During early stage 12, the primordia of the stomatogastric nervous system (stp) evaginate as three unpaired, linearly arranged pockets from the roof of the esophagus primordium (es; Poulson 1950). During stage 13, these pockets pinch off the esophagus epithelium and form closed vesicles with an inner lumen; up until stage 14, they stay in contact with the esophagus and move posteriorly. During late stage 14, the primordia of the stomatogastric nervous system lose their epithelial characteristics. Cells that were included in the anterior-most pocket migrate anteriorly (Campos-Ortega and Hartenstein 1985) and end up as the frontal ganglion (fg), which in Drosophila is a paired structure attached to the anterior surface of the brain (br). The middle and posterior pockets give rise to at least two different neural structures: the hypocerebral ganglion (hcg) and a group of neurons that migrate along the esophagus and proventriculus (pv) and might correspond to the ventricular ganglion (vgl) of other insects. At stage 17, many neurons of the stomatogastric nervous system have differentiated and formed axons. Neurons of the frontal ganglion project axons to the brain and to the pharynx musculature (frontal nerve,fn). Axons of the hypocerebral ganglion send their axons to the frontal ganglion (recurrent nerve, rn).

The ring gland (rgl) encircles the anterior tip of the dorsal vessel (dv). It is a complex structure that includes three different endocrine organs (Bodenstein 1950; Poulson 1950): the corpus allatum (produces juvenile hormone), the thoracic glands (produce ecdysone; see Riddiford, this volume), and the corpora cardiaca. The different components of the ring gland are of diverse origin. The corpora cardiaca (ventral part of ring gland) probably derive from the esophagus primordium. The corpus allatum seems to originate from a bilateral group of mesoderm cells that converge during dorsal closure above the tip of the dorsal vessel (A.E. Rugendorff et al., in prep.). The thoracic gland forms the lateral parts of the ring gland. It derives from the dorsal part of the prothorax (V. Hartenstein, unpubl.; A.E. Rugendorff et al., in prep.).

The postembryonic development of the Drosophila stomatogastric nervous system has not been studied in depth. The thoracic gland degenerates after metamorphosis; the corpus allatum, corpora cardiaca, and hypocerebral ganglion form the ring gland of the adult. (mg) Midgut; (ph) pharynx; (sec) supraesophageal commissure.