A scanning electron micrograph showing the wing scales of a brimstone butterfly, Gonepteryx rhamni. A butterfly wing consists of a membraneous base from which the scales grow from trichogenic cells. At maturity the scale is attached by a stalk - the pedicel - to a socket in the membrane; visble at the base of some of the scales here. Each scale is hollow, with a flattened lower surface, and an upper surface (shown here) with a series of ridges and ribs. In G. rhamni, the scales are pigmented; in species with metallic colouration, the appearance of colour is derived by the scattering of light by diffraction at the narrow striations on the scale surface. The function of the scales may include signalling - both to predators or potential mates - camouflage, and insulation. They are easily detached, and not replaceable; some loss with age need not impair flight.Mag x250 at 10x8