How does the ptilinum help some adult flies emerge from the puparium?

In the more advanced flies belonging to the group Schizophora, the ptilinum is a specialized, inflatable membranous sac located on the front of the head. To emerge from the puparium—the rigid outer shell formed from the hardened larval skin—the adult fly inflates this sac to exert pressure against the case. This action assists in rupturing the puparium, allowing the fly to escape. Once the fly has successfully emerged, the ptilinum shrivels away, leaving behind a characteristic horseshoe-shaped groove known as the ptilinal suture.12