Qualitative correlation between structural chirality through the cuticle of Chrysina aurigans scarabs and left-handed circular polarization of the reflected light

Abstract

Measured reflection spectra from elytra of C. aurigans scarabs are reported for wavelengths between 300 and 1100 nm. They show a broad reflection band for wavelengths from 525 to 950 nm with a ripple structure consisting of a sequence of maxima and minima reflection values superimposed on a mean value of around 30% of the reflection band. To our knowledge, this is one the first reports on measurements of a natural broad band reflector in which the spectral features of the band are completely contained in the measuring range, including the mentioned ripple structure. What seems to be a multilayer structure of the cuticle of the C. aurigans is displayed from SEM analysis showing a layer’s thickness dependence with the perpendicular depth through the procuticle. Additional optical measurements are carried out to establish the polarization of the reflected light which is circularly polarized to the left, with lower contributions of diffuse and non-coherent light. These findings require an interpretation of the structure displayed by the SEM images, in terms of a Bouligand-type (twisted helical) structure characterized by a depth-dependent spatial period distribution more complex than those previously reported in the literature for others biological systems.