Psocoptera are an order of insects that are commonly known as booklice, barklice or barkflies. They first appeared in the Permian period, 295–248 million years ago. They are often regarded as the most primitive of the hemipteroids (O'Toole 2002). Their name originates from the Greek word psokos meaning gnawed or rubbed and ptera meaning wings (Meyer 2005). There are more than 5,500 species in 41 families in three suborders. Many of these species have only been described in recent years (García Aldrete 2006).
They range in size from 1–10 millimeters (0.04–0.4 in) in length.
The species known as booklice received their common name because they are commonly found amongst old books—they feed upon the paste used in binding. The barklice are found harmlessly on trees, feeding on algae and lichen. No member of this order is currently considered endangered; in fact, in 2007, Atlantopsocus adustus, a species native to Madeira and the Canary Islands, was found to have colonized the mild Cornish coast of southwest England (BBC News 2007).
Recent mophological (Yoshizawa & Johnson 2006) and molecular evidence (Johnson et al. 2004) has shown that the parasitic lice (Phthiraptera) evolved from within the psocopteran suborder Troctomorpha. In modern systematics, Psocoptera and Phthiraptera are therefore treated together in the order Psocodea (Bess et al. 2006).