Selection for imperfection: A review of asymmetric genitalia in araneomorph spiders (Araneae: Araneomorphae)

Bilateral asymmetry in the genitalia is a rare but widely dispersed phenomenon in the animal tree of life. In arthropods, occurrences vary greatly from one group to another and there seems to be no common explanation for all the independent origins. In spiders, genital asymmetry appears to be especially rare. Few examples have been studied in detail but isolated reports are scattered in the taxonomic literature. Based on a broad literature study, we found several species in thirteen families with evidence of genital asymmetry, mostly expressed only in females. Our review suggests that spider genital asymmetries, although rare, are more common than previously thought and taxonomic descriptions and illustrations are a useful but not entirely reliable tool for studying them. Here we also document thoroughly the case of the liocranid spider Teutamus politus. We collected live specimens to observe male-female interactions and document their genital morphology. We consider T. politus to be the first known case of directional asymmetry and the first report of developmentally asymmetric male genitals in Entelegynae spiders. Generalities, evolution and categorization of asymmetry in spiders are further discussed.