Iridescence in beetles is usually due to interference, in which light is scattered as it passes through cuticular layers of varying density. In a few families, however, iridescence is due to diffraction gratings formed by cuticular surface sculpture. In these unusual taxa, patches of spectral iridescence are visible under direct light, usually at the elytral apices. Cuticular diffracting structures vary widely in form; some resemble flattened setae, while others take the shape of minute combs or tabs perpendicular to the surface of the elytron. This iridescence may be a form of crypsis, an intraspecific cue, or may simply be an artifact of the plastron-like pubescence related to moist environments. This phenomenon has hitherto been observed only in Scarabaeidae, Carabidae, Gyrinidae, Phalacridae and Torridincollidae; recently, several iridescent species in Leiodidae, Chrysomelidae, and Hydrophilidae were discovered. Of these, only the leiodids possess more than one microsculptural diffraction mechanism within the family. In order to elucidate the evolutionary origins and possible functions of these diffraction gratings, representatives of each family were examined with both light photography and scanning electron microscopy. Cuticular diffraction gratings appear to be linked to moist or aquatic habitats, but are likely visual cues as well; most of the iridescent species are diurnal and occur near the equator, where the angle of incident light approaches zero.