the “skin,” or exoskeleton (chitinous body wall). Actually it is not the skin but some part of the nervous system below it that can sense light. The American cockroach, and probably other cockroaches and many other insects, Harold Ball reported, has a similar light sensor at the tail end of the abdomen. There, a ganglion—a bundle of nerve cells and a part of the ventral nerve cord, which is, roughly speaking, the equivalent of our spinal cord—perceives light that passes through the translucent skin above it. Ball and other researchers demonstrated the existence of a dermal light sense. The American cockroach and other insects can tell light from dark even if the eyes on their heads have been covered with black paint.
Some insects, like cockroaches, rush to a hiding place if sufficiently alarmed. Others, such as the houseflies you startle in your kitchen, fly away rapidly but alight in plain sight on another wall. In either case, the fleeing insect was probably well served by an early warning system. “For species that are palatable,” Malcolm Edmunds pointed out, “it will be of advantage if they can detect their predators before the predators detect them, and if they can initiate their active defence (flight) before, or as soon as possible after, the predator has noticed them.”
Early warnings may be perceived by the organs of touch, vision, or hearing. At the tail end of the abdomen, noted R. F. Chapman, insects with gradual metamorphosis, except the true bugs, bear a pair of antenna-like sensory appendages, tactile receptors called cerci. Each cercus is clothed with fine hairs ultrasensitive to air currents or touch. This is, of course, an early warning system that alerts the insect to the approach from behind of something that might be a threat. Kenneth Roeder, an entomologist and neurophysiologist, described how the early warning signal of the cerci can be triggered. He suggested that “at night when cockroaches are most active, the observer should slowly approach a single insect standing motionless near the center of an unobstructed area such as a wall or floor. A short puff of air directed at the cerci…will send the roach scurrying off and probably out of sight.”
The early warning signal, a nerve impulse, travels along the insect's ventral nerve cord from the cerci to the ganglion in the thorax that controls the legs and launches the insect on its escape to safety. The faster the warning signal travels, the better. Among the many nerve fibers that constitute the ventral nerve cord of some insects, including the cockroach, are six to eight giant fibers as much as fourteen times as thick as the others. The virtue of the giant fibers is that they conduct nerve impulses much more rapidly, according to Roeder, at a rate of close to 23 feet per second rather than the other fibers' rate of no more than 2 feet per second.
Their ability to perceive distant objects often makes the eyes the most effective of the early warning systems. Most adult and nymphal insects have two compound eyes on the head, and many also have simple eyes (ocelli) between the compound eyes. Larvae have only simple eyes. The unusual structure of an insect's compound eye—radically different from that of our eyes or those of other vertebrates—gives it an exceptional ability to sense movements. A compound eye is an aggregation of closely packed but separate light-sensitive elements. “The system of small units…which constitute the compound eye,” Chapman explained, “lends itself to the perception of changes in stimulation resulting from small movements of [an] object.” This translates into a highly sensitive early warning system. For example, if you've ever tried to snatch a sitting fly, you know that it will, unless you are very fast, dash away long before your hand can reach it.
Except in the case of certain singing insects, Robert and Janice Matthews observed, “one does not usually think of insects as possessing ears.” Male singing insects—cicadas, crickets, and katydids are among the most familiar—belt out “love songs” to attract a mate. Females obviously must have ears, but males have them too, so they can listen for competing males. Most insects that don't make sounds do not have ears. But some mute insects of several unrelated groups—moths, lacewings, praying mantises—do have ears. If they are not listening to each other, what are 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
