This is a study of the functional morphology of the muscular and reproductive systems of the adult beetle, Atomaria ruficornis (Marsham) made by means of serial sections and dissections. An account is given of the skeleto-muscular system of the head, thorax and abdomen with reference to previous work in this sphere. Wherever possible, the muscles described have been homologized with those of other Coleoptera. In a small number of cases, it has been found necessary to use new names for muscles. The probable mode of action of the mouth-parts and foregut during feeding is described. An account is given of the reproductive organs and genitalia of both male and female beetles; the muscular system of the genitalia in both sexes is described fully, and the functioning of the genitalia is discussed.