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{{Short description|Infraorder of insects}}
{{Redirect|Lice}}
{{pp-vandalism|small=yes}}
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{{Use British English|date=November 2015}}
{{Automatic taxobox
| name = Phthiraptera
| fossil_range = {{fossilrange|Cenomanian|present|earliest=Cretaceous}}
| image = Fahrenholzia pinnata.JPG
| image_caption = [[Light micrograph]] of ''[[Fahrenholzia pinnata]]''
| taxon = Phthiraptera
| authority = [[Haeckel]], 1896
| display_parents = 2
| subdivision_ranks = Parvorders
| subdivision =
*[[Amblycera]]
*[[Anoplura]]
*[[Rhynchophthirina]]
*[[Ischnocera]]
*[[Trichodectera]]?
| subdivision_ref = <ref name="deMoya2021"/>
}}

'''Louse''' ({{plural form}}: '''lice''') is the common name for any member of the [[infraorder]] '''Phthiraptera''', which contains nearly 5,000 species of wingless [[parasitic]] [[insect]]s. Phthiraptera was previously recognized as an [[order (biology)|order]], until a 2021 genetic study determined that they are a highly modified lineage of the order [[Psocodea]], whose members are commonly known as booklice, barklice or barkflies.

Lice are [[obligate parasite]]s, living externally on warm-blooded [[Host (biology)|hosts]], which include every species of [[bird]] and [[mammal]], except for [[monotreme]]s, [[pangolin]]s, and [[bat]]s. Chewing lice live among the hairs or feathers of their host and feed on skin and debris, whereas 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 [[Head louse#Eggs/Nits|nits]], to hairs or feathers. The eggs hatch into [[Nymph (biology)|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]]''; and the [[pubic louse]], ''[[Pthirus]] pubis''. Lice are [[Vector (epidemiology)|vectors]] of diseases such as [[typhus]]. 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]]''.

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.  They commonly feature in the psychiatric disorder [[delusional parasitosis]]. A louse was one of the early subjects of [[Microscope|microscopy]], appearing in [[Robert Hooke]]'s 1667 book, ''[[Micrographia]]''. 

The oldest known fossil lice are from the [[Cretaceous]].<ref name=Archimenopon>{{Cite journal|last1=Zhang |first1=Y. |last2=Rasnitsyn |first2=A. P. |last3=Zhang |first3=W. |last4=Song |first4=F. |last5=Shih |first5=C. |last6=Ren |first6=D. |last7=Wang |first7=Y. |last8=Li |first8=H. |last9=Gao |first9=T. |year=2024 |title=Stem chewing lice on Cretaceous feathers preserved in amber |journal=Current Biology |volume=34 |issue=4 |pages=916–922.e1 |doi=10.1016/j.cub.2024.01.027 |pmid=38320551 |bibcode=2024CBio...34E.916Z |doi-access=free }}</ref>

==Classification==
Lice are classified as an [[infraorder]] called Phthiraptera, named by [[Ernst Haeckel]] in 1896. Phthiraptera was previously recognized as an [[order (biology)|order]], and was considered the [[sister taxon]] to the order [[Psocoptera]], which contains booklice, barklice and barkflies, all within the greater group called [[Psocodea]]. However, a 2021 genetic study determined that Phthiraptera lice are a highly modified lineage within the Psocoptera group, thus rendering Psocoptera [[paraphyletic]] and necessating a classification change. Phthiraptera is now nested within the [[suborder]] [[Troctomorpha]], belonging to the now-ranked order [[Psocodea]].<ref name="deMoya2021">{{Cite journal|last1=de Moya|first1=Robert S|last2=Yoshizawa|first2=Kazunori|last3=Walden|first3=Kimberly K O|last4=Sweet|first4=Andrew D|last5=Dietrich|first5=Christopher H|last6=Kevin P|first6=Johnson|date=2021-06-16|editor-last=Buckley|editor-first=Thomas|title=Phylogenomics of Parasitic and Nonparasitic Lice (Insecta: Psocodea): Combining Sequence Data and Exploring Compositional Bias Solutions in Next Generation Data Sets|url=https://academic.oup.com/sysbio/article/70/4/719/5912026|journal=Systematic Biology|language=en|volume=70|issue=4|pages=719–738|doi=10.1093/sysbio/syaa075|pmid=32979270|issn=1063-5157|doi-access=free}}</ref><ref name="PsocodeaSpeciesFile">{{Cite web| website = Psocodea Species File |title = Anoplura Leach, 1815 |last1 = Hopkins |first1 = H. |last2 = Johnson |first2 = K. P. |last3 = Smith |first3 = V. S. |url = https://psocodea.speciesfile.org/otus/878997/overview |access-date = 12 May 2025 }}</ref> Within Psocodea, the Phthiraptera lice are most closely related to [[family (biology)|family]] [[Liposcelididae]].<ref>{{cite journal | vauthors = Light JE, Smith VS, Allen JM, Durden LA, Reed DL | title = Evolutionary history of mammalian sucking lice (Phthiraptera: Anoplura) | journal = BMC Evolutionary Biology | volume = 10 | issue = 1 | pages = 292 | date = September 2010 | pmid = 20860811 | pmc = 2949877 | doi = 10.1186/1471-2148-10-292 | doi-access = free | bibcode = 2010BMCEE..10..292L }}</ref>

