Summary
Legged locomotion is the most common behavior of terrestrial animals and it is assumed to have become highly optimized during evolution. Quadrupeds, for instance, use distinct gaits which are optimal with regard to metabolic cost and have characteristic kinematic features and patterns of inter-leg coordination. In insects, the situation is not as clear. In general, insects are able to alter inter-leg coordination systematically with locomotion speed, producing a continuum of coordination patterns. This notion, however, is based on the study of not one but several insect species. These species differ greatly in size and weight and each species tends to walk at a rather narrow range of speeds. We have addressed these issues and examined four strains of Drosophila, which are similar in size and weight, but tend to walk at different speed ranges. Our data suggest that Drosophila controls its walking speed almost exclusively via step frequency. At high walking speeds we invariably found tripod coordination the quality of which increased with speed as indicated by a simple measure of tripod coordination strength (TCS). At low speeds we also observed tetrapod coordination and wave gait-like walking patterns. These findings suggest not only a clear speed dependence of inter-leg coordination, but imply that inter-leg coordination is flex-ible. This was further supported by amputation experiments in which we examined walking behavior in animals after the removal of a hind leg. These animals show immediate adaptations in body posture, leg kinematics, and inter-leg coordination thereby maintaining their ability to walk.
