Solitary, small to large subterranean nesting bees. Generally the cuticle is black except for some males with yellow markings on the head and some species with red markings on the gaster. Nests may be isolated from each other or close together as an aggregation. The cells are lined with a wax-like substance. The pollen balls are smooth and more or less spherical. One egg is laid on each pollen ball. Most species have a single generation a year, although a few have two generations a year when seasonal dimorphism may be shown. Spring species over-winter as adults, whereas summer species over-winter as diapausing prepupae. The two sexes emerge more-or-less at the same time, the males a few days earlier than the females. Most species are polylectic, but some are oligolectic for pollen sources. Nationally 70 species in two genera (1 species restricted to the Channel Islands).