[1][2][3] Lice are obligate parasites, living externally on warm-blooded hosts which include every species of bird and mammal, except for monotremes, pangolins, and bats.

Genetic evidence indicates that lice are a highly modified lineage of Psocoptera (now called Psocodea), commonly known as booklice, barklice or barkflies.

They appear in folktales, songs such as The Kilkenny Louse House, and novels such as James Joyce's Finnegans Wake.

[7][9] A louse's colour varies from pale beige to dark grey; however, if feeding on blood, it may become considerably darker.

Lice inhabiting birds, however, may simply leave their eggs in parts of the body inaccessible to preening, such as the interior of feather shafts.

[10] The average number of lice per host tends to be higher in large-bodied bird species than in small ones.

[12] Host organisms that dive under water to feed on aquatic prey harbour fewer taxa of lice.

[7] Ischnoceran lice may reduce the thermoregulation effect of the plumage; thus heavily infested birds lose more heat than others.

[26] The oldest confirmed fossil louse is Archimenopon myanmarensis, an amblyceran from the Cretaceous amber from Myanmar.

[4] Another early representative of the group is a bird louse, Megamenopon rasnitsyni, from Eckfelder Maar, Germany, which dates to the Eocene, around 44 million years ago.

[27] Saurodectes vrsanskyi from the Early Cretaceous (Aptian) Zaza Formation of Buryatia, Russia, has also been suggested to be a louse, but this is tentative.

[28] Placental mammal lice had a single common ancestor that lived on Afrotheria with this arising from host-switching from an ancient avian host.

[29] Cladogram showing the position of Phthiraptera within Psocodea:[1] Philopteridae Anoplura Rhynchophthirina Trichodectidae Amblycera Liposcelididae Pachytroctidae Sphaeropsocidae Amphientometae Homilopsocidea Caeciliusetae Psocetae Epipsocetae Philotarsetae Archipsocetae Atropetae Psyllipsocetae Prionoglaridetae (paraphyletic) Phthiraptera is clearly a monophyletic grouping, united as the members are by a number of derived features including their parasitism on warm-blooded vertebrates and the combination of their metathoracic ganglia with their abdominal ganglia to form a single ventral nerve junction.

[1] These changes were accepted by Psocodea Species File and others, with the exception of placing Phthiraptera under the infraorder Nanopsocetae, as a parvorder, with the four subgroups listed above.

Using phylogenetic and cophylogenetic analysis, Reed et al. hypothesized that Pediculus and Pthirus are sister taxa and monophyletic.

Genetic evidence suggests that human ancestors acquired pubic lice from gorillas approximately 3-4 million years ago.

At the death of Thomas Becket, Archbishop of Canterbury in 1170, it was recorded that "The vermin boiled over like water in a simmering cauldron, and the onlookers burst into alternate weeping and laughing".

[40] Robert Hooke's 1667 book, Micrographia: or some physiological descriptions of minute bodies made by magnifying glasses with observations and Inquiries thereupon, illustrated a human louse, drawn as seen down an early microscope.

[41] Margaret Cavendish's satirical The Description of a New World, Called The Blazing-World (1668) has "Lice-men" as "mathematicians", investigating nature by trying to weigh the air like the real scientist Robert Boyle.

[46] In the psychiatric disorder delusional parasitosis, patients express a persistent irrational fear of animals such as lice and mites, imagining that they are continually infested and complaining of itching, with "an unshakable false belief that live organisms are present in the skin".

With their simple life history and small genomes, the pair make ideal model organisms to study the molecular mechanisms behind the transmission of pathogens and vector competence.

[49] James Joyce's 1939 book Finnegans Wake has the character Shem the Penman infested with "foxtrotting fleas, the lieabed lice, ... bats in his belfry".

[52] Clifford E. Trafzer's A Chemehuevi Song: The Resilience of a Southern Paiute Tribe retells the story of Sinawavi (Coyote)'s love for Poowavi (Louse).

Hearing voices coming from it, however, Coyote opens the basket and the people, the world's first human beings, pour out of it in all directions.

[54][55] Robert Burns dedicated a poem to the louse, inspired by witnessing one on a lady's bonnet in church: "Ye ugly, creepin, blastid wonner, Detested, shunn'd, by saint and sinner, How dare ye set your fit upon her, sae fine lady!

John Ray recorded a Scottish proverb, "Gie a beggar a bed and he'll repay you with a Louse."