This is the author accepted manuscript. The final version is available from the Royal Society via the DOI in this record.Helicoidal architectures comprising various polysaccharides such as chitin and cellulose have
been reported in biological systems. In some cases, these architectures exhibit stunning optical
properties analogous to ordered cholesteric liquid crystal phases. In this work, we characterise
the circularly polarised reflectance and optical scattering from the cuticle of the beetle
Chalcothea smaragdina (Coleoptera: Scarabaeidae: Cetoniinae) using optical experiments, simulations
and structural analysis. The selective reflection of left-handed circularly polarised light
is attributed to a Bouligand-type helicoidal morphology within the beetle’s exocuticle. Using
electron microscopy to inform electromagnetic simulations of this anisotropic stratified medium,
the inextricable connection between the colour appearance of C. smaragdina and the periodicity
of its helicoidal rotation is shown. A close agreement between the model and the measured
reflectance spectra is obtained. In addition, the elytral surface of C. smaragdina possesses
a blazed diffraction grating-like surface structure, which affects the diffuse appearance of the
beetle’s reflected colour, and therefore potentially enhances crypsis amongst the dense foliage
of its rainforest habitat.This work was financially supported by the National Centre of Competence in Research BioInspired
Materials and the Ambizione program of the Swiss National Science Foundation SNSF
(PZ00P2 168223, to BDW)
