Nat. Commun. 11, 1294 (2020)

We often admire butterflies for the splendid colours on their wings, but a dark background can make them shine even brighter. Just like strong colours in nature are often the result of diffraction from microstructures rather than pigmentation, the near-perfect absorption we perceive as black can have a similar origin. Alexander Davis and colleagues have now shown that several butterfly species have independently evolved wing-scale structures with less than one per cent reflection, which together with melanin pigments create ultra-black colouration.

Credit: Krys Bailey / Alamy Stock Photo

The team compared the optical response and the wing-scale structures of different black and ultra-black species. They only found low reflectance in scales with pronounced trabeculae — rod-shaped structures — and holes on the order of a few hundred nanometres in size, regardless of the details of their quasi-periodic arrangement. Numerical modelling of the different structures confirmed these conclusions. Although the evolutionary pressures behind ultra-black colouration are unclear, Davis and colleagues believe it improves the contrast for signalling displays in bright-light conditions.