Variation and possible function of egg sac coloration in spiders

Coloration of the egg sacs of spiders varies widely to the human eye, both across and within taxonomic groups. These differences in coloration are expected with differences in the biology and ecology of different species. Here we measure the spectral properties of the egg sacs of 15 species in six families. Ultraviolet chroma, red chroma, and particularly overall brightness vary widely across and within taxonomic groups. We discuss the spectral properties of the silk of the egg sacs in the context of the physical characteristics of the silk, the reflective properties of the background, the environmental illumination, and the natural history and behavior of the spider species. In most cases, the spectral characteristics of the egg sacs seem to reduce their conspicuousness against the background and in those cases in which the coloration does not reduce the contrast in relation to the background, the low environmental light available may help to camouflage the egg sacs