Atlas of olfactory organs of Drosophila melanogaster - 1. Types, external organization, innervation and distribution of olfactory sensilla

The third antennal segment (funiculus) and the maxillary palp are the main and accessory olfactory sense organs of Drosophila melanogaster. Cryofixed antennae and palps revealed a superior preservation of the sensory dendrite s and other cellular details as compared to conventional chemical fixation. Extensive cross-section series through funiculus and palp were studied in order to obtain as complete an evaluation as possible of the sensillar comp lement on these appendages. About 75% of all sensilla on the male and femal e funiculus were individually studied and their position on the antennal su rface mapped. Dimensions of the cuticular apparatus of the various types of sensilla are provided as well as the number of innervating receptor neuron s. Particular attention has been paid to the cuticular gores important for stimulus transport and to the sensory dendrites. On the funiculus surface, all sensilla have multiple wall pores: sensilla (s.) trichodea and s. basic onica are single-walled, a. coeloconica are double-walled. The distribution of s. trichodea and s. basiconica follows opposing gradients along a diago nal axis parallel to the axis of the arista from proximo-medial to disto-la teral. In this disto-lateral direction the density of a. trichodea increase s while that of the s. basiconica decreases. S. trichodea occur in three su btypes with one, two or three receptor neurons. Basiconic sensilla can be s ubdivided into three subtypes of large s, basiconica (with two or four rece ptor neurons), three subtypes of thin s. basiconica (with mostly two, rarel y four neurons), and one subtype of small s. basiconica with two receptor n eurons. Large s. basiconica occur only in the most proximal region (the 'LB -zone'); thin s. basiconica are most common in a belt that borders the LB-z one distally, while small s. basiconica are most numerous even further dist ally along the mentioned diagonal axis in between the s. trichodea. S. inte rmedia are single-walled, multiporous sensilla which combine features of s. trichodea and s. basiconica; they are found in two subtypes with two or th ree receptor neurons, in the same region where s. trichodea abound. The s. coeloconica are irregularly distributed over the funicular surface, and occ ur in two subtypes with two or three receptor neurons. Sexual dimorphism on the antenna is moderate, the female funiculus is a bit longer and carries a larger number of small s. basiconica and large s. basiconica of the LB-I subtype; the male funiculus, however, has more s. trichodea than the female . On the maxillary palp, besides mechanoreceptive s, chaetica, there are on ly s. basiconica with two receptor neurons. According to the fine structure of their sensory dendrites, three subtypes can be discriminated which are distributed in a random pattern. The functional significance of the describ ed structures and distribution awaits future immunocytochemical and electro physiological experiments. (C) 2000 Elsevier Science Ltd. All rights reserv ed.