N. B. Davies wrote, “we expect natural selection to increase the efficiency with which predators detect and capture prey. On the other hand, we would also expect selection to improve the prey's ability to avoid detection and to escape. The complex adaptations and counter-adaptations we see between predators and their prey are testament to their long coexistence and reflect the result of an arms race over evolutionary time.”
These adaptations and counteradaptations are multitudinous, diverse, and sometimes so extraordinary that they defy belief. A spider lures certain male moths to their deaths by counterfeiting the chemical sex attractant of females of the victims' species. A burrowing owl uses bait to attract the beetles that it eats. A praying mantis attracts its prey, nectar-seeking bees and flies, by masquerading as a large, colorful flower. Some ant lions and a few other insects dig pitfall traps and quietly wait, unseen, in the bottom to devour careless insects that stumble into the pit.
A few caterpillars hoodwink insectivorous birds by posing as repulsive bird droppings. Certain nerve fibers of some fleet-footed insects are greatly enlarged to hasten the arrival at the central nervous system of nerve impulses, generated by the perception of a warning signal, that will trigger flight. Most moths are active only at night, and during the day many hide in plain sight, not making the slightest movement as they cling to the bark of a tree or some other surface that matches their remarkably deceptive camouflage. Equally astonishing are the strategies of some predators. For instance, as Malcolm Edmunds wrote, the Old World birds known as bee-eaters “defang” a stinging insect by rubbing its abdomen against its perch to “squeeze the venom out of its sting gland.”
The ten chapters that follow elucidate the strategies and counter-strategies in the everlasting evolutionary arms race between predators and prey.
NOTE TO READER
As you read this book, you will come across quotations from or references to the work of scientists and other writers. It is only fair that you have the opportunity to read and evaluate these publications on your own. In Selected References at the back of the book, you will find bibliographic citations, listed by author, that will lead you to these published sources for each chapter. People who provided me with unpublished information are identified in the text but are not listed in Selected References.
ACKNOWLEDGMENTS
I am greatly indebted to the many friends and colleagues whose support and expertise improved this book, making it much better than it would have been without their help. Outstanding among them are my wife, Phyllis Cooper Waldbauer, and my dear friend Nancy Clemente. Phyllis read several drafts of the manuscript and made many constructive suggestions. Nancy, who masterfully edited the five of my books published by Harvard University Press and who is now retired, volunteered to edit this manuscript pro bono. Many others freely shared their scientific expertise: May Berenbaum, the late John Bouseman, Lincoln Brower, Sydney Cameron, Larry Hanks, Steven Malcolm, James Nardi, David Seigler, and James Sternburg. With great patience, Karen Trame and Elizabeth Berry typed the manuscript. My agent, Edward Knappman of New England Publishing Associates, was, as always, very helpful.
ONE
Insects in the Web of Life
Insects constitute by far the largest amount of animal food available to flesh eaters both on dry land and in freshwater. The one quarter of the earth that is not covered by the oceans and seas is inhabited by an immense and not yet completely censused population of insects. The 900,000 currently known insect species (at least three million are yet to be discovered and named, according to reasonable estimates [Stephen Marshall]) constitute about 75 percent of the currently known 1,200,000 animal species on land, in freshwater, and in the oceans. The Canadian entomologist Brian Hocking made the daring but educated guess that the world population of insects is about one quintillion (1 followed by eighteen zeroes) individuals. Even if he overestimated by trillions, that would still be a stupendous population.
Although insects are small, they are generally so numerous in most terrestrial and freshwater ecosystems that, on a per-acre basis, they not only outnumber but also outweigh all the other animals—including deer and moose—combined. On the face of it, this is hard to believe. But keep in mind that a single acre of land may be home to many millions of insects of hundreds or even thousands of species. By contrast, the home territory of one small bird is likely to encompass as much as an acre, and that of a large mammal, such as a thousand-pound moose, several hundred or even thousand acres. Thus the biomass of an animal that weighs hundreds of pounds may be much less than one pound per acre. Also keep in mind that most people notice only a few of the many insects around them, perhaps a ladybird beetle or a large and beautiful butterfly, but more often the insects that sting, bite, or otherwise annoy them. Yet the other insects, by far the vast majority in almost any ecosystem, go unnoticed. Not only are they small, but many are difficult to see because they are camouflaged, and many are out of sight because they live in the roots, stems, or other parts of plants; as parasites within the bodies of insects and many other animals; or in the soil or other cracks and crevices of the environment.
Figure 1. A thousand-pound moose (representing all mammals, birds, and other vertebrates in its ecosystem) and a tiny beetle (representing all insects in the same ecosystem) drawn to a scale representing their biomass as pounds per acre.
Insects are, either directly or indirectly, the most plentiful source of flesh for animals that don't eat plants. But they are important to these predators not just because of their abundance. Plant-feeding insects, estimated to be about 450,000 species, and the insects and other animals that eat them are by far the most important link between green plants and animals that don't eat plants, a conduit through which predators receive the energy of the sun, which green plants—and only green plants—can capture and make available to animals via photosynthesis, in the form of sugars. Insect-eating insects play another significant, although less important, role. By eating tiny organisms and incorporating their prey's nutrients in their own bodies, large insects become “nutrient packages” for large insectivores that cannot profitably pursue and eat tiny organisms themselves.
Data gathered by Eugene Odum and other ecologists show just how important a part of the food chain insects are in specific ecosystems. For example, in a field of herbaceous plants in North Carolina, the biomass of the plant-feeding insects alone—not including any predaceous, parasitic, or scavenging species—was nine times greater than that of sparrows and mice, the larger and more conspicuous and by far the most numerous of the vertebrates in that field. On an East African plain, just two species of ants—only those two, among hundreds of other kinds of insects—were about equal in weight, per acre, to the combined weight of the large grazing animals, such as wildebeests, zebras, and antelopes. In these two habitats and in almost all others, insects are by far the most abundant of the prey animals in both numbers and biomass. As is to be expected, and as we will see in the next chapter, hundreds of thousands of different kinds of animals exploit this nutritious, protein-rich food: spiders, scorpions, insects, frogs, toads, lizards, birds, mammals.
The insects almost certainly have more different lifestyles, ways of surviving and “making a living,” than do any other group of animals. One species or another occupies every—or nearly every—ecological niche. An ecological niche is not just a place; it includes all of the resources, food, nesting sites, hiding places, and so on, required by an organism. Except for aquatic species, insects that undergo gradual metamorphosis occupy essentially the same niche throughout their lives. Those with complete metamorphosis often occupy two very different niches in their larval and adult stages.
Dragonflies, grasshoppers, cockroaches, mantises, true bugs (order Hemiptera), and lice are some insects that gradually metamorphose. A newly hatched
