In a crayfish walking on a motor-driven belt at constant speed, the power stroke (stance phase) of an individual leg is experimentally prolonged for a short time; the ways the legs retain their normal coordination are then observed. The results are shown in a modified phase-response curve. Only ipsilateral coupling is considered in detail.

Two coordinating mechanisms are necessary and sufficient to describe the experimental results. (1) The forward-directed influence is only active when the controlling (posterior) leg performs a power stroke. If the controlled leg is in power stroke, this influence leads it to interrupt the power stroke and start a return stroke. If the leg is in return stroke, it is influenced to continue the return stroke, the duration of which is thereby prolonged. The speed of the return stroke is also decreased. (2) The backward-directed influence is active only during the last part of the power stroke and the first part of the return stroke of the controlling (forward) leg. If the controlled leg is in return stroke, it is influenced to interrupt the return stroke and start a power stroke. If it is at the end of its power stroke, it is influenced to continue this stroke.

For legs 3 and 4, each of which possesses both anterior and posterior neighbours, the influences exerted by these two coordinating mechanisms are assumed to be additive. A model calculation shows that the two mechanisms are sufficient to describe the behaviour observed. The results are compared with previous findings from the literature.

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