A [[cladogram]] showing the position of Phthiraptera within [[Psocodea]] is shown below:<ref name="deMoya2021"/>
{{clade| style=font-size:85%; line-height:100%
 |label1='''[[Psocodea]]'''
 |1={{clade
  |1={{clade
   |label1=[[Troctomorpha]]
   |1={{clade
    |1={{clade
     |1={{clade
      |label1='''Phthiraptera'''
      |1={{clade
       |1={{clade
        |1=[[Philopteridae]]
        |2={{clade
         |1={{clade
          |1=[[Anoplura]]
          |2=[[Rhynchophthirina]] }}
         |2=[[Trichodectidae]] }} }}
       |2=[[Amblycera]] }}
      |2=[[Liposcelididae]] }}
     |2={{clade
      |1=[[Pachytroctidae]]
      |2=[[Sphaeropsocidae]] }} }}
    |2=[[Amphientometae]] }}
   |label2=[[Psocomorpha]]
   |2={{clade
    |1={{clade
     |1={{clade
      |1=[[Homilopsocidea]]
      |2=[[Caeciliusetae]] }}
     |2={{clade
      |1={{clade
       |1=[[Psocetae]]
       |2=[[Epipsocetae]] }}
      |2=[[Philotarsetae]] }} }}
    |2=[[Archipsocetae]] }} }}
  |label2=[[Trogiomorpha]]
  |2={{clade
   |1={{clade
    |1=[[Atropetae]]
    |2=[[Psyllipsocetae]] }}
   |2=[[Prionoglaridetae]] ([[paraphyletic]]) }} }} }}

===Internal classification===
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 [[Metathorax|metathoracic]] [[ganglia]] with their [[abdominal]] ganglia to form a single ventral nerve junction.<ref name=Ax>{{cite book| vauthors = Ax P |title=Multicellular Animals: Volume II: The Phylogenetic System of the Metazoa |url=https://books.google.com/books?id=x7vtCAAAQBAJ&pg=PA303 |year=2013|publisher=Springer Science & Business Media |isbn=978-3-662-10396-8 |pages=303–307 }}</ref>

The order has traditionally been divided into two suborders, the sucking lice ([[Anoplura]]) and the chewing lice ([[Mallophaga]]). However, subsequent classifications indicated that the Mallophaga are [[paraphyletic]], and four suborders were then recognized.<ref name=info>{{cite web |url=http://phthiraptera.info/ |title=Phthiraptera.info | vauthors = Smith V |publisher=International Society of Phthirapterists |access-date=25 October 2015}}</ref>

Upon finding that Phthiraptera was nested within [[Psocoptera]] in 2021, de Moya ''et al.'' reduced the rank of Phthiraptera to [[infraorder]], and the [[suborder]]s were reduced down to [[parvorder]].<ref name="deMoya2021"/><ref name="PsocodeaSpeciesFile"/> Also, based on the [[paraphyletic]] nature of one of the parvorders -  [[Ischnocera]] - they proposed creation of a fifth parvorder [[Trichodectera]] to be created and split off from Ischnocera. Here are the five proposed parvorders within Phthiraptera:<ref name="deMoya2021"/>
* [[Amblycera]]: primitive chewing lice, widespread on birds, and also occurring on South American and Australian mammals
* [[Anoplura]]: sucking lice, occurring on mammals exclusively
* [[Rhynchophthirina]]: parasites of elephants and [[warthog]]s
* [[Ischnocera]]: avian chewing lice, and contains the large family [[Philopteridae]]
* [[Trichodectera]]: parasitizing mammals, split off from Ischnocera, and contains the family [[Trichodectidae]]

