Louse (plural: lice) is the common name for members of the order Phthiraptera, which contains nearly 5,000 species of wingless insect. Lice are obligate parasites, living externally on warm-blooded hosts which include every species of bird and mammal, except for monotremes, pangolins, and bats. Lice are vectors of diseases such as typhus.

Chewing lice live among the hairs or feathers of their host and feed on skin and debris, while sucking lice pierce the host's skin and feed on blood and other secretions. They usually spend their whole life on a single host, cementing their eggs, called nits, to hairs or feathers. The eggs hatch into nymphs, which moult three times before becoming fully grown, a process that takes about four weeks.

Humans host two species of louse—the head louse and the body louse are subspecies of Pediculus humanus; 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.

Fahrenholzia pinnata
Light micrograph of Fahrenholzia pinnata
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Superorder: Psocodea
Order: Phthiraptera
Haeckel, 1896


Morphology and diversity

Humans host three different kinds of lice: head lice, body lice, and pubic lice. Lice infestations can be controlled with lice combs, and medicated shampoos or washes.[1]

Sucking lice are small wingless insects ranging from 0.5 to 5 mm (0.02 to 0.20 in) in length. 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 mouth parts, which are retractable into their head, are adapted for piercing and sucking.[2] There is a cibarial pump at the start of the gut; it is powered by muscles attached to the inside of the cuticle of the head. The mouthparts consist of a proboscis which is toothed, and a set of stylets arranged in a cylinder inside the proboscis, containing a salivary canal (ventrally) and a food canal (dorsally).[3] 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.[2]

Chewing lice are also flattened and can be slightly larger than sucking lice, ranging in length from 0.5 to 6 mm (0.02 to 0.24 in). 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. Many lice are specific to a single species of host and have co-evolved with it. They are usually cryptically coloured to match the fur or feathers of the host.[2][4]

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. Most are found on only specific types of animals, and, in some cases, 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. For example, in humans, different species of louse inhabit the scalp and pubic hair. Lice generally cannot survive for long if removed from their host.[5] 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. If their host dies, lice can opportunistically use phoresis to hitch a ride on a fly and attempt to find a new host.[6]

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.[5] Lice are exopterygotes, being born as miniature versions of the adult, known as nymphs. The young moult three times before reaching the final adult form, usually within a month after hatching.[5]


The average number of lice per host tends to be higher in large-bodied bird species than in small ones.[7] 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.[8] Host organisms that dive under water to feed on aquatic prey harbor fewer taxa of lice.[9][10] 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.[11][12] Reductions in the size of host populations may cause a long-lasting reduction of louse taxonomic richness,[13] for example, birds introduced into New Zealand host fewer species of lice there than in Europe.[14][15] 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.[16] 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.[13]

Lice may reduce host life expectancy if the infestation is heavy,[17] 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.[2] Lice may transmit microbial diseases and helminth parasites,[18] but most individuals spend their whole life cycle on a single host and are only able to transfer to a new host opportunistically.[2] Ischnoceran lice may reduce the thermoregulation effect of the plumage; thus heavily infested birds lose more heat than others.[19] Lice infestation is a disadvantage in the context of sexual rivalry.[20][21]



The order 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.[22] The order has traditionally been divided into two suborders, the sucking lice (Anoplura) and the chewing lice (Mallophaga); however, recent classifications suggest that the Mallophaga are paraphyletic and four suborders are now recognized:[23]

  • Anoplura: sucking lice, occurring on mammals exclusively
  • Rhynchophthirina: parasites of elephants and warthogs
  • Ischnocera: mostly avian chewing lice, however, one family parasitizes mammals
  • Amblycera: a primitive suborder of chewing lice, widespread on birds, however, they also live on South American and Australian mammals

Nearly 5,000 species of louse have been identified, about 4,000 being parasitic on birds and 800 on mammals. Lice are present on every continent in all the habitats that their host animals and birds occupy.[23] They are found even in the Antarctic, where penguins carry 15 species of lice (in the genera Austrogonoides and Nesiotinus).[24]

Ricinus bombycillae (Denny, 1842)

Ricinus bombycillae, an Amblyceran louse from the bohemian waxwing

Trinoton anserinum, an Amblyceran louse from a mute swan

Lice image01

Damalinia limbata is an Ischnoceran louse from goats. The species is sexually dimorphic, with the male smaller than the female.


