Medium-sized larva, about 6 mm long. Head capsule rounded; cephalic index 1.00. Median ventral suture narrow. Ventral cephalic setal pattern rhomboidal: SSm medial, S9 anterior to close-by S10, located midway between SSm and VP which is lateralmost. Dorsally S5 anterodorsal to S7, S7 arises from bigger pit than close-by S8, both near-aligned with posteriorly-placed DP. SSm multibranched, remaining setae simple.
Short, only 25% longer than mandible. Antennal ratio 7.6-8.0. Basal segment about 4x as long as basal width, with ring organ very distally placed; segment 2 short, scarcely 2 as long as wide; peg sensilla short, c 5µm; segment 3 short, as long as wide; apical segment slender, 2x as long as segment 3. Style extending to mid-segment 3. Blade stubby, extending to apex of segment 3. Accessory blade subequal in length to blade, reaching segment 3.
Slender, smoothly curved. Apical tooth 2 x as long as basal width, 30% length of mandible. Molar tooth and small inner tooth together appearing bifid; slender long seta subdentalis arises from pit between them. Ventrolateral seta 1 simple, 2 and 3 with 4 branches; 1 and 2 approximated; ring organ midway between 2 and 3.
Basal segment of palp about 2x as long as wide, with ring organ at 0.45.
With fringe of swim-setae. With 4 conical anal tubules. Procercus about 4x as long as wide, with 10-12 apical setae. Posterior parapod with 16 claws, some with appressed points along inner side, near middle and apex, none near base; at least 1 claws about 1/2 with dorsal spine; smaller claws moderately expanded.
Radotanypus belongs to the Macropelopiini; it can be distinguished by the rotund head capsule; ring organ near middle of maxillary palp; ligula with inner teeth directed forward; dorsomental plates with 5 large teeth. Separable from the similar Brundiniella by the dorsomentum lacking pointed inner lobes, the granulation of the pseudoradula being largest proximally (not medially), the pecten hypopharyngis having 16-20 teeth with the apical (most median) tooth not broadened, the base of the smallest posterior parapod claws not expanded laterally, and the presence of 16 claws on each parapod. The rhomboidal arrangement of SSm, S9, S10, and VP is quite different from that of Brundiniella, in which SSm, S9, S10 are aligned at a shallow angle, with VP retracted posterolaterally. The dorsal pit in Radotanypus is less retracted posteriorly compared to Brundiniella.
Two species have been named: R. florens (Johannsen) and R. submarginella (Sublette), but synonymy suggested by Bilyj (in Epler, 2001) gives priority to R. florens.
Radotanypus larvae live in slow flowing creeks, backwaters of rivers and low-order streams. Radotanypus is known from California, Colorado, Ohio, Oregon and Wyoming. Found in a stream and a small river in Wyoming (Epler, 1986) was abundant in beaver ponds in Colorado (Epler pers. obs.); in Ohio R. florens is considered characteristic of cold headwaters (Bolton, 1992). In California larvae occur in a cool stream draining a Darlingtonia pitcher plant swamp (P.S. Cranston pers. obs.). The genus may be widespread in cold upland North American bogs.