Comparison of the ultrastructure of stimulated and unstimulated mechanoreceptors in the taste hairs of the blowfly Phaenicia serricata.

The structure of mechanoreceptors at the base of labellar taste hairs of the blowfly Phaenicia serricata were examined in stimulated and unstimulated conditions (i.e. with the hair bent or unbent). Physiological recordings from the mechano-receptor showed that the receptors responded when the hair is bent dorsally or ventrally and when the hair is bent at extreme angles. These conditions are the same as those placed on hairs in the anatomical studies. Bending the hair toward the ventral labellar surface caused the hair base to compress and indent the tubular body and its surrounding membrane and sheath at the distal end of the mechanoreceptor dendrite. In compressed tubular bodies, microtubules oriented longitudinally were bent and separated a greater distance from each other. Separation as much as 70 nm was observed in compressed tubular bodies as compared with a maximum of 26 nm between micro-tubules in tubular bodies of unbent hairs. The dense amorphous material between microtubules of compressed tubular bodies formed prominent bridges 18 nm thick connecting the microtubules at intervals of 48-74 nm. Thin 10 nm filaments were also evident in the spaces between microtubules. When the hair was bent toward the proximal end of the proboscis, the tip of the tubular body was bent about 15 degrees. The tubular body appears to function as a firm but resilient structure over which the dendritic membrane can be stretched during mechanostimulation. Comparison of morphology of bent and unbent hairs suggests a means by which mechanical force from the movement of the hair is transferred to the receptors by structures in the hair socket region. No differences were found in ciliary structures of stimulated and unstimulated receptors.