Medium-sized to large, red, 9-5 mm long.
Frontoclypeal apotome without frontal pit with coarse reticulate structure, anteriorly narrowed, Anterior margin weakly concave, undulating. labrum divided into two sclerites, lateral labral sclerites fragmentary.
Antenna with 5 segments, flagellum as long or longer than basal segment. Lauterborn organs opposite on apex of segment 2, style apical on segment 2. Ring organ in proximal quarter of segment 1, seta absent. Blade not extending to apex of antenna, accessory blade short.
Labrum with 2 lobes extending beyond labrum distally, bearing many long, thin, simple lamellae, possibly homologous with chaetuae. Setae labralis and neighbouring structures covered by this labral brush. SI-SIII simple, very thin and short, SIV unusually strongly developed and sunk into the distal margin of labral sclerite 2. Seta premandibularis simple. Labral lamellae consisting of 2 slender spines. Pecten epipharyngis of three long platelets, finely toothed distally. Premandible without definite teeth.
Mandible without dorsal tooth, with very strongly developed apical tooth and 3 minute inner teeth all brown. Pecten mandibularis well developed with up to 20 lamellae. Seta subdentalis inserted on ventral surface, long, simple, broad, extending to apical mandibular tooth. Seta interna inserted on dorsal surface, plumose, 4 branched.
Mentum frequently very worn, with recessed trifid median tooth with high central part and 2 smaller outer teeth, and 7 lateral teeth, 1st broad and tall, 2nd narrow and low, remainder decreasingly evenly. Ventromental plate close together medially, about 0.5 width of mentum, scarcely curved and with continuous fine setae. Setae submenti simple, divided in 1 African species.
Lateral and ventral tubules absent.
Xenochironomus larvae possess an unusual character on the labrum - a dense, bipartite lamellar brush extending beyond the margin of the head. In combination with the characteristic arrangement of teeth on the mentum and labral sclerite 1, which is divided into 2 plates, this makes the larvae of this genus impossible to confuse with any other Chironomini.
As a result of mining in fresh-water sponges the mouthparts are usually very badly worn or damaged.
Larvae of Xenochironomus are obligate miners in freshwater sponges in both standing and flowing waters.
The genus has one Holarctic species, X. xenolabis, and a few further, uninvestigated species in the eastern Palaearctic. Xenochironomus species are known from other zoogeographical regions, but the possibility that some may belong to Axarus remains to be investigated.