The morphology of insect genitalia is often highly species-specific, and its variation has been suggested as an important impetus for evolution. Structural variation of the male genitalia and the female spermathecae in Phlebotomine sand flies is unique among the blood sucking Diptera. We describe the fine structures involved in mating for Phlebotomus papatasi (Scopoli). Relationships among the length of the spermathecal duct and aedeagal filament were studied in 26 species of Old World phelebotomine sand flies comprising 12 subgenera. In most taxa the aedeagal filaments were very long allowing direct insemination into the spermathecae, indicating that intrasexual competition among males occurs as sperm precedence. There was a positive correlation between the lengths of the spermathecal ducts and aedeagal filaments, indicating that an evolutionary "arms race" occurs between males and females over the control of fertilization. The pattern of genitalia variation in phlebotomine sand flies also indicated that differences in the lengths of the spermathecal ducts and aedeagal filaments are distributed unevenly among closely related species. Genital differences do not seem to occur in populations of the Phlebotomus argentipes Annandale & Brunetti complex, whereas differences showed strongly in closely related Phlebotomus papatasi and P. bergeroti Parrot, and in sympatric Phlebotomus martini Parrot and P. celiae Minter.