of a leaf, under debris on the ground, or in some other nook. Some, however, will leave their hiding place to flee if an intruder comes too close—within a critical distance whose length will vary with the species of the prey insect. The next chapter considers running away and hiding as ways to escape from predators, to avoid becoming a meal for a bird, a mantis, a mouse, or some other insect eater.
THREE
Fleeing and Staying under Cover
A well-hidden insect will be safe from many, if not most, insect-eating predators. But since natural selection is inexorable, predators will inevitably evolve with the anatomical and behavioral specializations needed to find and capture even the most thoroughly concealed insects. For example, if you hear what sounds like the blows of an ax in a winter woodland, it may well be a pileated woodpecker, the largest of our surviving North American woodpeckers, using its powerful, chisel-like bill to chop out chips of wood the size of a child's hand as it works to get at the larva of a long-horned beetle hidden deep in the trunk of a tree. An insect burrowing in the soil, such as a wireworm or a white grub, may be found by a mole or the probing bill of a grackle or some other bird. Nevertheless, hiding—although not always successful—can be advantageous, and insects of all sorts, and other animals too, have adopted this strategy for survival.
Natural selection favors—often very much so—an insect's normal lifestyle, especially its feeding behavior, if it keeps the insect out of sight and thereby protected from at least some potential predators. Usually only the larvae, insects in the immature stage, bore into plant tissues, burrow in the soil, or are otherwise hidden, and the usually immobile pupae generally remain hidden in the larval tunnel or burrow. The much more active adults are exposed to many more predators—spiders, insects, birds, mice, shrews, bats—as they fly and run about searching for nectar or other food, for a mate, and for appropriate places to lay their eggs. Most female insects lay hundreds of eggs, and many are exposed to predators as they fly long distances to distribute their eggs one by one or in small clutches on widely dispersed plants, often of only one or a few closely related species.
Figure 3. Disturbed by a predator, a grasshopper leaps into the air and flies off to make its escape.
In July and August we hear, high in the trees, the loud, shrill drone, the “love call,” of male dog-day cicadas—even in cities and towns. The females are frequently on the move as they disperse their eggs in small clutches laid in small cavities slashed into woody twigs by their sharp ovipositors, their egg-laying appendages. After hatching, the tiny nymphs drop to the ground and burrow deep into the soil, where they suck sap from roots until they emerge from the soil as adults about 2 inches long two or more years later. (Because the generations overlap, some cicadas emerge every year.)
Although the nymphs are relatively safe in the soil, the adults are eaten by birds of many kinds. Large, scary-looking but harmless solitary (nonsocial) wasps called cicada killers also search for them in the trees. The wasps inject them with a paralyzing but nonlethal venom, stock each of several small chambers in their underground nests with two or three of them, and lay a single egg in each chamber. (All but a very few nonparasitic wasps feed their larvae insect or spider prey.) The wasp larvae feed on the paralyzed cicadas but remain in the ground—as safely hidden from predators as are cicada nymphs—until they emerge from the soil as adult cicada killers the following summer.
Like the cicada killers, thousands of species of solitary wasps and bees prepare a shelter for their offspring. Most, like cicada killer larvae, live in burrows in the soil, but other parents build aboveground structures that shelter their offspring. (The Nobel laureate Karl von Frisch nicely described and illustrated some of these shelters in Animal Architecture) Some potter wasps (family Eumenidae), for example, build juglike nests of mud that they stock with paralyzed caterpillars, but other wasps of this family are not potters at all and instead nest in hollow plant stems.
Other insects also prepare concealed nurseries to hide their offspring from predators. Carpenter bees (subfamily Xylocopinae), some of which look like large bumblebees, excavate nesting tunnels as much as a foot long in solid wood—on one occasion in the unpainted cedar siding on my house, although they soon gave up because the inch-thick siding was too thin for them. Little brown solitary bees (family Andrenidae) hurry from blossom to blossom in early spring, gathering nectar and pollen from spring beauties. They dig long tunnels in the soil and provision small cells that branch off from the main tunnel with their harvest, which feeds the larvae that hatch from single eggs laid in separate cells. A remarkable mason bee (Osmia bicolor) of Europe prepares an individual nest for each of her larvae in the empty shells of land snails, perhaps even those that housed that gourmet's delight the escargot. After finding a shell, stocking it with food, on which she lays only one egg, and blocking the shell's opening, the bee, Frisch explained, “makes a series of flights to collect all kinds of dry stalks, blades of grass, thin twiglets, or even pine needles[;]…from this material, she builds a tentshaped roof over the snail shell, which eventually hides it completely.” Like all bees, both solitary and social, she provides her larval offspring with bee bread, a mixture of pollen and honey.
Their activities, seeking mates or places to lay their eggs, make it difficult or impossible for adults to always be hidden. Adult Japanese beetles, June beetles, and other herbivorous relatives of the scarabs feed on the foliage of shrubs and trees. Groups of metallic green and bronze Japanese beetles cluster shoulder to shoulder in conspicuous groups on a leaf. But both of these beetles and related species lay their eggs deep in the soil, including under our lawns. The chubby, C-shaped larvae, known as white grubs, live belowground, feeding on roots. Although well hidden and plagued by far fewer predators than the adults, they are preyed upon by some insects, birds, and moles. Among these predators are wasps of the family Scoliidae, which have no common name. John Henry Comstock noted that these wasps “do not exhibit as much intelligence as do most digger wasps; for they do not build nests and do not transport prey to them for their carnivorous larvae.” After locating a white grub in the soil, the female scoliid paralyzes it with a sting, “work[s] out a crude cell about it, and attaches an egg to…the grub.” The scoliid larva eats the grub, spins a cocoon, and completes its development in its underground cell.
Some immature insects hide in plant matter. The tiny leaf-mining larvae of some beetles, moths, flies, and wasps tunnel in the narrow space between the upper and lower epidermal layers of a leaf, feeding as they go. Their tunnels are clearly visible beneath the translucent epidermis. The tiny apple leaf miner moths glue their eggs to the undersides of leaves. When the larvae hatch, they pass through the egg shell directly into the leaf. Many beetle larvae and moth caterpillars, such as European corn borers, tunnel in the stems of nonwoody plants. Some snout beetles (weevils) gnaw a tunnel into an acorn or other nut with the mandibles at the end of their long, thin snouts and then turn around to place an egg in the tunnel and then move on to lay more eggs. When the full-grown larva emerges from the fallen acorn, it burrows into the soil to pupate. Some fly larvae, such as the apple maggot, and caterpillars, such as codling moth larvae—the infamous worm in the apple—burrow in fleshy fruits, but fly maggots leave the fruit to pupate in the soil, and codling moth caterpillars move away to pupate in a cocoon, often under a flake of bark on a tree trunk.
The sloth moths have what may be the most unusual lifestyle of all the insects, one that keeps them hidden from most, perhaps all, predators throughout the egg, larval, and pupal stages and exposes them only briefly during the adult stage. As adults, the four species of sloth moths, distant relatives of the European corn borer (family Pyralidae), hide in the dense growth of hair on sloths, slow-moving mammals of New World tropical forests that live high in the trees, feeding on foliage. Anywhere from a few to more than a hundred of these little moths may occupy a single sloth.
When sloth moths were first discovered in the nineteenth
