A model of leg coordination in the stick insect, Carausius morosus

Abstract

Mechanisms dependent upon leg position co- ordinate the alternate stepping of adjacent ipsilateral and contralateral legs in the stick insect. In this insect, swing duration and step amplitude are independent of walking speed. A simple geometrical model of the leg controller is used here to test different mechanisms for compatibility with these two invariant features. Leg position is the state variable of a relaxation oscillator and position thresholds determine the transitions be- tween swing and stance. The coordination mechanisms alter these thresholds. The position-dependent mecha- nisms considered differ either in the form or the speed- dependence of the function relating the shift in the posterior threshold of the receiving leg to the position of the sending leg. The results identify parameter com- binations leading to alternate stepping with symmetric or asymmetric phase distributions, to shifts in the pos- terior extreme position as a function of speed, to double stepping or to in-phase stepping. An optimal position- dependent excitatory mechanism is described. Finally the consequences of adding either inhibitory influences or time-dependent excitatory influences are analyzed.