The evolution of cryptic spider silk: a behavioral test

Phylogenetic patterns of change in spider silk coloration provide insight into the selective pressures directing evolution of silks. Trends toward evolution of silks with low reflectance of ultraviolet (UV) light suggest that reduced UV reflectance may be an adaptation to reduce visibility of webs to insect prey. However, a test of the visibility of primitive and derived spider silks is lacking. Several genera of orb-weaving spiders include conspicuous designs of silk, called "stabilimenta," at the center of their webs. Due to their large size, stabilimenta present signals that insects can use to avoid webs. Unlike other silks in the orb web, which reflect little UV light, evolutionarily derived stabilimentum silk retains a bright UV reflectance. But, unlike primitive silks, stabilimentum silk also reflects large amounts of blue and green light. We compared the visibility of primitive tarantula silks and derived stabilimentum silks to insects by using the ability of honey bees to learn to forage at targets of spider silk. We found that the unique spectral properties of stabilimentum silk render it cryptic to insects and that primitive silks are more visible to bees. Our findings support a hypothesis that the coloration of stabilimentum silk is an adaptation to reduce the ability of insects to avoid webs and that ancient biases in the color vision of insects have acted upon the evolution of spider silk coloration through sensory drive. But our findings question the emphasis on UV reflectance alone for visibility of spider silks to insects.