1. An optimal energy partitioning model is used to predict the optimum adult sizes of the soma, gonads and flight apparatus in insects. A comparison of the investment in gonads and lipoidal reserves in winged and unwinged individuals in species from three families of aphids is used to test the predictions and define the cost of developing a flight apparatus. 2. In accordance with theoretical predictions winged aphids had smaller gonads than wingless aphids and there appeared to be a trade-off between the investment in gonads and lipoidal reserves in both morphs. 3. Developing a flight apparatus prolongs development and resulted in a 20% reduction in gonad size