Leg movements of stick insects walking with five legs on a treadwheel and with one leg on a motor-driven belt. II. Leg coordination when step-frequencies differ from leg to leg.

Stick insects walking with five legs on a self-propelled treadwheel and with the left hindleg (L3) on a motor-driven belt may move the "belt" leg L3 and the "wheel" legs with different frequencies. When L3 made less steps than L2, that step of L2, which was performed during the swing phase of L3, is prolonged. The time interval between the end of swing phase of L3 and the onset of the following swing phase of L2 was remarkably constant. When L3 made more steps than L2, that step of L3, which was performed during the swing phase of L2, is prolonged. Again, the time interval between the end of swing phase of L3 preceding a L2 swing phase and the onset of the L2 swing phase was relatively constant. For both kinds of walking situations phase response curves were drawn. They show that two types of coordinating channels exist: An anteriorly directed type is more dependent on absolute time than on phase. A posteriorly directed type is phase-dependent. Both inhibit the transition from stance to swing for some time. The results are compared with the existing coordination models.