Lice have been the subject of significant DNA research in the 2000s that led to discoveries on human evolution. The three species of sucking lice that parasitize human beings belong to two genera, Pediculus and Pthirus: head lice (Pediculus humanus capitis), body lice (Pediculus humanus humanus), and pubic lice (Pthirus pubis). Human head and body lice (genus Pediculus) share a common ancestor with chimpanzee lice, while pubic lice (genus Pthirus) share a common ancestor with gorilla lice. Using phylogenetic and cophylogenetic analysis, Reed et al. hypothesized that Pediculus and Pthirus are sister taxa and monophyletic.[25] In other words, the two genera descended from the same common ancestor. The age of divergence between Pediculus and its common ancestor is estimated to be 6-7 million years ago, which matches the age predicted by chimpanzee-hominid divergence.[25] Because parasites rely on their hosts, host–parasite cospeciation events are likely.

Genetic evidence suggests that our human ancestors acquired pubic lice from gorillas approximately 3-4 million years ago.[25] Unlike the genus Pediculus, the divergence in Pthirus does not match the age of host divergence that likely occurred 7 million years ago. Reed et al. propose a Pthirus species host-switch around 3-4 million years ago. While it is difficult to determine if a parasite–host switch occurred in evolutionary history, this explanation is the most parsimonious (containing the fewest evolutionary changes).[25]

Additionally, the DNA differences between head lice and body lice provide corroborating evidence that humans used clothing between 80,000 and 170,000 years ago, before leaving Africa.[26] Human head and body lice occupy distinct ecological zones: head lice live and feed on the scalp, while body lice live on clothing and feed on the body. Because body lice require clothing to survive, the divergence of head and body lice from their common ancestor provides an estimate of the date of introduction of clothing in human evolutionary history.[26][27]

The mitochondrial genome of the human species of the body louse (Pediculus humanus humanus), the head louse (Pediculus humanus capitis) and the pubic louse (Pthirus pubis) fragmented into a number of minichromosomes, at least seven million years ago.[28] Analysis of mitochondrial DNA in human body and hair lice reveals that greater genetic diversity existed in African than in non-African lice.[27][29] Human lice can also shed light on human migratory patterns in prehistory. The dominating theory of anthropologists regarding human migration is the Out of Africa Hypothesis. Genetic diversity accumulates over time, and mutations occur at a relatively constant rate. Because there is more genetic diversity in African lice, the lice and their human hosts must have existed in Africa before anywhere else.[29]

The phylogeny of the Phthiraptera is shown, not yet fully resolved, in the cladogram:[30]


webspinners, cockroaches, stick insects and allies


(angel insects)






(scaly-winged barklice)




Phthiraptera (lice)


(booklice, barklice)


beetles, wasps, flies, butterflies, and allies

In human culture

In social history

Louse diagram, Micrographia, Robert Hooke, 1667
Drawing of a louse clinging to a human hair. Robert Hooke, Micrographia, 1667

Lice have been intimately associated with human society throughout history. In the Middle Ages, they were essentially ubiquitous. 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".[31] A mediaeval treatment for lice was an ointment made from pork grease, incense, lead, and aloe.[32]

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.[33]

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.[34][35]

In 1935 the Harvard medical researcher Hans Zinsser wrote the book Rats, Lice and History, alleging that both body and head lice transmit typhus between humans.[36] Despite this, the modern view is that only the body louse can transmit the disease.[37]

Jan Siberechts "Cour de ferme" détail Scène d'épouillage
Detail showing delousing from Jan Siberechts' painting Cour de ferme ("Farmyard"), 1662

Soldiers in the trenches of the First World War suffered severely from lice, and the typhus they carried. The Germans boasted that they had lice under effective control, but themselves suffered badly from lice in the Second World War on the Eastern Front, especially in the Battle of Stalingrad. "Delousing" became a grim euphemism for the extermination of Jews in concentration camps such as Auschwitz under the Nazi regime.[38]

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".[39]

In science

The human body louse Pediculus humanus humanus has (2010) the smallest insect genome known.[40] 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.[41]

In literature and folklore

Mother Louse, Alewife Wellcome L0000658
Mother Louse, a notorious Alewife in Oxford during the mid-18th century. Her crest includes three lice. Image by David Loggan.[42][43]

