Seasonal time constraints do not explain exceptions to the temperature size rule in ectotherms

A negative relationship between rearing temperature and adult size is widespread in ectotherms yet its causes remain poorly understood. It has been suggested that summer generations of some aquatic insects provide exceptions to this temperature size rule (TSR). The hypothesis that seasonal time constraints are responsible for these exceptions was tested using the mayfly Cloeon dipterum. Simulating early-season conditions by varying photoperiod and temperature, we found that this species followed the TSR. However, under simulated seasonal time constraints, the species omitted most of its summer adult generation (delaying completion of nymphal development till the next year), which limited the scope and applicability of the hypothesis for this species. Even those animals that completed nymphal development before winter showed no weakening of the negative temperature size relationship that was predicted by the hypothesis. Moreover, these animals did not increase rates of development or growth in response to simulated time constraints. We then analysed further the study that originally indicated aquatic insect exceptions to the TSR and suggest that gaps in the data and/or experimental design can explain the apparent exceptions. From these analyses and a survey of other exceptions, we conclude that time constraints do not account for any reported counter-TSR relationship.