Louse
Louse is the common name for any member of the infraorder Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera was previously recognized as an 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 parasites, living externally on warm-blooded hosts, which include most species of bird and mammal, notably excluding monotremes, pangolins, bats, sirenians, tapirs, rhinoceroses, and members of the clade and suborder Whippomorpha. 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 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; and the pubic louse, Pthirus pubis. Lice are 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 microscopy, appearing in Robert Hooke's 1667 book, Micrographia.
The oldest known fossil lice are from the Cretaceous.
Classification
Lice are classified as an infraorder called Phthiraptera, named by Ernst Haeckel in 1896. Phthiraptera was previously recognized as an 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. Within Psocodea, the Phthiraptera lice are most closely related to family Liposcelididae.A cladogram showing the position of Phthiraptera within Psocodea is shown below:
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 metathoracic ganglia with their abdominal ganglia to form a single ventral nerve junction.The order has traditionally been divided into two suborders, the sucking lice and the chewing lice. However, subsequent classifications indicated that the Mallophaga are paraphyletic, and four suborders were then recognized.
Upon finding that Phthiraptera was nested within Psocoptera in 2021, de Moya et al. reduced the rank of Phthiraptera to infraorder, and the suborders were reduced down to parvorder. 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:
- 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 warthogs
- Ischnocera: avian chewing lice, and contains the large family Philopteridae
- Trichodectera: parasitizing mammals, split off from Ischnocera, and contains the family Trichodectidae
Evolutionary history
Placental mammal lice had a single common ancestor that lived on Afrotheria with this arising from host-switching from an ancient avian host.
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. They are found even in the Antarctic, where penguins carry 15 species of lice.Lice are divided into two groups: sucking lice, which obtain their nourishment from feeding on the sebaceous secretions and body fluids of their host; and chewing lice, which are scavengers, feeding on skin, fragments of feathers or hair, and debris found on the host's body. Many lice are specific to a single species of host and have co-evolved with it. In some cases, they live on only a particular part of the body. Some animals are known to host up to fifteen different species, although one to three is typical for mammals, and two to six for birds. Lice generally cannot survive for long if removed from their host. If their host dies, lice can opportunistically use phoresis to hitch a ride on a fly and attempt to find a new host.
Sucking lice range in length from. They have narrow heads and oval, flattened bodies. They have no ocelli, and their compound eyes are reduced in size or absent. Their antennae are short with three to five segments, and their mouthparts, which are retractable into their head, are adapted for piercing and sucking. 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 and a food canal. The thoracic segments are fused, the abdominal segments are separate, and there is a single large claw at the tip of each of the six legs.
Chewing lice are also flattened and can be slightly larger than sucking lice, ranging in length from. They are similar to sucking lice in form but the head is wider than the thorax and all species have compound eyes. There are no ocelli and the mouthparts are adapted for chewing. The antennae have three to five segments and are slender in the suborder Ischnocera, but club-shaped in the suborder Amblycera. The legs are short and robust, and terminated by one or two claws. Some species of chewing lice house symbiotic bacteria in bacteriocytes in their bodies. These may assist in digestion because if the insect is deprived of them, it will die.
Lice are usually cryptically coloured to match the fur or feathers of the host. A louse's 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 preening, such as the interior of feather shafts. Living louse eggs tend to be pale whitish, whereas dead louse eggs are yellower. 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.
Humans host three different kinds of lice: head lice, body lice, and 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 lice combs, and medicated shampoos or washes.
Ecology
The average number of lice per host tends to be higher in large-bodied bird species than in small ones. Lice have an aggregated distribution across bird individuals, i.e. most lice live on a few birds, while most birds are relatively free of lice. This pattern is more pronounced in territorial than in colonial—more social—bird species. Host organisms that dive under water to feed on aquatic prey harbour fewer taxa of lice. Bird taxa that are capable of exerting stronger antiparasitic defence—such as stronger T cell immune response or larger uropygial glands—harbour more taxa of Amblyceran lice than others. Reductions in the size of host populations may cause a long-lasting reduction of louse taxonomic richness, for example, birds introduced into New Zealand host fewer species of lice there than in Europe. Louse sex ratios are more balanced in more social hosts and more female-biased in less social hosts, presumably due to the stronger isolation among louse subpopulations in the latter case. The extinction of a species results in the extinction of its host-specific lice. Host-switching is a random event that would seem very rarely likely to be successful, but speciation has occurred over evolutionary time-scales so it must be successfully accomplished sometimes.Lice may reduce host life expectancy if the infestation is heavy, 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. Lice may transmit microbial diseases and helminth parasites, but most individuals spend their whole life cycle on a single host and are only able to transfer to a new host opportunistically. Ischnoceran lice may reduce the thermoregulation effect of the plumage; thus heavily infested birds lose more heat than others. Lice infestation is a disadvantage in the context of sexual rivalry.