James Joyce's 1939 book Finnegans Wake has the character Shem the Penman infested with "foxtrotting fleas, the lieabed lice, ... bats in his belfry".[44]

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.[45]

The Irish songwriter John Lyons (b. 1934) wrote the popular[46] song The Kilkenny Louse House. The song contains the lines "Well we went up the stairs and we put out the light, Sure in less than five minutes, I had to show fight. For the fleas and the bugs they collected to march, And over me stomach they formed a great arch". It has been recorded by Christie Purcell (1952), Mary Delaney on From Puck to Appleby (2003), and the Dubliners on Double Dubliners (1972) among others.[46][47]

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."[48]

See also


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  3. ^ Gullan, P. J.; Cranston, P. S. (2014). The Insects: An Outline of Entomology. Wiley. pp. 41–42. ISBN 978-1-118-84615-5.
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  6. ^ University of Utah (2008). Ecology and Evolution of Transmission in Feather-feeding Lice (Phthiraptera: Ischnocera). ProQuest. pp. 83–87. ISBN 978-0-549-46429-7.
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  10. ^ Felső B; et al. (2007). "Diving behaviour reduces genera richness of lice (Insecta: Phthiraptera) of mammals" (PDF). Acta Parasitologica. 52: 82–85. doi:10.2478/s11686-007-0006-3.
  11. ^ Møller AP; et al. (2005). "Parasite biodiversity and host defenses: Chewing lice and immune response of their avian hosts" (PDF). Oecologia. 142 (2): 169–176. doi:10.1007/s00442-004-1735-8. PMID 15503162.
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  16. ^ Rózsa L; et al. (1996). "Relationship of host coloniality to the population ecology of avian lice (Insecta: Phthiraptera)" (PDF). Journal of Animal Ecology. 65 (2): 242–248. doi:10.2307/5727. JSTOR 5727.
  17. ^ Brown CR; et al. (1995). "Ectoparasites reduce long-term survivorship of their avian host" (PDF). Proceedings of the Royal Society of London B. 262 (1365): 313–319. doi:10.1098/rspb.1995.0211.
  18. ^ Barlett, C.M. (1993). "Lice (Amblycera and Ischnocera) as vectors of Eulimdana spp. (Nematoda: Filarioidea) in Charadriiform birds and the necessity of short reproductive periods in adult worms". Journal of Parasitology. 75 (1): 85–91. doi:10.2307/3283282. JSTOR 3283282.
  19. ^ Booth DT; et al. (1993). "Experimental demonstration of the energetic cost of parasitism in free-ranging hosts" (PDF). Proceedings of the Royal Society of London B. 253 (1337): 125–129. doi:10.1098/rspb.1993.0091.
  20. ^ Clayton (1990). "Mate choice in experimentally parasitized rock doves: lousy males lose" (PDF). American Zoologist. 30 (2): 251–262. doi:10.1093/icb/30.2.251.
  21. ^ Garamszegi LZ; et al. (2005). "Age-dependent health status and song characteristics". Behavioral Ecology. 16 (3): 580–591. doi:10.1093/beheco/ari029.
  22. ^ Ax, Peter (2013). Multicellular Animals: Volume II: The Phylogenetic System of the Metazoa. Springer Science & Business Media. pp. 303–307. ISBN 978-3-662-10396-8.
  23. ^ a b Smith, Vince. "Phthiraptera.info". International Society of Phthirapterists. Retrieved 25 October 2015.
  24. ^ Banks, Jonathan C.; Paterson, Adrian M. (2004). "A penguin-chewing louse (Insecta : Phthiraptera) phylogeny derived from morphology". Invertebrate Systematics. 18 (1): 89–100. doi:10.1071/IS03022.
  25. ^ a b c d Reed D.L.; Light, J.E.; Allen, J.M.; Kirchman, J.J. (2007). "Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice". BMC Biology. 5 (7): 7. doi:10.1186/1741-7007-5-7. PMC 1828715. PMID 17343749.
  26. ^ a b Parry, Wynne (7 November 2013). "Lice Reveal Clues to Human Evolution". LiveScience. Retrieved 25 October 2015.
