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, callednits, to hairs or feathers. The eggs hatch intonymphs, which moult three times before becoming fully grown, a process that takes about four weeks.Genetic evidence indicates that lice are a highly modified lineage ofPsocoptera (now calledPsocodea), commonly known as booklice, barklice or barkflies. The oldest known fossil lice are from theCretaceous.[4]
Lice are divided into two groups: sucking lice, which obtain their nourishment from feeding on thesebaceous secretions and body fluids of their host; and chewing lice, which arescavengers, feeding onskin, 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.[5] If their host dies, lice can opportunistically usephoresis to hitch a ride on a fly and attempt to find a new host.[6]
Sucking lice range in length from 0.5 to 5 mm (1⁄64 to13⁄64 in). They have narrow heads and oval, flattened bodies. They have noocelli, and their compound eyes are reduced in size or absent. Theirantennae are short with three to five segments, and their mouthparts, which are retractable into their head, are adapted for piercing and sucking.[7] 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).[8] 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.[7]
Chewing lice are also flattened and can be slightly larger than sucking lice, ranging in length from0.5 to 6 mm (1⁄64 to15⁄64 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 suborderIschnocera, but club-shaped in the suborderAmblycera. The legs are short and robust, and terminated by one or two claws. Some species of chewing lice housesymbiotic bacteria inbacteriocytes in their bodies. These may assist in digestion because if the insect is deprived of them, it will die.Lice are usuallycryptically coloured to match the fur or feathers of the host.[7][9] 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 areparthenogenetic, with young developing from unfertilized eggs. A louse'segg is commonly called a nit. Many lice attach their eggs to their hosts' hair with specializedsaliva; 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 topreening, such as the interior of feather shafts. Living louse eggs tend to be pale whitish, whereas dead louse eggs are yellower.[5] Lice areexopterygotes, being born as miniature versions of the adult, known asnymphs. The young moult three times before reaching the final adult form, usually within a month after hatching.[5]
Humans host three different kinds of lice:head lice,body lice, andpubic lice. Head lice and body lice are subspecies ofPediculus humanus, and pubic lice are a separate species,Pthirus pubis. Lice infestations can be controlled withlice combs, and medicated shampoos or washes.[10]
Ecology
The average number of lice per host tends to be higher in large-bodied bird species than in small ones.[11] 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.[12]Host organisms that dive under water to feed on aquatic prey harbour fewer taxa of lice.[13][14]Bird taxa that are capable of exerting stronger antiparasitic defence—such as strongerT cell immune response or largeruropygial glands—harbour more taxa of Amblyceran lice than others.[15][16]Reductions in the size of host populations may cause a long-lasting reduction of louse taxonomic richness,[17] for example, birds introduced intoNew Zealand host fewer species of lice there than in Europe.[18][19] 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.[20] 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, butspeciation has occurred over evolutionary time-scales so it must be successfully accomplished sometimes.[17]
Lice may reduce host life expectancy if the infestation is heavy,[21] but most seem to have little effect on their host. The habit of dust bathing indomestic hens is probably an attempt by the birds to rid themselves of lice.[7] Lice may transmit microbial diseases andhelminth parasites,[22] but most individuals spend their whole life cycle on a single host and are only able to transfer to a new host opportunistically.[7] Ischnoceran lice may reduce thethermoregulation effect of the plumage; thus heavily infested birds lose more heat than others.[23]Lice infestation is a disadvantage in the context of sexual rivalry.[24][25]
Phthiraptera is clearly amonophyletic grouping, united as the members are by a number of derived features including their parasitism onwarm-blooded vertebrates and the combination of theirmetathoracicganglia with theirabdominal ganglia to form a single ventral nerve junction.[30] The order has traditionally been divided into two suborders, the sucking lice (Anoplura) and thechewing lice (Mallophaga); however, subsequent classifications suggest that the Mallophaga areparaphyletic and four suborders were then recognized:[31]
Ischnocera: mostly avian chewing lice, with one family parasitizing mammals
Amblycera: a primitive suborder of chewing lice, widespread on birds, and also occurring on South American and Australian mammals
Upon finding that Phthiraptera was nested withinPsocoptera, Phthiraptera, in 2021 de Moya et al. proposed reducing the rank of Phthiraptera toinfraorder, and the foursuborders toparvorder.[1] These changes were accepted by Psocodea Species File and others, with the exception of placing Phthiraptera under the infraorderNanopsocetae, as a parvorder, with the four subgroups listed above. These classifications are likely to change in the future as a result of ongoingphylogenetic research.[2][3]
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.[31] They are found even in theAntarctic, wherepenguins carry 15 species of lice (in the generaAustrogonoides andNesiotinus).[32] The oldest known record of the group isMegamenopon rasnitsyni from the Eocene of Germany, but it is essentially a modern form, belonging to Amblycera, so the group as a whole likely has an origin in the Mesozoic.[27]
Lice have been the subject of significantDNA 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 andPthirus: head lice (Pediculus humanus capitis), body lice (Pediculus humanus humanus), and pubic lice (Pthirus pubis). Human head and body lice (genusPediculus) share a common ancestor with chimpanzee lice, while pubic lice (genusPthirus) share a common ancestor with gorilla lice. Using phylogenetic and cophylogenetic analysis, Reed et al. hypothesized thatPediculus andPthirus are sister taxa and monophyletic.[33] In other words, the two genera descended from the same common ancestor. The age of divergence betweenPediculus and its common ancestor is estimated to be 6-7 million years ago, which matches the age predicted bychimpanzee-hominid divergence.[33] Because parasites rely on their hosts, host–parasite cospeciation events are likely.
