Key events: gastric caecae are formed, somatic musculature becomes visible

Stage 16 lasts for about 3 hours (13-16 h). Stage 16 begins when the intersegmental grooves can be distinguished at mid-dorsal levels and ends when the dorsal ridge has completely overgrown the tip of the clypeolabrum, which is thereby included in the atrium. During this stage several constrictions appear in the midgut. Shortening of the ventral nerve cord occurs to bring its posterior tip to about 60% egg length (0% egg length = posterior pole).

The final stages of cytodifferentiation take place during stage 16. Secretion of cuticle has begun in the epidermis, in the tracheal tree, and in fore- and hindgut. The pharyngeal ridges appear. The fat body can be seen extending laterally from the gonads to anterior thoracic levels. The larval pattern of somatic musculature and sensory organs becomes distinguishable. The heart has also formed at a middorsal position.

The gastric caecae appear. The proventriculus is well differentiated and demarcates the junction of fore- and midgut. The oesophagus, with secreted cuticle in its lumen, inserts deeply into the proventriculus to make contact with the epithelium of the midgut, which is itself devoid of cuticle; this then folds back caudally to surround the oesophagus, finally becoming the anterior portion of the midgut, the so-called ventriculus. In the ventriculus, four evaginations appear, which are called gastric caecae.