The colouration toolkit of the Pipevine Swallowtail butterfly, Battus philenor: thin films, papiliochromes, and melanin

The ventral hindwings of Pipevine Swallowtail butterflies, Battus philenor, display a colourful pattern, created by variously coloured wing scales. Reflectance and transmittance measurements of single scales indicate that the cream and orange scales contain papiliochrome pigments, while brown, black and blue scales contain melanin. Microspectrophotometry and scatterometry of both sides of the wing scales show that the lower lamina acts as a thin film, with reflection properties dependent on the scale’s pigmentation. Notably in the orange scales, the reflectance spectrum of the lower lamina is tuned to the pigment’s absorbance spectrum. The dorsal hindwings of the male (but not the female) B. philenor are blue-green iridescent. At oblique illumination, the light reflected by the male’s dorsal hindwings can be highly polarised, which may have a function in intersexual signalling.