The head louse is a tiny, soft-bodied insect that is somewhat flat. The adult stage is approximately 1/16 inch long—about the size of a sesame seed. Immature stages (nymphs) are smaller. The head louse has a narrow head that is equipped with sucking mouthparts. Individuals often are grayish-white to tan but may become reddish-brown after feeding on human blood. The head louse is wingless and has six legs, each ending in a small claw (hook) that is designed for grasping hairs. These claws are better able to grasp hair that is round in cross-section, which is typical of Caucasians. Adults and nymphs use their claws to cling to hair, and they do not fall off hair easily.
Fig.1. A head louse (left) and crab louse (right) are distinctly different.
Head lice do not jump, hop, or fly. They do crawl a lot, and they can move quickly. They may be found anywhere on the human scalp and hair. The most typical locations are the crown of the head, above and behind the ears, and the nape of the neck.
Lice eggs are referred to as nits. Nits are oblong and usually pearly white, but they may darken to a tan or coffee-color as the young louse completes its development within the egg and nears hatching. Each nit is about 1/30 inch long (less than the width of a #2 pencil lead). Each nit is attached (cemented) onto a hair shaft, typically very close to the scalp. Although nits are laid close to the scalp, they eventually may be found further down the hair shaft as the hair grows and elongates. Nits may occur on any region of the head, but often are found behind the ears or near the hairline at the back of the neck. Nits are not easily dislodged, and they may remain attached to hair strands that have been shed.