnotata: New York state’s official insect) have plunged, apparently because of introduced species of Eristalis and Coccinella that have spread and overwhelmed them. In Hawaii, the introduced yellowjacket wasp, Vespula pensylvanica, is killing off native insects. Yellowjackets butcher insects that they bring back to the nest to feed to larvae. Rats and mongoose have always posed serious threats to insects, particularly large, flightless ones on islands.

Protection Protecting native populations of insects is a serious challenge, one reason being that insects are not just unknown to most people, they are chronically misunderstood. Humans have cast their judgment based on a small proportion of species that feed on crops, gardens, and blood. Even North America’s most iconic

↑ If protection is inadequate for even

FOLLOWING PAGES The strands

iconic species like monarch butterflies,

connecting the web of life are

here at their winter roosts, what does

comprised mostly of insects. Their

the future hold for the natural

future will determine the future of life

diversity of the world’s insects?

on land. Here, a worker ant looks up from its perch on a forest mushroom.

insect, the monarch butterfly (Danaus plexippus), is not yet federally protected, despite both east and west coast populations having plummeted in the past several decades. Protection of a beetle—or worse, a fly, even if a pollinating species—is a political powderkeg. Monitoring of declining and endangered insect populations is seriously understaffed and underfunded, but essential. The most effective approach to protecting insects is probably to save their natural habitats. The “30-by-30” (30 percent of world’s natural areas by 2030) and “Half Earth” proposals are ambitious, but achievable. Further, the world also needs to wean itself off of biocides and fossil fuels if we are to prosper and learn from the greatest radiation in the 3.5-billion-year history of life on Earth, the insects.

INSECT DECLINES

351

Glossary Aculeate: Hymenopteran insects in

Batesian mimicry: A form of

Crypsis (n.), cryptic (adj.): The

which the ovipositor is modified into a defensive weapon as a sting; includes the stinging wasps, ants, and bees.

mimicry in which harmless species converge on a common appearance with one or more species with effective defenses, thus duping potential predators into believing the harmless species are actually harmful.

ability of an organism to be hidden or avoid detection due to a resemblance or blending in with a background (can include visual, olfactory, or auditory concealment).

Biocides: These include pesticides,

decaying organic matter, particularly plant detritus.

Ametabolous: Insects undergoing post-embryonic development where juveniles only grow in size and do not develop wings. This type of metamorphosis is found in silverfish (Zygentoma), for example.

Anemophily: The process by which pollen is transported between plants by air currents.

Angiosperm: A flowering plant. Anthophilous: Insects associated with flowers.

Apneustic: Insect respiratory systems in which the tracheal system is closed owing to the absence of functional spiracles.

Aposematism: Conspicuous warning, for example, by bright color, that an animal is toxic or otherwise unsuitable as prey.

Apterygote: An insect or other hexapod that lacks wings because it belongs to a group that evolved before the appearance of wings (for example, silverfish, bristletails, springtails).

Arbovirus: Short for arthropodborn virus, or one transmitted by ticks, insects, or other kinds of arthropods. Examples include yellow fever, dengue, and West Nile viruses, among many others.

Arthropod: Invertebrate animals in the phylum Arthropoda that possess a chitinous exoskeleton, segmented body, and paired jointed appendages.

herbicides, insecticides, and fungicides, among others, which are poisons that target certain kinds of organisms, but that can sicken and sometimes kill non-target ones.

Calyptrate: Possessing a calypter, the membranous flap at the base of the wing on some dipteran insects.

Cercus, pl. cerci: The posteriormost pair of appendages, typically sensory, on the insect abdomen.

Clypeus: A sclerite of the anterior

THE COMPLETE INSECT

Diapause: A state in which the development of an insect (at any stage) is arrested and its metabolism greatly lowered, usually during cold, dry, or other periods of stress.

Dimorphic: Meaning “two forms,” such as between minor and major workers of social insects, or between the male and female if they have obvious external differences (sexual dimorphism).

region of the head, below the frons, at the base of the mouthparts, and that articulates with the labrum.

Diploid: A cell is diploid when it

Community: Referring to a

Eclosion: Emergence of a juvenile in

biological community, or a group of species living in the same area and that interact either directly or indirectly through other species.

egg-hatching or an adult emerging from its pupa in complete metamorphosis (holometaboly).

Convergence: The independent

undergoes development on the exterior of the host’s body.

origin of derived features that are similar but that evolved in unrelated taxa.

Coriolis force: An inertial force that occurs as a result of simultaneous motion in a rotating or oscillating reference frame and motion in another plane.

Coxa: The segment of an insect leg closest to the body.

Cretaceous: The geological period that lasted from 145 to 66 million years ago.

354

Detritivore: An insect that feeds on

contains two sets of chromosomes, one from each parent.

Ectoparasite: A parasite that

Endemism: Referring to living in a certain area, such as a species occuring only in a certain lake or island.

Endoparasitoid: A parasitoid that undergoes development within the body of its host.

Entognath: Meaning “inner chewing,” a group that includes the springtails (Collembola), Diplura, and Protura, that have their mouthparts recessed into a pouch.

Entomophily: The process by which

Hexapod: A six-legged arthropod;

Lignocellulose: The woody material

pollen is transported between plants by insects.

a group that includes entognaths and the true insects.

Elytra: The hardened forewings

Holometabolous: Insects

that gives plants their rigidity, composed of the three polymers cellulose, hemicellulose, and lignin.

of beetles.

undergoing post-embryonic development where juveniles grow in size before going through a pupal stage in which wings develop. This type of metamorphosis is also called complete metamorphosis.

Maxilla: A segment of the insect

Homologous: Sharing a common

Median caudal filament: A long,

evolutionary origin.

segmented, thread-like structure at the apex of the abdomen that occurs between the cerci of Archaeognatha, Zygentoma, and many immature and some adult Ephemeroptera.

Eusocial: Societies in which there is an overlap of generations, cooperative care of the young, and a reproductive division of labor, with distinct queen and worker castes.

Femur: The third segment of the insect leg, between the trochanter and the tibia.

Gamete: A reproductive cell generally containing one set of chromosomes, resulting from meiosis.

Haltere: The reduced hindwings of dipteran insects.

Haplodiploid: A genetic system where females carry two sets of chromosomes while males carry only one set of chromosomes. Haplodiploid systems can be found in Hymenoptera.

Haploid: A cell is haploid when it contains only one set of chromosomes. These cells are generally generated during meiosis (gametes) or are found in some male insect species (see also, haplodiploid).

Haustellum: A form of proboscis in which the mouthparts function by the suction of liquids.

Hematophagous: Insects feeding on blood.

Hemimetabolous: Insects undergoing post-embryonic development where juveniles grow in size and develop wing buds throughout their successive molts. This type of metamorphosis is also called incomplete metamorphosis.

Hormone: In insects, a molecule secreted by specialized internal glands that affects distant cells in the body, which in turn affect metabolism and development.

Imago (n.), imaginal (adj.): The adult, or sexually mature stage of an insect.

Inquiline: Any organism that exploits the nest of another insect species.

Instar: The immature stage in insects between molts. A newly hatched insect is the first instar, which then molts into a larger, second instar, and so on.

Karyotype: The complete set of chromosomes in an individual organism, including their number and form.

Labellum: One of the insect mouthparts. In some flies, the labellum is a spongy mouthpart used for tasting and sucking up blood.

Labium: One of the insect mouthparts, often modified for specific diets.

Larva: A juvenile of insects undergoing complete metamorphosis or holometaboly.

Hemolymph: Arthropod blood,

Larviform: An adult (generally a

composed of a fluid plasma in which the hemolymph cells (hemocytes) are suspended.

female) that resembles a larva.

head adjacent and posterior to the mandibles that bears paired appendages composed of the cardo, stipes, galea, lacinia, and maxillary palpus.

Meiosis: Cell division process that results in the production of gametes containing only half the number of chromosomes compared to the original cell.

Metamere: One of the similar body segments into which earthworms, crayfish, and similar animals are divided longitudinally.

Mitosis: A cell division process that results in two cells with the same number of chromosomes as the original cell.

Morphology: Shape and structure, or the study of shapes and structures.

Müllerian mimicry: A form of mimicry in which two or more species with effective defenses converge on a common appearance.

mya: An abbreviation for millions of years ago.

Natural selection: Differential survival and reproduction of members of a species depending on specific, heritable characteristics.

Neoptera: The large group of winged insects that can fold their wings over their abdomen.

Larviparous: Insects that give birth to larvae instead of laying eggs.

GLOSSARY

355

Ocellus (sing.), ocelli (pl): The

Plastron: Specially structured cuticle

Spiracle: The pairs of small, valved

small, photosensitive “simple” eyes, each bearing a single lens, found on top of the head in most adult insects.

or areas of distinct, dense setae that trap a thin layer of air against the insect body in aquatic insects. It allows respiration either by replacing the air film at the water surface or by diffusion of gases into the film.

openings of the respiratory (tracheal) system on the side of each or some post-cephalic segments in certain arthropod groups, including insects.

Oocyte: A gamete created by a female.

Ootheca (sing.), oothecae (pl.): An egg case containing eggs and found in cockroaches and mantises.

Oviparous: An insect that lays eggs. Ovipositor: The egg-laying

different behavioral tasks between individuals within an insect society, or at different times during the life of an individual within the colony.

structure of female insects that consists of pairs of appendages from two abdominal segments, each appendage primitively composed of a gonocoxa and gonapophysis.

Polytene: Refers to strongly

Palp: A long, segmented, sensory

Pretarsus: The final segment of the

amplified forms of interphase chromosomes that provide high levels of function in certain tissues such as salivary glands.

aculeate Hymenoptera, which functions as a defensive weapon in association with venom glands.

Subimago: An adult-like stage but that does not mate. Mayflies have a winged subimago, which then molts into the reproductive adult.

Symbiont: Any organism that lives in

structure (telopodite) that is part of the maxillary and labial appendages.

tarsus and any structures attached to it, including claws, bristles, or pads.

close association with another, usually larger, unrelated one. These include parasites, commensals (which derive benefit from a host without harming it), and mutualists (where the host also benefits).

Pancrustacea: A group that

Pterygote: The subclass of insects

Tarsus: The final segment of the

includes Crustacea plus the insects, which are closely related to certain kinds of crustaceans.

with wings (including those that are secondarily wingless).

insect leg; can end in claws or pads.

Pupa: A post-embryonic stage found

Parasite: An insect that lives in or

in holometabolous insects, where the tissue of the juvenile is reorganized completely to give rise to a completely different adult.

of the insect body; legs and wings are attached to the thorax.

on a host and that deprives the host of nutrients but does not kill the host.

Parasitoid: An insect that lives in or on a host and that eventually kills the host as it completes its development.

Parthenogenetic: Reproducing without sex, in which case there are only female individuals. Various insects have evolved parthenogenesis, caused by different genetic mechanisms.

Pheromone: A secreted chemical compound that is dispersed to the air or laid on substrate and is used for intraspecific (within the same species) communication.

Phylogeny: A scheme of evolutionary relationships among species or other groups. Relationships are determined using DNA, anatomy, and other kinds of data.

Phytophagous: Insects feeding on plants.

356

Polyethism: The presence of

Sting: The modified ovipositor of

THE COMPLETE INSECT

Radiation: The rapid, simultaneous origin of many species over a short period of geological time.

