Evolution theory posits that sexual selection, more so than natural selection, drives sex differences, such as bright males and inter-sexual mimicry. However, female-limited dimorphism has not been fully explained by sexual selection even though this phenomenon includes what appears to be intersexual mimicry of bright males. This dissertation presents evidence that natural selection over an ecological gradient may explain sexual dimorphism and female-limited dimorphism in a damselfly species, as well as macroevolutionary patterns of sexual dimorphism throughout a damselfly genus.

I documented new cases of female-limited color dimorphism in the endemic Hawaiian Megalagrion damselflies. Two species, M. calliphya and M. hawaiiense, contain a typical, green female morph (gynomorph) as well a red female morph (andromorph) that is similar in color to the males. In M. calliphya, I describe habitat differences between the sexes and patterns of color variation in males and females over an elevational gradient on Hawaii Island that varies in temperature and solar radiation. Using transplant experiments, I determined that body temperature does not differ between the female morphs according to hue, although it is affected by body brightness. Therefore, environmental temperature is not a selective agent on the female-limited dimorphism. However, solar radiation may select for red individuals, including andromorphs, under exposed conditions. A comparison of antioxidant ability between the female morphs as well between males over an elevational gradient indicated that red pigmentation is an effective antioxidant that may enable red individuals to exist where free radicals are produced from high radiation levels. Finally, I extend the ecological explanation for the female-limited dimorphism to patterns of sex differences throughout the Megalagrion genus as well a case of female-limited dimorphism in a distantly related species.