Medium sized larvae, up to 8 mm long. Head capsule narrowed anteriorly and posteriorly, slightly retractable into thorax.
With 4-5 tubercles, ranging in shape from long angular tubercles to low protrusions; tubercles with or without setae. Frontoclypeolabral apotome present.
Borne on short pedestal; sometimes pedestal with inner process. With 5 segments; annulated segment 3 longer than second, and fourth very short; segment 2 sometimes with uneven margin. Style and Lauterborn organs less than 1/2 length of segment 3.
Wth SI seta-like; SIII simple. Labral lamellae consisting of 2 broad pectinate lobes. With 6-7 pairs of broad, apically pectinate chaetae; chaeta media not differentiated. One to 2 pairs of small pointed spinulae present. Pecten epipharyngis consisting of 5 broad, elongate, apically blunt scales. Chaetulae laterales (always present?) covered by pecten epipharyngis. Ungula narrowly U-shaped; basal sclerite absent. Premandible apically broad with 6-8 blunt teeth; lateral spine apically branched or plumose.
Apical tooth partly dorsal to and subequal in size to first inner tooth; 4 inner teeth present. Seta subdentalis extending beyond apex of third inner tooth, apically pointed. Seta interna consisting of 10-16 simple to weakly serrate branches.
Narrow, with 1 or 2 median and 5-7 pairs of lateral teeth; convex with teeth decreasing in size from median to lateral or truncated with 4-6 teeth across apex subequal in size and larger than teeth along sides. Ventromental plate elongate and narrow. Seta submenti arising closer to mentum than to postoccipital margin.
With palp with a lateral sensillum divided into 5-6 pointed lamellae. Chaetulae of palpiger setose. Lamellae of galea setose. Medial seta maxillaris (SM-1) bifid, remaining setae (SM-2 to SM-4) simple.
With anterior parapods fused, apically divided into 2 lobes, with each lobe bearing a circle of claws. Posterior parapods partly fused basally, each bearing circle of claws. Anal end bent ventrally. Procercus very short to absent, on lateral margin of preanal segment; 6-8 short anal setae present; no setae present adjacent to procercus or to anal setal bases. Two pairs of anal tubules present; shorter than posterior parapods; 1 tubule directed distoventrally, often extending ventrally to posterior parapods. Dorsum of thorax and abdomen with minute, closely set simple-, double-, or multi-pointed spines usually forming regular pattern on thorax and 2 transverse bands on each abdominal segment; sometimes black body pattern consisting of 4-5 coarse bars arising from a clear central circle and articulating with similar bars on surrounding area. Abdominal segments sometimes each with 2 pairs of small dorsolateral tubercles.
Boreoheptagyia is the sole member of the Diamesinae tribe Boreoheptagyiini. The larvae of Boreoheptagyia are distinctive amongst the Diamesinae due to the dorsal tubercles of the head capsule and areas of simple to multi-pointed spines or 4-5 black bars intermeshed with adjacent bars. The circle of claws on the parapods is also distinctive.
The larvae of Boreoheptagyia are found in cool, clear or glacial-fed streams and boulders in the splash zone. Twenty-two species are reported: most occur in the Holarctic, with one from Nepal, one from Tibet, one from Yunnan and Bhutan, and 3 from Oriental China (Sichuan). The larvae of 3 were described by Saunders (1928, 1930).