The size of an organism is one of the most influential traits from an ecological and evolutionary perspective. During ontogenesis, immature insects can increase approx. 10,000 fold in size and accompanying this increase, is often altered morphology, physiology and behaviour which has ecological consequences. Despite this, the majority of our knowledge of specific nutritional requirements of insects has been restricted to the final nymphal instar. Although scaling form with function is not well understood, due to allometric constraints, it is predicted smaller animals require a diet that is of a ‘higher’ quality than that for larger animals. For grasshoppers, changes in host plants eaten and diet breadth have been associated with ontogenetic growth. The Australian plague locust, Chortoicetes terminifera, develops in an environment dominated by the perennial Mitchell grass and annual Button grass. Feeding trials were undertaken on all C. terminifera instars, except the first to (1) ascertain their nutrtitional requirements, (2) their performance on the two host grasses, and (3) elucidate the mechanisms that resulted in differences in growth and development between the different instars.
We report the results of experiments that demonstrate that early and late nymphal instars C. terminifera, required the same ratio of the major macronutrients found in plants, protein and carbohydrate. And although the early instar nymphs performed equally on both grasses, due to an interaction between nymphal size and leaf biomechanical properties that influenced the amounts of macronutrients absorbed, the later instar nymphs performed more poorly on Mitchell grass than Button grass. Analysis revealed that when feeding on both grasses, locusts absorbed protein with equal efficiency but the amount of carbohydrate absorbed declined with increasing age and that this decrease in the relative amount of carbohydrate assimilated was more pronounced for nymphs feeding on Mitchell grass. Decreases in the relative amount of carbohydrate absorbed was associated with altered chewing behaviour, decreased fractionation of a meal and increased time the food was retained in the gut. Our results suggest that ontogenetic shifts in diet may result not only from spatial and temporal variation in plant quality but for an herbivorous chewing insect, leaves differ nutritionally with body size. Thus, because of an interaction between the biomechanical properties of leaves and locust food processing apparatus, as locusts increase in size, their nutritional ‘world’ may become more heterogeneous.