Sand Swimming Desert Polyphagidae

During their active period, fossorial desert Polyphagidae form temporary subsurface trails as they "swim" through the superficial layers of the substrate. Their activities generate a low rise on the surface as the loosely packed sand collapses in their wake. The resultant serpentine ridges look like little mole runs (Fig. 2.6) (Hawke and Farley, 1973). During the heat of day, the cockroaches (Areni-vaga) may burrow to a depth of 60 cm (Hawke and Farley, 1973).The bodies of adult females and nymphs are streamlined, with a convex thorax and sharp-edged pronotum. Tibial spines on the short, stout legs facilitate their pushing ability and serve as the principal digging tools. These spines are often flattened or serrated, with sharp tips. Anterior spines are sometimes united around the apex in a whorl, forming a powerful shovel (Chopard, 1929; Friauf and Edney, 1969). Eremoblatta subdiaphana, for example, has seven spines projecting from the front tibiae (Helfer, 1953). Also aiding subterranean move-

Fig. 2.6 Tracks (2-3 cm wide) of Arenivaga sp. at the base of a mesquite shrub near Indigo, California. Females and nymphs burrow just beneath the surface at night. From Hawke and Farley (1973), courtesy of Scott Hawke. Inset: Ventral view of female Arenivaga cerverae carrying an egg case. The orientation of the egg case is likely an adaptation for carrying it while the female "swims" through the sand. Note well-developed tibial spines. Photo by L.M. Roth and E.R. Willis.

Fig. 2.6 Tracks (2-3 cm wide) of Arenivaga sp. at the base of a mesquite shrub near Indigo, California. Females and nymphs burrow just beneath the surface at night. From Hawke and Farley (1973), courtesy of Scott Hawke. Inset: Ventral view of female Arenivaga cerverae carrying an egg case. The orientation of the egg case is likely an adaptation for carrying it while the female "swims" through the sand. Note well-developed tibial spines. Photo by L.M. Roth and E.R. Willis.

Fig. 2.7 Sensory organs on cerci of adult male Arenivaga sp.

(A) Ventral view of insect, with the cerci indicated by arrows.

(B) Posterior end of the abdomen showing the orthogonal position of the cerci and rows of tricholiths. (C) Cross section through the left cercus to illustrate that the cerci are rotated laterally from the horizontal plane. (D-E) Scanning electron micrographs showing details of tricoliths on the cerci. (D) Ventral view of left cercus; note two parallel rows of tricholiths. (E) View from the distal end of the tricholith (tl) rows showing sen-silla chaetica (sc) and a trichobothrium (tb). Courtesy of H. Bernard Hartman. From Hartman et al. (1987), with permission from Springer Verlag.

Fig. 2.7 Sensory organs on cerci of adult male Arenivaga sp.

(A) Ventral view of insect, with the cerci indicated by arrows.

(B) Posterior end of the abdomen showing the orthogonal position of the cerci and rows of tricholiths. (C) Cross section through the left cercus to illustrate that the cerci are rotated laterally from the horizontal plane. (D-E) Scanning electron micrographs showing details of tricoliths on the cerci. (D) Ventral view of left cercus; note two parallel rows of tricholiths. (E) View from the distal end of the tricholith (tl) rows showing sen-silla chaetica (sc) and a trichobothrium (tb). Courtesy of H. Bernard Hartman. From Hartman et al. (1987), with permission from Springer Verlag.

ments are large spherical sense organs (tricholiths) on the ventral surface of the cerci in Arenivaga and other polyphagids (Roth and Slifer, 1973). These act like tiny plumb bobs in assisting orientation of the cockroaches while they move through their quasifluid environment (Walthall and Hartman, 1981; Hartman et al., 1987) (Fig. 2.7). First instars of Arenivaga have only one tricholith on each cercus; new ones are added at each molt. Adult females have six pairs and males have seven pairs (Hartman et al., 1987).

Head-Raising (Blaberus craniifer)

In studying the burrowing tendencies of Blab. craniifer, Simpson et al. (1986) supplied the cockroaches with a mixture of peat moss and topsoil, then filmed them as they dug into the substrate. The insects were able to bury themselves in just a few seconds using a rapid movement of the legs, combined with a stereotyped dorsal-ventral flexion of the head and pronotum. The combined head-raising, leg-pushing behavior seems well suited to digging in light, loose substrates (litter, dust, guano), but may also facilitate expanding existing crevices, like those in compacted leaf litter or under bark. This digging technique does not require the profound body modifications exhibited by cockroaches specialized for burrowing in hard substrates, and is therefore compatible with the ability to run rapidly. Indeed, the behavior seems well suited to the "standard" cockroach body type displayed by Blab. crani-ifer: an expanded, hard-edged pronotum, inflexed head, slick, flattened, rather light body, and moderately strong, spined legs.

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