Medium-sized to largish larvae, up to 9 mm long.
Clypeus and frontal apotome weakly to strongly separate. All head capsule setae simple.
Antenna with 4 segments, each consecutively shorter; segment 2 divided into 2 parts with proximal part about 1/3 length of distal part. Ring organ in basal 1/3 of segment. Blade shorter or longer than flagellum. Lauterborn organs distinct, on apex of distal part of segment 2. Style on apical part of distal part of segment 2.
SI plumose, remaining S setae simple. Chaetae simple to branched, spinulae simple or with apical notch. Labral lamellae consisting of 2 broad, pectinate lobes. Pecten epipharyngis of 3 short scales. Premandible with broad with weakly bifid apical tooth; brush absent.
Mandible with apical tooth shorter than combined width of 4 inner teeth. Seta subdentalis long, apically pointed and slightly curved. Seta interna with 6-7 serrate to plumose branches.
Mentum with two long median teeth and 5 pairs of lateral teeth; a small tooth usually present between bases of median teeth; 4th and 5th lateral teeth sometimes appressed to each other. Ventromental plate small; beard absent. Setae submenti arising on posterior half of submentum.
Anterior and posterior parapods separate, each bearing an apical groups of claws. Procercus about 2x as long as wide with 8 anal setae. Anal tubules shorter than length of posterior parapods. Body setae simple.
The taxon and group previously known as B. marcidus should be called B. bifida; the other group is the flavifrons group.
Oliver and Roussel (1983) key North American species.
Larval Brillia fall into two general ecological groupings, those that mine immersed wood and those that live in leaf packs. There are records from springs, running waters, lake littoral shores and the hygropetric zone, but larvae are not usually found away from wood/leaf habitats.
The genus is known from the Holarctic, Neotropics and Oriental regions. Differentiation of past larval allocations to Brillia from a "radiation" of relatives such as Euryhapsis Oliver and adult-based Irisobrillia Oliver, Tokyobrillia creates some uncertainty concerning true limits of distribution and immature stage habitats of Brillia.