Structural origin of the green iridescence on the chelicerae of the red-backed jumping spider, Phidippus johnsoni (Salticidae: Araneae)

Abstract

The structurally coloured chelicerae of jumping spiders (Salticidae) display some of the most striking of all colours in this family, in which they predominantly occur. Remarkably, however, the source of this iridescence has not been studied. For this reason, we chose to investigate the green iridescent chelicerae of the red-backed jumping spider, Phidippus johnsoni. The colour is restricted to the dorsal region of the basal chelicera segment--the paturon. This was confirmed by reflectance measurements taken at normal incidence and in backscatter, which gave a peak reflectance in the green (520 nm), arising from the first harmonic of a Bragg resonance in the near infrared. Transmission electron microscope analysis of the paturon cuticle revealed a stack of 86 layers of alternating low and high density materials, identified as air and chitin respectively. Simulations based on a periodic multilayer model of the ten outermost layers of this structure gave theoretical reflectance spectra, closely matching those observed, suggesting that the stack functions as a multilayer reflector for green. The colour is thought to function as a conspecific signal, since studies of vision in a closely related species, also displaying green chelicerae, have shown that the eyes have a peak spectral sensitivity, matching that of the chelicerae.