  27. ^ a b Kittler, R.; Kayser, M.; Stoneking, M. (2003). "Molecular Evolution of Pediculus humanus and the Origin of Clothing". Current Biology. 13 (16): 1414–1417. doi:10.1016/S0960-9822(03)00507-4. PMID 12932325.
  28. ^ Shao, R.; Zhu, X.Q.; Barker, S.C.; Herd, K. (2012). "Evolution of extensively fragmented mitochondrial genomes in the lice of humans". Genome Biology and Evolution. 4 (11): 1088–1101. doi:10.1093/gbe/evs088. PMC 3514963. PMID 23042553.
  29. ^ a b Light, J.E.; Allen, J.M.; Long, L.M.; Carter, T.E.; Barrow, L.; Suren, G.; Raoult, D.; Reed, D.L (2008). "Geographic distribution and origins of human head lice (Pediculus humanus capitis) based on mitochondrial data". Journal of Parasitology. 94 (6): 1275–1281. doi:10.1645/GE-1618.1. PMID 18576877.
  30. ^ (Kluge 2005, Kluge 2010, Kluge 2012):
  31. ^ Kowalski, Todd J.; Agger, William A. (2009). "Art Supports New Plague Science". Clin. Infect. Dis. 48 (1): 137–138. doi:10.1086/595557. PMID 19067623.
  32. ^ Elliott, Lynne (2004). Clothing in the Middle Ages. Crabtree. p. 29. ISBN 978-0-7787-1351-7.
  33. ^ Hooke, Robert. "Microscopic view of a louse". The Royal Society. Retrieved 17 October 2015.
  34. ^ Sarasohn, Lisa T. (2010). The Natural Philosophy of Margaret Cavendish: Reason and Fancy During the Scientific Revolution. JHU Press. pp. 165–167. ISBN 978-0-8018-9443-5. The Bear-men were to be her Experimental Philosophers, the Bird-men her Astronomers, the Fly- Worm- and Fish-men her Natural Philosophers, the Ape-men her Chymists, the Satyrs her Galenick Physicians, the Fox-men her Politicians, the Spider- and Lice-men her Mathematicians, the Jackdaw- Magpie- and Parrot-men her Orators and Logicians, the Gyants her Architects, &c.
  35. ^ Cavendish, Margaret (1668). The Description of a New World, Called the Blazing-World. A. Maxwell.
  36. ^ Zinsser, Hans (2007) [1935]. Rats, Lice and History. Transaction Publishers. ISBN 978-1-4128-0672-5.
  37. ^ Altschuler, Deborah Z. (1990). "Zinsser, Lice and History". HeadLice.org. Retrieved 19 October 2015.
  38. ^ Evans, Richard J. "The Great Unwashed". Gresham College. Retrieved 17 October 2015.
  39. ^ Weinstein, Phillip. "Entomophobia/Delusionary Parasitosis; Illusionary Parasitosis". University of Sydney Department of Medical Entomology. Retrieved 17 October 2015.
  40. ^ Kirkness, Ewen F.; et al. (2010). "Genome sequences of the human body louse and its primary endosymbiont provide insights into the permanent parasitic lifestyle". Proceedings of the National Academy of Sciences of the United States of America. 107 (27): 12168–12173. doi:10.1073/pnas.1003379107. PMC 2901460. PMID 20566863.
  41. ^ Pittendrigh, Barry R.; Berenbaum, May R.; Seufferheld, Manfredo J.; Margam, Venu M.; Strycharz, Joseph P.; Yoon, Kyong S.; Sun, Weilin; Reenan, Robert; Lee, Si Hyeock; Clark, John M. (2011). "Simplify, simplify: Lifestyle and compact genome of the body louse provide a unique functional genomics opportunity". Communicative and Integrative Biology. 4 (2): 188–191. doi:10.4161/cib.4.2.14279. PMC 3104575. PMID 21655436.
  42. ^ William White (1859). Notes & Queries. Oxford University Press. pp. 275–276.
  43. ^ Pierce, Helen (2004). "Unseemly pictures: political graphic satire in England, c. 1600-c. 1650" (PDF). University of York.
  44. ^ Joyce, James (1939). Finnegans Wake. Faber. p. 180.
  45. ^ Trafzer, Clifford E. (2015). A Chemehuevi Song: The Resilience of a Southern Paiute Tribe. University of Washington Press. pp. 22–25. ISBN 978-0-295-80582-5.
  46. ^ a b Carroll, Jim. "Songs of Clare: The Kilkenny Louse House". Clare Library. Retrieved 19 October 2015.
  47. ^ Scott, Bruce (2013). "My Colleen by the Shore" (PDF). Veteran. Retrieved 25 October 2015.
  48. ^ Twinn, Cecil Raymond (1942). Insect Life in the Poetry and Drama of England: With Special Reference to Poetry. University of Ottawa (PhD Thesis). hdl:10393/21088.

