Medium-sized to large larvae, up to 12 mm long.
Antenna with 5 segments; sometimes 4 segmented; segments consecutively shorter or segments 3 and 4 subequal in length. Ring organ on basal 1/3 of segment 1. Blade usually not extending beyond flagellum. Lauterborn organs small to large. Style usually as long as third segment.
Labrum with SI bifid; remaining S setae simple. Labral lamellae absent. Chaetae with simple long branches; spinulae with simple to branched apices. Pecten epipharyngis consisting of 3 scales. Chaetulae laterales usually simple, occasionally serrate; chaetulae basales with branched apices. Ungula U-shaped with an oval to quadrate basal sclerite. Premandible with 1 or 2 apical teeth, if 1, frequently notched; brush present or absent.
Apical tooth usually shorter than combined width of 3 inner teeth; longer in some species of O. Euorthocladius. Seta subdentalis apically expanded, pointed or notched. Seta interna consisting of 5-8 branches ranging from all simple to all serrate; absent in some species of O. Eudactylocladius and O. Euorthocladius. Outer margin either smooth or crenulate. Mola without spines.
Mentum with 1 median and usually 6 pairs of lateral teeth present; 7-9 pairs present in some species of O. Euorthocladius. Ventromental plate narrow; short, sparse beard present in O. Pogonocladius and some other species. Seta maxillaris simple.
Maxilla with chaetulae of palpiger triangular to leaf-like. Galea with simple lamellae anteriorly and several rows of pectinate lamellae dorsally; pectinate lamellae weak on some species of O. Euorthocladius; weak pecten galearis present in O. Pogonocladius and absent in other subgenera. Setae maxillaris simple.
Body with anterior and posterior parapods separate, each bearing an apical crown of claws. Procercus usually slightly longer than wide, without spur, and with 5-7 anal setae. Anal tubules shorter than, rarely as long as, posterior parapods. Body setae simple.
The larvae of Orthocladius are found in all kinds of aquatic habitats of the northern hemisphere. No records from outside this region have been validated. Generally larvae are most diverse and abundant in flowing waters, but O. (Eudactylocladius) larvae are found in hygropetric seeps, temporary waters and perhaps moist soil, whereas O. (Pogonocladius) larvae are in lakes and ponds.