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The Gerridae (Hemiptera: Insecta) is a worldwide family whose constituent species exhibit dramatic inter- and intra-specific variation in the degree of winglessness (Brinkhurst, 1960; Vepsäläinen, 1978; Calabrese, 1979). At one extreme, the family contains species which are fully winged in all populations and during all seasons, while several species consist almost exclusively of wingless morphs over large geographical ranges and during all seasons. Many species exhibit the intermediate case of wingpolymorphism: the occurrence of various combinations of fully winged, partially winged and/ or wingless morphs in the same population at the same time. Various wing-polymorphic species show differing patterns of spatial and/or temporal changes in morph ratios and patterns may vary both inter- and intraspecifically.
The dramatic differences in frequency of winged morphs pose intriguing questions regarding the evolutionary forces responsible for degree of winglessness and the relationship between degree of winglessness and genetic structure of water-strider species. One might expect genetic structure to be strongly influenced by degree of winglessness via reduction of flight dispersal ability and consequent reduced gene flow. Thus, species composed almost exclusively of wingless individuals should exhibit patterns of marked genetic differentiation and reduced levels of within-population variability typically found in organisms with reduced dispersal (Avise and Selander, 1972; Laing et al., 1976; Selander, 1976). However, additional factors may counteract the effects of reduced dispersal by flight. Gene flow among populations may occur via alternate modes of dispersal, including passive stream drift and overland dispersal (Riley, 1920). Furthermore, marked genetic differentiation among populations is not a necessary consequence of severely reduced dispersal if locality-independent balancing selection is operating (McKechnie et al., 1975).
In this study I compare patterns of spatial variation of polymorphic enzyme-loci and levels of variability in two species of waterstriders (Gerridae: Hemiptera) with differing degrees of winglessness: the nearly wingless Gerris remigis and the wing-polymorphic Limnoporus canaliculatus.