Mating positions and the evolution of asymmetric insect genitalia.

Genital asymmetry is a recurring phenomenon in insect morphology and current data suggest that it has arisen multiple times independently in several neopteran orders. Various explanations have been proposed, including space constraints, ecological constraints, sexual selection via antagonistic coevolution, and sexual selection via changed mating positions. Each of these hypotheses may best explain individual cases, but only the last seems to account for the large majority of insect genital asymmetries. Here I summarize the basic assumptions and evolutionary steps implied in this model and review the evidence for each of them. Several components of this scenario can be easily tested, for example by including genital asymmetries and mating positions in phylogenetic analyses. Others require in-depth analyses of the function of asymmetric genital structures, targeted comparative analyses (e.g., of taxa with sex-role reversal, taxa with reversal to symmetry, etc.), and of female genital neuroanatomy.