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. Their name originates from the Greek word ψῶχος, psokhos meaning gnawed or rubbed and πτερά, ptera meaning wings. There are more than 5,500 species in 41 families in three suborders. Many of these species have only been described in recent years.
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.
In the 2000s, morphological and molecular evidence 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.
Psocids are small, scavenging insects with a relatively generalized body plan. They feed primarily on fungi, algae, lichen, and organic detritus in nature but are also known to feed on starch-based household items like grains, wallpaper glue and book bindings. They have chewing mandibles, and the central lobe of the maxilla is modified into a slender rod. This rod is used to brace the insect while it scrapes up detritus with its mandibles. They also have a swollen forehead, large compound eyes, and three ocelli. Their bodies are soft with a segmented abdomen. Some species can spin silk from glands in their mouth. They may festoon large sections of trunk and branches in dense swathes of silk.
Some psocids have forewings that are up to 1.5 times as long as the hindwings, and all four wings have a relatively simple venation pattern, with few cross-veins. The wings, if present, are held tent-like over the body. The legs are slender and adapted for walking, rather than gripping, as in the true lice. The abdomen has nine segments, and no cerci.
There is often considerable variation in the appearance of individuals within the same species. Many have no wings or ocelli, and may have a different shape to the thorax. Other, more subtle, variations are also known, such as changes to the development of the setae. The significance of such changes is uncertain, but their function appears to be different from similar variations in, for example, aphids. Like aphids, however, many psocids are parthenogenic, and the presence of males may even vary between different races of the same species.
Psocids lay their eggs in minute crevices or on foliage, although a few species are known to be viviparous. The young are born as miniature, wingless versions of the adult. These nymphs typically molt six times before reaching full adulthood. The total lifespan of a psocid is rarely more than a few months.
Booklice range from approximately 1mm to 2mm in length (1/25" to 1/13"). Some species are wingless and they are easily mistaken for bedbug nymphs and vice versa. Booklouse eggs take 2 to 4 weeks to hatch and can reach adulthood approximately 2 months later. Adult booklice can live for six months. Besides damaging books, they also sometimes infest food storage areas, where they feed on dry, starchy materials. Although some psocids feed on starchy household products, the majority of psocids are woodland insects with little to no contact with humans, therefore they are of little economic importance. They are scavengers and do not bite humans.
Psocids can affect the ecosystems in which they reside. Many psocids can affect decomposition by feeding on detritus, especially in environments with lower densities of predacious micro arthropods that may eat psocids. The nymph of a psocid species, Psilopsocus mimulus, is the first known wood-boring psocopteran. These nymphs make their own burrows in woody material, rather than inhabiting vacated, existing burrows. This boring activity can create habitats that other organisms may use.
Some species of psocids, such as Liposcelis bostrychophila, are common pests of stored products. Psocids, among other arthropods, have been studied to develop new pest control techniques in food manufacturing. One study found that modified atmospheres during packing (MAP) help control the reoccurrence of pests during the manufacturing process as well as a way to prevent further infestation in the final products that go to consumers.
The Order Psocoptera is divided into three suborders.
Trogiomorpha have antennae with many segments (22–50 antennomeres) and always three-segmented tarsi.
Trogiomorpha is the smallest suborder of the Psocoptera sensu stricto (i.e. excluding Phthiraptera), with about 340 species in 7 families, ranging from the monospecific fossil family Archaeotropidae to the speciose Lepidopsocidae (over 200 species). Trogiomorpha comprises infraorder Atropetae (extant families Lepidopsocidae, Psoquillidae and Trogiidae, and fossil families Archaeotropidae and Empheriidae) and infraorder Psocathropetae (families Psyllipsocidae and Prionoglarididae).
Troctomorpha have antennae with 15–17 segments and two-segmented tarsi.
Troctomorpha comprises the Infraorder Amphientometae (families Amphientomidae, Compsocidae, Electrentomidae, Musapsocidae, Protroctopsocidae and Troctopsocidae) and Infraorder Nanopsocetae (families Liposcelididae, Pachytroctidae and Sphaeropsocidae). Troctomorpha are now known to also contain the order Phthiraptera (lice), and are therefore paraphyletic, as are Psocoptera as a whole.
Some Troctomorpha, such as Liposcelis (which are similar to lice in morphology), 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.
Psocomorpha are notable for having antennae with 13 segments. They have two- or three-segmented tarsi, this condition being constant (e.g. Psocidae) or variable (e.g. Pseudocaeciliidae) within families. Their wing venation is variable, the most common type being that found in the genus Caecilius (rounded, free areola postica, thickened, free pterostigma, r+s two-branched, m three-branched). Additional veins are found in some families and genera (Dicropsocus and Goja in Epipsocidae, many Calopsocidae, etc.)
Psocomorpha is the largest suborder of the Psocoptera sensu stricto (i.e. excluding Phthiraptera), with about 3,600 species in 24 families, ranging from the species-poor Bryopsocidae (2 spp.) to the speciose Psocidae (about 900 spp). Psocomorpha comprises Infraorder Epipsocetae (families Cladiopsocidae, Dolabellopsocidae, Epipsocidae, Neurostigmatidae and Ptiloneuridae), Infraorder Caeciliusetae (families Amphipsocidae, Asiopsocidae, Caeciliusidae, Dasydemellidae and Stenopsocidae), Infraorder Homilopsocidea (families Archipsocidae, Bryopsocidae, Calopsocidae, Ectopsocidae, Elipsocidae, Lachesillidae, Mesopsocidae, Peripsocidae, Philotarsidae, Pseudocaeciliidae and Trichopsocidae) and Infraorder Psocetae (families Hemipsocidae, Myopsocidae, Psilopsocidae and Psocidae).