The relationship between leg stepping pattern and yaw torque oscillations in curve walking of two crayfish species

Curve walking in two species of crayfish, Procambarus clarkii and Astacus l eptodactylus, was investigated to test whether the mechanism underlying cur ve walking is the synchronous action of a centrally pre-programmed leg trip od or whether it is the action of one principal leg that produces the main body yaw torque. Curve walking was induced by an optomotor visual stimulus, and the yaw torque produced by the tethered animals was measured in open-l oop conditions, Our main results suggest that the yaw torque oscillations i n both P, clarkii and A. leptodactylus are related to the movement of outer leg 4 (i.e. leg 4 on the outside of the turn). That is, the peaks in the y aw torque occur, on average, in synchrony with the power stroke of outer le g 4. When comparing the results of this open-loop experiment on P. clarkii with results previously obtained for curve walking in untethered individual s of the same species, we found a much higher variability in leg coordinati on in the open-loop situation. Similarly, here we did not find the same lev el of synchrony in the tripod (formed by outer leg 4 and inner legs 2 and 5 ) observed during untethered free walking, Therefore, we suggest that tethe red conditions mag diminish the need for stability and thus allow outer leg 3 to produce a body rotation regardless of the leg stepping configuration, The characteristics of leg 4 are in line with its major role in turning. A ccording to previous studies, legs 4 provide the largest force and the larg est step amplitude during walking, and their force includes both a pulling and a pushing component which can facilitate the control of turning. Althou gh it is apparent that outer leg 4 is not the only leg that can produce an inward yaw torque, its major role in modulating the yaw torque suggests tha t there may be a specific, centrally generated control of outer leg 4 durin g curve walking in crayfish.