An ultrastructural study of the tips of four classical bimodal sensilla with one mechanosensitive and several chemosensitive receptor cells

Abstract

1. The ultrastructure of the tips of four insect sensory hairs was examined in serial section. They are characterized as follows: a. an apical pore opening in the dendrite-containing canal (dC); b. a double lumen in the hair shaft; c. multiple and bimodal innervation. 2. Variation was found in the form and cross-sectional area of the apical pores. 3. The two lumina, one of which contains the dendrites, do not appear to communicate with one another; openings were not found. 4. A tubular body in one of the receptor cells, whose dendrite terminates at the base of the hair shaft, identified it as mechanosensitive. The dendrites of the other (2–5) receptor cells lack apparent modal-specific structures and terminate at somewhat varying levels below the apical pore. 5. The three-layered cuticle (endocuticle, interstitial layer, exocuticle) of the hair shaft is traversed by narrow perforations which, as demonstrated by tracer experiments, are penetrated by small silver particles. Although the interstitial layer is the thinnest layer at the base of the hair, it becomes the most prominent layer at the tip of the hair. 6. An intensely stained substance coats part or all of the tip of the hair. 7. The ultrastructure of the tips of these sensory hairs, two of which have been identified electrophysiologically as both mechanoreceptive and chemoreceptive, is discussed in relation to their function.