Earwigs (Insecta, Dermaptera) show astonishing diversity in penis morphology. In several families, males have a single penis (termed virga), whereas males of other families possess two functional virgae. Taxonomists have assumed that the two-virgae state is ancestral; however, ecological reasons why the ancestor acquired two virgae have not been explored. This study investigated in detail male and female genital structures, mating behaviour and insemination processes in the earwig Diplatys flavicollis (Diplatyidae). Diplatyidae are considered to be the most primitive family of earwigs. SEM and light-microscopy revealed that males of this species have two gonopores on each of two virgae, similar to those reported in other diplatyids (i.e. two double-barrelled penises), while females have four to six independent sperm-storage organs (spermathecae). Rapid fixation of mating pairs and insemination success of males from which one virga had been removed clearly revealed that only one virga was used for mating and was usually sufficient for inseminating multiple spermathecae. This finding rejects the one-to-one correspondence between male gonopores and female spermathecae. Based on allometric analysis of spermathecal variation, the possible significance of multiple spermathecae in relation to sperm-storage strategies of females is discussed. Compared to studies of male genital morphology, few studies have described spermathecal morphology. Based on compiled data of spermathecal and virgal morphology among earwigs, a parallel evolutionary trend between spermathecae and virgae from complicated (multiple) to simple (single) ones is suggested, and further investigation of the phylogeny, female genital morphology and insemination processes among earwigs is encouraged.