The ptilinum is an eversible pouch on the head, above the base of the

antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

in schizophoran flies (a section of muscomorphan and cyclorrhaphan flies). It is used to force off the end of the

puparium A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their ...

in order for the fly to emerge, and after this inflation at emergence, the ptilinum collapses back inside the head, marked thereafter only by the ptilinal suture or frontal suture (which defines the aperture through which it everts).

References

Bugguide.net. ptilinum, ptilinal suture
Insect anatomy {{insect-anatomy-stub