Remipedia: Small crustaceans that may be the closest relatives of hexapods. Remipedes live near islands in the Caribbean, Canary Islands, and Australia, in “blue holes” (sinkholes partially fed with fresh water).

Sclerite: A hardened, generally external plate of the exoskeleton present on individual segments.

Sequester: To accumulate and concentrate in the body, such as an insect that sequesters toxins from its host plant for defense.

Thorax: The second main segment

Tibia: The fourth segment of the insect leg.

Trachea: The ringed, finely branching tubes of the respiratory system that pass air to and from the tissues, opening to the outside of the body via spiracles or to gills in some aquatic immature stages.

Triungulin: The active, wandering first instar larva of parasitoid meloid and rhipiphorid beetles, the later immature stages of which become sessile.

Trochanter: The small segment of the leg between the coxa and femur.

Viviparous: An insect giving birth to live juveniles instead of laying eggs.

Further Reading American Museum of Natural History. 2022. Extinct and Endangered: Insects in Peril. Harry N. Abrams.

Grimaldi, D. A. and M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press.

Mohr, S. E. 2018. First in Fly: Drosophila Research and Biological Discovery. Harvard University Press.

Blackmon, Heath, Laura Ross, and Doris Bachtrog. 2017. “Sex determination, sex chromosomes, and karyotype evolution in insects.” Journal of Heredity 108.1: 78–93.

Gullen, P. F. and P. S. Cranston. 2014. The Insects: An Outline of Entomology. John Wiley & Sons.

Proctor, M., P. Yeo, and A. Lack. 1996. The Natural History of Pollination. Timber Press.

Heming, B. 2018. Insect Development and Evolution. Cornell University Press.

Resh, V. H. and R. T. Cardé. 2009. Encyclopedia of Insects. Academic Press.

Capinera, J. L., R. D. Scott, and T. J. Walker. 2004. Field Guide to the Grasshoppers, Katydids, and Crickets of the United States. Cornell University Press. Chapman, R. F. 1998. The Insects: Structure and Function. Cambridge University Press. Church, Samuel H., et al. 2019. “Insect egg size and shape evolve with ecology but not developmental rate.” Nature 571.7763: 58–62. Clausen, C. P. 1940. Entomophagous Insects. McGraw-Hill. Dudley, R. 2002. The Biomechanics of Insect Flight: Form, Function, Evolution. Princeton University Press. Eisner, T. 2003. For Love of Insects. Belknap Press. Engel, M. S. 2018. Innumerable Insects. Sterling Press. Flatt, Thomas, and Andreas Heyland (eds.). 2011. Mechanisms of Life History Evolution: the Genetics and Physiology of Life History Traits and Trade-offs. Oxford University Press. Gauld, I. and B. Bolton (eds.). 1988. The Hymenoptera. Oxford University Press.

Hölldobler, B. and E. O. Wilson. 1990. The Ants. Harvard University Press. Hölldobler, B. and Kwapich, C. L. 2022. The Guests of Ants: How Myrmecophiles Interact with Their Hosts. Harvard University Press. Klowden, M. J. 2013. Physiological Systems in Insects. Academic Press. Lawrence, P. A. 1992. The Making of a Fly: The Genetics of Animal Design. Wiley-Blackwell. Leather, Simon R. 2018. Insect Reproduction. CRC Press. Lehane, M. J. 1991. Biology of BloodSucking Insects. Harper Collins. Marshall, S. A. 2006. Insects, Their Natural History and Diversity, with a photographic guide to insects of eastern North America. Firefly Books. Marshall, S. A. 2012. Flies, The Natural History and Diversity of Diptera. Firefly Books. Mason, A. C., Popper, A. N., Pollack, G. S., and Fay, R. R. 2016. Insect Hearing. Springer.

Scoble, M. 1992. The Lepidoptera: Form, Function and Diversity. Oxford University Press. Shuker, D. M. and Simmons, L. W. 2014. The Evolution of Insect Mating Systems. Oxford University Press. Snodgrass, R. E. 1935. Principles of Insect Morphology. McGraw-Hill (reprinted by Cornell University Press). Snodgrass, R. E. 1956. Anatomy of the Honey Bee. Cornell University Press. Stehr, F. W. 1987, 1991. Immature Insects, vols. 1 and 2. Kendall/Hunt. Strausfeld, N.J. 2012. Arthropod Brains: Evolution, Functional Elegance, and Historical Significance. Belknap Press. Truman, James W. 2019. “The evolution of insect metamorphosis.” Current Biology 29.23: R1252–R1268. Wilson, E. O. 1971. The Insect Societies. Harvard University Press.

Michener, C. D. 2007. The Bees of the World, 2nd edition. Johns Hopkins University Press.

FURTHER READING

357

Index Numbers in bold refer to main entries and their illustrations; italic numbers refer to illustrations or their captions. A abdomen 14, 15, 42, 42, 50, 51, 59–61, 118, 189 movement 76 nymphs 204 ruddering 160 Abedus indentatus 263 Acalyptratae 318 accessory glands 78, 79, 298 Acropyga 281 Aculeata 31 adaptation 212–13, 226–7 plant–insect interactions 294, 308–9, 314–15, 318 adults See imagoes Aedes 324–6, 326, 337, 345, 350 Aenictus 281 aeropyles 66, 193, 193 Agaonidae 307, 307 alarm signals 112, 112, 268–9 alary muscles 101, 101 alimentary canal 78, 79, 104 allantoic acid 108 allelochemicals 112–13 altricial species 276, 276 amber 24, 24 ametabolous orders 46, 67, 97, 97, 197, 198 ammonia 108 amplificatory structures 58 Andrena 256 Anopheles 328–9, 329, 345 Anoplura 15 antennae 14, 15, 18, 21, 50–1, 50, 51, 54, 56, 56 chemoreceptors 88 heart 101, 101 imaginal discs 222, 223 Johnston’s organ 90, 91 mechanosensors 154, 154 metamorphosis 199 mosquitoes 154, 154 moths 182 nerves 81 olfaction 80, 88, 182 antennal segment 54 Antennapedia complex 42, 189 Anthonomus grandis 323 Anthophila 284 Anthophora plumipes 285 antlions 23, 31, 56, 176, 241, 241, 318 ants 15, 23, 24–5, 31, 43, 126, 141, 195 See also Hymenoptera aphid- and scale-tending 281, 281, 299 army 281, 321 bullet 331

358

THE COMPLETE INSECT

communication 268–9, 279 dracula 281 ecological significance 262, 290–1 exaggerated traits 224, 224 excavators, as 319, 319 fungus gardens 280, 281 haplodiploidy 178, 281, 347 inquiline species 45 leaf-cutter 228, 279, 281 myrmecomorphs 321 myrmecophiles 320 myrmecophytes 320 queen 272–3, 279, 281, 346 silk glands 117 sociality 262, 268, 268, 272–3, 279–81, 321, 346, 347 socially parasitic 281 soldiers 224, 224, 279, 279 stings 331 trail pheromones 112, 268, 268 trap-jaw 280, 281 velvet 331 weaver 352–3 workers 279, 346 anus 106, 106, 117 aorta 101 Aphelinidae 333 aphids 23, 31, 110, 112, 181, 241, 245, 245, 323 aphid-tending ants 281, 299 biological control 333 eusocial 286, 288–9 gall-inducing 288, 288 honeydew 299 reproduction 148, 181, 181, 192 wings 148, 149 Apis mellifera 287, 287, 338–9 apneustic species 94 Apocrita 58 apodeme 76, 123 aposematism 342 appendages 14, 15, 20, 41, 42–3, 44, 50–3, 54 abdomen 59–61, 59 appendage-like extensions 53, 53, 59 circulatory system 100 exaggerated 224–5 head 55, 56, 56 hornlike 52, 52 mouthparts 14, 18, 42, 50–1, 50, 56, 56, 61 movement 76 ovipositor 11, 14, 18, 19, 50, 61 sexually dimorphic 51, 52 thorax 58 apterous species 132, 134 Apterygota 19, 60, 197 aquatic species 89, 95, 95, 229–39 evolution 232, 238 respiratory system 95, 97

Arachnida 14, 21, 96, 96, 230 Archaeognatha 18, 23, 26, 46, 46, 51, 59, 69, 97, 97, 145, 197, 198 Arixenia esau 190, 257, 257 Arixeniidae 257, 257 arrhenotoky 180–1 arthrodial membrane 63 Arthropoda 14, 21 Asbolus verrucosus 128–9 assassin bugs 92, 113, 246, 246, 292–3, 318, 324, 329, 329 Athericidae 236 Atta laevigata 280 Auchenorrhyncha 31 Austroplatypus incompertus 289, 289 autosomes 177 autotomy 245 axon 87 Azteca 272–3, 281 B bed bugs 120, 184, 249, 249, 254, 324, 325 bees 22, 23, 31, 55, 72, 138, 184, 185 See also Hymenoptera allodapine 284 bumble 148, 284, 286, 312 carpenter 284 cleptoparasitic 256, 256, 284 communication 268, 270, 347 cuckoo 256, 256 haplodiploidy 178, 178, 281, 347 honey See honey bees orchid 309, 312, 320 pollinating 305, 305, 309, 312, 320 sex determination 347 silk glands 117 sociality 268, 270–1, 284–7, 347, 347 solitary 284, 285 squash 312 stingless 284, 286–7, 312 stings 331 sweat 57, 264, 265, 284 wax glands 116 wings 139, 164, 165 beetles 15, 23, 32, 51, 59, 113, 138, 318 bark 32, 336, 337 biological control 333 blister 202, 202, 311 burying 158 carrion 140, 263 checkered 311 Colorado potato 322–3, 322 cucumber 314 diving 89, 235, 235, 239 dung 214, 214

elytra 32, 92, 92, 126, 142–3, 147, 158–9, 164 eusocial 289, 289 flour 143, 221, 221, 339 goliath 125, 125 ground 32, 56 hercules 125, 126 iridescence 74–5 ironclad 128–9, 158, 158 Japanese 35 jewel 311 lady 351 leaf 32, 318, 322 longhorn 32, 56, 311, 311, 318, 331, 350 metamorphosis 110, 196, 199 migration 148 nocturnal species 85 ovaries 185 parasitic species 32 parasitoid species 260, 260 pleuron 58 pollinating 311 pupa 199 rhinoceros 225, 225 rove 45, 290, 290–1 sap 311 silk glands 117 soldier 311 stag 32, 52, 52, 224 stridulation 92 titan 125 tortoise 72, 72 tumbling flower 311 water 235, 235, 263 wedge-shaped 260 whirligig 235 wingless 140, 159, 159 behavioral features 22 bicoid gene 188 Bicyrtes quadrifasciatus 282 biocides 349, 351 biological pest control 333 bioluminescence 98–9, 108, 108–9, 242 firefly courtship 182, 182 biomedical research 338–41 Biston betularia 345, 345 Bithorax complex 42, 189 Bittacidae 246–7 Blaberus giganteus 340 blastoderm 188, 189 Blattella germanica 221, 221, 340 Blattodea 23, 30, 44, 44, 47, 174, 177, 186, 189, 197 bloodworms 340 body size 125–6, 125, 126 critical size 222, 223 determination 222 growth 222–3 nutrition and 214, 222 terminal growth period 222, 223

