Medium sized to large, up to 12 mm long. Red larvae. With 2 pairs of separate eyes. Head capsule shining.
Frontal apotome separated from narrow clypeus by straight anterior margin, without frontal fenestra; labrum present, remaining labral area fragmentary.
Antenna with 5 segments. Lauterborn organs about as long as segment 3, opposite on apex of 2nd segment, with style. Ring organ in basal 1/3 of segment 1, seta absent. Blade longer than flagellum, accessory blade short.
SI and SII plumose on both sides; SIII, SIV and labral lamellae normally developed. Seta premandibularis simple. Pecten epipharyngis divided into 3 plates each serrate with several rows of toothlets on dorsal surface. Premandible with 2 teeth.
Mandible with pale dorsal tooth, apical tooth and 3 inner teeth dark, middle inner tooth may be smaller. Pecten mandibularis modestly developed. Seta subdentalis inserted on ventral surface, simple, usually curved distally. Seta interna inserted on dorsal surface, plumose, multi-branched.
Mentum with indistinct dorsomentum of 4 median teeth, subequal in breadth, median pair slightly smaller than next; 6 pairs of lateral teeth on each side, with 1st lower than 2nd, then evenly decreasing in size laterally. Ventromental plates medially not quite 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 fine, interrupted in dorsal band. Setae submenti simple.
Lateral and ventral tubules absent.
Tribelos larvae may be recognized and separated from the closely related genera Endochironomus, Phaenopsectra and Sergentia by the following combination of characters: frontal apotome and labral sclerite 1 and 2 present (Phaenopsectra and Sergentia have frontoclypeal apotome and labral sclerite 2), trifid pecten epipharyngis with additional teeth on the surface (Phaenopsectra and Sergentia without such teeth), seta praemandibularis simple (distally divided only in Endochironomus), only 2 of the 4 median teeth of mentum delineated from rest of mentum (in Phaenopsectra and Sergentia all 4 median teeth are delineated) and mandible with 3 inner teeth as in Phaenopsectra (Sergentia with 4 inner teeth).
As a result of the almost identical mentum, larvae of Tribelos species have usually been erroneously identified as Endochironomus or Phaenopsectra and vice versa. There is now sufficient material available for these genera to be distinguished as larvae.
Presumably as a deformity, 5 instead of 4 median teeth may occur on the mentum (cf. Moller Pillot 1978a, fig. IV. 13.c).
Of the six Nearctic species assigned to Tribelos only the type species of the genus, T. jucundus, can be accepted on imaginal characters. The generic position of the other species is an open question.
Larvae of Tribelos occur in littoral sediments of lakes.
The genus is Holarctic in distribution, ranging from subarctic to subtropical zones. One Palaearctic species (T. intextus) and six Nearctic species are referred to Tribelos.