Geographical variation in wing polymorphism of the waterstrider Aquarius najas (Heteroptera, Gerridae)

The waterstrider Aquarius najas is wingless in Northern Europe, while winge d individuals occur frequently in Central and Southern Europe. To test if t he latitudinal difference is genetically controlled, we collected mature in dividuals from 10 different populations and raised their offspring in 'comm on garden' laboratory conditions. Half of these populations were from south ern and the other half from central Finland. Daylength and temperature do i nfluence wing development among other species of waterstriders, and thus we maintained a similar short daylength and warm conditions for all populatio ns. These conditions should be favourable for wing development in general. Among laboratory-bred individuals several winged individuals appeared, and their proportion varied between populations. The relative frequency of wing ed individuals was highest in the southern populations. Thus, apart from ph enotypic plasticity there seems to be some genetic control over the occurre nce of wings, and the latitudinal trend coincides with the direction in nat ural populations over a larger European scale. Overwinter survival in our l aboratory conditions was higher among the wingless individuals. The surviva l cost may explain why the proportion of winged individuals was lower in th e northern populations with more extreme overwintering conditions than in t he southern ones.