Small larvae, up to 5 mm long. Head yellow to brownish; antenna, especially segment 2, brown; body pale, even white. Head capsule oval; cephalic index about 0.6. Dorsally, DP small, not larger than dorsal setal pits; S8 and S7 very close, unaligned with S5 or DP. Ventrally S10 lateral to posterolateral to S9; VP larger than setal pits, posterior, posterolateral or mediolateral to S9, closer to S9 than S10, arranged diagonally, transversely, or in triangle, distant anteriorly to SSm. S5, S7 and SSm 3-4 multibranched, S8, S9 and S10 simple.
Long, 60% length of head, almost 3.5x as long as mandible. Antennal ratio 3.5-4.5. Basal segment about 10x as long as basal width, with ring organ at 0.45-0.65; segment 2 about 9.5x as long as wide, 2x as long as terminal segment. Style extending beyond middle of segment 4. Peg sensilla 1/2 length of segment 3. Blade and accessory blade of equal length, almost as long as flagellum; basal ring 2x as high as wide.
Strongly curved, gradually narrowed towards apex. Apical tooth 2.5x as long as maximum width, rather more than 1/3 mandible length. Inner tooth well developed; mola developed as large, low tooth; seta subdentalis large. Ventrolateral setae and ring organ aligned on outer margin of proximal 1/2 of mandible; setae 1 and 2 very close together.
Palp arising from substantial hyaline base; basal segment 4x as long as wide, with ring organ 0.5-0.6.
Without fringe of swim-setae. Anal tubules 3x as long as wide. Procercus 4-6x as long as wide, with 7 apical setae. Claws of posterior parapod all yellow and simple: larger claws with small spines on inner and/or outer margin.
Larsia belongs to the Pentaneurini: the pupa and adult of Larsia appear close to Zavrelimyia, but the larva less so. The simple small claws of the posterior parapod differentiate from many, but not all Zavrelimyia (q.v.). The much longer antenna in Larsia (3.5x mandible length v 2x) separates it from the otherwise very similar Telopelopia, The straight teeth of the ligula resembles those of Natarsia, as does the strong molar tooth of the mandible; however the well-developed inner mandibular tooth in Larsia differs. Typically for a pentaneurine, Larsia has S10 lateralmost, whereas Natarsia has the ventral pit lateral to S9 and S10. The arrangement of S9, S10 and VP, the ring organ position on the palp, and the ligula shape are unusually variable in Larsia.
Epler (2001) keys 6 named and unnamed species from s.e. USA, and provides notes on their ecology.
In the Holarctic, larvae of Larsia are moderately cold-stenothermic and occur in many habitats including springs, ditches, streams, small standing waters and the littoral zone of lakes. In the southern hemisphere, by contrast, Larsia larvae are associated with warm waters, both lotic and standing. In Florida, L. decolorata is associated often with eutrophic conditions.
The genus is distributed worldwide with 21 described species (Niitsuma, 2003). There are 7 named species from the Nearctic but more can be differentiated on the immature stages (Epler 2001). Nearctic L. planensis is known also from Hawai’i and central America. Larsia atrocincta and L. curticalcar are known from the western Palaearctic, and 2 (of the 7 in total) Afrotropical species, L. rutshuruiensis and L. teesdalei, are recorded also from Saudi Arabia. Larsia miyagasensis, from submerged fallen leaves in cold, clear streams in Fukushima and Kanagawa Prefectures, is the first Larsia reported from Japan (Niitsuma, 2001; 2003).
Three named species have been recorded and keyed from Central America (Sublette and Sasa, 1994), and two from the neotropics (Argentina, Brazil) but the immature stages are unknown for all. From south India, Roback and Coffman (1989) record a pupal exuvial Larsia and 2 exuviae referable to 'Genus nr. Larsia’. A widespread eurythermic Larsia species in Australia seems to be identical to the Oriental L. albiceps, which is abundant in warm waters in s.e. Asia.