A Spider’s Sense of Touch: What to Do with Myriads of Tactile Hairs?

Abstract

Some spiders are densely covered by an intriguingly large number of mechanoreceptive hairs on their exoskeleton, the wandering spider Cupiennius salei being the main example examined here. All of these hairs represent first-order lever arms, whose deflection triggers nervous impulses in the sensory cells ending at their base. Their sensitivities differ greatly. By far the most sensitive hairs are the trichobothria. They respond to the frictional forces contained in the slightest movement of air. The large majority of the hairs, however, are much less sensitive. They represent touch receptors, including proprioreceptive hairs, which monitor the movements of joints. The mechanical properties of the hairs such as their resistance to deflection and their directional properties vary as do details of their morphology (like structure of socket and outer hair shaft, length, angle of hair insertion). Although such differences are graduated, the distributions of some main morphological types form stereotyped patterns on the spider exoskeleton. The functional significance of these patterns in regard to particular behaviors is largely unknown. The enormous versatility of the tactile sense nevertheless clearly emerges from the analysis of prominent examples of hairs and their relation to behavior. Like in other senses, stimulus transformation turns out to be a most important evolutionary playground for biologically applied physics and to a large extent to be responsible for the fine-tuned match between the sensor and the adequate stimulus patterns which it is meant to receive for different behavioral tasks.