Stick insects walking along inclined surfaces.

In the experiments stick insects walk on an inclined substrate such that the legs of one side of the body point uphill and the legs of the other side point downhill. In this situation the vertical axis of the body is rotated against the inclination of the substrate as if to compensate for the effect of substrate inclination. A very small effect has been found when the experiment was performed with animals standing on a tilted platform which shows that the effect depends on the behavioral context. When, however, animals first walked along the inclined surface and then, before measurement, stopped walking spontaneously, a rotation of the body has been observed similar to that in walking animals. In a second experiment it was tested whether the observed body rotation is caused by the change of direction of gravity vector or by the fact that on an inclined surface gravity necessarily has a component pulling the body sideways. Experiments with animals standing on horizontal ground and additional weights applied pulling the body to the side showed similar body rotations supporting the latter idea. In a simulation study it could be shown that the combined activity of proportional feedback controllers in the leg joints is sufficient to explain the observed behavior. This is however only possible if the gain factors of coxa-trochanter joint controller and of femur-tibia joint controller show a ratio in the order of 1 : 0.05 to 1 : 1.8. In order to describe the behavior of animals standing on a tilted platform, a ratio of 1 : 1.7 is necessary. In walking animals, this body rotation requires to change the trajectories of stance and swing movements. The latter have been studied in more detail. During swing, the femur-tibia joint is more extended in the uphill legs. Conversely, the coxa-trochanter joint appears to be more elevated in the downhill legs which compensates the smaller lift in the femur-tibia joint. The results are discussed in the context of different hypotheses.