Vibration sensitivity and a computational theory for prey-localizing behavior in 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 (200</<500 Hz) to be biologically detectable. Eight acceleration-sensitive receptors (slit sensilla) at the tips of the scorpion's circularly arranged legs detect surface waves generated by prey movements or juddering signals from other scorpions. From this input alone, direction of the disturbance source is 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 is most JUcely the cue that determines target location. Given the simplicity of this sensory system, 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 vibration receptor and inhibition from the triad of receptors opposite to it in the eight-element array. Input from a passing surface wave opens and closes a time widow, the width of which determines the firing probability of second-order neurons. Target direction is encoded as the relative excitation of these neurons, and stochastic optimization tunes the relative strengths of excitatory and inhibitory inputs for accuracy of response. The excellent agreement between predictions of the model and observed behavior of sand scorpions confirms a simple theory for computational mapping of surface vibration space.