Both sexes fully winged. Body and legs dark brown, tarsi slightly paler, antennal segment III with pedicel yellow; major setae light brown, tergite IX setae paler; fore wing deeply shaded but slightly paler in distal area. Head longer than wide with eyes not protruding; maxillary stylets retracted to eyes, close together medially; post ocular setae shorter than dorsal length of eyes, with apices rounded; mouth cone long and pointed, extending beyond fore coxae. Antennae 8-segmented; segment III with one sensorium, IV with three sensoria; VIII short and broad at base. Pronotum with five pairs of major setae with rounded or weakly capitate apices; epimeral sutures complete; prosternal basantra not developed, ferna present, mesopresternum divided into paired lateral triangles. Fore tarsus without a tooth. Metanotum with elongate reticulations medially, median setae not long. Fore wing parallel sided, with about six duplicated cilia; three weakly capitate sub-basal setae sub-equal in length. Tergite IX setae S1 bluntly pointed, slightly shorter than tube.
Male similar to female; tergite IX setae S2 short and bluntly pointed; sternite VIII without a pore plate.
L. invisus is unusual in this genus because of the short and dark third antennal segment. The head shape is also unusual, with the cheeks and compound eyes forming a single smooth outline, and the elongate mouth cone extending well beyond the fore coxae. Currently, there are 290 species listed in the genus Liothrips, although 30 of these are placed in two sub-genera known only from Asia. As a result, this is larger than either Thrips or Haplothrips, these three being the largest genera of Thysanoptera. However, in comparison to both Thrips and Haplothrips there are far greater problems in Liothrips in species recognition. A particularly high proportion of the described species are known from single samples, or even single individuals, resulting in little knowledge of variation within and between species, and thus the general assumption that most members of the genus are host-specific requires extensive testing. Stannard (1957) listed 32 species of Liothrips from North America, and subsequently (Stannard, 1968) included 14 of these in his keys to the Illinois fauna. Cott (1957) treated 11 species from California, two of which he placed in Rhynchothrips, but currently from this State there are 13 Liothrips species listed (Hoddle et al., 2004) of which several cannot at present be recognized.
Liothrips invisus (Cott)
Probably feeding on green tissues, adults and larvae being recorded in association with green oak galls.
Probably Quercus sp. (Fagaceae).