The Booklice and Barklice
The order Psocoptera is comprised of the booklice and barklice. They are not true lice, but resemble them superficially. The ordinal name Psocoptera is derived from the Greek words, “psoco” (rub small); and “ptera” (wing). This references the tented appearance of the wings when held at rest, on those species that have them that is. These "lice" are tiny scavengers, thriving in microhabitats at the size level of mold. They are so small (1-10mm /0.04-0.4") that they feed on pollen, spores, single-celled algae, and lichens, as well as all manner of plant and insect tissue fragments. They evolved from the early Hemiptera, with the earliest known fossil Psocoptera occurring in the Permian period (225-280 million years ago). Psocopterans, perhaps, later gave rise to the true parasitic lice. The booklice are the smallest of the Psocopterans. Capable of eking out a living almost anywhere, they can be found inside and around human dwellings where they feed on the detritus of humanity. Their diet varies, from pollen to skin cells, with some species fond of paper and flour products, giving rise to the common name "booklice". They seldom occur in large numbers but outbreaks can occur in poorly maintained or undisturbed stored products (like those encyclopedias rotting in your basement/garage/attic). The barklice are the outdoor equivalent of booklice. Although larger than booklice, and often winged, barklice are far less noticeable because they live on the bark and foliage of trees and shrubs, as well as beneath stones and ground litter. Some live communally, but due to their tiny size they generally go unnoticed, as they cause no harm. There are about 2,000 known species of Psocopterans.