Arthropod touch reception

Arthropods like insects and spiders carry a multitude of cuticular hairs on their body surface. In some spiders there are several hundreds of thousands of these hairs which are practically all innervated and mostly serving a tactile sense. Their sensory cells respond to hair deflection. The night active wandering spider Cupiennius uses its tactile sense in many ways during prey capture, copulation , building the egg sac and other behaviors. In complete darkness the first legs serves as feelers, intensely probing their immediate environment during locomotion. We used this behavior and the tactile stimuli associated with it to elucidate basic properties of the hardly understood arthropod tactile sense. Our approach was organismic. It involved the collaboration with engineers of the Vienna Technical University in appreciation of the complex mechanical phenomena to be analysed and with the possibility in mind to eventually develop biology-inspired technical tactile sensors (MEMS) for industrial applications. The knowledge now at hand clusters around three main topics: (i) Structure and distribution of tactile hairs; (ii) processes of stimulus transformation, and (iii) physiological response characteristics. (i) The density of the hairy coat covering the spider is as high as > 300 hairs / mm2 . The hairs come in widely differing shapes and mechanical properties, reflecting their particular function such as the representation of the outer boundary of the tactile space or the proprioreceptive monitoring of joint position. It is impossible, however, to assign the hairs to distinct morphological classes, which are all variants of the same bauplan and linked by intermediate forms. Remarkably, the coupling of the tips of the sensory cells (three for most hairs) to the inner hairshaft clearly differs from that know for insects, signalling significant differences in stimulus uptake. (ii) The hairshaft forms a lever with a long outer arm and a very short inner arm. Their length ratio of ca. 750:1 implies an enormous down scaling of the deflection of the hair at its tip and a concomitant increase in force. The sensory cells end very close to the hair`s axis of rotation, underlining the effect and showing that the hair is "designed" for both high sensitivity and protection from being overloaded. When stimulating tarsal tactile hairs from above, a situation common during the spider`s guide stick walk, additional effects reflect the mechanical refinement of a seemingly simple structure. With increasing load from above the point of contact of the force moves towards the hairbase due to the hairshaft`s bending. Thus the effective lever arm shortens considerably extending the sensors mechanical working range and assuring particularly high sensitivity for small deflections (ca. 5x10-5 N/). According to a Finite-Elements-Model the hair is a structure of equal strength. Within the limits of the actual material properties of spider cuticle the axial stresses due to bending do not exceed a maximum value (ca. 1x105 N/m 2 ), independent of stimulus load. A method was also developed to measure miniature forces (N) necessary to deflect the tiny tactile hairs under the microscope and using image analysis in situ in order to study the mechanical properties of the hair`s suspension. The torsional restoring moments are between 10-8 and 10-9 Nm. This is about 104 times more than in case of the trichobothria. (iii) According to their nervous response the tactile hairs are movement detectors, not or hardly responding to static hair deflection. They are suited to signal the onset or mere presence of a stimulus but not its spatial and temporal details. Threshold deflection angles are about 1. Reminding of the mammalian skin there are fast and slow adapting cells. Their responses to ramp and hold stimuli follow power functions with k-values around 0.5. Ensembles of sensilla and the application of an across fiber code seem to be necessary for the analysis of two- and threedimensional tactile patterns.