Substrate Type Affects Partial Prey Consumption by Larvae of the Antlion Myrmeleon Acer (Neuroptera: Myrmeleontidae)

Three types of hypotheses have been used to explain why predators often do not fully consume their prey. First, parts of the prey may be inedible. Second, physical or physiological constraints may be important. For example, gut capacity might limit the amount that can be taken in one meal, which may result in partial prey consumption. Third, individuals may choose to terminate feeding before an absolute constraint is reached. For example, some optimality models assume that partial prey consumption maximises rate of energy gain. Distinguishing between these hypotheses can be difficult because they may make qualitatively similar predictions. For example, as prey encounter rate increases, a smaller proportion of each prey item is consumed. We attempted to test between 'constraint' and 'choice' hypotheses by manipulating the expectation of capture independently of the actual capture rate for larvae of the antlion Myrmeleon acer. Ants escaped from artificial pits made in coarse sand more quickly than from pits made in fine sand, suggesting that capture success for antlions would be lower in coarse sand. Furthermore, antlions given the choice of these two substrates built their pits in fine sand. We therefore proposed that an antlion's 'expectation' of mean capture success will be lower when it is forced to build its pit in coarse sand compared with fine sand. We found that antlions in coarse sand consumed a greater proportion of their prey than did antlions in fine sand, contrary to the constraint hypotheses, but in support of choice models. Handling time did not differ between substrates, suggesting that antlion larvae may be able to modify digestion such that they can digest prey at a greater rate in poor conditions.