Diffractive optics in spiders

The mechanisms behind the metallic colours of two species of spider are examined here with particular reference to the surface structures involved. Cosmophasis thalassina possesses a combination of a first-order diffraction grating and an underlying broadband multilayer reflector. The diffraction grating causes one spectral component to be selectively directed away from the direction of incident illumination. The laminar structure beneath produces a broadband ('white') reflectance, and with a significant proportion of the blue light being removed from the incident white light by the grating, the reflectance observed at most angles is yellow (white minus blue). Castaneira sp. possesses a zero-order diffraction grating, resulting in the antireflection of ultraviolet light. Beneath this lies a quarter-wave stack, reflecting saturated colours that change with angle of observation. The reflections of both species appear conspicuous against their natural environments, probably facilitating conspecific recognition (and/or mimicry of ants in Castaneira sp.), and constitute the first cases of structural colours analysed in spiders.