Vibratory signals and spider behavior: How do the sensory inputs from the eight legs interact in orientation?

Summary

When the wandering spiderCupiennius salei is standing with one or several of its legs on a platform that begins to vibrate, it turns toward the platform if the stimulus resembles prey-generated vibration. With application of simultaneous, uniformly intense substrate vibration, the turning angle depends on the combination of legs stimulated.

When the forelegs are stimulated the mean error angle is smaller than when the stimulus is applied to the hindlegs (Figs. 3, 4a, 6, 7); accordingly, the sensory information from the forelegs is weighted more strongly in establishing turning angle. The relation of turning angle to the stimulated leg combination can be described by the following three properties of the central nervous interaction of the sensory inputs from the eight legs. 1. The stimulus angles are multiplied by aconstant weighting factor (F) specific to each leg (1st leg,F=1.2; 2nd leg, 0.6; 3rd and 4th legs, 0.4). 2. The stimulation of an anterior leg reduces theF of all posterior legs on the same side by 0.1 (additiveipsilateral inhibition). 3. When legs are stimulated on both sides there is a reciprocalcontralateral inhibition acting exclusively from front to back. This inhibition is multiplicative, in all cases decreasingF by the factor 0.5.

During experiments in which stimuli with different amplitudes(p ₁∶p ₂=1∶3.3) or onset times (Δt = 4 ms) are produced by two vibrators, the spiders always move in the direction of the legs stimulated first or with larger amplitude (Figs. 5, 6, 7), as though only these legs were stimulated.

The turning angles resulting from airborne stimulation alone (buzzing fly) or substrate vibration alone (vibrating platform, trichobothria removed) do not differ significantly from those found with simultaneous stimulation by way of both air and substrate (P>0.05, F-test). They are distinctly smaller than the stimulus angles. The error angles increase as the stimulus site moves further posterior (Figs. 2, 8). Asymmetrical removal of trichobothria (only the 3rd and 4th right legs intact) reveals that these receptors interact in the CNS with the substrate-vibration receptors: when stimluated by way of the substrate and the air together (buzzing fly between the intact legs) the animals turn further (in the direction of the intact legs) than do animals entirely lacking trichobothria (Figs. 5a, 6, 7).