Large larvae, up to 16 mm long.
With 5 segments; segment 5 longer than 4; segments 3 and 4 weakly separated, segment 3 annulated. Lauterborn organs about as long as combined length of segments 3 and 4. Style longer than combined length of segments 3 and 4.
With SI seta-like to lamelliform, sometimes apex bifid; SIII simple. Labral lamellae present, consisting of 2 broad pectinate lobes, or absent. Chaetae setose, some branched; chaeta media distinct, apically branched; 5-8 pairs of blunt, triangular, usually overlapping spinulae present. Pecten epipharyngis consisting of 7 long, blunt scales overlapping chaetulae laterales. Median pair of chaetulae laterales elongate. Ungula broadly U-shaped; basal sclerite vestigial. Premandible apically broad with 8-11 blunt teeth; long simple lateral spine present.
Apical tooth shorter than combined width of 4 inner teeth. Seta subdentalis very short. Seta interna with 13-17 simple to weakly serrate branches.
With one median and 7 pairs of lateral teeth; median tooth triangular and separated from first lateral tooth by deep V-shaped notch; second lateral tooth, sometimes with lateral notch, lying partly dorsal to first lateral; fifth and seventh lateral teeth shorter than sixth; only 2-4 lateral teeth visible in unflattened mentum. Ventrolateral plate large, extending laterally to median teeth and apparently continuous with first lateral and median teeth. Setae submenti arising closer to mentum than to postoccipital margin.
With palp about 2x as long as wide. Chaetulae of palpiger ranging from lamelliform to triangular pointed scales. Galea with setose and sometimes lamelliform scales. Setae maxillaris simple.
With anterior and posterior parapods separate, each bearing an apical crown of claws. Procercus longer than wide; posterior margin strongly sclerotized and sometimes nodulose, bearing 6-8 anal and 2 subapical setae. Supraanal setae weak. Body setae pale and indistinct.
The mentum of larval Pseudodiamesa with median tooth with V-shaped notches is distinctive. The median and first lateral teeth appear continuous with the ventromental plates and anterior margin of the mentum may be formed by fused ventromental plates.
Two subgenera are recognised: Pachydiamesa has pectinate labral lamellae, SI seta-like and procercus with 8 anal setae; Pseudodiamesa s.s. lacks labral lamellae, has a lamelliform or apically bifid SI seta and only 6-7 anal setae on the procercus. Ilyashuk et al. (2010) describe and illustrate features which distinguish larvae of a P. branicki group from those of the P. nivosa group.
Larvae of Pseudodiamesa inhabit springs, streams and lakes, including the profundal of oligotrophic lakes.
Twelve species are known, all from the Holarctic region, including 2 from the northern Oriental region. Larvae of several species are known but some associations are somewhat tenuous.