Louse

Phthiraptera
	Fahrenholzia pinnata
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Subclass:	Pterygota
Infraclass:	Neoptera
Order:	Phthiraptera; Haeckel, 1896

Lice (singular: louse), are wingless insects that live in hair. They are all external parasites on every species of bird and most mammalian orders. They are not found on Monotremes and a few eutherian orders, namely bats (Chiroptera), whales, dolphins and porpoises (Cetacea) and pangolins (Pholidota). There are more than 3,000 different species; three are classified as human lice.

Description

Lice spend their whole life on the host. They are adapted to keep close contact with the host. These adaptations are reflected in their size (0.5–8 mm), stout legs, and claws which are adapted to clinging tightly to hair, fur and feathers. They are also wingless and flattened.

Lice feed on skin (epidermal) debris, feather parts, sebaceous secretions and blood. A louse's color varies from pale beige to dark grey; however, if feeding on blood, it may become considerably darker.

A louse's egg is commonly called a nit. Lice attach their eggs to their host's hair with specialized saliva which results in a bond that is very difficult to separate without specialized products. Living lice eggs tend to be pale white. Dead lice eggs are more yellow. Lice are very annoying and are difficult to remove, but not impossible. The process is called nit-picking, and is often done with a close-toothed metal comb. For humans, anti-insect shampoos are available.

Classification

The order has traditionally been divided into two suborders; the sucking lice (Anoplura) and chewing lice (Mallophaga). Four suborders are now recognised:

Anoplura: sucking lice, including head and pubic lice
Rhyncophthirina: parasites of elephants and warthogs
Ischnocera: avian lice
Amblycera: chewing lice, a primitive order of lice
Amblycera: jumping lice have very strong hind legs and can jump a distance of three feet

It has been suggested that the order is contained by the Troctomorpha suborder of Psocoptera. What this means is that lice may have evolved from free-living species of that group.

Lice and humans

Humans are unique in that they host three different species of lice: head lice, body lice (which live mainly in clothing), and pubic lice. The DNA differences between head lice and body lice provide corroborating evidence that humans started wearing clothes at approximately 70,000 BCE.^[1]

Recent DNA evidence suggests that pubic lice spread to the ancestors of humans approximately 3.3 million years ago from the ancestors of gorillas by sharing the same bed or other communal areas with them, and are more closely related to lice endemic to gorillas than to other lice species infesting humans.^[2]

Gallery

The chewing louse Damalinia limbata is found on Angora goats. The male louse (right) is typically smaller than the female (left), whose posterior margin of the abdomen is more rounded than those of male lice.
Diagram of a louse, by Robert Hooke, 1667.
Trichodectes canis, the biting dog louse