Butterfly wing scales. Coloured scanning electron micrograph (SEM) of scales from the wing of a glassy tiger (Parantica aglea) butterfly, showing the 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.