Medium-sized larvae, 6-7 mm long. Head yellowish, posterior margin darkened on either side of pale gular margin, becoming lighter dorsally; body pale yellow. Elongate-oval, cephalic index 0.6-0.7. Dorsally, DP present, Dorsally S5 anterior to DP, S8 close to S7 with S8 slightly mesial, both anterolateral to DP. Ventrally SSm lateral and slightly posteromesial to S9, and VP; S9, VP and SSm form oblique line to longitudinal axis, with S10 anterior to VP. Ventral setae S10 and SSm weak, multi-branched; S9 strong simple and much longer than S10.
Almost 1/2 as long as head. Antennal ratio 4.0-6.0. Basal segment 11-13x as long as basal width, 2x as long as mandible, with ring organ 0.75-0.8 from base; apex of basal segment produced into point on outer side; segment 2, 9-11x as long as wide; apical segment about 3/4 length of segment 3. Style capitate, reaching to base of apical segment. Peg sensilla about 1/2 as long as segment 3. Blade somewhat shorter than flagellum; basal ring 1.5-2x as as high as wide. Accessory blade slightly shorter than blade.
Gradually curved, strongly narrowed in distal 2/3, about 2x as long as broad. Apical tooth about 2x as long as basal width, slightly more than 1/4 length of mandible, distal 1/3 blackish brown. Inner tooth small, but distinguishable, pointed and deeply recessed from inner margin; mola broad, slightly expanded from inner margin of mandible. Ventrolateral setae located adjacent to outer margin of basal 1/2 of mandible; seta 1 small peg-shaped, 2 simple, 3 divided; gap between setae 2 and 3, about 2x as great as that between setae 1 and 2; ring organ situated proximal to seta 3.
Basal segment of palp 3.5-4.5x as long as wide, with ring organ about 0.8 from base; b sensillum 2-segmented, basal segment 3x as long as apical segment.
Abdominal segments without lateral setal fringe, but possessing long scattered setae, as long as about 1/3 width of segment. Anal tubules slender, spindle-shaped, about 5x as long as wide. Procercus just over 5x as long as wide, with 7 apical setae. Subbasal seta simple. Posterior parapod with 16 simple claws; margins of some longer claws weakly spinulate; four smaller claws somewhat darkened, with one having clear lamella; one large claw distinctly more developed than the rest and bent downward at an acute angle to base.
Amnihayesomyia belongs to the tribe Pentaneurini, and lies within the Thienemannimyia-group of genera. It belongs in the subgroup consisting of Arctopelopia, Meropelopia, Thienemannimyia, Hayesomyia and some Rheopelopia, in having the maxillary b sensillum 2-segmented. Amnihayesomyia can be differentiated from all preceding genera by the different arrangement of the ventral cephalic seta, particularly the more posterior location of S9 which does not fit the typical arrangement for the Thienemannimyia-group as a whole. Other characters that set it apart are the mandible with a longer narrow distal part and the distinctly more developed long claw of the posterior parapod. One species has been described from Japan (Niitsuma, 2007). The Conchapelopia nepalicola group of four species with unknown immature stages, described by Murray (1976) from Nepal, has been transferred to this recently-described genus.
The larva of Amnihayesomyia ikawensis has been collected from a spring with submerged moss.