Another way to be green: pigmented liquid ink in butterfly wings – a novel form of colouration

Abstract A novel mechanism underlying butterfly wing colouration was discovered in two distantly related butterfly species, Siproeta stelenes (Nympalinae) and Philaethria diatonica (Heliconiinae). These butterflies have bright green wings, but the colour is not derived from pigments or nano-structures of the scales or the colour of the cuticular membrane, but from an ink-like liquid retained in the wings. Wing structure differs between the green and non-green areas. In the non-green region, the upper and lower cuticular membranes are attached to each other, whereas in the green region there is a space of 5–10 microns where liquid ink is held and living cells were observed. The green ink was composed of pigments similar to those in the hemolymph, but enriched with carotenoid lutein. This novel discovery sheds a new light on our understanding of the evolution of wing colouration in butterflies.