Structural colors in spiders

Abstract

Structural colors are described and analyzed in theraphosid and salticid spiders. Some theraphosids are brightly blue: this is caused by special hairs with a lamellated wall that causes an interference of the incoming light. The incident white light is then reflected as a deep blue. In some cases, the hairs are bright yellow: there the hair wall exhibits a fine cuticular meshwork of slightly different dimensions than in the blue hairs but also results in interference, and the reflected light appears yellow. In salticids, iridescent colors are produced by flattened hairs (scales). Golden scales have a rather simple structure of two thin cuticular layers on the outside and a narrow air space in between. Blue iridescent scales are more complex, with multilayered scale walls and fine ridges on the surface that act as a diffraction grating. The body cuticle (e.g., chelicerae and eye lenses) may also be brightly colored due to light interference on many thin cuticular layers. The biological significance of structural colors in spiders is well understood in the diurnal salticids where optical signals are exchanged in courtship, usually in bright daylight. In contrast, theraphosids are mostly active at night and visual communication hardly plays any role. Their coloration may be of advantage at dawn during their defensive behavior, when the brightly colored blue and yellow legs are raised toward an aggressor.