(A-D) Time-varying curvatures of the worm’s head region, measured from experiments (red, 5047 cycles using 116 worms) or produced by models (black). The four models match the experimental curvature with MSEs ≈ 0.18, 0.44, 0.18, and 0.39, respectively. (Inset table) (fraction of period of bending toward the ventral or dorsal directions) and (fraction of period of straightening toward a straight posture), for experiments or models, respectively. (E-H) Phase portrait graphs of the free-running dynamics shown in (A-D), measured from experiments (red) or produced by models (black). Arrow indicates clockwise movement. (I-L) Phase plots showing each model perturbed (indicated by purple arrow) away from the stable limit cycle and then recovering toward the equilibrium. (M-P) Phase response curves with respect to both-side muscle inhibition, measured from experiments (blue, 991 trials using 337 worms) or produced by models (orange). Four models match the experimental PRC with MSE ≈ 0.12, 0.21, 0.37, and 0.72, respectively.