Medium to large-sized larva, 6-12 mm long, red in life. Head capsule rounded-oval; cephalic index 0.70-0.80. Dorsally, S5 anteromesal to S7; S7, S8, DP posteromedial to S8, non-aligned with S7. Ventrally S10 posterolateral to S9, VP posterolateral to SSm, SSm, S9, S10 non-aligned, S9, S10, VP near transversely aligned. SSm plumose; S7 plumose, S8 thin, simple; S9 plumose; S10 simple, thin.
1.2-1.4 times length of mandible. Antennal ratio 6.8-7.9. Basal segment about 4.5-6.3x as long as basal width, with ring organ at 0.77-0.82; segment 2 short, 2-3x as long as wide; segment 3 small, as long as wide; apical segment short subequal to segment 3. Style reaching segment 4. Blade shorter than flagellum, accessory blade same length as blade.
Slender, smoothly curved. Apical tooth ca 2 x as long as basal width, about 1/4 length of mandible. Mola extending into point, with small accessory tooth; long seta subdentalis arises between mola and accessory tooth. Ventrolateral seta 1 simple, 2 and 3 multi-branched; Ring organ midway between SII and SIII.
Basal segment of palp elongate, 3-4x as long as wide, with ring organ ca .25 from base.
With sparse fringe of swim-setae. With 4 anal tubules. Procercus about 3.8-5x as long as wide, with 8 apical setae. Posterior parapod with 11 large claws, 2 small claws depressed with expanded bases.
Bilyjomyia is a member of the tribe Macropelopiini, together with Apsectrotanypus, Bethbilbeckia, Brundiniella, Macropelopia and Radotanypus. Bilyjomyia shares with Brundiniella the uniquely widened base of some small posterior parapod claws, but is distinguished from it by the elongate (not squat) basal palp segment and the rounded innermost tooth of the pecten hypopharyngis. The simple S10 seta and the plumose S9 seta is the reverse for all other Macropelopiini in which S9 is strong and simple, S10 plumose. The few large teeth on the dorsomentum of Apsectrotanypus and Radotanypus distinguishes these genera from the others.
Larvae of Bilyjomyia, like those of most other Macropelopiini, live in depositional substrates of springs, seeps and headwater streams. Bilyjomyia algens has been found in mud in shallow pools in Pacific n.w. USA. The second known species, B. fontana, occurs in a summer-cool spring with mud bottom in Fukushima, Japan.