The epithelium of the larval gut degenerates completely and is replaced during the prepupal and early pupal period by imaginal cells (Robertson 1936; Bodenstein 1950). Precursors of the imaginal gut (drawn in dark blue or red) are integrated into the larval gut epithelium (drawn in light blue or red, respectively); they can already be identified at early larval stages (Hartenstein and Jan 1992). In the third instar larva, precursors of the adult foregut are concentrated in a so-called imaginal ring (imr) located in the proventriculus (pv), The pharynx is replaced by cells of the labial imaginal discs (id), The adult salivary duct (sd) and glands (sg) originate from a pair of imaginal rings (imr) surrounding the proximal tips of the larval salivary glands. The midgut is replaced by midgut histoblasts (mhi) scattered throughout the larval midgut epithelium (mg). Precursors of the adult hindgut (hg) lie in an imaginal ring located at the junction between larval hindgut and midgut; the posterior hindgut is replaced by cells originating in the genital disc (gd). The larval Malpighian tubules (mp) persist into the adult.

At the end of the prepupal stage (12 hr apf), most of the larval gut has been replaced by imaginal cells. The esophagus (es) forms a straight, narrow tube. The larval salivary gland is still in place; it degenerates at 15 hr apf and will be replaced by imaginal cells in the first half of the pupal period. The primordium of the adult midgut forms a cylindrical chamber that encloses the remnants of the larval midgut (yellow body). The hindgut has been partly replaced.

The alimentary tract of the adult fly is shown on page 35. Components that had not been present in the larva are the crop (cr), an unpaired outgrowth of the esophagus, and the rectal ampulla (amp), a specialization of the posterior hindgut. Conversely, the gastric caeca (gc), out-growths of the anterior larval midgut, are not replaced in the adult gut. (air) Air bubble; (ph) pharynx (also called cibarium in the adult); (pv) proventriculus (also called cardia in the adult).