Ontogenetic Scaling of Jump Performance in the African Desert Locust (Schistocerca Gregaria)

Abstract

ABSTRACT Ontogenetic growth was used as a model for the effect of body size on jumping performance of the African desert locust (Schistocerca gregaria). Using models that generated relationships between morphology and body size proposed by McMahon and the relationships between morphology and performance described by Hill, we generated testable predictions of how jumping performance measures may change as a function of body mass. Data were collected over an ontogenetic sequence that ranged from 1-day-old first instars to 45-day-old adults. Performance was quantified using a high-sensitivity, three-dimensional force plate. Performance parameters quantified included the force, acceleration, take-off velocity, kinetic energy and power output. With the exception of power output, each measure of performance scaled to body mass in a manner consistent with the predictions of the elastic similarity model. Power output scaled to body mass in a manner consistent with the predictions of the constant stress similarity model. As we noted previously for the scaling of flexural stiffness of the metathoracic tibiae, the elastic similarity model is approximated by the performance of the locust in spite of the morphological design that deviates from that model’s predictions. These results indicate that the jump has separate functions in the flightless juvenile instars and in the flying adult stage of the life history. Juvenile locusts produce take-off velocities of between 0.9 and 1.2 ms−1 that are relatively scale-independent. The take-off velocity in juveniles produces a distance of ballistic travel that averages between 20 and 30cm. In adults, the take-off velocity is also relatively scale-independent at a level approximately twice as high as in juveniles (i.e. 2.5 ms−1). This velocity is coincident with the minimum flight speed reported by Weis-Fogh and a minimum flight speed that we have estimated using actuator disc theory. We suggest that, in juveniles, the jump is designed to achieve a characteristic distance travelled and in adults the jump is designed to achieve a minimum velocity necessary to fly.