Chelostoma Latreille 1809

The bees of this Holarctic genus are among the smallest of the Nearctic megachilids. Two European species are successfully established in the eastern United States (New York). Our native species occur in the western United States and adjacent northern Mexico (Baja California). All of these native species are found in California, but two of them range northward into Oregon and Washington and thence eastward into the Rocky Mountains. They are chiefly vernal bees and visit flowers of the Hydrophyllaceae, especially the genera Phacelia and Eriodictyon, and possibly collect pollen exclusively from these flowers. Virtually nothing is known about the nesting habits of the Nearctic species, although one species, Chelostoma phaceliae Michener, has been induced to nest in prebored stems of Sambucus set out as trap-nests. In Europe, members of this genus make their nests in holes and abandoned insect tunnels in wood. Such a nesting site was discovered in an old log barn in Finland where foraging studies at the flowers of buttercup (Ranunculus) were conducted on a marked population of Chelostoma florisomnis (Linnaeus) by Kapyla (1978. Ent. Fenn., Ann. 44: 63-64). ~The Nearctic members of this genus are slender black bees which are more finely punctate than those in the genus Heriades. The thorax is the most elongate of any megachilid except Prochelostoma, the minimum distance between the tegulae being less than the length of the mesoscutum. The propodeum has a horizontal basal area about as long as the metanotum and is finely sculptured medially and sometimes is pitted laterally. The basal area is not sharply separated from the posterior surface of the propodeum. The first metasomal tergum is convex, as seen in profile, and thus lacks a well-defined anterior face, but does have a longitudinal median sulcus basally. The metasoma of the male is less telescoped than in most allied genera, there being seven exposed terga and the seventh in our species is strongly tridentate or quadridentate. The pterostigma, as in Heriades, is quite large and the inner ventral angle of each posterior coxa is carinate. The monotypic genus Prochelostoma of the eastern United States could perhaps be justifiably considered a subgenus of Chelostoma, but since so little is known about the biologies of these taxa their current taxonomic status is maintained in this treatment.