Large, red coloured, up to 11mm long.
Frontoclypeal apotome present with convex anterior margin, without frontal pit; with labral sclerites fragmentary.
Antenna with 6 segments Lauterborn organs well developed, alternate on apex of segment 2 and subapex of 3, short style subapical on segment 3. Ring organ in basal third of segment 1, seta absent, basal segment decribed with pouch-like structure between ring organ and base (but not visible in sw USA or Australian taxa). Blade extending to apex of antenna, accessory blade short.
SI broad, shovel-shaped, plumose only distally, basally with broad fusion; SII apparently plumose on both sides; SIII narrow, short; SIVa small. Seta premandibularis simple. Labral lamellae normal. Pecten epipharyngis tripartite, each plate distally with several blunt teeth. Premandible with 2 teeth and strong beard.
Pale dorsal tooth present, with small subapical tooth, small apical tooth and 2 inner teeth, with only indication of 3rd. Pecten mandibularis weak. Seta subdentalis broad, apically hooked. Seta interna inserted on dorsal surface, plumose, 4-5 branched.
Mentum with ventromental component of 6 teeth, median pair small, first outer larger, next slightly narrower, but slightly more protruding, then the first of 5 dorsomental teeth which evenly decrease in size laterally; all teeth similarly dark brown/black, median teeth perhaps slightly paler. Ventromental plates separated medially by 1/3 width of ventromentum, about width of mentum, curved and continuously striated. Setae submenti simple.
Lateral and ventral tubules absent.
Omisus larvae are very similar to those of Paratendipes. They also have a 6 segmented antenna with alternate Lauterborn organs, only 2 inner mandibular teeth and a pale dorsal tooth. However, the median teeth of the mentum are usually dark, the central pair lower and more slender than the outer pair, whereas in Paratendipes all 4 median teeth are similar in height or, exceptionally, the central pair may be taller.
Pupal exuviae show that Microtendipes caledonicus (from Bavaria, Reiss 1982) is an Omisus species. Other findings are from near Lake Baikal (Chernovski 1937x, sub Stictochironomus rezvoi; Chernovski 1949b, sub Microchironomus ? rezvoi) and northern Germany (Kreuzer 1940, sub Paratendipes sp.).
Larvae of Omisus occur in small, partly dystrophic lakes and in dystrophic small water bodies, e.g. peat cuttings.
The genus has a Holarctic distribution with one described species each from North America and Europe and possibly further species in the Nearctic.