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).

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