Body Size Distribution in Predatory Ladybird Beetles Reflects That of Their Prey

The size distribution of the species of a wide range of plants, herbivores, and carnivores are similar in form, i.e., right skewed when size is plotted logarithmically. In addition to differential extinction and speciation rates, it is argued that allometric constraints determine the efficiency with which resources are converted into offspring, which in turn determines the frequency of species of different body sizes. In looking for a general explanation for the size distribution shown by all organisms, theorists currently tend to favor explanations based on physiological rather than ecological constraints. Of the body size distributions of predatory ladybird beetles in the Palearctic, Nearctic, Ethiopian, and Australian regions, only that for the Nearctic is significantly right skewed. Even within the Palearctic, the form of the distribution differs among countries, with that for Japan significantly right skewed and that for Central Europe significantly left skewed. An analysis of the prey of ladybirds indicates that increasing ladybird size is associated with increase in size and/or mobility of their prey. The smallest species feed on mites, and the largest on caterpillars and beetle larvae. In addition, the ratio of the numbers of species of aphidophagous to coccidophagous ladybirds in the Nearctic and Palearctic regions reflects the ratio of the numbers of species of aphids to coccids in these two regions. The relationships between egg and adult volumes for 61 species, and egg and adult masses for 26 species both indicate that large species lay larger eggs than small species. In particular, the predators of large and/or active prey lay larger eggs than the predators of small and/or slow moving prey. The relevance of these findings to our understanding of the factors that have shaped body size frequency distributions is discussed. In the case of predatory ladybirds it is concluded that the shape of their body size distribution curves is determined by the nature and the relative abundance of their prey, that is, by ecological rather than physiological constraints.