part of develops from adult thorax adult tracheal air sac dorsal air sac fly_anatomy.ontology FBbt:00007132 Large, multi-lobed air sac of the dorsal adult thorax. It arises from an outgrowth of the lateral trunk between the branches to the second and third legs. It comprises the postnotal, lateroscutal, medioscutal, scutellar, pleural and notopleural air sacs. dorsal air sac primordium