A visually induced switch in mode of locomotion of a spider

Abstract

The spider Cupiennius salei Keys, (Araneac, Ctenidae) is a nocturnal hunter living on monocotyledones. It does not build webs for prey capture. During the day it remains in its retreat and during dusk it begins to hunt for prey or to search for mates. C. salei is well equipped with mechanosensory systems to detect air- or substrate- borne vibrations elicited by prey or predators. If none of them produce either air movements or substrate vibrations, and the light intensity is below the threshold (0.1 Ix), the animal is virtually 'blind'. Therefore a hypothetical, additional sensory input should exist, which is used only in complete darkness. The animal was tested on a locomotion compensator were it performs constant walks towards a visual target. Three different light intensities were used (bright 200 1x. dim 0.1 1x. and dark at 950 nm, which is outside the spectral sensitivity range of the animal). At bright and dim illumination the animal walked in the alternating tetrapod gait towards the target. In complete 'darkness' the walk was no longer directed and the animal changed its gait and continued walking on only six legs using the first pair as guide-sticks. If the first pair is missing, the second cannot replace this function. This shows a twofold use of the first pair of legs as ordinary walking legs and as guide-sticks or 'antennae. Therefore one can assume that visual input causes a behavioral change, which can not bc explained by a fixed locomotion pattern but by adaptive changes caused by visual inputs.