a pure colour – at any given angle, but other wavelengths – different colours – at different angles. So, as a thin-film structure such as a butterfly's wing or bird's feather moves, it seems to change colour. Mechanisms Fixed structures A number of fixed structures can create structural colours, by mechanisms including diffraction gratings, selective mirrors, photonic crystals, crystal fibres and deformed matrices. Structures can be far more elaborate than a single thin film: films can be stacked up to give strong iridescence, to combine two colours, or to balance out the inevitable change of colour with angle to give a more diffuse, less iridescent effect. Each mechanism offers a specific solution to the problem of creating a bright colour or combination of colours visible from different directions. A diffraction grating constructed of layers of chitin and air gives rise to the iridescent colours of various butterfly wing scales as well as to the tail feathers of birds such as the peacock. Hooke and Newton were correct in their claim that the peacock's colours are created by interference, but the structures responsible, being close to the wavelength of light in scale (see micrographs), were smaller than the striated structures they could see with their light microscopes. Another way to produce a diffraction grating is with tree-shaped arrays of chitin, as in the wing scales of some of the brilliantly coloured tropical Morpho butterflies (see drawing). Yet another variant exists in Parotia lawesii, Lawes's parotia, a bird of paradise. The barbules of the feathers of its brightly coloured breast patch are V-shaped, creating thin-film microstructures that strongly reflect two different colours, bright blue-green and orange-yellow. When the bird moves the colour switches sharply between these two colours, rather than drifting iridescently. During courtship, the male bird systematically makes small movements to attract females, so the structures must have evolved through sexual selection. Photonic crystals can be formed in different ways. In Parides sesostris, the emerald-patched cattleheart butterfly, photonic crystals are formed of arrays of nano-sized holes in the chitin of the wing scales. The holes have a diameter of about 150 nanometres and are about the same distance apart. The holes are arranged regularly in small patches; neighbouring patches contain arrays with differing orientations. The result is that these emerald-patched cattleheart scales reflect green light evenly at different angles instead of being iridescent. In Lamprocyphus augustus, a weevil from Brazil, the