Meeting Abstract
Tube-dwelling webspinners are among the few insects with front legs that produce silk, and the only taxa that routinely locomote using those highly modified limbs. Severe constraints on leg morphology likely impose constraints on locomotor mechanics that result in a strong preference for backwards running. Embiids, Antipaluria urichi (0.04 g; 13 mm body length), disturbed outside of their web tubes attained maximum speeds that were over 50% faster than when running forwards. Center of mass motion (COM) was highly exaggerated. Animals lost almost all their speed during each stance period, as their bodies frequently crashed against the ground. Even more strangely, COM oscillations took place at half the stepping frequency, with the body rising on one stance period and falling on the next. Unlike a walk, where the center of mass vaults over a stiff leg like an inverted pendulum, or a run, where the limb compresses mid-stride like a pogo stick, the embiid’s gait had features of both, alternating between vaulting and compression phases in a single stride period. With an average Froude number (Fr=2.7) over double the theoretical limit for walking, embiids appear to fly off their stiff leg and crash to the ground. Despite the clumsy features of their locomotor dynamics, embiid escape speeds fell in the range of dynamic running gaits of other small insects (10-20 body lengths per second; 11 cm/s average speed, 20 cm/s peak instantaneous). The usual running gait of these tube-specialized, backwards running animals defies our definitions of effective running, but certainly appears just-good-enough to escape predators.
