Cockroaches are found in a continuum of dark, humid, poorly ventilated, and often cramped spaces either continuously or when sheltering during their non-active period. Although certain species may be associated with a particular crevice type like the voids beneath rocks or the space beneath loose bark, others are commonly found in more than one of these habitat subdivisions. Many species exploit the interconnectivity of dark, enclosed spaces wherever there is suitable food and moisture, and a distinctive classification of cockroaches as either obligate or facultative inhabitants of caves, litter, or soils is not always a natural one (Peck, 1990). The cave and the forest floor differ far more from the open-air habitat than they do from each other (Darlington, 1970). In closely grown tropical and subtropical forest almost all atmospheric movements are inhibited, surface evaporation of the leaves maintains a high humidity, and the canopy shields the forest floor from the direct rays of the sun. Cockroaches that live in the maze of hiding places that exist in suspended soils or on the forest floor live in a doubly blanketed environment, as moist plant litter further dampens the small fluctuations of light, temperature, and humidity that prevail throughout the forest (Lawrence, 1953). Caves, on the other hand, encompass a continuum of various sized dark, humid voids. To an arthropod, these could range from a few millimeters in size to the largest caverns, and may occur in soil layers, fractured rock, lava tubes, and talus slopes (Howarth, 1983). All of these spaces, whether created by the insect or naturally occurring in soil, leaf litter, guano, debris, rotten wood, or rock are similar in that they are dark, often humid, and buffered from temperature fluctuations.
It is obvious that a crevice-seeking/burrowing lifestyle is suited to a wide range of habitats, as long as dark, humid spaces are present or the substrate allows for their creation. Burrowing, the act of manufacturing or enlarging a space for shelter, is common among Blattaria, but there is a fine line of distinction between a cockroach forcing itself into an existing void, such as one under loose bark, and actually tunneling into the soft, rotted wood beneath. Both photonegativity and positive thig-motaxis predispose cockroaches to burrowing behavior. Beebe (1925, p. 147) offers a vivid definition of positive thigmotaxis: "having the irresistible desire to touch or be touched by something, above, below, and—a thigmotac's greatest joy—on all sides at once" (Fig. 3.7). Additional traits that favor successful colonization of dark, dank habitats include the use of non-visual cues in detecting food, mates, and predators, a lack of highly specialized feeding habits, and physiological adaptations to food scarcity (Darlington, 1970; Culver, 1982; Langecker, 2000).
A subterranean niche offers a relatively simple habitat, with climatic stability and a degree of protection from predators. These benefits are countered by physical and physiological challenges that must be met for successful occupancy. Costs may be incurred in obtaining or constructing burrows and shelters. The insect must cope with an environment that is aphotic, low in production, and high in humidity, endo- and ectoparasites, and pathogens (Nevo, 1999). Suboptimum O2 and toxic CO2 levels are also common in burrows, in caves, in wet, decaying logs, at high altitudes, and when insects are encased in snow and ice (Mani, 1968; Cohen and Cohen, 1981; Hoback and Stanley, 2001).
For our discussion of cockroach habitats, we recognize five broad subdivisions: (1) cockroaches that shelter in
loose substrates (plant litter, guano, uncompacted soil, dust); (2) crevice fauna (under logs, bark, stones, and clumps of earth, in rolled leaves, leaf bases, bark crevices, scree); (3) those that excavate burrows in a solid substrate (wood, soil); (4) those that make use of existing nests or burrows (active or abandoned nests of social insects and small vertebrates); and (5) those in large burrows: caves and cave-like habitats like sewers and mines. We then address cockroaches found in three rather specialized habitats: deserts, aquatic environments, and the forest canopy. We are aware that there are difficulties in adhering to these distinctions, as the subdivisions grade into each other and species often span categories. Many cockroaches that do not routinely inhabit a burrow, for example, may construct underground chambers for rearing the young, for hibernation, for aestivation, or for molting. Many species travel between shelter and sites of feeding and reproductive activity; others (especially those in categories 3 and 4) live their entire life in shelter, except for brief dispersal periods. Some cockroaches never leave sheltered spaces (some cases of category 5). Those in category 3 actively create their living space, while those in the other four categories generally choose advantageous locations among existing alternatives. In each category, variation exists that is rooted in resource quality, quantity, and location.
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