part of adult hindgut adult posterior hindgut Posterior-most region of the adult hindgut. It consists of a broadly expanded rectal sac (rectal ampulla) that tapers posteriorly to the narrow rectum proper, terminating at the anus. Protruding from the walls into the lumen of the rectal sac are two pairs of conical rectal papillae. adult rectum FBbt:00003144 posterior intestine fly_anatomy.ontology rectum