  Biophotonic nanostructures in butterfly wing scales remain fascinating
examples of biological functional materials, with intriguing open questions in
regards to formation and evolutionary function. One particularly interesting
butterfly species, Erora opisena (Lycaenidae: Theclinae), develops wing scales
that contain three-dimensional photonic crystals that closely resemble a single
gyroid geometry. Unlike most other gyroid forming butterflies, E. opisena
develops discrete gyroid crystallites with a pronounced size gradient hinting
at a developmental sequence frozen in time. Here, we use a hyperspectral
(wavelength-resolved) microscopy technique to investigate the ultrastructural
organisation of these gyroid crystallites in dry, adult wing scales. We show
that reflectance corresponds to crystallite size, where larger crystallites
reflect green wavelengths more intensely; this relationship could be used to
infer size from the optical signal. We further successfully resolve the
red-shifted reflectance signal from wing scales immersed in refractive index
oils with varying refractive index, including values similar to water or
cytosol. Such photonic crystals with lower refractive index contrast may be
similar to the hypothesized nanostructural forms in the developing butterfly
scales. The ability to resolve these fainter signals hints at the potential of
this facile light microscopy method for in vivo analysis of nanostructure
formation in developing butterflies.
