Sheep keds, which superficially resemble ticks rather than flies, are wingless, brown, stout-bodied insects that pass their entire life cycle in the wool of their only host, sheep.
Adults creep about in the wool, feeding whenever hungry.
Sheep keds are members of a small family of flies that are all ectoparasites, mostly of birds. Usually called louse flies, they are strongly flattened from top to bottom, presumably the easier to creep about through fur or feathers. Most species of Hippoboscidae are winged, and thus are more readily recognizable as flies. None attacks domestic fowl, although wild game birds are sometimes afflicted. One species of hippoboscid, Lipoptena cervi, found on deer in British Columbia, starts its adult life with a useable pair of wings, then sheds them as soon as it has found a host, possibly because the wings interfere with movement through the hair.