very much like its parents but lacks wings. As it grows, it molts several times, and its developing wings, which are external, can be seen gradually increasing in size until the hopper stops growing and molts for the last time to become an adult with flightworthy wings. Insects with gradual metamorphosis have only three life stages: the egg; the nymph, the growing stage; and the adult, the egg-laying reproductive stage. Nymphs look and behave much like adults, except in most aquatic species. For instance, adult dragonflies are aerial acrobats that pursue flying insects. But the nymphs are aquatic, don't look at all like the adults, and are fierce predators that eat aquatic insects and even small fish.
Beetles, fleas, flies, wasps, bees, moths, and butterflies are among the many insects that undergo a complete metamorphosis. The baby butterfly just hatched from the egg is a wormlike larva that does not at all resemble its parents. A biologist from another planet might think that the larva and the butterfly are two quite different kinds of animals, as dissimilar as birds and snakes. Complete metamorphosis proceeds in four life stages: the egg; the wingless larva, called a caterpillar in the case of butterflies and moths; the pupa, the transition stage in which the larva metamorphoses into the adult; and the winged reproductive stage. Larvae not only look different than their parents but also usually behave very differently. Caterpillars, for example, have chewing mouthparts, and most feed on plants, usually the leaves. The wingless pupae, with only a few exceptions, can squirm but cannot walk or crawl and are usually tucked away in a safe place, perhaps in the soil, under bark, or in a silken cocoon. The adult, a butterfly or moth for example, has large wings that developed internally in the pupa, as do its long, soda straw-like siphoning mouthparts, used for sucking nectar from blossoms.
The larvae and the adults are specialists, anatomically and behaviorally equipped to do particular tasks. The larvae eat, grow, and do their best to foil predators. The adults suck the sugary nectar that supplies energy to fuel their frequent flights as they seek mates and as the females distribute their eggs. Most butterflies and moths, as well as many other insects, glue their eggs only to one of the few plant species their fussy—host plant-specific—offspring will be willing to eat. The larvae are “eating machines,” and the adults are flying gonads or, as Carroll Williams said, “flying machines devoted to sex.” Because of the benefits of such specialization, complete metamorphosis has been an evolutionarily more successful strategy for survival than gradual metamorphosis, at least as judged by the number of known insect species on earth today—only about 135,000 (15 percent) with gradual metamorphosis, while 765,000 (85 percent) have complete metamorphosis.
Insects with either type of metamorphosis can occupy similar ecological niches. Grasshoppers, Japanese beetles, June beetles, and many other insects occupy fairly commonplace niches. The adults feed on foliage and lay their eggs in the soil. However, after hatching from the egg, nymphal grasshoppers immediately make their way to the surface and feed on the leaves of grasses or herbaceous plants, while the white C-shaped larvae, known as grubs, of the two beetle types, which both have complete metamorphosis, remain in the soil and feed on plant roots. They pupate in the soil, and the adults dig their way up to the surface after they have shed the pupal skin.
Other insects have more complex and elaborate lifestyles. For example, a male burying beetle whose search for a dead animal has been successful sits on the body of the small dead animal he has discovered and releases a scent, a sex-attractant pheromone. A female soon joins him, and, working together, they burrow back and forth beneath the dead body until it sinks deep enough into the ground so that they can cover it with soil. Then they create an open space underground around the buried carcass and cover the dead animal with a secretion that kills bacteria, thereby delaying decomposition. The female then lays as many as thirty eggs in the soil near the carrion. After the larvae hatch, they crawl to a nest prepared by their parents, who feed them by regurgitating predigested carrion. Eventually the larvae feed on their own. The father then leaves, but the mother guards her young until they are ready to pupate. In a few weeks her offspring emerge from the soil as adults and begin another cycle.
Elsewhere, a tiny female gall wasp inserts an egg into an oak leaf. As May Berenbaum wrote in Bugs in the System, gall makers commandeer “the plant's hormonal system in such a way that the plant is induced to produce bizarre and unusual growths [galls], which provide the insect with a place to live and with nice nutritious tissue on which to feed.” The larva that hatches from her egg causes the oak leaf to form an abnormal growth, in this case an “oak apple,” a light tan spherical gall that may be as large as a table tennis ball. Another gall maker, a moth, lays an egg in the stem of a growing goldenrod in spring, causing the plant to produce an egg-shaped thickening almost an inch in diameter. In summer the caterpillar grows to full size. The following spring it gnaws an exit hole through which, after it pupates, it will emerge as a moth. But it may not survive that long. In winter a hungry downy woodpecker may peck a hole in the gall, pull out the caterpillar, and make a meal of it.
These few examples give no more than an inkling of the many different ways in which insects conduct their lives. Insectivores must, of course, have the anatomical and behavioral adaptations required to catch their prey. A bird, for example, can snatch an adult grasshopper, beetle, gall wasp, or gallfly from the air with its beak, but only a tunneling animal or a bird that probes in the soil is likely to find subterranean eggs, grubs, or pupae. Only a woodpecker is likely to get at a larva in a gall or burrowing under the bark of a tree.
The evolution of the millions of different kinds of insects that live on earth now and the many extinct species that we know only as ancient fossils began about four hundred million years ago, when the first insects-to-be were gradually leaving the water, where life began, to move onto the land. They probably reached the shore via moist organic debris at the edges of freshwater ponds and once on land probably continued to feed on soft rotting organic matter, which they ate with their primitive, unspecialized mouthparts, the organs of ingestion. From these simple creatures evolved the diverse assortment of modern insects, as different from one another as grasshoppers with mouthparts specialized for chewing on plants, butterflies with tubelike mouthparts for sucking nectar from flowers, and mosquitoes with piercing-sucking mouthparts for consuming the blood of birds, mammals, or reptiles.
Plants and animals, of course, continue to evolve. But how does evolution work? Charles Darwin had the brilliant insight that natural selection is the driving force of evolution, producing new species just as breeders produce new dog breeds through artificial selection, by selecting animals with desirable traits to be the parents of the next generation. (Keep in mind that all breeds, from the tiny Chihuahuas to the huge Saint Bernards, are descended from the wolf.) Natural selection, while tending to cull poorly adapted individuals, favors those better adapted to avoid hazards and to take advantage of opportunities. For example, an individual with even slightly better camouflage than others will be somewhat less likely to be noticed by a predator and, consequently, somewhat more likely to survive and become a parent. Heritable adaptive traits are passed on to future generations and given enough time will spread to all members of a population. As the centuries or millennia pass, more favorable mutations accumulate in a population until those who have them are so different from the other members of their species that they become a separate, distinct, reproductively isolated species, one whose members do not breed with members of other species.
These new adaptive traits constantly arise as genetic mutations caused by means such as radioactivity, ultraviolet light, cosmic rays, or intrinsic factors in DNA, the genetic material itself. Mutations are random, some favorable and many unfavorable. However, evolution is by no means a random process; it is directed by natural selection, which tends to eliminate unfavorable mutations and generally perpetuates favorable mutations. Think of a prospector panning for gold. He scoops up a mixed assortment of sand, pebbles, and—with luck—a few bits of gold. But only the heavier flakes and nuggets of the valuable gold survive the panning. They are not, unlike the lighter, valueless mixture of sand and gravel, washed out of the pan as he swirls the water. In a similar way, natural selection preserves favorable genes and eliminates deleterious genes.
Some of the insects' most important adaptations are responses to insectivores, a numerous
