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

are usually lost in older female instars and are absent in
adult females. Juveniles have undeveloped and poorly
sclerotized genitalia and they often lack other characters
useful in species identification. Nymphs of Australian
soil-burrowing cockroaches, for example, are difficult to
tell apart because the pronotal and tergal features that
distinguish the various species are not fully developed
(Walker et al., 1994). In some taxa, nymphal coloration
and markings differ markedly from those of adults, making
them scarcely recognizable as the same species (e.g.,
Australian Polyzosteria spp.—Tepper, 1893; Mackerras,
1965a). In general, the first few instars of a given species
can be distinguished from each other on the basis of nonoverlapping
measurements of sclerotized morphological
features such as head width or leg segments. In older
stages, however, accumulated variation results in overlap
of these measurements, making it difficult to determine
the stage of a given nymph. This variation results from intermolt
periods that differ greatly from individual to individual,
not only in different stages, but also within a
stage (Scharrer, 1946; Bodenstein, 1953; Takagi, 1978;
Zervos, 1987). The difficulty in distinguishing different
developmental stages within a species and the nymphs of
different species from each other often makes young developmental
stages intractable to study in the field. Consequently,
the natural history of cockroach juveniles is
virtually unknown.
Dimorphism
In addition to dimorphism in the presence of wings
(Chapter 2) and overall body size (discussed below), male
and female cockroaches may differ in the color and shape
of the body or in the size, color, and shape of specific body
parts. The general shape of the male, particularly the abdomen,
is often more attenuated than that of the female.
Several sex-specific morphological differences suggest
that the demands of finding and winning a mate are
highly influential in cockroach morphological evolution.
Dimorphism is most pronounced in species where males
are active, aerial insects, but the females have reduced
wings or are apterous. These males may have large,
bulging, nearly contiguous eyes while those of the more
sedentary female are flattened and farther apart, for example,
several species of Laxta and Neolaxta (Mackerras,
1968b; Roth, 1987a, 1992) and Colapteroblatta compsa
(Roth, 1998a). Male morphology in the blattellid genera
Escala and Robshelfordia is completely different from that
of the opposite sex (Roth, 1991b). Such strong sexual dimorphism
makes associating the sexes difficult, particularly
when related species are sympatric (Roth, 1992); as
a result, conspecific males and females are sometimes
described as separate species. Additional sexual dimorphisms
include the presence of tergal glands on males of
many species, and the size and shape of the pronotum.
Asymmetry
Cockroaches tend to have an unusually high level of fluctuating
asymmetry (Hanitsch, 1923), defined as small,
random differences in bilateral characters. The cockroach
tarsus is normally composed of five segments, but on one
leg it may have just four. Spines on the femora also may
vary in number between the right and left sides of the
same individual. In both characters a reduction more often
occurs on the left side of the body. Wing veins may be
simple on one side and bifurcated on the other. This tendency
often makes it difficult to interpret the fossil record,
where so much of our information is based on wings.
Asymmetries of this type are widely used as a measure of
fitness because they result from developmental instability,
the ability of an organism to withstand developmental
perturbation. Of late, fluctuating asymmetry has become
a major but controversial topic in evolutionary
biology (e.g., Markow, 1995; Nosil, 2001), but is unstudied
in the Blattaria. Less subtle bilateral asymmetries also
occur in cockroaches; gynandromorphs are reported in
Periplaneta americana, Byrsotria fumigata (Willis and
Roth, 1959), Blattella germanica (Ross and Cochran,
1967), and Gromphadorhina portentosa (Graves et al.,
1986).
Pronotum
The large, shield-shaped pronotum is a defining characteristic
of cockroaches and its size, shape, curvature, and
protuberances have systematic value in certain groups
(e.g., Perisphaeriinae, Panesthiinae). Some cockroaches
are more strongly hooded than others, that is, the head
ranges from completely covered by the pronotum to almost
entirely exposed. In some species the pronotum is
flat, in others it has varying degrees of declivity. At its extreme
it may form a cowl, shaped like an upside down U
in section. The border of the pronotum may be recurved
to varying degrees, forming a gutter around the sides,
which sometimes continues into the cephalic margin.
The majority of species of Colapteroblatta, for example,
have the lateral wings of the pronotum deflexed and the
edges may be ridged or swollen (Hebard, 1920 [1919];
Roth, 1998a, Fig. 1-6). In a few cases the pronotum can
resemble the headpiece of certain orders of nuns (Fig.
1.1A). Some species of Cyrtotria have pronota perforated
with large, semilunar pores in both sexes; these may be
the openings of glands (Fig. 1.1B) (Shelford, 1908). The
shape of the pronotum can vary within a species, with
distinct forms correlated with varying degrees of wing re-
2 COCKROACHES