Large larvae, up to 11 mm long.
With segment 3 longer than 2 and 5 longer than 4. Style and Lauterborn organs subequal in length, about 1/2 as long as segment 3.
With SI lamelliform; SIII simple. Labral lamellae consisting of a transverse row of narrow, apically bifid scales. Chaetae with setose branches; chaeta media distinct. Pecten epipharyngis consisting of 5 elongate, helmet-shaped scales. Ungula broadly U-shaped; basal sclerite elongate. Premandible with 6 teeth; lateral spine apically branched.
Apical tooth slightly larger than first inner tooth. Seta interna with 22-25 weakly serrate branches.
With one median and 9-10 pairs of lateral teeth; median tooth slightly wider than first lateral tooth. Ventromental plate small. Setae submenti arising closer to mentum than to postoccipital margin.
With chaetulae of palpiger and lamellae of galea setose, similar to Diamesa. Setae maxillaris simple.
With anterior and posterior parapods separate, each bearing an apical crown of claws. Procercus shorter than wide with posteroapical and long basal spur; bearing 5-6 anal and 2 subapical setae. Body setae pale.
Larvae of Syndiamesa resemble those of some Diamesa, especially in the structure of the mouthparts. The 3rd antennal segment is longer than the 2nd in Syndiamesa whereas it is shorter in Diamesa. The two genera are distinguished primarily as larvae in the procercus: in Syndiamesa it bears 5 or 6 anal and 2 subapical setae. Typical Diamesa have a small or no procercus with only 4 anal setae. The supraanal setae in Syndiamesa are stronger than in Diamesa. Larvae assigned to Diamesa with procerci that bear 5 or more anal setae may be distinguishable only on the basis of the antennae.
Larvae of Syndiamesa inhabit springs, small bodies of flowing water and the hygropetric zone. The genus is Holarctic: Five species have been recorded from Europe and North Africa, about seven species from the eastern Palearctic region including Japan (Endo, 2007), Russia and China, and one unnamed species from North America. Only 1 larva is known amongst the 3 described species.