The caterpillar is the larva of a moth or butterfly. Larvae are the juvenile forms of insects.
Caterpillars hatch from insect eggs. The eggs are laid by adult moths or butterflies.
An egg will hatch in about two weeks. After hatching the caterpillar will feed for about 25 days. Caterpillars typically eat leaves.
During the weeks the caterpillar feeds it can grow to as much as 10,000 times its initial weight. The caterpillar will molt or shed its skin four times as its body grows larger.
After about four weeks, the caterpillar is ready to change from a caterpillar to an adult moth or butterfly. The process of change from young insect to adult is called metamorphosis. That change is dramatic.

When fully grown the caterpillar will either form a cocoon of silk around itself or its skin will split to reveal a pupa; the caterpillar has molted the final time. This transformation is controlled by the insect’s hormones.
The pupa is a hard case held in place on a leaf or branch by a thin loop of silk spun by the caterpillar before its metamorphosis begins. Inside the pupa, the caterpillar’s body is transformed from a caterpillar into the adult moth or butterfly. Clusters of cells that were inside the caterpillar become active. These cells form a new body and the old body dissolves. After two weeks in the pupa, the adult emerges.
The whole life cycle from egg to caterpillar takes about 10 weeks.