OPTIMUM BODY-SIZE IN APHIDS

1. Smallness in British plant-sucking bugs is associated with feeding on the contents of individual plant cells, especially phloem. 2. The sizes of species of aphids living in similar habitats and the rates of water loss from those that have and lack a filter chamber do not support the contention that small size in aphids is a means of maximizing the loss of water by cuticular transpiration. 3. An optimum energy partitioning model, previously developed for aphids, was used to predict the interspecific relation between adult weight and birth weight that would maximize the population growth rate, r(m). Using the observed r(m)/RGR ratio (0.8-0.9) and that the gonads are smaller than the soma at birth the predicted relation between the logarithm of the adult weight and the logarithm of the birth weight is linear and has a slope of 1. 4. The relation between the logarithm of the adult weight and the logarithm of the birth weight for eighteen species of aphids is linear and has a slope of 1. 5. Birth size differs between species of aphids and in each species is assumed to be the minimum size necessary for feeding. Accepting this constraint then the optimum adult size for maximizing r(m) is approximately 15 times the birth size.