body structure and function 14, 15, 41–3 appendages See appendages cuticle See cuticle developmental changes 46–9 exaggerated traits 224–5 exoskeleton See exoskeleton integument 62–72 metamorphosis See metamorphosis modifications 44–5 muscles See muscles pupal stage 199 segmentation 14, 15, 60, 63, 135, 142, 188–9, 189, 204 Boganiidae 308 Bolboleaus hiaticollis 195 Bombyliidae 311, 318 Bombyx mori 338–9 boring insects 32, 289, 297, 297 Braconidae 140, 164, 255 brain 80, 168, 218 Branchiopoda 21 Braulidae 162 breathing 94 bristles 45, 54, 58, 68, 86 See also hairs; setae bristletails 18, 19, 23, 26, 26, 46, 59, 97, 97, 132, 145, 198, 200 brood balls 214, 214 bugs 23, 31, 158, 185, 221 Buprestidae 311 butterflies 15, 18, 23, 33, 33 See also caterpillars; chrysalises; Lepidoptera brush-footed 313 Cairns birdwing 73 camouflage 155 coloration 73, 133, 152, 152 diet 203 flight 144, 145, 148 Imperial hairstreak 194 life cycle 173 metamorphosis 48, 196, 199 migration 148–9, 150–1 mimicry in 133, 156, 342–3 monarch See monarch butterfly ovaries 185 owl 133, 156 parasitic 257, 257 pollinating 305, 312–13 scales 68–9, 72 skipper 313 swallowtail 108, 112–13, 196, 196 C caddisflies 23, 33, 33, 244, 261 aquatic stage 95, 206, 232, 236, 237 Calliphora 340 Calliphoridae 310, 331 Calyptratae 165, 324, 325, 325 calyx 122 camouflage 29, 30, 68, 74–5, 108, 108, 130, 154–7, 158, 244, 244 predatory species 241, 241, 244, 292–3 campaniform sensilla 86, 86, 88, 160–1, 162–3, 164 Cantharidae 311

capitula 195 Carabidae 113, 318 carbon dioxide 94, 148 cardiomyocytes 78 cardo 55 caterpillars 33, 108, 112–13, 173, 199, 206, 206, 319, 322 See also larvae; pupae biological control 333 chrysalises 33, 170, 173, 199 diapause 226 diet 33, 203, 295, 296 metamorphosis 170 overwintering 103 pupae See pupae silk glands 117 stemmata 82, 84 subsocial 265 tent 265, 318, 318 tracheal system 98, 98 urticating setae 113 venomous 331, 331, 342 woolly bear 103, 127, 226 caudal gene 188 Cecidomyiidae 114 cecidozoans 299 Cecropia 281 cell division 174–5, 186, 188 cellularization 188 Celyphidae 40 centromere 174, 174 Cephalocarida 20, 21 Cerambycidae 311, 311 Ceratopogonidae 236, 324 cerci 14, 15, 27, 52, 59 heart 101 cerebrum 80 Chagas disease 216 Chaoboridae 236 chelicerae 21, 96 chelicerates 230 chemical dispersal 68 chemoreceptors 86, 88, 89, 160 Chironomidae 227, 227, 236 Chironomus tentans 340 chitin 63, 123 chitinase 65 Chlerogella 57 chordotonal organ 90–1, 161 Choreutidae 156 chorion 64, 66, 122, 185, 193 chromatids 174–5 chromosome 188 chromosomes 172, 174–5 autosomes 177 diploidy 178 haplodiploidy 178, 178 heteromorphic 177 holocentric 174–5, 174 paternal genome elimination 179, 179 paternal-sex-ratio (PSR) 179, 179 sex determination 176–9 chrysalises 33, 170, 173, 199 See also pupae mimicry in 199 Chrysocoris stollii 37 Chrysomelidae 113, 318, 322 Chrysomya bezziana 331 Chrysopidae 241, 241, 333

cibarium 27, 78 cicadas 23, 31, 90, 98, 147 life span 125 metamorphosis 199, 218 nymphs 204, 204 tymbals 92, 93, 182 cilium 87 Cimicidae 249, 325 circadian rhythm maintenance 84 circulatory system 100–3 dorsal pericardial sinus 100 hemolymph 100–3 medial perivisceral sinus 100 muscles 100, 100, 101 ventral perineural 100 visceral muscles 78, 79 circumoesophageal vessel ring 101 Cirripedia 21 Cixiidae 59 cladistics 347 claws 14, 21, 45 imaginal discs 222 cleptoparasitism 256, 256, 284 Cleridae 311 climate change 227, 350 clypeus 27, 55 coarctate 202 Coccinelidae 158 Coccinella novemnotata 351 Coccinellidae 333, 333 Cochliomyia 337 C. hominivorax 331 cockroaches 30, 30, 44, 44, 106, 108, 122, 185, 190 chromosomes 177 life span 125 model experimental organisms, as 340 nymphs 204, 205 cocoons 33, 56, 117, 135, 136, 137, 201, 319 See also pupae Coleoptera 15, 23, 32, 43, 59, 69, 114, 174, 182, 197, 206, 299, 301 body undulation 77 chromosomes 177 diapause 227 elytra 32, 92, 92, 126, 142–3, 147, 158–9, 164 exaggerated traits 224 larvae 206, 207 metamorphosis 48, 202 Colias UV patterns 152, 152 Collembola 16, 17, 23, 69, 184, 230, 231 colleterial glands 117 colon 106 coloration 62, 68, 70–5 butterflies 73, 133, 152, 152 camouflage See camouflage chemical compounds/pigments 70, 70, 71, 73 color changes 72 courtship displays 152–4 crypsis 154–7, 244, 244 developmental plasticity 212–13 diet and 71 eye pigment cells 82–4, 82, 83, 84, 85 iridescence 72, 74–5

metallic 72 mimicry See mimicry photonic crystals 69, 72 scales 69, 71, 72 structural 70, 72 UV patterns 152, 152 warning 68, 73, 113, 158, 159, 330, 342 columnar (principal) cells 105, 106 commercial harvesting 348, 350 communication alarm signals 112, 112, 268–9 chemical 112–13, 268, 268, 269, 270, 279 recognition signals 269 social species 112, 268–73, 347 stridulation 28, 58, 90, 92, 92, 147, 182, 269 waggle dance 270 competition metamorphosis and 203 compound eyes See eyes condyles 26 Condylognatha 197 Copepoda 21 Coptotermes 276 copulation 120–1 corbicula 284, 284, 312 coremata 113 cornicles 112, 245 corpora allata 81, 218 corpora cardiaca 81, 93, 117 corpora pedunculata 80 Corydalidae 52 Cossidae 123 courtship 152–4, 168–9, 182 nuptual gifts 182, 183, 183, 246–7 coxa 14, 222 Crabronidae 282, 282 crane flies 236 Crataerina hirundinis 252 crickets 18, 23, 28, 43, 90 chromosomes 177 courtship songs 168, 169 house 43, 47 legs 224 mole 28 stridulation 92 two spotted 194 wingless 140 crop 78, 104, 105, 312 crustaceans 20, 21, 230 cryoprotectants 127 crypsis 14, 45, 116, 154–7, 244–6 parasitic species 251 cryptobiosis 127 Cryptomyzus galeopsidis 181 cryptonephridial system 106–7 cuckoos 256, 256 Culex 236, 239, 326, 329 Culicidae 61, 236, 324 Curculionidae 318, 322, 337 Curculionoidea 57, 57 Cuterebrinae 331 cuticle 38, 62, 63–8, 126 adult 67 cement layer 63, 63 chorion 64, 66, 122, 185, 193 color See coloration

INDEX

359

development 117 digestion during molting 62, 64–5 ecdysial lines 65 endocuticle 63–5, 63, 67 envelope 63, 65 epicuticle 63–5, 63 exocuticle 63–5, 63, 67 exoskeleton 63 extensions and modifications 53, 53, 62, 66–8 gills 238, 238 growth and 196 gut 105 hardening 137 larval 64, 65 macrochaetae See bristles; scales mesocuticle 63–5, 63 molting See molting muscle attachments 76, 78 pleuron 58, 135–6, 136, 141 polymerization 63 pore canals 63, 65 procuticle 63, 63 proteins 63 pupa 64 respiratory system 94–5 sclerotization 64, 65, 67, 68, 69 secretion 62, 63, 64, 66, 68, 69 sensilla 67, 68, 86–9 taenidia 94 tracheal system 94–5 wax filaments and layer 63, 63, 65, 116 weevils 67, 110 cuticulin 63 cutworms 322 Cyrtobagous salviniae 333 D daddy longlegs 96 damselflies 23, 27, 82, 219, 318 aquatic stage 95, 232–3 copulation 183, 183 Danaus plexippus 351, 351 Decticus albifrons 190 defenses 56, 112 See also camouflage; crypsis alarm signals 112, 112, 268–9 aposematism 342 autotomy 245 chemical 113, 245, 278 concealment 244 iridescence 74–5 mimicry 108, 108, 133, 342–3 natural selection 345, 347 parental care 263, 264 pheromones 112 phragmotic species 279 physical 245 plant defenses against insects 302–4, 315 recognition signals 268 sociality as 264, 268, 276, 278, 279, 286 stings 18, 31, 113–15, 113, 115, 282–3, 318, 331, 342 stink glands 113, 113 toxins See toxic species warning coloration 68, 73, 113,

360

THE COMPLETE INSECT

158, 159, 330, 342 wax glands 116 dehydration 62 Dennyus hirundinis 253 density polyphenism 212 Derbidae 156 Dermaptera 23, 27, 43, 52, 52, 190, 197, 300 Dermatobia hominis 331 detritivores 20, 33 developmental changes 46–7 See also growth; metamorphosis postembryonic 172, 196, 198–9, 213, 217, 219, 220, 222 developmental plasticity 212–15 castes 210, 211, 215 diapause 38, 117, 202, 226–7 facultative 227 obligatory 226, 227 Diaphorina citri 322 diaphragms 101, 101 Diapriidae 237, 237 Dicondylia 26 Dictyoptera 27, 30, 300 diet and nutrition 104, 105, 112 coloration and 71 decomposers 319 detritivores 20, 33 developmental and adult stages 203, 204 digestive system See digestive system fluid-feeders 78, 298–9, 300, 325, 326 hematophagy (blood-feeding) 88, 248–53, 324–6 monophagy 295 oligophagy 295, 309 phytophagy (herbivory) 139, 294–305, 318, 320, 322–3 plant–insect interactions 292–305, 318 predators See predators scavengers 20, 27 size and nutrition 214, 222, 225 sucking insects 298–9 symbionts 110 termites 274, 276–7, 319 digestive system 104–11, 298–9 abdomen 59 digestive fluids 112 muscles 105, 109 peritrophic matrix 105 stomatogastric nervous system 81, 93 symbionts 106, 108, 110–11 visceral muscles 78, 79 Diplolepis rosae 299 Diploptera punctata 190 Diplura 16–17, 18, 20, 23, 197 Diptera 15, 23, 33, 43, 48, 51, 109, 114, 186, 189, 197, 299, 311, 318, 347 See also flies chromosomes 174, 175, 177 diapause 227 disease vectors 324–5 exaggerated traits 224 flight muscles 145

