Collections of the predominantly wingless urothripine Thysanoptera from leaf litter in Australia show little variation in external morphology within any one population, although the variation between populations is frequently very considerable. Morphological segregates in which winged individuals are known show little variation between populations, and these segregates can be regarded as distinct species. However, morphological segregates in which winged individuals are not known exist in the field as a complex of similar but distinctive local forms. Taxonomy is concerned with the interpretation in biological terms of morphological difference, but the significance of these distinctive populations cannot be determined at present, whether they are species, subspecies, or local variants. However, this high rate of incipient speciation of the Urothripini is evidently related, at least in part, to the low degree of dispersive activity of the wingless forms. The resulting complicated pattern of variation at the species level is reflected in the unsatisfactory generic classification, and several of the characters which have been used for distinguishing genera are here shown to be of little value. As a result four genera, Bournieria, Ramachandraiella, Transithrips, and Verrucothrips, are here placed in synonymy with Baenothrips, and a key is provided to the remaining 13 genera of Urothripini from the world. An account is given of the eight species in five genera known from Australia, including two new species: Baenothrips caenosus (Stannard), comb. nov. ; Baenothrips moundi (Stannard), comb. nov. ; Biconothrips reedi Stannard; Habrothrips curiosus Ananthakrishnan; Octurothrips pulcher Priesner; Stephanothrips occidentalis Hood & Williams; S. barretti, sp, nov.; S. ferrari, sp. nov. In addition, knechteli Priesner is transferred to Amphibolothrips, comb. nov., and erythrinus Pelikan is transferred to Baenothrips, comb. nov.

The available biological information on the association, unique to Australia, between gall-inducing thrips and the phyllodes of Acacia species is summarised. Identification keys to the three genera and 21 gall-inducing species involved are presented, together with descriptions of nine new species (Kladothrips harpophyllae, Kladothrips maslini, Kladothrips xiphius, Oncothrips morrisi, Oncothrips schwarzi, Oncothrips sterni, Oncothrips torus, Onychothrips pilbara and Onychothrips zygus).

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Twelve species with unusually long, sometimes highly convoluted, maxillary stylets are discussed under four genera. Ten of these species have been collected from Casuarina trees. It is suggested that the elongate stylets are adapted to feeding on the chlorophyllous tissue of Casuavina trees, which lies deep in longitudinal furrows of the stem protected by sclerenchyma. Keys are given to the Phlaeothripine genera and species from Casuarina trees. The systematic changes include four new synonyms, five new combinations, six new species, and three new genera; notes are also given on the genera Pygmaeothrips and Teuchothrips which are affected by these changes.

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