LOUSE, a name commonly applied to small wingless insects parasitic upon mammals and birds and belonging to the order Anoplura. They are flattened creatures with short to 5-jointed antennae, the eyes reduced or wanting and the tarsi 1- or 2 jointed with claws strongly developed for clinging to their hosts. The eggs are attached to the hairs or feathers and the young lice are active as soon as they emerge : they undergo several moults during growth but pass through no true metamorphosis, and when numerous, cause great irritation to the hosts upon whose bodies their whole life is passed. The Anoplura are divided into two sub-orders, viz., the Siphunculata (Anoplura of many author ities) or sucking lice and the Mallophaga or biting lice.
The sucking lice, or true lice, have the mouth-parts adapted for piercing and sucking and live by imbibing the blood of mam mals. About 150 species are known and, in addition to man and domestic animals, a wide range of other mammals are infested by these insects. The best known species is Pediculus humanus, the common louse of man, which infests people living under unhy gienic conditions. It exists as two races (formerly regarded as distinct species), viz., P. capitis, the head louse, and P. corporis,
the body louse. The human louse is concerned in the trans mission, from man to man, of the pathogenic agents of typhus, trench fever (during the World War) and relapsing fever, and there is now a vast literature on the subject. The other louse infesting man is the crab louse (Phthirus pubis) while the genus Haematopinus occurs on pigs, cattle and other Ungulates and Haematomyzus is found on elephants.
The biting lice or bird-lice (q.v.) have biting mouth-parts and chiefly infest birds, a smaller number living on mammals : about 1,700 species are known. The most notorious member of the group is the chicken louse illenopon pallidum, while pigeons are nearly always infested by Lipeurus baculus. The species of Tricho dectes live on dogs, cats and other mammals.
The word louse is also applied in a popular sense to many other animals of a different nature from the true lice, e.g., wood louse (q.v.), fish louse (see CRUSTACEA), book-louse, plant louse (see APHIDES) and bark louse (see SCALE INSECT).