Suboesophageal Ganglion

The suboesophageal neuropil comprises 3 fused ganglia, each innervated by nerves from the mouthparts. Primitively, these are the mandibular, maxillary and labial nerves. In Drosophila, parts of the maxillae, and especially the labium, are fused into a proboscis, which terminates in a richly innervated structure specialized for taste, the labellum. There are no mandibles and there is no hypopharynx.

The names used for suboesophageal nerves in Drosophila depart from the standard terminology. Dethier identifies a labrofrontal and labial nerve in the fly Phormia. Miller (in Demerec) names three major nerves. These are, from anterior to posterior, the pharyngeal nerve, the accessory pharyngeal nerve, and the labial nerve. The pharyngeal and accessory pharyngeal nerves are likely to be, partially homologous to the maxillary nerve of a basal taxon such as the cockroach. The labrofrontal and labial nerves are thought to be equivalent to the maxillary and labral nerves.

Descriptions by Shanbag and Singh (1992: Cell and Tissue Res, 267:273-282) indicate that all the labellar sensory axons are carried to the suboesophageal ganglion via the labial nerve. These authors suggest that six discrete glomerular-like neuropils are innervated by labellar receptor terminals. These neuropils are: ventro-lateral glomeruli flanking ventro-medial glomeruli, a pair of dorso-anterior neuropils, and a medial neuropil between them. The Golgi images reflect, only to some degree, these divisions. Golgi impregnations also show evidence of stratified neuropils associated with the labrofrontal nerve and the pharyngeal nerves. The peripheral origin of these receptor terminals is, as yet, unknown but chemoreceptors are likely to arise from the pharynx, and both chemo- and mechanoreceptors from the maxillary palp and the lacinia (the vestigial maxillae).

There are probably no more than 22 muscle pairs associated with the proboscis. Their motor neurons arise in the suboesophageal ganglia, and their axons are carried by labial and labrofrontal nerves.

Suboesophageal neuropils innervated by receptor terminals of the mouthparts, reaching the suboesophageal ganglion through the pharyngeal, accessory pharyngeal, labrofrontal and labral nerves. Click on squares for related images. Equivalent levels in the frontal silver series are: Y= -6 to -7.5.

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