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Louse eggs on the hair very close to the scalp are the primary sign of an active infestation. A female can lay up to 50-150 eggs in its life, but it can lay up to 100 eggs in 30 days. She also glues her eggs, sometimes called "nits", which look like tiny white beads, to hair shafts very close to the scalp (usually about a centimeter or two [half an inch to three-quarters of an inch] from the scalp). Eggs are very small, about the size of a period "." (full stop) in normal printing. Eggs may appear white, yellowish, brownish or greyish, but are almost always lighter colored. An egg normally undergoes 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 darker, or raisin-like, as they dry out. "Nits" of this sort are usually found over one centimeter (approximately one-half inch) or more away from the scalp and are not considered a sign of active infestation. Some people refer to lice eggs, live or dead, as "nits." In common usage, an egg that is still incubating may also be called a "nit."

Head lice have no wings and move primarily by crawling. They have no adaptations for jumping.

Whether a louse is male or female is not apparent until it is nearly mature. It takes about 10 days for a head louse to grow into an adult. Fertilization of eggs takes place once the female is mature. The female can then lay 3-7 eggs each day for the next 28 to 30 days, her normal life span. Females may lay multiple eggs on one hair or lay an egg on a hair and then move on.

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

* Nit: [1]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. Most head lice at this stage are almost transparent until they have eaten (sucked blood from a human skin). Then they can be seen more easily since it will be possible to see the tiny amount of

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,(which makes them insects) but their two front legs are very large in order to grab onto the hair shafts. Head lice are tan to greyish-white in color. Headlice are easy to notice in blonde or ginger hair,because yellow and orange are light colours. Human lice do not feed on the blood of other animals, only humans.