: Egg and larval development times of Gerris odontogaster (Zett.) (COD), G. lateralis Schumm. (LAT), G. lacustris (L.) (LAC), G. thoracicus Schumm. (THO) and G. rufoscutellatus Lt. (RU) were studied. In species OD they are dependent on temperature. The roles of photoperiod, crowding and food remain open, even if continuous light seems to have a retarding effect on the larval development of OD and LAC. The development times of the subsequent instars of OD, LAT and LAC were shown to be of unequal lengths. When the development rates of different species were compared in 25―26°C, the species sequence from fastest to slowest was LAT, THO, LAC, OD and RU, i.e., disregarding RU and THO, just the opposite of the order experienced in nature in Finland. This raised the question of why the fastest species have not displaced OD in the microclimatically warmest habitats. The answer is supposed to lie in the seasonal, environmentally determined alary dimorphism of OD, which adapts it better than other species to microclimatically warm but at the same time temporary habitats.