A pupa is an immature insect, in the stage of development just before adulthood. A butterfly or moth pupa is known as a "chrysalis."
Some kinds of insects go through dramatic, distinct stages as they grow, otherwise known as metamorphosis. For these insects, the egg and larval stages are followed by a period of time as a pupa that can last weeks, or even years, depending on the insect species and what the temperature is. At the end of their time as a pupa, insects break open their shell or case and emerge, fully grown.