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Evolution and systematics
Phylogenetically, the order Psocoptera (psocids, barklice, and booklice) is closely related to the Phthiraptera (lice), Thysanoptera (thrips), and Hemiptera (bugs, cicadas, aphids, etc). These four orders compose a monophyletic group, the Paraneoptera (hemipteroid insects), and psocids retain the most primitive features in the group. Within the Paraneoptera, psocids are most closely related to lice, with which they compose a monophyletic taxon, Psocodea. Furthermore, the family Liposcelididae (booklice) of the Psocoptera is regarded as the sister group of the Phthiraptera. Therefore, the order Psocoptera is probably a paraphyletic taxon. The Psocoptera has been divided into three suborders, Trogiomorpha, Troctomorpha, and Psocomorpha. The Trogiomorpha consists of two infraorders and five families. The Troctomorpha, in which the Liposcelididae is classified, consists of two infraorders and eight families. The Psocomorpha, the largest suborder in the Psocoptera, consists of four or six infraorders and 23 or 24 families. Monophyly of suborders other than the Troctomorpha is well supported by morphology and DNA data. The oldest fossil psocid is recorded from the lower Permian deposits in Kansas (about 290 million years ago). Fossil psocopterans in the Permian to Cretaceous periods represent extinct families. In contrast, fossil psocopterans in the Oligocene to Holocene periods are all assigned to extant families.
Psocids range from small (about 0.04 in/1 mm) to medium (about 0.4 in/10 mm) in size. Most of them are brownish or whitish with blackish brown markings, but some species of Caeciliusidae, Amphipsocidae, and Stenopsocidae have very colorful bodies and wings with markings. Morphologically, psocids are characterized by a well-developed postclypeus, long antennae, pick-like laciniae, a reduced prothorax, a welldeveloped pterothorax, and membranous wings (when present) held rooflike over the abdomen. The biting-type mouthparts with pick-like lacinia in psocids are considered to represent the intermediate condition between initial biting mouthparts in orthopteroids and the piercing and sucking type mouthparts in thrips and bugs, and this characteristic strongly supports the monophyly of the Paraneoptera. Psocids and lice share the specialized hypopharynx to uptake water vapor from the air, which supports monophyly of the Psocodea (psocids and lice). Most psocids are macropterous (fully winged), but brachypterous (short winged) or apterous (without wings) forms are also known.
Psocids have been recorded from all zoogeographical regions, from tropical to subarctic zones.
Psocids are found on a wide range of terrestrial habitat, such as on dead or living leaves, on stone or bark surfaces, and in leaf litter. Cave dwellers (such as Prionoglaris stygia) and wood borers (Psilopsocus mimulus) are also known. Some psocids are frequently discovered from food storage or other domestic environments.
Most psocids are solitary, but the aggregation of larvae (e.g., Sigmatoneura, Psococerastis, and Metylophorus of Psocidae) or colony-forming psocids (including subsociality of Archipsocus) are also known. Some psocids weave silk nests of various shapes and sizes, and from one to numerous individuals may live under it. Sound production in psocids is widely known. The sound is generally considered to be a courtship song, but this behavior is very poorly understood.
Feeding ecology and diet
Psocids feed chiefly on lichen, fungi, or organic debris such as flour and scurf (skin flakes).
Most psocids are bisexual, but parthenogenesis (thelytoky) is also known in 12 families of all three suborders. Courtship involves various behavioral patterns. Eggs are laid singly or in groups, and may be bare, covered with silk webbing, or encrusted with fecal material. A few psocids are viviparous (such as Archipsocopsis and Phallocaecilius species). Larvae hatch from eggs using a specialized egg-burster. The usual number of instars is six, but this is sometimes reduced to five, four, or rarely three. Psocids have no pupal stage.
No species of psocids are included on the IUCN Red List. Most leafand bark-dwelling psocids are common and seem to endure human influence. However, some psocids are clearly sensitive to human impact. For example, many psocids are endemic to a single small island; some cave-dwelling psocids are known only from a single cave; all species of the genus Psilopsocus are known to be rare, possibly because of their specialized wood-boring behavior.
Significance to humans
Most psocids live in the wild and are thus harmless to humans. Some domestic psocids, such as species of Liposcelis, are common household insects. Under warm, humid conditions, they can reproduce very rapidly, becoming serious pantry pests. Psocids also occur in stored food, and at high densities can contaminate foodstuffs. Domiciliary psocids are known to cause allergic responses in sensitized people.