In this paper the wonderful metallic colouring of certain iridescent scaleless beetles will be considered. Many previous writers have discussed this subject, and have taken widely divergent views as to the cause of the colours. For a summary, reference may be made to a paper by my father, the late Lord Rayleigh. As there set forth, the discussion turns mainly on whether the iridescent color is due to repeated thin plates, as in the case of chlorate crystals, or whether it is of the same kind as the surface reflexion of aniline dyes. The latter view was supported by the authority of Prof. Michelson, but I understand that he thinks that the last word has not been said on the subject.