Fine structure and primary sensory projections of sensilla on the maxillary palp of Drosophila melanogaster Meigen (Diptera : Drosophilidae)

Three types of hairs were identified on the maxillary palp of Drosophila melanogaster Meigen (Diptera : Drosophilidae): (i) single-walled, multiporous sensilla basiconica, which constitute 75% of the innervated hairs; (ii) thick walled non-porous sensilla trichodea, which make up the remaining 25% of the innervated hairs; and (iii) numerous spinules, which are un-innervated. These sensilla basiconica uniformly contain 2 bipolar sense cells, whereas sensilla trichodea have a single dendrite with a tubular body at the base of each hair. A majority of the sensilla basiconica is located on the distal half of the dorsal surface, whereas sensilla trichodea are positioned on the tip and entire ventrolateral ridge of the palp. Approximately 125 axons of the sense cells join to form a single nerve. The structure of sensilla basiconica and sensilla trichodea suggests that they are olfactory and mechanosensory respectively. The contact chemoreceptors (gustatory sensilla) are conspicuously absent on the maxillary palp.Golgi silver impregnations and cobalt fills show that the primary sensory fibres from sensilla trichodea and sensilla basiconica on the maxillary palp project in the posterior suboesophageal ganglion (SOG) and the antennal lobe respectively. A single fibre projects separately either in the SOG or in the antennal lobe. In the antennal lobe, the input received from sensilla basiconica is usually bilateral and at least 5 glomeruli are innervated symmetrically on either side from both the palps.This study suggests that the sensory neurons are capable of making selective projections in the specific regions of the brain. Accordingly, the fibres from a sensillum project to the brain with respect to their functions and the individual glomeruli represent functional units of the brain, receiving inputs in a characteristic combination.