Deserts

While cockroaches are generally associated with humid habitats, there are a number of species that have settled deserts, scrub, grassland, and other arid environments. These habitats vary in temperature, from hot subtropical deserts to colder deserts found at high latitudes or high elevations. In each, however, low precipitation plays a major role in controlling biological productivity. Many polyphagids, some blattellids, and a few blattids inhabit these xeric landscapes. Polyphagidae are most diverse in the deserts of North Africa and South Central Asia (Bei-Bienko, 1950), and best studied in Egypt (Ghabbour et al., 1977; Ghabbour and Mikhail, 1978; Ghabbour and Shakir, 1980) and Saudi Arabia (Bei-Bienko, 1950; Grandcolas, 1995a). The cockroaches can be very abundant, comprising nearly a third of the mesofaunal biomass collected in surveys of soil arthropods in the desert of northern Egypt (Ayyad and Ghabbour, 1977). In North America, polyphagid cockroaches occur in the southwestern United States, with one species (Arenivaga flori-densis) found in Florida.

Desert-dwelling cockroaches exhibit morphological, behavioral, and physiological adaptations for maintaining water balance, avoiding or tolerating extreme temperatures, and finding food in habitats with sparse primary productivity. Behavioral tactics for coping with these extreme conditions include diurnal and seasonal shifts in spatial location and prudent choice of microhabitat. Like many desert arthropods, the sand-swimming Polyphagidae take advantage of the more salubrious conditions beneath the surface of desert soil. Arenivaga investigata migrates vertically in loose sand on a diel basis. In spring and summer, activity near the surface commences 2 hr after darkness and continues for most of the night (Edney et al., 1974). In winter, activity corresponds to peaks in nighttime surface temperature (Hawke and Farley, 1973). The insects move about just beneath the sand (Fig. 2.6), making them less susceptible to predators (e.g., scorpions) as they forage for dead leaves, roots, and other food. Throughout the year A. investigata can find a relative humidity of about 82% by descending 45 cm in the sand, and can avoid temperatures above 40°C by moving no lower than 15 cm (Edney et al., 1974). The cockroaches descend deeper in the sand in summer than in winter (Edney et al., 1974) (Fig. 3.10). In July, all developmental stages except adult males range 2.5-30 cm below the surface, with a mode at 12.5 cm. In November the insects are found no deeper than 15 cm, with most occurring at 5 cm or less. It is possible that the maximum depth to which these cockroaches burrow may be limited by hypoxia (Cohen and Cohen, 1981).

Although deserts can be very hot, very dry, and sometimes very cold, they have numerous microhabitats where the climate is much less extreme. In addition to the depths of loose sand, these include the burrows of small vertebrates, under boulders, in caves, and amid decaying organic material in dry stream beds, at the base of tussocks, in rock crevices, and under shrubs or trees (Roth and Willis, 1960). Some cockroach species are consistently associated with one of these microhabitats, and others move freely between them. Arenivaga grata is found under stones and rocks in scrub oak, oak-pine, and oak manzanita forests in Texas (Tinkham, 1948), but has been reported from bat guano in a cave in Arizona (Ball et al., 1942). Sand-swimming and Australian burrowing cockroaches are frequently found in the root zones of plants. Arenivaga investigata is most commonly associated with the shrubs Larrea tridentata, Atriplex canescens, and Croton californicus (Edney et al., 1978). The burrows of desert vertebrates utilized by some cockroach species are also typically found near desert plants. In the desert, vegetation is a source of shade and food, and subterranean root systems concentrate available moisture (Wallwork, 1976).

About half the desert cockroaches for which we have any information live in the burrows of vertebrates (Roth and Willis, 1960). Various species of Arenivaga and Poly-phaga live in the excavations of desert turtles, prairie dogs, ground squirrels, wood rats, gerbils, and white-footed mice (Roth and Willis, 1960; Krivokhatskii, 1985). In some species, burrows are just one of several utilized microhabitats. The blattellid Euthlastoblatta abortiva can be found in both wood rat nests and leaves and dry litter on the ground along the Rio Grande River in Texas (Helfer, 1953). Arenivaga floridensis has been observed in the burrows of mice, burrowing freely in loose sand, and amid vegetation in sandhill and scrub communities (Atkinson et al., 1991). Occasionally only females (e.g., Arenivaga erratica—Vorhies and Taylor, 1922) or

Fig. 3.10 Distribution of Arenivaga sp. in relation to depth below the surface (A,C) and temperature (B,D). In (A) and (C) the insects are scored according to size: open columns = 1st-3rd instar; striped columns = 4th-6th instars; solid columns = 7th-9th instars and adults. Adult males were rarely found below the surface and are not included in the data set. After Edney et al. (1974). Reprinted by permission of the Ecological Society of America.

