Ground Locomotion Speed

Periplaneta americana typifies a cockroach built to cover ground quickly and is, relative to its mass, one of the fastest invertebrates studied. It has a lightly built, somewhat fragile body and elongated, gracile legs capable of lengthy strides. The musculature is typical of running insects, but the orientation of the appendages with respect to the body differs. The middle and hind pairs point obliquely backward, and the leg articulations are placed more ventrally than in most insects (Hughes, 1952; Full and Tu, 1991). Periplaneta americana has a smooth, efficient stride, and at most speeds, utilizes an alternating tripod gait, that is, three legs are always in contact with the ground. The insect can stop at any point in the walking pattern because its center of gravity is always within the support area provided by the legs. At a very slow walk the gait grades into a metachronal wave, moving from back to front, that is, left 3-2-1, then right 3-2-1 (Hughes, 1952; Del-

Fig. 2.1 Ground reaction force pattern for Periplaneta americana running bipedally, with the metathoracic legs propelling the body. Vertical forces periodically decrease to zero, indicating that all six legs are off the ground in an aerial phase. From Full and Tu (1991), with the permission of Robert J. Full and Company of Biologists Ltd.

Fig. 2.1 Ground reaction force pattern for Periplaneta americana running bipedally, with the metathoracic legs propelling the body. Vertical forces periodically decrease to zero, indicating that all six legs are off the ground in an aerial phase. From Full and Tu (1991), with the permission of Robert J. Full and Company of Biologists Ltd.

comyn, 1971; Spirito and Mushrush, 1979). At its highest speed, P. americana shifts its body weight posteriorly and becomes bipedal by sprinting on its hind legs. The body is raised well off the ground and an aerial phase is incorporated into each step in a manner remarkably similar to bipedal lizards (Fig. 2.1). Periplaneta can cover 50 body lengths/sec in this manner (Full and Tu, 1991). As pointed out by Heinrich (2001), by that measure they can run four times faster than a cheetah. Other studied cockroaches are slower and less efficient. The maximum speed for Blaberus discoidalis, for example, is less than half of that of P. americana. The former is a more awkward runner, with a great deal of wasted motion (Full and Tu, 1991). Speed is known to vary with temperature (Blab. discoidalis), substrate type, sex, and developmental stage (B. germanica) (Wille, 1920; Full and Tullis, 1990). Hughes and Mill (1974) note that it is the ability to change direction very rapidly that often gives the impression of great speed. The ability to run swiftly and to fly effectively are not mutually exclusive. Imblattella panamae, a species that lives among the roots of epiphytic orchids, is fast moving both on wing and on foot (Rentz, 1987, pers. comm. to CAN). Hebard (1916a) noted that Cari-blatta, a genus of diminutive insects, "ran about with great speed and took wing readily, though usually flying but short distances. When in flight, they appeared very much like small brownish moths." As a group, blattellids are generally very fast moving, especially when pursued.

Most are long-legged with the ventral surfaces of the tarsi spined (Rentz, 1996).

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