Psocids are small insects with soft bodies, usually no more than a few millimetres long. They have longish antennae and chewing mouthparts. Psocoptera got their common name of book lice because they sometimes live in the bindings of books and superficially they resenble lice, They are in their own order though and are only distantly related to the chewing lice order, Mallophaga. There are about 1700 species world-wide, grouped in 21 families and none are parasitic.
Most psocids live on lichens, in foliage and under bark and leaf litter. The common name of these species is barklice. Some are winged while other species are wingless. They are saprophytes mostly, living on fungi, algae, plus plant and animal tissue fragments. Most prefer damp conditions and they cannot cope with drying out. Some species have adapted to humans and their surrounds. These so-called book lice can be found in stored foodstuffs such as flour products in houses, barns and grain stores. However, they are not considered to be economically important. These are usually the wingless species while the species found in forests have wings. In a few species only one sex has wings.
Many psocids are gregarious and live in large groups. They lay their eggs in silken webs which can be quite large and have been described as ' unsightly' so they must be quite messy. The silk is produced by labial silk glands. Their life cycle is simple and incomplete, with eggs hatching into young that basically resemble the adults. There is no metamorphosis. The males perform nuptial dances to attract the females and then transfer sperm in sperm packets. The females then lay the eggs singly, in clumps or in silken nests. The nymphs have fewer antennal segments than the adults and undeveloped wings. There are two to three generations produced each year and they overwinter as eggs.
Psocids can be fast runners which is just as well as they are preyed upon by spiders, ants and ant-lions. They can also be infected by fungi and parasitic nematodes.
Psocopterans are probably descended from early true bugs of the order Hemiptera. There are a few fossils which date from the late Cretaceous period, so they have been around for over a hundred million years.
References: O'Toole 1986. The Encyclopedia of Insects. George, Allen and Unwin.
Borror and Delong 1971. Introduction to the Study of Insects. Holt, Rinehart and Winston.
for pictures, check out http://www.ento.csiro.au/education/insects/psocoptera.html