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 isolationis unclear. Males of the millipede genus Parafontaria use gonopods(modified eighth legs) charged with sperm from the genitalopenings of the second legs as intromittent organs. Males performboth preliminary and true intromission during mating. During preliminaryintromission, a male attempts to insert his gonopods intothe female genitalia before charging the gonopods with sperm. If thisintromission is completed, it is followed by the ejaculation of spermto the gonopods and true intromission for insemination. In twosympatric species of Parafontaria that lack effective precopulatoryisolation, copulation was terminated without insemination becauseof preliminary intromission failure caused by mismatched genitaland body sizes. Thus, mechanical isolation between these sympatricspecies resulted from morphological differentiation mediated by theobligatory preliminary intromission. These findings demonstrate theproximate importance of genital and body size differences for reproductiveisolation within this genus of millipede