Large larvae, up to 13 mm long. Head capsule rounded-oval; cephalic index 0.95. Dorsally DP posterior to triangular arrangement of S5 (anteriormost), S7 and S8. Ventrally SSm mediolateral between S9 and S10, VP lateral to posterolateral of S10. All setae simple.
About as long as mandible. Antennal ratio 6.0-7.5. Basal segment about 5X as long as basal width, with ring organ at 0.8; segment 2 3X as long as wide; segment 3 scarcely longer than wide, somewhat shorter than apical segment. Style extending roughly to middle of segment 3, in common with peg sensilla arising at sides of segment 2, proximal to distal margin. Blade as long as or somewhat shorter than flagellum. Accessory blade about as long as segment 2.
Uniformly curved. With several patches of teeth, variable in size and irregularly distributed dorsally, in distal 1/2 of mandible, extending onto apical tooth. Apical tooth compact, about 2X as long as basal width, with proximal lamella divided into 2-3 low 'teeth’; mola with 2 to 6 small, adjacent, saw-like teeth on inner margin, distally forming dilate molar tooth, at base of arises long seta subdentalis. Ventrolateral setae simple, RIng organ located midway between approximated setae 1, 2 and more basal seta 3.
Basal segment of palp about 2.5X as long as wide; ring organ at 0.65.
With fringe of swim-setae. With 4 conical anal tubules. Procercus 3.25-4X as long as wide; with 18 apical setae. Some claws of posterior parapod with appressed points along inner and outer margins; smallest claws variable, elongate with associated flat hooks, recurved hooked ends or simply curved.
Derotanypus differs from the closely related Psectrotanypus in the shape of outer teeth of ligula, the apices of which curve outwards, the teeth on the outer mandible and in the strongly concave row of dorsomental teeth, the outermost being partially fused into an anteriorly-projecting compound tooth. In Psectrotanypus the tooth row is only weakly concave and outer ends have small, posteriorly situated teeth.
Larvae of Derotanypus favour small, cold, standing and flowing water bodies, where they live in silt (Sergeeva and Makarchenko, 1999). Apparently the genus is restricted to northern and montane regions of the Holarctic. Three species are known from the Nearctic: Derotanypus alaskensis is described in all developmental stages from Alaska (and Siberia), and additional taxa are suspected. In the Palaearctic, sibiricus (Kruglova & Chernovski), previously placed in Psectrotanypus, and limbatus (Lundström) previously placed in Anatopynia, belong in Derotanypus. Larvae of 2 or 3 further species are known from China and the northern Alps.