Large-sized larvae, to 20 mm long.
Frontal apotome separate from clypeus / labral sclerite 1, isolated labral sclerite 2, labral sclerites 3 and 4 fragmentary.
Antenna with 5 segments. Ring organ in basal 1/3 of 1st antennal segment. Lauterborn organs not visible. Antennal blade subequal to flagellum, accessory blade about 1/2 as long as segment 2. Style on apex of 2nd segment, about as long as segment 3.
SI seta serrate on each side, not plumose, SII and SIII fine, simple, SIVa simple sensillum. 8-10 long thin spinulae divided apically into short points, overhanging tormal bar. Chaetae deeply serrate on inner margin. Labral lamellae normal. Pecten epipharyngis simple, of 25-30 alternately large and small teeth. Premandible with 2 long apical teeth, 4 shorter, pointed inner teeth, brush strong; seta premandibularis simple.
Mandible slender, dorsal tooth absent, apical tooth slender, with 4 flattened inner teeth. Seta subdentalis long, extending to base of apical tooth, with inner side irregularly and finely serrate. Seta interna of 4 plumose branches. Pecten mandibularis with toothed lamellae.
Mentum with trifid median tooth with outer toothlets small, 6 pair of laterals, regularly decreasing in size. Ventromental plates contiguous medially, very wide, almost straight with coarse striae restricted to narrow transverse band.
Procercus somewhat longer than broad, with 8 long pale anal setae. Lateral and ventral tubules absent. Anal tubules almost as long as posterior parapods.
Axarus larvae are distinguishable from most other genera by their long, slender ventromental plates which are narrowly separated medially. This character also occurs in Lipiniella but Axarus larvae differ in that the mandible has 4 flattened inner teeth, a toothed seta subdentalis and no dorsal tooth. In addition labral sclerite 1 is present in Axarus. Axarus was erected by Roback (1963a) as a subgenus of Xenochironomus for 4 species occurring in the Nearctic. Larval characters as well as pupal and imaginal morphology show that Axarus merits generic status.
The rogersi and festivus species groups of Roback (1963a) are not supported by the larvae. Axarus and Lipiniella larvae have several, probably apomorphic characters in common (form of mentum and ventromental plates, numerous premandibular teeth, pecten mandibularis with distally divided lamellae, etc.) so that a close relationship between the two genera is to be expected.
Larvae of Axarus occur in littoral to sublittoral soft sediments of lakes and in river sediments and clay varves. Four species are known from North America, one (unpublished) from Europe. Several species occur in the Neotropical region.