External links


The family Argulidae contains the carp lice or fish lice – a group of parasitic crustaceans of uncertain position within the Maxillopoda. Although they are thought to be primitive forms, they have no fossil record. The Argulidae are the only family in the order Arguloida (occasionally "Arguloidea"), although a second family, the Dipteropeltidae, has been proposed.

Body louse

The body louse (Pediculus humanus humanus, sometimes called Pediculus humanus corporis) is a hematophagic louse that infests humans. The condition of being infested with head lice, body lice, or pubic lice is known as pediculosis. Body lice are vectors for the transmission of the human diseases epidemic typhus, trench fever, and relapsing fever. The body louse genome sequence analysis was published in 2010.

Bookworm (insect)

Bookworm is a general name for any insect that is said to bore through books.The damage to books that is commonly attributed to "bookworms" is, in truth, not caused by any species of worm. Often, the larvae of various types of insects including beetles, moths and cockroaches, which may bore or chew through books seeking food, are responsible. Some such larvae exhibit a superficial resemblance to worms and are the likely inspiration for the term, though they are not true worms. In other cases, termites, carpenter ants, and woodboring beetles will first infest wooden bookshelves and later feed on books placed upon the shelves, attracted by the wood-pulp paper used in most commercial book production.

True book-borers are uncommon. The primary food sources for many "bookworms" are the leather or cloth bindings of a book, the glue used in the binding process, or molds and fungi that grow on or inside books. When the pages themselves are attacked, a gradual encroachment across the surface of one page or a small number of pages is typical, rather than the boring of holes through the entire book (see images on right).The term has come to have a second, idiomatic use, indicative of a person who reads a great deal or to perceived excess: someone who devours books metaphorically.


Braulidae, or bee louse, is a family of fly (Diptera) with seven species in two genera, Braula and Megabraula. Found in honey bee colonies, these most unusual wingless and small flies, are not a true bee parasite, and are barely recognizable as Diptera, as they have the superficial appearance of mites or lice.

Crab louse

The crab louse or pubic louse (Pthirus pubis) is an insect that is an obligate ectoparasite of humans, feeding exclusively on blood. The crab louse usually is found in the person's pubic hair. Although the louse cannot jump, it can also live in other areas of the body that are covered with coarse hair, such as the eyelashes. It is of the order Psocodea.

Humans are the only known hosts of the crab louse, although a closely related species, Pthirus gorillae, infects gorilla populations. The human parasite diverged from Pthirus gorillae approximately 3.3 million years ago. It is more distantly related to the genus Pediculus, which contains the human head and body lice and a louse that affects chimpanzees and bonobos.


Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such familiar animals as crabs, lobsters, crayfish, shrimps, prawns, krill, woodlice, and barnacles.

The crustacean group is usually treated as a class under subphylum Mandibulata and because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, and comprises all animals in the Pancrustacea clade other than hexapods. Some crustaceans are more closely related to insects and other hexapods than they are to certain other crustaceans.

The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 3.8 m (12.5 ft) and a mass of 20 kg (44 lb). Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by their larval forms, such as the nauplius stage of branchiopods and copepods.

Most crustaceans are free-living aquatic animals, but some are terrestrial (e.g. woodlice), some are parasitic (e.g. Rhizocephala, fish lice, tongue worms) and some are sessile (e.g. barnacles). The group has an extensive fossil record, reaching back to the Cambrian, and includes living fossils such as Triops cancriformis, which has existed apparently unchanged since the Triassic period. More than 7.9 million tons of crustaceans per year are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Krill and copepods are not as widely fished, but may be the animals with the greatest biomass on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as carcinology (alternatively, malacostracology, crustaceology or crustalogy), and a scientist who works in carcinology is a carcinologist.

