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.