===Evolutionary history===
The oldest confirmed fossil louse is ''[[Archimenopon myanmarensis]]'', an [[amblycera]]n from the Cretaceous amber from [[Myanmar]].<ref name=Archimenopon /> 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.<ref name="Scratching an ancient itch: an Eoce">{{cite journal | vauthors = Wappler T, Smith VS, Dalgleish RC | title = Scratching an ancient itch: an Eocene bird louse fossil | journal = Proceedings. Biological Sciences | volume = 271 Suppl 5 | issue = suppl_5 | pages = S255-8 | date = August 2004 | pmid = 15503987 | pmc = 1810061 | doi = 10.1098/rsbl.2003.0158 }}</ref> ''[[Saurodectes vrsanskyi]]'' from the Early Cretaceous ([[Aptian]]) [[Zaza Formation]] of [[Buryatia]], Russia, has also been suggested to be a louse, but this is tentative.<ref>{{cite journal | vauthors = Rasnitsyn AP, Zherikhin VV | title = First fossil chewing louse from the lower Cretaceous of Baissa, Transbaikalia (Insecta, Pediculida= Phthiriaptera, Saurodectidae fam. n.). | journal = Russian Entomological Journal | date = 1999 | volume = 8 | issue = 4 | pages = 253–5 }}</ref>

Placental mammal lice had a single common ancestor that lived on [[Afrotheria]] with this arising from host-switching from an ancient avian host.<ref name="p273">{{cite journal |last1=Johnson |first1=Kevin P. |last2=Matthee |first2=Conrad |last3=Doña |first3=Jorge |date=2022 |title=Phylogenomics reveals the origin of mammal lice out of Afrotheria |journal=Nature Ecology & Evolution |volume=6 |issue=8 |pages=1205–1210 |doi=10.1038/s41559-022-01803-1 |pmid=35788706 |bibcode=2022NatEE...6.1205J |issn=2397-334X|hdl=10481/100956 |hdl-access=free }}</ref>

==Description==
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 occupy.<ref name=info/> They are found even in the [[Antarctic]], where [[penguin]]s carry 15 species of lice (in the genera ''[[Austrogonoides]]'' and ''[[Nesiotinus]]'').<ref>{{cite journal | vauthors = Banks JC, Paterson AM |title=A penguin-chewing louse (Insecta : Phthiraptera) phylogeny derived from morphology |journal=Invertebrate Systematics |date=2004 |volume=18 |issue=1 |pages=89–100 |doi=10.1071/IS03022 |s2cid=53516244 }}</ref>

<gallery mode="nolines">
File:Ricinus bombycillae (Denny, 1842).JPG|''Ricinus bombycillae'', an amblyceran louse from a [[Bohemian waxwing]]
File:Trinoton anserinum (Fabricius, 1805).JPG|''Trinoton anserinum'', an amblyceran louse from a [[mute swan]]
File:Lice image01.jpg|''[[Bovicola]] limbata'', an ischnoceran louse from [[goat]]s. The species is [[sexually dimorphic]], with the male smaller than the female.
</gallery>

Lice are divided into two groups: sucking lice, which obtain their nourishment from feeding on the [[Sebaceous gland|sebaceous secretions]] and body fluids of their host; and chewing lice, which are [[scavenger]]s, 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.<ref name=IIBD>{{cite book | vauthors = Hoell HV, Doyen JT, Purcell AH |year=1998 |title=Introduction to Insect Biology and Diversity |edition=2nd |publisher= [[Oxford University Press]] |pages= 407–409|isbn= 978-0-19-510033-4}}</ref> If their host dies, lice can opportunistically use [[phoresis]] to hitch a ride on a fly and attempt to find a new host.<ref>{{cite thesis | vauthors = Harbison CW | date = 2008 |title=Ecology and Evolution of Transmission in Feather-feeding Lice (Phthiraptera: Ischnocera) | degree = Doctoral  | work = Department of Biology | publisher = University of Utah |url=https://books.google.com/books?id=RUfFjqPoQTEC&pg=PA83 |isbn=978-0-549-46429-7 |pages=83–87}}</ref>

Sucking lice range in length from {{convert|0.5|to|5|mm|in|frac=64|abbr=on}}. They have narrow heads and oval, flattened bodies. They have no [[ocelli]], and their compound eyes are reduced in size or absent. Their [[Antenna (biology)|antennae]] are short with three to five segments, and their mouthparts, which are retractable into their head, are adapted for piercing and sucking.<ref name=Capinera/> 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 ([[ventral]]ly) and a food canal ([[dorsally]]).<ref name=GullanCranston>{{cite book| vauthors = Gullan PJ, Cranston PS |title=The Insects: An Outline of Entomology |date=2014 |publisher=Wiley |isbn=978-1-118-84615-5 |pages=41–42}}</ref> 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.<ref name=Capinera>{{cite book| vauthors = Capinera JL |title=Encyclopedia of Entomology |url=https://books.google.com/books?id=i9ITMiiohVQC&pg=PA838 |year=2008 |publisher=Springer Science & Business Media |isbn=978-1-4020-6242-1 |pages=838–844}}</ref>

