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Butterfly wing scales. Coloured scanning electron micrograph (SEM) of scales from the wing of a common emigrant (Catopsilia pomona) butterfly, showing their photonic crystal structure. The colour of a butterfly's wing can arise either from a pigment contained in the scales or from the actual structure of the scales themselves, as here. Photonic crystals are nanostructures that affect the motion of photons in light, producing a specific colouration. (KEYSTONE/SCIENCE PHOTO LIBRARY/PETR JAN JURACKA/SCIENCE PHOTO LIBRARY)