Medium sized larvae, up to 10 mm long. Head brownish, posterior margin dark brown; body yellowish white. Head capsule longish-oval; cephalic index 0.65-0.70. Dorsally S5 anterior to DP, S8 close to S7, pit of S8 large. Ventrally SSm posteromesial to clustered S9, S10, VP, which is much larger than ventral setal pits; S10 lateral or posteromesial to S9 + VP; S9, S10 and SSm form oblique line to longitudinal axis.
Almost 1/2 as long as head and almost 2x as long as mandible. Antennal ratio 4.0-6.0. Basal segment 6-7x as long as basal width, with ring organ at base of apical 1/3; apex of basal segment may be produced into point on outer side; segment 2 4.5-10x as long as wide; last segment about 3/5 length of segment 3. Style clubbed apically, reaching to middle of last segment. Peg sensilla very small. Blade scarcely longer than flagellum, 4.5-6.5x as long as height of basal ring; basal ring 1-2x as high as wide. Accessory blade extending to level of last segment.
Gradually curved, strongly narrowed in apical 1/2, this part about 3x as long as basal width, basal 1/2 strongly broadened. Apical tooth 2x as long as basal width, 1/4 length of mandible, distal 1/2 blackish brown. Inner tooth bluntly rounded, extending somewhat over seta subdentalis; molar expansion inconspicuous, low and apically directed. Ventrolateral setae approximated on outer margin of basal 1/2 of mandible; ring organ situated proximal thereto, on ventral side; ventrolateral seta 1 small, peg-like, seta 2 simple, 3 divided.
Basal segment of palp almost 3.5x as long as wide, with ring organ at base of apical 1/3; b sensilla in 2 sections, basal part 2-3x as long as distal part.
Without fringe of swim-setae. Anal tubules slender, spindle-shaped, about 5x as long as wide. Subbasal seta of posterior parapod simple. Procercus almost 3.5x as long as wide, with 7 apical setae. Claws of posterior parapod simple, some larger claws with small spinules on inner and/or outer margin; some of smaller claws may be darker than remainder.
Thienemannimyia is a Pentaneurini, and gives its name to a group of very similar genera: the Thienemannimyia-group, comprising Arctopelopia, Conchapelopia, Hayesomyia, Helopelopia, Meropelopia, Rheopelopia, Telopelopia. and Thienemannimyia (Murray and Fittkau, 1985). In common with Thienemannimyia, Arctopelopia, Hayesomyia, Meropelopia and Telopelopia have a b sensilla of the maxillary palp with 2 sections, but differ by having a strongly broadened pseudoradula and many teeth on the pecten hypopharyngis (Arctopelopia) or in having a pale posterior margin on the head and widely-separated mandibular ventrolateral setae 2 and 3 (Meropelopia). Species groups have been erected for the pupal and imaginal stages of Thienemannimyia, but not only may these not tally with the larvae, but some larvae cannot even be reliably allocated to genus in this group without assistance of pupal thoracic horn features in pharate 4th instars. Normally diagnostic generic characters vary excessively, as for example the proportions of the antennal and palp segments (Murray and Fittkau, 1985) and generic boundaries need revision with the aid of molecular data and cladistic analyses. At present, at least Hayesomyia cannot be separated successfully from Thienemannimyia in the larval stage.
The larvae of Thienemannimyia are found in both lotic and lentic waters, in streams, rivers and lakes. In temperate latitudes and montane regions they inhabit the littoral and profundal zones of oligotrophic lakes. Sandy-muddy substrata are preferred in streams, springs are avoided.
Four species are known from the Nearctic but species diversity is highest in the Palaearctic, where 11 species are known. Based on adults, at least two species occur in Costa Rica which may be conspecific with the western Nearctic T. barberi and T. fusciceps. A larva is recorded from Nepal (Roback and Coffman, 1987), and Thienemannimyia is recorded also from the Afrotropical, Oriental and Australasian regions, including one species known from New Guinea.