Developmental changes in locomotory ability may affect many aspects of organismal behaviour and ecological success. While juvenile vertebrates have lower power output and endurance than older animals, the effects of ontogeny on locomotory performance in invertebrates are unknown. Among insects, grasshoppers serve as an ideal model to study ontogeny of locomotory behaviour. Ecological and behavioural studies show that adult and juvenile grasshoppers have different predators and dispersal behaviours, suggesting different locomotory abilities. In addition, older American locust grasshoppers, Schistocerca americana, have improved oxygen delivery, which may enhance endurance. In this study, we forced American locust grasshoppers of different ages to jump repeatedly to exhaustion and quantified jump frequencies and distances. We found that, in contrast to vertebrates, grasshoppers appear to develop from low-power, high-endurance juveniles to high-power, low-endurance adults. These developmental changes in physiology match the ontogeny of life-history behaviours. Adult grasshopper jumping legs produce the high power output required to initiate flight for escape from vertebrate predators and migration, whereas high-endurance juvenile grasshoppers repeatedly jump to escape invertebrate predators and may also be forced to disperse long distances to find food.