The shining world of beetles - 19/11/19

Doi : 10.1016/j.crvi.2019.09.005

Michel Mitov
Centre d’élaboration de matériaux et d’études structurales, CEMES, CNRS, Toulouse, France

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Résumé

Twisted liquid-crystalline organizations are ubiquitous in living matter [1, 2]. Many insects own a tessellated carapace with bumps, pits, indentations, stripes or spots (Fig. 1). These geometric variation often exhibit iridescent colors, which are the consequence of a complex twist of chitin macromolecules. Such optical information with vivid structural colors might be of paramount importance in the life and the evolution of most day-living insects. Many biological functions are yet debated. They are related to conspecific or intra-species communication, thermoregulation, camouflage, survival or navigation [1, 3].

For example, the cuticle of the scarab beetle Chrysina gloriosa exhibits two bands (Fig. 2). The green band serves as a wavelength-selective (green) diffuser due to the set of polygons arranged on a curved surface and, at the mesoscopic scale, it behaves as an array of wavelength-selective micromirrors [4]. The silver stripe plays the role of a flat metallic reflector operating over the visible spectrum and into the infrared spectrum [4]. The orientation of the helicoidal structure varies in green stripes (Fig. 3), whereas it is fixed in silver stripes.

The outermost part of insect cuticles is very often covered with wax, which restricts water loss, prevents desiccation, may lend superhydrophobic and self-cleaning properties to the cuticle, and serves for chemical communication in many species. While cuticular waxes are considered isotropic, the wax layer of the cuticle of C. gloriosa exhibits an intriguing nanoscale laminate texture, as revealed by electron microscopy of the transverse sections [5].

Micro-textured cuticles of scarabs may inspire researchers and engineers to make their replicas as optical materials. Potential applications are in the field of wavelength-specific light modulators in routing technologies, broadband reflectors for energy savings, coatings for cryptography purposes (wavelength-dependent and polarization-dependent micro- and nanoscale patterns), camouflage (suits with a broad reflection in the IR spectrum identical to the one of the background) or thermoregulation for buildings.

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