Small larvae, up to 5 mm long. With brown thoracic striations.
Antenna with 5 segments; segments consecutively smaller. Ring organ low on basal segment. Blade shorter than flagellum. Lauterborn organs well developed, but shorter than segment 3.
Labrum with SI either simple, with weak apical serrations, or bifid in apical half or at extreme apex, equally wide over its whole length, or slightly widened at apex. Labral lamellae absent. Other labral setae simple. Chaetae weakly serrated; 4-5 pairs. Pecten epipharyngis consisting of 3 broad, pointed scales. Chaetulae laterales and basales all smooth. Premandible simple; brush absent or replaced by a single serrate seta.
Apical tooth shorter than combined width of 3 inner teeth. Seta interna with plumose branches.
Mentum with three median teeth higher than remaining 4 pairs of lateral teeth; median tooth about twice as wide as first lateral tooth. Ventromental plate weak; beard absent.
Maxilla with anterior lacinial chaeta very broad and short, rectangular. Pecten galearis well developed.
Body with well developed parapods; at least some claws of anterior parapods weakly serrated. Procercus with strong preapical spur and smaller basal spur; longer than wide; either with 3 strong and 2 weak or 5 strong anal setae. Supraanal seta less than 1/2 as long as anal setae. Anal tubules slender, digitiform, slightly to much longer than posterior parapods. Body segments with one long seta (about half as long as the segment) on each side.
Among larvae with the SI seta simple, slightly frayed or bifid, and lacking a beard, the presence of procercal spurs and a body beset only with simple setae allows identification from other orthoclads. The median tooth of the mentum has been treated as simple, rounded, but a median 'nipple' is evident from fresh material from Thailand, and is indicated in a Japanese species (Niitsuma, 1990: Fig. 9).
Niitsuma (1990) provides a key to the larvae and pupae of species of the genus.
Larvae live in mosses, fallen leaves and algae in streams and seepages, generally in high quality water in forested areas.
Paracricotopus are known from the Holarctic, with one undescribed species, known as larva and unassociated pupa, from Thailand. Described species are: niger (Kieffer, syn. uliginosa Brundin), glaber (Saether), mozleyi (Steiner), irregularis Niitsuma, oyabeangularis (Sasa, Kawai & Ueno).