halteres 33, 58, 88, 108, 143, 145, 162–5 hypermetamorphosis 202 larvae 207 mouthparts 61 scales 69 disease vectors 216–17, 249, 324–9 Ditrysia 312 diuresis 117 dobsonflies 31, 52, 59 aquatic stage 95, 232 Dolichopodidae 152, 153, 236 domatia 281 dorsal appendages 51 Dorylus 281 dragonflies 15, 23, 27, 27, 82, 134, 138, 232–3 copulation 183 larva 107, 107, 241 migration 148 Paleozoic 148 predation by 138, 147, 241, 243, 243, 318 wing sensors 161 wings and flight 146, 147, 166–7 Drosophila 344, 345 See also fruit flies D. melanogaster 221, 317, 338, 344 D. pseudoobscura 344 growth 222 larvae 222 Drosophilidae 38, 38, 143, 146, 152, 184 See also fruit flies Dytiscidae 89, 235, 235 E earwigs 23, 27, 27, 43, 52, 52, 140, 190, 191, 219, 257, 257, 262 ecdysial lines 65 ecdysone 217–21, 222 ecdysteroids 117 Eciton burchellii 321 ecological niches adaptation to 172 metamorphosis and 203 ecological significance of insects 262, 262, 290–1, 317–21 ectoderm 86, 222 eggs 46, 60, 79, 122–3, 171 See also gametes; reproduction aeropyles 66, 193, 193 atrial opening 193 brood balls 214, 214 chorion (cuticle) 64, 66, 122, 185, 193 development 172, 173 diapause 227 ectoparasitoids 251 egg cases 123 embryo 189, 193 endochorion 193, 193 exochorion 193, 193 fertilization 123, 180, 185, 186, 188, 190 follicular epithelium 68 gas exchange 193 hatching 198–9 holometabolous species 199

laying 172, 173, 190–2, 193 micropylar atrium 193 micropyle 123, 188 mimicry in 195, 195 ootheca 122, 192 ovipositor See ovipositor ovisac 192, 192 oxygenation 193 parasitic species 61, 251, 254, 255, 318 parasitoid species 237, 237, 251, 255, 318 parental care 262–3, 264 periplasm 193 plastron 66, 95 structure, size, and shape 193–5 vitelline membrane 193, 193 ejaculatory duct 118–19 elytra 32, 92, 92, 126, 142–3, 147, 158–9, 164 Embioptera 23, 29, 117, 197 embryo 172 de-embryonization hypothesis 203 diapause 227 embryogenesis 179, 180, 188–9, 222 segmentation 188–9, 189 Empididae 236, 247, 318 Empis snoddyi 183, 183 Encyrtidae 333 endangered species 245, 317, 350–1 endochorion 193, 193 endocrine system 81, 93, 105, 117 endocrine glands 112, 117, 218–19 endoparasitoid’s host 255 growth control 222 metamorphosis, regulation 216–21 secretory neurons 93 Endopterygota 197 endosymbionts 179 Entognatha 16, 18, 197, 230 environmental conditions adaptation to 212–13, 226–7 climate change 227, 350 enzymes 65, 102, 105 Ephemeroptera 23, 26, 27, 51, 59, 174, 186, 197, 198, 200, 232, 234 Ephydridae 236, 237 epidermis 62, 63, 87 color 72 cuticle secretion and molting 62, 63, 64–5 larval 62 epithelium 62, 105, 112, 116 bristles and scales 68–9 coloration 62, 71 imaginal discs 48, 62, 172, 199, 203, 222–3 sensilla 68 tracheal system 94 epitracheal (peritracheal) glands 117 Erebidae 127 Eristalis brousii 351 escape mechanisms 68 esophageal nerves 93 esophagus 104 Eudryas 155

Eurema mandarina 179 Eurydema ornate 36 Eurypterida 21 eusocial species See sociality eversible vesicles 14, 51 evolution 13, 14, 19–25, 38–9 adaptation 212–13, 226–7 aquatic species 232, 238 body modifications 44–5 coevolution 144, 342 convergent 41, 56, 61, 69, 96, 114, 164 exaggerated traits 224–5 genetic variation 344, 344 hematophages 248–9, 253, 324 metamorphosis and 196–7, 196, 203 natural selection 342, 345, 347 New Synthesis 344 plant–insect interactions 294, 308–9, 314–15, 318 radiations 344 sociality 274, 278, 281, 284, 290, 347 speciation 305, 314 study of 342–7 systematics 22, 347 tracheal system 96 transition to land 230 wings 58, 132–43 exaggerated traits 224–5 excretion and waste 59, 71, 78, 93, 104–9 exochorion 193, 193 exocrine glands 112–13, 115, 116–17, 119 exoskeleton 14, 21, 41, 62 body size and 126 cuticle See cuticle formation 63 growth and 196, 222 macrochaetae See bristles; scales molting See molting skeletal muscles 76–7 extinction rates 38, 39 eyes apposition 85, 85 central nervous system 80 compound 14, 18, 54, 54, 55, 80, 82, 84, 148 imaginal discs 222, 223 integument 62 larval 54, 82, 84 ommatidia 82, 85 optic nerves 81 pigment cells 82–4, 82, 84, 85 rhabdomeres/rhabdom 82, 82, 84, 85 simple 82, 83–4 stalked 45, 53, 224 stemmata 54, 82, 84 superposition 85, 85 eyespots 133, 156, 156 F female reproductive system 122–3 femur 14 fermentation chamber 106 fighting 52, 56, 224 fireflies 32, 78, 79

bioluminescence 98, 99, 108, 108–9, 242, 242 courtship light patterns 182, 182, 242 heart 100 fleas 15, 23, 32, 32, 117, 251–3, 324, 327, 327 flies 18, 23, 33, 33, 40, 43, 134, 137, 185 See also Diptera alderflies 232 aquatic 236, 236, 239 balloon 247 bee 311, 318 black 33, 248, 250, 324, 329, 333 blow 165, 166, 310, 331, 340–1 bot 254, 254, 331 brachyceran 51 bristles 45, 54, 58 compound eyes 54 courtship displays 152, 153, 154, 154, 168–9 cyclorrhaphan 207 dance 183, 183, 236, 236 disease vectors 248, 248, 250, 324–31 drone 351 flesh 165, 190 fruit See fruit flies halteres 33, 58, 88, 108, 143, 145, 162–5, 223 Hawaiian 194 head 54, 55 hematophages 248, 248, 250, 324–6 horse 33, 324 ibis 236 labial palps 33 louse 190, 252 maggots 199, 207, 207, 331 mantis 202 metamorphosis 196, 199 migration 148 moth 236 mouthparts 54 nitrogen fixation 110 owlflies 240 parasitic 33, 162, 324–5 parasitoid 260, 260, 318, 333 pollinating 305, 310–11 predatory 246–7 robber 12, 120, 138, 318 sand 324, 326, 327, 329 scorpionflies 23, 32, 32, 54, 57, 59, 61, 61, 183, 246–7, 253 scuttle 318 sewer 236 shore 236, 237 snakeflies 23, 31, 199 soldier 97 spongillaflies 232, 257 stable 236, 324 stalk-eyed 45 stoneflies 23, 28, 92, 232, 234 thistle gall 153 tsetse 33, 190, 324, 337 water snipe 236

flight 68, 144–9 See also wings advantages 138–40 control 80 energy use 148 evolution 230, 230 gliding 144, 145 halteres 88, 108, 143, 145, 162–5 hovering 144 migration 148–9 muscles 76, 77, 98, 98, 100, 144–7, 161, 166 optomotor response 166–7 ruddering abdomen 160 stability 162–5 steering 146, 166–7 fluid-feeders 78, 298–9, 300, 325, 326 food source, insects as 348 Formicidae 279 fossils 24–5, 39, 203, 230, 230, 231, 300–1, 308–9, 318 frass 109, 109 frons 55 fruit flies 38, 38, 42, 43, 54, 143, 146, 184, 344, 345 brain 80 courtship signals 154, 154, 168–9 eggs 190 embryogenesis 188, 188 flight 166 genetic model, as 221, 317, 338 growth 222–3 Hawaiian 344, 345 imaginal discs 223 larva 87, 88, 222, 223 metamorphosis 217, 221, 221 mimicry by 156–7, 157 model experimental organism, as 338, 344 ovaries 185 pest control 337 sensory hairs 160 sex determination 178 sperm 187, 187 fulgorids 72 furcula 16 G galea 55 gall-inducing insects 114–15, 288, 288, 299 gall wasps 114, 115, 295, 299, 299 gametes 174–7, 180, 185–6, 305, 318 See also eggs; sperm haploid 175 heterogamety 176, 177 pollen See pollinators ganglia 77, 80–1, 93, 168, 218 thoracic 80, 168 gas exchange 94, 148 aquatic species 238, 239 eggs 193 Gasterophilinae 331 gastrulation 188 gena 55 genetic manipulation and engineering 333, 337, 338 genetics 338

genetic variation 344 genitalia 47, 51, 51, 60, 79, 118–23, 183 imaginal discs 222, 223 Geometridae 313 German cockroach 221, 221, 340 germarium 122, 122, 185, 186 gills 51, 59, 95, 233, 238, 238 rectal 106–7, 107, 238 glands 112–17 eversible 112–13 prothoracic 218 pygidial 113 silk glands 117 stink glands 113, 113 wax glands 116 glassworms 236 glial cells 90 Glossinidae 324 goblet cells 105 gonapophysis 60, 61 gonocoxite 60 gonopore 122 gonostyle 60 grasshoppers 19, 23, 28, 28, 58, 348 camouflage 155 chromosomes 177 legs 224, 225 ovaries 185 romaleid 113 stridulation 92, 182 grooming 56, 263, 265, 271 growth 172, 222–3 critical size 222, 223 endocrine control 222 exaggerated traits 224–5 molting and 196, 222 nutrition influencing 214, 222, 225 terminal growth period 222, 223 grubs 206, 206 Grylloblattodea 23, 28, 197 gustatory sensilla 88, 89 gut 104–11 cuticle, molting 64 lining 62 symbionts 106, 108, 110–11 visceral muscles 78 Gynaephora groelandica 226 Gyrinidae 235 H habitat loss 349, 349, 351 Haidomyrmecinae 24 hairs See also bristles; setae scales See scales sensory 68, 86, 87, 160–1 stinging 113 wings, on 30, 33, 160–1 Halictinae 284 Halobates 20 halteres 33, 58, 88, 108, 143, 145, 162–5 imaginal discs 223 haplodiploidy 178–9, 178, 180 social insects 281, 347 harlequin bug 213, 213 harvestmen 96

INDEX

361

haustellum 33, 61 head 15, 42, 42, 50–1, 54–7, 189 cibarium 27, 78 eyes See eyes nervous system 80, 81 nymphs 204 oversized 224, 224 hearing 58, 86, 90–1, 98, 99, 161, 242 heart 78, 93, 100–3 accessory hearts 101, 101 muscles 100, 100, 101 heartbeat 100 Heliconius 152, 184, 342–3 Helicoverpa armigera 323 hematophagy 88, 248–53, 324–6 Hemimeridae 257 hemimetaboly See metamorphosis Hemiptera 23, 31, 43, 51, 61, 69, 104, 109, 114, 158, 179, 182, 186, 189, 197, 249, 298, 299, 301, 322, 324 chromosomes 174, 175 diapause 227 eggs 190, 227 exaggerated traits 224 head 55 mouthparts 61 stridulation 92 hemocytes 100, 102 hemolymph 72, 93, 98, 100–3, 105, 113, 117, 120, 148 delivery of hormones 216–18, 220 osmoregulation 105, 106 hermaphrodism 181 Hesperiidae 313 heterogamety 176, 177 heteromorphic chromosomes 177 Heteroptera 31 Heterotermes 276 Hexapoda 14, 18, 21, 230 noninsect 16 Hippoboscidae 324 Hodotermopsis sjostedti 215 Holometabola 27, 31, 48–9, 54, 97 holometaboly See metamorphosis homeobox (Hox) genes 42–3, 142–3 homeotic genes 188, 189 homologs 175 Homoptera 43 honey bees 139, 284–7, 312, 338–9 communication 268, 270, 347 model experimental organism, as 338–9 nurses 266 queen 266, 267, 268 spermatheca 180 waggle dance 270 workers 267, 268, 270–1, 287 honeydew 299 Hormaphidinae 288 hormones 93, 112, 117, 137 bursicon 137 delivery 216–18, 220 ecdysone 217–21, 222 juvenile hormone 215, 217–21, 225, 255, 333 metamorphosis, regulation 216–21

