Scott Richard Shaw is professor of entomology and Insect Museum curator at the University of Wyoming, Laramie.
The University of Chicago Press, Chicago 60637
The University of Chicago Press, Ltd., London
© 2014 by The University of Chicago
All rights reserved. Published 2014.
Printed in the United States of America
23 22 21 20 19 18 17 16 15 14 1 2 3 4 5
ISBN-13: 978-0-226-16361-1 (cloth)
ISBN-13: 978-0-226-16375-8 (e-book)
DOI: 10.7208/chicago/9780226163758.001.0001
Library of Congress Cataloging-in-Publication Data
Shaw, Scott R. (Scott Richard), author.
Planet of the bugs : evolution and the rise of insects / Scott Richard Shaw.
pages cm
Includes bibliographical references and index.
ISBN 978-0-226-16361-1 (cloth : alk. paper) — ISBN 978-0-226-16375-8 (e-book)
1. Insects—Evolution. I. Title.
QL468.7.S53 2014
595.7—dc23
2013050775
This paper meets the requirements of ANSI/NISO Z39.48–1992 (Permanence of Paper).
PLANET OF THE BUGS
EVOLUTION AND THE RISE OF INSECTS
Scott Richard Shaw
THE UNIVERSITY OF CHICAGO PRESS
CHICAGO AND LONDON
This book is dedicated to my wife, Marilyn, for her patience, support, and most especially understanding. Thanks for letting me keep bugs in the freezer. I couldn’t have done it without you.
Contents
Prologue. Time Travel with Insects
1. THE BUGGY PLANET
2. RISE OF THE ARTHROPODS
The Cambrian period, 541–485 million years ago, and the Ordovician period, 485–444 million years ago
3. SILURIAN LANDFALL
The Silurian period, 444–419 million years ago
4. SIX FEET UNDER THE MOSS
The Devonian period, 419–359 million years ago
5. DANCING ON AIR
The Carboniferous period, 359–299 million years ago
6. PALEOZOIC HOLOCAUST
The Permian period, 299–252 million years ago
7. TRIASSIC SPRING
The Triassic period, 252–201 million years ago
8. PICNICKING IN JURASSIC PARK
The Jurassic period, 201–145 million years ago
9. CRETACEOUS BLOOM AND DOOM
The Cretaceous period, 145–66 million years ago
10. CENOZOIC REFLECTIONS
The Cenozoic era, 66 million years ago to the present day
Postscript. The Buggy Universe Hypothesis
Acknowledgments
About the Author
Notes
Suggested Reading
Index
Color gallery follows Chapter 6.
A long-horned beetle (family Cerambycidae) perches on a melastome leaf after a recent rain at San Ramon forest in Costa Rica.
Prologue: Time Travel with Insects
Time flies like an arrow. Fruit flies like bananas.
GROUCHO MARX
Late one October afternoon I was walking along a trail through the rain forest at San Ramon Biological Reserve in Costa Rica, pondering the nature of time and wishing for a time machine. The earth’s tropical wet forests do not display obvious seasonality so you would never guess the day, month, or year by looking at the surrounding mossy vegetation, and they emanate such an ancient and timeless green aura that it’s easy to imagine traveling back in time, thousands, hundreds of thousands, or even millions of years. The scent of decaying vegetation and fungus permeated the wet air, and the forest was both figuratively and literally crawling with insects. Around my feet, abundant small flies and other insects enjoyed the bounty of rotting fruits fallen to the forest floor. I recalled Groucho’s classic punch line in Duck Soup: “Fruit flies like bananas.”
As the songs of frogs, katydids, crickets, and cicadas emanated from the forest, my boots sloshed along the pathway. Typical of San Ramon, it had been raining all day, the trail oozed treacherously slick with slippery mud, and water was everywhere. On mushroom caps sprouting from a rotting log by the trail, silvery droplets rolled to the edge, clung briefly shimmering—then fell away. The sounds of water were all around, bubbling and gurgling over mossy rocks in the river, chattering in nameless streams and rivulets. A light mist was still falling, and the emerald vegetation, dappled in a hundred shades of green, was dripping and glistening with raindrops. The trees at San Ramon were matted with mosses, lichens, and ferns that had absorbed water, sponge-like, later releasing it gradually long after the rain had stopped. Up in the forest canopy, bromeliads had collected rainwater in their concave leaf bases, forming miniature ponds for numerous tree frogs, salamanders, and hundreds of kinds of aquatic insects. Water, everywhere water, was dripping.
