Aquatic Habitats

Most amphibious and quasi-aquatic cockroaches fall into two basic groups: those that live in phytotelmata (small pools of water within or upon plants) and those associated with rivers, streams, and ponds. In both cases, the insects live at the surface of the water or on solid substrate in its immediate vicinity, but submerge to hunt for food or to escape predators. About 62 species (25 genera) of cockroaches have been collected from the leaf bases of bromeliads (Roth and Willis, 1960; Rocha e Silva Albuquerque and Lopes, 1976), but it is unknown how many of these are restricted to this habitat. One example is Dryadoblatta scotti, a large, handsome, Trinidadian cockroach found in considerable numbers in epiphytic bromeliads; they rest just above the surface of the water or are partly immersed in it (Princis and Kevan, 1955). Nymphs of Litopeltis sp. are encountered during the day at all times of the year in the erect bracts of Heliconia, which collect and hold water even during the dry season of Costa Rica. The cockroaches forage at night on the outer and inner surfaces of the bracts, feeding on mold and decayed areas (Seifert and Seifert, 1976).

Numerous species in at least six genera of Epilampri-nae live near streams or pools, usually in association with rotting vegetation amid rocks along the edge of the water. Poeciloderrhis cribrosa verticalis in Rio de Janeiro (Rocha e Silva Albuquerque et al., 1976) and Rhabdoblatta an-nandalei in Thailand (LMR, pers. obs.) occur near swift-

moving streams, and Rhabdoblatta stipata in Liberia occurs on logs or mats floating directly in the current (Weidner, 1969). The cockroaches submerge in response to disturbance or when a shadow passes overhead, and swim rapidly below the surface for a minute or two. They then cling to submerged vegetation for up to 15 min before climbing to the surface (e.g., Epilampra maya [reported as Ep. abdomennigrum] in Panama—Crowell, 1946).

It has been debated as to whether aquatic cockroaches have morphological adaptations that enable underwater respiration. In most species observed to date, it appears that the insects use the abdominal tip as a snorkel, use a bubble of air as an accessory gill, or both. Weidner (1969) writes that individuals of Rha. stipata inspire via spiracles located on conical projections adjacent to the cerci, and die in 6 -12 hr if the abdominal tip is held under water. Opisthoplatia maculata also has spiracular openings at the tip of abdominal projections, and these are protected by long hairs on the ventral surface of the cerci (Taka-hashi, 1926). Annandale (1906) suggested that the position of these posterior spiracles is an adaptation to an aquatic lifestyle; however, Shelford (1907) and Chopard (1938) point out that this character is present in many terrestrial cockroach species. Scanning electron micrographs of Ep. abdomennigrum reveal no unique adaptations of the terminal spiracles; they appear to be identical to those elsewhere on the body (WJB, unpubl. obs.). There are distinct patches of hairs on the ventral side of the cerci in older nymphs that that are absent in other Epilampra species examined; however, these hairs are quite distant from the terminal spiracles. The tracheal systems of aquatic and terrestrial cockroaches are morphologically distinct. The tracheae of the latter are thread-like, silvery in appearance, and dilated to their maximum with air. The tracheae of amphibious cockroaches are strap-like, not silvery, and contain just a few scattered air bubbles. Shelford (1916) suggested that the differences are rooted in the need for the amphibious species to be "sinkable," which would be prevented by internal accumulated air.

A large bubble is apparent beneath the pronotal shield of several aquatic species when they are submerged. The air is trapped by easily wetted, long hairs on the underside of the thorax (Takahashi, 1926; Crowell, 1946); these hairs also occur on terrestrial species. Some observers suggest that the bubble is formed by air taken in through the terminal abdominal spiracles, which then issues from the prothoracic spiracles in Ep. maya and O. orientalis (Shelford, 1907; Takahashi, 1926). Although this may explain the formation of the thoracic air bubble, air usually moves posteriorly through the tracheal system of bla-berids (Miller, 1981), and recent observations suggest a different source of the bubble. WJB (unpubl. obs.) ob served 48 dives of Ep. abdomennigrum nymphs in an aquarium in Costa Rica. When a nymph swimming on the surface is disturbed, it flips 180 degrees, with the venter of the body briefly facing upward. While supine the cockroach envelops an air bubble with its antennae and front legs, and holds the bubble beneath the thorax; the antennae remain extended posteriorly between the legs. As the cockroach dives below the surface, it turns again, righting itself, with the bubble held ventrally. Once underwater, it either grasps vegetation to remain submerged, or floats slowly to the surface. The median time totally submerged was 80 sec (range 20-1507 sec). While floating to the surface, the abdomen is extended upward, lifting the terminal spiracles out of the water. The insect remains motionless while floating on the air bubble for up to 30 min as the abdomen pulses slowly, at 1 or 2 pulses/10 sec.

Was this article helpful?

0 0

Post a comment