Cockroache; Ecology, behavior & history - W.J. Bell

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).
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; Hüppop,
2000; Gilbert and Deharveng, 2002). Second, troglomorphic
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, expensive
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). Photoreception
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 plesiomorphic
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
SHAPE, COLOR, AND SIZE 15