Genetic evidence suggests that human ancestors acquired pubic lice fromgorillas approximately 3-4 million years ago.[33] Unlike the genusPediculus, the divergence inPthirus does not match the age of host divergence that likely occurred 7 million years ago. Reed et al. propose aPthirus 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).[33]
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.[34] 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.[34][35]
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.[36] Analysis of mitochondrial DNA in human body and hair lice reveals that greater genetic diversity existed in African than in non-African lice.[35][37] Human lice can also shed light on human migratory patterns in prehistory. The dominating theory ofanthropologists regarding human migration is theOut 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.[37]
Lice have been intimately associated with human society throughout history. In theMiddle Ages, they were essentially ubiquitous. At the death ofThomas 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".[38] 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.[39] A mediaeval treatment for lice was anointment made from pork grease,incense,lead, andaloe.[40]
In 1935 the Harvard medical researcherHans Zinsser wrote the bookRats, Lice and History, alleging that both body and head lice transmit typhus between humans.[44] Despite this, the modern view is that only the body louse can transmit the disease.[45]
Detail showing delousing fromJan Siberechts' paintingCour de ferme ("Farmyard"), 1662
In the psychiatric disorderdelusional 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".[47]
In science
The human body lousePediculus humanus humanus has (2010) the smallest insectgenome known.[48] 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 idealmodel organisms to study themolecular mechanisms behind the transmission ofpathogens andvectorcompetence.[49]
Clifford E. Trafzer'sAChemehuevi Song: The Resilience of aSouthern 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.[53]
The Irish songwriter John Lyons (b. 1934) wrote the popular[54] songThe 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 onFrom Puck to Appleby (2003), and theDubliners onDouble Dubliners (1972) among others.[54][55]
Robert Burns dedicated a poem to the louse, inspired by witnessing one on a lady's bonnet in church: "Ye ugly, creepin, blastid wonner, Detested, shunn'd, by saint and sinner, How dare ye set your fit upon her, sae fine lady! Gae somewhere else, and seek your dinner on some poor body."John Milton inParadise 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."InShakespeare'sTroilus and Cressida,Thersites comparesMenelaus, brother ofAgamemnon, 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."[56]
^Felsõ B, Rózsa L (August 2006). "Reduced taxonomic richness of lice (Insecta: Phthiraptera) in diving birds".The Journal of Parasitology.92 (4):867–9.doi:10.1645/ge-849.1.PMID16995408.S2CID31000581.
^abRòzsa L (November 1993). "Speciation patterns of ectoparasites and "straggling" lice".International Journal for Parasitology.23 (7):859–64.doi:10.1016/0020-7519(93)90050-9.PMID8314369.
^Barlett CM (1993). "Lice (Amblycera andIschnocera) as vectors ofEulimdana 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.JSTOR3283282.
^abLight JE, Allen JM, Long LM, Carter TE, Barrow L, Suren G, et al. (December 2008). "Geographic distributions and origins of human head lice (Pediculus humanus capitis) based on mitochondrial data".The Journal of Parasitology.94 (6):1275–81.doi:10.1645/GE-1618.1.PMID18576877.S2CID9456340.
^Sarasohn LT (2010).The Natural Philosophy of Margaret Cavendish: Reason and Fancy During the Scientific Revolution. JHU Press. pp. 165–167.ISBN978-0-8018-9443-5.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.
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