Unquestionably, this group is closely related to the Crabronidae, where most authorities have placed it; but, to me, the quiet different shaped head, the absence of the basal abscissa of the cubitus in the front wings, and the remarkable formation of the scutellum and postscutellum, characters not found in any other group, seem to justify one in separating it from the Crabronidae and treating it as a distinct family.

The group was first recognized as a subfamily in 1874, by c. G. Thomson, in his Skandinaviens Hymenoptera, Vol. III., p. 256.