Stability and Balance

Impressive locomotor performances are not limited to flat surfaces; cockroaches can scamper over uneven ground and small obstacles with agility and speed. Their vertically oriented joint axes act in concert with a sprawled posture to allow the legs to perform like damped springs during locomotion. As much as 50% of the energy used to displace a leg is stored as elastic strain energy, then returned (Spirito and Mushrush, 1979; Dudek and Full, 2000;Wat-son et al., 2002). In experiments on rough terrain, running P. americana maintained their speed and their alternating tripod gait while experiencing pitch, yaw, and roll nearly 10-fold greater than on flat surfaces (Full et al., 1998). Blaberus discoidalis scaled small objects (5.5 mm) with little change in running movements. Larger (11 mm) objects, however, required some changes in kinematics. The insects first assessed the obstacle, then reared up, placed their front tarsi on it, elevated their center of mass to the top of the object, then leveled off. The thorax was capable of substantial ventral flexion during these movements (Watson et al., 2002).

In a remarkable and no doubt entertaining series of experiments, Jindrich and Full (2002) studied self-stabilization in Blab. discoidalis by outfitting cockroaches with miniature cannons glued to the thorax. They then triggered a 10 ms lateral blast designed to knock the cockroach suddenly off balance in mid-run (Fig. 2.2). The insects successfully regained their footing in the course of a single step, never breaking stride. Stabilization occurred too quickly to be controlled by the nervous system; the mechanical properties of the muscles and exoskeleton were sufficient to account for the preservation of balance.

Fig. 2.2 Blaberus discoidalis with an exploding cannon backpack attempting to knock it off balance. Photo courtesy of Devin Jindrich.

There is some concern over gangs of these armed research cockroaches escaping and riddling the ankles of unsuspecting homeowners with small-bore cannon fire (Barry, 2002).

A healthy cockroach flipped onto its back is generally successful in regaining its footing. In most instances righting involves body torsion toward one side, flailing movements of the legs on the same side, and extension of the opposite hind leg against the substrate to form a strut. The turn may be made to either the right or left, but some individuals were markedly biased toward one side. In some cases a cockroach will right itself by employing a forward somersault, a circus technique particularly favored by B. germanica (Guthrie and Tindall, 1968; Full et al., 1995). If flipped onto its back on a smooth surface Macropanesthia rhinoceros is unable to right itself and will die (H. Rose, pers. comm. to CAN).

Aging cockroaches tend to dodder. There is a decrease in spontaneous locomotion, the gait is altered, slipping is more common, and there is a tendency for the protho-racic leg to "catch" on the metathoracic leg. The elderly insects develop a stumbling gait, and have difficulty climbing an incline and righting themselves (Ridgel et al., 2003).

The recent spate of sophisticated research on mechanisms of cockroach balance and control during locomotion is in part the result of collaborative efforts between robotic engineers and insect biologists to develop blattoid walking robots. The ultimate goal of this "army of biologically inspired robots" (Taubes, 2000) is to carry sensory and communication devices to and from areas that are difficult or dangerous for humans to enter, including buildings collapsed by earthquakes, bombs, or catastrophic weather events. In some cases living cockroaches have been outfitted with small sensory and communication backpacks ("biobots"), and their movement steered via electrodes inserted into the bases of the antennae (Moore et al., 1998). Gromphadorhina portentosa was the species selected for these experiments because they are large, strong enough to carry a reasonable communications payload, easy to maintain, and "no one would get too upset if we were mean to them" (T. E. Moore, pers. comm. to LMR). One limitation is that biobots could be employed only in the tropics or during the summer in temperate zones. Perhaps engineers should start thinking about making warm clothing for them, modeled after spacesuits (LMR, pers. obs.).

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