SPAGNA, J.C.*; GOLDMAN, D.I.; FULL, R.J.; Univ. of California, Berkeley; Univ. of California, Berkeley; Univ. of California, Berkeley: Contribution of a Distributed Foot to Running Performance in Arthropods
Arthropods are capable of stable running at high speeds over irregular surfaces. Foot-substrate interactions must contribute to this ability, yet foot placement during dynamic movement remains largely unknown. We measured the horizontal speed and regions of foot placement of grass-spiders (Hololena adnexa) and cockroaches (Periplaneta americana) on wire-mesh and flat surfaces. Spiders and cockroaches crossed meshes rapidly and continuously with only 1% of the surface area remaining. In cockroaches, speeds never fell below half the mean speed (29 body lengths / s) on the mesh surface due to missed steps. The same was true for 87% of the spider runs on mesh (mean speed 39 body lengths / s). Surprisingly, the mesh leg-surface interactions were not concentrated at the animals’ terminal leg-segments. Rather, interactions were distributed along last three leg-segments of both species, with “hot spots” of high contact frequency (38% of all leg-surface interactions for spiders, 25% for cockroaches) at the joint between two the most-distal leg segments. Both species have large hairs or cuticular spines distributed along the legs, found in higher densities at the “hot spots.” These hairs/spines are oriented at a 70° angle to the leg axis, and share an asymmetrical flexibility– they bend easily toward the leg axis, but resist movement in the opposite direction. We hypothesize that this asymmetry favors gripping of surfaces during foot placement, but allows release during swing-phase. We conclude that a “distributed foot” with spines and hairs allows the entire limb to interact with substrates when careful foot-placement is improbable. A �distributed foot� may be a useful, general leg-design principle for fast-moving, stable animals that must traverse challenging surfaces.
