The transition between AP and IP states is mediated by internal spring constants. Mean and standard deviation of as functions of kx and ky for external rotor separations (a) , (b) , and (c) . White dotted lines correspond to the blue bifurcation plot in figure 5c. For small kx, the system is capable of supporting intermediate phase-locked states, with . However, for larger kx, an abrupt transition between AP and IP occurs. (Online version in colour.)