A Pupa (plural: pupae or pupas) is the life stage of some insects undergoing transformation. The pupal stage only occurs in insect that undergo a complete metamorphosis. It follows the larval stage and preceeds adulthood (imago). Pupae are sessile and have a hard protective coating. During the time of pupation, the adult structures of the insect are formed whilst the larval structures are broken down.
There are two ways in which an insect emerges from the pupa. One is the splitting of the pupal skin. When doing so, the insect chews its way through the skin. The other way is when the insect secrets a fluid that softens the cocoon to a degree where the insect can leave the pupa. The whole process of pupation is controlled by the insects hormons.
The chrysalis of a butterfly and the cocoon of a moth (Lepidoptera) are common examples of pupae. The pupal stage of the butterfly (cocoon) is most commonly recognized. In these cases the caterpillar is transformed into the adult stage of the insect under the protective covering of the cocoon. Cocoons can be fouund on bushes where they hang from twigs or are hidden in rolled leaves. Some cocoons can be found in underground litter or in burrows. There are insects that spend the entire winter in the pupal stage.