Confined Locomotion - Wedging through Cracks and Gaps

A composite exoskeletal system with an integrated array of sensors and muscles enables arthropods to locomote through the most restrictive environments. Here we found that the tough yet compressible exoskeleton of the cockroach, Periplaneta americana, enabled the animal to run through confined spaces less than a third of its standing height (12-15mm). We ran animals through a variable ceiling height rectangular tunnel at 4, 6, 9 and 12mm heights. Surprisingly, animals ran within the vertically restricted space with equal ease at high speeds (52.15±2.68cm/s), only showing a decrease at the lowest height of 4mm (12.56±2.45cm/s, P<0.01). Further, animals maintained a tripod gait at all heights except 4mm when feet often slipped on the surface (medium-grit sandpaper) and stereotyped leg trajectories were altered. Kinematic analysis revealed no significant change of leg cycling frequency (16.12±1.24Hz, P>0.05) across the ceiling heights. However, cockroaches used significantly (P<0.01) shorter stride lengths at 4mm. At the smallest ceiling height, animals chose a more serpentine path of travel and lost foothold traction in 40.2±3.49% (P<0.01) of the strides leading to significantly less effective propulsion. Although navigating through confined spaces likely increases the normal load, remarkably animals showed limited adjustments of the tarsal (hind leg) extreme positions relative to the body centerline, contrary to our expectations. Insights obtained into strategies of high-speed, confined space navigation not only increases our understanding of the mechanical design principles of these organisms, but it also is inspiring the development of novel robots that will go where no robot can at present. 

Effect of Friction

We hypothesized that limit on the smallest traversable gap could be determined by the animal’s ability to generate effective force production driving forward motion. To test the importance of force limitations, we changed the friction on the ceiling (top) and running surface (bottom) of a variable ceiling height rectangular tunnel at gap heights of 4, 6, 9, 12mm. For the top surface, we used uncoated Plexi-glas, P40 grit sandpaper and graphite powder as control, high, and low friction surfaces, respectively. Increasing friction on the top surface decreased running speeds of animals across all gap heights by 5-40% with the greatest decrease at the smallest gap. Decreased speed resulted from a decrease in stride length and a more tortuous path. Decreasing friction did not show any significant change in performance. For the bottom surface, we used Plexi-glas lined with P40, P20, and P100 grit sandpaper as control, high, and low friction surfaces, respectively. Increasing friction did not change performance at 6, 9 or 12mm, but produced decreased speeds at 4mm gaps. The decrease in speed resulted from a decrease in stride length accompanied by increased slipping. Decreasing friction did not change performance at 9 or 12mm, but resulted in discontinuous motion with frequent foot slippage at 4 and 6mm. A simple theoretical model suggests that modifying the spring loaded inverted pendulum-like motion typical of unrestricted locomotion imposes significant forces on the body in confined environments that hinders forward locomotion.

Publications

1. Jayaram, K, Springthorpe D, Haldane D, McKinley S, DiRocco A, Full R.  2013.  Navigation in confined spaces by the American cockroach, 6 January . Society of Integrative and Comparative Biology Annual Meeting. , San Francisco

2. Jayaram, K, Goldman D, Full R.  2014.  Effect of friction on cockroaches running in confined spaces, 6 January . Society of Integrative and Comparative Biology Annual Meeting. , Austin, Texas