Louse, a name used to refer to a group of bloodsucking or biting insects. All are wingless parasites, living on the bodies of warmblooded animals. They are to a great measure controlled easily by the application of delousing powders or ointments on the host.

The louseThe louse is a wingless parasite living on the bodies of warm-blooded animals.

True lice suck blood and spend their entire lives as parasites. The eggs, or nits, are usually attached to the hair of the host. Three kinds can infest humans: the body louse (or cootie), the head louse, and the crab louse. The body louse can transmit several dangerous diseases such as typhus and relapsing fever. Body lice, which are about one-eighth of an inch (3 mm) long, are usually found in clothing. The head louse is usually found in the hair of the head; the crab louse, in the hair of the pubic area. Both are much smaller than the body louse. Lice are transmitted by direct contact with infested persons or their clothing. Other true lice infest domestic animals such as dogs, horses, cattle, and hogs. Biting lice feed on the feathers and skins of poultry, pigeons, and other birds.

Other parasitic insects are sometimes called lice; aphids, for example, are also called plant lice. The book louse and the wood louse are called lice because of their superficial resemblance to true lice.

The true louse is of the order Anoplura; the biting louse, Mallophaga. The crab louse is Phthirus pubis; body, Pediculus humanus corporis; head, P. humanus capitis.