A head vesicle or bladder-like expansion of the head of a fly emerging from a pupa.

There is a special structure which is used to help an emerging adult fly break free of the puparium. This structure is an inflatable membranous sac called the ptilinum which protrudes from the face, above the antennae.

The inflation of the ptilinum (using fluid hemolymph, rather than air) creates pressure along the line of weakness in the puparium (pupa casing or resting stage), which then bursts open along the seam to allow an adult fly to escape.