Wingless, permanent obligate ectoparasites of mammals and birds. Highly host specific. Body dorso-ventrally flattened, adults 0.35 – 11 mm long. Eyes variable, antennae 3-5 segmented, mouthparts mandibulate or highly modified for sucking and/or piercing. Pronotum usually free. Meso- and metanotum complete or partially fused. Legs usually short/stout bearing single or paired pretarsal claws. Abdomen comprising 8-11 visible segments variably sclerotized, cerci absent. Colouration usually cryptic matching host pelage or plumage. Three nymphal instars.