Although I took my time walking through the slick mud, placing each step with care, maybe I should have been daydreaming less. The San Ramon forest contains thousands of insect species, many of them still new and undiscovered, but unfortunately, it also contains lots of venomous snakes and some deadly ones as well—and like Indiana Jones, I hate snakes. Even so, I was enjoying myself immensely, finding fascinating plants and insects all around. Then unexpectedly, just past a bend, down a gentle slope near a small gurgling stream, I came across a small melastome tree. There, on a large leaf about four feet off the ground, I found my time machine. It was about three inches long and deep mahogany brown, with long curved antennae adorned with dewdrops from the recent rain. Standing motionless on the leaf, it rested securely, supported by its six multijointed legs that formed two perfect tripods. My time machine was a long-horned beetle.
As the rain began to fall more heavily at San Ramon, new beads of water started dripping from the beetle. Its tough armored exterior would not allow them to penetrate. The beetle’s body plan is extremely different from our own, with its rigid skeleton on the outside rather than the inside, and this external skeleton, as well as the beetle’s multijointed legs, send us a message from the shallow oceans of the Cambrian period, roughly 541 million years ago. Atmospheric oxygen levels increased at that time, animal metabolism accelerated, and marine predators became faster and nastier. In response, the common ancestor of all living arthropods (insects and their nearest relatives) evolved external skeletons, which provide protection from the environment, defense against predators, and sites of muscle attachment that increase mobility. They evolved jointed legs as well, which are also exceptionally useful for defending against predators, as well as for food-gathering, mating, and dispersal. Both features have been quite fashionable ever since.
The beetle remained motionless on the melastome leaf, but on close inspection I noticed that its abdominal body segments were contracting then expanding slightly. The beetle was breathing. Along the sides of its body, minute holes (spiracles) allowed molecules of air to rush in. If we could shrink ourselves down to the size of a few microns, we might follow those oxygen molecules on their travels through the beetle’s spiracles, through a series of large tubes (tracheae), and finally through a series of smaller tubes (tracheoles) that branch out in all directions, and get smaller and smaller until they approach all the living cells in the insect’s body. The fact that the beetle breathes air tells us that in the Silurian period, roughly 444 million years ago, ancestral arthropods first colonized the land and developed air-breathing tracheal respiration systems that have been passed onwards to all modern insects, even those that live in fresh water and breathe through gills. Their innovativ
e breathing apparatus allowed Silurian arthropods to exit the oceans to avoid marine predators, forage for food along the shorelines, and use those unoccupied beaches as a safe place to mate and lay eggs. It also prepositioned the arthropods to be the first animal group to successfully diversify among the earliest Lilliputian plant communities, which developed during the Late Silurian.
I examined the beetle’s form more closely. Its body was divided into three functional regions: a head up front, from which its hornlike antennae swept to the sides, and small multifaceted eyes gazed back at me; a central thorax, to which its legs were attached; and lastly, a multisegmented abdomen. These are the features that define an insect. They evolved sometime in the Late Silurian or Early Devonian, about 419 million years ago. During this time, plant communities became taller and more diverse, the first insects evolved among the mossy soils at the bases of the first archaic trees, and six-leggedness established itself as a versatile and stable means of walking, standing, and running on this planet, allowing the insects to become highly successful scavengers. By the Late Devonian period, 360 million years ago, springtails, jumping bristletails, and silverfish were abundant among the accumulating litter of decaying plant materials. The feeding activities of these scavengers created and conditioned organic soils, which enabled the earth’s forests to evolve and expand into the interior from the shorelines. With tiny but triumphant steps, microscopic insects marched inland as the plant communities extended their colonization of the continents.
