No evidence for costs of being large in females of Orgyia spp. (Lepidoptera, Lymantriidae): larger is always better.

Strong correlation between female body size and potential fecundity is often observed in insects. Directional selection favouring increased body sizes is thus predicted in the absence of opposing selection pressure. The evolutionary forces capable of counterbalancing such a 'fecundity advantage' are poorly documented. This study focuses on revealing the costs of large body size in the wingless females of Orgyia antiqua and O. leucostigma, two related species of lymantriid moths. Extreme behavioural simplicity of these animals allows systematic assessment of various fitness components in conditions that are close to natural. A linear relationship between pupal weight and potential fecundity was observed. This association was found to be independent of particular rearing conditions. There was no evidence that the relationship between fecundity and body mass becomes asymptotic when body sizes increases. No component of fitness showed a negative phenotypic correlation with body size; some displayed a weakly positive one. In particular, pupal mortality, adult longevity, mating and oviposition success, as well as egg hatching rate and egg size, were established as independent of body size in a series of field and laboratory experiments. There was a very high overall efficiency of converting resources accumulated during the larval stage to egg masses. With no costs of large adult size, selective forces balancing the fecundity advantage should operate in the course of immature development. The strong dependence of realized fecundity on body size is considered characteristic of the capital breeding strategy.