The seasonal changes in the frequency of long-winged individuals and individuals with reduced flight organs in bivoltine populations of Gerris lacustris (L.) and odontogaster Zett. were investigated by means of regular sampling of adults and rearings of nymphs. Both species have a high frequency of short-winged individuals in the first generation. During the late summer and autumn there is an increase in lacustris nymphs moulting into short-winged adults, while nearly all odontogaster become long-winged. The females of the early summer generation have reduced ventral pigmentation and attain reproductive maturity just after their final moult. Their offspring—the partial second generation—together with the nonreproducing adults of the late summer generation constitute the overwintering population. During the teneral development of newly moulted, long-winged adults lasting for one or two weeks there is a nearly fourfold increase in the mean fibre diameter of flight muscles, a significant growth of the thoracic phragmata, and a pigmentation of mesonotum associated with the development of flight ability. The flight muscles are histolysed during the period of reproductive activity in spring. These observations promote a new hypothesis regarding the adaptive significance of alary polymorphism in aquatic Hemiptera.