By sticking tiny cannons on the backs of cockroaches to see how their recoil jars the insects' balance, researchers have added firepower to a new mathematical model that explains how roaches move so nimbly. The model has already helped them design a better robot bug.
Cockroaches such as the 44-millimeter-long Blaberus discoidalis from South and Central America can scamper over rugged terrain with remarkable agility and speed. How do they keep their balance? Integrative biologist Robert Full of the University of California, Berkeley, suspected that some of the insect's balance-preserving reflexes may be too fast to be controlled by its nervous system alone and are also built into its mechanical structure. Full calls these responses "preflexes." Working with a team of Princeton University mathematicians, Full developed a mechanical model of the roach with its legs acting like springs. Just the mechanical properties of muscles and exoskeleton, they found, could account for the insect's stability.
The obvious way to check the model--by throwing real roaches off balance and observing how they regained their footing--wasn't so simple. After trying everything from magnets to spring-loaded projectiles, integrative biologist Devin Jindrich, now at the Harvard School of Public Health in Boston, decided on jetpacks of sorts. These 2-centimeter-long plastic tubes filled with explosives, triggered by electrical wires, delivered 10-millisecond bursts. Gluing these to roaches' backs, they observed how running bugs reacted when a quick blast knocked a leg off balance. The performance was astonishing. "They didn't even break their stride," Jindrich says.
This rapid recovery rivals the fastest nervous reflexes and thus bolsters their model, the scientists report in the 15 September Journal of Experimental Biology. Comparative biomechanist Andrew Biewener of Harvard University's Concord Field Station in Bedford, Massachusetts, agrees. Furthermore, he suggests that "a lot of the principles of movement that apply to invertebrates here are going to apply to vertebrates as well," meaning that intrinsic properties of muscles and skeleton may be more important to animal movement than thought.
Full and engineer Daniel Koditschek at the University of Michigan, Ann Arbor, have designed a breadbox-sized robot with springy limbs like a cockroach. It scrambles at 3 meters per second over rough terrain. Full says this simple model for stability has helped liberate computing power that can now be used for navigation and path-planning.