Pupa

The pupa is the third of four life stages, egg, larva, pupa and adult, in insects with complete metamorphosis, a group called the Holometabola that includes beetles, flies, butterflies, moths, bees, wasps and ants. It forms when a fully grown larva molts for the last time. A pupa does not feed and usually moves little, so it is often hidden or protected: many moth caterpillars spin a silk cocoon, a butterfly pupa, the chrysalis, hangs in the open inside a hardened skin, and many fly pupae develop within the toughened skin of the last larval stage, called a puparium.

The stillness outside hides intense activity within. Many larval tissues are broken down, while clusters of cells set aside earlier, called imaginal discs in flies, grow into wings, legs, eyes and other adult parts. Hormones steer the change: the steroid ecdysone triggers each molt, and the loss of juvenile hormone near the end of larval life lets the body switch to the pupal and then the adult program. Finally the adult splits the pupal case and emerges.

Dividing life between a feeding larva and a reproducing adult, joined by the pupa, is thought to help explain why these insects are so diverse. Grasshoppers and true bugs have no pupa; their nymphs grow gradually into adults.