Thin-film structural coloration from simple fused scales in moths

The metallic colouration of insects often originates from diverse nanostructures ranging from simple thin films to complex three-dimensional photonic crystals. In Lepidoptera, structural colouration is widely present and seems to be abundant in extant species. However, even some basal moths exhibit metallic colouration. Here, we have investigated the origin of the vivid metallic colours of the wing scales of the basal moth M. aureatella by spectrophotometry and scanning electron microscopy. The metallic gold-, bronze- and purple-coloured scales share a similar anatomy formed of a fused lower and upper lamina resulting in a single thin film. The optical response of this thin film scale can be attributed to thin-film interference of incident light, resulting in the colour variations that correlate with film thickness. Subtle variations in the wing scale thickness result in large visible colour changes that gives Micropterix moths their colourful wing patterns. This simple colouration mechanism could provide a hint to understand the evolution of structural colouration in Lepidoptera