Small larvae, up to 4 mm long.
Antenna 5 segmented; segment 3 shorter than fourth. Ring organ situated in basal 1/2 of basal segment. Blade slightly shorter or longer than flagellum. Lauterborn organs well developed.
Labrum with SI plumose; other S setae normal. With 2 serrated median labral lamellae (in T. gracilis) or labral lamella absent in most species. About 7-9 more of less plumose chaetae; 0-2 spinulae. Pecten epipharyngis consisting of 3 smooth, scale-like spines. About 7 chaetulae laterales, 3 anteromedian ones scale-like, next 3 weak and more or less plumose, last one spine-like; chaetulae basales small, peg-like, not bifid. Premandible with 2 apical and 2 inner teeth; brush well developed (T. gracilis) to small in most species.
Mandible with apical tooth shorter than combined width of 3 in most species (4 in T. gracilis) inner teeth. Seta subdentalis present. Seta interna consisting of 7 smooth to weakly serrate branches.
Mentum with two median teeth slightly higher (in most species) to lower (in T. gracilis) than first of the 5 pairs of lateral teeth. Base of mentum with tooth or notch. Ventromental plate weak; beard absent.
Maxilla with anterior lacinial chaeta conspicuously broad, pointed. Pecten galearis absent.
Body with parapods well developed. Procercus short, rounded, weakly sclerotized, without spur, with 6 or 7 apical anal setae longer than posterior parapods. Supraanal seta long. Anal tubules normal, pointed, about half as long as posterior parapods.
Thienemannia larvae rather resembles both Metriocnemus and Limnophyes, but is best differentiated by the antenna never being reduced in size, and never possesing labral lamellae and the ventromental plates being very narrow and never 'tooth-like' as for Limnophyes and relatives. Note that Fig. 9.76B, D, G, H & J in Cranston et al. (1983) is a Limnophyes sp.
The larvae of Thienemannia are found in springs and hygropetric situations in streams. Larval T. gracilis have been observed feeding on trichopteran egg masses.
Four species are known from the Palaearctic region, one from the Holarctic.