Chewing lice are also flattened and can be slightly larger than sucking lice, ranging in length from {{convert|0.5|to|6|mm|in|frac=64|abbr=on}}. 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 [[bacteriocyte]]s in their bodies. These may assist in digestion because if the insect is deprived of them, it will die.
Lice are usually [[Crypsis|cryptically coloured]] to match the fur or feathers of the host.<ref name=Capinera/><ref>{{cite web |url=http://phthiraptera.info/classification/23 |title=Phthiraptera: Summary |vauthors=Smith V |publisher=Phthiraptera.info |access-date=25 October 2015 }}{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> A louse's colour varies from pale beige to dark grey; 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 [[Personal grooming|preening]], such as the interior of feather shafts. Living louse eggs tend to be pale whitish, whereas dead louse eggs are yellower.<ref name=IIBD/> Lice are [[exopterygote]]s, being born as miniature versions of the adult, known as [[nymph (biology)|nymphs]]. The young moult three times before reaching the final adult form, usually within a month after hatching.<ref name=IIBD/>

Humans host three different kinds of lice: [[head lice]], [[body lice]], and [[Crab louse|pubic lice]]. Head lice and body lice are subspecies of ''Pediculus humanus'', and pubic lice are a separate species, ''Pthirus pubis''. Lice infestations can be controlled with [[Nit comb|lice combs]], and medicated shampoos or washes.<ref>{{cite journal | vauthors = Mumcuoglu KY | title = Prevention and treatment of head lice in children | journal = Paediatric Drugs | volume = 1 | issue = 3 | pages = 211–8 | year = 1999 | pmid = 10937452 | doi = 10.2165/00128072-199901030-00005 | s2cid = 13547569 }}</ref>

==Ecology==
The average number of lice per host tends to be higher in large-bodied bird species than in small ones.<ref>{{cite journal | vauthors = Rózsa  | year = 1997| title = Patterns in the abundance of avian lice (Phthiraptera: Amblycera, Ischnocera) | url = http://www.zoologia.hu/list/patterns%20in%20the%20abundance.pdf | journal = Journal of Avian Biology | volume = 28 | issue = 3| pages = 249–254 | doi=10.2307/3676976| jstor = 3676976}}</ref> 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.<ref>{{cite journal | vauthors = Rékási J, Rozsa L, Kiss BJ | year = 1997 | title = Patterns in the distribution of avian lice (Phthiraptera: Amblycera, Ischnocera) | url = http://www.zoologia.hu/list/patterns%20in%20the%20distribution.pdf | journal = Journal of Avian Biology | volume = 28 | issue = 2| pages = 150–156 | doi=10.2307/3677308 | jstor = 3677308 | citeseerx = 10.1.1.506.730 }}</ref> Host organisms that dive under water to feed on aquatic prey harbour fewer taxa of lice.<ref>{{cite journal | vauthors = Felsõ B, Rózsa L | title = Reduced taxonomic richness of lice (Insecta: Phthiraptera) in diving birds | journal = The Journal of Parasitology | volume = 92 | issue = 4 | pages = 867–9 | date = August 2006 | pmid = 16995408 | doi = 10.1645/ge-849.1 | s2cid = 31000581 }}</ref><ref>{{cite journal | vauthors = Felső B, Rózsa L | year = 2007 | title = Diving behaviour reduces genera richness of lice (Insecta: Phthiraptera) of mammals | url = http://www.zoologia.hu/list/felso_rozsa2.pdf | journal = Acta Parasitologica | volume = 52 | pages = 82–85 | doi=10.2478/s11686-007-0006-3| s2cid = 38683871 }}</ref> Bird taxa that are capable of exerting stronger antiparasitic defence—such as stronger [[T cell]] immune response or larger [[uropygial gland]]s—harbour more taxa of Amblyceran lice than others.<ref>{{cite journal | vauthors = Møller AP, Rózsa L | title = Parasite biodiversity and host defenses: chewing lice and immune response of their avian hosts | journal = Oecologia | volume = 142 | issue = 2 | pages = 169–76 | date = January 2005 | pmid = 15503162 | doi = 10.1007/s00442-004-1735-8 | s2cid = 12992951 | bibcode = 2005Oecol.142..169M }}</ref><ref>{{cite journal | vauthors = Møller AP, Erritzøe J, Rózsa L | title = Ectoparasites, uropygial glands and hatching success in birds | journal = Oecologia | volume = 163 | issue = 2 | pages = 303–11 | date = June 2010 | pmid = 20043177 | doi = 10.1007/s00442-009-1548-x | s2cid = 11433594 | bibcode = 2010Oecol.163..303M }}</ref> Reductions in the size of host populations may cause a long-lasting reduction of louse taxonomic richness,<ref name=Rozsa1993>{{cite journal | vauthors = Ròzsa L | title = Speciation patterns of ectoparasites and "straggling" lice | journal = International Journal for Parasitology | volume = 23 | issue = 7 | pages = 859–64 | date = November 1993 | pmid = 8314369 | doi = 10.1016/0020-7519(93)90050-9 }}</ref> for example, birds introduced into [[New Zealand]] host fewer species of lice there than in Europe.<ref>{{cite journal | vauthors = Paterson AM, Palma RL, Gray RD | year = 1999 | title = How Frequently Do Avian Lice Miss the Boat? Implications for Coevolutionary Studies | journal = Systematic Biology | volume = 48 | pages = 214–223 | doi=10.1080/106351599260544 | doi-access = free }}</ref><ref>{{cite journal | vauthors = MacLeod CJ, Paterson AM, Tompkins DM, Duncan RP | title = Parasites lost - do invaders miss the boat or drown on arrival? | journal = Ecology Letters | volume = 13 | issue = 4 | pages = 516–27 | date = April 2010 | pmid = 20455925 | doi = 10.1111/j.1461-0248.2010.01446.x | bibcode = 2010EcolL..13..516M }}</ref> 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.<ref>{{cite journal | vauthors = Rózsa L, Rékási J, Reiczigel J | year = 1996 | title = Relationship of host coloniality to the population ecology of avian lice (Insecta: Phthiraptera) | url = http://www.zoologia.hu/list/relationship%20of%20host%20coloniality.pdf | journal = Journal of Animal Ecology | volume = 65 | issue = 2| pages = 242–248 | doi=10.2307/5727 | jstor = 5727 | bibcode = 1996JAnEc..65..242R }}</ref> 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.<ref name=Rozsa1993/>

