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 on only specific types of animals, and, in some cases, 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. 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, usually 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 has been studied more intensively than that of mammal lice.
A few major trends
The average number of lice per host tends to be higher in large-bodied bird species than in small ones.
Louse individuals exhibit 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 taxa that dive under the water surface to feed on aquatic prey harbor fewer taxa of lice.
Bird taxa that are capable of exerting stronger antiparasitic defense—such as stronger T cell immune response or larger uropygial glands—harbor more taxa of Amblyceran lice than others.
Temporal bottlenecks in host population size may cause a long-lasting reduction of louse taxonomic richness. E.g., 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 (living on separate birds) in the latter case.
Lice may transmit microbial diseases or helminth parasites.
Ischnoceran lice may reduce the thermoregulation effect of the plumage; thus heavily infested birds lose more heat than other ones.
Lice infestation 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 recognized:
Lice have been the subject of significant DNA research in the 2000s that led to discoveries on human evolution. For example, genetic evidence suggests that our human ancestors acquired pubic lice from gorillas approximately 3-4 million years ago. Additionally, the DNA differences between head lice and body lice provide corroborating evidence that humans started losing body hair about 2 million years ago.
The mitochondrial genome of the human species of body lice (Pediculus humanus humanus), the head louse (Pediculus humanus capitis) and the pubic louse (Pthirus pubis) is fragmented into a number of minichromosomes. This fragmentation appears to have been present for at least 7 million years. The body louse evolved from the head louse ~107,000 years ago and since body lice require clothing to survive in the absence of thick body hair, this latter date has been suggested as the approximate time when humans began to wear clothing, perhaps as adaption to some groups of humans having moved to cooler climates than those of Africa.