Large larvae, up to 14 mm long. Head yellowish, occipital margin may be darkened; body red in life. Head capsule rounded-oval, cephalic index 0.73-80. Dorsally S7 aligned medial to slightly postero-medial to S8, moderately separated; S5 far anterodorsal, DP posterodorsal, unaligned with S7,S8. Ventrally SSm lies medial to rather closely approximated S9 and S10; VP distantly posterolateral. SSm, S5 and S7 multi-branched; S9, S10 stout, simple.
Somewhat longer than mandible. Antennal ratio about 6.5. Basal segment about 5. 5x as long as basal width, with ring organ 0.75: segment 2 about 3.5x as long as wide: segment 3 scarcely longer than wide, somewhat shorter than terminal segment; intersegmental region between segments 2 and 3 large, as long as peg sensilla. Style extending past segment 3. Blade as long as or shorter than flagellum. Accessory blade reaching to about segment 3.
Slender. moderately curved. Apical tooth 3x as long as basal width. almost 1/3 length of mandible. Mola with 1 distal and with 1 or more marginal teeth (notata group), with small dorsolateral tooth. Seta subdentalis slender. Ventrolateral seta 1 simple, 2 and 3 with 3-4 branches.
Basal segment of palp at least 3x as long as wide, with ring organ between 0.3 and 0.45.
With fringe of swim-setae. With 4 rather long, conical anal tubules. Procercus about 3-4.5x as long as wide, with 9-15 apical setae. Claws of posterior parapod simple, sometimes with fine, appressed points on inner and/or outer side; smallest claws simple, weakly curved.
Macropelopia belongs in the Macropelopiini, distinctive sharing the proximal location of the ring organ of the maxillary palp with Bethbilbeckia. The number of large teeth in the dorsomental plates differs - excluding the innermost and outermost teeth each plate has 6 large teeth in Macropelopia, 5 in Bethbilbeckia. From related genera Alotanypus, Apsectrotanypus and Brundiniella, Macropelopia differs in features of the antenna, dorsomentum and pseudoradula. The third antennal segment of Macropelopia uniquely is as long as broad. Apsectrotanypus differs in having the second antennal segment shortened and few, unusually-sized dorsomental teeth. Alotanypus and Macropelopia have comparable numbers of dorsomental teeth. The pseudoradula is uniformly slender with weak granulation over basal 1/2; in this feature it cannot be confused with other genera. The notata and nebulosa groups, which were erected originally on the basis of pupal characters, may be differentiated by the inner ligula teeth being distinctly curved towards outwards in the notata group, rather than straight or even incurved in the nebulosa group. The mesal apex of the pecten hypopharyngis, a broad hump in the notata group, is less broadened in the nebulosa group.
Larvae of Macropelopia live in fine sediments in cool water bodies (springs, brooks, lakes and bogs). The abundance of species in the Palaearctic (about 10) and their weak representation in the Nearctic (three species, including M. decedens and 2 further species known from immature stages) is striking. The two species of Macropelopia described form the Himalayas of India (Hazra and Chaudhuri, 2001) and those redescribed from Japan by Niitsuma et al. (2004) may represent the records closest to the Oriental region, where the genus appears absent. Macropelopia as presently defined, appears absent also from Australia and New Zealand, where Apsectrotanypus and Alotanypus are the common Macropelopiini. The situation in South America is unclear but Macropelopia sp. 2 (Trivinho-Strixino and Strixino, 1993) may represent the genus as defined here.