The standard cockroach body is flattened and broadly oval, with a large, shield-like pronotum covering the head, ventrally deployed, chewing mouthparts, and long, highly segmented antennae. The forewings (tegmina) are typically leathery and the hindwings more delicate and hyaline. The coxae are flattened and modified to house the femur, so that when the legs are tucked in close to the body the combined thickness of the two segments is reduced. A comprehensive discussion of the morphological features of cockroaches, particularly those of importance in recognizing and describing species, is given in Roth (2003c).
Like other hemimetabolous insects, cockroach nymphs generally resemble adults except for the absence of tegmina and wings; these structures are, however, sometimes indicated by non-articulated, lobe-like extensions of the meso- and metanotum in later developmental stages. Early instars of both sexes have styles on the subgenital plate; these 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 non-overlapping 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 in-termolt 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.
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