A detailed study of the holotype of Sphecomyrma canadensis Wilson, 1985 (Hymenoptera: Formicidae) from Canadian amber has led to the conclusion that the specimen belongs to a new genus, here named Boltonimecia gen.n. Since the taxonomy of stem-group ants is not well understood, in order to fi nd the taxonomic position of this genus, it is necessary to review the classifi cation of stem-group ants in a study of their relation to crown-group ants. In the absence of data for traditional taxonomic approaches, a statistical study was done based on a morphometric analysis of antennae. Scape elongation is believed to play an important role in the evolution of eusociality in ants; however, this hypothesis has never been confi rmed statistically. The statistical analysis presented herein lends support to the view that antennal morphology reliably distinguishes stem-group ants from crown-group ants, to determine whether a species belongs to one or the other group. This, in turn, may indicate a relationship exists between eusociality and scape elongation. A review of Cretaceous records of ants is made and the higher classifi cation of Formicidae with defi nitions of stem and crown groups is proposed. Newly obtained data are discussed focusing particularly on the origin, evolution and diversity of ants.