Medium sized larvae, up to 7 mm long.
With 5 segments; segment 5 longer than fourth; apical part of segment 2 annulated and not always clearly separated from annulated segment 3. Lauterborn organs shorter and style longer than segment 3.
With SI lamelliform; SIII simple. Labral lamellae weak, probably consisting of 2 broad transparent lobes. Chaetae multibranched, with long narrow branches; chaeta media distinct; 4 pairs of pointed spinulae present. Pecten epipharyngis consisting of 3 long pointed scales, lateral pair partly overlapping median scale. Chaetulae laterales forming an arched group on either side of pecten epipharyngis. Ungula semicircular; basal sclerite absent. Premandible narrow, with a large apical tooth and 2-3 small appressed inner teeth; apical half usually brownish; with a simple lateral spine present.
Apical tooth shorter than combined width of 4 inner teeth, sometimes apical tooth subequal in size to first inner tooth. Seta interna with 12-21 simple to weakly serrate branches.
With one median and 8-10 pairs of lateral teeth; median tooth dome-shaped or weakly bilobed and 2-3 x as wide as first lateral tooth; median and first lateral tooth lighter in color than remaining brownish lateral teeth. Ventrolateral plate extending lateral to mental teeth and ending about level of first lateral tooth. Setae submenti arising closer to mentum than to postoccipital margin.
With palp about as long as wide, with curved sides. Chaetulae of palpiger consisting of flat scales. Galea with long, setose lamellae. Setae maxillaris simple.
With anterior and posterior parapods separate, each bearing an apical crown of claws. Procercus shorter than wide, posterior margin usually strongly sclerotized; bearing 7 anal and 2 subapical setae. Body setae pale and indistinct.
Larvae of Sympotthastia resemble those of P. gaedii group (q.v.): both have brown-tipped mandibles and indistinctly separated second and third antennal segments with the apical part of the 2nd segment annulate The mentum and ventromental plates are if similar structure but the median tooth is wider in the P. gaedii group. The labrum, particularly the labral lamellae, pecten epipharyngis and arrangement of the chaetulae laterales also appear very similar. All specimens of Sympotthastia examined for Entomologica scandinavica key were poorly preserved. However Sympotthastia may be distinguished from the P. gaedii group by maxillary structures; the row of peg-like lamellae present in the P. gaedii group are absent from those of Sympotthastia.
Four species of Sympotthastia are reported from the Holarctic region and 1 from the Oriental.