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 i nvolved in feeding in Drosophila melanogaster Meigen (Diptera:Drosophi lidae) are organized. The motor neurons have been visualized both by G olgi-silver impregnation and by intramuscular injection of horseradish peroxidase, and analyzed in light of the existing information on tast e sensory system and the feeding behaviour. The motor neurons have bee n broadly classified into the following types: labial nerve motor neur ons, pharyngeal nerve motor neurons, and accessory pharyngeal nerve mo tor neurons, depending on the nerve through which their axons exit. Th e arborization of all the motor neurons is confined to the suboesophag eal ganglion (SOG). All of them have predominantly ipsilateral and som e contralateral arborizations. Their dendrites predominantly occupy th e ventral region of the neuropil of the SOG and partially overlap the taste sensory projections, thereby providing an opportunity for intera ction with the taste sensory input. The pharyngeal motor neurons arbor ize more extensively in the ventral tritocerebrum, anteroventral, and mid-ventral neuropil, whereas the dendritic fields of labial motor neu rons are confined to the mid-ventral neuropil. There is a functional s egregation in motor neuron organization: cibarial muscles involved in sucking are innervated by pharyngeal motor neurons, while the probosci s muscles involved in positioning of the proboscis are innervated by l abial motor neurons. We have also observed projections of the stomodae al nerve in the tritocerebrum.