Arthropod touch reception: structure and mechanics of the basal part of a spider tactile hair.

Our previous studies on spider tactile hairs concentrated on the mechanical behavior of the hair shaft and the electrophysiological properties of the sensory cells. Here we focus on the structure and mechanical properties of the coupling of the hair shaft and the sensory terminals. 1. The functional "design" of the coupling provides for a combination of high sensitivity and protection against mechanical damage and overstimulation. The dendritic sheath is not directly coupled to the hair shaft. Rather, there is "terminal connecting material" between the dendrites and the hair shaft. 2. The hair shaft forms a first-order lever. Its acentric axis of rotation is located ca.3.5 microm from its inner end. Displacement of the hair tip is scaled down by a factor of ca.750:1, not even considering the outer hair shaft's bending. 3. At threshold the dendrite sheath displacement is ca. 0.05 microm by forces in the order of 0.4-4x10(-6) N. 4. The hair shaft bends within the socket even before contacting it. The elasticities representing its suspension and bending in the socket can be described quantitatively by measuring the hair's restoring moments (range: 10(-9) Nm) and bending at different degrees of deflection.