For comparison, CP reflectance was modelled for a Bouligand structure (18 half-pitches of 118 nm, an average refractive index of 1.56 [79], in-plane birefringence of 0.018 [66]) covered by a 180 nm-thick isotropic film (n = 1.75), calculated for incidence and emergence angles ranging from 0° to 75° in 15° increments in cross-CP configuration (a) and at normal incidence in co- and cross-CP configuration (b). Modelled CP reflectance spectra in (a) appear as broad as the experimental spectra measure from Calliphora vicina (Fig. 3a). The left-left co-CP (LCP-LCP) spectrum exhibits a small peak in the UV range. In the real photonic structure of C. vicina, positional disorder [44], light absorption, and scattering would likely reduce this co-CP signal to a very weak level.