By stage 15, the presumptive midgut (mg) has closed ventrally and dorsally. A constriction in its posterior third gives the midgut a heart-like shape. Anteriorly and posteriorly, the midgut lumen has become continuous with the lumen of the presumptive proventriculus (pv) and hindgut (hg), respectively. The different parts of the foregut have progressed in their differentiation (see Skaer, this volume). The pharynx (ph) forms a wide, vertically flattened structure; the esophagus (es) represents a round tube that is bent like an S. A circular constriction separates the presumptive proventriculus into an anterior and posterior chamber. The posterior chamber opens into the midgut. During later development (stage 16), the anterior chamber invaginates into the posterior chamber. The salivary duct (sd) has formed. Its external opening has reached the ventral lip of the stomodeum.

Stage 16 is characterized by three circular midgut constrictions that subdivide the midgut into four chambers (1-4), Four evaginations, the gastric caeca (gc), develop from the first midgut chamber. During later development, all four chambers will constrict further, resulting in the long, highly convoluted tube representing the larval midgut. Anteriorly, parts of the gnathal segments have invaginated through the stomodeum, forming the atrium (at) of the foregut. The drawing at the bottom depicts the fully developed alimentary tract of a stage-17 embryo. (ap) Anal pads; (mp) Malpighian tubules; (vm) visceral musculature.