Does the 'investment principle' model explain moulting strategies in lepidopteran larvae?

The aim of the present study was to evaluate the applicability of a general optimality model of insect development in the case of lepidopteran larvae. According to the model, larvae moult mainly to increase the size of mouthparts, which is assumed to limit consumption rates. The assumptions of this model being met, one should expect a dependence of growth rates on head capsule size, constancy of absolute growth rates within an instar and higher growth rates in older instars. The validity of these predictions is tested on two species of lymantriid moths. Head capsule size has only a weak effect on growth rates in one of the species, and no effect in the other. Absolute growth rates tend to increase during the development within an instar. Higher growth rates of older instar larvae are explainable by an allometric relationship that extrapolates from growth within the preceding instar. Thus, there is no evidence of an extra increase in growth performance attributable to moulting into the subsequent instar. These results support the idea that growth rates of lepidopteran larvae are limited by nutrient absorption rates rather than by the rates of consumption. There is a considerable cost of moulting in terms of lost growing time: the larvae would double their weights during an instar if they were able to develop without the premoult decrease in growth rates. The investment principle model does not appear to be directly usable to explain moulting strategies in the lepidopteran larvae studied. However, it may well be applicable after some adjustment (i.e. assuming that larvae should moult before consumption rates become limiting).