Ontogenetic Scaling and Mechanical Behaviour of the Tibiae of the African Desert Locust (Schistocerca Gregaria)

Current models for scaling of skeletal morphology were examined to test their applicability to the ontogenetic growth of an exoskeletal animal, the African desert locust (Schistocerca gregaria). It was found that the tibial leg segments of both the mesothoracic (i.e. non-jumping) and the metathoracic (jumping) legs scaled in a manner that produced relatively longer, more slender skeletal elements as the animal grew. Metathoracic tibial length scaled to tibial diameter raised to the power 1.21. This result deviates both from isometric (i.e. geometric similarity) and distortive (constant stress, elastic similarity) allometric models. The mechanical properties of the metathoracic tibiae were measured using a dynamic, three-point bending technique. The flexural stiffness of metathoracic tibiae scaled to body mass raised to the power 1.53. This was intermediate to the predictions made by constant stress and elastic similarity models. Thus, the mechanical properties scaled as predicted by mechanical scaling expectations in spite of the morphological developmental programme. It may be that the thin-walled skeleton developed in the exoskeletal body plan has allowed a finer control over the distribution of load-bearing material in the leg. Such a distribution may be responsible for the observed increase in mechanical stiffness of legs that exhibit an unexpectedly spindly morphology. The rationale for the observed morphological programme may be a design that takes advantage of the inherent deformability of long, slender beams. Thus, it may be that the tibiae, which have been treated as rigid levers, are in fact flexible springs. Calculations indicate that the energy stored in the substantial deflection of the adult, metathoracic tibiae during a jump may be as high as 10 % of the total kinetic energy of the jump.