Small larvae, to 5 mm long.
Antenna with 5 segments on a moderate to tall pedestal lacking any basal tooth or spur. Antennal ratio c 1. Ring organ proximal on 1st antennal segment. Seta well developed. Lauterborn organs modest to well-developed, on pedestals ranging in length from subequal to segment 3 to extending almost to antennal apex, opposite on apex of elongate, apically slightly dilate 2nd segment. 3rd segment shorter than 2nd, 4th and 5th short. Antennal blade usually as long as 2nd segment, accessory blade fused to base of blade, short. Style on apex of 2nd segment, shorter than segment 3.
SI seta comb-like, fused at base, SII on large pedestal, distally plumose, SIII short, simple, SIVa present. Labral lamellae well developed. Pecten epipharyngis either a simple broad comb, or incompletely divided into 3 parts. Premandible bifid, brush strong; seta premandibularis simple.
Mandible with brown dorsal tooth, apical tooth and 2-3 pointed inner teeth. Seta subdentalis long, curved, reaching to tip of apical tooth. Seta interna of 4 plumose branches. Pecten mandibularis well developed with many branched lamellae.
Mentum with rounded median tooth, or laterally median tooth with 1-2 notches, sometimes almost trifid, 5 pairs of laterals, regularly decreasing in size. Ventromental plates close together medially, subequal in width to mentum, strongly curved anterior margin, with coarse striae producing an often distinctive chunky appearance.
Without tubules. Claws of posterior parapod simple.
Unlike in any other genus of Tanytarsini the pecten epipharyngis of Rheotanytarsus is a single, broad, distally serrated plate, which may however be deeply divided into 3 lobes. The cases constructed by larvae also are characteristic, bearing several arm-like extensions at one end.
Descriptions of larvae are given by Bause (1913a) and Fittkau (1960a).
Larvae of Rheotanytarsus are rheobiontic, being found in small rivers and streams (rhithron) and also in larger rivers (potamon). Characteristically they construct cases with arm-like extensions between which strands of silk are suspended to catch particles of suspended detritus which are utilized both as food and for case-building. Cases are usually attached to stones or to submerged macrophytes.