Lice may reduce host life expectancy if the infestation is heavy,<ref>{{cite journal | vauthors = Brown CR, Brown MB, Rannala B | year = 1995 | title = Ectoparasites reduce long-term survivorship of their avian host | url = http://www.rannala.org/reprints/1995/Brown1995a.pdf | journal = Proceedings of the Royal Society of London B | volume = 262 | issue = 1365| pages = 313–319 | doi=10.1098/rspb.1995.0211| s2cid = 13869042 }}</ref> but most seem to have little effect on their host. The habit of dust bathing in [[Chicken|domestic hens]] is probably an attempt by the birds to rid themselves of lice.<ref name=Capinera/> Lice may transmit microbial diseases and [[helminth]] parasites,<ref>{{cite journal | vauthors = Barlett CM |year=1993 |title=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=Journal of Parasitology |volume=75 |issue=1 |pages=85–91  |jstor=3283282 |doi=10.2307/3283282|doi-access=free }}</ref> but most individuals spend their whole life cycle on a single host and are only able to transfer to a new host opportunistically.<ref name=Capinera/> Ischnoceran lice may reduce the [[thermoregulation]] effect of the plumage; thus heavily infested birds lose more heat than others.<ref>{{cite journal | vauthors = Booth DT, Clayton DH, Block BA | year = 1993 | title = Experimental demonstration of the energetic cost of parasitism in free-ranging hosts | url = http://darwin.biology.utah.edu/PubsHTML/PDF-Files/18.pdf | journal = Proceedings of the Royal Society of London B | volume = 253 | issue = 1337 | pages = 125–129 | doi = 10.1098/rspb.1993.0091 | bibcode = 1993RSPSB.253..125B | s2cid = 85731062 | access-date = 2010-08-14 | archive-url = https://web.archive.org/web/20100610162227/http://darwin.biology.utah.edu/PubsHTML/PDF-Files/18.pdf | archive-date = 2010-06-10 | url-status = dead }}</ref> Lice infestation is a disadvantage in the context of sexual rivalry.<ref>{{cite journal | vauthors = Clayton | year = 1990 | title = Mate choice in experimentally parasitized rock doves: lousy males lose | url = http://darwin.biology.utah.edu/PubsHTML/PDF-Files/11.pdf | journal = American Zoologist | volume = 30 | issue = 2 | pages = 251–262 | doi = 10.1093/icb/30.2.251 | access-date = 2010-08-14 | archive-url = https://web.archive.org/web/20100610164747/http://darwin.biology.utah.edu/PubsHTML/PDF-Files/11.pdf | archive-date = 2010-06-10 | url-status = dead | doi-access = free }}</ref><ref>{{cite journal | vauthors = Garamszegi LZ, Heylen D, Møller AP, Eens M, De Lope F | year = 2005 | title = Age-dependent health status and song characteristics | journal = Behavioral Ecology | volume = 16 | issue = 3| pages = 580–591 | doi=10.1093/beheco/ari029 | doi-access = free }}</ref>

==In human culture==
{{About||information about human infestation|Pediculosis|information on treatment|Treatment of human head lice}}

