The sessile abdomen, always without a constriction between the first and second segments, but above all the very large, free, triangularly elongated labrum, which is always much longer than wide at base, the sinuate or ≀-shaped transverse median nervure in the hind wings, and the aborted ocelli, at once distinguished the family.

Most authorities have confused, or at least included this family with the family Stizidæ, which also has a more or less prominent labrum; but in the Stizidæ the ocelli are always distinct, normal, the labrum is most frequently semicircular, always wider than long, while the middle tibiæ have two distinct apical spurs, characters of great taxonomic value, readily recognizable, and which at once separate them from the Bembicidæ.