Small larvae, up to 4.5 mm long.
Antenna about 2/3 length of mandible; 5 segmented; segment 2 longer than first; segment 4 longer than third; segment 2 with weakly sclerotized portion at proximal 1/3 of segment. Blade long, extending beyond flagellum by length of flagellum. Lauterborn organs present, as long as segment 3.
Labrum with SI bearing 4-7 branches; remaining S setae simple; SII very broad. Pecten epipharyngis consisting of 3 broad spines; 5-6 pairs of chaetulae laterales present; almost lamellar in shape. Premandible with 3-4 teeth; brush absent.
Apical tooth shorter than combined width of 3 inner teeth. Seta subdentalis small. Seta interna with many fine branches.
Mentum with paired broad median teeth divided by broad U-shaped notch; 5 pairs of lateral teeth present; fourth pair smaller than third or fifth. Distinct notch or tooth at base of mentum. Ventromental plate well developed but not extending beyond outer mental tooth; beard absent.
Maxilla with small palp with well developed sensillae and setae. Numerous simple lamellae present on galea and palpiger. Sensillae on galea well developed; pecten galearis present. Setae maxillaris simple and serrate. Appendix seta not visible.
Body with anterior and posterior parapods well developed with anterior parapod claws bluntly serrate; posterior claws smooth. Procercus well developed, bearing 5 subequal anal setae. Lower lateral procercal seta long, as is supraanal seta. Anal tubules not visible on exuviae examined. No body setae.
Larval Heleniella are rheophilic, living in typically cold running waters of the northern hemisphere, with some reports from warmer rivers of south-east Asia. There are 9-10 species known, predominantly from western Europe with some species extending to, and others indigenous to, China. Adults with picture wings are known from Japan (2 species) and from elevation in montane northern Thailand (Andersen and Wang, 1997).