362

THE COMPLETE INSECT

molting hormone 218–19 prothoracicotropic 117, 218 sesquiterpenoid 218 trehalose 148 hornets 282, 283 horns 52–3, 52, 62, 126, 214, 224–5, 224 hover flies 22, 138, 148, 311 larva 207 hunchback gene 188 Hydriella 237, 237 hydrofuge setae 95, 238 hygroreceptors 86, 88 Hyles lineata 313 Hymenopodidae 241 Hymenoptera 15, 23, 30, 31, 42, 43, 44, 44, 47, 58, 59, 61, 69, 186, 189, 197, 260, 279, 284, 299, 301 See also ants; bees; sawflies; wasps arrhenotokous parthenogenesis 180–1 body undulation 77 chromosomes 174, 178, 178 diapause 227 exaggerated traits 224, 224 haplodiploidy 347 larvae 104, 207 metamorphosis 48, 202 sociality See sociality stings 18, 31, 113, 114, 115, 282–3, 318, 331, 342 wings 145 hypermetamorphosis 202 hyperplasia 299 hypertrophy 299 Hypothenemus hampei 323 I ice crawler 23, 28 Icerya purchasi 181, 181 Ichneumonidae 11, 190, 191, 255, 255, 256, 313, 313 Idiomyia picticornis 38 ileum 106, 106 imaginal discs 48, 62, 172, 199, 203, 222–3 imagoes 27, 171, 196, 198–9, 218, 219 immune system 102 hosts, of 115, 254, 255 inchworms 313 inclusive fitness 281, 347 inquilines 45, 290 insecticides 221, 332, 339, 349 resistance to 345 insulation 68 insulin pathway 222, 225 integument 62–72 basement membrane 63 cuticle See cuticle epidermis 62, 63 epithelium 62 larval 62 intercalary segment 54 intestine 104 intromittent organ 119 invasive species 287, 322, 337, 351 iridescence 72, 74–5 Isoptera 30, 43, 174, 197 Ixodes 327

J japygids 16 jewel bug 37 Johnston’s organ 90, 91 juvenile hormone 215, 217–21, 225, 255, 333 juvenoids 333 juvenile-like insects 210–11 juvenile stages 172, 203, 204–9, 218–19 See also larvae; nymphs castes 215 growth 221, 222–3 holometabolous species 207 K Kallima inachus 130 karyotypes 175, 176 katydids 18, 23, 28, 90 camouflage 155, 244 hearing 99 reproduction 181 stridulation 92 keratin 117 kinetochores 174 kissing bugs 216–17, 249, 249 L labellum 33, 55 labial glands 117 labial palpus 55 labial segment 54 labium 55, 56, 61 labrum 55, 61 lacewings 23, 31, 31, 59, 232, 318 biological control, use as 333 eggs 66 larvae 77, 206, 241, 241, 246, 247, 257, 318 metamorphosis 136 osmylid 232, 232 parasitoid 261, 261 percussive sounds 92 silk glands 117 ladybugs 32, 147, 158, 159 biological control, use as 333, 333 Laemophloeidae 57 larvae 27, 38, 48, 48, 172, 198–9, 206–7 See also caterpillars aquatic 207–9, 236–7, 237 camouflage 240, 241, 241, 244 campodeiform 206 critical size 222, 223 cuticle 64, 67 diapause 226–7 diet 33, 203, 295, 296 elateriform 206, 206 epidermis 62 eruciform 206, 206 growth 222 hypermetamorphosis 202 imaginal discs 222–3 integument 62 lacewings 77, 206, 241, 241, 246, 318 Lepidoptera 59, 59 locomotion 102 metamorphosis 106, 198–9, 203 mimicry 199, 241

molting 199 ocelli 148 parasitic species 253, 254, 257 parasitoid species 251, 254, 255, 261, 261 parental care 262–3 predatory 148, 206, 240 scarabeiform 206, 206 stemmata 54, 82, 84 vermiform 207, 207 larviparity 123, 190 leaf-footed bug 224, 224 leafhoppers 179 leaf insects 23, 245 leg remnants 50 legs 14, 15, 42, 50, 50, 51, 51, 58 autotomy 245 exaggerated traits 224 imaginal discs 222, 223 metamorphosis 199 nymphs 204 soldier caste 278 Lepidoptera 15, 23, 33, 43, 48, 48, 51, 114, 121, 186, 189, 197, 299, 301, 312–13, 318, 322 See also butterflies; moths aposematism 342, 343 biological control 333 body undulation 77 camouflage 155 chromosomes 174, 175, 177 chrysalises See chrysalises coloration 68, 69, 71, 73 diapause 226, 227 larva 59, 59 life cycle 173 metamorphosis 48 proboscis 55, 56, 61 prolegs 59, 59 scales 33, 68–9, 71 Lepisma saccharina 198 Leptinotarsa decemlineata 322–3, 322 Leptocentrus 175 lice 15, 23, 30–1, 31, 43, 252–3, 324, 324, 326 bark 30, 31, 253 book 30–1 coevolution 342–3 head 204, 324 jumping plant 43 life cycle 170–3 See also metamorphosis diapause 38, 117, 202, 226–7 life span 124, 125 Liposcelis 31 lobula plate tangential cells 167 locomotion 14, 58, 83, 100, 102 See also movement; muscles locusts 58, 148, 148, 294, 334–5 polyphenism 212 Lonomia 331 lorum 55 Lucanidae 52, 52 luciferin 182 Lucilia 340 Lycidae 57 Lygaeidae 340 Lygaeus equestris 184, 184 Lymantria dispar 336, 337

M macrochaetae See bristles; scales Macrodontia cervicornis 350, 350 Macrotermes falciger 318 Maculinea arion 257, 257 maggots 199, 207, 207, 331 Malacosoma americanum 318 Malacostraca 21 male reproductive system 118–19 malillary palpus 55 Malpighian tubules 78, 104, 106–7, 106, 107, 117 mandibles 14, 18, 19, 21, 26, 55, 56, 61, 297 hornlike 52, 52 skeletal muscles 77 mandibular segment 54 Manduca sexta 164 mantises 23, 27, 30, 30, 72, 122, 123, 246, 318 camouflage 155, 246 praying 30, 30, 224, 225, 225 Mantispa styriaca 136 Mantispinae 261, 261 Mantodea 23, 30, 197, 246 Mantophasmatodea 23, 29, 197 marine insects 20 maternal effect genes 188 maxilla 55, 56, 61 maxillary segment 54 mayflies 19, 23, 26, 27, 59 aquatic stage 95, 95, 232, 234, 238 cuticle 67 life span 124, 125 prometaboly 198, 200 mealybugs 116, 179, 190, 281, 281 mechanoreceptors 58, 68, 86–7, 90 mechanosensors 154, 160 Mecoptera 23, 32, 57, 59, 61, 84, 84, 197, 253 median caudal filament 19 Megaloptera 31, 59, 197 Meganeuropsis 126 Megarhyssa macrurus 190, 191 meiosis 174, 175, 186, 188 inverted 175 melanization 102, 102, 213 Meloidae 311 memory 80 mentum 55 meroistic ovaries 185, 185 mesoderm 188 mesothorax 14, 135 metameres 42 metamorphosis 117, 170–3, 196–203 ametabolous 46, 67, 97, 97, 197, 198 antennae 199 competition and 203 critical size 222, 223 cuticle 137 development 203 ecological niches and 203 endocrine regulation 93, 216–21 evolution and 196–7, 196, 203 growth regulators 221

hemimetaboly (incomplete) 47, 54, 62, 67, 97, 197, 198, 199, 203, 220 holometaboly (complete) 13, 27, 42, 48–9, 62, 62, 67, 94, 97, 97, 135, 196, 197, 198–9, 203, 204, 206–7, 220, 222 hypermetamorphosis 202 imaginal discs 48, 62, 172, 199, 203, 222–3 imago 198–9 juvenile stages 172, 203, 204–9, 222 larval stage 106, 198–9, 203, 204, 207–9 legs 199 molting and 171, 172, 196, 198–9, 218–21 neometaboly 201 nymphs 198–201, 203, 204–5, 207 prometaboly 198, 200 pupal stage 106, 198–9, 203 tracheal system 94, 97 wings 136–7, 136, 171, 196, 198–9, 203, 204 metathorax 14, 58, 113, 113, 135, 143 Metaxyphloeus 57 Metoecus paradoxus 260 Metriocnemus knabi 304 micropyle 123, 188 Microstigmus 282, 282 microtrichia 67 midges 114, 236, 304, 340 wingless 140 migration 117, 148–9 milkweed bug 98, 194261, 194, 340 mimicry 33, 45, 45, 68, 108, 108, 130, 133, 154–7, 311, 342–3 aggressive 155, 241–2, 241, 242, 246, 246 Batesian 342 crypsis 14, 45, 116, 154–7, 244, 244 eggs, in 195, 195 endoparasitoids, by 255, 261 larval stage, in 199, 241, 241 Müllerian 342–3 myrmecomorphs 321 natural selection 342, 345, 347 parasites, by 257 mites 96, 114 mitochondria 186, 187 mitosis 174, 186 model experimental organisms 221, 317, 338–41, 344 molting 19, 27, 46, 47, 62, 64–5, 68, 97, 117, 171 digestion of cuticle 62, 64–5 ecdysone 217–21, 222 growth and 196, 222 gut cuticle 105 hemolymph 102 hormonal regulation 217–21, 222 hypermetamorphosis 202 larvae 199 metamorphosis and 171, 172, 196, 198–9 molting fluid 65 neometaboly 201

