We tested the hypothesis that increased prey availability, which results in increased energy consumption and probably lower energetic costs of foraging, increases reproductive output. Larvae ofMetasyrphus corollae(F.) (formerlySrphus corollaeF.), which are obligate predators of aphids, were individually reared at 19.5 ± 1°C, with 10, 20, 40, or 60 pea aphids offered each day on plants with a surface area of 7.5, 35, or 73 cm2. When more aphids were offered, larvae consumed more and produced heavier pupae. Fecundity of adults reared from these pupae was extremely variable (range, 0–1,488 eggs laid) and depended mostly on adult longevity, not on consumption or pupal weight. More eggs were laid in the first 15 days of adult life by larger adults. Adult longevity depended on the size of the cage in which the adults were kept. Flight had a higher cost against reproduction in small fliesthan in large ones. Differencesbetween the strategy of energy allocation of insects which acquire most of their energy while they are larvae and that of insects which feed while reproducing are discussed.