Sex on the Beach: Sexual Size Dimorphism Among Aquatic Bugs (Nepomorpha) Inhabiting Different Habitats

Aquatic bugs (Nepomorpha) are a species-rich taxon of insects inhabiting all types of freshwaters, often at high densities, and some are among the dominant predators in aquatic food webs. In contrast to the vertebrates and some other arthropod taxa, sexual dimorphism in aquatic bugs is a poorly investigated issue; the present work is the first comprehensive analysis in this field. Morphological differences between the sexes are generally minor, except that female body size is usually longer than that of males. There is no information on the extent of sexual dimorphism in Nepomorpha, except in Belostomatidae and Corixidae. We examined the patterns of sexual size dimorphism (SSD) of 1195 species and subspecific taxa (about 45% of the known species) and whether they conform to Rensch's rule (RR) at the infraorder, superfamily, family and genus levels. We attempted to identify the potential causes of the observed patterns. Our assumption that females are longer-bodied in most aquatic bug species is falsified. Species living in habitats near the edges of water bodies had higher SSD than in all other habitats. The most likely reason for the large intersexual size difference here is that, unlike the species living in other habitats, the species here are sit-and-wait predators. Our results indicate that sexual selection (including fertility selection) plays an important role in the development of SSD. Moreover, it is probable that water-edge habitats and the associated sit-and-wait lifestyle increase sexual selection pressure and the degree of SSD in Nepomorpha. Converse RR, typical for many insects, characterises Nepomorpha as a whole, and two of its superfamilies. At the family and genus levels, however, roughly half of the taxa obey RR and half of them obey converse RR.