Average duration of the larval stage of Syrphus corollae Fab. reared at 20 ± 1 °C decreased linearly with increase in amount consumed from 9.2 days with 40.9 cal consumed/larva to 7.9 days at 78.1 cal/larva. Mean weight of pupae and percentage emergence of adults increased with calories consumed. No eggs were laid by flies developed from larvae which consumed less than 54.6 cal of energy, equivalent to 38 aphids. Average fecundity increased linearly with consumption up to 81.6 cal/larva (= 58 aphids). Mean reproductive output was 3.2% of larval consumption above 55 cal but decreased as larval consumption increased. When food was scarce, fecundity decreased because pupae, and therefore ovaries, were smaller. Larvae consumed as much as possible in the time available rather than pupating after consuming a minimal amount.