nymphs 216–17 prometaboly 200 pseudergates 215, 215 wings and 134, 144, 171, 172 monarch butterfly 43, 144, 145, 295, 304, 313, 351, 351 life cycle 173 migration 149–51 monocentric chromosomes 174–5, 174 Mordellidae 311 morphological diversity 13, 22, 171, 196, 199 mosquitoes 15, 33, 43, 55, 61, 184, 236, 238, 239, 250, 250, 304, 324–6, 328–9, 329, 331 antennae 154, 154 carbon dioxide receptors 88 climate change and 350 cuticle 67 insecticide resistance 345 larvae 207–9 saglin, secrete 249 Sterile Insect Technique control 337 moths 18, 23, 33, 55, 58, 59, 85 See also caterpillars; chrysalises; Lepidoptera Ailanthus silkmoth 176 antenna 182 atlas 125 camouflage 155 carpenter 119, 123 cinnabar 226 diamondback 333, 345 flight 148 hawk 164, 292–3, 296, 313 larva 84 metalmark 156 mimicry 156, 156 owl 313 peppered 345, 345 percussive sounds 92 pollinating 305, 312–13 puss 59 silkworm 338–9 sphinx 313 spongy 295, 295, 336, 337 tiger 113, 121 tymbals 92, 93 tympanal organs 91 wings 152, 164 wing sensors 160, 160 yucca 309, 313, 315, 315 mouth 104 mouthparts 14, 18, 42, 50–1, 50, 56, 56, 61, 104, 105, 297 fluid-feeders 298–9, 325 movement 14, 76 mRNA 188 Murgantia histrionica 213, 213 Muscidae 236, 324 muscles 76–9 asynchronous (fibrillar) 76–7, 147, 161 attachment 14, 62, 76, 76, 77, 78 body undulation 77 digestive system 93, 105, 109 flight muscles 76, 77, 98, 98, 100, 144–7, 161, 166

heart and circulatory system 100, 100, 101 nerve impulses 76, 77, 86 sarcomeres 76, 78 skeletal 62, 76–7, 79, 100 synchronous 76–7 visceral 76, 78–9 mutation 38, 141, 143, 314, 327, 329, 332, 345 Mutillidae 331 mutualism 305–7 fungus gardens 280, 281, 289 pollinators 32, 144, 148, 241, 284, 292–3, 305–13 Mymaridae 125, 318 Myriapoda 20, 21, 96, 96, 230 myrmecomorphs 321 Myrmica sabuleti 257, 257 N naiads 27, 233, 234, 241 nanos gene 188 Nasonia vitripennis 179, 179 natural selection 342, 345, 347 Naupheta cinerea 340 nematodes 114 neo-chromosomes 176 neometaboly 201 Neoptera 27 neotenics 210–11 nervous system asynchronous muscles 77 automatic 93 body segmentation and 14 brain/cerebrum 80, 168 central 80–5, 86, 93 chemoreceptors 86, 86, 88, 89, 160 dendrites 87 ganglia 77, 80, 80, 81, 93, 168 hearing and sound production 90–1 interneurons 80, 80, 166 lobula plate tangential cells 167 mechanoreceptors 58, 68, 86–7, 90, 160 motor neurons 76, 80, 80, 86, 147, 166 optomotor response 166–7 peripheral (sensory) 80, 86–93 photoreception See eyes; ocelli secretory neurons 117 sensilla/sensory neurons 56, 67, 68, 86–91, 160–7 steering in flight 166–7 stomatogastric 81, 93 synchronous muscles 76 thoracic ganglion 80, 168 ventral nerve cord 80 visceral 80, 93 wings 137, 160–9 nests silk 117 social species 262, 264–5, 267, 269, 271, 271, 275, 284, 290 neuropil 80 Neuroptera 31, 56, 59, 104, 176, 177, 197, 202, 206, 232 diapause 227 Neuropterida 23, 31, 318

INDEX

363

Nicrophorous 158 Nitidulidae 311 nitrogen fixation 110, 110, 111, 318 Noctuidae 313, 322 nocturnal species 56, 68, 85 nodulation 102, 102 Nomada goodeniana 256, 256 nonwinged insects 14, 19, 26, 198 parasitic species 251 no-see-ums 33, 324 notum 135–6, 136 nuptual gifts 182, 183, 183, 246–7 nurse (trophic) cells 185, 185 nutrition See diet and nutrition Nycteribiidae 324 Nymphalidae 313 nymphs 27, 46, 172, 198, 199, 203, 204–5 aquatic 207, 232–3 diet 204 eyes 83, 83, 84 legs 204 molting hormones 216–17 neometaboly 201 prometaboly 198, 200 soldiers 215 O oak apple 115 ocelli 14, 18, 54, 55, 80, 82, 83, 148 ocellar nerves 61 pigment cells 83, 83 ocular segment 54 Odonata 15, 23, 27, 47, 51, 97, 120, 197, 232–3 chromosomes 174 flight muscles 144 wings and flight 146, 147 olfaction 56, 80, 86, 88, 160, 182, 242, 270 See also pheromones Oligotoma nigra 265 ommatidia 82, 85 Oncopeltus fasciatus 340 Onthophagus 214, 214 Onychocerus albitaris 331 Onychophora 21, 96 oocytes 66, 122, 122, 180–1, 186, 188, 190 oogenesis 185, 193 oogonia 185 oothecae 27, 66, 122, 192 Orthezia urticae 192 Orthoptera 23, 28, 43, 47, 55, 61, 69, 90, 92, 118, 174, 182, 189, 197, 300–1 chromosomes 177 diapause 227 Orthotrichia muscari 261 Orussidae 158 osmeterium 112–13 osmoregulation 105, 106 ostia 101, 101 Ostracoda 21 Ostrinia 323 ovaries 122, 185, 185 ovarioles 122, 122, 185, 185 oviduct 122, 122, 185, 188, 190 oviparity 123, 190

364

THE COMPLETE INSECT

ovipositor 11, 14, 18, 19, 50, 61, 122, 123, 190, 190, 191, 300, 320 appendicular 122 heart 101 parasitoids 155, 158–9, 190–1, 318 venom, delivery by 331 ovisac 192, 192 ovitestis 181 ovoviviparity 123 oxygen and oxygenation 94, 98–9 aquatic species 238, 239 eggs 193 flying insects 148 Oxytrigona 287 P Pachyrhynchus 158 pads 14 Palaeodictyoptera 232, 301 Palaeodictyopterida 53, 61, 126, 298, 300–1 Paleoptera 27, 197 Palpares 176 palpus 55 Pancrustacea 20, 21 panoistic ovaries 185, 185 Panorpidae 32 Papilio machaon 196, 196 Papilionidae 313 Paradoxidae 311 Paraneoptera 27 Paraponera 331 parasitic species 248–57 beetles 32 biological control, use as 333 cleptoparasitism 256, 256, 284 crypsis by 251 ectoparasites 27, 45, 251–3, 257, 257 eggs 61, 251, 254, 255, 318 endoparasites 254 fleas 15, 23, 32, 32, 117, 251–3, 324, 327, 327 flies 33, 162, 324–5 hematophages 88, 248–53, 324–6 host’s immune system 102, 254, 255 larvae 253, 254, 257 Lepidoptera 33 lice 15, 23, 30–1, 31, 43, 252–3, 324, 324, 326 mimicry by 257, 261 obligate endoparasites 211, 211 protection from 62, 102 scale insects 210 specialization 253 superparasites 254 twisted-wing insects 32, 32, 140, 163, 164, 211 vectors 216–17, 249, 324–9 wingless 140, 140 parasitoids 31, 32, 248, 258–61, 302, 318 beetles 260, 260 ectoparasitoids 251, 260 eggs 237, 237, 251, 255, 318 encapsulation 255 endoparasitoids 33, 254, 255

flies 260, 260, 318, 333 host conformers 255 host regulators 255 host’s immune system 102 hyperparasitoids 254 juvenile hormone, secretion by 255 larvae 251, 254, 255, 261, 261 manipulation of hosts by 256 mimicry of host’s tissues 255 wasps See wasps parental care 262–3, 264, 276, 276 parsimony 22 parthenogenesis 148, 172, 180–1, 191, 192, 288 paternal genome elimination 179, 179 pauropods 20 Paussinae 56 pedicel 122 Pediculus 326–7 Pelecotominae 260 Pemphiginae 288 Pemphigus spyrothecae 289 penis 60 Pentatomidae 36–7, 38 Pepsidae 330, 331 percussive sounds 92 perilrophic membrane 104 Periplaneta americana 340 peritrophic matrix 105 phagocytosis 102, 102, 255 pharynx 78, 104, 105 Phasmatodea 23, 29, 43, 141, 175, 197, 244 phenoloxidases 102 pheromones 56, 112–13, 116, 182, 242 alarm 112, 112, 268–9 pest control using 337 recognition signals 268 trail 112, 268, 268 Phlebotominae 324 Phloeodes 158, 158 Phobaeticus serratipes 141 Phoridae 318 Photuris 242, 242 Phthirus 326–7 phylogenetic systematics 22, 347 phylogeny 21–2 phytohormones 114 phytophagy (herbivory) 139, 294–305, 318, 320, 322–3 planthoppers 31, 59, 156 nymphs 205 plant–insect interactions 293–305, 318, 320, 320 plasmatocytes 102, 102 plastron 66, 95, 238 Platycnemis 183 Platygaster vernalis 195 Plecoptera 23, 28, 197, 232, 234 pleuron 58, 135–6, 136, 141 Plutella xylostella 333 Poduromorpha 230 Pogonomyrmex 318 Polistes metricus 136 Polistinae 282

pollinators 32, 144, 148, 241, 284, 284, 292–3, 305–13, 316, 317, 318, 319, 320, 320, 351 bees 305, 305, 309, 312, 320 beetles 311 butterflies 305, 312–13 flies 305, 310–11 moths 305, 312–13 predators of 155 wasps 305, 307, 313, 320, 320 polydnaviruses 115, 255 polyethism 271 Polyneoptera 27, 197 Polypedilum vanderplanki 227, 227 polyphenism 117, 212, 219 polyphenols 63 Pontomyia 20 population declines 317, 349–51 predators 20, 240–8, 318 active hunting 242, 242, 243 aggressive mimicry 155, 241–2, 241, 242, 246, 246 ambush predators 240, 241, 246, 246 assassin bugs 113, 292–3 avoiding 132, 138, 145, 148, 155–7, 195 biological control, use as 333 bioluminescence 242, 242 camouflage 241, 241, 244, 292–3 defenses against See defenses dragonflies 138, 147, 241, 243, 243 exaggerated traits 225, 225 intimidation of 133, 156–7, 156, 157 mimicry of 156–7, 157, 199 predatory larvae 148, 206, 235, 235, 241, 241, 246, 247 trap-jaw ants 280, 281 pre-mentum 55 pretarsus 14 proboscis 55, 56, 61, 246 gustatory sensilla 89 haustellum 33, 61 projapygids 16 prolegs 59, 59, 206 prometaboly 198, 200 pronotum 136, 142, 142 proprioceptors 86, 91, 161 protease 65 prothoracicotropic hormone 117, 218 prothorax 14, 58 Protodonata 126 Protura 16, 17, 23, 186, 197 proventriculus 78, 104, 105 pseudergates 215, 215, 276 Pseudococcus longispinus 179 pseudocopulation 305 Pseudopolycentropodidae 61 pseudotracheae 55 Psocodea 23, 30–1, 31, 43, 61, 197, 253 Psocoptera 69 Psocopterans 117 Psychodidae 236, 324 Pterygota 18, 27, 48, 58, 60, 60

