Medium-sized larvae, up to 9 mm long. Posterior margin or posterior 2/3 of head blackish brown; body yellowish white. Head capsule longish-oval; cephalic index about 0.65. Dorsally S5 anterior to DP, S7 and S8 very close together; DP posteromesial to S7, S8. Ventrally S10 lateral or posteromesial to S9 + VP, S9 and S10 clustered, S9 much stronger than S10; SSm posteromesial. VSR = 2.22-2.65. Setae: S5, S8, S10 plumose, S7 single or few branched, S9 very long, strong.
2.5-3x as long as mandible. Antennal ratio 3.4-3.8. Basal segment about 10x as long as basal width, with ring organ at 0.6-0.7; segment 2 about 11x as long as wide; last segment about 1/2 as long as third. Style clubbed apically, extending to middle of last segment. Peg sensilla very small. Blade and accessory blade extending considerably beyond flagellum. Blade almost 4x as long as height of basal ring; basal ring 2.5x as high as wide.
Apical 1/2 strongly narrowed and curved, about 3.5x as long as basal width; basal 1/2 strongly broadened. Apical tooth 2x as long as wide, somewhat less than 1/4 length of mandible. Minute accessory or inner tooth visible only at highest magnification; mola scarcely expanded; seta subdentalis follows inner margin of apical tooth. Ventrolateral setae equidistant, situated in close row at beginning of basal 1/2 of mandible; ring organ located proximal thereto; ventrolateral seta 1 scarcely discernible peg, 2 simple, 3 divided.
Basal segment of palp 3.5x as long as wide or somewhat longer, with ring organ at 0.6; b sensilla in 2- or 3-sections, when 3 then middle 2x as long as basal or apical section.
Without fringe of swim-setae, but with long, thick, spirally arranged body setae. Anal tubules spindle-shaped, about 3x as long as wide. Subbasal seta of posterior parapod simple or unequally forked. Procercus 3x as long as wide, with 7 apical setae. Claws of posterior parapod simple; larger claws with small, fine points on inner and/or outer margin; some smaller claws may be darker than others.
Rheopelopia is a Pentaneurini, clearly belonging in the Thienemannimyia-group, within which it may be differentiated by the strongly curved apical 1/2 of the mandible, with no molar expansion and little evidence of an inner or accessory tooth. The antennal ratio, which does not exceed 3.8, is unusually low. The gradual tapering of the pseudoradula from base to apex may be distinctive. The eastern North American species R. paramaculipennis and the unreared larva of a potentially different species from the southeastern USA have b sensillum of the maxillary palp in 2 sections, rather than the otherwise typical 3 (Epler, 2001). Several species groups have been distinguished from imagines and/or pupae, but no larval features have been found for these groups.
Epler (2001) keys five species and provides ecological notes for species from s.e. USA.
Larvae of Rheopelopia live almost exclusively in fast flowing, well aerated lotic waters. The larvae are thus polyoxybiontic and rheophilic and have also been recorded from the potamon of larger rivers. North American species prefer small, fast flowing streams, whereas European species avoid the rhithral environment and first appear in the potamon, where they associate with stable substrata.
Rheopelopia species are known almost exclusively from the Holarctic - one larval record from Colombia (Ruiz et al., 2000) is the only exception. Two possibly synonymous species, R. acra and R. perda are described from North America, but immature stages are known of potentially 5 further taxa (Epler 2001). From the Palaearctic 5 species are described, including R. maculipennis and R. ornata widespread from Europe and Russia to Japan and R. murrayi from n.w. Africa.