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. Fe males of many species are dimorphic, being either smooth or structured dors ally. Darwin suggested the female structures were an aid for the male but i n this study we investigate these characteristics in the light of sexual co nflicts. The intraspecific variation in the numbers and size distribution o f male tarsal discs, and in body measurements were recorded for three dytis cine species, all with dimorphic females. The number of protarsal discs in the two Dytiscus species varied much more than previously reported. In addi tion, only a small part of the variation could be explained by body size. I n Graphoderus we found highly significant differences in male secondary sex ual characters among populations. A multivariate analysis significantly cor related male secondary sexual characters with the proportion of granulate f emales in the populations. These observations are consistent with the theor y of arms races and female counter adaptations. Covariation between male an d female characters is predicted from a framework of sexual conflict over m ating rate. At the same time our study gives a new perspective on the funct ion of dytiscine female dorsal irregularities debated ever since Darwin. (C ) 2001 The Linnean Society of London.