An eversible endophallus is present in most groups of bees. Its presence or absence and its morphological variation were studied in 122 species, representing nine families of bees. An eversible endophallus is absent in the sphecoid wasps and other aculeate Hymenoptera, as well as in the bee families Colletidae and Oxaeidae, and part of the subfamily Andreninae of the Andrenidae. It is suggested that the families Colletidae and Oxaeidae may represent early offshoots in bee phylogeny. Extremely complex endophalli occur in some groups of bees: a positive correlation between complex endophalli and polyandrous mating system was found, suggesting that sexual selection may be the evolutionary force shaping endophallic morphology. The function of the endophallus, as well as of other parts of the external genitalia, was studied in specimensin copulaofNomia nevadensisandPerdita albipennis.The study of the mechanics of copulation was extended to other species, showing that the several sclerotized parts of the bee genital capsule function as clasping devices.