Cymothoa exigua

Cymothoa exigua, or the tongue-eating louse, is a parasitic isopod of the family Cymothoidae. This parasite enters fish through the gills. The female attaches to the tongue and the male attaches on the gill arches beneath and behind the female. Females are 8–29 millimetres (0.3–1.1 in) long and 4–14 mm (0.16–0.55 in) in maximum width. Males are approximately 7.5–15 mm (0.3–0.6 in) long and 3–7 mm (0.12–0.28 in) wide. The parasite severs the blood vessels in the fish's tongue, causing the tongue to fall off. It then attaches itself to the remaining stub of the tongue and becomes the fish's new tongue.

Epidemic typhus

Epidemic typhus is a form of typhus so named because the disease often causes epidemics following wars and natural disasters. The causative organism is Rickettsia prowazekii, transmitted by the human body louse (Pediculus humanus corporis).

Head lice infestation

Head lice infestation, also known as pediculosis capitis and nits, is the infection of the head hair and scalp by the head louse (Pediculus humanus capitis). Itching from lice bites is common. During a person's first infection, the itch may not develop for up to six weeks. If a person is infected again, symptoms may begin much more quickly. The itch may cause problems with sleeping. Generally, however, it is not a serious condition. While head lice appear to spread some other diseases in Africa, they do not appear to do so in Europe or North America.Head lice are spread by direct contact with the hair of someone who is infected. The cause of head lice infestations are not related to cleanliness. Other animals, such as cats and dogs, do not play a role in transmission. Head lice feed only on human blood and are only able to survive on human head hair. When adults, they are about 2 to 3 mm long. When not attached to a human, they are unable to live beyond three days. Humans can also become infected with two other lice – the body louse and the crab louse. To make the diagnosis, live lice must be found. Using a comb can help with detection. Empty eggshells (known as nits) are not sufficient for the diagnosis.Possible treatments include: combing the hair frequently with a fine tooth comb or shaving the head completely. A number of topical medications are also effective, including malathion, ivermectin, and dimethicone. Dimethicone, which is a silicone oil, is often preferred due to the low risk of side effects. Pyrethroids such as permethrin have been commonly used; however, they have become less effective due to increasing pesticide resistance. There is little evidence for alternative medicines.Head-lice infestations are common, especially in children. In Europe, they infect between 1 and 20% of different groups of people. In the United States, between 6 and 12 million children are infected a year. They occur more often in girls than boys. It has been suggested that historically, head lice infection were beneficial, as they protected against the more dangerous body louse. Infestations may cause stigmatization of the infected individual.

Head louse

The head louse (Pediculus humanus capitis) is an obligate ectoparasite of humans that causes head lice infestation (pediculosis capitis).Head lice are wingless insects spending their entire lives on the human scalp and feeding exclusively on human blood. Humans are the only known hosts of this specific parasite, while chimpanzees host a closely related species, Pediculus schaeffi. Other species of lice infest most orders of mammals and all orders of birds, as well as other parts of the human body.

Lice differ from other hematophagic ectoparasites such as fleas in spending their entire lifecycle on a host. Head lice cannot fly, and their short, stumpy legs render them incapable of jumping, or even walking efficiently on flat surfaces.The non-disease-carrying head louse differs from the related disease-carrying body louse (Pediculus humanus humanus) in preferring to attach eggs to scalp hair rather than to clothing. The two subspecies are morphologically almost identical, but do not normally interbreed, although they will do so in laboratory conditions. From genetic studies, they are thought to have diverged as subspecies about 30,000–110,000 years ago, when many humans began to wear a significant amount of clothing. A much more distantly related species of hair-clinging louse, the pubic or crab louse (Pthirus pubis), also infests humans. It is visually different from the other two species and is much closer in appearance to the lice which infest other primates. Lice infestation of any part of the body is known as pediculosis.Head lice (especially in children) have been, and still are, subject to various eradication campaigns. Unlike body lice, head lice are not the vectors of any known diseases. Except for rare secondary infections that result from scratching at bites, head lice are harmless, and they have been regarded by some as essentially a cosmetic rather than a medical problem. Head lice infestations might be beneficial in helping to foster a natural immune response against lice which helps humans in defense against the far more dangerous body louse, which is capable of transmission of dangerous diseases.


