Anoplura

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A small group of insects usually considered to constitute an order. They are commonly known as the sucking lice. All are parasites living in the covering hair of mammals. About 250 species are now known, and these comprise probably about half of the species in the world, but the group is known in detail to few persons.

These lice are distinguishable from the Mallophaga by their mouthparts and manner of feeding. The mouthparts consist of three very slender stylets which form a tube. When at rest, they are retracted in a pocket which lies just behind the mouth. The antennae are usually five-segmented, rarely three-segmented. The thoracic segments are always very closely fused and lack wings. The claw is one-segmented, and on at least one pair of legs this claw is enlarged and can be folded into a process from the tibia to grasp a hair of the host. In some species two and in others all of the legs are thus modified. The ovipositor is very much reduced. It consists of but little more than two flaps which are able to close around a hair. Eyes are commonly lacking, and in those species in which they do occur, they are reduced to a pair of simple lenses or two light-receptive spots. All the species are quite small; the largest scarcely exceeds 0.2 in. (5 mm) in length and the smallest does not attain 0.04 in. (1 mm).

Feeding is accomplished by thrusting forth the tube formed by the stylets, piercing the skin of the host, and sucking blood by a pump in the throat of the insect. This habit of sucking blood gives the Anoplura a special importance. They take up any disease-producing organisms in the host's blood and may transfer these organisms to another individual. Thus, they transfer the organisms which cause epidemic typhus, relapsing fever, and trench fever. The transfer of a louse, from one host to another, can occur only when the hosts are in close bodily contact, as in the nest or at the time of mating. In the case of humans, the lice may become detached and enter the clothing or the bedding. In times of social disturbance, such as war or when people are crowded together as formerly occurred on ships, or in jails or slums, the opportunities for an exchange of these parasites are enormously increased. The close restriction of each species of lice to its special host is sufficient to account for the fact that diseases are not transferred from one species of mammal to another.

There are two species of lice which occur as common ectoparasites upon humans. One of them is Pediculus humanus, the head and body louse which transmits the diseases mentioned above (see illustration). The other species is Phthirius pubis, known as the crab louse. It transmits no known disease. The lice of the Old World monkeys are referred to another genus, Pedicinus. See also Insecta; Mallophaga.

ventral views.">
The louse Pediculus humanus, female, shown in (a) dorsal and (b) ventral views.

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