Spider joint hair sensilla: adaptation to proprioreceptive stimulation
Adding to previous efforts towards a better understanding of the remarkable diversity of spider mechanosensitive hair sensilla, this study examines hairs of Cupiennius salei most likely serving a proprioreceptive function. At the tibia–metatarsus joint of all walking legs, there are two opposing groups of hairs ventrally on the tibia (20 hairs) and metatarsus (75 hairs), respectively. These hairs deflect each other when the joint flexes during locomotion, reversibly interlocking by microtrichs on their hair shafts. The torque resisting the hair deflection into the direction of natural stimulation is smaller by up to two powers of ten than that for the other directions. The torsional restoring constant S of the hair suspension is about 10−10 Nm rad−1 in the preferred direction, up to a hair deflection angle of 30° (mean of natural deflection angles). Joint movements were imposed in ranges and at rates measured in walking spiders and sensory action potentials recorded. Within the natural step frequencies (0.3–3 Hz) the rate of action potentials follows the velocity of hair deflection. All findings point to the morphological, mechanical, and physiological adaptedness of the joint hair sensilla to their proprioreceptive stimulation during locomotion.