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The organisation of the tritocerebrum of Drosophila melanogaster was studied by Bodian-Protargol reduced silver staining, Golgi-silver impregnation, horseradish peroxidase (HRP), and cobalt-chloride labelling of neurones and transmission electron microscopy. Nerve fibres of six categories were found to project to the tritocerebrum. (1 and 2) The sensory fibres from the internal mouthpart sensilla known to course along pharyngeal and accessory pharyngeal nerves were found to project in mainly two tiers, in the tritocerebrum. (3) Stomodaeal nerve fibres also project along the pharyngeal nerve, to the tritocerebrum. (4) Cells of the pars intercerebralis (PI) project along the median bundle and arborise in the tritocerebrum. HRP labelling and subsequent examination by transmission electron microscopy indicated their neurosecretory nature. (5 and 6) Two tracts of ascending fibres, designated as dorsal and ventral ascending tracts, were found to project to the tritocerebrum. Some of the sensory fibres from the labial nerve extend close to the sensory projections of the tritocerebrum, suggesting a possible convergence of the two sensory inputs. In the tritocerebrum, the sensory input, the stomodaeal input, the neurosecretory fibres of PI, and the ascending fibres were found to have overlapping fields, suggesting mutual interaction. The medial subesophageal ganglion and the tritocerebrum may interact through the ventral ascending tract.

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Cobalt fills from small, defined regions of the antenna in D. melanogaster show that the three types of sensilla on the third segment, the flagellum, and a fourth sensillum located in the arista, project into the glomeruli of the antennal lobe. We have identified 19 glomeruli in each lobe, according to their location, shape, and size. At least ten of these represent major projection areas of flagellar or aristal sensilla. The large majority of glomeruli is innervated from both antennae, but a small group of five receive exclusively ipsilateral input. A particular sensory fiber appears to terminate only in one specific glomerulus, either in the ipsilateral or in both lobes. Fills from flagellar regions bearing a single type of sensillum, yield a specific pattern of glomeruli containing stained terminals. Aristal projections remain strictly ipsilateral, whereas those from the other sensilla consist of an ipsilateral and a bilateral component. When filling from different points in an area bearing one type of sensillum, similar projections are produced, suggesting that projection patterns observed reflect predominantly the type of sensillum rather than its location on the flagellum. Accordingly, individual glomeruli might represent functional units, each receiving antennal input in a characteristic combination.