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Light micrograph of scales on the wing of a butterfly. The colour of a butterfly or moth's wing can arise either from a pigment contained in the scales or from iridescence produced by the actual structure of the scales themselves. This iridescence is produced by the diffraction of light by the microscopic structure of these scales. Magnification: x100 when printed at 10 centimetres across. (KEYSTONE/SCIENCE PHOTO LIBRARY/FRANK FOX/SCIENCE PHOTO LIBRARY)