The Human Head Louse is a human-specific parasite that lives only on the scalp. This is different from the other two species of human louse, the Pubic "Crab" Louse and the Human Body Louse, which occur in the pubic region and the body (exculding the head and pubic region) respectively. The lifecycles of the three species differ and therefore control measures also differ depending upon which infestation is occuring.
Human Head Louse eggs are called nits, and are often found on hair around a person’s ears and back of the head. You cannot "catch” nits; the eggs must be laid on the hair by live head lice. Nits hatch into nymphs, immature forms which look like small adults. Both nymphs and adults have piercing-sucking mouthparts to pierce the skin for a blood meal. Adult head lice are approximately 2-3 mm length (about the size of a sesame seed) and range in color from white to brown to dark gray. They do not have wings or powerful jumping legs, so they move about by clinging on hairs with specially adapted claw-like legs. Adult head lice are swift-moving and tend to avoid light. They are usually spread by head-to-head contact with an infested person's hair or by sharing hats, combs, earphones, jackets or hair accessories.

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