The head louse (Pediculus humanus capitis) is an obligate ectoparasite of humans. Head lice are wingless insects spending their entire life on human scalp and feeding exclusively on human blood. Humans are the only known host of this specific parasite, but many other species of lice are known which infest most orders of mammals and also birds
Human head lice are closely related to human body lice (Pediculus humanus humanus) which also infest humans. Genetic analysis suggests that the human body louse may have originated about 107,000 years ago from the head louse after the invention of clothing, with the ancestor of all human lice emerging about 770,000 years ago. A more distantly-related species of louse, the pubic or crab louse (Pthirus pubis), also infests humans. Lice infestation of any part of the body is known as pediculosis.
The head louse (and lice in general) differ from other hematophagic ectoparasites such as the flea in that lice spend their entire life cycle on a host. Head lice cannot fly, and their short stumpy legs render them incapable of jumping, or even walking efficiently on flat surfaces.
One pair of antenna, each with five segments, protrude from the insect's head. Head lice also have one pair of eyes. Eyes are present in all species within Pediculidae (the family of which the head louse is a member), but are reduced or absent in most other members of the Anoplura suborder. Like other members of Anoplura, head lice mouthparts are highly adapted for piercing skin and sucking blood. These mouthparts are retracted into the insect's head except during feeding.
Like other members of Anoplura, wings are entirely absent.
Like most insects, head lice are oviparous. Louse eggs contain a single embryo, and are attached near the base of a host hair shaft. Eggs are generally laid within 1 cm of the scalp surface. To attach each egg, the adult female secretes a glue from her reproductive organ. This glue quickly hardens into a "nit sheath" that covers the hair shaft and the entire egg except for the operculum – a cap through which the embryo breathes. The glue was previously thought to be chitin-based, but more recent studies have shown it to be made of proteins similar to hair keratin.
Each egg is oval-shaped and about 0.8 mm in length. They are brown so long as they contain an embryo, but appear white after hatching. Viable louse eggs hatch six to nine days after oviposition. After hatching, the louse nymph leaves behind its egg shell, still attached to the hair shaft.
This has produced some confusion in, for example, school policy (see The "no-nit" policy) because, of the three items listed above, only eggs containing viable embryos have the potential to infest or re-infest a host. Some authors have reacted to this confusion by restricting the definition of nit to describe only a hatched or nonviable egg:
Others have retained the broad definition while simultaneously attempting to clarify its relevance to infestation:
Note that all these quotations appear to reject the notion that louse nymphs are nits, and may indicate that the nit definition is currently in flux.
Head lice, like other insects of the order Phthiraptera, are hemimetabolous. Newly hatched nymphs will molt three times before reaching the sexually-mature adult stage. Thus, mobile head lice populations contain members of up to four developmental stages: three nymphal instars, and the adult (imago). Metamorphosis during head lice development is subtle. The only visible differences between different instars and the adult, other than size, is the relative length of the abdomen, which increases with each molt. Aside from reproduction, nymph behavior is similar to the adult. Nymphs feed only on human blood (hematophagia), and cannot survive long away from a host.
Like adult head lice, the nymph cannot fly or jump. However, there are reports of the light-weighted nymphs being blown by wind. Similarly, after a molt, the discarded exoskeleton can be later shed by the host, and may be mistakenly interpreted as a viable louse.
The time required for head lice to complete their nymph development to the imago depends on feeding conditions. At minimum, eight to nine days are required for lice having continuous access to a human host. This experimental condition is most representative of head lice conditions in the wild. Experimental conditions where the nymph has more limited access to blood produces more prolonged development, ranging from 12 to 24 days.
Nymph mortality is reportedly high -- about 38% -- especially within the first two days of life. Nymph hazards are numerous. Failure to completely hatch from the egg is invariably fatal, and may be dependent on the humidity of the egg's environment. Death during molting can also occur, although it is reportedly uncommon. During feeding, the nymph gut can rupture, dispersing the host's blood throughout the insect. This results in death within a day or two. It is unclear if the high mortality recorded under experimental conditions is representative of conditions in the wild.
Adult head lice reproduce sexually, and copulation is necessary for the female to produce fertile eggs. Parthenogenesis, the production of viable offspring by virgin females, does not occur in Pediculus humanus. Pairing can begin within the first 10 hours of adult life. After 24 hours, adult lice copulate frequently, with mating occurring during any period of the night or day. Mating attachment frequently lasts more than an hour. Young males can successfully pair with older females, and vice versa.
Experiments with Pediculus humanus humanus (body lice) emphasize the attendant hazards of lice copulation. A single young female, confined with six or more males, will die in a few days, having laid very few eggs. Similarly, death of a virgin female was reported after admitting a male to her confinement. The female laid only one egg after mating, and her entire body was tinged with red -- a condition attributed to rupture of the alimentary canal during the sexual act. Old females frequently die following, if not during, intercourse.
Females lay an average of three to four eggs daily. During its lifespan of four weeks a female louse lays 50–150 eggs (nits).
The number of children per family, the sharing of beds and closets, hair washing habits, local customs and social contacts, healthcare in a particular area (e.g. school) and socio-economic status were found to be significant factors in head louse infestation. Girls are two to four times more frequently infested than boys. Children between 4 and 13 years of age are the most frequently infested group.
Analysis of the DNA of lice found on Peruvian mummies have led to some surprising findings. The research indicates that some diseases (like typhus) may have passed from the New World to the Old World, instead of the other way around.