Role of ageing and temperature in shaping reaction norms and fecundity functions in insects

The existing energy partitioning models assume that fecundity is constant throughout adult life. In insects, however, fecundity is a triangular function of time: after maturation, it initially sharply increases and after reaching its maximum it slowly declines as the mother ages. These models also fail to explain that empirical data generally indicate an increase in juvenile growth rate caused by improvement in food quality results in larger adults, whereas that caused by an increase in ambient temperature results in smaller adults. This 'life history puzzle' has worried many biologists for a long time. An energy-partitioning model for insects is presented with soma and gonads as its components, which - contrary to other models - assumes ageing of soma. This model explains the triangular shape of the fecundity function, and also offers an explanation of the 'life history puzzle'. The differential response in adult size to changes in food quality and temperature in nature may result from the differential responses of our model's parameters to changes in these environmental parameters. Better food quality results in bigger adults, because food quality affects the assimilation rate, but not the rate of conversion of gonadal biomass into offspring, or the rate of senescence. In contrast, an increase in temperature speeds up all the processes. That is, temperature affects the assimilation rate, the conversion rate of gonadal biomass into offspring, and the rate of senescence equally. Therefore, an increase in temperature results in larger or smaller adults, depending on the shape of the senescence function.