The insects in this family have an unattractive, flat body and a crab-like gait and they cling to their victims with powerful claws. At first sight they do not look very much like flies. They are specialised for living on mammals or birds, where they crawl around in the fur or among the feathers and suck blood.
The proboscis is somewhat like that of a stable fly.. The larvae develop within the female’s body and pupate immediately they are released by the female.
Two of the species which live on birds may wander into houses from the nests. These are Crataerina pallida and Stenepteryx hirundinis, which occur on swifts, martins and swallows.