BUTTERFLIES have a hitherto unknown way of making black on their wings. Instead of relying solely on light-absorbing pigments, as it was thought every black animal did, the insects use specially structured wing scales to absorb light.
Different scales absorb, scatter and diffract light in ways that produce colours, such as the intense blue of the male Papilio ulysses (above). On this species these blue areas are enhanced by a black frame, which Pete Vukusic of the University of Exeter in the UK and his colleagues have found reflects only about 5 per cent of light that hits it. 鈥淚t鈥檚 a good, deep black,鈥� says Vukusic.
To find out what proportion of this absorption is due to pigments, the researchers immersed black scales from male P. ulysses into a liquid with the same refractive index as the scales themselves. That prevents any light being diffracted or scattered by the scales鈥� structure. By comparing these scales with those suspended in air, they calculated that only about half the light falling on the scales is absorbed by pigment.
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Vukusic鈥檚 team says that the rest is absorbed because the nano-structure of the scales prevents light from being reflected or scattered, helping to make the wing appear a deeper black (Biology Letters, DOI: 10.1098/rsbl.2003.0510).