A caterpillar starts to become a butterfly as soon as it enters the chrysalis stage, by wrapping itself in a cocoon. During this phase, known as pupation, the insect gradually develops all the butterfly characteristics, including wings. For some butterflies the transformation may be complete in one or two weeks. But others need many months before they are ready to leave the cocoon and fly away.
From the time it is hatched to the time it starts the pupation stage, a caterpillar or larva does little more than eat, grow larger and moult several times. This larval stage varies according to the species. Small caterpillars will complete their development in a week, but some large varieties will take up to two years or more.
When this period is over the caterpillar spins a cocoon under a leaf or even underground, and enters it to begin the transformation. Many cocoons are of silk.