Female genital dimorphism in a spider -: (Araneae: Pholcidae)

Discontinuous individual variation among members of the same sex, in the same life stage, within a population, is an important phenomenon for our understanding of more fundamental issues in evolutionary biology. One set of morphological characters is conspicuously rare in the literature on intrasexual polymorphisms: genitalia. In this paper we report the first case of female genital polymorphism, the first not only for spiders but apparently for animals in general. We argue that the overwhelming use of genitalia in species identification practiced by many invertebrate taxonomists presents a strong bias against discovering polymorphisms in genitalia. Further methodological and practical aspects of invertebrate taxonomy are discussed that, taken together, make the discovery of genital polymorphisms unlikely, no matter whether or not the phenomenon is rare.