Butterfly wing scale. Coloured scanning electron micrograph (SEM) of a scale from the wing of a swallowtail (Papilio sp.) butterfly. A butterfly wing is covered in many tiny scales. The scales are modified hairs, or setae, and are made of chitin, a common substance in insect exoskeletons. The iridescence of the colours of a butterfly's wings is produced by the diffraction of light by the microscopic ridges on these scales.