Reduction and Loss of Genitalic Structures

The genital phallomeres of some blaberid cockroaches are lightly sclerotized, considerably reduced, or in some cases, altogether absent. The Panchlorinae are characterized by the absence of a genital hook, and if the remaining two phallomeres are present, they are markedly reduced (Roth, 1971b). Likewise, one or more phallomeres may be reduced or absent in many Panesthiinae (including Geoscapheini) (Fig. 6.11D) (Roth, 1977). Macro-panesthia rhinoceros and M. heppleorum males completely lack a genital hook, and sclerites L1 and L2d are also missing. Some of the Australian soil-burrowing cockroaches exhibit intraspecific variation in the reduction of phallomeres (Walker and Rose, 1998). The occurrence of poorly developed male genitalia in cockroaches corresponds very well with copulatory behavior. A reduced or absent genital hook is strong evidence of type III mating behavior, that is, the male backs into the female to initiate mating (Roth, 1971b, 1977).

Simple genital structure in males is predicted by the cryptic choice hypothesis if females are monandrous, because sexual selection by female choice is possible only if females make genitalic contact with more than one male (Eberhard, 1985, 1996). In monandrous females, the choice of sire is settled prior to copulation, via mechanisms such as premating courtship or male-male contests. The mating strategy in cockroaches with reduced genitalia is not known well enough to determine if that is the case here; however, one male is usually present in social groups of Panesthia. Panesthia cribrata typically lives in aggregations, often (29%) comprised of a single adult male, a number of adult females, and nymphs of various sizes (Rugg and Rose, 1984a).

Additional correlates of reduced male genitalia in cockroaches also must be considered. Among the Panes-thiinae species studied, the absence of an oothecal covering around the eggs is correlated with the absence or reduction of male genital structures (Walker and Rose, 1998). All of the species for which we have information also exhibit a burrowing lifestyle, tunneling in soil, rotted wood, or rotted palms. How all these threads connect (burrowing lifestyle, mating system, copulatory behavior, male genital morphology, and absence of egg case) awaits further study. It is of interest (Chapter 9), however, that termites are monogamous (Nalepa and Jones, 1991) and that isopteran males are largely unencumbered by genitalia (Roonwal, 1970). Termites also live in burrows, mate by backing into each other, and except for Mastotermes, have lost the casing around their eggs. Species in the Cryptocercidae, the sister group of termites, live in burrows and are apparently monandrous, but male genitalia are not markedly reduced; they do, however, exhibit a number of paedomorphic characters (Klass, 1997).

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