Cave Dwellers

Cave-adapted cockroaches exhibit a suite of morphological characters common to cave-dwelling taxa around the world. These include depigmentation and thinning of cuticle, the reduction or loss of eyes, the reduction or loss of tegmina and wings, the elongation and attenuation of appendages, and a more slender body form (Howarth, 1983; Gilbert and Deharveng, 2002). A large nymph of the genus Nelipophygus collected in Chiapas, Mexico, for example, cannot survive outside of its cave and is colorless, slender, and 20 mm long; it has extremely long antennae and limbs, and has no trace of compound eyes or pigment (Fisk, 1977). Males of Alluaudellina cavernicola exhibit a remarkable parallel reduction of eyes and wings (Fig. 1.17) (Chopard, 1932). Eye size ranges from well developed to just three ommatidia, with intermediates between. Individuals of Nocticola australiensis from the Chillagoe region of Australia also show a consistent gradation of forms, from less troglomorphic in southern caves to more troglomorphic in the north (Stone, 1988). The pattern of variation is very regular, unlike the more complex variation seen in some other taxa. The Australian species Paratemnopteryx howarthi, for example, also demonstrates the entire range of morphological variation, but both the reduced-eye, brachypterous forms and the large-eyed, winged morphs can occur in the same cave (Chopard, 1932; Roth, 1990b).

One consequence of regressive evolution of visual structures in cave-adapted animals is that orientation and communication have to be mediated by non-visual systems. Thus, the loss of the visual modality is often complemented by the hypertrophy of other sensory organs (Nevo, 1999; Langecker, 2000). In cockroaches, this may include the elongation of the legs, antennae, and palps (Fig. 1.18). In All. cavernicola the antennae are three times the length of the body (Vandel, 1965), and both Noc. australiensis and Neotrogloblattella chapmani have very long,

Fig. 1.17 Variation in eye and wing development in cave-dwelling Alluaudellina cavernicola. (A,B) Eye development in macropterous males; (C) eye development in a micropterous males; (D,E,F) eye development in wingless females. After Chopard (1938).

Macropterous male Macropterous male

Fig. 1.17 Variation in eye and wing development in cave-dwelling Alluaudellina cavernicola. (A,B) Eye development in macropterous males; (C) eye development in a micropterous males; (D,E,F) eye development in wingless females. After Chopard (1938).

slender legs and elongated maxillary palps. Palps are long in Ischnoptera peckorum as well (Roth, 1980, 1988). In nymphs of some species of Spelaeoblatta from Thailand it is only the front pair of legs that is elongated, which together with their narrow, elongated pronotum confers a mantid-like appearance (Vidlicka et al., 2003). Long legs are adaptive in reaching across gaps, negotiating irregular substrates, and covering larger areas per unit of expended energy (Howarth, 1983). Elongated antennae and palps function in extending the sensory organs, allowing the insects to detect food and mates faster and at a greater distance from their bodies. Consequently, less energy is required for resource finding (Hüppop, 2000), a decided advantage in a habitat where food may be scarce and population densities low. Cave-dwelling Paratemnopteryx exhibit subtle shifts in the number and type of antennal and mouthpart sensilla as compared to surface-dwelling relatives (Bland et al., 1998a, 1998b). There is a moderate reduction in the mechano-contact receptors and an increase in the number of olfactory sensilla in the cave dwellers when compared to similar sized epigean species. The elongation of appendages is typically correlated with a behavioral change. Troglomorphic cockroaches move with slow deliberation while probing with their long appendages. They "thereby avoid entering voids from which no escape is possible" (Howarth, 1983). Weinstein and Slaney (1995) found that highly troglomorphic species of

Fig. 1.18 Male of the Western Australian troglobitic cockroach Nocticola flabella from a cave in the Cape Range, Western Australia (Roth, 1991c). Top, dorsal view; bottom, grooming its metathoracic leg.; photo courtesy of the Western Australia Museum, via W.F. Humphreys.

Paratemnopteryx were able to avoid baited pitfall traps, but the slightly troglomorphic species readily entered them. Overall, cockroaches may experience less selection pressure for improved non-visual sensory organs than many other insects; cave colonizers that are already nocturnal may require little sensory improvement (Langecker, 2000).

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