An extremely specialized organ that characterize the higher fly taxon Schizophora.
It is a sac-like organ which is evaded at the very beginning of the adult life to facilitate tearing up the puparium and escaping from the puparial locality.
In older flies only the ptilinal fissure (head structure.pct, Li trifolii frons.pct) is visible, marking the border line between ptilinum (lunule) and frons. In older adult flies the area where the ptilinum had evaded is called lunule. (head structure.pct)
The following figures of heads of Liriomyza trifolii show an only partly evaded ptilinumLi trifolii ptilinum.pct and a head with fully retracted ptilinum Li trifolii frons.pct.

Alternative form for ptilinum : ptilinal fissure.