During the quiescent fourth larval instar the larva of Drosophila becomes loosened from the puparium and takes on the form of a pupa. A large gas bubble is formed within the body during this prepupal period. At the end of this stadium, which lasts 11 1/2 hours at 25°C., it pupates, shedding the prepupal cuticle. The gas bubble is used as a mechanism to separate the cuticie from its hypodermis. Practically all of the tissues undergo histolysis and are replaced by imaginal structures derived from specialized cells. Some of the muscles persist longer than the general mass and perform special functions in the pupa. The sequence of histolytic and histogenetic changes is accurately timed at a constant temperature.