Arthropod touch reception: spider hair sensilla as rapid touch detectors

Wandering spiders like Cupiennius salei are densely covered by tactile hairs. In darkness Cupiennius uses its front legs as tactile feelers. We selected easily identifiable hairs on the tarsus and metatarsus which are stimulated during this behavior to study tactile hair properties. Both the mechanical and electrophysiological hair properties are largely independent of the direction of hair displacement. Restoring torques measure 10 ^-9 to 10 ^-8 Nm. The torsional restoring constant S changes non-linearly with deflection angle. It is of the order of 10 ^-8 Nm/rad, which is about 10,000 times larger than for trichobothria. Angular thresholds for the generation of action potentials are ca.1°. Electrophysiology reveals a slow and a fast sensory cell, differing in adaptation time. Both cells are movement detectors mainly responding to the dynamic phase (velocity) of a stimulus. When applying behaviorally relevant stimulus velocities (up to 11 cm s ^-1) threshold deflection for the elicitation of action potentials and maximum response frequency are reached as early as 1.2 ms after stimulus onset and followed by a rapid decline of impulse frequency. Obviously these hairs inform the spider on the mere presence of a stimulus but not on details of its time-course and spatial orientation.