Small larvae, up to 3 mm long.
Antenna 3/4 length of head capsule; 5 segmented; segment 2 as long as first, unevenly sclerotized and with apparently weak alternate Lauterborn organs, possibly 3 in number although apical one may be expanded style; segment 4 longer than third; segment 5 vestigial. Blade extending only to about apex of segment 2.
Labrum with only the well developed SIV visible on preparation. Palatum difficult to interpret in the specimen. Pecten epipharyngis not visible but whole area covered with well developed spines, all apparently simple. Premandible with up to 10 fine teeth and distinct inner brush.
Mandible with apical tooth shorter than combined width of 5-6 inner teeth, innermost unclearly delimited from mola. Seta subdentalis a strong simple spine, placed further posterior than usual. Seta interna with 5 simple branches.
Mentum with 3 subequal median teeth and 6 pairs of lateral teeth of decreasing size such that the outermost are very small (some described larvae have about 8 median teeth and 6 pairs of lateral teeth). Ventromental plate indistinct and not extending beyond outermost lateral teeth; beard absent.
Maxilla with palp well developed with segmentation distinct although sensillae appear to be reduced. Galea indistinct. Setae maxillaris simple and fine.
Body with parapods separate, bearing serrate claws on the anterior, simple on the posterior. Procercus as long as wide and bearing 5-6 anal setae. Supraanal setae weak. Anal tubules unconstricted, half length of posterior parapods. No distinct setae present.
Larvae of Rheosmittia are probably always psammophilic (sand-dwelling) in rivers.
The genus is known from the Holarctic only.