Insects running on elastic surfaces

A J Spence, Shai Revzen, J Seipel, C Mullens, R J Full

Journal of Experimental Biology, vol. 213, no. 11, pp. 1907–1920, 2010

Abstract

In nature, cockroaches run rapidly over complex terrain such as leaf litter. These substrates are rarely rigid, and are frequently very compliant. Whether and how compliant surfaces change the dynamics of rapid insect locomotion has not been investigated to date; largely due to experimental limitations. Here we test the hypothesis that a running insect can maintain average forward speed over an extremely soft elastic surface (10 N/m) equal to 2/3 of its virtual leg stiffness (15 N/m). Cockroaches (Blaberus discoidalis) running from a rigid surface onto an elastic surface were able to maintain forward speed (39.6 +/- 0.7 cm/s, rigid substrate, versus 43.3 +/- 1.1 cm/s, elastic substrate; p = 0.0064). Step frequency was unchanged (25.5 +/- 0.3 steps/sec, rigid surface, versus 26.1 +/- 0.5 steps/sec, elastic surface; p = 0.28). To uncover the mechanism we measured the animal’s COM dynamics using a novel miniature backpack, consisting chiefly of a 3-axis MEMs accelerometer attached very near the COM. Vertical acceleration of the COM on the elastic surface had smaller peak-to-peak amplitudes (13.95 +/- 0.41 m/s2, rigid, vs. 10.07 +/- 0.50 m/s2 on the elastic surface; p < 0.0001). The observed change in COM acceleration over an elastic surface requires no change in effective stiffness when duty factor and ground stiffness are taken into account. Due to the lowering of the COM towards the elastic surface the swing leg lands earlier and increases the period of double support. A simple feedforward control model explains the experimental result and provides one plausible model mechanism. We find no evidence for active neural control of effective leg stiffness.

BibTeX

@article{spence-2009-JEB,
  author = {Spence, A J and Revzen, Shai and Seipel, J and Mullens, C and Full, R J},
  title = {Insects running on elastic surfaces},
  journal = {Journal of Experimental Biology},
  year = {2010},
  volume = {213},
  number = {11},
  pages = {1907--1920},
  doi = {10.1242/jeb.042515},
  cvtopic = {science}
}