The locomotion of many soft-bodied animals is driven by the propagation of
rhythmic waves of contraction and extension along the body. These waves
are classically attributed to globally synchronized periodic patterns in the
nervous system embodied in a central pattern generator (CPG). However, in
many primitive organisms such as earthworms and insect larvae, the evidence
for a CPG is weak, or even non-existent. We propose a neuromechanical model
for rhythmically coordinated crawling that obviates the need for a CPG, by
locally coupling the local neuro-muscular dynamics in the body to the mechanics of the body as it interacts frictionally with the substrate. We analyse our
model using a combination of analytical and numerical methods to determine
the parameter regimes where coordinated crawling is possible and compare
our results with experimental data. Our theory naturally suggests mechanisms
for how these movements might arise in developing organisms and how they
are maintained in adults, and also suggests a robust design principle for
engineered motility in soft systems.