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Booklice and Barklice (Order: Psocoptera)

A photograph of an adult Barklouse (_Graphopsocus cruciatus_)

Adult barklice (like this Graphopsocus cruciatus) are often winged.
Photograph by Jean-Jacques Milan, used under GFDL.

Although they are called lice, the Psocoptera are free-living insects, not parasites. The scientific name comes from the Greek psocus (to grind) and pteron (wing) and refers to the psocopteran jaws, which are shaped to grind food, rather like a pestle and mortar.

These insects can be conveniently, if somewhat arbitrarily, discussed in two groups, barklice and booklice, based on whether they tend to live outdoors or in human habitations. Most of them live outdoors.

Barklice are usually found in moist places, such as leaf litter, beneath stones, on vegetation or under tree bark. They have long antennae, broad heads and 'bulging' eyes. They feed on algae, lichens, fungi and various plant products, such as pollen.

Barklice may grow up to 10mm in length, though they're usually less than 6mm, and the adults are often winged. The wings are held roof-like over their bodies. Winged barklice look a little like aphids but their long antennae, broad heads and biting jaws show that they are not.

Some species are gregarious, living in small colonies beneath a gossamer blanket spun with silk from labial glands in their mouth. Interestingly, both adults and larvae can spin silk from these glands. Sometimes the colonies seem to move in coordinated fashion, rather like sheep.

Booklice are wingless and are much smaller than barklice (less than 2 mm). They are most commonly found in human dwellings and warehouses. Many of them are in the family Trogiidae. Most booklice species feed on stored grain, book bindings, wallpaper paste and other starchy products, and on the minute traces of mould found in old books. They can sometimes cause damage to stored museum collections of insects and plants. A few, such as Liposcelis decolor, can be a pest in grain stores.

Indoor living Psocoptera have poor eyesight, and some of them seem to communicate using sound. Such species tap with the end of their abdomens and produce a faint ticking noise, so they have been called 'ticking spiders'. Liposcelis divinatorius is one example of a 'ticking spider' - although of course it is an insect, not a spider!

A few Psocoptera live in bird nests. Those that do this feed on the remains of feathers or on skin cells, never on the birds themselves.

Classification

Psocoptera have incomplete metamorphosis (i.e. they have eggs, nymphs and adults) and the order is divided into three suborders - Trogiomorpha, Troctomorpha, and Psocomorpha. These are distinguished by the number of segments in the antennae, tarsi, and labial palps.

Psocoptera are regarded as the most primitive amongst the hemipteroids (true bugs, thrips and lice) because their mouthparts show the least modification from those of the earliest known fossils.

Phthiraptera (parasitic lice) may have evolved directly from commensal barklice, and a close relationship between barklice and parasitic lice is also supported by similarities in the structure of their mouthparts.

There are over 1,000 species of Psocoptera in Europe, and around 100 in the British Isles. The World Catalogue published by the Geneva Natural History Museum lists 41 families, 371 genera and 4,408 species as having been described worldwide, up to the end of the year 2000.

How to find them

Outdoor species are easy to find using sweep nets and beating trays, or by looking under bark or in old birds' nests. Indoor species can be found by looking in musty-smelling, neglected corners - including, perhaps, your prized insect collection!

Related links: Booklice and Barklice (Order: Psocoptera)

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