VISUAL BRIGHTNESS-DISCRIMINATION OF THE JUMPING SPIDER MENEMERUS-BIVITTATUS (ARANEAE, SALTICIDAE)

It was observed that the jumping spider Menemerus bivittatus lives on light surfaces as well as on dark surfaces, hunting prey which is ligh ter or darker than the surface the spider is on. From these observatio ns arises the question about the brightness or contrast discrimination abilities of this spider. The orientation response was recorded for 1 4 spiders to a moving circular prey-stimulus varying from white, throu gh grey, to black, against a white, grey or black background. When the stimulus was darker than the background, there was a rapid increase i n response as the stimulus gets darker. This rapid change in response with stimulus brightness did not occur when the stimulus was lighter t han the background. These results reveal a high contrast discriminatio n ability and also a dependence of the response on the overall stimula tion conditions.