Fig. 3.10 Distribution of Arenivaga sp. in relation to depth below the surface (A,C) and temperature (B,D). In (A) and (C) the insects are scored according to size: open columns = 1st-3rd instar; striped columns = 4th-6th instars; solid columns = 7th-9th instars and adults. Adult males were rarely found below the surface and are not included in the data set. After Edney et al. (1974). Reprinted by permission of the Ecological Society of America.

nymphs (e.g., Car. lutea—Hubbell and Goff, 1939) are collected from burrows.

Animal burrows generally offer a more favorable microclimate than surface habitats. A higher humidity is maintained by the respiration of the vertebrate occupant (Tracy and Walsberg, 2002), and because of enhanced air circulation in burrows, cockroaches that utilize them avoid the hypoxic conditions that may be encountered by sand-swimming species (Cohen and Cohen, 1981). Richards (1971) indicates that animal burrows have a microclimate that is intermediate between that of caves and that of surface habitats. Recent studies, however, suggest that animal burrows are not always cool and humid refugia from surface conditions. For more than 100 days of the year soil temperatures rose to over 30°C at depths of 2 m in burrows of Dipodomys in the Sonoran desert (Tracy and Walsberg, 2002).

In a remarkable case of niche construction, at least one cockroach species mitigates conditions within vertebrate burrows by building a home within a home. In southeastern Arizona Arenivaga apacha is a permanent inhabitant of mounds of the banner-tailed kangaroo rat (Dipodomys spectabilis) and builds a microenvironment of small burrows ("shelves") within the main burrow of the rat (Cohen and Cohen, 1976). The mini-burrows are tightly packed with the grasses that were dragged into the main burrow by the rat for use as nesting material. Although the rodent burrows extend much deeper, most of the cockroaches were found 30-45 cm below the sand surface. Surface temperatures reached as high as 60°C, burrow temperatures reached 48°C , but the temperature of the grass-lined cockroach shelves averaged 16.5°C. Hu midity of the burrows was as low as 20%, but the shelves remained nearly saturated at all times; 91% was the lowest reading. Conditions within the vertebrate burrow were nearly as harsh as the open desert and were made tolerable only by the alterations in the microenvironment made by the cockroaches; the insects died in 3-5 min if subjected to temperatures above 40°C. These cockroaches feed on the stored seeds of their host. "With this stored food available throughout the year and the very stable environmental conditions, the cockroaches have an ideal kind of oasis in the midst of a harsh desert environment" (Cohen and Cohen, 1976).

While A. apacha exhibits striking behavioral strategies for living in the harsh desert environment, its closely related congener, the Colorado Desert sand swimming A. investigata, relies heavily on well-developed physiological mechanisms. Arenivaga investigata has a higher temperature tolerance and lower rates of water loss and oxygen consumption than A. apacha (Cohen and Cohen, 1981). This is due in large part to the predominance of long chain wax esters in the cuticle that are effective in waterproofing the insect (Jackson, 1983). Arenivaga investigata is also able to tolerate a water loss of 25-30% without lethal effects (Edney, 1967) and is able to absorb water vapor from the surrounding air at ^ 82% relative humidity (RH) (Edney, 1966). This level of RH is available at 45 cm below the ground surface (Edney et al., 1974). Thus, descending to that level assures the cockroach a predictable source of water. Water vapor is absorbed by means of a unique system of specialized structures on the head and mouthparts (O'Donnell, 1977a, 1977b). A thin layer of hygroscopic fluid is spread on the surface of two eversible

Fig. 3.11 Morphological structures associated with capturing atmospheric water in Arenivaga investigata. Top, photograph of head showing the two dark, spherical bladders protruding from the mouth. Note hairs around edge of pronotum. From O'Donnell (1977b), courtesy of M.J. O'Donnell. Bottom, sagittal view of the head with portions removed to show details of structures; redrawn from O'Donnell (1981), with permission of M.J. O'Donnell. The frontal body secretes a fluid that spreads over everted hypopharyngeal bladders. Atmospheric water condenses in the fluid and both liquids then flow toward the esophagus and are swallowed. Arrows indicate route of fluid movement from site of production in the frontal bodies to the esophagus.

