Papiliochrome II pigment reduces the angle dependency of structural wing colouration in nireus group papilionids

The wings of four papilionid butterfly species of the nireus group, Papilio bromius, P. epiphorbas, P. nireus and P. oribazus, are marked by blue-green coloured bands surrounded by black margins. The cover scales in the coloured bands contain a violet-absorbing, blue-fluorescing pigment. The fluorescence and absorbance spectra of the nireus group wings are very similar to those of the wings of the Japanese yellow swallowtail, Papilio xuthus, and thus the pigment is presumably papiliochrome II. The scale structures of P. xuthus are arranged irregularly, and both the fluorescence and light reflection are diffuse. In the nireus papilionids, the spatial fluorescence distribution of the scales is also diffuse, but the reflection is specular. The scales have a multilayered structure, consisting of two main laminae. We show that the papiliochrome II pigment in the upper lamina of the scales functions as a violet-blocking long-pass filter in front of the lower lamina, thus limiting the reflectance spectrum to the blue-green wavelength range. Optical modelling showed that the papiliochrome II filter effectively removes the angle dependency of the reflectance spectra - that is, it reduces the wing iridescence. The contribution of the fluorescence signal to the visual appearance is minor.