Psocoptera

(Book lice)

Class Insecta

Order Psocoptera

Number of families 36 or 37

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.

Physical characteristics

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 well-developed 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.

Distribution

Psocids have been recorded from all zoogeographical regions, from tropical to subarctic zones.

Habitat

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.

Behavior

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).

Reproductive biology

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.

Conservation status

No species of psocids are included on the IUCN Red List. Most leaf- and 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.

Species accounts

List of Species

No common name

Psoquilla marginepunctata

family

Psoquillidae

taxonomy

Psoquilla marginepunctata Hagen, 1865, Germany.

other common names

None known.

physical characteristics

Small (about 0.04 in/1 mm long). Body and wings black in basal color, with white distinctive markings. Wing polymorphism is known.

distribution

Primarily wet tropics. Introduced by humans to some countries in the temperate zone, including Europe, the United States, and Japan.

habitat

Primarily under bark or bird nests; also found in greenhouse and domestic environments.

behavior

Crowding provides increased numbers of macropterous forms and females.

feeding ecology and diet

Feeds on fungi and organic debris.

reproductive biology

Bisexual. Egg laid singly and covered by debris. Six larval instars.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Liposcelis bostrychophila

family

Liposcelididae

taxonomy

Liposcelis bostrychophila Badonnel, 1931, Mozambique.

other common names

None known.

physical characteristics

Small, about 0.04 in (1 mm) long; flattened dorsoventrally, pale brown in color, apterous.

distribution

Worldwide.

habitat

Common in stored food found in domestic environments; also found in the wild in bird nests, etc.

behavior

Variations in color, size, egg production, and tolerance to pyrethroid insecticides are known among different populations.

feeding ecology and diet

Feeds on fungi and organic debris.

reproductive biology

Parthenogenesis (thelytoky) is caused by Wolbachia infection. Eggs laid singly or in small batches and covered with powdery dusts. Four larval instars.

conservation status

Not listed by the IUCN.

significance to humans

Primarily nuisance and stored food pest.

No common name

Mesopsocus unipunctatus

family

Mesopsocidae

taxonomy

Hemerobius unipunctatus Müller, 1764, Europe.

other common names

None known.

physical characteristics

Relatively large psocid, about 0.2 in (5 mm) long. Sexual dimorphism present: male macropterous, female micropterous. Dimorphic in body color, light and melanic morphs.

distribution

Holarctic, north to Alaska.

habitat

Found in tree bark.

behavior

Industrial melanism is known.

feeding ecology and diet

Feeds on lichens.

reproductive biology

Bisexual. Eggs laid in groups, covered with incrustation and silk webs. Six larval instars.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Lienhard, C. "Psocopteres Euro-Mediterraneens." In Faune de France No. 83. Paris: Fédération Française des Sociétés de Sciences Naturelles, 1998.

Lienhard, C., and C. N. Smithers. "Psocoptera: World Catalogue and Bibliography." In Instrumenta Biodiversitatis, Vol. 5. Geneva: Museum d'histoire naturelle, 2002.

Mockford, E. L. "North American Psocoptera." In Flora & Fauna Handbook, No. 10. Gainesville, FL: Sandhill Crane Press, 1993.

Periodicals

New, T. R. "Biology of the Psocoptera." Oriental Insects 21 (1987): 1–109.

Smithers, C. N. "Keys to the Families and Genera of Psocoptera (Arthropoda: Insecta)." Technical Reports of the Australian Museum no. 2 (1990): 1–82.

Yoshizawa, K. "Phylogeny and Higher Classification of Suborder Psocomorpha (Insecta: Psocodea: "Psocoptera")." Zoological Journal of the Linnean Society 136 (2002): 371–400.

Other

"Psocid as Pests" [March 31, 2003]. <http://www.kcl.ac.uk/ip/bryanturner/other/index-psocids.html>.

"PsocoNet" [March 31, 2003]. <http://insect3.agr.hokudai.ac.jp/psoco-web/psoc.html>.

Kazunori Yoshizawa, PhD