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3. The specular spectral reflectance( quantified in terms of reflection factor) from Calliphora vicina blow fly peaks at 437 ± 8 nm at normal incidence( a), corresponding to a blue visual appearance, as observed on a chromaticity diagram( b). With increasing incidence and viewing angles, the reflectance peak blue-shifts( a) giving rise to violet colourations( b). In addition, a narrowing of the peak FWHM is observed. Lucilia richardsi blow fly’ sreflectance peak is located at 537 ± 6 nm at normal incidence( c), leading to a green colouration( d). With grazing incidence and observation angles, the reflectance peak narrows and blue-shifts( c) leading to blue and violet colours( d).
Figure 4. The spectral reflectance peaks measured from the abdomens of both Calliphora vicina and Lucilia richardsi( and quantified here in terms of reflection factor) blue-shift with increasing incidence and observation angles( in absolute values). Such a behaviour is typical of a multilayer reflector.
numerous species from the order Coleoptera and are known for their light circularly polarising properties [ 6, 63 – 68 ]. They consist of incrementally rotated birefringent layers stacked upon each other. Dipteran vision is known to be sensitive to light linear polarisation [ 15 ] and circular polarisation vision was demonstrated in other insects such as beetles [ 69 ], dragonflies [ 70 ], and damselflies [ 70 ], and more generally, in arthropods [ 71 ] including stomatopod crustaceans [ 72 ]. Polarised-light experimental techniques were used in order to measure circularly polarised( CP) reflectance components( further detail available in the electronic supplementary material). The possibility of the presence of a Bouligand structure may undermine the conclusion that the one-dimensional photonic structure is a non-chiral multilayer. Both co-CP and cross-CP configurations were hence investigated, where co-CP corresponds to incident and reflected CP components with the same handedness( left or right) and cross-CP refers to different handedness of the incident and reflected CP components. Despite this TEM observation, only cross-CP responses, similar to that of a planar mirror, displayed significant intensities( Fig. S2). The absence of the typical Bouligand structure’ s optical response might be explained by the small number of layers with respect to what was observed in other insects [ 6, 64 – 67 ]. Another likely explanation is that the birefringence contrast in refractive indices along the ordinary and extraordinary axes is not sufficiently high in order to give rise to co-CP reflection. A last possibility involving a thin film overlaying the Bouligand structure is discussed at the end of this section.
As mentioned above, the CIE chromaticity diagram is not valid for the colour perception of other animals. We therefore analysed the colour sensation from the