Psocid (order Psocoptera), any of a group of about 5,000 species of soft-bodied insects, usually less than 5 mm (0.2 inch) long. Its slender antennae are at least as long as its body, and wing venation is simple, with no crossveins. Mouthparts are adapted for chewing, with the upper jaw usually elongated and chisel-like. Psocids eat fungi (including molds), cereals, pollen, and organic debris.
The best-known species, the booklouse, is a pale, wingless insect usually found indoors among old books and papers, on dusty shelves, or in cereals.
The majority of psocids, usually called barklice, generally have four membranous wings that are held rooflike over the body when at rest. They are found on tree bark and foliage, under stones, or in ground litter.