Hippoboscidae, the louse flies or keds, are obligate parasites of mammals and birds. In this family, the winged species can fly at least reasonably well, though others with vestigial or no wings are flightless and highly apomorphic. As usual in their superfamily Hippoboscoidea, most of the larval development takes place within the mother's body, and pupation occurs almost immediately.

The sheep ked, Melophagus ovinus, is a wingless, reddish-brown fly that parasitizes sheep. The Neotropical deer ked, Lipoptena mazamae, is a common ectoparasite of white-tailed deer (Odocoileus virginianus) in the southeastern United States. Both winged and wingless forms may be seen. A common winged species is Hippobosca equina, called "the louse fly" among riders. Species in other genera are found on birds; for example, Ornithomya bequaerti has been collected from birds in Alaska. Two species of the Hippoboscidae – Ornithoica (Ornithoica) podargi and Ornithomya fuscipennis are also common parasites of the tawny frogmouth (Podargus strigoides) of Australia.

Pseudolynchia canariensis is commonly found on pigeons and doves, and can serve as the vector of "pigeon malaria" (Haemoproteus columbae). Louse flies of birds may transmit other parasites such as those in the genus Plasmodium or other Haemoproteus parasites. Some evidence indicates that other Hippoboscidae can serve as vectors of disease agents to mammals. For example, a louse fly of the species Icosta americana was found with West Nile Virus infection from an American Kestrel


The Mallophaga are a suborder of lice, known as chewing lice, biting lice or bird lice, containing more than 3000 species. These lice are external parasites that feed mainly on birds, although some species also feed on mammals. They infest both domestic and wild animals and birds, and cause considerable irritation to their hosts. They have paurometabolis or incomplete metamorphosis.


Psocoptera are an order of insects that are commonly known as booklice, barklice or barkflies. They first appeared in the Permian period, 295–248 million years ago. They are often regarded as the most primitive of the hemipteroids. Their name originates from the Greek word ψῶχος, psokhos meaning gnawed or rubbed and πτερά, ptera meaning wings. There are more than 5,500 species in 41 families in three suborders. Many of these species have only been described in recent years.They range in size from 1–10 millimeters (0.04–0.4 in) in length.

The species known as booklice received their common name because they are commonly found amongst old books—they feed upon the paste used in binding. The barklice are found on trees, harmlessly feeding on algae and lichen. No member of this order is currently considered endangered; in fact, in 2007, Atlantopsocus adustus, a species native to Madeira and the Canary Islands, was found to have colonized the mild Cornish coast of southwest England.In the 2000s, morphological and molecular evidence has shown that the parasitic lice (Phthiraptera) evolved from within the psocopteran suborder Troctomorpha. In modern systematics, Psocoptera and Phthiraptera are therefore treated together in the order Psocodea.


Psyllidae, the jumping plant lice or psyllids, are a family of small plant-feeding insects that tend to be very host-specific, i.e. each plant-louse species only feeds on one plant species (monophagous) or feeds on a few closely related plants (oligophagous). Together with aphids, phylloxerans, scale insects and whiteflies, they form the group called Sternorrhyncha, which is considered to be the most "primitive" group within the true bugs (Hemiptera). They have traditionally been considered a single family, Psyllidae, but recent classifications divide the group into a total of seven families; the present restricted definition still includes more than 70 genera in the Psyllidae.

Psyllid fossils have been found from the Early Permian before the flowering plants evolved. The explosive diversification of the flowering plants in the Cretaceous was paralleled by a massive diversification of associated insects, and many of the morphological and metabolic characters that the flowering plants exhibit may have evolved as defenses against herbivorous insects.

Several genera of psyllids, especially among the Australian fauna, secrete coverings called "lerps" over their bodies, presumably to conceal them from predators and parasites.

Relapsing fever

Relapsing fever is a vector-borne disease caused by infection with certain bacteria in the genus Borrelia, which is transmitted through the bites of lice or soft-bodied ticks (genus Ornithodoros).

Salmon louse

The salmon louse (Lepeophtheirus salmonis) is a species of copepod in the genus Lepeophtheirus. It is a sea louse, a parasite living mostly on salmon, particularly on Pacific and Atlantic salmon and sea trout, but is also sometimes found on the three-spined stickleback. It lives off the mucus, skin and blood of the fish. They are natural marine parasites of fish such as adult salmon. Once detached, they can be blown by wind across the surface of the sea, like plankton. When they encounter a suitable marine fish host, they may adhere themselves to the skin, fins, or gills of the fish, and feed off the mucus or skin. Sea lice only affect fish and are not harmful to humans.Salmon lice are natural ectoparasites of salmon, in the 1980s, high levels of salmon lice were observed on pink salmon smolts. Salmon lice are found in the Pacific and Atlantic Oceans; they infect pink salmon, Atlantic salmon, and chum salmon.

