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Order of insects

Automatic taxobox
| name = Phthiraptera
| fossil range =
| image = Fahrenholzia pinnata. JPG
| image caption = Light micrograph of Fahrenholzia pinnata
| taxon = Phthiraptera
| authority = Haeckel, 1896
| display parents = 2
| subdivision ranks = Clades
| subdivision =
Amblycera
Anoplura
Rhyncophthirina
Ischnocera
Trichodectera? | subdivision ref =

Humans host two species of louse—the head louse and the body louse are subspecies of Pediculus humanus; and the pubic louse, Pthirus pubis. The body louse has the smallest genome of any known insect; it has been used as a model organism and has been the subject of much research. Lice were ubiquitous in human society until at least the Middle Ages. They appear in folktales, songs such as The Kilkenny Louse House, and novels such as James Joyce's Finnegans Wake. They commonly feature in the psychiatric disorder delusional parasitosis. A louse was one of the early subjects of microscopy, appearing in Robert Hooke's 1667 book, Micrographia.

Lice are divided into two groups: sucking lice, which obtain their nourishment from feeding on the sebaceous secretions and body fluids of their host; and chewing lice, which are scavengers, feeding on skin, fragments of feathers or hair, and debris found on the host's body. Many lice are specific to a single species of host and have co evolved with it. In some cases, they live on only a particular part of the body. Some animals are known to host up to fifteen different species, although one to three is typical for mammals, and two to six for birds. Lice generally cannot survive for long if removed from their host. If their host dies, lice can opportunistically use phoresis to hitch a ride on a fly and attempt to find a new host. Sucking lice range in length from They have narrow heads and oval, flattened bodies. They have no ocelli, and their compound eyes are reduced in size or absent. Their antennae are short with three to five segments, and their mouthparts, which are retractable into their head, are adapted for piercing and sucking. The thoracic segments are fused, the abdominal segments are separate, and there is a single large claw at the tip of each of the six legs. Chewing lice are also flattened and can be slightly larger than sucking lice, ranging in length from They are similar to sucking lice in form but the head is wider than the thorax and all species have compound eyes. There are no ocelli and the mouthparts are adapted for chewing. The antennae have three to five segments and are slender in the suborder Ischnocera, but club shaped in the suborder Amblycera. The legs are short and robust, and terminated by one or two claws. Some species of chewing lice house symbiotic bacteria in bacteriocytes in their bodies. These may assist in digestion because if the insect is deprived of them, it will die. Lice are usually cryptically coloured to match the fur or feathers of the host. A louse's color varies from pale beige to dark gray; however, if feeding on blood, it may become considerably darker. Female lice are usually more common than males, and some species are parthenogenetic, with young developing from unfertilized eggs. A louse's egg is commonly called a nit. Many lice attach their eggs to their hosts' hair with specialized saliva; the saliva/hair bond is very difficult to sever without specialized products. Lice inhabiting birds, however, may simply leave their eggs in parts of the body inaccessible to preening, such as the interior of feather shafts. Living louse eggs tend to be pale whitish, whereas dead louse eggs are yellower.

The average number of lice per host tends to be higher in large bodied bird species than in small ones. Lice have an aggregated distribution across bird individuals, i. e. most lice live on a few birds, while most birds are relatively free of lice. This pattern is more pronounced in territorial than in colonial—more social—bird species. Host organisms that dive under water to feed on aquatic prey harbor fewer taxa of lice. Bird taxa that are capable of exerting stronger antiparasitic defense—such as stronger T cell immune response or larger uropygial glands—harbor more taxa of Amblyceran lice than others. Reductions in the size of host populations may cause a long lasting reduction of louse taxonomic richness, for example, birds introduced into New Zealand host fewer species of lice there than in Europe. Louse sex ratios are more balanced in more social hosts and more female biased in less social hosts, presumably due to the stronger isolation among louse subpopulations (living on separate birds) in the latter case. The extinction of a species results in the extinction of its host specific lice. Host switching is a random event that would seem very rarely likely to be successful, but speciation has occurred over evolutionary time scales so it must be successfully accomplished sometimes. but most seem to have little effect on their host. The habit of dust bathing in domestic hens is probably an attempt by the birds to rid themselves of lice. but most individuals spend their whole life cycle on a single host and are only able to transfer to a new host opportunistically. Lice infestation is a disadvantage in the context of sexual rivalry.

The human body louse Pediculus humanus humanus has (2010) the smallest insect genome known. This louse can transmit certain diseases while the human head louse (P. h. capitis), to which it is closely related, cannot. 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.

James Joyce's 1939 book Finnegans Wake has the character Shem the Penman infested with "foxtrotting fleas, the lieabed lice,  bats in his belfry". 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). Her eggs are sealed in a basket woven by her mother, who gives it to Coyote, instructing him not to open it before he reaches home. 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. The Irish songwriter John Lyons (b. 1934) wrote the popular

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! Gae somewhere else, and seek your dinner on some poor body." John Milton in Paradise Lost mentioned the biblical plague of lice visited upon pharaoh: "Frogs, lice, and flies must all his palace fill with loathed intrusion, and filled all the land." John Ray recorded a Scottish proverb, "Gie a beggar a bed and he'll repay you with a Louse." In Shakespeare's Troilus and Cressida, Thersites compares Menelaus, brother of Agamemnon, to a louse: "Ask me not what I would be, if I were not Thersites; for I care not to be the louse of a lazar, so I were not Menelaus."

The name woodlouse or wood louse is given to crustaceans of the suborder Oniscidea within the order Isopoda, unrelated to lice. The Oxford English Dictionarys earliest citation of this usage is from 1611.