Micro-optics in the cuticle of matt-green Chrysina beetles create spatially projected images

Green Chrysina beetles show left-handed circularly polarized light reflections produced by a helicoidal chitin-protein fibre structure, but, unlike reflections of the well-studied golden and silver species, their reflections look diffuse as a result of having an exocuticle with small craters that spread local specular reflections in many directions. At the micrometre scale, the curved reflective structure at the craters behaves as concave reflectors projecting real images above their surfaces, while the convex rims separating the craters form instead virtual images below. The result is an optical illusion where the real topography is inverted: the craters look like hills and the rims like valleys. We have successfully modelled the resulting pattern of multi-coloured reflections, imagining the curved, helicoidal structure as a stack of wavelength-selective micro-reflectors. This spatially projected imaging interpretation becomes relevant for optical characterization studies of such structures at the microscale.