Selection Pressures

Food limitation is most commonly suggested as the selective basis of the syndrome of characters associated with cave-dwelling organisms. First, many of the characters are directed toward improved food detection (e.g., elongation of appendages) and food utilization (e.g., lower metabolic and growth rate, starvation resistance, slow movement, fewer eggs) (Poulson and White, 1969; Hüp-pop, 2000; Gilbert and Deharveng, 2002). Second, troglo-morphic species are more often found in caves that lack sources of vertebrate guano (Vandel, 1965; Culver, 1982). It is the combination of scarce food and the consistently dark, humid environment of deep caves, however, that best accounts for the reductions and losses that characterize troglomorphism. Eyes are complex organs, expen sive to develop and maintain. Animals rarely have sophisticated visual systems unless there is substantial selection pressure to favor them (Prokopy, 1983). Optical sensors are useless in the inky blackness of deep caves and "compete" with non-visual systems for available metabolites and energy (Culver, 1982; Nevo, 1999). Photore-ception is also related to a complex of behavioral and morphological traits that become functionless in the permanent darkness of a cave. These include visually guided flight and signaling behavior based on cuticular pigmentation (Langecker, 2000). Cave-dwelling cockroaches in north Queensland, Australia, display a remarkable degree of correlation between levels of troglomorphy and the cave zone in which they occur. In the genera Nocticola and Paratemnopteryx, the most modified species described by LMR are found only in the stagnant air zones of deep caves, while the slightly troglomorphic species of Paratemnopteryx are concentrated in twilight transition zones (Howarth, 1988; Stone, 1988). Because cockroaches live in a variety of stable, dark, humid, organic, living spaces, however, reductive evolutionary trends are not restricted to cavernicolous species (discussed in Chapter 3). Nocticola (= Paraloboptera) rohini from Sri Lanka, for example, lives under stones and fallen tree trunks. The female is apterous; the males have small, lateral tegminal lobes but lack wings, and the eyes are represented by just a few ommatidia (Fernando, 1957).

Many cave cockroaches diverge from the standard character suite associated with cave-adapted insects. They may exhibit no obvious troglomorphies, or display some characters, but not others. Blattella cavernicola is a habitual cave dweller but shows no structural modifications for a cave habitat (Roth, 1985). Neither does the premise that some cave organisms diverge from the morphological profile because they live in energy-rich environments such as guano piles (Culver et al., 1995) always hold true for cockroaches. Paratemnopteryx kookabinnensis and Para. weinsteini are associated with bats (Slaney, 2001), yet both show eye and wing reduction. Heterogeneity in these characters may occur for a variety of reasons. The surface-dwelling ancestor may have exhibited varying levels of morphological reduction or loss prior to becoming established in the cave (i.e., some losses are ple-siomorphic traits) (Humphreys, 2000a). Such is likely the case for the two species of Paratemnopteryx mentioned above; most species in the genus have reduced eyes, lack pulvilli, and are apparently "pre-adapted" for cave dwelling (Roth, 1990b). Species also may be at different stages of adaptation to the underground environment (Peck, 1998). Generally, regression increases and variability decreases with phylogenetic age (Culver et al., 1995; Langecker, 2000). Nocticola flabella is probably the most troglobitic cockroach known (Fig. 1.18); the male is 4-5

mm long, eyeless, with reduced tegmina and no hind-wings, has very long legs and antennae, and is colorless except for amber mouthparts and tegmina (Roth, 1991c). This high level of regressive evolution is also found in other species found in deep caves of the Cape Range in western Australia and is consistent with the apparent great age of this fauna (Humphreys, 2000b). Other sources of variation that may play a role include ecological differences within and among caves, continued gene flow between epigean and cave populations, the accumulation of neutral mutations, developmental constraints, or some combination of these (Culver, 1982; Slaney and Weinstein, 1997b; Huppop, 2000; Langecker, 2000).

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