Small to medium sized larvae, up to 5 mm long.
Antenna 5 segmented; segment 1 slightly less than twice as long as wide; segment 2 elongate; segment 4 subequal to or longer than third. Blade subequal to flagellum, divided into 2 parts, the inner weak. Lauterborn organs weak, as long as segment 3. Style present.
Labrum with SI elongate with 4-10 lateral branches; remaining S setae simple; SII and SIII stouter than usual. Strongly sclerotized labral lamellae present with single tooth in T. thalassophila and up to 5 teeth in Nearctic species. Pecten epipharyngis consisting of 3 subequal scales, partially fused at bases. Premandible with 1 pointed apical tooth and 2 rounded inner teeth; brush absent.
Mandible with apical tooth shorter than combined width of 4 inner teeth. Seta subdentalis pointed. Seta interna with 6-7 simple and serrate branches.
Mentum with single rounded median tooth and 4 pairs of lateral teeth. Ventromental plate weak, not extending beyond outermost lateral tooth; beard absent.
Maxilla with tetrahedral lamellae on palpiger well developed. Lamellae on galea either low and rounded or (maybe) absent. Setae maxillaris simple. Appendix seta absent.
Body with parapods present, divided and short. Claws of anterior parapods simple and serrate; those of posterior parapods simple. Procercus absent; 2 or 3 (rarely 1) anal setae present. Supraanal setae subequal to posterior parapods. Anal tubules absent.
The larvae of Thalassosmittia are separable from those of Tethymyia by the 3-toothed premandible in the former. The Antarctic genus Belgica has similar larvae.
Five Holarctic species of Thalassosmittia are from the marine littoral, one other was described from adults from inland montane China (Wang and Saether, 1993). The population dynamics and life histories of 3 Nearctic species were discussed by Morley and Ring (1972), who key the species as Saunderia. The western Palaearctic species T. thalassophila was studied by Strenzke and Remmert (1957), and a fifth species is known from Japan.