SEXUAL DIMORPHISM AND PREDACEOUS FEEDING-HABITS OF THE WATERSTRIDER GERRIS-REMIGIS SAY (HETEROPTERA, GERRIDAE)

The feeding habits of Gerris remigis were examined in a southeastern Ontario stream. Gerrids with prey items were collected from two pools along a 15-m length of the site. The size distribution of potential prey available in the surface drift during the survey was estimated. Gerrid prey consisted mainly of terrestrial insects; coleopterans, dipterans, homopterans, and hymenopterans represented the predominant orders. The median size of prey taken by all gerrids was significantly larger than that available in the surface drift. A sexual dimorphism in gerrid body size was found. The length of prey taken did not differ between the sexes; however, the diet of the larger bodied females consisted of prey with a greater median body width than the diet of smaller males. Females also took a greater size range of prey. These results support a size-dependent predation model which predicts that the upper size range of prey that can be successfully captured is correlated with predator size.