The following material taken with permission from: Mitchell, T.B. 1962. Bees of the Eastern United States, Volume II. North Carolina Agricultural Experiment Station. Tech. Bul. No.152, 557 p.
This is a very large genus of wasp-like, parasitic bees, usually red or black in color, with yellow or red integumental maculations. They are parasitic chiefly on Andrena. In the front wing stigma is well developed, the marginal cell is elongate, acute at tip and not divergent from the costal margin, and usually there are three submarginal cells. The 1st submarginal cell equals the 2nd and 3rd combined in length. The pubescence is usually very short, thin and inconspicuous. The maxillary palpi are 6-segmented. The abdomen differs from that in most other genera in the form of the basal tergum, the ateral margins of which are produced as free flaps that over lie the margins of the sternum. In the female, sternum 6 is broadened and exposed apically, and conspicuously spinose on each lateroapical angle. The pygidium, is well developed in both sexes ' but is usually hidden by tergum 5 in the female, this plate having a more or less conspicuous pseudo pygidial area apically.
Ten subgenera of Nomada have been recognized in the Nearctic region. Some of these (Gnathias, Holonomada, Pachynomada, Callinomada, Micronomada and Nomadula) are quite distinct and easy to recognize, once the characters are comprehended and observed. The subgenus Centrias is represented by a single species which shows marked similarities to the species of Nomadula, and its recognition as a distinct group seems questionable. Heminomada is a rather large group of species having complete yellow abdominal bands in both sexes, but the line of demarkation separating it from the subgenus Nomada is obscure. Furthermore, if a species is known in but one sex, it may be difficult to place it with confidence in one or the other of these groups. Phor is a very small group, closely related to Nomada and its status seems to this writer to be questionable.
Of the 87 species which are keyed out and described in the following pages, 40 are either assigned to the subgenus Nomada or cannot be placed with certainty in any of the other subgenera. For this reason, a number of them are not placed in any subgeneric group. Their future inclusion in any of these depends upon a more intensive and comprehensive study of the genus, and upon more collecting at the nesting sites of the hosts, with a consequent basis for correlation of the sexes and the degree of variation within the species. This variation is very marked in certain of the subgenera, notably in Gnathias, where specific limits are very difficult to determine. No character, or combination of characters, seems entirely reliable, and the degree of sexual dimorphism makes correlation of the sexes difficult.
Because of these difficulties, no new species are proposed in the subgenus Gnathias. The types of those species or forms which occur in the Eastern United States have been studied and some opinions regarding synonymy have been formed. Those species which seem valid have been included in the comprehensive key to species of the genus, and specimens which agree reasonably well with these type specimens should be identifiable. Others, however, which are less typical in character, will need to be indicated as of doubtful identity.