===In social history===

[[File:Louse diagram, Micrographia, Robert Hooke, 1667.jpg|thumb|upright=0.7|left|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".<ref name=Kowalski>{{cite journal | vauthors = Kowalski TJ, Agger WA | title = Art supports new plague science | journal = Clinical Infectious Diseases | volume = 48 | issue = 1 | pages = 137–8 | date = January 2009 | pmid = 19067623 | doi = 10.1086/595557 | doi-access = free }}</ref> The clergy often saw lice and other parasites as a constant reminder of human frailty and weakness. Monks and nuns would purposely ignore grooming themselves and suffer from infestations to express their religious devotion.<ref>{{Cite web |last=Harvey |first=Katherine |date=2019-04-09 |title=Medieval people were surprisingly clean (apart from the clergy) |url=https://aeon.co/essays/medieval-people-were-surprisingly-clean-apart-from-the-clergy |access-date=2022-10-13 |website=[[Aeon magazine|Aeon]] |language=en}}</ref> A mediaeval treatment for lice was an [[ointment]] made from pork grease, [[incense]], [[lead]], and [[aloe]].<ref>{{cite book | vauthors = Elliott L |title=Clothing in the Middle Ages |url=https://books.google.com/books?id=qtq_WSpdo0gC&pg=PT29 |year=2004 |publisher=Crabtree |isbn=978-0-7787-1351-7 |page=29}}</ref>

[[Robert Hooke]]'s 1667 book, ''[[Micrographia|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]].<ref>{{cite web | vauthors = Hooke R |title=Microscopic view of a louse |url=http://prints.royalsociety.org/art/580831/microscopic-view-of-a-louse |publisher=The Royal Society |access-date=17 October 2015}}</ref>

[[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]].<ref name="Sarasohn2010">{{cite book | vauthors = Sarasohn LT |title=The Natural Philosophy of Margaret Cavendish: Reason and Fancy During the Scientific Revolution |url=https://books.google.com/books?id=sE0XnNgF9ZoC&pg=PA167 |year=2010 |publisher=JHU Press |isbn=978-0-8018-9443-5|pages=165–167 |quote=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.}}</ref><ref>{{cite book| vauthors = Cavendish M |title=The Description of a New World, Called the Blazing-World|date=1668|publisher=A. Maxwell|url=http://digital.library.upenn.edu/women/newcastle/blazing/blazing.html}}</ref>

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.<ref>{{cite book | vauthors = Zinsser H |title=Rats, Lice and History |date=2007 |orig-year=1935 |publisher=Transaction Publishers |isbn=978-1-4128-0672-5}}</ref> Despite this, the modern view is that only the body louse can transmit the disease.<ref>{{cite web | vauthors = Altschuler DZ |title=Zinsser, Lice and History |url=http://www.headlice.org/faq/disease/zinsser.htm |publisher=HeadLice.org |access-date=19 October 2015 |date=1990}}</ref>

[[File:Jan Siberechts "Cour de ferme" détail Scène d'épouillage.jpg|thumb|upright|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 [[euphemism]] for the extermination of Jews in [[concentration camp]]s such as [[Auschwitz]] under the Nazi regime.<ref>{{cite web | vauthors = Evans RJ |author-link1=Richard J. Evans |title=The Great Unwashed |url=http://www.gresham.ac.uk/lectures-and-events/the-great-unwashed |publisher=Gresham College |access-date=17 October 2015}}</ref>

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".<ref>{{cite web | vauthors = Weinstein P | date = 26 February 2013 | title = 'The Great Unwashed': Entomophobia/Delusionary Parasitosis; Illusionary Parasitosis | url = http://medent.usyd.edu.au/fact/delpara.htm | publisher = University of Sydney Department of Medical Entomology | access-date = 17 October 2015 | series = The Great Plagues: Epidemics in History from the Middle Ages to the Present Day | archive-date = 17 May 2016 | archive-url = https://web.archive.org/web/20160517005233/http://medent.usyd.edu.au/fact/delpara.htm | url-status = dead }}</ref>

===In literature and folklore===
[[File:Mother Louse, Alewife Wellcome L0000658.jpg|thumb|left|upright|Mother Louse, a notorious [[Alewife (trade)|alewife]] in Oxford during the mid-18th century, shown with three lice as a [[coat of arms]]. Image by [[David Loggan]].<ref name="White1859">{{cite book|url=https://books.google.com/books?id=l68ZhT2agbEC&pg=PA275|title=Notes & Queries|publisher=Oxford University Press|year=1859|pages=275–276| vauthors = White W }}</ref><ref>{{Cite web|publisher=University of York| vauthors = Pierce H |title=Unseemly pictures: political graphic satire in England, c. 1600-c. 1650|date=2004|url=http://etheses.whiterose.ac.uk/9864/1/423697_vol1.pdf}}</ref>]]

