feeding behavior of a sit-and wait-predator, Ranatra dispar (Heteroptera: Nepidae): optimal foraging and feeding dynamics

The feeding behaviour of R. dispar was examined with respect to the proportion of prey contents used, the time between successive captures (intercatch interval) and the feeding time. The feeding process consisted of three stages. (1) Injection of venom, (2) breakdown of tissue/digestive stage and (3) extraction of food. The rate of extraction from an individual prey decreases as its contents are depleted, but was shown to increase significantly during the first 15 min before decreasing. Even after 30 min the extraction rate was still marginally higher than the initial extraction rate. This phenomenon is quite different to what has previously been reported for sucking bugs. There was a negative relationship between increasing prey density and prey depletion, with the predators being significantly more 'wasteful', (i.e. prey were discarded before all extractable food was removed) at the two higher prey densities compared with those at the lower densities. As the prey density decreased from 60 to 1 prey per container, so the resultant intercatch interval, feeding time, and the average dry weight extracted per prey increased. No correlation was found between individual intercatch interval and subsequent feeding time when examined throughout a sequence of eight captures. This is taken to support the optimal feeding model in which the predator reacts to the average profitability of the environment (i.e. mean intercatch interval) rather than as reflected by the amount of food in the gut. The effect of the changing rate of extraction of food during a meal allows Ranatra, when exposed to high prey density, to feed for less than half the time on each prey item that it spends at low densities, and yet still obtain 60% of available food.