The Secret Life of Caterpillars

Adaptive caterpillars play a key part in forest, fields, and ecosystems everywhere.

| July/August 2015

One foot in front of the other – and a lot of feet at that. Plodding along, the caterpillar’s means of locomotion depends upon its several pairs of legs. Equipped with tiny claws designed for superior traction on surfaces ranging from smooth new leaves to rough tree bark and narrow pine needles, these versatile animals really get around. With as many as 4,000 muscles throughout a caterpillar’s body, compared to only 629 in the human body, it’s quite understandable that these small creatures are not only extremely flexible, but capable of gripping firmly onto severely windswept tree boughs, like high-wire performers in a circus act.

The immature or larval stage of either a butterfly or moth (these are combined into the scientific order known as Lepidopterans), a caterpillar is a remarkably well-adapted piece of biological engineering. Although most start out with more legs than a small dance company, only their six true legs, a trait of all insects, remain with them when they transform into adults, through the miracle of metamorphosis.

Because of their laid back lifestyles, caterpillars frequently attempt to blend into their surroundings. Often resembling brown twigs or green stems to which they cling, many have evolved with plantlike growths on their bodies. Some have gone so far as to develop weird shapes that appear as partially eaten leaves, accented by strange patterns of leaf decay stenciled along their lengths.

However, as a whole, our North American species display every color of the rainbow, ranging from lemon yellow to powder blue, and even shocking pink in one southern variety. Many sport racing stripes, bands, dots and bizarre hairstyles, and with these outlandish designs, loudly proclaim their distastefulness to birds and other predators. Those with the most vibrant colors and showy markings, such as the brightly banded larva of our revered monarch butterfly, are likely recognized by many birds as being toxic.

With irritating hairs or spines that can cause mild to severe allergic reactions in human skin, some species of North American caterpillars are best left untouched. One group of moth caterpillars, endemic to the Amazonian region of South America, is so toxic that a number of human deaths have actually been attributed to direct contact with them.

Born Mimics

Some Lepidopteran larvae have even evolved to mimic other animals, like snakes or wide-eyed mammals, intimidating their aggressors with artificial eyes that stare back at them. The larvae of both the tiger and spicebush swallowtails are good examples of this phenomenon. In its last instar, or growth stage, before changing to a chrysalis, the mature spicebush caterpillar turns yellow or orange and, viewed from head on, resembles a jack-o’-lantern, with its oversized, glaring “pupils” impossible to miss. Below a black lip marking is the true head of the insect, curled out of sight. As a backup strategy to avoid being eaten, all swallowtail larvae also produce, on demand, an inflatable, forked organ when disturbed. This brightly colored gland, called the osmeterium, mimics a snake’s tongue, and, when extended, gives off a foul odor to boot. A gently handled specimen, though harmless to humans, will very likely put on a show.

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