The genus Psithyrus Lepeletier is closely related to the genus Bombus Latreille. A large number of similar and dissimilar characters of these genera have been enumerated (Richards, 1927), but the major modification of Psithyrus to an obligate, parasitic mode of life has been the loss of the pollen-collecting apparatus and the lack of a worker caste. However, Psithyrus still visits flowers for nectar, and the general morphology of the “tongue” has not been modified as has the pollen-collecting apparatus. Detailed measurements on the labium of Psithyrus have not been reported previously. This paper gives data on 10 species. The information adds to basic knowledge on Psithyrus, and may be of value in relation to the question of the monophyletic or polyphyletic derivation of species of Psithyrus. Also, as with Bombus, considerable variation exists in the sizes of the parasitic bees, which is associated with the nutrition of the larvae. The measurements of the radial cell of the forewing show size relations within a species. The length of the radial cell (Fig. 1) has been selected as the most convenient method, although the size of an individual can be expressed also by the measurement of either its wing length or weight.