House flies and other Schizophoran flies form their pupae inside the cuticle of the final larval stage. The adult develops inside this hard cuticle and must escape its confines when fully formed. Most insects digest a straight line of cuticle from the back of the head through the thorax. They escape by pushing against the cuticle until it breaks, creating a straight line opening along the dorsum to facilitate emergence.
House flies have another mechanism of escape. The head of a house fly has an eversible sac called the ptilinum. The emerging fly inflates the ptilinum to apply force to the front of its puparium. This causes the end of the puparium to pop off, creating a round emergence hole for the emerging adult, a larger and cleaner opening. Once it has emerged, the ptilinum deflates, realigns to the fly head, and hardens to form a rigid part of the head that can no longer evert.