Intraspecific variation and intersexual correlation in secondary sexual characters of three diving beetles (Coleoptera: Dytiscidae)

Male diving beetles of the subfamily Dytiscinae possess tarsi with adhesive discs that they strike on the female dorsum during mating interactions. Females of many species are dimorphic, being either smooth or structured dorsally. Darwin suggested the female structures were an aid for the male but in this study we investigate these characteristics in the light of sexual conflicts. The intraspecific variation in the numbers and size distribution of male tarsal discs, and in body measurements were recorded for three dytiscine species, all with dimorphic females. The number of protarsal discs in the two Dytiscus species varied much more than previously reported. In addition, only a small part of the variation could be explained by body size. In Graphoderus we found highly significant differences in male secondary sexual characters among populations. A multivariate analysis significantly correlated male secondary sexual characters with the proportion of granulate females in the populations. These observations are consistent with the theory of arms races and female counter adaptations. Covariation between male and female characters is predicted from a framework of sexual conflict over mating rate. At the same time our study gives a new perspective on the function of dytiscine female dorsal irregularities debated ever since Darwin.