The larvae of Drosophila lack a crop, a food storage sack in the adult and in most other insects. Drosophila larvae are immersed in their food, can feed constantly and have no need to store food. The foregut of the Drosophila larva consists of a pharynx, an esophagus and proventriculus. On the lumen side, epithelial cells secrete a cuticle. On the hemolymph side, the epithelium is surrounded by muscles. At the junction with the midgut, the foregut fits inside the midgut and folds back at the junction to create a proventriculus with two foregut layers, one facing the lumen and one facing the midgut.
Adult Drosophila are powered by sugar secretions and feed intermittently. They need a crop to store food and the foregut arrangement in the larva is unsuitable. At the molt to pupa, the muscle cells surrounding the foregut are lost. A new, different, foregut begins to grow from both anterior and posterior ends. A pharynx, better suited to pumping fluid, replaces the larval structure, better suited to pushing solid food. After the foregut tube of the adult forms, the crop begins as a growth on the foregut ventrally that remains attached to the esophagus by a narrow stalk. At the posterior end, the crop attaches to the new proventriculus. The narrow anterior stalk expands so that the bulkier crop of the adult will be located in the abdomen, leaving more room in the thorax for flight muscles. The walls of the crop are folded many times such that the crop in the pupa occupies minimal space with no air inside. Once the adult emerges, the crop will greatly distend to its full size within the fly.