pupae 27, 48, 49, 135, 173, 198–9 See also caterpillars; chrysalises; cocoons aquatic species 236 cuticle 64 diapause 226–7 growth before 222 hypermetamorphosis 202 imaginal discs 222 metamorphosis 106, 199, 203 pupation chamber 56 Pycnogonida 21 pyloric valve 78, 104, 106 Pyralidae 322 Pyriproxyfen 221 Pyrrhocoris apterus 221 Q quinones 64 R Raphidioptera 31, 199 rectum 78, 79, 104, 106–7, 109 rectal gills 106–7, 107, 238 rectal pads 106, 106, 109 Reduviidae 113, 246, 249 Remipedia 20, 21 reproduction 171, 172 abdomen 59 accessory glands 78, 79 arrhenotoky 180–1 asexual 172, 180–1 copulation 60, 78, 79, 120–1, 180, 182, 183–4 courtship 152–4, 168–9, 182–3 eggs See eggs embryogenesis 179, 180, 188–9, 222 eusocial societies 266–7, 276, 281, 282, 347 female reproductive system 122–3 fertilization 180, 182, 185, 186, 188, 190 genitalia 47, 51, 51, 60, 79, 118–23, 183 hermaphrodism 181 inclusive fitness 281, 347 male reproductive system 118–19 mating 152–5, 168–9, 182–3 neotenics 210–11 oogenesis 185–7 parthenogenesis 148, 172, 180–1, 192, 288 pheromones 112, 113 post-copulation 182, 184–5, 184 reproductive appendages 42, 51 sexual 118–23, 172, 180–7 spermatogenesis 179, 185–7 thelytoky 181 visceral muscles 78, 79, 79 resilin 67 respiratory system 14, 62, 94–9 aquatic species 95, 97, 238, 239 bioluminescence 98, 99 flying insects 148 spiracles 94–7 tracheal 94–9, 238 tracheal tufts 98 Reticulitermes 276

rhabdomeres/rhabdom 82, 82, 84, 85 Rhaphidioptera 61, 197 Rhodnius 329, 329 R. prolixus 216–17 Ripiphoridae 260 roaches 23, 27, 30, 30, 260, 262 See also cockroaches rock crawler 23, 29, 29 Rodolia cardinalis 333, 333 rostrum 57, 57, 76 Rutpela maculata 311 S salivary glands 104, 105, 114–15, 117, 298, 299 Salpingidae 57 Samia cynthia 176 sarcomeres 76, 78 Sarcophagidae 190 sawflies 31, 114, 116 See also Hymenoptera scale insects 31, 43, 116, 179, 181, 181, 192, 210, 317, 323 ant farms 281 biological control 333, 333 cottony cushion 181, 333, 333 ensign 192, 192 honeydew 299 iceryine 116 mealybugs 116, 179, 190, 281, 281 neometaboly 201 parasites, as 210 paternal genome elimination 179 scales 62, 68–9, 152 See also bristles coloration 71 photonic crystals 69, 72 Scaptotrigona xanthotricha 286 Scarabaeidae 32, 43, 106, 313, 318 Scarabeoidea 206, 206 scavengers 20, 27 scent marking 112, 245, 245 trail pheromones 112, 268, 268 Schistocerca 212, 212, 294 Schwann cells 87 sclerites 14, 27, 63, 67, 146 sclerotization 64, 65, 67, 68, 69 scolopale 87 scolopidium 90–1 Scolytinae 336, 337 Screwworms 331, 337 seed bugs 340 segmentation genes 188–9, 189 seminal vesicles 119 senses 80 sensilla 56, 67, 68, 86–91 wings 160–7 Sepsidae 59, 152 serosa 66 setae 62, 67, 68, 69, 86, 87, 89 See also hairs; bristles hydrofuge 95, 238 urticating 113 sex determination 172, 176–9, 188 bees 347 endosymbionts 179 social Hymenoptera 281 sexual dimorphism 51, 52, 140, 225 sexual selection 45, 45, 119, 172,

224 shield bugs 36, 260 silk 29, 29, 33, 56, 112, 117 silk button gall 114 silverfish 18, 19, 19, 23, 26, 26, 44, 44, 46, 46, 58, 59, 132, 134, 198, 200 nymphs 204 Simuliidae 248, 324 Simulium 329 Siphonaptera 15, 23, 32, 61, 197, 253 siphons 238, 239 Sisyra fuscata 257 Sisyridae 257 snout 57, 57 sociality 30, 31, 80, 228, 262–91, 321 ants 262, 268, 268, 272–3, 279–81, 347 aphids 286, 288–9 bees 264, 265, 266, 267, 268, 270–1, 284–7, 347, 347 beetles 289, 289 castes 210, 211, 215, 271, 276–9, 282, 286–7, 288 communication 112, 112, 268–73, 279, 347 defense, as 112, 112, 264, 268, 269, 276, 278, 279, 286 ecological significance 262, 262, 290 eusociality 267, 347 evolution 274, 281, 284, 290, 347 exaggerated traits 124, 124, 125 grooming 263, 265, 271 haplodiploidy 281, 347 inclusive fitness 281, 347 king 267 nesting 262, 264–5, 267, 269, 271, 271, 275, 284, 290, 319, 321 nurse bees 266 parental care 262–3, 264, 276, 276 polyethism 271 pseudergates 276 quasisocial societies 267, 284 queen 266, 267, 268, 272–3, 276, 279, 286, 346 reproductive division of labour 266–7, 276, 281, 282, 347 semisocial societies 267, 284 socially parasitic species 281 sociobiology 347 soldiers 113, 215, 224–5, 224, 276–7, 278–9, 287, 288 subsocial societies 264, 265, 265, 284 super-organisms 347 termites 113, 210–11, 215, 224, 225, 262, 267, 268, 274–8, 319, 321 thrips 286 trail pheromones 112, 268, 268 wasps 268, 279, 282–3 wingless 141 workers 266, 267, 268, 270–1, 276, 286–7, 346 sociobiology 347

soil health 318 somata 80 sound production 90, 92–3, 98, 147 courtship songs 168, 182 speciation 305, 314 species 22–3 naming 11 number of 38 preserving specimens 22, 34–5 sperm 79, 118–19, 123, 179, 180, 186–7 See also gametes; reproduction spermatogenesis 179, 186–7 spermatophore 118, 119, 180, 181, 184 spermatophylax 181 spermatozoa 180, 185, 187, 188 Sphingidae 313 spines 53, 53 spiracles 94–7, 98, 117, 238 springtails 14, 16, 17, 23, 184 evolution 230, 231 Staphylinidae 57, 318 stemmata 54, 82, 84 Stenocara 159, 159 Sterile Insect Technique (SIT) 337 Sternorrhyncha 31, 201, 323, 333 stick insects 23, 29, 29, 125, 141, 141, 175, 244–5, 245 camouflage 155, 195, 244 Lord Howe Island 245, 245 nymphs 205 stinging insects 18, 31, 113, 114, 115, 282–3, 318, 331, 342 stink bug 31, 43, 263 stink glands 113 stipes 55 stomodal valve 104 Stratiomyidae 97 Streblidae 162, 324 Strepsiptera 23, 32, 140, 163, 164, 177, 197, 202, 211, 256 stridulation 28, 58, 90, 92, 92, 147, 182, 269 Stripsiptera 163 stylets 55, 61, 61 styli 14, 51 subgenual organs 91, 98 subimago (dun) 27, 200 sub-mentum 55 super-organisms 347 supraesophageal ganglion 80, 81, 93 symbionts 19, 106, 108, 110–11, 114 Austroplatypus incompertus 289 endosymbionts 179 fungus gardens 280, 281, 289 Symphrasinae 261 symphylans 20 Syrphidae 311 systematics 22, 347 T Tabanidae 61, 82, 324 Tachinidae 260, 260, 318 tagma 42, 54, 58 tagmata 42 Tardigrada 21 tarsomeres 18 tarsus 14, 18 temperature regulation 68

INDEX

365

temperature tolerance 103, 103, 126–7, 127, 128–9, 159, 172, 212 developmental plasticity 213 diapause 226–7, 227 tendons 62, 76, 76 Tephritidae 152, 153, 156–7, 157, 337 tergum 58, 141, 145 terminal growth period 222, 223 terminalia 50, 51 termites 23, 27, 30, 30, 43, 106, 141 alarm signals 268 alloparental care 276, 276 castes 210, 211, 215, 276–8 chemical defences 278 diet 274, 276–7, 319 ecological significance 262, 262, 290 evolution 274, 278 exaggerated traits 224 inquiline species 45 life span 125 neotenics 210–11 nests 267, 275, 290, 319, 321 nitrogen fixation 110, 111 queen 276 skeletal muscles 77 sociality 113, 210–11, 215, 224, 225, 262, 267, 268, 274–8, 319, 321 soldiers 113, 215, 224, 225, 276–7, 278 symbionts 110 trail pheromones 268 testes 118–19, 186, 186 Tettigoniidae 181 thecogen 86 thelytoky 181 Theretra oldenlandiae 296 thermoreceptors 86, 88 thorax 14, 15, 42, 42, 50, 51, 58, 189 appendages 58, 142, 142 notum 135–6, 136 nymphs 204 pleuron 58, 135–6, 136, 141 pronotum 136, 142, 142 tergum 58, 141, 145 thoracic ganglion 80, 168 wings See flight; wings thrips 23, 30, 30, 43, 61, 178, 299, 305, 313 eusocial 286 neometaboly 201 Thysanoptera 23, 30, 43, 61, 114, 117, 174, 197, 201 haplodiploidy 178–9 tibia 14 ticks 251, 327 Tipulidae 236 tormogen 86 toxic species aculeate wasps 18, 31, 113, 115, 282–3, 318, 331 mimicry of 33, 342 plants 302–4, 320 sequestration 342, 343 study of 342 termites 278

366

THE COMPLETE INSECT

venoms 79, 112, 113, 115, 331 warning coloration 113, 330, 331, 342 tracheal system 94–9 bioluminescence 98, 99 cuticle, molting 64 evolution 96 flying insects 148 metamorphosis 94, 97 respiration 94–7, 148, 238 specialized modifications 98–9 spiracles 94–7 tracheal tufts 98 wings 137 tracheoles 63, 148 trail pheromones 112, 268, 268 treehoppers 62, 142, 142, 244 horned 175 trehalose 148 Triatoma 324, 329, 329 Triatominae 249 Tribolium castaneum 143, 221, 221, 339 trichobothria 86 trichogen 86, 87 Trichogrammatidae 318 Trichopria columbiana 237, 237 Trichoptera 23, 33, 174, 177, 186, 197, 206, 236 Trilobites 21 Trilobium 158, 338 triungulin 202, 202, 260 trochanter 14 Tubulifera 288–9 twisted-wing insects 23, 32, 32, 140, 163, 164, 202, 211 tymbals 92, 93, 182 tympanal organs 91 tympanum 58, 91, 93, 98, 99 Tyria jacobaeae 226 tyrosine 110 U Ulidiidae 152, 153 ultrabithorax 142–3 uric acid 71, 107, 108, 108–9 Urophora cardui 153 V vas deferens 119 vectors 216–17, 248, 249, 324–9 velvet worms 96 venoms 79, 112, 113, 115 ventral appendages 51 ventral nerve cord 80 ventriculus 104 Vespidae 279, 282, 347 Vespinae 282 Vespula pensylvanica 351 visceral tissues 76, 78–9 vision 80, 81, 82–3, 86, 182 See also eyes; ocelli light reception 88 optic flow 166–7, 167 optomotor response 166–7 vitellarium 122, 122, 185 vitelline envelope 66 viviparity 123, 181, 190