[[James Joyce]]'s 1939 book ''[[Finnegans Wake]]'' has the character Shem the Penman infested with "[[foxtrot]]ting fleas, the lieabed lice,&nbsp;... bats in his belfry".<ref>{{cite book | vauthors = Joyce J | author-link1 = James Joyce |title=Finnegans Wake |date=1939 |publisher=Faber |page=180}}</ref>

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.<ref name="Trafzer2015">{{cite book | vauthors = Trafzer CE |title=A Chemehuevi Song: The Resilience of a Southern Paiute Tribe |url=https://books.google.com/books?id=GA81CgAAQBAJ&pg=PA24 |date=2015 |publisher=University of Washington Press |isbn=978-0-295-80582-5 |pages=22–25}}</ref>

The Irish songwriter John Lyons (b. 1934) wrote the popular<ref name=Clare/> 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.<ref name=Clare>{{cite web | vauthors = Carroll J |title=Songs of Clare: The Kilkenny Louse House |url=http://www.clarelibrary.ie/eolas/coclare/songs/cmc/the_kilkenny_louse_house_jlyons.htm |publisher=Clare Library |access-date=19 October 2015}}</ref><ref>{{cite web |vauthors=Scott B |title=My Colleen by the Shore |url=http://www.veteran.co.uk/VT149CD%20Paginated%20booklet%20pages.pdf |publisher=Veteran |year=2013 |access-date=25 October 2015 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304000329/http://www.veteran.co.uk/VT149CD%20Paginated%20booklet%20pages.pdf |url-status=dead }}</ref>

[[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."<ref>{{Cite thesis |title=Insect Life in the Poetry and Drama of England: With Special Reference to Poetry | vauthors = Twinn CR |publisher=University of Ottawa | degree= PhD |year=1942 |hdl=10393/21088  }}</ref>

===In science===
The human [[body louse]] ''Pediculus humanus humanus'' has the smallest insect [[genome]] known,<ref>{{cite journal | vauthors = Kirkness EF, Haas BJ, Sun W, Braig HR, Perotti MA, Clark JM, Lee SH, Robertson HM, Kennedy RC, Elhaik E, Gerlach D, Kriventseva EV, Elsik CG, Graur D, Hill CA, Veenstra JA, Walenz B, Tubío JM, Ribeiro JM, Rozas J, Johnston JS, Reese JT, Popadic A, Tojo M, Raoult D, Reed DL, Tomoyasu Y, Kraus E, Krause E, Mittapalli O, Margam VM, Li HM, Meyer JM, Johnson RM, Romero-Severson J, Vanzee JP, Alvarez-Ponce D, Vieira FG, Aguadé M, Guirao-Rico S, Anzola JM, Yoon KS, Strycharz JP, Unger MF, Christley S, Lobo NF, Seufferheld MJ, Wang N, Dasch GA, Struchiner CJ, Madey G, Hannick LI, Bidwell S, Joardar V, Caler E, Shao R, Barker SC, Cameron S, Bruggner RV, Regier A, Johnson J, Viswanathan L, Utterback TR, Sutton GG, Lawson D, Waterhouse RM, Venter JC, Strausberg RL, Berenbaum MR, Collins FH, Zdobnov EM, Pittendrigh BR | display-authors = 6 | title = Genome sequences of the human body louse and its primary endosymbiont provide insights into the permanent parasitic lifestyle | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 27 | pages = 12168–73 | date = July 2010 | pmid = 20566863 | pmc = 2901460 | doi = 10.1073/pnas.1003379107 | bibcode = 2010PNAS..10712168K | doi-access = free }}</ref> and this louse can transmit certain diseases that the closely related human [[head louse]] (''P. h. capitis'') cannot. With their simple life history and small genomes, the pair make ideal [[model organism]]s to study the [[Molecular biology|molecular mechanisms]] behind the transmission of [[pathogen]]s and [[Vector (epidemiology)|vector]] [[Natural competence|competence]].<ref>{{cite journal | vauthors = Pittendrigh BR, Berenbaum MR, Seufferheld MJ, Margam VM, Strycharz JP, Yoon KS, Sun W, Reenan R, Lee SH, Clark JM | display-authors = 6 | title = Simplify, simplify: Lifestyle and compact genome of the body louse provide a unique functional genomics opportunity | journal = Communicative & Integrative Biology | volume = 4 | issue = 2 | pages = 188–91 | date = March 2011 | pmid = 21655436 | pmc = 3104575 | doi = 10.4161/cib.4.2.14279 }}</ref>

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.<ref name="Reed D.L. 2007"/> 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–human last common ancestor|chimpanzee-hominid divergence]].<ref name="Reed D.L. 2007"/> Because parasites rely on their hosts, host{{endash}}parasite cospeciation events are likely.

