Facial structure was compared in over 925 of the 1,945 Nearctic genera and subgenera of the order Hymenoptera. The clypeus is considered as the facial region between the labrum and a definite system of strengthening inflections associated with the tentorium. The primitive position of the anterior tentorial pits was probably at the level of the ventral edges of the genae. In many of the species examined they lie well above the ventral edges of the genae. This modification was probably produced by ventral extension of the genae, and not by dorsal migration of the clypeus. In many species, however, the pits lie at the ventral rim of the face. This modification was probably produced first by enclosure of the clypeus between ventral extensions of the genae, and then by ventral migration of the tentorial pits. The formation of new sclerites by the secretion of additional cuticular material was observed and described in several groups. Similarities in facial structure between groups widely separated by taxonomists are pointed out.