Fig. 3.11 Morphological structures associated with capturing atmospheric water in Arenivaga investigata. Top, photograph of head showing the two dark, spherical bladders protruding from the mouth. Note hairs around edge of pronotum. From O'Donnell (1977b), courtesy of M.J. O'Donnell. Bottom, sagittal view of the head with portions removed to show details of structures; redrawn from O'Donnell (1981), with permission of M.J. O'Donnell. The frontal body secretes a fluid that spreads over everted hypopharyngeal bladders. Atmospheric water condenses in the fluid and both liquids then flow toward the esophagus and are swallowed. Arrows indicate route of fluid movement from site of production in the frontal bodies to the esophagus.

Table 3.4. Water balance in Arenivaga. Data are in mg/100 mg/ day at 25°C for a 320 mg nymph. From Edney (1966).

Dry air 88% RH

Water loss

Feces 0.19 0.19

Cuticular and spiracular 5.43 0.65

Total 5.62 0.84 Water gain

Food 0.22 0.44

Metabolism 0.87 0.87

Vapor absorption 0 2.14

Total 1.09 3.45

bladders, one on each side of the mouth (Fig. 3.11). These are coated with a thick layer of cuticular hairs that hold and distribute the fluid via capillary action. The fluid is supplied to the bladders by two glands located on the inside of the labrum and embedded in a massive muscular complex that can be seen oscillating when the glands are secreting fluid. Atmospheric water condenses on the bladders and is then transferred to the digestive system, where it is absorbed. The capture of atmospheric moisture is a solute-independent system, based on the hy-drophilic properties of the cuticular hairs on the bladders (O'Donnell, 1981,1982). As a result of this water uptake system, A. investigata can maintain water balance even if no free water is available and food contains only 20% water, provided that air at 82% RH or above is available (Table 3.4). Females and nymphs are capable of absorbing water vapor,but males are not (Edney, 1967). Females are apterous, but males are winged and may be capable of seeking out free water and higher humidity surface habitats.

The Egyptian species Heterogamisca syriaca is similarly adapted to desert life. A lipid layer effective up to 56°C protects against evaporation, and the cockroach can extract water vapor from unsaturated air between 20 and 40°C and RH > 75% (Vannier and Ghabbour, 1983). Humid air is available at a depth of 50 cm and at the surface during the night. Water absorption presumably occurs via hypopharyngeal bladders, as these have been observed in H. chopardi (Grandcolas, 1994a). Under the harshest conditions of water stress, H. syriaca may fast to generate metabolic water from fat reserves, which are abundant during the summer months (references in Vannier and Ghabbour, 1983).

Cockroaches that live in arid zones are rich in potential for research into behavioral ecology and physiology. Thorax porcellana living in suspended litter in dry forests of India, for example, do not actively seek or drink water when maintained in laboratory culture (Reuben, 1988), and nothing is known about the many diurnal Australian species that enjoy sunbasking. Perhaps as in some birds (Dean and Williams, 1999) the added heat helps speed digestion of a cellulose-based diet. Juvenile Phyllodromica maculata live on the dry, grassy hillsides of Bavaria, prefer low humidity, and do not aggregate (Gaim and Seelinger, 1984). Studies of laboratory-bred cockroaches indicate a variety of methods for dealing with heat and water stress. Periplaneta americana, B. germanica, and Blatta orientalis can withstand a body weight loss of 30% and still recover successfully when given an opportunity to drink water (Gunn, 1935). Periplaneta fuliginosa and R. maderae nymphs use the salivary glands as water storage organs (Laird et al., 1972; Appel and Smith, 2002). Gromphadorhina brauneri and P. americana maintain body temperatures below that of surrounding air by evaporative cooling (Janiszewski and Wysocki, 1986), and there is some evidence that P. americana can close dermal gland openings to conserve water (Machin et al., 1994). The physiology of water regulation in cockroaches is addressed in detail by Edney (1977), Mullins (1982), and Hadley (1994).

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