Genetic evidence suggests that human ancestors acquired pubic lice from [[gorilla]]s approximately 3-4 million years ago.<ref name="Reed D.L. 2007">{{cite journal | vauthors = Reed DL, Light JE, Allen JM, Kirchman JJ | title = Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice | journal = BMC Biology | volume = 5 | issue = 7 | pages = 7 | date = March 2007 | pmid = 17343749 | pmc = 1828715 | doi = 10.1186/1741-7007-5-7 | doi-access = free }}</ref> 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{{endash}}host switch occurred in evolutionary history, this explanation is the most parsimonious (containing the fewest evolutionary changes).<ref name="Reed D.L. 2007"/>

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.<ref name=Parry>{{cite web | vauthors = Parry W |title=Lice Reveal Clues to Human Evolution |url=http://www.livescience.com/41028-lice-reveal-clues-to-human-evolution.html |publisher=LiveScience |access-date=25 October 2015 |date=7 November 2013}}</ref> 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.<ref name=Parry/><ref name="Kittler R. 2003">{{cite journal | vauthors = Kittler R, Kayser M, Stoneking M | title = Molecular evolution of Pediculus humanus and the origin of clothing | journal = Current Biology | volume = 13 | issue = 16 | pages = 1414–7 | date = August 2003 | pmid = 12932325 | doi = 10.1016/S0960-9822(03)00507-4 | s2cid = 15277254 | doi-access = free | bibcode = 2003CBio...13.1414K }}</ref>

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 [[minichromosome]]s at least seven million years ago.<ref name=Shao2012>{{cite journal | vauthors = Shao R, Zhu XQ, Barker SC, Herd K | title = Evolution of extensively fragmented mitochondrial genomes in the lice of humans | journal = Genome Biology and Evolution | volume = 4 | issue = 11 | pages = 1088–101 | year = 2012 | pmid = 23042553 | pmc = 3514963 | doi = 10.1093/gbe/evs088 }}</ref> Analysis of mitochondrial DNA in human body and hair lice reveals that greater genetic diversity existed in African than in non-African lice.<ref name="Kittler R. 2003"/><ref name=Light>{{cite journal | vauthors = Light JE, Allen JM, Long LM, Carter TE, Barrow L, Suren G, Raoult D, Reed DL | display-authors = 6 | title = Geographic distributions and origins of human head lice (Pediculus humanus capitis) based on mitochondrial data | journal = The Journal of Parasitology | volume = 94 | issue = 6 | pages = 1275–81 | date = December 2008 | pmid = 18576877 | doi = 10.1645/GE-1618.1 | s2cid = 9456340 }}</ref> Human lice can also shed light on human migratory patterns in prehistory. The dominating theory of [[Anthropology|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.<ref name=Light/>

=== Woodlouse ===
The name [[woodlouse]] or wood-louse is given to [[crustacean]]s of the [[suborder]] Oniscidea within the [[order (biology)|order]] [[Isopoda]], unrelated to lice. The ''[[Oxford English Dictionary]]''{{'}}s earliest citation of this usage is from 1611.<ref>{{Cite OED|Woodlouse}}</ref>

== See also ==
*[[Pest (organism)]]
*[[Use of DNA in forensic entomology]]

== References ==
{{Reflist}}

== External links ==
{{Commons category|Phthiraptera}}
{{Wikispecies|Phthiraptera}}
{{Wiktionary|louse}}
*[http://npic.orst.edu/pest/lice.html National Pesticide Information Center – Understanding and Controlling Lice]
*[http://entomology.ifas.ufl.edu/creatures/urban/human_lice.htm body and head lice] on the [[University of Florida]]/[[Institute of Food and Agricultural Sciences]] Featured Creatures Web site
*[http://entomology.ifas.ufl.edu/creatures/urban/crab_louse.htm crab louse] on the [[University of Florida]]/[[Institute of Food and Agricultural Sciences]] Featured Creatures Web site
*[https://web.archive.org/web/20150715200253/http://www.metapathogen.com/lice/phumanus/ ''Pediculus humanus capitis'' head louse facts, myths, life cycle at MetaPathogen]
* [https://en.wikibooks.org/wiki/Parasitic_Insects,_Mites_and_Ticks:_Genera_of_Medical_and_Veterinary_Importance/Sucking_lice Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance] Wikibooks
{{Human lice}}
{{Psocodea|2}}
{{Insects in culture}}
{{Taxonbar|from=Q6481228}}
{{Authority control}}

[[Category:Lice| ]]
[[Category:Insects in culture]]
[[Category:Taxa named by Ernst Haeckel]]
[[Category:Delusional parasitosis]]
[[Category:Nanopsocetae]]