Different instars of Gerris argentatus Schumm. were most abundant each in a different habitat in a pond. The older stages were found on more open water surface than the younger ones. Totally open water was avoided. Hind femurs of G. argentatus and sympatric G. lacustris (L.) were measured. The ratio of the measurements of two subsequent developmental stages is about 1.3—1.6. The measurements of G. lacustris instars lie between those of different G. argentatus size classes. The observations are interpreted to indicate different habitat preferences. The length of the leg reflects ease of movement in dense vegetation, correlating with efficiency of foraging. Changes in abundance of the two species in the study pond are discussed.