Many-legged maneuverability: Dynamics of turning in hexapods

Remarkable similarities in the vertical plane of forward motion exist among diverse legged runners. The effect of differences in posture may be reflec ted instead in maneuverability occurring in the horizontal plane. The maneu ver me selected was turning during rapid running by the cockroach Blaberus discoidalis, a sprawled-postured arthropod, Executing a turn successfully i nvolves at least two requirements. The animal's mean heading (the direction of the mean velocity vector of the center of mass) must be deflected, and the animal's body must rotate to keep the body axis aligned with the headin g. We used two-dimensional kinematics to estimate net forces and rotational torques, and a photoelastic technique to estimate single-leg ground-reacti on forces during turning. Stride frequencies and duty factors did not diffe r among legs during turning. The inside legs ended their steps closer to th e body than during straight-ahead running, suggesting that they contributed to turning the body. However, the inside legs did not contribute forces or torques to turning the body, but actively pushed against the turn, Legs fa rther from the center of rotation on the outside of the turn contributed th e majority of force and torque impulse which caused the body to turn, The d ynamics of turning could not be predicted from kinematic measurements alone , To interpret the single-leg forces observed during turning, we have devel oped a general model that relates leg force production and leg position to turning performance, The model predicts that all legs could turn the body. Front legs can contribute most effectively to turning by producing forces n early perpendicular to the heading, whereas middle and hind legs must produ ce additional force parallel to the heading. The force production necessary to turn required only minor alterations in the force hexapods generate dur ing dynamically stable, straight-ahead locomotion. A consideration of maneu verability in the horizontal plane revealed that a sprawled-postured, hexap odal body design may provide exceptional performance with simplified contro l.