Optimum Diet and Body Size in Backswimmers (Heteroptera: Notonectidae, Pleidae)

Abstract

Relations between predator and prey sizes and profitability of the prey to the predator were analyzed for cost-benefit in the backswimmers Notonecta lunata Hungerford, N. undulata Say, N. irrorata Uhler, N. insulata Kirby (Notonectidae) and the pigmy backswimmer Neoplea striola (Fieber) (Pleidae). As costs of pursuing prey are often great (i.e., subjecting predator to predation), the probability of prey capture (p) was incorporated multiplicatively in a cost-benefit function. Attack distances of Notonecta spp. increased with prey size and decreased with predator size; attacks by Neoplea were the same length on all prey sizes. In both genera, handling time and the amount of a prey item eaten depended on both predator and prey size. Pursuit costs of time and energy appear too low to affect the profitability of prey. Generally, larger predators obtained less profit from prey. The profitability of prey appeared normally distributed with reference to prey size in N. lunata, N. undulata, and N. irrorata, but generally remained constant in N. insulata and Neoplea striola. Prey density did not affect the sizes in the diet of Neoplea but starvation increased the proportion of large prey eaten. This tendency poorly fits an hypothesis of diet optimization. Prey-sizes selected by Notonecta spp. reveal little or no diet optimization. Alternatives to diet optimization are examined.