ORGANIZATION OF MOTOR-NEURONS INNERVATING THE PROBOSCIS MUSCULATURE IN DROSOPHILA-MELANOGASTER MEIGEN (DIPTERA, DROSOPHILIDAE)

The present study addresses the question as to how the motor neurons involved in feeding in Drosophila melanogaster Meigen (Diptera:Drosophilidae) are organized. The motor neurons have been visualized both by Golgi-silver impregnation and by intramuscular injection of horseradish peroxidase, and analyzed in light of the existing information on taste sensory system and the feeding behaviour. The motor neurons have been broadly classified into the following types: labial nerve motor neurons, pharyngeal nerve motor neurons, and accessory pharyngeal nerve motor neurons, depending on the nerve through which their axons exit. The arborization of all the motor neurons is confined to the suboesophageal ganglion (SOG). All of them have predominantly ipsilateral and some contralateral arborizations. Their dendrites predominantly occupy the ventral region of the neuropil of the SOG and partially overlap the taste sensory projections, thereby providing an opportunity for interaction with the taste sensory input. The pharyngeal motor neurons arborize more extensively in the ventral tritocerebrum, anteroventral, and mid-ventral neuropil, whereas the dendritic fields of labial motor neurons are confined to the mid-ventral neuropil. There is a functional segregation in motor neuron organization: cibarial muscles involved in sucking are innervated by pharyngeal motor neurons, while the proboscis muscles involved in positioning of the proboscis are innervated by labial motor neurons. We have also observed projections of the stomodaeal nerve in the tritocerebrum.