Medium sized to large larvae, up to 11.5 mm long.
Antenna with 5 segments; segment 5 longer than fourth; segment 3 annulated, very rarely annulations absent. Lauterborn organs vestigial to small. Style about as long as segment 3.
Labrum with SI seta-like to lamelliform; S III usually simple, occasionally bifid with simple or divided branches. Labral lamellae consisting of narrow, elongate scales with pointed, serrated, or branched apices, often appearing to arise in groups. Group of branched chaetae on each side; chaeta media usually distinct and apically branched; 3-5 pairs of spinulae present, each apically divided into 2 or more points. Pecten epipharyngis consisting of 5 elongate, helmet-shaped scales. Two or 3 pairs of chaetulae laterales partly covered by pecten epipharyngis. Ungula broadly U-shaped; basal sclerite elongate. Premandible apically broad with 5-7 teeth; lateral spine present, frequently branched.
Mandible with apical tooth variably developed, usually shorter than combined width of four distinct inner teeth. Seta subdentalis weakly developed, not reaching innermost tooth. Seta interna very strongly branched, with between 12 and over 20 weakly serrate branches.
Mentum with single or double median and 7-11 pairs of lateral teeth. Ventromental plate vestigial to small. Setae submenti arising closer to mentum than to postoccipital margin.
Maxilla with palp usually about as long as broad and with curved sides. Chaetulae of palpiger and lamellae of galea setose. Setae maxillaris simple.
Body with anterior and posterior parapods separate, each bearing an apical crown of claws. Procercus very reduced to absent, when present, without lateral setae; 4 anal setae present, arising from body wall when procercus absent. One seta arising anterior to procercus or bases of anal setae. Supraanal setae weak. Body setae pale and indistinct.
Typically larvae of Diamesa are distinguished by having the procerci absent or very reduced, and with a maximum of 4 anal setae. These arise from the body wall if the procercus is absent. When present, procerci lack lateral setae. The seta arising anterior to the procercus or anal setae probably is a displaced subapical seta. The diagnosis refers to larvae having reduced or no procercus and 4 anal setae.
Several species (e.g. D. dampfi Kieffer, D. permacer (Walker) and D. appendiculatus Lundstrom) that have relatively well-developed procerci bearing 5 or more anal setae and 2 subapical setae have been assigned to Diamesa. The first two named appear to belong better to Syndiamesa and are distinguished by having the median tooth of the mentum much wider than a lateral tooth. D. appendiculatus has 2 small deeply recessed median teeth on the mentum, premandible with 3 teeth and no basal sclerite and is almost certainly not a Diamesa.
The systematic position of several other species is more problematic, emphasising the need for a revision of Diamesa including the larva. D. baicalensis Chernovski has attributes of a typical Diamesa including the reduced procercus and number of anal setae, but the mentum has only 7 teeth. D. carpatica and D. heterodontus, described by Botnariuc and Cure, are more like Potthastia, except D. carpatica has a bifid SI.
Larvae of Diamesa generally are cool-adapted, inhabiting flowing water, springs and to a lesser extent shallow still water and the hygropetric zone.
Over 100 species are known, predominantly from the Holarctic region, 7 from the Oriental (India, Nepal, Tibet, Afghanistan and Mount Kinabalu in Sabah, Borneo) and 3 from the Afrotropical mountains. About 1/3 of the larvae are known and some species have been based on larvae alone.