Swimming

It seems logical that cockroaches are not easily drowned, as they are members of a taxon whose ancestors were associated with swamp habitats and "almost certainly able to swim" (North, 1929). As anyone who has tried to flush a cockroach down the toilet can verify, these insects have positive buoyancy and will bob to the surface of the water if forced under. A water-repellent cuticle aids surface tension in keeping them afloat (Baudoin, 1955). Peri-planeta americana is a fine swimmer, and can move in a straight line at 10 cm/sec. The body is usually arched, with the antennae held clear of the water and moving in normal exploratory fashion. If the antennae touch a solid substrate, the insect turns toward the source of stimulation and swims faster. While swimming, the legs are coordinated in the same alternating tripod pattern seen while walking on land; this differs from the pattern of synchronous leg pairs seen in other terrestrial and aquatic insects in water. Articulated spines on the tibia of each leg are strongly stimulated by movement through water and may provide feedback in regulating swimming behavior. All developmental stages can swim, but the youngest instars are hampered by surface tension (Lawson, 1965; Cocatre-Zilgein and Delcomyn, 1990).

Most P. americana isolated on an artificial island will escape within 10 min, with escape more rapid in experienced insects (Lawson, 1965). Two strategies are employed, reminiscent of those seen in humans at any swimming pool. (1) Gradual immersion (the "wader"): the surface of the water is first explored with the forefeet (Fig.

Fig. 2.8 (A) Periplaneta americana testing the water with forelegs before (B) taking the plunge. Courtesy of R.M. Dob-son.

2.8). The middle legs then attempt to reach the bottom beneath the water, while clinging to the island with the rear legs and with the front of the body afloat. Finally, the cockroach releases the hind legs, enters completely, and swims away. (2) The "cannonball" strategy: after initial exploration, the insect retires slightly from the edge, crouches, then jumps in, often while fluttering the wings.

The legs of amphibious cockroaches do not exhibit any morphological adaptations for swimming and are no different from those of non-aquatic species (Shelford, 1909; Takahashi, 1926). Nymphs of many Epilampra spp. swim rapidly below the surface (Crowell, 1946; Wolcott, 1950); newborn nymphs as well as adults of Ep. wheeleri (= Ep. abdomennigrum) swim easily and remain under water a good deal of the time (Sein, 1923). Individuals of Poe-ciloderrhis cribrosa verticalis can swim against a current velocity of 0.15 m/sec (Rocha e Silva Albuquerque et al., 1976). Opisthoplatia maculata, on the other hand, rarely swims, but instead walks on submerged rocks along stream bottoms (Takahashi, 1926).

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