Medium-sized to large larvae, up to 9.5 mm long.
Antenna with 7 segments; segment 3 much smaller than fourth; segment 7 hair-like, vestigial. Ring organ in basal 1/4 of segment 1. Blade shorter to longer than flagellum. Lauterborn organs absent or vestigial.
Labrum with SI plumose. Two labral lamellae rounded, pointed, or triangular, weakly sclerotized. Pecten epipharyngis weakly sclerotized, consisting of 3 serrated scale-like spines. Chaetulae laterales simple. Premandible with 2 indistinct teeth; brush absent.
Apical tooth shorter than combined width of 3-4 inner teeth. Seta interna with 6-7 branches; 1-2 smooth, remaining broad and plumose.
Mentum with 1-2 median teeth and 5 pairs of lateral teeth; median teeth with or without accessory teeth. Ventromental plate distinct, extended beyond margin of flattened mentum; beard absent.
Maxilla with broad anterior lacinial chaeta, sometimes differentiated from other chaetae. Pecten galearis present, with distinct teeth.
Body with well developed procercus. Anal tubules shorter than posterior parapods. Body setae inconspicuous.
Larval Heterotrissocladius occur in lakes (littoral to profundal), ponds, puddles, and in all sizes of flowing waters. Typically subpilosus group and maeaeri group larvae indicate oligotrophy. Members of the marcidus group are commonly found in oligotrophic lakes, as well as acid streams.
The genus is Holarctic, with up to 15 species, but also reported from the Ecuadorean Andes.