(also Psocoptera), an order of small insects (1–5 mm) marked by incomplete metamorphosis. The antennae are filamentous. The mouthparts are intermediate between chewing and sucking types. The lacinia of the lower jaw is separated to form a long, hard rod that can move vertically like the spiny bristles in the proboscis of true bugs (Hemiptera). The legs are cursorial, and the tarsi are two- or three-jointed. The membranous wings are colorless or have dark spots and bands; the forewings are longer than the hind wings. At rest the wings fold in such a way as to resemble a roof. Sometimes the wings are reduced or absent. The abdomen is short and, in females, has a small ovipositor.
There are more than 1,500 species, distributed throughout the world. The insects are especially widespread in the tropics and subtropics. There are about 60 species in the USSR. The insects are found among plants and plant remains, on soil, under rocks, in human dwellings and in the nests of birds and other insects. The eggs are often covered with a web, secreted by the silk glands, which open into the oral cavity. The insects feed on lichens, fungi, or organic remains. The book louse (Troctes divinatorius) and Trogium pulsatorium often damage books, herbarium plants, and insect collections. Copeognatha are known from the Tertiary; fossils have been found in Eocene amber.