Complex copulatory behavior and the proximate effect of genital and body size differences on mechanical reproductive isolation in the millipede genus Parafontaria.

The role of species-specific genitalia in reproductive isolation is unclear. Males of the millipede genus Parafontaria use gonopods (modified eighth legs) charged with sperm from the genital openings of the second legs as intromittent organs. Males perform both preliminary and true intromission during mating. During preliminary intromission, a male attempts to insert his gonopods into the female genitalia before charging the gonopods with sperm. If this intromission is completed, it is followed by the ejaculation of sperm to the gonopods and true intromission for insemination. In two sympatric species of Parafontaria that lack effective precopulatory isolation, copulation was terminated without insemination because of preliminary intromission failure caused by mismatched genital and body sizes. Thus, mechanical isolation between these sympatric species resulted from morphological differentiation mediated by the obligatory preliminary intromission. These findings demonstrate the proximate importance of genital and body size differences for reproductive isolation within this genus of millipede.