There is a great amount of variation in microsculpture on the surface of Carabid Beetles. On the elytra, subject of this study, microsculpture may be entirely lacking, but as a rule consists of engraved microlines, either forming a network of meshes from isodiametric to more or less stretched, or of very dense transverse lines, a system of diffraction gratings causing iridescence. Since this phenomenon appears in many non-related genera and groups, for instance within the large genus Bembidion, it is concluded that diffraction grating is clearly polyphyletic in the Carabid family. An investigation of Baltic Amber Carabidae showed that the different types of microsculpture were developed already in early Oligocene and gave no support to the hypothesis-plausible in itself and confirmed in genus Bembidion-that diffraction grating is a derivative condition. The biological importance of iridescence is doubtful. In certain situations it may have a repulsive effect on predators, for instance birds. But, since it occurs also in certain cavernicolous species never exposed to light, this cannot be the only valid explanation. Other possible effects of iridescence, such as protection against over-heating or ultraviolet light, were doubted. It seems that no generally valid selective value of diffraction grating can be applied and that, in many cases, it may be preserved only as "not harmful".