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Lepidopterans: Allure and Illusion
by Michael H. Robinson

If all of the ants in the Amazonia forest were lumped together, their mass would be greater than all of the land-living amphibians, reptiles, birds, and mammals combined. In fact, insects dominate the land everywhere both in numbers of species and numbers of individuals. Insects are present in all major ecosystems and habitats with the notable exception of perpetually frigid areas and the seas and oceans. There, their crustacean cousins are the dominant arthropods, a group that comprises all the animals with a jointed exoskeleton, from tiny mites to giant crabs, and including spiders, the great slayers of insects.

Among the multitudinous insects, the Lepidoptera--butterflies and their mainly nocturnal close kin, the moths--stand out as a relatively well known and highly successful group, with more than 125,000 species so far described. Butterflies and moths are conspicuous flyers in many parts of the world. In general, butterflies and moths are fairly slow-flying flutterers, but some are quite swift on the wing, and some are fast enough to elude birds and bats in flight. Many adult butterflies and moths are large and brightly colored, beautiful creatures that visit flowers for food and do good in their role as plant pollinators. But first impressions, based on the familiar members of this group, are misleading. Not all lepidopterans are brightly colored, not all are large as insects go, and not all are flower visitors as adults. Some are quite dull in color, as camouflage against predators, while others disguise themselves as leaves or even bird droppings. Some are tiny--about the size of a sesame seed--while the birdwing butterflies, with wings that would cover both pages of this magazine, are the largest known insects. In rare cases, some butterflies ignore flower nectar and drink tears and blood instead.

The larvae of butterflies and moths are known as caterpillars. These tubular creatures, sometimes fuzzy, sometimes bearing strange spikes, sometimes garishly colored, often provide children with their first memorable encounters with as odd an animal as an invertebrate. Unlike the mostly benign adults that they turn into, most caterpillars feed on leaves and can be relentless plant pests. The massive defoliation wrought by gypsy moth caterpillars is a perfect example. On the other hand, the caterpillars of the silk moth--silkworms--are the only domesticated insect and provide one of our most luxuriant and prized fabrics. Silkworms are raised on mulberry leaves and spin the precious silk to form the cocoon that encloses the pupa (the stage in a lepidopteran's life cycle before the adult butterfly or moth emerges.)

Cases of Mistaken Identity

If a butterfly or moth is large, conspicuous, and slow flying, the risks of predation are great. Butterflies are eaten by birds, lizards, spiders, and a wide range of other insects. Haunting the night skies, moths are pursued by bats and trapped by spiders. To defend themselves, some butterflies are downright nasty. Monarchs, for instance, taste terrible and make birds that eat them vomit violently. The compounds, called cardiac gylcosides, responsible for this effect actually come from the plants on which monarch caterpillars feed. These chemicals are part of the defensive system of milkweed, although they don't seem to deter the caterpillars, which eat the leaves, store the chemicals, and pass them along to the adult form. Birds, however, associate the butterfly's conspicuous orange and black coloration with their traumatic experience and avoid encounters with monarchs thereafter. (Animals, including these butterflies, coral snakes, and dart poison frogs, that advertise their nastiness with memorably vivid colors that are conspicuous even to our eyes are said to be warningly colored.)

As so often happens in nature, there are some cheats that cash in on the monarch's defenses. Harmless themselves, viceroy butterflies look like monarchs and so are avoided by birds. In some butterflies, the match between the harmful species and the cheating, harmless one is uncannily exact, with color and pattern identical down to the last detail. The benefit to the cheater, of course, is protection from predation without the expense of producing or storing poison itself.

This type of deception was first illuminated by English naturalist Henry W. Bates, a butterfly and beetle enthusiast. In 1863, at the age of 22, Bates embarked on an exploration of the Amazon that was to last 11 years. From this adventure came a marvelous book, The Naturalist on the River Amazons, and the immortalization of his name: When a harmless animal mimics a harmful one, it is known as Batesian mimicry. He also collected specimens of more than 14,700 species of animals, almost 8,000 of them new to science. A testament both to his interest and their abundance--some 14,000 of the specimens were insects!

Mimicry is not limited to resembling other, poisonous species. To avoid predation, some butterflies are marvelously convincing mimics of leaves, even folding their wings together when at rest so the markings on their undersides look like leaf veins. Other butterflies and moths have paired wing markings that resemble eyes. These may be paired on hind wings and closely resemble owl's eyes, complete with oval pupils and owlish highlights. Hidden when the moth is at rest, the sudden flash of the eyes startles approaching would-be predators, giving the insect time to escape. Small eye markings on wing edges appear to deflect the bites of pursuing predators from the vital head and thorax. Some species of hairstreak butterflies go a step farther: the edges of their hind wings form a false head, which even has fake antennae to catch a predator's attention. Losing a bit of expendable wing is much better than losing one's head!

