Few insects other than moths, as Thomas Eisner and his coworkers discovered, manage to escape from the webs of orb-weaving spiders. Moths are sometimes saved by the tiny, easily detached scales that cover their wings and bodies. (The colored “powder” that clings to your fingers when you handle a moth or a butterfly consists of these scales.) Moths that blundered into a web, Eisner noted, “were detained only momentarily, and usually flew off seemingly unaffected by the encounter. However, they invariably left behind, stuck to the particular viscid threads…that bore the impact, some of the scales that ordinarily cover their wings and bodies…. Coated with scales, the threads are no longer adhesive, and the moth is free to escape.”
On a sunny June day in northern Michigan, I spotted an interesting-looking insect on a large white blossom of a Canada anemone. (It turned out to be a fly that mimics a yellow jacket wasp. You will read a lot more about mimicry in later chapters.) This insect, oddly enough, wasn't moving at all, and its posture seemed unnatural. Looking more closely, I saw that it was clutched by a white crab spider that was all but invisible on the flower. Many crab spiders, like this one, are ambushers that lurk motionless in a blossom as they wait for their prey to land—a fly, bee, or other insect. Some can, in the course of about a week, change from white to yellow or yellow to white to match the color of the blossom on which they are lurking. “Fortified with extremely potent venom in compensation for weak Chelicerae [pincers], the small crab spiders,” Foelix commented, “are formidable creatures that attack insects and other spiders much larger than themselves.”
The fact that these spiders evolved the ability to change their color suggests that at least some insects are wary and will not land on a flower occupied by something that might be a predator. In The World of Spiders, W. S. Bristowe described a simple experiment which showed that this seems to be so. On half of sixteen yellow dandelion blossoms that he had placed on a lawn, he put a black pebble about the size of a crab spider; on the other half he placed a pebble that was about the same size but matched the dandelion blossom's color. As Bristowe watched these blossoms for half an hour, only seven insects visited the ones with black pebbles, while fifty-six flies and bees visited the blossoms with yellow pebbles.
Some insects, such as the familiar praying mantis, are predators that, like a tiger or a wolf, during their lives will capture many prey animals and more or less immediately kill and eat them. Other insects, mainly many of the wasps and all of the flies of the family Tachinidae, according to Richard Askew, are parasites that develop from egg to full-grown larva within the body of just one host, usually an immature insect—perhaps a beetle grub, a caterpillar, or a grasshopper. However, parasitic insects are best referred to as parasitoids. True parasites, such as the intestinal worms of humans and other mammals, usually do not kill their hosts. But although a parasitoid begins relatively benignly, just absorbing nutrients from its host's blood, it eventually becomes a predator, killing its host by devouring its tissues.
Among the many thousands of insects that parasitize other insects, the wasps of the genus Trichogramma (they have no common name) are particularly interesting because of their habits and because agriculturalists disperse them in fields to protect various crops from leaf-eating caterpillars. These tiny wasps—the largest are only about 0.04 inches long—insert their eggs into the eggs of many different kinds of moths and butterflies. The Trichogramma larvae destroy the host egg by consuming its contents. The size and even the anatomy of the adult parasite that emerges from the egg, Askew explained, varies with the size and species of the host egg. Wasps reared from the large eggs of a cutworm moth were nearly twice the size of those of the same species reared from the much smaller eggs of a grain-feeding moth. Males of another species reared from a moth egg had normal wings, but those reared from an alderfly egg had no wings at all. Trichogramma wasps can be raised by the millions in the eggs of a domesticated colony of grain moths. If a crop, perhaps cotton, is likely to be seriously damaged by the caterpillar progeny of egg-laying moths, Trichogramma are often released in the field in the form of larvae developing in grain moth eggs that had been glued to slips of paper by their mothers.
Many nonparasitic insects that feed on other insects are hunters that actively pursue their prey. Robber flies (family Asilidae) dart from a perch, perhaps the tip of a twig on a low shrub, to snatch flying insects from the air. When the fly comes back to land, it sucks its victim dry after injecting it with a secretion containing poisons that kill it and enzymes that liquefy its inner tissues. Almost all of the more than 2,600 species of ground beetles in North America are insectivores, such as the colorful, inch-long, tree-climbing caterpillar hunters (genus Calosoma). Adult cicada killers (Sphecius speciosis)—wasps barred with yellow and black and about 1.5 inches long—are vegetarians that sip nectar from flowers but search trees for the dog-day cicadas that they will feed to their larvae ensconced in underground cells.
Ambushers, by contrast, are stealthy insects that are generally well camouflaged and sit motionless as they wait patiently for their prey to come along. Like the white crab spider, ambush bugs (family Reduviidae) lurk on flowers, often goldenrods, waiting to snatch up with their raptorial front legs a visiting nectar feeder, such as a bee, a wasp, or a butterfly. As do all of the other true bugs, they have piercing-sucking mouthparts, which the nonvegetarians use to suck their prey dry, rather than masticate it like the mantises and other insects with chewing mouthparts.
One of the most deceptive of the ambushing insects is a southeast Asian praying mantis (Hymenopus bicornis) that masquerades as a pink flower of the straits rhododendron, a shrub known as the Sendudok in the Malay language. Hugh Cott summarized Nelson Annandale's 1900 account of the appearance and behavior of this remarkable insectivore in the report of the University of Cambridge's expedition to the Malay Peninsula. The bright pink mantis, the color of the blossoms among which it rests, is patient and motionless as it waits for its next meal to come close enough to be snatched. Its deception is enhanced by wide pink petal-like flanges on its middle and hind legs. Insects, as Wolfgang Wickler noted, actually land on the mantis's body and probe for nectar, “for which they pay with their lives.” Its “alluring colouration,” as Hugh Cott put it, is a bait that attracts the insects that it eats. This is one of many examples of aggressive mimicry, the duping of potential victims, causing them to relax their guard. The mantis's disguise may also save its life. Insectivores such as lizards and birds are likely to pass it up because it is so deceptively camouflaged.
The mantis's ruse is most effective when it lurks among blossoms that it resembles. Consider Nelson Annandale's account of the dogged persistence of one such insect as it searched for an appropriate resting place. A captive mantis put on the ground near a large branch of a Sendudok
deliberately walked towards the branch, swaying its whole body from side to side as it progressed, and commenced to climb one of the twigs. This twig, however, bore only green buds and unripe fruit. When the Mantis reached the tip of the twig and found no flowers, it remained still for a few seconds, and then turned and descended with the same staggering gait. It proceeded to climb another twig. This also bore no flowers. The Mantis descended from it and mounted a third twig which was topped by a large bunch of full-blown blossoms. To these it clung by means of the claws of the two posterior pairs of limbs. For a few minutes it remained perfectly still, and then began swaying its body from side to side, as it had done while walking.
Annandale, quoted by Cott, tells us how well this mantis's aggressive mimicry deceives the insects that it will eat.
Almost as soon as the Mantis had settled itself on the inflorescence, a small, dark…[fly] of a kind very commonly seen on the flowers of this species of [shrub] alighted on one of its hinder legs. It was soon joined by others, apparently of the same species as itself. They settled quite indiscriminately on the petals and on the body and limbs of the Mantis…. The Mantis made no attempt either to drive off or to capture the small flies, for its motions seemed to attract rather than to repel them. After a short time a larger [fly,] as big as a common house-fly, alighted on the inflorescence within reach of the predatory limbs. Then the Mantis became active immediately; the fly was seized,
