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.
ADULTS. Head dorso-ventrally compressed and more-or-less prognathous with reduced or greatly modified mouthparts. Annular antennae short, small, or rarely absent, comprising a scape, pedicel, and three terminal flagellomeres, the latter two of which bear sensilla. Terminal flagellomeres may be fused and are often sexually dimorphic. Variable compound eyes (may be absent) and no ocelli. Tentorium reduced or absent. Clypeus variably extended displacing the labrum to the ventral surface. This forms a conspicuous hyaline pad-like protrusion (the pulvinus) which sits anterior to the mandibular mouthparts of most chewing lice (i.e. Amblycera, Ischnocera and Rhynchophthirina) but is absent in sucking lice (Anoplura). When present the pulvinus and mandibles assist in anchoring the louse to its host. Maxillae greatly reduced in chewing lice but highly modified as stylet guides in sucking lice. Labium and hypopharynx distinct in Amblycera and Ischnocera but highly modified in Rhynchophthirina (not present in Greenland) and Anoplura. Pronotum reduced and usually distinct except in Anoplura. Meso- and metanotum usually indivisibly fused except in some Amblycera. Leg articulations with the thorax, pleuro- (i.e. lateral) or sternocoxal (i.e. ventral), usually both. Legs variously modified for locomotion and attachment to host. Tarsus subdivided into two tarsomeres that are variably fused. Pretarsus bears two claws for lice parasitizing birds, or one claw for lice parasitizing mammals (no exceptions in Greenland). Abdomen comprises eleven segments, although one or more are always partly or wholly suppressed. Typically nine segments are visible, the first comprising segment I and II (true segment number indicated by Roman numerals) and the ninth comprising fused segments IX, X and XI. Tergal, sternal and tergopleural sclerotized plates are variably distributed over the abdomen. Six abdominal spiracles are normally borne from segments III-VIII, although reduction has occurred in many mammal infesting species – usually sequentially from segment VIII. The posterior margin of the sternum VII forms the ventral margin of the vulva. Gonapophyses may be present on segment VIII and a female genital lobe of uncertain homology on segment IX. The male genital opening is posterior to sternum IX. Male external genitalia of Phthiraptera are highly variable.
As permanent obligate ectoparasites, louse distribution essentially mirrors that of the mammal and bird species they parasitize. Consequently, it is the diversity of these hosts rather than any geographic or ecological correlate that is the best predictor of louse diversity. Whilst Greenland is home to just 13 species of terrestrial/semi-terrestrial mammal, the countries rich avifauna, comprising an unusual mix of Arctic species combined with European and North American vagrants provides a varied habitat for the many species of lice that infest these birds.