Medium-sized larvae, up to 7 mm long. Head brownish, posterior margin pale brown; body yellowish-white. Head capsule longish, oval; cephalic index 0.67.
Almost 1/2 as long as head, at least 2x as long as mandible. Antennal ratio 4.7-5.8. Basal segment about 15x as long as basal width, with ring organ at 0.6-0.7; segment 2 about 9x as long as wide; last segment about 3/4 as long as third. Style clubbed apically, extending to middle of last segment. Peg sensilla very small. Blade only slightly longer than flagellum. Accessory blade extending beyond segment 3, about 6 x as long as height of basal ring.
Uniformly curved; distal 1/2 strongly narrowed, almost 2.5x as long as basal width. Proximal 1/2 strongly broadened. Apical tooth 2x as long as basal width, 1/4 length of mandible; distal 1/2 blackish brown. Inner tooth deeply inset, small, bluntly-rounded, scarcely overlying seta subdentalis; mola essentially flat, not protruding beyond inner margin of mandible. Ventrolateral setae aligned on outer margin of basal 1/2 of mandible; seta 1 small peg, 2 simple, 3 divided; setae 1 and 2 approximated; distance between setae 2 and 3 greater than that between seta 3 and the proximal and more ventrally located ring organ.
Basal segment of palp about 4.5x as long as wide, with ring organ at 0.7; b sensilla in 2 sections, basal section 3x as long as apical section.
Without fringe of swim-setae. Anal tubules slender, spindle-shaped, about 5x as long as wide. Subbasal seta of posterior parapod simple. Procercus more than 3.5x as long as wide, with 7 apical setae. Claws of posterior parapod simple, larger claws with small spinules on inner and/or outer margin; some small claws darker than remainder.
Meropelopia belongs in the Thienemannimyia group of the Pentaneurini. The history of treatment as subgenus of Arctopelopia (Roback, 1971), subgenus of Conchapelopia (Roback 1981) and then elevation to genus (Fittkau and Roback, 1983) has resulted in the only two included species [Nearctic M. americana (Fittkau) and M. flavifrons (Johannsen)] being virtually inseparable in the larval stage from others in the Thienemannimyia group. The major, perhaps only, differentiating feature from Conchapelopia is the b sensillum of the maxillary palp being in 2 sections, a feature shared by Arctopelopia, Hayesomyia, Telopelopia, Thienemannimyia and some Rheopelopia (q.v.). An Antennal Ratio greater than 4.5 may separate Meropelopia from Hayesomyia and Telopelopia (which has the molar tooth distinctively large for this group), the lower number of pecten hypopharyngis teeth (c 22 v 25+) may distinguish from Arctopelopia. Separation from the Rheopelopia with b sensilla in 2-sections, and from Thienemannimyia may be possible (at least in southeastern. USA) by meristic features: the basal maxillary palp segment is equal to or longer than the second antennal segment, the ratio of lengths of the basal antennal segment to mandible > 1.8 (Epler, 2001).
Larvae of the two described species of Meropelopia live in small to medium-sized streams and rivers in North America. In a second order steam in Québec, Canada, Meropelopia larvae appeared to graze on diatoms on rocks (Tal et al., 2006). Given identification difficulties, isolated larval records from elsewhere (China, Russia) require confirmation. Pupal records from Guatemala (Sublette and Sasa, 1994) and Costa Rica (Spies et al., 2008) imply a presence at least in Central America.