Small to medium sized larvae, up to 5 mm long.
Antenna about 1/3 length of mandible; 5 segmented; segment 1 slightly less than twice as long as wide; 3 terminal segments subequal. Blade as long as flagellum. Lauterborn organs extending at least to base of segment 4. Elongate style present.
Labrum with SI elongate, pectinate with up to 10 branches; remaining S setae simple. Labral lamellae present, strongly sclerotized and with 2-3 teeth. Pecten epipharyngis with 2 broad outer scales and a characteristically bulbous median scale. Premandible with 2 strong teeth and 2 weaker 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 one single median tooth, perhaps with a weak lateral notch; 4 pairs of lateral teeth present. Ventromental plate weak, not extending beyond outermost lateral tooth; beard absent.
Maxilla with two lamellae at base of palpiger developed as spines. Bisensillae of palp elongate. No lamellae present on galea; pecten galearis absent. Antiaxial chaeta bluntly plumose. Appendix seta absent.
Body with parapods present, divided and short; claws of anterior parapods both simple and serrate; those of posterior parapods simple and dark. Procercus absent; 2-3 short anal setae present. Supraanal setae as long as posterior parapods. Anal tubules absent.
Tethymyia larvae are very similar to those of Thalassosmittia and are separable only on the number of premandibular teeth (4 in Tethymyia, 3 in Thalassosmittia). The bulbous median scale of the pecten epipharyngis of Tethymyia may also be distinctive. The Antarctic genus Belgica has similar larvae.
Larval Tethymyia are marine, living in the inter- and subtidal, building tubes on intertidal rocks. The sole species, T. aptera Wirth, is known only from the Californian coast.