Hyperspectral Topography of the Twisted, Cholesteric Patterns of an Insect Cuticle under Various Conditions of Helix Obliquity

Twisted cholesteric liquid crystal patterns are found in the iridescent chitin-containing cuticles of many insects. They may exhibit spatial variation in the helical pitch and in the orientation of the helix axis, as in the two-band, green and silver cuticle of the scarab beetle, Chrysina gloriosa, which is the focus of the present study. The silver bands are pattern-free, whereas the green bands exhibit an array of microcells. By hyperspectral microscopy with 6 nm spectral resolution in the range of 400 nm–1000 nm, we investigated the topography of twisted cuticular structures under various orientations of the helical axis, from an orientation close to the normal to the cuticle surface in the silver bands to a spatially varying orientation in the microcells. We deal with two aspects of the topography: the shape of the surface at the interface between the transparent wax layer and the chitin-protein matrix, which is a challenging task since the cuticle is covered by the wax layer preventing direct topographic measurements by standard techniques, and the features of the deep layers in the cuticle, by addressing the distribution of the helical pitch of the twisted structure at a variable depth. We demonstrate that both types of topographic information are embedded in the hyperspectral data. Specifically, we show that the spatial distribution of the spectral center of mass portrays the geometrical shape of the hexagonal microcells. Furthermore, we evidence the spectral signature of the structural patterns located at the edges of the microcells.