FACTORS INFLUENCING SEXUAL SIZE DIMORPHISM IN TEMPERATE WATERSTRIDERS

Sexual size ratios (mean female length divided by mean male length) of 12 species of temperate waterstriders in the subfamily Gerrinae (Hemiptera, Gerridae) are analyzed to test both potentially nonadaptive and adaptive hypotheses for the evolution of sexual size dimorphism. Females tend to be larger than males, and regression of female length on male length (both log-transformed) reveals significant hypoallometry for size dimorphism. Only a nonadaptive model is consistent with the available data. Two closely related genera, Limnoporus and Gerris, are included in the data set, but covariance analysis reveals no significant effect of phylogeny on patterns of variation in size dimorphism. The basic female bias may reflect descent from a common, highly dimorphic ancestor, rather than adaptation within this taxon. Patterns of size dimorphism may be influenced by at least 3, potentially interacting, selective processes: selection for dispersal by flight, selection for reduced loading during prolonged pairing, and sexual selection. -from Author