Please refer also to the following link for details on this group:
Melittidae = Link 1
Melittidae. -- Melittids are small, dark bees that are not very often encountered, as they are relatively rare. They have similar nesting habits to the Andrenidae. They are distinguished by having the jugal lobe of the hind wing shorter than the submedian cell, and the segments of the labial palps are similar and cylindrical. North American species nest in burrows in the soil.There were more than 105 species of Melittidae known as of 2011. Some species of the American Dasypodinae (Hesperapis) differ from similar bees in their softer integument. They have two submarginal cells; the base of the second cell is at right angles to vein M. The American Melittinae have either 2 or 3 submarginal cells; those with 3 cells differ from similar bees like Andrena in that the 3rd submarginal cell (2Rs) is pointed at its apex on vein M.
This is a small bee family, with some 60 species in 4 genera, restricted to Africa and the northern temperate zone. Historically, the family has included the Dasypodaidae and Meganomiidae as subfamilies, but recent molecular studies indicate that Melittidae (sensu lato) was paraphyletic, so each of the three historical subfamilies is now accorded family status, with Dasypodaidae as the basal group of bees, followed by Meganomiids and Melittids, which are sister taxa.
Most species are small to moderate-sized with bushy scopae, and are often oligolectic. A few species utilize floral oils as larval food rather than pollen; e.g., Rediviva emdeorum, which is unique by having forelegs longer than the entire body. These are adapted for sponging the floral oil at the end of elongated corolla spurs of the host plant, Diascia.
Melittidae nest in soil crevices, and females transport pollen on their hind legs, in a scopa that is restricted to the tibia.
A key reference is Michener (1981).
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Danforth, B.N., Sipes, S., Fang, J., Brady, S.G. (2006) The history of early bee diversification based on five genes plus morphology. Proceedings of the National Academy of Sciences 103: 15118-15123.
Michener, C.D. 1974. The Social Behavior of the Bees. Harvard University Press. 404 pp.
Michener, C.D. 2000. The Bees of the World. Johns Hopkins University Press. 913 pp.