Lice, True

(Anoplura, or Siphunculata), an order of bloodsucking insects parasitic on man and animals. The body of lice is flattened and 1-5 mm long; the head and thorax are small, the abdomen is relatively large, and wings are absent. Certain species have two simple eyes; many are blind. There are three pairs of legs, which are strong and clinging. The mouth parts are adapted for piercing and sucking. The oral opening is surrounded by a soft extruding tube with a crown of hooks. With this the louse clamps itself to the skin in sucking. When a louse bites an animal or human being, saliva is injected into the wound, causing an irritation of the skin and itching. The lice eggs (nits) are attached to hairs or threads of clothing by a special sticky substance secreted by the female. Larvae hatch from the eggs, and the larvae differ from the adult lice only in their smaller size. The entire life cycle of lice is spent on the host. There are around 200 species.

Parasitic on man are the human louse and the crab louse. The human louse (Pediculus humanus) is known in two forms. The head louse (P. h. capitis) has dark pigmented spots along the sides of the body and relatively deep lateral notches between the segments of the abdomen. It reaches 4 mm in length and lives mostly in the hair of the head. The body louse (P. h. vestimenti) is larger, up to 4.75 mm (the males are smaller). It usually keeps to the folds of clothing and underwear. The nits are attached to the fibers of the material or to hairs. The human louse feeds on blood two or three times a day. The head louse lays up to 140 eggs during its life, and the body louse up to 300. Development of the embryo takes four to eight days at 36°-37° C and 16 days at 23° C. The larvae do not hatch at temperatures under 22° C and over 40° C. Unhygienic living conditions cause lousiness. Aside from their direct effect on man, lice can also serve as the specific vectors of a number of infectious diseases (exanthematous fever, recurrent typhus, trench fever, and others).

The crab louse (Phthirus pubis) has a short, broad, and greatly flattened body up to 1.5 mm long. It lives mostly in the pubic hairs and more rarely in the armpit, beard, and eyebrows. It attaches itself closely to the body, causing itching. The female lays up to 50 eggs over its lifetime, and development of the embryo takes five to eight days at 36° C. Infestation with crab lice occurs with direct contact, usually sexual.

Many lice species are parasitic on mammals, and various animals have their own lice species. The hog louse (Haematopinus suis) lives on pigs; H. asini lives on horses and donkeys; H. eurysternus and others live on cattle; Linognathus setosus is found on dogs; and species from the genus Hoplopleura live on Muridae. Animal lice are strictly specific parasites, and are not transmitted to man.