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The light reflectors in the beetles Calloodes grayanus and Anoplognathus parvulus are examined. Contrasting multilayer reflectors are revealed. Calloodes grayanus appears a weak green colour, matching its background leaves, while A. parvulus is strongly metallic-gold coloured. The former reflection is diffuse, as the result of a structure causing scattering that overlies the multilayer reflector, whereas the latter reflection is strongly directional. The green colour of C. grayanus is achieved by a multilayer reflector with a fixed spatial periodicity, here termed 'regular', which is far removed from the quarterwave, or physically 'ideal', condition. The gold colour of A. parvulus is achieved by a type of reflector which involves systematically changing optical thicknesses of the component layers with depth in the structure. A layer of melanin underlies the reflector of C. grayanus to absorb the transmitted portion of light and prevent its back-reflectance, which would otherwise alter the green colour. The resultant structural reflectance from C. grayanus effectively matches green pigments, which are rare in beetles.
Parker et al. (Fri,) studied this question.