A MODEL OF LEG COORDINATION IN THE STICK INSECT, CARAUSIUS-MOROSUS .2. DESCRIPTION OF THE KINEMATIC MODEL AND SIMULATION OF NORMAL STEP PATTERNS

An information-processing model of leg control is presented. The model incorporates the four primary mechanisms of interleg coordination in the stick insect (Fig. 1). It is formulated in terms of kinematic parameters of leg movement. The step pattern generator for each leg is represented as a relaxation oscillator with two states - swing and stance. The variable of integration corresponds to leg position; the oscillation corresponds to protraction and retraction of the leg between the end-points of swing and stance. Coordination mechanisms from adjacent legs modulate the movement end-points of the controlled leg, the receiver; coordinating influences depend upon the state, the position and the retraction velocity of the controlling leg, the sender (Fig. 2). The simulations demonstrate that the four mechanisms produce a stable metachronal coordination and rapidly correct perturbations. When all six legs retract at the same speed, the simulated leg movements show a stable coordination over a wide range of step periods (Fig. 3). However, both the shape of the simulated leg movements (Fig. 3) and the inability to quantitatively simulate ipsilateral lag intervals (Fig. 4) point out the limitations of a model restricted to kinematic parameters.