Medium-sized larvae, up to 10 mm long. Head brownish; body red. Head capsule longish-oval; cephalic index about 0.6-0.7. Dorsally DP ventral to S5, both quite anterior to widely-separated S7-S8. Ventrally VP round, postero-lateral to S9 and S10, and antero-lateral to SSm, which is posteriormost; S10 more mesial and rather distant from S9. SSm and S7 multibranched, remaining setae simple.
About 1/3 head length, 2x as long as mandible. Antennal ratio 1.7-3.0. Basal segment about 10x as long as basal width, with ring organ 0.65; length of segment 2 variable, 6-8x as long as wide; broad intersegmental section between segments 2 and 3. Style 2x as long as segment 3 and peg sensilla. Blade may be considerably longer or shorter than flagellum, 10-12x as long as height of basal ring, basal ring as high as wide. Accessory blade longer than or considerably shorter than blade.
Gradually curved; apical 1/2 strongly narrowed, almost 2.5x as long as wide; basal 1/2 strongly broadened. Apical tooth 2x as long as basal width, 1/4 length of mandible, distal 1/2 blackish-brown. Elongate lamella on margin of apical tooth extends to conical accessory tooth, lying on inner surface of strongly expanded mola which forms large broad tooth; seta subdentalis robust. Ventrolateral setae simple, aligned on outer margin of basal 1/2 of mandible; setae 1 and 2 very close together; distance between setae 2 and 3 3x as great as that between setae 1 and 2; ring organ proximal to seta 3, somewhat displaced ventrally.
Basal segment of palp 2.5x as long as wide; b seta 2-segmented, basal segment 2x as long as apical segment. Ring organ at 0.8.
Without fringe of swim-setae. Anal tubules bluntly spindle-shaped, about 4x as long as wide. Procercus 6-7x as long as wide, with 7 apical setae. Claws of posterior parapod simple, about 13-15; large claws with small points on inner and/or outer side.
Natarsia, placed in its own isolated tribe Natarsiini, intermediate between Pentaneurini and Macropelopiini, is easily distinguished from larvae of other Tanypodinae lacking a toothed dorsomental plate (Pentaneurini) by the relatively thick and short antenna (only about 1/3 the head length) and the large molar tooth of the mandible. The specific differences in length of antennal blade and accessory blade are noteworthy, as is the configuration of the teeth of the ligula, which may be deeply concave or straight. The placement of the VP on the ventral head capsule, aligned directly posterior to S9, is intermediate between the situation in Pentaneurini (VP mesial to S9) and that of all other taxa (VP more lateral than S9).
Larvae of the North American species of Natarsia live in small running waters, perhaps favouring cool water. European species inhabit streams, springs and the littoral zone of montane or northern lakes, but perhaps most especially show hygropetric behaviour in small standing waters including Sphagnum bogs (Vallenduuk and Moller-Pillot, 2008). Natarsia larvae were reported from 'wet soils' in the Czech Republic (Frouz and Matěna, 2000).
The genus is essentially Holarctic in distribution. Two species are named from the Nearctic, with some additional hidden diversity suggested by Epler (2001). Three are known from the Palaearctic: N. nugax, widespread in Europe, Russia (including Siberia); N. punctata, widespread in Europe, Russia, Japan and the Japanese N. tokunagai (Kobayashi and Endo, 2008). The genus is recorded from the Oriental region in China (Cheng and Wang, 2006). Natarsia found were found in more than 70% of 18 lakes studied in the Yucatan Peninsula, Mexico (Vinogradova and Riss, 2007).