Psocodea

Psocodea is a taxonomic group of insects comprising the bark lice, book lice and parasitic lice. It was formerly considered a superorder, but is now generally considered by entomologists as an order. Despite the greatly differing appearance of parasitic lice, they are believed to have evolved from within the former order Psocoptera, which contained the bark lice and book lice, now found to be paraphyletic.
Psocodea contains around 11,000 species, divided among three suborders and more than 70 families. They range in size from 1–10 millimetres 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 on trees, feeding on algae and lichen.

External phylogeny

Psocodea has been recovered as a monophyletic group in recent studies. Their next closest relatives are traditionally recognized as the monophyletic grouping Condylognatha that contains Hemiptera and Thysanoptera, which all combined form the group Paraneoptera. However, this is somewhat unclear, as analysis has shown that Psocodea could instead be the sister taxon to Holometabola, which would render Paraneoptera as paraphyletic.
Here is a simple cladogram showing the traditional relationships with a monophyletic Paraneoptera:
Here is an alternative cladogram showing Paraneoptera as paraphyletic, with Psocodea as sister taxon to Holometabola:

Internal phylogeny

Here is a cladogram showing the relationships within Psocodea:

Classification

The order Psocodea is divided into three extant suborders.

Suborder Trogiomorpha

have antennae with many segments and always three-segmented tarsi.
Trogiomorpha is the smallest suborder of the Psocoptera sensu stricto, with about 340 species in 7 families, ranging from the fossil family Archaeatropidae with only a handful of species to the speciose Lepidopsocidae.
Trogiomorpha comprises infraorder Atropetae and infraorder Psocathropetae.

Suborder Troctomorpha

have antennae with 15–17 segments and two-segmented tarsi.
Troctomorpha comprises the Infraorder Amphientometae and Infraorder Nanopsocetae. Troctomorpha are now known to also contain the order Phthiraptera, and are therefore paraphyletic, as are Psocoptera as a whole.
Some Troctomorpha, such as Liposcelis, are often found in birds' nests, and it is possible that a similar behavior in the ancestors of lice is at the origin of the parasitism seen today.

Suborder Psocomorpha

are notable for having antennae with 13 segments. They have two- or three-segmented tarsi, this condition being constant or variable within families. Their wing venation is variable, the most common type being that found in the genus Caecilius. Additional veins are found in some families and genera
Psocomorpha is the largest suborder of the Psocoptera sensu stricto, with about 3,600 species in 24 families, ranging from the species-poor Bryopsocidae to the speciose Psocidae.
Psocomorpha comprises Infraorder Epipsocetae, Infraorder Caeciliusetae, Infraorder Homilopsocidea and Infraorder Psocetae.