Colour and polarization signatures of Southland chafer beetles

Abstract

The beautiful structural colours and metallic sheens of scarab beetles are well known, but the polarizing properties of cuticular reflectors are less so. In a recent article Goldstein reviewed early observations of the state of polarization of light reflected from beetles illuminated by unpolarized light. The physicist A.A. Michelson, better known for his measurements of the speed of light and the Michelson-Morley experiment that preceded Einstein's theory of relativity, is credited with the discovery that the light reflected from many scarab beetles is circularly polarized. In 1924 P. Gaubert attributed left-handedness to the reflections, and Britton showed that the polarizing effect is most common in the subfamilies Rutelinae and Cetoniinae. The structural features that cause the left- handed reflection were investigated by Caveney and Neville. In their models the reflections are from multilayer interference and the basic dielectric period is formed of sublayers of aligned fibres that twist steadily plane-by-plane with depth through half a turn. The pitch determines the wavelength of the reflected light (known as the circular Bragg wavelength) and the handedness of the light is the same as the handedness of the twisting. Thus it follows from optical observations that the sublayers of beetles always twist in the left-handed sense. The multilayered fibrous structure adds strength to beetle cuticle but it is not known if the beetles make use of the circular polarization property for recognition or survival functions. Here we consider the circular polarization signature for a group of beetles and determine the influence that it has on colour.