Large, to 18 mm long. Red larvae. With 2 pairs of separate eyes.
Frontoclypeal apotome without frontal fenestra, separated from labrum by convex anterior margin, distinct labral sclerite 5, remaining area fragmentary.
Antenna with 5 segments. Lauterborn organs opposite on apex of 2nd segment, with style. Ring organ in basal 1/3 of segment 1, seta absent. Blade subequal to flagellum, accessory blade very short.
SI and SII plumose on both sides; SIII short, slender; SIV normally developed. Seta premandibularis simple. Labral lamellae normal. Pecten epipharyngis divided into 3 parts, each platelet distally serrate. Premandible with 2-4 teeth.
Mandible with all teeth dull brown. Dorsal tooth relatively short, apical tooth and 4 inner teeth. Pecten mandibularis modestly developed, lamellae distally divided. Seta subdentalis inserted on ventral surface, simple, long, slender, curved distally. Seta interna inserted on dorsal surface, plumose, multi-branched.
Mentum dull brown, with distinct dorsomentum of 4 anteriorly-projecting equally large median teeth, or with median pair lower; 6 pairs of lateral teeth on each side, with 1st lower than 2nd, then decreasing in size laterally to small apressed pair of outermost teeth. Ventromental plates medially connected to median mentum, and thus separated medially by width of median section, broad and semi-lunar, with smooth anterior margin; striae continuous. Setae submenti simple.
Lateral and ventral tubules absent.
Sergentia larvae may be recognized by the 4 more or less equally broad, pointed and widely separated median mental teeth which are set above the remainder of the mentum, and also by the 4 inner mandibular teeth. In addition the anterior margin of the frontoclypeal apotome is convex, not straight as in Polypedilum.
Sergentia was regarded by all North American authors as a synonym of Phaenopsectra. However, at least the larvae and male imagines possess characters which establish Sergentia as a separate genus. The mandible of the Sergentia larva has 4 inner teeth, regularly decreasing in size (only 3 in Phaenopsectra, with a deep notch before the basal one), brown mental teeth (shining black in described Phaenopsectra), as well as 4 separate, equally broad median mental teeth and 6 pairs of scarcely differing lateral teeth (in Phaenopsectra 2 small, sunken, closely adjacent teeth centrally and 2 large outer median teeth, together with 6 pairs of lateral teeth, very strongly differentiated in size).
Baicalosergentia Linevich was cited by the author herself as a subgenus of Sergentia or as a valid genus (Linevich 1977a). On the basis of the descriptions the larvae cannot be distinguished from Sergentia s.str.
Larvae of Sergentia occur in the sublittoral and profundal of oligotrophic to mesotrophic lakes. They are probably cold-stenothermic.
The genus is distributed within subarctic to temperate and montane regions of the Holarctic. About six species occur in the Palaearctic (including Baicalosergentia), at least one species in the Nearctic.