How Not to Be Eaten. Dr. Gilbert Waldbauer
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bag, and die.
A cocoon protects many insects, most famously moths, during the pupal stage, when they are especially vulnerable to predators because being virtually immobile, they cannot run away or defend themselves. Before molting to the pupal stage, Comstock noted, caterpillars “make provision for this helpless period by spinning a silken armor about their bodies.” As we will see in chapter 9, several of the giant silkworm moths (family Saturniidae) of North America spin very large tough-walled cocoons, in which they spend the winter in the pupal stage. The huge cecropia caterpillar, for example, constructs a double-walled cocoon 3 or more inches long and immovably attached along its length to a sturdy twig.
In 1978, one of the two homes of the Green Revolution, the International Rice Research Institute (IRRI) on the island of Luzon in the Philippines, invited me to visit and develop a method for testing many thousands of rice varieties for resistance to the rice leaf folder, a moth of the family Pyralidae and an important rice pest. The main problem was figuring out a way to raise large numbers of leaf folder caterpillars in the laboratory so that the varietal tests could be done in a greenhouse. If the varieties to be tested are planted outdoors, the results of the test may be inconclusive, because as luck is likely to have it, the natural population of leaf folders will be too small or almost nonexistent that year.
Gottfried Fraenkel, who years before had supervised my PhD research, was invited to do the very same thing with rice leaf folders by the Central Agriculture Research Institute of Sri Lanka. About six months after I arrived at IRRI, Gottfried stopped off to see me on his way to Sri Lanka. When he asked me if I had made any progress, I oneupped my former boss, handing him a copy of a manuscript ready for publication that described my then recently devised method for testing rice plants for resistance to the leaf folder.
In Sri Lanka, Gottfried did other research projects with this insect, including a masterly study published in a Dutch journal in which he and Faheema Fallil wrote of the leaf folder, “Its characteristic behaviour is to spin a rice leaf longitudinally into a roll, by stitching together opposite rims of the leaf, and to feed inside this roll, leaving the epidermis on the outside of the roll intact,” as camouflage. Lying aligned with the long axis of the long, narrow leaf, the caterpillar swings the head end of its body from side to side in the same spot as many as one hundred times, forming a thick band of silk fibers that joins the edges of the leaf together. By repeating this procedure as it advances short distances along the leaf, the caterpillar forms as many as thirty such crossbands. “A newly woven band,” Fraenkel and Fallil found, “quickly becomes shorter by a process of contraction…bringing the rims of the leaf blade closer together…. With each succeeding band, this distance becomes shorter until the leaf is completely rolled up.”
If an insect's lifestyle does not commit it to living under cover, in hiding—or if it lacks an effective physical or chemical defense—it will most likely have another way of protecting itself against predators. Some insects, as we will see in chapters 4 and 5, are camouflaged, blend in with the background, or resemble an inedible object, but generally speaking, most defenseless species flee to a hiding place when they feel threatened—even camouflaged individuals whose cover is blown.
In 2008, Oswald Schmitz reported that red-legged grasshoppers (Melanoplus femurrubrum) respond differently to ambushing “sit and wait” spiders and to “roaming, actively hunting” spiders. Grasshoppers respond to sit and wait spiders, but not to roaming spiders, by shifting from their preferred food plant, a nutritious grass, to goldenrod, which is not a favorite food but on which they are less likely to be killed by a spider.
Most cockroaches, as Thomas Eisner and his coauthors so aptly put it, “crave concealment. Anyone who has shared a kitchen with cockroaches knows that they seek shelter by day and that they are driven to flee for cover at night if a light is turned on.” This is the way of not only the tiny minority of cockroaches that have become household pests but also most of the world's almost four thousand other cockroach species, which live in natural settings.
An insect that hides in a crevice or under a fallen leaf, a flake of bark, a rock, or a clod of soil would, ideally, have eyes not only on its head but also on its tail end so that it could tell if all of its body was safely tucked away in the dark of its hiding place. No insect or other arthropod has eyes on its tail end, but according to M. S. Bruno and D. Kennedy, a crayfish, a spiny lobster, and a shrimp have what Sir Vincent Wigglesworth called a “dermal light sense” at the tail end of the abdomen; in other words, in 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