Pupa is the stage of final growth before a fly becomes an adult. The pupae of mosquitoes and some other kinds of flies that develop in water are active swimmers. Most pupae that live on land remain quiet. The larvae of some flies build a strong oval-shaped case called a puparium around their bodies. Black fly larvae spin a cocoon for protection. Inside, the larva gradually loses its wormlike look and takes on the shape of the adult fly. Then the adult fly bursts one end of the pupal case or splits the pupal skin down the back and crawls out. The pupal stage of a house fly lasts from three to six days in hot weather, and longer in cool weather. The length of time varies among the different species.