Medium-sized larvae. Body frequently striped, often violet or bluish or luminous greenish.
Antenna with 5 segments; quite variable in structure; segment 3 shorter than 4th if antenna not strongly reduced. Blade much longer than flagellum if antenna reduced, slightly longer or shorter with normal antenna. Lauterborn organs absent or indistinct to longer than segment 3.
Labrum with SI plumose or occasionally (M. fuscipes Meigen) simple, stubby and peg-like; remaining S setae normally developed. Labral lamella present slightly anterior to bases of SI, consisting of bell-shaped structure more or less deeply divided and plumose or sometimes simple at apex. Only 2 spinulae present; some chaetae simple, others serrate. Pecten epipharyngis consisting of 3 smooth spines. Chaetulae laterales reduced; only median pair as large as scales of pecten epipharyngis. Premandible with 2-4 apical teeth and distinct to indistinct brush.
Apical tooth shorter than combined width of 4-5 inner teeth. Seta subdentalis present. Seta interna consisting of 6-7 branches which may be broad or narrow, when broad at least longest seta smooth, the others plumose.
Mentum with 1-4 median teeth; usually median teeth lower than first of 5-6 pairs of lateral teeth. Ventromental plate very weak; beard absent.
Maxilla with well developed sensillae, lamellae, and chaetae. Anterior lacinial chaeta not differentiated. Pecten galearis apparently absent.
Body with anterior parapods usually divided. Posterior parapods normal. Procercus well developed or very small (M. terrester Pagast) with 5-6 relatively short anal setae; often with posterior spur or uneven sclerotization. Supraanal setae short. Anal tubules 1/2 as long to much longer than posterior parapods. Body setae short.
Larval Metriocnemus are found in a wide variety of habitats: damp moss, phytotelms (plant-held waters), springs, ditches and streams. One species occurs in lakes, another in marine splash zone pools. The genus is Holarctic, and also found in phytotelmata in Jamaica, and high altitude tropical Africa.