Many insects own a tessellated carapace with bumps, pits or indentations. Little is known on the physical properties of these geometric variations, and biological functions are unknown or still debated. Beetles from the genus Chrysina show vivid reflections from bright green to metallic silver-gold as a consequence of the cholesteric liquid crystal organization of chitin molecules. Particularly, the carapace of Chrysina gloriosa exhibits green and silver stripes.

(a) Chrysina gloriosa beetle. (b) Interface between green and silver stripes (reflection mode, bright field, unpolarized light). (c) Magnified view of polygons in a green stripe.

© CEMES / CNRS

By combining confocal microscopy and spectrophotometry, scanning electron microscopy and (FDTD) numerical simulations, the relationship between the reflectance and the structural parameters for both stripes at the micro- and nanoscales is established. Over the visible and near IR spectra, polygonal cells in tessellated green stripes behave as multiwavelength selective micro-mirrors and the silver stripes as specular broadband mirrors. With its green stripes, Chrysina gloriosa simultaneously realizes the selective reflection of multiple wavelength ranges at normal incidence and the generation of diffusive reflections at a wide angular range. These seemingly conflicting requirements are fulfilled by combining, on a curved surface, appropriate regions of regularity and irregularity at different length scales. Conspecifics or intra-species communication, thermoregulation or camouflage against predators are discussed as advanced functions. As a prerequisite to bio-inspired artificial replicas, the physical characteristics of the polygonal texture in Chrysina gloriosa carapace are compared to their equivalents in synthetic cholesteric oligomers and their fundamental differences are ascertained, contrary to expectation and assumption in the literature. The present results may provide inspiration for fabricating spatial wavelength-specific light modulators and optical packet switching in routing technologies.

Scanning electron microscopy images of cross-sections in (a) green and (b) silver stripes. In a green stripe and below each polygon the fingerprint texture presents concave nested lines. In contrast, the lines are regularly parallel in a silver stripe. The distance between two lines of equal contrast is related to the helicoidal pitch of the twisted structure, while the helicoidal axis is perpendicular to the lines.

© CEMES-CNRS

Mapping of reflected intensity at the focus plane of polygons for three wavelength ranges related to three critical patterns: (a) donut, (b) spot and (c) continuum background. A spectrometer equipped with a confocal microscope is used.

© CEMES-CNRS

Reference

Multiwavelength micromirrors in the cuticle of scarab beetle Chrysina Gloriosa , G. Agez, C. Bayon and M. Mitov, Acta Biomaterialia , 48, pp. 357–367 (2017).

http://www.sciencedirect.com/science/article/pii/S1742706116306298

Contact

Michel Mitov : mitov at cemes.fr