BROWNELL, P.H.*; VAN HEMMEN, J.L: Vibration Sensitivity and Prey-localizing Behavior of Sand Scorpions
As burrowing, nocturnal predators of small arthropods, sand scorpions have evolved exquisite sensitivity to vibrational information that comes to them through the substrate they live on –dry sand. Over distances of a few decimeters, sand conducts low velocity (~50 m/sec) surface (Rayleigh) waves of sufficient amplitude and bandwidth (fmax ~350 Hz) to be biologically detectable. Eight acceleration-sensitive receptors (slit sensilla) at the tips of the scorpion’s circularly arranged legs detect surface vibrations generated by prey movements or ‘juddering’ signals from other scorpions. From this input alone, target direction is accurately calculated up to 20 cm distance. By ablating slit sensilla in various combinations on the eight legs, the contribution each makes in computing target location can be assessed. Other behavioral experiments show that differential timing of surface wave arrival at each sensor, and not relative amplitude of stimulation, is the cue that determines target location. A computational theory to account for wave source localization has been developed using a population of second-order neurons, each receiving excitatory input from one sensor and inhibition from a triad of sensors opposite to it in a circular, eight-element array. Input from a passing surface wave opens a time window whose width determines the firing probability of second-order neurons. Stochastic resonance (optimization) tunes the relative strengths of excitatory and inhibitory inputs to second-order neurons, and target direction is encoded as the relative excitation of cells within the population. The excellent agreement between theory and behavioral observations confirms the mechanistic simplicity of the sand scorpion’s sensory system for computational mapping of vibration source location.