Along the backside of the beetle ran a hairline seam, which indicated the presence of wings: a message from the common ancestor of all flying insects, which first evolved in the Carboniferous period, about 354 million years ago. When the insects invented wings there were no other flying animals—no birds, bats, pterodactyls, or gliding squirrels—and they completely mastered the air for more than 150 million years before any other organisms evolved the ability to fly or could chase them in the air. The advantage of flight was certainly one of the foremost factors contributing to the explosive proliferation of insect species on this planet. But wings not only allowed insects to disperse and colonize distant new habitats, they also played important roles in courtship and mating, predator avoidance, food acquisition, and thermoregulation. By the late Carboniferous Period, tall forests had spread across the Earth’s continents. They were a glittering fairyland of curious flying insects: banded, spotted, and net-winged paleodicytoperans; dragonfly-like griffenflies and protodonatans; ancestral mayflies; and even sundry forms of flying roaches that fluttered and glided among the treetops.
Preparing to fly, the beetle extended its rigid, shell-like front wings outward and unfolded its membranous hind wings. This style of wings is, in fact, the beetles’ key innovation, evolved some 260 million years ago in the Permian period. Only the hind wings power beetle flight; the modified armored front wings allow the delicate hind wings to be put away, hidden and protected, when not in use. These hard front wings, unique to beetles, also protect the beetles when they’re not flying and give them a more streamlined body profile, which enables them to crawl into cracks and crevices, under loose bark, in leaf litter, and among woody debris. While the beetles seem to fly clumsily with the unfolded hind wings, they are efficient enough to allow the beetles to disperse widely in search of mates and food, and to colonize new habitats.
Some of those ancient Permian beetle ancestors also evolved a very useful feeding behavior, retained even today by the modern long-horned wood-boring beetle. Their immature forms, larvae, developed the habit of boring deep into the woody trunks of dead trees to feed there on the fungal growth and decaying plant tissues. This sheltered them from increasingly adept warm-blooded predators throughout the Permian, and on through the age of the dinosaurs, the Mesozoic era from the Triassic through the Cretaceous periods. It may have also served to buffer them from environmental change. At the end of the Permian, about 252 million years ago, the ancient continents collided to form the supercontinent of Pangaea. Coastlines and marine habitats were severely disrupted, perhaps triggering a mass extinction of species greater than any other extinction event so far. Terrestrial habitats became hotter and drier than before, but this only seemed to stimulate the beetles’ success. While there were only 5 families of primitive beetle-like insects in the Late Permian, by the Late Triassic (around 220 million years ago) at least 20 families consisting of 250 species of true beetles had evolved. Over the course of the following Jurassic period, despite living among hungry dinosaurs, beetle numbers continued to skyrocket; at least 600 species in 35 families have been identified in middle-Mesozoic fossils.
Back along the trail at San Ramon, the beetle flexed its wings again and took a short, buzzing flight to a nearby yellow flower. After a few moments, it slowly began to chew, lazily indulging in a high-protein pollen meal. The sight of insects feeding on flowers is so commonplace in our modern world that we tend to forget how unusual it really is, geologically speaking. The ancient Permian proto-beetles didn’t visit flowers because they didn’t exist yet, at least not in the sense that we understand them. The flowering plants that currently dominate the landscape, known by botanists as angiosperms, did not evolve until the Early Cretaceous period, around 126 million years ago. Perhaps sometime during that period an ancient beetle first visited a flower, maybe in the shadow of a Tyrannosaurus or Triceratops dinosaur, and found the pollen tasty. Over the Late Cretaceous, this time in tandem with the flowering plants’ early diversification, the richness of beetle species skyrocketed again, particularly among many of the plant-feeding beetle groups that survived the period and now dominate our modern world: leaf beetles, weevils, scarabs, click beetles, metallic wood-boring beetles, and, notably, the long-horned beetles, the family into which our time-machine beetle is classified.
