Aphidophagous insects all exploit aphids as food, but there appears to be no association between the size of the aphidophagous predator and that of the species or the age structure of the aphid colonies they exploit. Aphid colonies generally increase, peak and decline in abundance, and are exploited by a sequence of predators, which is consistent from year to year. The objective of this study was to determine the rules underlying this temporal pattern. For example, in the field, aphid colonies are often first attacked by a small and then a larger species of ladybird. Theory based on the geometry and physiology of ladybirds predicts that the quantity of food required for oviposition and the area searched per unit time should scale with body weight, with exponents of 1 and 0.66, respectively. An analysis of empirical data supports these predictions. Thus, in relative terms a 35 mg ladybird requires 1.5 times more aphids per unit area for oviposition than a 10 mg ladybird. That is, the temporal pattern in oviposition is possibly mainly determined by geometrical and physiological constraints associated with body size, with small species of ladybird able to lay eggs at lower aphid population densities than large species. Cannibalism is thought to be the mechanism by which these predators are able to coexist.