Population structure in rock-pool Corixids (Hemiptera, Corixidae) during the reproductive season

Populations of Arctocorisa carinata (Sahlb.) and Callicorixa producta (Reut.) were studied by regular removal netting during three successive summers. The length of the reproductive period is about 4 months. The numbers of larvae fluctuated considerably and in an apparently regular manner. In most pools the numbers of small larvae reached two distinct peaks, in June and in July — August, but sometimes there was a single peak in mid-summer. Corresponding changes were observed in the numbers of larger larvae. The numbers of adults reflected their movements from pool to pool, and considerable densities were recorded in pools with low densities of larvae. In most pools the maximal densities of large larvae were similar, and presumably often attained or exceeded the limit set by the prey populations. As the larvae developed, total biomass increased rapidly, maximal values usually coinciding with the peak numbers of the last-stage larvae. At the end of development mortality seemed to be heavy. Fluctuations in numbers were probably due mainly to elimination of small larvae by older individuals either by cannibalism or by competition. When the numbers of small larvae declined, preying behaviour changed; the older larvae grew less large and their mortality was higher. In the permanent pools, A. carinata was dominant and had a lower mortality. When the temporary pools refilled in July, they were rapidly colonized by C. producta; thus in late summer the two species partly occupied different habitats.