Shine and Hide : Biological Photonic Crystals on the Wings of Weevils
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Shine and Hide: Biological Photonic Crystals on the Wings of Weevils Bodo D. Wilts1, Natasja IJbema1, Kristel Michielsen2, Hans De Raedt1 and Doekele G. Stavenga1 1 Computational Physics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, NL-9747 AG Groningen, The Netherlands, EU 2 Institute for Advanced Simulation, Jülich Supercomputing Centre, Research Centre Jülich, D52425 Jülich, Germany, EU ABSTRACT The body and elytra of the diamond weevil, Entimus imperialis, is studded with numerous brightly colored scales. The scales exhibit brilliant reflections because they contain unusually large diamond-type photonic crystals. The scales are concentrated in pits on the otherwise black elytra. This framing enhances the color contrast when the weevil is observed from nearby. From a distance the diamond weevil looks green, alike green foliage. Another weevil, Eupholus cuvieri, has also scales with green reflective photonic crystals, but here the scales are arranged closely apposed on the planar elytra. Both weevils use photonic crystals for camouflage, but the display methods are different. INTRODUCTION The brilliant, iridescent body colors of many beetles, butterflies, fish and birds are due to (coherent) scattering of light by nanostructured materials present in the integument [1, 2]. When the refractive index of the biological structures is periodically modulated on the length scale of visible light (i.e. of the order of ~200 nm) with spatial variations in one, two or three dimensions, the structures are acting as photonic crystals. Of extreme interest are the three-dimensional photonic crystals found in insects that are often shaped into one of the three simplest, triplyperiodic, bicontinuous-cubic minimal surfaces: primitive cubic (P), diamond (D) or gyroid (G) [3]. Given the low refractive index palette of the biological materials employed in the photonic structures, the reflectance of the biological photonic crystals has become nearly optimal during the long process of evolution [4-6]. In the insect order Coleoptera (beetles and weevils), a diversity of photonic structures has been found [6-8]. Many beetles have multilayers in the elytra (wings) and body, making them highly reflective. In scarabs, helicoidal layered elytra cause bright circular polarized reflections [9]. Even more sophisticated structures exist in weevils, which have three-dimensional, diamondtype photonic crystals. Compared to other structures, diamond-type photonic crystals have the largest photonic bandgaps and the weevils’ photonic crystals therefore form a promising candidate for bio-inspired applications [10, 11]. A most intriguing animal is the Neotropical diamond weevil, Entimus imperialis. The elytra, body and even legs are studded by brilliantly reflective scales [6, 12]. By electron microscopy, we found that the scales consist of large domains of diamond-type photonic crystals. The domains are randomly oriented, resulting in multi-colored reflections, a unique characteristic of Entimus imperialis [12]. Here w
