Conflicts over mating decisions characterize the sexual behaviour of many insects,in particular when males encounter females that already carry enough sperm to fertilize their eggs, since a mating often will inflict greater costs than benefits upon females. Therefore, coevolutionary models predict adaptation and counter-adaptation by the sexes in a battle to control the outcome of sexual encounters. A phylogenetic analysis was performed on patterns of sexual dimorphism and mating systems within water striders (Hemiptera, Gerridae). Phylogenetic effects or «constraints» have significantly shaped patterns of sexual dimorphism in female/male size ratios, legs and genitalia of males, and the structure of the female abdomen. Males of ancestral gerrids were probably slightly smaller than conspecific females, had powerful fore legs adapted to grasp the female's thorax during mating, and had clasping genitalic structures suited to grasp or pinch the female posteriorly. Most gerrids have a female/male size ratio between 1.05 and 1.14, but more pronounced sexual size ratios (above 1.25) have independently evolved several times in the family, usually in association with extended post-copulatory mate guarding. The comparative, phylogenetic analysis suggests coevolution of female anticlasper and male clasping devices for the clade comprising the subfamilies Cylindrostethinae, Ptilomerinae, and Halobatinae while female anticlasper devices have evolved in the absence of male clasping genitalia in the Gerrinae. The ancestral and most common mating system in gerrids is «scramble competition polygyny» from which has evolved «resource defence polygyny» at least four times independently of each other. The phylogenetic effects on patterns of mating behaviour are much less obvious, as exemplified by the large amount of interspecific variation in some genera.