Sucking louse

Sucking lice (Anoplura, formerly known as Siphunculata) have around 500 species and represent the smaller of the two traditional suborders of lice. As opposed to the paraphyletic chewing lice, which are now divided among three suborders, the sucking lice are monophyletic.

The Anoplura are all blood-feeding ectoparasites of mammals. They only occur on about 20% of all placentalian mammal species, and are unknown from several orders of mammals (Monotremata, Edentata, Pholidota, Chiroptera, Cetacea, Sirenia and Proboscidea). They can cause localized skin irritations and are vectors of several blood-borne diseases. Children appear particularly susceptible to attracting lice, possibly due to their fine hair.

At least three species or subspecies of Anoplura are parasites of humans; the human condition of being infested with sucking lice is called pediculosis. Pediculus humanus is divided into two subspecies, Pediculus humanus humanus, or the body louse, sometimes nicknamed "the seam squirrel" for its habit of laying of eggs in the seams of clothing, and Pediculus humanus capitis, or the head louse. Pthirus pubis (the crab louse) is the cause of the condition known as crabs.


Typhus, also known as typhus fever, is a group of infectious diseases that include epidemic typhus, scrub typhus, and murine typhus. Common symptoms include fever, headache, and a rash. Typically these begin one to two weeks after exposure.The diseases are caused by specific types of bacterial infection. Epidemic typhus is due to Rickettsia prowazekii spread by body lice, scrub typhus is due to Orientia tsutsugamushi spread by chiggers, and murine typhus is due to Rickettsia typhi spread by fleas.Currently no vaccine is commercially available. Prevention is by reducing exposure to the organisms that spread the disease. Treatment is with the antibiotic doxycycline. Epidemic typhus generally occurs in outbreaks when poor sanitary conditions and crowding are present. While once common, it is now rare. Scrub typhus occurs in Southeast Asia, Japan, and northern Australia. Murine typhus occurs in tropical and subtropical areas of the world.Typhus has been described since at least 1528 AD. The name comes from the Greek tûphos (τύφος) meaning hazy, describing the state of mind of those infected. While "typhoid" means "typhus-like", typhus and typhoid fever are distinct diseases caused by different types of bacteria.


A woodlouse (plural woodlice) is a crustacean from the monophyletic suborder Oniscidea within the isopods. The first woodlice were marine isopods which are presumed to have colonised land in the Carboniferous. They have many common names and although often referred to as 'terrestrial Isopods' some species live semiterrestrially or have recolonised aquatic environments. Woodlice in the families Armadillidae, Armadillidiidae, Eubelidae, Tylidae and some other genera can roll up into an almost perfect sphere as a defensive mechanism; others have partial rolling ability but most cannot conglobate at all.

Woodlice have a basic morphology of a segmented, dorso-ventrally flattened body with 7 pairs of jointed legs, specialised appendages for respiration and like other peracarids, females carry fertilised eggs in their marsupium, through which they provide developing embryos with water, oxygen and nutrients. The immature young hatch as mancae and receive further maternal care in some species. Juveniles then go through a series of moults before reaching maturity.

While the broader phylogeny of the Oniscideans has not been settled, five Infraorders/Sections are agreed on with 3637 species validated in scientific literature in 2004 and 3710 species in 2014 out of an estimated total of 5000–7000 species extant worldwide. Key adaptations to terrestrial life have led to a highly diverse set of animals; from the marine littoral zone and subterranean lakes to arid deserts and mountain slopes 4,725m above sea-level, woodlice have established themselves in most terrestrial biomes and represent the full range of transitional forms and behaviours for living on land.

Woodlice are widely studied in the contexts of evolutionary biology, behavioural ecology and nutrient cycling. They are popular as terrarium pets because of their varied colour and texture forms, conglobating ability and ease of care.

Human lice and pediculosis
Other terms of interest
Extant Psocodea families
of insects
in culture


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