The evolution of cryptic spider silk: a behavioral test

Phylogenetic patterns of change in spider silk coloration provideinsight into the selective pressures directing evolution ofsilks. Trends toward evolution of silks with low reflectanceof ultraviolet (UV) light suggest that reduced UV reflectancemay be an adaptation to reduce visibility of webs to insectprey. However, a test of the visibility of primitive and derivedspider silks is lacking. Several genera of orb-weaving spidersinclude conspicuous designs of silk, called "stabilimenta,"at the center of their webs. Due to their large size, stabilimentapresent signals that insects can use to avoid webs. Unlikeother silks in the orb web, which reflect little UV light,evolutionarily derived stabilimentum silk retains a bright UVreflectance. But, unlike primitive silks, stabilimentum silkalso reflects large amounts of blue and green light. We comparedthe visibility of primitive tarantula silks and derived stabilimentumsilks to insects by using the ability of honey bees to learnto forage at targets of spider silk. We found that the uniquespectral properties of stabilimentum silk render it crypticto insects and that primitive silks are more visible to bees.Our findings support a hypothesis that the coloration of stabilimentumsilk is an adaptation to reduce the ability of insects to avoidwebs and that ancient biases in the color vision of insectshave acted upon the evolution of spider silk coloration throughsensory drive. But our findings question the emphasis on UVreflectance alone for visibility of spider silks to insects.