Medium-sized to large, up to 17 mm long. Dark red to orange larvae.
Frontal apotome separated from equally-wide clypeus; labrum present, remaining labral area fragmentary. Without frontal fenestra.
Antenna with 5 segments, diminishing in size distally Lauterborn organs about as long as segment 3, opposite on apex of 2nd segment, with style. Ring organ deep, near base of segment 1, seta absent. Blade shorter than flagellum.
SI characteristic, slender, triangular in outline, plumose on inner side only; SII slender, feathered on both sides; SIII short, slender; SIV normally developed. Seta premandibularis distally divided. Labral lamellae normal. Pecten epipharyngis divided into 3 parts, each plate with several strong distal teeth and numerous further teeth on surface. Premandible with 3 teeth.
Mandible with obscure dorsal tooth, with small tooth attached on inner side; apical tooth slender, and 3-4 inner teeth. Pecten mandibularis modestly developed, lamellae distally divided. Seta subdentalis inserted on ventral surface, simple, usually curved distally. Seta interna inserted on dorsal surface, plumose, multi-branched.
Mentum with distinct dorsomentum of 3-4 median teeth, of which central 1-2 are lower than outer pair; 6 pairs of lateral teeth on each side, with 1st equal to or lower than 2nd, then evenly decreasing in size laterally. Ventromental plates medially connected to median mentum, and thus separated medially at least by width of median section, narrow and strongly curved, with smooth to coarsely toothed anterior margin; striae interrupted medially. Setae submenti simple.
Lateral and ventral tubules absent.
Larvae of Endochironomus have very uniform characters, so that separation into species groups is not appropriate. The species previously included in the intextus group are now placed in Tribelos.
A frontal apotome and an equally broad clypeus are present in Endochironomus larvae. The form of the SI is specific to the genus, being slender, triangular and feathered only along the inner side; the seta praemandibularis is always divided distally. The pecten epipharyngis consists of 3, distally toothed plates with toothlets also on their faces. The dorsal tooth of the mandible does not extend beyond the outer margin. Tribelos and Phaenopsectra larvae are frequently confused with those of Endochironomus because of their very similar mentum. However, in both of these genera the SI is feathered along both sides, the seta praemandibularis is simple and the dorsal tooth protrudes beyond the outer margin of the mandible. In addition Phaenopsectra larvae have a frontoclypeal apotome and a pecten epipharyngis which lacks toothlets on the surface, whilst Tribelos has a labral sclerite 1 in place of the clypeus.
Larvae of Endochironomus occur in "Aufwuchs" of living and dead substrata in almost all types of still water or mine in leaves and stems of macrophytes with a tendency to preferential occurrence in small, eutrophic standing waters. Some species are tolerant of brackish water. A few species overwinter in coccoons in sediments.
Distributed primarily in temperate regions of the Holarctic. About 15 species are known from the Holarctic, four from the Afrotropical and one from the Oriental region.