Small larvae, up to 4.5 mm long.
Antenna scarcely longer than 1/2 mandible length; 5 segmented; basal segment shorter than flagellum; segment 3 much shorter than 2nd and 4th. Ring organ at basal 1/3 of segment 1. Blade nearly 2 times as long as flagellum. Lauterborn organs small. Style broad and rounded.
Labrum SI bifid with 1 strong and 1 weak branch. Other labral setae normal, simple. One long and 2 minute chaetae, 2 spinulae and a labral rod present. Pecten epipharyngis consisting of 3 simple scales; 5 pairs of simple chaetulae laterales and 2 apically notched chaetulae basales present. Premandible with 1 apical tooth; brush absent.
Apical tooth about as long as combined width of 3 apical inner teeth; a fourth tooth indicated. Seta subdentalis long and slender. Seta interna with plumose branches.
Mentum with 2 wide median teeth, longer than first lateral tooth; 5 pairs of lateral teeth present; fifth lateral tooth longer than fourth. Ventromental plate small; beard absent.
Maxilla normally developed. Pecten galearis fine with long teeth. Anterior lacinial chaeta not differentiated. Appendix seta present.
Body with anterior and posterior parapods separate. All claws of anterior parapods with serrations. Procercus higher than wide, with 2 lateral and 7 anal setae. Supraanal seta long and strong. Anal tubules shorter than posterior parapods, digitiform.
Diagnosis based on tentatively associated material from Finnish, Norwegian, and Canadian lakes.
Larval Mesocricotopus were known first from oligo-mesotrophic lake profundal zones in n.w. Europe and Russia. A Nearctic species, M. loticus, has been reared from streams in Georgia and Ohio, USA. Larvae were found in winter to early spring in 2nd and 3rd order streams, perhaps from depositional substrates. One Georgia stream was ephemeral with no summer flow (Caldwell, 1996).