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Despite a long tradition of research, our understanding of mechanisms driving prey selectivity in predatory insects is limited. According to optimal foraging theory, predators should prefer prey which provides the highest amount of energy per unit time. However, prey selectivity may also depend on previous diet and specific nutritional demands of the predator. From the long-term perspective, diet composition affects predator fitness. An open question is whether short-term selectivity of predators provides a diet which is optimal in the long-term. To shed more light on these issues, we conducted laboratory experiments on prey selectivity and its long-term consequences in larvae of the dragonfly  Sympetrum sanguineum . We conditioned the larvae to one of two prey types, the cladoceran  Daphnia magna  and larvae of a non-biting midge  Chironomus  sp., and then exposed them to various combinations of the two prey types. We found that dragonfly larvae conditioned to  Chironomus  larvae consumed the same amount of  D. magna , but significantly less  Chironomus  larvae compared to dragonfly larvae conditioned to  D. magna . However, there was no effect of previous diet on their success of capture and handling time, suggesting a limited role of learning in their ability to process prey. We then tested the long-term effects of diets with different proportions of both prey for survival and growth of the dragonfly larvae. Individuals fed  Chironomus -only diet had higher mortality and slower growth than dragonflies fed  D. magna , while larvae fed a mixed diet had the highest survival and growth rate. In conclusion, we show that dragonfly larvae fed by  Chironomus  larvae performed poorly and compensated by preferring  D. magna  when both prey types were available. The superiority of the mixed diet suggests that a diverse diet may be needed to satisfy nutritional demands in  S. sanguineum  larvae. We demonstrate that merging short-term predation experiments with relevant data on predator fitness may provide better understanding of predator-prey interactions and conclude that detailed information on the (mis)matches between prey composition and predator nutritional demands is needed for further progress.

Prey selectivity and the effect of diet on growth and development of a dragonfly, Sympetrum sanguineum