Subordines: Amblycera - Anoplura - Ischnocera - Rhyncophthirina
* Phtiraptera Central
Lice (singular: louse) is the common name for over 3000 species of wingless insects of the order Phthiraptera; three of which are classified as human disease agents. They are obligate ectoparasites of every avian and mammalian order except for Monotremes (the platypus and echidnas), bats, whales, dolphins, porpoises and pangolins.
Most lice are scavengers, feeding on skin and other debris found on the host's body, but some species feed on sebaceous secretions and blood. Most are found only on specific types of animal, and, in some cases, only to 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.
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 the males, and some species are even known to be parthenogenetic. A louse's egg is commonly called a nit. Many lice attach their eggs to their host's 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 lice eggs tend to be pale white. Dead lice eggs are more yellow.
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, which they usually reach within a month of hatching.
Lice are optimal model organisms to study the ecology of contagious pathogens since their quantities, sex-ratios etc. are easier to quantify than those of other pathogens. The ecology of avian lice have been studied more intensively than that of mammal lice.
A few major trends
1. The average number of lice per host tends to be higher in large-bodied bird species than in small ones.
2. Louse individuals exhibit an aggregated distribution across bird individuals, i.e. most lice live on a few bird, while most birds are relatively free of lice. This pattern is more pronounced in territorial than in colonial – more social – bird species.
3. Host taxa that dive under the water surface to feed on aquatic prey harbour fewer taxa of lice.
4. Bird taxa that are capable to exert stronger antiparasitic defense – such as stronger T cell immune response or larger uropygial glands – harbour more taxa of Amblyceran lice than others.
5. Temporal bottlenecks in host population size may couse a long-lasting reduction of louse taxonomic richness. E.g., birds introduced into New Zealand host fewer species of lice there than in Europe.
6. 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.
A few effects of lousiness upon the host
1. Lice may reduce host life expectancy.
2. Lice may transmit microbial diseases or helminth parasites.
3. Ischnoceran lice may reduce the thermoregulation effect of the plumage, thus heavily infected birds loss more heat than other ones.
4. Lousiness is a disadvantage in the context of sexual rivalry.
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 recognised:
* Anoplura: sucking lice, occurring on mammals exclusively
* Rhyncophthirina: parasites of elephants and warthogs
* Ischnocera: mostly avian chewing lice, however, one family parasitize mammals
* Amblycera: a primitive suborder of chewing lice, widspread on birds, however, also live on South-American And Australian mammals
It has been suggested that the order is contained by the Troctomorpha suborder of Psocoptera.
Lice in humans
For information about human infestation, see Pediculosis. For information on treatment, see Treatment of human head lice.
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. Adult and nymphal lice can survive on sheep-shearers' moccasins for up to 10 days, but microwaving the footwear for five minutes in a plastic bag will kill the lice.
Human lice and DNA discoveries
Lice have been the subject of significant DNA research that has led to discoveries on human evolution. For example, recent DNA evidence suggests that pubic lice spread to humans approximately 2,000,000 years ago from gorillas. Additionally, the DNA differences between head lice and body lice provide corroborating evidence that humans started losing body hair, also about 2,000,000 years ago.
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