Sensilla on the tarsal segments and mouthparts of adult Drosophila melanogaster meigen (Diptera : Drosophilidae)

Abstract

Sensilla on the tarsal segments of legs and on the external and internal mouthparts of adult Drosophila melanogaster Meigen (Diptera : Drosophilidae) were studied by light and electron microscopy. Tarsi are invested with bracteate and bractless bristles. Ethanolic silver nitrate stain distinguishes dendrite-containing bristles from other hairs on the tarsi. Stained bristles are bractless. Males have more silver-staining bristles than females on the I st 4 tarsal segments of prothoracic legs. There are 33 – 34 taste bristles on each half of the labellum. Many of them are 2-pronged. Both prongs have a pore at the tip, but chemosensory dendrites enter only in one of them. The taste bristles in the 2 medial rows on each half of the labellum are predominantly associated with 4 chemosensory neurons and the peripheral bristles usually contain 2 chemosensory neurons each. Five groups of paired sense organs are known to exist on the internal mouthparts. Amongst 9 sensilla present in the labral sense organ, electron microscopy shows that sensilla numbers 1 – 6 have one mechanosensory neuron each. Sensillum number 7 has 8 neurons with dendrites arranged in 3 groups: 2 triplets and a pair. Sensilla 8 and 9 have 2 neurons each. The ventral cibarial sense organ (VCSO) has 2 sensilla, one with 2 and the other with 4 neurons. Rows of hollow satellite bristles forming 2 sensory systems, dorsal and ventral to the VCSO have one mechanosensory neuron each. The dorsal cibarial sense organ has 2 sensilla on either side of the midline, each containing 3 neurons. The paper describes the fine structural morphology of taste receptors and enumerates axons of all the prominent nerves of mouthparts at different levels. An external nerve loop connecting directly 2 regions of the brain, the tritocerebrum and suboesophageal ganglion, is reported.