Booklouse, common name for any of an order of small, delicate insects known for feeding on dead plant and animal matter or starchy substances such as bookbinding paste. Booklice are also called barklice or psocids. The Latin name of their order, Psocoptera, means "small rub" (psoco) and "winged" (-ptera), terms that refer to the abrasionlike damage caused by their feeding and the fact that some have wings. They vaguely resemble true lice but do not feed on other live animals. Booklice are found throughout the world but are most diverse in tropical regions. About 2600 species are known worldwide, and 245 species are found in North America.

Booklice are soft-bodied insects less than 6 mm (0.25 in) long; most are minute, only 1 to 2 mm (0.4 to 0.8 in) in length. They are usually gray or yellowish. Adults have wings in some species and are wingless in others. In some species, both adult forms occur. Psocids are sometimes confused with similar-looking psyllids. However, psocids have chewing mouthparts and commonly run or fly away when disturbed; psyllids, like aphids, suck plant juices and jump when alarmed. When viewed through a hand lens, booklice can be recognized by the distinctly swollen, bulbous area at the front of their face between the two widely spaced antennae.

Females lay pale, oval eggs singly or in clusters, sometimes covered with silk webbing or bits of soil or leaves. Booklice have incomplete metamorphosis. Most species develop through six nymphal instars, or growth stages, as they gradually change from egg to nymph (an immature form with some adult characteristics) and then to adult. Some species live in colonies under protective silken webs. These webs resemble sheetlike spiderwebs and may cover rough tree bark. Booklice also live on leaves, rocks, and in soil. Entomologists collect psocids by sifting soil debris, by sweeping trees and shrubs with a net, or by shaking branches to dislodge them onto a sheet of white paper or other collecting surface held underneath the plant.

Most booklice are of relatively little economic importance, as they feed only on pollen; various fungi, such as sooty mold; and decaying plant or animal tissue. The cereal psocid is an occasional pest. It feeds on cereal, wallpaper, bookbindings, and collections of dead insects, especially if these materials are slightly damp. The cereal psocid looks like a minute brown speck, less than 1 mm (0.04 in) long.

Scientific classification: Booklice make up the order Psocoptera, which is divided into several families. The cereal psocid belongs to the family Liposcelidae and is classified as Liposcelis divinatorius.

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