Medium-sized larvae, up to 10 mm long. Head brownish, posterior margin dark brown; body yellowish white. Head capsule longish-oval; cephalic index 0.5-0.7. Dorsally S5 anterior to DP, S8 close to S7, pit twice as large as any ventral setal socket. Ventrally SSm posteromesial to clustered S9, S10, VP; S10 lateral or posteromesial to S9 + VP; S9, S10 and SSm form oblique line to longitudinal axis. S5, S7 setae multibranched; S8, S9 simple, S9 very strong, S10 unknown.
About 1/2 as long as head, almost 2x as long as mandible. Antennal ratio 4.0-5.1. Basal segment 8x as long as basal width, with ring organ at 0.60, distal margin of basal segment flat; segment 2 10x as long as wide; last segment about 60% length of segment 3. Style clubbed apically, reaching middle of last segment. Peg sensilla very small. Blade scarcely longer than flagellum, 6.5x as long as height of basal ring; basal ring 2x as high as wide. Accessory blade extending to level of last segment.
Uniformly curved, strongly narrowed in distal 1/2, this part about 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 short, bluntly rounded, extending slightly over long, slender seta subdentalis; mola with little or no expansion. Ventrolateral setae situated close together on outer margin of basal 1/2 of mandible; sensillum minusculum situated proximal thereto, on ventral side; ventrolateral seta 1 small, peg-like, seta 2 simple, 3 divided. Setae 1 and 2 close, distance between seta 2 and 3 less than between seta 3 and ring organ.
Basal segment of palp almost 3.5x as long as wide, with ring organ at 0.6; b sensillum in 2-sections, basal section 3x as long as distal section.
Without fringe of swim-setae, but with scattered long setae. Anal tubules slender, spindle-shaped, about 5x as long as wide. Subbasal seta of posterior parapod simple. Procercus almost 3.5x as long as wide, with 7 apical setae. Claws of posterior parapod simple, some larger claws with small spinules on inner and/or outer margin; two smaller claws darker than remainder.
Hayesomyia belongs in the Thienemannimyia group of the Pentaneurini. This diagnosis is based on H. senata (Walley), which was placed previously in Thienemannimyia (Roback, 1981). Since this remains the only species with known larva, distinction from some other members of the Thienemannimyia group is uncertain. Although the pupa is distinct, Kobayashi (2003) renders the distinctiveness of the adult male tenuous, especially separation from Rheopelopia.
Epler (2001) suggests that larval H. senata may be distinguished by its b sensillum of the maxillary palp being in 2 sections, the ring organ located in distal third of the maxillary palp, the AR <5.0 with second antennal segment length of about 50-60 µm, the ratio of lengths of first antennal segment to mandible being less than or equal to 1.8, the sclerotized antennal blade base being almost twice as long as wide, and the long scattered body setae. However, as the range of variation is revealed for more Thienemannimyia larvae, Hayesomyia seems less distinguishable and would be better treated as a synonym.
In contrast to the larvae of many of the Thienemannimyia group of genera that live in small to medium sized, shallow streams, larvae of Hayesomyia tend to occur in larger rivers with moderate flow. The genus has a largely Holarctic distribution with the continent-wide H. senata recorded from the Nearctic and H. tripunctata (Goetghebuer) ranging from the western Palaearctic to Japan. The genus is not found in the more northern latitudes but unexpected diversity based on adult males has been reported from China, including from the Oriental part (Cheng & Wang, 2006b). There is one pupal record from West Africa (Coffman et al., 1992), and two taxa are recognised as pupal exuviae from Australia (Cranston, 1996).