Head louse

Head lice (Pediculus humanus capitis) are one of the many varieties of sucking lice (singular “louse”) specialized to live on different areas of various animals.

As the name implies, head lice are specialized to live among the hair present on the human head and are exquisitely adapted to living mainly on the scalp and neck hairs of their human host. Lice present on other body parts covered by hair are not head lice but are either Pubic lice (Pthirus pubis) or Body lice (Pediculus humanus humanus).


The adult head louse resembles a miniature ant that appears flat when viewed from the side through a strong magnifying glass. Head lice have a head, thorax and abdomen with six legs, but their two front legs are very large in order to grab onto the hair shafts. Head lice are tan to grayish-white in color.

Life cycle

Lice eggs on the hair very close to the scalp are the primary sign of an active infestation. The female louse glues her eggs, sometimes called “nits”, which look like tiny white beads, to hair shafts very close to the scalp. Eggs are very small and may appear yellowish, brownish or grayish, but almost always lighter colored. Eggs normally undergo a 7-9 day incubation before hatching as a baby nymph. Classically, a louse egg does not become a “nit” until after it has completed its incubation stage, thus leaving a “nit”. A “nit” is either the empty shell remaining after the nymph has departed or the dead egg that remains if incubation was not successful. Dead eggs will appear dark, or raisin-like, as they dry out. “Nits” are usually found one-half inch or more away from the scalp and are not considered a sign of an active infestation.

There are three nymph instar stages as the baby louse matures, with the louse shedding its exoskeleton at the end of each stage as it gets larger. The nymph stage typically lasts 10 to 12 days.

Whether a louse is male or female is not apparent until they are nearly mature. Fertilization of eggs takes place once as the female reaches the mature stage. The female can then lay 3-7 eggs each day for the next 28 to 30 days, her normal life span.

There are three main stages in the life of a head louse: the nit, the nymph, and the adult.

  • Nit: Nits are head lice eggs. They are hard to see and are found firmly attached to the hair shaft. They are oval and usually yellow to white. Nits take about 1 week to hatch.
  • Nymph: The nit hatches into a baby louse called a nymph. It looks like an adult head louse, but is smaller. Nymphs mature into adults about 7 days after hatching. To live, the nymph must feed on blood. It metamorphoses 3 times before it reaches the adult stage.
  • Adult. Females lay nits (a few hundreds of eggs); they are usually larger than males. To live, adult lice need to feed on blood. If the louse falls off a person, it dies within 1-2 days.


The louse feeds on human blood, and the bite causes itching. Bites can become secondarily infected; scratching may break the skin and help cause this secondary infection.

Head lice are normally spread by close contact but can also be spread by sharing clothes.


The most common Western treatment is with chemical insecticides such as pyrethrin, however there is increasing controversy over possible toxic side effects.

Use in Archaeogenetics

Lice are also important in the field of Archaeogenetics. Because most “modern” human diseases have in fact recently jumped species from animals into humans through close agricultural contact, and also given fact that neolithic human populations were too scattered to support contagious “crowd” diseases, lice (along with such parasites as intestinal tapeworms) are considered to be one of the few ancestral disease infestations of humans and hominids. As such, analysis of mitochondrial lice DNA has been used to map early human and archaic human migrations and living conditions. Because lice can only survive for a few hours or days without a human host, and because lice species are so specific to certain species or areas of the body, the evolutionary history of lice reveals much about human history. It has been demonstrated, for example, that some varieties of human lice went through a population bottleneck about 100,000 years ago (supporting the Single origin hypothesis), and also that hominid lice lineages diverged around 1.18 million years ago (probably infesting Homo erectus) before re-uniting around 100,000 years ago. This recent merging seems to argue against the Multi-regional origin of modern human evolution and argues instead for a close proximity replacement of archaic humans by a migration of anatomically modern humans, either through sexual contact, fighting, or cannibalism.

Photo courtesy of the Centers for Disease Control and Prevention (CDC)/Dr. Dennis D. Juranek