ptilinum
A bladder-like pouch on the head of schizophoran flies that by expanding enables the fly to emerge from its puparium.
As for the structure of the ptilinum itself, Strickland (1953) reported that the ptilina of Conopidae 'are voluminous and represent the most rugged type of ptilinum which we have encountered'.
Where it comes from
Borrowed from New Latin ptilīnum, itself an adaptation of the French ptiline.
Adapted from the Wiktionary entry ptilinum (CC BY-SA 4.0).