Medium-sized to large larvae, up to 12 mm long. Head capsule rounded-oval; cephalic index 0.8-1.0. Dorsally S5 variably antero-median to S7, DP, S7, and S8 nearly aligned anterolateral to posterodorsal. Ventrally S9 and S10 anterolateral to SSm, S9 anterior and slightly more lateral to S10, VP posterolateral to SSm, S9, S10. All setae multibranched, or S9 simple.
50% longer than mandible. Antennal ratio 5.8-8.1. Basal segment about 5-7x as long as basal width, with ring organ at 0.65; segment 2 about 4x as long as wide; segment 3 distinctly longer than terminal segment. Style seated on apical margin of segment 2, extending somewhat beyond apex of segment 3. Blade about as long as flagellum; accessory blade barely reaching or extending beyond segment 3.
Slender, moderately curved, progressively broadening towards base. Apical tooth 2.5x as long as basal width, 1/4 length of mandible. Two inner teeth, 1 or 2 additional small points on subapex of mola; mola extended into pointed tooth; seta subdentalis long. Ventrolateral setae all simple.
Basal segment of palp 3x as long as wide with ring organ near middle.
With sparse fringe of swim-setae. With 4 conical anal tubules. Procercus 4-4.6x as long as wide, with 13 apical setae. Some claws of posterior parapod with small, fine, appressed points; smallest claws simple, curved.
Alotanypus belongs to the tribe Macropelopiini, differing from other members in having all ventrolateral setae of the mandible simple. Furthermore, it is separable from other macropelopiines with a centrally-placed ring organ of the basal maxillary palp by the uniformly granular pseudoradula being sharply defined over the entire length and only minimally expanded apically, the 3rd antennal segment longer than 4th, and the inner tooth of ligula being sharply outcurved.
Larvae appear to tolerate a broad range of conditions including very acidic waters (pH 3.9-4.0) in springs, seeps and bogs, a coffee pond and an urban stream. Alotanypus venustus (Coquillett) is reported in eastern USA from Ohio (Bolton, 2007) and distributed from western North America at least as far south as Costa Rica (Roback, 1971; Watson and Heyn, 1993; Sublette and Sasa, 1994). Alotanypus aris, known from s.e. USA from West Virginia to Florida (Epler, 2001), is still unknown as a larva. A closely similar 3rd pupal species was reported from New Hampshire (Roback, 1987). In Japan, larvae of A. kuroberobustus were collected from neutral standing waters in Toyama and Shizuoka Prefectures (Niitsuma, 2005). In Brazil Alotanypus is abundant in wetlands, including especially palustrine, emergent and permanent wetlands. Alotanypus oliveirai comb. n. (as Guassutanypus) were collected from depositional pool and exfiltration areas of the riparian zone of a first order, perennial stream in Cerrado, and pupae were collected in Amazonas (Roque and Trivinho-Strixino, 2003). In Australia A. dalyupensis have been reared from acidic lakes and granitic pools.