life stage of some insects undergoing transformation

A pupa (plural: pupae or pupas) is part of becoming an adult for some insects, like some butterflies, moths, beetles, flies, and bees. The pupal stage comes after being a larva; the next stage is adulthood.

A Peacock Butterfly (Inachis io) larva (caterpillar) becoming a pupa. Time: Nine minutes
An adult Peacock Butterfly

The pupa stage of a butterfly, called a chrysalis (plural "chrysalids"), is a well-known example of a pupa because they are often found in the open.