Summer distribution patterns of the backswimmer, Anisops wakefieldi (Hemiptera : Notonectidae), in a New Zealand pond

The relative abundance of different-sized Anisops wakefieldi in a small, fi shless pond was determined from sweep-net samples taken along one open-wate r and two shoreline transects at noon on two dates, and at noon and midnigh t on a third date. During the day, large individuals occurred only along th e shore, whereas smaller ones occurred both along the shore and in open wat er. At night, large individuals also occurred in the open water. The diurna l size-class segregation suggests that young individuals migrate into less preferred, open water to avoid interference and cannibalism from larger ind ividuals. The shoreline may be preferred during the day because it provides greater food resources or a refuge from visually feeding avian predators. Laboratory experiments in containers with floating aquatic plants at one en d complemented the field data. When alone, both small and large A. wakefiel di sought cover during the day and moved into open water at night. During t he day, the presence of large individuals induced small ones to move into o pen water. The results of this study on a notonectid population are consist ent with some earlier ones in demonstrating that interactions between large and small size classes may lead to pronounced spatial segregation of size classes, and show for the first time that spatial distributional patterns d uring the day and night may be very different.