How Not to Be Eaten. Dr. Gilbert Waldbauer
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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 century, it was supposed that both the adults and the caterpillars lived on sloths and that the caterpillars fed on the plentiful growth of green algae on the sloths' hair or ate the hair itself. But in 1976 Jeffrey Waage and G. Gene Montgomery reported that although they found many adult moths on sloths, they found no eggs, caterpillars, or pupae. But they did find caterpillars feeding on sloth dung. About once a week a sloth descends to the ground to defecate. Hanging from a vine, it scoops out a pit with the long, curved claws on its hind legs, deposits about a cupful of fecal pellets in the pit, and covers it with leaf litter. Female moths briefly leave the sloth to lay their eggs on its yet-to-be-covered dung. The caterpillars eat dung, and when the moths emerge from the pupae in the dung pit, they fly up into the trees to find a sloth. They mate on the sloth, and gravid females leave the animal only long enough to lay their eggs.
Some insects construct their own hiding places. Working together, several hundred newly hatched tent caterpillars (Malacosoma americanum) of eastern North America spin a small tent of silk. As the caterpillars grow, they continuously enlarge the tent until it is about 2 feet long. Shaped like upside-down pyramids in the crotches of wild cherry trees, these tents are a common sight along country roads in spring. At night and during the cool parts of the day—early morning and late afternoon—the caterpillars shelter in the tent, where they are protected from many parasites and predators, Terrence Fitzgerald explained in The Tent Caterpillars. When it is warm enough, they leave the tent en masse and march nose to tail in single file to a leafy branch to feed, laying down a pheromone trail that will later guide them back to the nest.
Groups of other insects, mainly caterpillars, also cooperate to spin the communal silken nests in which they live. The messy nests of the fall webworm (Hyphantria cunea), constructed on the leafy branches of many kinds of trees, are a common sight in late summer in much of southern Canada and the United States. In spring and early summer, the webworm moths emerge from silken cocoons hidden under leaf litter or a flake of bark and lay their eggs in clusters of several hundred on the undersides of leaves. Upon hatching, the caterpillars immediately begin, as Ephraim Felt explained, “to spin a communal web under which they feed. This protecting web is extended to include more and more foliage till finally a considerable portion of a branch may be enclosed.” The caterpillars partly skeletonize leaves, eating only the upper surface, leaving the veins and the lower surface intact. “The skeletonized leaves within the nest soon dry, turn brown, and they, with the frass [excrement] and cast skins of the caterpillars, render the nests very unsightly objects.”
The caterpillars known as bagworms (family Psychidae) are well named. They live in cocoonlike pouches that they make of silk and decorate with bits of leaves and twigs. The head and thorax can be protruded through an opening in the bag, enabling the caterpillar to crawl and eat leaves. Fecal pellets are expelled through an opening at the other end of the bag. The familiar evergreen bagworm (Thyridopteryx ephemeraeformis) feeds mainly on junipers (red cedars) and arborvitae and, like other species of its family, spends virtually its entire life in its bag. The eggs laid by the wingless females overwinter in the bag. In spring, the newly hatched larvae leave the bag and immediately build their own bags, which they continually enlarge as they grow. In autumn, the fullgrown caterpillars pupate in their bags. The winged males emerge from the bags but, having vestigial mouthparts and unable to feed, live for only about a day. Drawn by a female's sex-attractant pheromone, a male thrusts his extensible abdomen far up into her bag and inseminates her. Only after laying their eggs do the larvalike adult females—which