Electromyograms from the elevator and depressor muscles, together with tarsal claw receptor activity, were recorded from the fourth legs of freely walking scorpions. The slope of the depressor burst duration versus step cycle time was less for short cycle times, below about 600 ms, than it was for longer cycles. The opposite was true for the elevator burst duration versus step cycle relationship, and the slope for longer cycle times was not significantly different from zero. The switching of motor activity between antagonists at the stance to swing phase transition was different from that of the swing to stance phase. The depressor burst invariably terminated before the elevator burst, while the elevator burst frequently did not terminate until after the depressor burst had begun. A similar asymmetry of the elevator/depressor motor programme has been described for insect and crustacean preparations. The termination of the depressor muscle burst represents the initial peripheral indicator that the decision to step has been made centrally. The latency between the central decision and the time when the leg is lifted, as determined by tarsal claw receptor burst termination, can be as much as 125 ms. This observation is of importance when considering both intrasegmental and intersegmental neural control mechanisms of scorpion locomotion.

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