Maybe, 66 million years ago, some Cretaceous beetles were busily feeding on pollen from ancient magnolia blossoms when a sound from above caused a nearby Tyrannosaurus to glance briefly skyward and see a massive asteroid hurtling toward the earth—a catastrophe which brought the time of the giant dinosaurs to a close and marked the end of the Cretaceous. Global winter ruled for a time, killing off not only dinosaurs but also perhaps many kinds of small marine organisms. But deep in rotting tree trunks and elsewhere, the larvae of many beetles survived, completed their metamorphoses, and emerged into a brave new world without giant dinosaurs. Over ensuing millions of years, some of those survivors lived on, evolved, and diversified to become the most species-rich animal group in the world today.
Our beetle flew into the variegated green of the primeval forest, perhaps to seek others of its kind, and in doing so, to replicate its many messages from the past. Its gentle buzzing was lost among the sounds of dripping water and the increasing rainfall. This beetle is gone now, but many others remain. We can only estimate their numbers. Studies by Smithsonian entomologist Terry Erwin indicate that there are millions, perhaps tens of millions, of different kinds of beetles in our tropical forests, most of them still unnamed. And that’s just one major insect order. Many other kinds of insects are hyperdiverse, such as the moths, butterflies, true flies, wasps, and true bugs. This hyperdiversity is the rich historical legacy of the Cenozoic era. Over the past 66 million years, as the insects have continued to diversify along with the flowering plants, our tropical rain forests have evolved into the most biologically complex and diverse ecosystems ever to arise. Why are there so many different kinds of insects, and why do they dominate terrestrial ecosystems? Science has unlocked an extraordinary number of mysteries, and the story of the insects’ rise can now be read over hundreds of millions of years of earth’s history. The messages are written there in the rocks, the forests, and the insects, for those who choose to read them.
1
The Buggy Planet
It is for me a stunning fact that while the physical surface of the earth has been thoroughly explored, so that virtually every hilltop, tributary, and submarine mount has been mapped and named, the livin
g world remains largely unknown. As few as ten percent of the species of insects and other invertebrate animals have been discovered and given scientific names.
EDWARD O. WILSON, The High Frontier
All things have a root and a top,
All events an end and a beginning;
Whoever understands correctly
What comes first and what follows
Draws nearer to Tao
BARRY HUGHART, Bridge of Birds
Earth is a very buggy planet. Nearly one million distinct living species, different kinds of insects, have been discovered and named so far. From A to Z, they overwhelm us with their diversity: ants, birdwing butterflies, cockroaches, dung beetles, earwigs, flies, grasshoppers, head lice, inchworms, June beetles, katydids, ladybugs, mantises, net-winged midges, owlflies, periodical cicadas, queen termites, royal palm bugs, sawflies, thrips, underwing moths, velvety shore bugs, webspinners, xyelid sawflies, ypsistocerine wasps, and zorapterans. But that is just the tip of the iceberg, the door to the hive. Most of the insect species haven’t even been given a name, and scientists estimate that the number of different kinds of insects living in tropical forests is perhaps in the tens of millions.1 Whether you adore them or abhor them, their diversity and ecological success is impressive.
Insects are so successful that it’s not much of an exaggeration to say that they literally rule the planet. Our egos allow us to think that we humans rule earth, with our cities, our technology, and our civilizations, but we seem to be doing more to destroy the planet than to improve it, and we are like one superabundant pest species run amok over the globe. If humans were to suddenly become extinct, the living conditions for most species would be greatly improved with only a few exceptions, such as human body lice and crab lice. On the other hand, if all the insects became extinct, in the words of Edward O. Wilson, the famous Harvard entomologist, “the terrestrial environment would collapse into chaos.”2 Human civilizations have only recently developed over the last several thousand years. Insects have successfully coevolved with terrestrial ecosystems over the last four hundred million years. They are ecologically essential as scavengers, nutrient recyclers, and soil producers, feeding on and utilizing virtually every kind of imaginable organic material. Six-legged detritivores consume dead plants, dead animals, and animal droppings, greatly increasing the rates at which these materials biodegrade. Insects, as both predators and parasitoids, are keystone organisms that feed upon and reduce populations of other kinds of plant-feeding and scavenging insects. They are also their own worst enemies: most kinds of insects have populations that are kept in check by the feeding activities of other insects.
Planet of the Bugs: Evolution and the Rise of Insects Page 1
