1911 Encyclopædia Britannica/Louse

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LOUSE (O. Eng. lús, cf. Du. luis, Ger. Laus, Dan. and Swed. lus), a term applied to small wingless insects, parasitic upon birds and mammals, and belonging strictly speaking to the order Anoplura, often included among the Hemiptera, though the term is frequently extended to the bird-lice constituting the suborder Mallophaga, formerly included among the Neuroptera. Both agree in having nothing that can be termed a metamorphosis; they are active from the time of their exit from the egg to their death, gradually increasing in size, and undergoing several moults or changes of skin. The true lice (or Anoplura) are found on the bodies of many Mammalia, and occasion by their presence intolerable irritation. The number of genera is few. Two species of Pediculus are found on the human body, and are known ordinarily as the head-louse (P. capitis) and the body-louse (P. vestimenti); P. capitis is found on the head, especially of children. The eggs, laid on the hairs, and known as “nits,” hatch in about eight days, and the lice are full grown in about a month. Such is their fecundity that it has been asserted that one female (probably of P. vestimenti) may in eight weeks produce five thousand descendants. Want of cleanliness favours their multiplication in a high degree—the idea once existed, and is probably still held by the very ignorant, that they are directly engendered from dirt. The irritation is caused by the rostrum of the insect being inserted into the skin, from which the blood is rapidly pumped up. A third human louse, known as the crab-louse (Phthirius pubis) is found amongst the hairs on other parts of the body, particularly those of the pubic region, but probably never on the head. The louse of monkeys is now generally considered as forming a separate genus (Pedicinus), but the greater part of those infesting domestic and wild quadrupeds are mostly grouped in the large genus Haematopinus, and very rarely is the same species found on different kinds of animals.

The bird-lice (Mallophaga) are far more numerous in species, although the number of genera is comparatively small. With the exception of the genus Trichodectes, the various species of which are found on mammalia, all infest birds (as their English names implies) (see Bird-Louse). Louse-infestation is known as phthiriasis in medical and veterinary terminology.

Authorities.—The following works are the most important: Denny, Monographia Anoplurorum Britanniae (London, 1843); Giebel, Insecta Epizoa (which contains the working-up of Nitzsch’s posthumous materials; Leipzig, 1874); van Beneden, Animal Parasites (London, 1876); Piaget, Les Pédiculines (Leiden, 1880); Mégnin, Les Parasites et les maladies parasitaires (Paris, 1880); Neumann, Parasites and Parasitic Diseases of Domesticated Animals (1892); Osborn, Pediculi and Mallophaga affecting Man and the Lower Animals (Washington, 1891; U.S. Dept. Agr.); Enderlein, “Läuse-Studien,” Zool. Anz. xxviii. (1904).