Small larvae, about 5 mm long, slender, but not as slender as those of Paraboreochlus; about 12 x longer than broad.
Antenna slender; indistinctly 5 segmented. Ring organ situated somewhat above middle of basal segment. Beyond segment 2 the antenna is annulated along a distance nearly equal to the length of the basal segment. Annulated portion at about mid length with a small Lauterborn organ. Blade very slender, much longer than segment 2, situated on a tubercle; accessory blade reduced or lacking.
Labrum with labral sensillae long and slender; SI and SII on high, somewhat clubbed tubercles, SIII on low tubercles; SIV A again on high tubercles; SIV B developed as a long, slender rod. Numerous apically divided chaetae present in front of SI. Pecten epipharyngis 3 lobed.
Mandible with 6 inner teeth.
Mentum with 1 protruding median tooth and 7-8 pairs of lateral teeth.
Body with procercus very slender, hyaline anteriorly, blackish posteriorly; with 5 anal setae, some very long. Anal segment without a dorsal pair of long setae (present in Paraboreochlus). Eleventh body segment dorsally with 2 small spiracular rings.
Larval Boreochlus and Paraboreochlus have bicoloured procerci, hyaline anteriorly, dark from base to apex posteriorly, as in most Boreochlini. A prominent, as opposed to recessed median mental tooth distinguishes from Trichotanypus, and the few lateral mental teeth (7) from Lasiodiamesa. Boreochlus and Paraboreochlus are distinguished from each other by the unique presence of two long, strong black setae lying dorsal to the anal tubules in Paraboreochlus, and the reduced number of anal setae (5 vs. 8) in Boreochlus.
Boreochlus is Holarctic in distribution, with southern outposts in northern Burma. Of the seven described species, four are known as pupae (Brundin 1966a). The immature stages live amongst mosses in springs and streams.