The three instars of Hydrocanthus sharpi Zimmermann, 1928 are described and illustrated, including morphometric and chaetotaxic analyses of the cephalic capsule, head appendages, legs, last abdominal segment, and urogomphus. Larvae of Hydrocanthus Say, 1823 are diagnosed by the following combination of characters: elongate body; posterior tentorial pits not contiguous to occipital foramen; antennomere 4 2–3 times shorter than antennomere 3; non-serrate inner mandibular margin; siphon short, slightly protruding; urogomphi not fused along inner margin. Hydrocanthus differs in chaetotaxy from Suphis Aubé, 1836 by the position of frontoclypeal seta FR10, antennal seta AN3, and maxillary sensilla MX11 and MXj, the presence of several additional setae on the trochanter, the absence of seta MX16 on the maxilla, of seta AB2 on abdominal segment VIII, and of pore MXd on the galea, the length of labial seta LA2 and of urogomphal seta UR4, and the shape of tarsal setae TA3, TA4, TA5, and TA6.