A caterpillar is the larva of a butterfly or moth: the worm-like stage between egg and adult. When a butterfly lays eggs on a leaf, tiny caterpillars hatch and immediately start eating. They munch constantly, growing bigger and bigger until they're sometimes hundreds of times heavier than when they hatched.

Caterpillars look nothing like the butterflies or moths they'll become. Most have long, soft bodies divided into segments, with several pairs of stubby legs. Some are smooth and green, blending perfectly with leaves. Others are fuzzy, spiky, or brilliantly colored to warn predators they taste terrible. The woolly bear caterpillar, with its black and orange bands, is a familiar sight crossing sidewalks in autumn.

After weeks of eating, a caterpillar stops moving and forms a hard shell called a chrysalis (butterflies) or cocoon (moths). Inside this protective case, something extraordinary happens: the caterpillar's body completely reorganizes itself. Weeks or months later, a fully formed butterfly or moth emerges, spreads its wings, and flies away.