A MODEL OF LEG COORDINATION IN THE STICK INSECT, CARAUSIUS-MOROSUS .3. RESPONSES TO PERTURBATIONS OF NORMAL COORDINATION

A model of interleg coordination presented in a separate report is evaluated here by perturbing the step pattern in three ways. First, when the initial leg configuration is varied, the simulated leg movements assume a stable coordination from natural starting configurations in a natural way (Fig. 1a). They also rapidly re-establish the normal coordination when started from unnatural configurations (Fig. 1b-d). An explicit hierarchy of natural frequencies for the legs of the three thoracic segments is not required. Second, when the coordination is perturbed by assigning one or more legs a retraction velocity different from the rest, gliding coordination or various integer step ratios can be produced (Figs. 2-4). Third, when the swing of one leg is obstructed, characteristic changes in the stepping of other legs occur (Fig. 5). Overall differences between the step patterns of the model and those of the stick insect are related to the form of the coordinating mechanisms. Errors made by the model, such as overlapping swings by adjacent legs or discrepancies in step timing and step end-points, point out the limitations of a model restricted to kinematic parameters.