Effects of water depth on choice of spatially separated prey by Notonecta glauca L

In laboratory experiments, the predator, Notonecta glauca L., was exposed to varying densities of surfacedwelling culicine mosquito larvae and the bottom-inhabiting isopod, Asellus aquaticus L., in either shallow or deep water at 20° C. At this temperature N. glauca spends most of the time at the water's surface, so, by changing water depth the accessibility of Asellus to the predator was manipulated relative to a consistent Culex presence.All N. glauca spent more time submerged in shallow (75 mm) than in deep (275 mm) water but submergence times were independent of exposure to different prey combinations. Mature females made more descents and remained submerged longer than males.All N. glauca captured more Asellus in shallow than in deep water but males and newly-moulted females captured Culex predominantly, in all treatments, regardless of water depth or prey availability. Mature females captured mostly Asellus in shallow water and Culex in deep water. When presented with small rather than large Asellus, mature females spent an equivalent amount of time submerged as in the large Asellus treatments and ate the same number of Asellus but more Culex.By foraging on Culex larvae, male and newly moulted female N. glauca maximise their rate of energy intake. In contrast, mature females may actively select Asellus to optimise something other than energy (e.g. specific nutrients). Alternatively their predation on Asellus may be simply a consequence of a high encounter rate with this prey type, reflecting habitat use determined by factors that do not concern prey capture directly.