The Corixidae (Hemipt.) of some Danish lakes

Abstract

1. As bays in lakes gradually fill with vegetation, the succession in soft water may pass from Phragmites to Carex and then Sphagnum, an oligotrophic succession; or, at the other extreme, found in hard-water lakes or near the mouths of rivers in soft-water lakes, from Phragmites and Typha to Salix and Alnus, a eutrophic succession. There are successions of intermediate type. These changes are accompanied by changes in the species of Corixidae. 2. In the hard-water Danish lakes studied, Phragmites is the main reedswamp plant and it persists long after the accumulation of its remains has raised the bottom above the average level of the water. Eventually it is replaced by Alnus. The weight of the trees, probably coupled with the rotting of the mat of Phragmites roots, depresses the soil level and forms pools. 3. Corixa striata is found in the outer parts of reed-beds; C. linnei near the landward edge of wide reed-beds; C. sahlbergi in pools in the Alnus woods. Evidently these species succeed one another as fen develops from open water. 4. C. falleni was found at many stations but not often in abundance. The records suggest that optimum conditions for it are outside reed-beds, in places where organic matter accumulates on the bottom. C. fossarum was found abundantly in two places where open water was due to human excavation in areas of fen. 5. General conclusions about Corixid successions are summarized in the diagram on p. 000 6. What has hitherto been known as Corixa striata is split into two species. The new one, which is the only one found in Great Britain, but which has also been recorded from France, Italy and Sweden, is named C. lacustris. C. lacustris is broader and rounder posteriorly than C. striata (fig. 4), and the pale lines on its corium are wider, particularly at the ends (fig. 3), but the variability of both characters renders difficult the certain distinction of some specimens by means of them. The outer row of palar pegs curves in suddenly at the inner end in C. striata, but not in C. lacustris (fig. 3). The only character on which the two species can be separated easily is the shape of the right and left genital forceps (figs. 5 and 6).