Small larvae, 4-5 mm long.
Apparently a single undivided sclerite.
Antenna nearly as long as head capsule, apparently 6-segmented, basal segment shorter than segments 2 + 3; 4th-6th segments short. Probably best interpreted as having 5 segments, with the se segment divided near medially (assessed as such by apical bearing of Lauterborn organ and style). Ring organ situated slightly distal to midpoint of basal segment. Blade arising nr middle of 2nd segment, extending to mid-'3rd' segment; accessory blade absent. Lauterborn organs apparently single on apex of apparent 3rd segment, extending to apex of penultimate segment, style distinct but short. Antennal seta absent.
SI slender and fine, closely approximated on rounded plate (that may represent a labral lamella), SII simple, elongate and broad, SIII very small, seta-like; SIVa long, 3-segmented, SIVb , 2 segmented. Pecten epipharyngis a triangular plate, with c10 spines/teeth. Premandible with 3 apical teeth,1 spine-like basal tooth, brush present.
Mandible lacking dorsal tooth, with short apical tooth and 3 inner teeth, the outermost pair angled relative to innermost. Seta subdentalis short, extending no further than basal tooth. Seta interna narrow, 6-7 simple setae. Pecten mandibularis of 2-3 lamellae.
Mentum with single median tooth though appearing trifid as 1st (of 6) lateral teeth somewhat appressed, 2nd and 3rd laterals also appressed to each other and distinct from median and outermost subequal three teeth. Mentum unicolourous pale. Ventromental plates coarsely striate, semilunar/rounded, shorter than mentum, with smooth anterior margin. Setae submenti simple, stout.
Palp short and squat with very reduced sensillae and spines, only one long, medially-directed lacinial chaeta. Hypopharynx distinctive, with strongly sclerotised inverted V-shaped support, median broad (tongue-like) lobe and few surrounding sensillae.
Anal setae seemingly arising directly from posterior margin of preceding segment and without distinct procercus, and extending posteriorly only slightly beyond posterior parapods. Posterior parapods slender with crown of minute claws. Anal tubules half length of posterior parapod.
Kloosia is a member of the Harnischia group of genera, with an apparent 6-segmented antenna, and mandibular seta interna present. It differs from most Harnischia-complex in having the SI seta very thin, relative to the more conventional lanceolate SII. It differs further from the similar Saetheria and Paracladopelma in having striae of the ventromental plates short, not reaching the anterior margin. From otherwise rather similar Hanochironomus and Olecryptotendipes, Kloosia may be separated in lacking a dorsal mandibular tooth and in seemingly lacking any procercus.
Seemingly there are few, but rather widely distributed species of this potamobiont genus, being found throughout the Holarctic, Afrotropical and Oriental regions (Reiss, 1988a). Larvae are found in sandy substrates of relatively clean, fast running waters with fine, including sandy, substrates.