Male genital allometry in scathophagidae (Diptera)

Male genital structures are extremely divergent across species and sexual selection is largely responsible. Many sexually selected traits show positive allometry and have high phenotypic coefficients of variation (CV). Sexually-selected genital traits that come into contact with females during copula may be an exception to this general pattern. We compared the within species size allometry of the genital claspers, mandibular palps, and testes in a comparative study across the Scathophagidae. We additionally compared the levels of phenotypic variation in these traits and in hind tibia length. Within species, claspers typically displayed negative allometry and had low CV, indicative of stabilizing selection. In contrast, testis size was more like sexually selected display traits, typically being positively allometric and having very large CV. Palps tended to be positively allometric or isometric, and intermediate in levels of phenotypic variation, much like leg length. In spite of intraspecific stabilizing selection on the genital claspers, there has been major divergence of these characters across species.