Ultraviolet Reflectance of Spiders Andtheir Webs

To determine the reflectance of spider webs and spiders under ultraviolet (UV) light, spiders and their webs were photographed under normal (white) light and under UV light. It was found that all silk in araneoid webs reflect slightly more UV light than white light; i.e., they had a positive UV-brightness. However, the often cited, particularly high UV-brightness of stabilimenta could not be confirmed. Spiders differed in their UV-brightness, with most spiders reflecting less UV light than white light. Based on the knowledge of the visual system of insects and invertebrates it is suggested that the main function of stabilimenta is predator defense. However, drawing a final conclusion requires more knowledge on the way potential predators and prey perceive spiders, spider webs and stabilimenta.