The caterpillar is the common name for the larva of a moth or butterfly. Caterpillars have distinct heads and are segmented and wormlike. They have three pairs of short, jointed legs (retained in the adult) on the thorax; in addition, they have unjointed, fleshy appendages, called prolegs, on some abdominal segments.
The prolegs end in clusters of tiny hooks. There is a row of simple eyes on either side of the body. Sawfly larvae are often mistaken for caterpillars, but their prolegs have no hooks and they have a single simple eye on each side. Almost all caterpillars are vegetarian and have strong jaws for chewing. The chewing mouth parts and the prolegs disappear during the pupa stage, as the larva is transformed into an adult.
Caterpillars have silk glands that open into a mouth part called the spinneret. The caterpillar exudes a silk strand continuously as it moves along; small caterpillars swing by the strand when dropping from a height. Many caterpillars use the thread to build a cocoon in which to pupate. Most molt their skin (to accommodate growth) five or six times before pupation. Some caterpillars have smooth skin; others are hairy, such as the woolly bear, or hedgehog, caterpillar of the Isabella tiger moth.
The caterpillars of the larger night-flying moths (e.g., the luna moth and polyphemus moth) are smooth and green and may be over 3 in. (7.5 cm) long. Caterpillars are equipped with various protective devices. The io moth caterpillar has sharp spines connected with glands that secrete an irritating substance. Others have irritating bristles, and the swallowtail butterfly larva emits a repellent odor when disturbed. Nevertheless, caterpillars form the major part of the diet of many birds and other animals.
Caterpillars are voracious eaters and some cause considerable economic damage. Among these are the appleworm, the cutworm, and the larvae of the bee moth, the codling moth, and the clothes moth. Some moths and butterflies remain caterpillars for two or three months, others for about 10 months, hibernating through the winter in this stage. In the Arctic are some forms that require two or three years to develop from egg to adult.