Exposing different in-depth pitches in the cuticle of the scarab beetle Cotinis mutabilis

The cuticle of the scarab beetle Cotinis mutabilis reflects left-handed polarized light indicating the presence of a helicoidal structure. Different in-depth pitches in the cuticle are corroborated by optical microscopy images of the cuticle which originally is yellowish or reddish but becomes greenish after gently scratching its top side. Using the Mueller-matrix formalism the degree of polarization (total and circular) of reflected light is determined for unpolarized incident light. The effects of the finite thickness of the cuticle on the broadening and strength of the selective Bragg reflection are discussed on the basis of dispersion relations for optical modes in helicoidal structures and simulated spectra. (C) 2017 The Author(s). Published by Elsevier Ltd.Funding Agencies|Conacyt-Mexico; Knut and Alice Wallenberg foundation; Swedish Research Council; Carl Tryggers foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]