Small, red, up to 4mm long. Two widely separated eyes.
Frontoclypeal apotome without frontal pit, anteriorly protruding medially; indistinct lateral labral sclerites, except with 5 palmately divided mesally.
Antenna with 5 segments. Basal segment twice length of flagellum, 3rd segment about as long as broad, chitinized only along narrow median ring. Lauterborn organs absent, style indistinguishable. Ring organ in basal 1/4 of segment 1, seta absent. Blade broad, extending beyond apex of antenna, accessory blade short.
SI-SIV closely approximated, SI and SII simple, very long and slender; SIII narrow; SIV indistinguishable only as pits. Seta premandibularis long and simple. Labral lamellae and pecten epipharyngis apparently absent. Premandible with 3 teeth, 1st and 3rd long, pointed, 2nd shorter and broad. Chaetulae basales and possibly laterales unusually long and distally plumose.
Narrow, all pale, with 2 dorsal teeth, apical tooth separated from 5 inner teeth. Pecten mandibularis present. Seta subdentalis fine, extending to base of apical mandibular tooth. Seta interna with long bare stalk, coarsely plumose distally.
Mentum medially with four teeth, central pair truncated, larger, outer pair very small, pointed, with 6 lateral teeth, first lateral twice width of second. Ventromental plates separated medially only width of median mental tooth. Ventromental plate narrow, strongly curved laterally, dorsally striate, with continuous coarse striae. Setae submenti simple, inserted basomedially on ventromental plates.
Lateral and ventral tubules absent. Body distinctly tapering posteriorly.
The very small larva of Pagastiella possesses a pale mentum and mandibular teeth. It differs from the similar Nilothauma larva in having widely separated eyes, a substantially longer basal antennal segment and characteristic mentum, together with completely striated ventro mental plate. In addition the setae submenti are unusually situated on the ventromental plate, and the mandible has 2 slender dorsal teeth. The palmately divided, presumed labral sclerite 5 is unique to Pagastiella.
Larvae of Pagastiella occur in littoral to sublittoral lake sediments.
The genus is distributed in subarctic to temperate zones of the Holarctic. One species is known from the Palaearctic (P. orophila) and one from the Nearctic (P. ostansa).