Figure 1.

(a) Schematic of a mechanical model for crawling. (b) Local feedback loop responsible for coordination. The body is represented as a chain of masses linked by linear springs with stiffness k and damping c. The muscles are represented by the actuators f and the interaction with the substrate by asymmetric solid friction F^f. The corresponding equation of motion is given by equation (2.2). The feedback loop represented here shows that the motor neuron θ gets activated when the strain ∂_xu reaches a given threshold and the muscle f contracts when the neuronal activity θ crosses a threshold (see equations (2.4) and (2.5)). The dash-dotted blocks indicate the potential presence of neural coupling (see equation (4.1)). (Online version in colour.)