Adults and larvae of Phlaeothripinae have the maxillary stylets slender, that is, the width of these stylets in their distal half is between 3 and 5 microns in diameter. It is essential to distinguish between the “maxillary stylets” and the closely associated “maxillary guides”. In some phlaeothripine species, the maxillary guides are much more robust in appearance than the stylets, with the result that the latter are easily overlooked and the maxillary guides misinterpreted as stylets. If the maxillary stylets have become dissociated and moved during slide-preparation, they are then particularly difficult to interpret. The other differences between members of the two subfamilies do not fully discriminate between these two groups. Males of Idolothripinae never have a pore plate on sternite VIII, whereas a pore plate is developed on this sternite in many (but not all) species of Phlaeothripinae. Moreover, setae S2 on tergite IX are shorter and stouter than setae S1 in most species of Phlaeothripinae, whereas these two pairs of setae are equal in size in species of Idolothripinae.
The supra-generic classification of the Phlaeothripinae is not satisfatory. One proposal involved the segregation of nine groups, each involving between one and 30 species, as distinct families (Bhatti, 1994, 1998). However, no satisfactory relationship has been proposed between these “families” and the 3000 species that remained in the Phlaeothripinae. These families thus do not seem to serve any useful systematic purpose (Mound & Morris, 2007). The two largest of these groups at first sight appear to be distinctive; one comprises a series of related species that have an elongate tenth tergite such as Stephanothrips and Urothrips, the other comprises a series of laterally compressed species including Hyidiothrips. However, the character states on which these groups are defined recur in other species, and neither group seems suitable for recognition at family level.
Within the Phlaeothripinae three “lineages” have been recognised (Mound & Marullo, 1996). The “Liothrips lineage” comprises most of the leaf-feeding species, and these usually have only one sensorium on the third antennal segment, and three sensoria on the fourth. The “Phlaeothrips lineage” is another large group, but these are commonly associated with dead leaves and dead branches and they feed on fungal hyphae; antennal segments three and four commonly have three and four sensoria respectively. In neither of these two major groups do adults have a pair of basantra (= praepectal plates) on the prosternum. The third major group in this subfamily is now recognised as the Tribe Haplothripini (Mound & Minaei, 2007); these species always have prosternal basantra, and the forewings are slightly constricted medially, but the number of antennal sensoria is variable.
Some groups of Phlaeothripinae, such as members of the large genus Haplothripsthat live in the florets of Asteraceae, are remarkably constant in general body form. Similarly constant in body form, including both sexes, are the leaf-feeding thrips of the most species-rich genus, Liothrips. In contrast, fungus-feeding phlaeothripine species are highly variable in structure, both between and within species. In many of them, large males look very different from small males and females, and this structural variation causes taxonomic problems. Only a few leaf-feeding species are particularly variable in body structure between large and small individuals, such as domicile creating species of the Australian genus Lichanothrips (Crespi et al. 2004). The last abdominal segment, the tube, varies in shape between genera, such that it is in the form of a moderately long simple tube in Haplothrips species, but almost as long as the rest of the abdomen in Leeuwenia species, and about as wide as long in some Dunatothrips species. Despite the implication of the name of the Order, Thysanoptera, adults of many thrips species are always wingless, and many others are wing polymorphic in one or both sexes.
Phlaeothripinae exhibit a wide range of biologies. Species of Haplothrips usually feed and breed within the florets of Asteraceae flowers, although some breed in grass florets (Mound & Minae, 2007). Despite this, lower-living is relatively rare amongst Phlaeothripidae, although one species of Haplothripini is known as the specific pollinator of Macaranga species in Southeast Asia (Moog et al., 2002). Species of Liothrips and Gynaikothrips, also many related genera, breed on leaves, and some of these induce galls (Mound, 2008). Gall-induction also occurs among species of the Neotropical genus Holopothrips (Cavalleri & Kaminski, 2007), and also the Australian genus Kladothrips, with some species in the latter genus exhibiting caste-determined eusocial behaviour (Crespi et al, 2004).
A few species of Phlaeothripinae, such as some species of Karnyothrips, are known to be predators, either on mites or the sedentary stages of coccoids and Aleyrodidae, or more rarely on small larvae of some Lepidoptera (Palmer & Mound, 1991). Species of one small genus, Lissothrips, are associated with mosses but possibly feed on small algae that grow at the base of the mosses. However, at least 40% of Phlaeothripinae species feed on fungal hyphae, and can be found living in leaf-litter, on dead branches and twigs, and on dead leaves. Some of these species exhibit similar patterns of structural variation to those of some Idolothripinae, with large males looking very different from small males and females of the same species. For example, complex variation of this sort resulted in one species, Ecacanthothrips tibialis, being described under at least 16 different names in three genera. In contrast, many other fungus-feeding species are effectively monomorphic, particularly species that live in leaf-litter, and some of these have unusually long major setae.