The body is mainly reddish-brown or yellow-brown. The head is yellow, quite large and inflated-looking, with a kind of bubble at the front, narrow cheeks and short antennae. The large eyes are reddish. When in resting position the abdomen is usually folded forward.
The adults grow up to 8–13 millimetres (0.31–0.51 in) long and can be encountered from May through September, feeding on nectar or pollen .
Their larvae are endoparasite of bumble bees of the genus Bombus (B. lapidarius, B. terrestris, B. hortorum, B. pascuorum, etc.). They pupate and overwinter in their victims.