Medium sized to large larvae, up to 11 mm long.
With 5 segments; segment 5 longer than fourth; last 3 segments unequal in length; segments 3 and 4 weakly separated; segment 3 annulated. Lauterborn organs as long as segment 3. Style as long as combined lengths of segments 3 and 4.
With SI seta-like to lamelliform; SIII simple. One pair of broad elongate, apically pectinate or branched labral lamellae present, usually located slightly lateral of bases of SI. Chaetae with long thin branches; chaeta media branched or pectinate; 5-6 pairs of pointed spinulae present. Pecten epipharyngis consisting of 3 narrow, pointed scales, 5 pairs of simple chaetulae laterales present, median pair broad. Ungula broadly U-shaped; basal sclerite absent. Premandible with 5-6 teeth; simple lateral spine present, either long or short and sinuate.
Apical tooth longer or shorter than combined width of 4 inner teeth; inner teeth arising from common base, forming a compound tooth, or arranged in normal row. Seta interna with 5 or 14-20 branches, branches weakly serrate. Mandible narrow with distolateral corner elongate.
With median area without teeth; 6-8 pairs of lateral teeth present, only 3 or 4 seen in unflattened mentum; outer lateral tooth sometimes very small. Ventromental plates ventrally fused beneath median area of dorsomentum, extending laterally and anteriorly to mental teeth; median area truncated with 4-6 median ridge-like projections. Seta submenti arising closer to postoccipital margin than to mentum.
With chaetulae of palpiger lamelliform, often not evident. Galea with setose lamellae. Setae maxillaris simple.
With anterior and posterior parapods separate, each bearing an apical crown of claws. Procercus slightly longer than wide, strongly sclerotized with posterobasal projection extending on adjacent body surface; 7-8 anal and 2 subapical setae present. Body setae pale to light brown.
The posterior (retracted) placement of the setae submenti and the broad fused ventromental plates with truncated apex bearing ridge-like projections are distinctive amongst the Diamesini. One larval type has the 4 inner mandibular teeth forming a weak compound tooth. Another type has the inner teeth in a normal row.
Larval Pagastia inhabit small to medium-sized bodies of flowing water. Two subgenera are recognised: Hesperodiamesa Sublette, for 1 species and Pagastia sensu stricto for 7 species. The larvae of one species is a dominant retreat builder in the Upper Colorada River (Monroe et al., 2005).
The genus is Holarctic in distribution with at least 9 species, of which the larvae of 6 are known (Makarchenko and Makarchenko, 2000). Endo (2004) described a new species Pagastia hidakamontana Endo and reported the previously Nearctic P. orthogonia Oliver from Japan, and reviewed the Japanese species P. lanceolata (Tokunaga) and P. nivis (Tokunaga).