part of develops from adult head capsule ptilinum Membranous sac that is part of the front of the head capsule. It is everted at eclosion and serves to break open the operculum at the end of the puparium. Following eclosion, it is withdrawn into the head where it collapses and becomes sclerotized, externally leaving the ptilinal suture. fly_anatomy.ontology FBbt:00004483 region of integument presumptive ptilinum