  Limbless locomotors, from microscopic worms to macroscopic snakes, traverse
complex, heterogeneous natural environments typically using undulatory body
wave propagation. Theoretical and robophysical models typically emphasize body
kinematics and active neural/electronic control. However, we contend that
because such approaches often neglect the role of passive, mechanically
controlled processes (those involving "mechanical intelligence"), they fail to
reproduce the performance of even the simplest organisms. To uncover principles
of how mechanical intelligence aids limbless locomotion in heterogeneous
terradynamic regimes, here we conduct a comparative study of locomotion in a
model of heterogeneous terrain (lattices of rigid posts). We used a model
biological system, the highly studied nematode worm Caenorhabditis elegans, and
a robophysical device whose bilateral actuator morphology models that of
limbless organisms across scales. The robot's kinematics quantitatively
reproduced the performance of the nematodes with purely open-loop control;
mechanical intelligence simplified control of obstacle navigation and
exploitation by reducing the need for active sensing and feedback. An active
behavior observed in C. elegans, undulatory wave reversal upon head collisions,
robustified locomotion via exploitation of the systems' mechanical
intelligence. Our study provides insights into how neurally simple limbless
organisms like nematodes can leverage mechanical intelligence via appropriately
tuned bilateral actuation to locomote in complex environments. These principles
likely apply to neurally more sophisticated organisms and also provide a design
and control paradigm for limbless robots for applications like search and
rescue and planetary exploration.
