Medium-sized to large wasps. Black, often with red, yellow or white markings. The larvae are cleptoparasites on solitary bees of the genera Osmia and Chelostoma. The female introduces an egg into the cell of its host by using her sting to penetrate the cell wall. On hatching, the first instar larva, although without legs, is active and destroys the host's egg with its large mandibles. The larva then moults into the next instar which has smaller mandibles and feeds on the host's provisions. Probably one generation a year. Nationally: 2 genera with 2 species.