Nature offers a plethora of possibilities for optical biomimetics. Some birds, butterflies, insects and other creatures exhibit brilliant colors as result of diverse optical phenomena produced by micro- and nanostructures at the near surface. Particularly, the shiny metallic colors reflected by the exoskeleton (the so called cuticle) of some beetles show elliptical polarization properties; most commonly of the left-handed type but also the right-handed type has been found [1]. These color and polarization properties have been related to a twisted plywood or Bouligand structure which is comprised by the clustering of chitin nanofibrils wrapped by proteins in a planar woven.

In this work we report the polarization properties of light reflected from the scarab beetle Cotinis mutabilis (Gory and Percheron 1833) and the microstructure of its cuticle. This species is found in Mexico and the southwestern part of the United States. The specimen under study was collected at the facilities of Cinvestav in Querétaro, Mexico where it is known as mayate (Mayatl in Náuhtl, the language of Aztecs). The Mayatl presents a green mate color in its dorsal side with red-orange stripes in the elytra. On the other hand, the abdominal side shows a shiny green metallic color which is subject of the present study. The polarization properties are investigated by the complete Mueller matrix (M) measured with a dual rotating compensator ellipsometer (J. A. Woollam Co., Inc.) at angles of incidence of 20-75° and wavelength range of 250-1000 nm. In particular, the elements m₄₁ and m₁₄ of M show that green left-handed polarized light is reflected when the beetle is illuminated at near-normal angles of incidence with unpolarized white light. As the angle of incidence increases the maximum of light reflected is blue-shifted. The degree of polarization, ellipticity, and azimuth calculated with relationships between m_j1 elements provide a full description of the polarization state of light reflected from the beetle’s cuticle for incident un-polarized light. Scanning electron microscopy images of the cuticle reveal that the epicuticle and exocuticle comprise a (hard) layer ca. 15 μm thick. Two regions can be differentiated in the exocuticle: the outer exocuticle without any clear structure and the inner exocuticle where a clear multilayer structure is observed. Beneath the inner exocuticle is the endocuticle which is comprised by unidirectional layers of microfibrils alternate with layers running at nearly right angles to each other in a pseudo-orthogonal arrangement. The cuticle also was imaged with a microscope coupled to a FTIR system allowing further identification of bands due to chitin.