W waggle dance 270 walkingstick 43 warning coloration 68, 73, 113, 158, 159, 330, 342 wasps 23, 30, 31, 138, 185 See also Hymenoptera aculeate (stinging) 18, 31, 113, 115, 282–3, 318, 331 biological control, use as 333 fairy 125 fig 307, 309, 320 haplodiploidy 178–9, 179, 347 horntail 190 jewel 72, 179, 179 metamorphosis 196 paper 136, 282, 283 parasitoid 11, 18, 44, 44, 51, 115, 190, 191, 195, 232, 237, 237, 255, 255, 258–9, 313, 313, 318, 319, 333 pollinating 305, 307, 313, 320, 320 predatory 247, 247 scorpionwasps 255, 255 silk glands 117 sociality 268, 279, 282–3, 347 spider 247, 247 spider hawk 330 wings 164, 165 wood 18, 44, 44, 158 water loss 159 eggs 193 Polypedilum vanderplanki 227, 227 waterproofing 68 wax cuticle 63, 63, 65, 192 glands 112, 116 webspinners 23, 29, 29, 117, 262, 265 weevils 32, 53, 57, 57, 158, 318, 322–3 bean 297 biological control, use as 333 boll 323 burying 158 clown 72 cuticle 67, 110 ganglia 80 larvae 49 leaf-rolling 53 nut 57 proventriculus 105, 105 respiratory system 94 rostrum 57, 57, 76 stridulation 92 symbionts 110 wetas 28, 125 whiteflies 323 Wigglesworth experiment 216–17, 340 winged insects 18, 19, 27, 38, 39, 47 wingless insects 139–41, 145 winglets 53, 132 wings 42, 50, 50, 51, 51, 58, 131 See also flight advantages of flight 138–40 camouflage, crypsis, and mimicry 133, 154–7, 158

chordotonal organ 90–1, 161 coupling 145 courtship displays and signalling 152–4, 168–9 disadvantages 140–1 elytra 32, 92, 92, 126, 142–3, 158–9, 164 evolution and structure 58, 132–43, 230, 230 expansion 137 fossilized 24, 24 imaginal discs 222, 223 losing and re-evolution 138–43 metamorphosis 136–7, 136, 171, 196, 198–9, 203, 204 modifications 142–3 molting and 134, 144, 171, 172 nervous system 137, 160–9 scales 152 sensory hairs 30, 33, 160–1 sensory structures, as 160–5 sexual dimorphism 140 sound frequencies created by 91 steering 166–7 tracheae 137 veins 137, 137 vertebrates 134 weight 77 Wolbachia 179 wound healing 102 Wyeomyia 304, 304 X Xenopsylla 327, 327 Xiphosura 21 Y yellow jackets 282, 351 Z Zoraptera 23, 28, 197, 262, 263, 265 zorspermatheca 122, 122, 180, 181 Zygentoma 18, 23, 26, 44, 44, 46, 46, 51, 58, 59, 69, 197, 198 Zyginidia pullula 179 zygote 188

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Simonsen, Robert S. Bradley, and Philip J. Withers, in “Metamorphosis revealed: time-lapse three-dimensional imaging inside a living chrysalis,” 2013; 204t Holger T.K./AdobeStock; 204bl, 206b Matt Bertone; 204br, 205t, 206t, 206mt, 206mb John & Kendra Abbott/Abbott Nature Photography; 205m Jeff W. Jarrett/ Shutterstock; 205b Matee Nuserm/Shutterstock; 207t Dirk Ercken/Shutterstock; 207m Ch.Photos/Shutterstock; 208–09 7th Son Studio/Shutterstock; 210, 211b Robert Hanus and Simon Hellemans/SpringerNature; 212t MirekKijewski/Getty Images; 213 SDym Photography/Alamy; 214 Neal Cooper/Dreamstime; 216, 300, 301tr, 301b The Natural History Museum/Alamy; 218 Akihito Yokoyama/Alamy; 221tl, 338 Anton Kozyrev/ Shutterstock; 221bl Eric Isselee/Shutterstock; 224t Clarence Holmes Wildlife/Alamy; 224b Papilio/Alamy; 225tl Larry Doherty/Alamy; 225tr GFC Collection/Alamy; 225b Hidekazu Kubo/Minden; 226t Sandra Standbridge/Shutterstock; 226b Galaxiid/Alamy; 227 Roger Eritja/Alamy; 230 Wikimedia Commons/JPWilson; 231b www.amberinclusions.eu; 232 Creative Commons/Donald Hobern, Canberra, Australia; 233 MP cz/Shutterstock; 234t Rostislav Stefanek/Shutterstock; 234b Izanbar/Dreamstime; 235 Slowmotiongli/ Dreamstime; 236 Kanyshev Andrey/Shutterstock; 237 FJAH/Shutterstock; 238 NNehring/ istockphoto; 239t Wikimedia Commons/James Gathany, CDC; 240 BIOSPHOTO/Alamy; 241l Tomas Vacek/Shutterstock; 241r Michal Fuglevic/Dreamstime; 242 Suzanne Tucker/ Shutterstock; 244tl Salparadis/Dreamstime; 245t Matt Cardy/Getty Images; 246l Sleepyhobbit/Dreamstime; 246r YoONSpY/Shutterstock; 247t, 267, 279t, 346 Pavel Krasensky/Shutterstock; 247b Siga Meze/Shutterstock; 248 Musat/istockphoto; 249t Mr. smith Chetanachan/Dreamstime; 249b ggw/Shutterstock; 250 Lamnoi Manas/ Shutterstock; 251b Sahara Frost/Shutterstock; 252 David Forster/Alamy; 256 grandaded/ istockphoto; 257r Margus Vilbas/Alamy; 258 Triwidana/Shutterstock; 259 Eileen Kumpf/ Shutterstock; 260t John Burnham/Alamy; 260b DeRebus/Shutterstock; 261t Creative Commons/Jean and Fred Hort; 261b Brett Hondow/Shutterstock; 262 Sharon Jones/ Dreamstime; 263t jaki good photography/Getty Images; 263b, 329 Vinicius R. Souza/ Shutterstock; 264 Joe Dlugo/Alamy; 265t Mircea Costina/Shutterstock; 265b Creative Commons/Jesse Rorabaugh; 266 Megan Kobe/Shutterstock; 268 Orapin Joonkhajohn/ Shutterstock; 269 Flukycliks/Shutterstock; 271 Daniela_B/Shutterstock; 274 ritfuse/ Shutterstock; 275 Offroad Media Productions/Shutterstock; 276 chaphot/Shutterstock; 277 Witsawat.S/Shutterstock; 279b Dr Morley Read/Shutterstock; 280b Creative Commons/Alex Wild; 281 Creative Commons/Steve Shattuck; 282t Elliotte Rusty Harold/ Shutterstock; 282b George Grall/Alamy; 283t TTstudio/Shutterstock; 284 Libor Fousek/ Shutterstock; 285 Ed Phillips/Shutterstock; 286t Krasowit/Shutterstock; 286b Wagner Campelo/Shutterstock; 288l Creative Commons/AfroBrazilian; 288r Kazakov Maksim/ Shutterstock; 291t Anton Sorokin/Alamy; 291b Kersti Lindstrom/Shutterstock; 292–93 Bidouze Stephane/Dreamstime; 294 Holger Kirk/Shutterstock; 295 Martynova Anna/ Shutterstock; 296b Alexander Denisenko/Shutterstock; 297 Nicola Dal Zotto/Shutterstock; 298t Andrei Shupilo/Dreamstime; 298b Mrehssani/Dreamstime; 299 guentermanaus/ Shutterstock; 301tl PB/YB/Alamy; 302 AjayTvm/Shutterstock; 303t JTKP/Shutterstock; 303b Fire-n/Shutterstock; 304 Creative Commons/Katja Schulz; 305 kckate16/ Shutterstock; 306 Gherzak/Shutterstock; 307 Creative Commons/Alandmanson; 308 Sabena Jane Blackbird/Alamy; 309 Corbin17/Alamy; 310 Drew Rawcliffe/Shutterstock; 311 Ian Redding/Dreamstime; 312 SashaMagic/Shutterstock; 313t Annette Shaff/Shutterstock; 318 Aubrey1/Dreamstime; 319t Stephen Bonk/Shutterstock; 319b Creative Commons/ Charles F. Badland; 320t Bagus upc/Shutterstock; 321t JohnCarnemolla/istockphoto; 321b Westend61 GmbH/Alamy; 322 Oleg Kachura/Dreamstime; 323t natthawut ngoensanthia/ Shutterstock; 323b moxumbic/Shutterstock; 325 Everett Collection/Shutterstock; 326b Creative Commons/CDC/ Frank Collins/Centers for Disease Control and Prevention’s Public Health Image Library; 327 Creative Commons/Katja ZSM; 328t IRD / VECTOPOLE SUD / PATRICK LANDMANN / SCIENCE PHOTO LIBRARY; 328b Salvador Aznar/Shutterstock; 330 Robert Briggs/Shutterstock; 332 Walter Arce/Dreamstime; 337 Ray Wilson/Alamy; 339t jxfzsy/istockphoto; 339m sruilk/Shutterstock; 339b komkrit Preechachanwate/ Shutterstock; 340 Gale Verhague/Dreamstime; 341 PeopleImages.com/Yuri A/ Shutterstock; 343t Survivalphotos/Alamy; 344 Creative Commons/Karl Magnacca; 345 Prisma by Dukas Presseagentur GmbH/Alamy; 347 Diyana Dimitrova/Shutterstock; 348t nicemyphoto/Shutterstock; 348b Faiz Dila/Shutterstock; 349 kakteen/Shutterstock; 350 Gil Wizen/naturepl.com; 351 JHVEPhoto/Shutterstock; 352–53 Robby Fakhriannur/ Shutterstock.

PICTURE CREDITS

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Acknowledgments Dr. David A. Grimaldi is grateful to Hollister Herhold for use of the CT scan images on pp. 46–49 and 95–99, to Aga Pierwola for photographs on pp. 22 and 343, and to Aga and Suzanne Green for assistance that freed David to work on the book. Gratitude is also extended to the editors of the series, in particular Abbie Sharman, Anna Southgate for skilled editing and great patience, and Jenny Quiggin for research on such striking images. David Grimaldi is also extremely grateful to Steve Davis, Michael S. Engel, Jessica Fox, and Isabelle Vea for contributing their expertise in writing and other content of the book. Dr. Steven R. Davis would like to express his gratitude to David Grimaldi for providing the opportunity to include his contribution and for influencing a general passion for comparative biology and all things entomology related. He would like to thank the American Museum of Natural History for supporting his stay and continued affiliation and its Microscopy and Imaging Facility (staff members Morgan Chase and Andrew Smith). Steven is also grateful to his previous doctoral advisor, Michael Engel, who permitted and encouraged unbounded entomological pursuits. Great appreciation is given to several researchers who contributed figures or assisted with them, namely Hollister Herhold, Igor Siwanowicz, Chun Han, and Kristin Dunn. Steven would also like to commend the publishing, project, editorial, design, art, and illustration staff of Quarto Publishing for their tireless efforts in the production of this work. Dr. Jessica Fox would like to acknowledge helpful discussions with Mark Willis and Cole Gilbert. Dr. Isabelle M. Vea would like to acknowledge Jacqui Sayers, Abbie Sharman, and Anna Southgate, the designer Tony Seddon and the illustrator Rob Brandt for their work on the chapter. Dr. Michael S. Engel is grateful to Dave Grimaldi for the invitation to provide a brief overview of insect natural history. Dave has always been that rising tide in entomology that lifts everyone simultaneously. Michael is further thankful to Anna Southgate and the entire production team for their tremendous assistance as the work came into final form.

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THE COMPLETE INSECT