Masters of Danger and Deception

Caterpillars also have their defenses, including warning coloration. This is the most vulnerable stage in the lepidopteran life cycle, as caterpillars are slow and often highly exposed on the leaves they eat. Some caterpillars cause intense skin irritation in mammals. I still remember the rash on my arms and legs that broke out after I handled a hairy black and red caterpillar when I was eight years old. It was agony! After that, I knew that black and red meant danger. The caterpillars of saddleback moths look cute, rather like small terriers wearing doggy jackets. They have lateral rows of stiff spines and four long projections, fore and aft, bearing clumps of spines. Merely brushing against the spines causes instant pain like an electric-shock jolt, presumably from some very potent substance.

The caterpillars of many species of swallowtail butterflies are equipped with a special pair of protrusible processes on their heads that release an extraordinarily malodorous substance. When provoked, for example by a pecking bird, the two yellow horns appear, along with an overpowering smell of sweaty socks. Many swallowtail caterpillars rest in clumps of up to 20 individuals. When they stink simultaneously, the smell is detectable many feet away.

Other caterpillars exhibit a range of camouflage defenses. One of these is counter-shading, a device that reduces the outline-enhancing effects of highlight and shadow. Most solid objects lit from above look solid to us because the top is highlighted and the bottom in shadow--think of how artists convey this by shading. Caterpillars that feed on the tops of leaves have light-colored underparts that cancel out the shadow and dark backs that cancel out the highlight. In caterpillars of species that habitually feed on the undersides of leaves, however, this counter-shading is reversed. Many camouflaged caterpillars are barred or blotched like the clothing of a soldier or hunter.

Other caterpillars disguise themselves as a defense. For example, bird-dropping mimicry is fairly common. In some swallowtail species the caterpillars are white with green blotches and have the slimy look of fresh bird feces. Some caterpillars even change their disguise as they molt and grow. First they rest on leaves and mimic feces; at a later stage, when they move to stems, they mimic bark. Others are stick mimics, such as the inchworms that clasp twigs with their posteriors and hang away at an angle like small side branches. Experiments with insect-eating birds show that they usually do not detect immobile stick mimics but eat them instantly if they move.

Another caterpillar defense is snake mimicry. Several kinds of moth caterpillars respond to disturbance by inflating the front of their body into a snake-like head complete with large imitation snake eyes. Thus transformed, they twitch from side to side in a startling manner. We have no experimental proof that this works with natural predators, but it surely scares naive humans! Similarly, hag moth caterpillars, sometimes called monkey slugs, look like hairy spiders, complete with pseudo-legs.

Hag moth caterpillars may even scare their parents away, as spiders are important butterfly and moth predators. Web-building spiders, especially nocturnal ones, set their traps for moths when they are potentially a rich food source. But moths, and, to a lesser extent, butterflies as well, have a non-stick or slippery defense. Wing scales are powdery structures that slip easily off the wings. When a moth or butterfly strikes an orb spider web, the scales stick to the web's gluey droplets and the moth or butterfly does not. In turn, spiders have evolved various specializations to increase their moth-trapping ability. The ultimate spider weapon, found in bola spiders and others, may be synthesizing a copy of a female moth's sex attractant perfume. This lures male moths to their doom.

Lepidopteran marvels abound. Here's another story:

Most adult butterflies and moths feed on nectar, but there are species that feed on the juices of decomposing organic matter, both plant and animal, others that sip tears from the eyes of vertebrates, and even some that suck the blood of mammals. Similarly, caterpillars are primarily plant eaters, but some eat flowers or ant larvae and pupae, and others eat dung. Dung eating brings up perhaps the most bizarre moth/mammal relationship known. Adult sloth moths--up to a hundred or more--live on the fur of two- and three-toed sloths. Sloths, as leaf eaters, retain the difficult-to-digest leaves in their gut for many days, giving bacteria time to break down the leaves' cellulose into starches and sugars. As a result, defecation takes place only at weekly or longer intervals. The feces, however, are a rich food resource for sloth moth caterpillars. When the sloth descends to the ground to defecate, gravid moth females leave the sloth to lay their eggs on the fresh dung. Riding on the sloth ensures that the female moth is around at those rare and short moments when the caterpillar food is voided.

Michael H. Robinson is the Director of the National Zoo.

(ZooGoer 25(2) 1996. Copyright 1996 Michael H. Robinson. All rights reserved.)

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