Long, smooth hair sensilla on the spider leg coxa: Sensory physiology, central projection pattern, and proprioceptive function (Arachnida, Araneida)

Abstract

Rows of long, smooth hair sensilla situated on both sides of the leg coxae were examined in the spider Cupiennius salei (Ctenidae). The hair shafts point into the space between adjacent legs and are deflected when the hairs of one coxa touch the cuticle of the neighboring coxa. 1. Unlike the serrated hair shafts of the ubiquitous tactile and chemosensitive setae of spiders, these hairs are entirely smooth. At their base they are articulated in a socket with an asymmetrical groove that determines the direction of hair deflection. Hair shafts are up to 1000 μm long. The exact grouping of smooth hairs in rows is typical of the coxae for each pair of legs. 2. Unlike the other, multiply innervated cuticular sensilla of spiders, smooth hairs are supplied by only a single mechanosensitive neuron. This is confirmed by electrophysiological recordings from single hairs. Threshold deflection to elicit a spike response lies near 1°. The response to maintained, step-like stimuli declines rapidly. 3. All central endings of these hair receptors in the fused segmental ganglia are confined to dorsal neuropil of the ipsilateral neuromere. The specific arborization pattern resembles an elongated, three-pronged fork with a long central prong. Topography, natural stimulus situation, and the “phasic” response characteristic of smooth hairs suggest that spiders use these sensilla to monitor the relative distance between leg coxae during locomotion.