Tactile Information Processing for the Orientation Behaviour of Sand Scorpions

Arachnids including sand scorpions and spiders use their tactile sense organs, called basitarsal compound slit sensilla (BCSS), to detect their prey. The sense organs consisting of mechanoreceptors are located at or near joints in the cuticle, and they can sense a vibrational signal caused by prey movement. The nocturnal sand scorpion Paruroctonus mesaensis has a distinguished capability of finding their prey only with these tactile senses. The sand scorpions show their orientation behaviour of positioning themselves directly towards their prey and then run into the prey when there is a vibration disturbance caused by the prey. According to the biological researches (Brownell & Farley, 1979, Brownell, 1984), it is presumed that the sand scorpions respond to Rayleigh waves, surface waves of sand, to detect the direction of a vibration source and possibly longitudinal vibrations to estimate the distance. Especially, the time delay between arrival of a vibration signal at the BCSS sense organs is an important cue to determine the direction of their prey (Brownell & Farley, 1979). The central nervous system should process stimulus-locked neuron firings of the sense organs on their eight legs to induce the orientation behaviour. How the nervous system is organized to handle the orientation behaviour is still an open question. Only a few studies explain this behaviour mechanism. Stuerzl et al. (2000) introduced a neuronal mechanism to support the orientation behaviour of scorpions, and it is based on the difference of the arrival time of stimulus signals produced at sense organs on their legs. They argued that the brain of sand scorpions receives sensor signals from mechanoreceptors on their eight legs and processes an inhibition mechanism among a set of command neurons projected from the sense organs on each leg. This inhibitory interaction leads to more accumulated firings of the command neurons near the prey source and less firings at the opposite sides. The accumulated neuron firings thus form a tuning curve for a specific resource direction. Then the distribution of neuron firings can vote to determine the resource direction. In fact, arachnids have sensory projections to the central nervous system for each leg (Babu & Barth, 1989, Anton & Barth, 1993). Previously Brownell and Farley (1979) suggested that eight command neurons (or eight clusters of neurons in the brain) accumulate neuron firings from mechanoreceptors on the eight legs of scorpions, respectively and also interact each other with triad inhibitions. According to the triad inhibition hypothesis, early arrival of vibration stimulus to mechanoreceptors on a leg excites the corresponding command O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg