Psocoptera (Psocids, Booklice) are found in all regions. They range from less than 1 to almost 10 mm in length and have a characteristic appearance due mainly to their having a round, mobile head, long antennae, enlarged pterothorax and, in many species, the wings held roofwise over the abdomen. Most species are winged as adults but alary polymorphism occurs and brachyptery and aptery in one or both sexes is common. The hypopharynx is of peculiar form, modified as part of an atmospheric water-vapour uptake system (Rudolf 1982).
Their relationships are not clear: their nearest living relatives appear to be the Phthiraptera but fossil evidence to link them is lacking. Psocoptera would seem to have been derived from primitive hemipteroid stock. Lyal (1985) has made the interesting suggestion that the family Liposcelidae may represent the sister group to the Phthiraptera in which case the Psocoptera as presently understood is a paraphyletic group. The extent to which the hypothesis rests on convergence of characters requires further investigation. The phylogenetic classification suggested by Smithers (1972) needs some modification in the light of more recent discoveries. Until these problems are resolved it is suggested that the classification currently in general use be retained for practical purposes.
Publications on the order to 1964 have been listed by Smithers (1965a) and a list of described species to 1965 has been published (Smithers 1967). General accounts of the order can be found in Badonnel (1951), Weidner (1972), Smithers (1972), Gunther (1974) and New (1974). Study techniques are described by Smithers (1978b).
Small, free-living, exopterygote Neoptera, with large, mobile head, filiform antennae and bulbous postclypeus; mandibles asymmetrical; maxillae with rod-shaped lacinia; labial palpi reduced; wings membranous, usually held roofwise over abdomen, venation reduced, brachyptery and aptery frequent; tarsi 2- or 3-segmented in adults, 2-segmented in nymphs; cerci absent.
All families that are found in Australia are also found elsewhere. Any remarks on the faunal relationships of the Australian species must be somewhat tentative as the fauna is not yet adequately known and those of adjacent territories to the north need further study.
Psocids are easily transported by air currents and have been found in samples of drifting insect populations of the upper air; some species which are common in stored products have been widely distributed by man.
There are, however, certain recognisable elements in the Australian fauna. There are the cosmopolitan species, some of which are associated with man and his products (e.g.
) whereas others are naturally widespread.
A second, typically Gondwanan, element consists of species having relationships to species of other Southern Hemisphere areas and includes the genus
, with a species in Tasmania and South Australia, a species in the Argentina, five in Chile and one in Angola. A related species has been found in Baltic amber.
Some of the northern Australian species of Psocoptera represent a more or less tropical, world-encircling element (e.g. Archipsocidae) whereas others are more definitely related to Papuan forms (e.g.
Finally, there is an element consisting of widespread, endemic, Australian species belonging to groups represented beyond Australia. Thornton (1985) has provided a useful review of the biogeography and ecology of arboreal species. There are only a few groups of Psocoptera the distribution of which might be related to plate tectonic events.