Medium-sized larva, up to 8 mm long. Head brownish, basal margin darker in colour; body yellowish white. Head capsule longish-oval; cephalic index 0.65. Dorsally S7 arises from large pit, close to S8, not aligned with rather posterior-located S5 and small DP. Ventrally SSm at inner end of transverse / diagonally aligned SSm-S9-S10; VP immediately posterior to S10. Setae simple.
Almost 1/2 as long as head, 2.3x as long as mandible. Antennal ratio about 4.5. Basal segment about 10x as long as basal width, with ring organ at 0.75, strongly narrowed distal to ring organ; segment 2 about 11x as long as wide; apical segment 1/2 as long as segment 3. Style club-shaped, extending beyond middle of terminal segment. Peg sensilla very small. Blade extending somewhat beyond flagellum, 5x as long as height of basal ring; basal ring 2.5x as high as wide. Accessory blade reaching middle of segment 2.
Uniformly weakly curved, apical 1/2 strongly narrowed, about 2x as long as basal width; basal 1/2 strongly broadened. Apical tooth 2x as long as basal width, about 1/4 length of mandible; distal 1/2 blackish brown. Inner tooth deeply inset, bluntly rounded, and easily distinguishable arises from ventral inner margin of apical tooth, immediately distal to mola, somewhat overlying long seta subdentalis. Mola not projecting beyond inner margin of mandible. Ventrolateral setae situated adjacently on outer margin of basal 1/2 of mandible; ring organ proximal to these and somewhat displaced ventrally; seta 1 small blade, distinguishable with difficulty, seta 2 simple, 3 divided; gap between setae 1 and 2 about 1/2 that between 2 and 3; seta 3 located near middle of basal 1/2 of mandible.
Basal segment of palp 4.5x as long as wide, with ring organ at 0.6-0.7; b seta 3-segmented, basal segment as long as wide, middle segment 5x as long as wide, last segment 2x as long as basal one.
Without fringe of swim-setae. Anal tubules slender, spindle-shaped, about 5x as long as wide. Subbasal seta of posterior parapod simple. Procercus about 3.5x as long as wide, with 7 apical setae. Claws of posterior parapod simple, about 15; larger claws with small points on inner and/or outer side; some smaller claws somewhat darker than remainder.
Helopelopia, which belongs to the Pentaneurini, and to the Thienemannimyia group of genera; was erected originally as a subgenus of Conchapelopia for the species H. cornuticaudata (Walley) and H. pilicaudata (Walley) by Roback (1971, 1981). At present the metamorphosis only of H. cornuticaudata is published, an undescribed species is known (M. Bolton pers. comm.): the pupa and larva are distinguishable from Conchapelopia only with difficulty. Conchapelopia are separable from Coffmania by the narrow pseudoradula in the latter genus, but Helopelopia shares this feature. Rheopelopia differs from Helopelopia and Conchapelopia in having a scarcely recognizable molar tooth. The other members of the Thienemannimyia group have a b seta in 2 sections.
This is a synonym of Conchapelopia as was - larval rational (Fittkau and Roback) is nonsense. Combine and key as subgenus.
Both H. cornuticaudata and H. pilicaudata occur only in the Nearctic. Larvae of the former species are primarily found in riffle areas of small to medium-sized streams. This genus is not recorded from Central America, but larvae with the b sensillum on the maxillary palp being in three parts, thus possibly belonging to Conchapelopia or Helopelopia, have been found in Costa Rica (C.N. Watson, unpublished).