How Not to Be Eaten. Dr. Gilbert Waldbauer
they listening to? The answer, Roeder demonstrated, is fierce predators: night-flying, insect-eating bats.
In chapter 2 we saw that bats find their way in the dark by means of echolocation, a discovery made by Donald Griffin. They sense obstacles and flying insects by using their keen sense of hearing—many have very large ears—to listen for the echoes of their own sounds, pitched too high for us to hear, that bounce back from these objects. Flying insects that hear bat cries, Roeder found, respond by taking evasive action, which differs with the species: power-diving, folding the wings and falling to the ground, changing course, flying faster and more erratically.
An insect may flee from a predator by running, jumping, swimming, or flying, but many, notably plant feeders, just drop to the ground. With few exceptions, the insect is most likely to survive if it drops as soon as possible. The shaking of the branches and leaves of a tree or shrub may signal the arrival of a predator, most likely a bird hopping from twig to twig as it scans the foliage for insects. In response to a disturbance of this sort, some insects immediately bail out by dropping to the ground, a disappearing act that may happen even before the bird notices the insect. Some insects, notably aphids, disturbed by a bird—or more likely a ladybird beetle or an aphid lion—may simply drop from the plant. But, as Malcolm Edmunds notes, “few aphids respond to a predator by dropping, [although] this is always a successful method of escape. One disadvantage of dropping is that the animal may have great difficulty in finding and climbing a suitable plant on which to feed, particularly if it is immature and has no wings.” This is not a problem for leaf-feeding caterpillars of many species—and some spiders—which lower themselves more carefully and may climb back up on a thin strand of silk.
Some beetles, particularly snout beetles, respond to a disturbance by tucking in their legs and letting themselves tumble to the ground, where they are likely to lie motionless for some time. Many, as seen in the beautiful color photographs in Stephen Marshall's encyclopedic Insects: Their Natural History and Diversity, look deceptively like dark fecal pellets or small clods of earth. Among the latter group is the well-disguised quarter-inch-long plum curculio (Conotrachelus nenuphar), which is brown with a few small white markings and four large humps on its back (wing covers). Fruit growers take advantage of these insects' escape behavior to find out if they are numerous enough in plum, peach, or apple orchards to justify the expense of an insecticide application. “Jarring the beetles from the trees in the early morning, on sheets placed on the ground,” explained Robert L. Metcalf and Robert A. Metcalf, “enables the grower to…[census the population] of these beetles.”
The forked fungus beetles (Bolitotherus), named for a pair of prominent “horns” that protrude from just behind the male's head, have an escape behavior similar to but even more impressive than the plum curculio's. Adults and larvae of this eastern North American species feed on the shelf, or bracket, fungi commonly seen protruding from the trunks of dead trees. If they are disturbed, Marshall observed, the adults' “first defensive response is to pull in their appendages and drop to the ground.” Legs and antennae protectively retract into special grooves. The beetles don't move, “feigning death,” and are difficult to spot because they look even more like clods of earth than the plum curculio. In addition, adult forked fungus beetles have a chemical defense, an irritating substance secreted by an eversible forked gland at the tip of the abdomen. But the most amazing thing about these beetles is their early warning system, described by Jeffrey Conner and his coworkers. The beetles recognize “mammalian breath on the basis of its temperature, moisture, and flow dynamics,” which causes them to evert the gland, but they do not respond to a mechanically produced air stream. The beetle's “ability to cue in on mammalian breath enables it to respond preemptively to a potentially lethal attack from a ground-foraging mammal, perhaps a deer mouse. Gland eversion can save the beetle by making it distasteful at the very moment that it is taken into the predator's mouth, before a bite is inflicted.”
Many insects, especially ground-dwelling species such as cockroaches and beetles, run away as rapidly as they can when alerted by their early warning system. There are, however, surprisingly few records of how fast they can go. My friend and colleague Fred Delcomyn, a neurophysiologist and an expert on the neural control of walking and running by insects, told me that it is very difficult to time running insects because they rarely go very far in a straight line. The American cockroach, according to G. M. Hughes and P.J. Mill, is one of the fastest insects, running at a maximum speed of 51 inches per second, or 2.9 miles per hour. This may seem slow, but consider the rate compared to body length. The American cockroach runs a distance of about thirtyfour times its body length of 1.5 inches in one second. This is equivalent to a coyote with a body length (excluding the tail) of 2.8 feet running 65 miles per hour. I doubt coyotes can run that fast. (I once clocked a panicked one going 45 miles per hour trying to outdistance my pursuing car as it ran along a ditch bank parallel to the road I was driving on.) It seems, then, that on the basis of a fair comparison, the American cockroach is probably a faster runner than a panicked coyote. Keep in mind that some of the thousands of cockroach species that live outdoors—and surely have many enemies other than irate householders—run at least as fast as and perhaps even faster than the pestiferous American cockroach.
A sudden leap into the air is another good way to escape from a predator. It is not surprising, then, that many different kinds of insects, members of several unrelated groups, have evolved this escape tactic independently of one another—and have jumping apparatuses fashioned in quite different ways, and even from different body structures. Grasshoppers, crickets, and katydids are closely related and probably inherited their jumping hind legs from a common ancestor. But fleas, flea beetles, and certain relatives of the aphids and cicadas, such as leafhoppers and planthoppers, have leaping hind legs that obviously evolved independently of each other and may be quite different in design. Click beetles and springtails, close relatives of the insects that I discuss below, leap into the air without using their legs.
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