A head louse has a life cycle of three stages: egg, nymph and adult. Difficult to see, nits are lice eggs and are often mistaken for dandruff or hair product. Adult female lice lay nits at the base of a hair shaft close to the scalp. Between 0.88 and 0.3 millimeters in size, the eggs are oval and generally yellow or white. They take approximately one week to hatch, with the viable eggs placed within 6 millimeters of the scalp.
Lice eggs hatch to release nymphs. Continuing to attach to the hair shaft, the nit shells become a more noticeable pale yellow. Smaller than an adult louse but very similar in appearance, the nymph is approximately the size of a pinhead. Nymphs transition through three stages of molts to become adults seven days after hatching.
The size of a sesame seed with six legs and claws, an adult louse varies between tan and a gray-white color. In dark hair, the louse appears darker. Larger than the males, females can lay up to eight nits each day. Adults can live up to a month on a person's head. To do so, they must feed on blood. Without blood meals, lice die within one to two days.