The adult female is 1.5 to 2 mm long. The male is about half that length. The nymph is smaller than the adult but resembles it in shape. The crab louse occurs in any area of the world inhabited by man. However, it is not as common as head and body lice.
Man is the only host on which crab lice are known to breed and survive.
More rarely, the scalp or eyelashes of young children may be infested. While gripping several hairs, the louse pierces the skin with its needle-like mouthparts and sucks blood. This feeding action causes the formation of blue spots 0.2 to 3.0 cm in diameter on the skin. These spots may not appear for several hours after the bite and may last several days. Reaction to a crab louse infestation varies with the individual.