A methodological constraint influencing measurement of food intake rates in sucking predators

In a number of studies which provide food extraction curves for sucking predators, data were obtained by separating the predator from the prey (i.e. by artificially interrupting feeding) at predetermined intervals within the total feeding time. The amount of food the predator had extracted at these time intervals was then determined by measuring either the mass gain in the predator or the mass loss in the prey. An implicit assumption of this method is that at the time feeding is interrupted, the food extracted by the predator is contained within its own digestive system and no part of the food has been released back into the prey. I found this was not the case with the crab spider Diaea sp. indet. feeding on the fruit fly Drosophila immigrans. The food Diaea extracts from prey is retained in its own digestive system only at times when the spider changes feeding sites on the prey and when it discards the prey when finished feeding. At other times it cycles the extracted food between itself and the prey (i.e. a sucking phase is alternated with a relaxing phase during which the extracted food is released back into the prey). Unless feeding is interrupted as close as possible to the end of the sucking phase, the mass change measured in the spider will be an underestimate of the actual amount of food extracted at this time. My results suggest that understanding how a sucking predator's feeding mechanism affects the transfer of food is necessary, not only in identifying constraints affecting feeding efficiency, but also in identifying how these constraints relate to the design of empirical tests. The precision of these tests will, in part, reflect the degree to which these mechanisms are incorporated into the test design.