Small larvae, to 5 mm long.
SIII simple or bifid, sometimes placed on prominent tubular pedestal. Dorsal surface often with 2 or more knob-like projections posteriorly.
Antenna with 5 segments on prominent pedestal bearing mesal palmate process, sometimes also a basal spur. Antennal ratio c 1.5-1.8. Ring organ proximal on 1st antennal segment. Seta weakly developed. Segment 2 wedge-shaped, equal to or slightly longer than 3rd segment, apically bearing style and well-developed, large Lauterborn organs placed on short pedestals. Antennal blade inserted apically on 1st segment, extending beyond antennal apex; accessory blade fused to base of blade, short.
SI seta comb-like, fused at base, SII plumose, situated on large pedestal. SIII short, simple, SIV present. Labral lamellae well developed. Pecten epipharyngis divided into 3 separate, slender, distally-pointed scales. Premandible with 5 teeth, brush strong.
Mandible with yellow dorsal tooth, apical tooth and 2 pointed inner teeth brown or yellow. Seta subdentalis long, curved, reaching beyond apical tooth. Seta interna of 4 plumose branches. Pecten mandibularis well developed with about 10 lamellae.
Mentum with pale rounded, median tooth or slightly notched laterally, 6 pairs of yellow or darker laterals, regularly decreasing in size. Ventromental plates fan-shaped, widely separated medially, subequal in width to mentum, anterior margin smooth or weakly crenulate.
Without tubules. Claws of posterior parapod simple. Procercus with strong anal and lateral setae, some anal setae bifid to plumose; more or less strongly sclerotised distally, sometimes with strongly sclerotised, occasionally palmate spines.
Larvae of Stempellina are easily distinguished from those of any other genus by the large, palmate mesal, or apical, expansion which occurs on the antennal pedestal.
Re-examination of the material identified by Thienemann as S. montivaga and upon which his description of the immature stages was based, has shown that the adult was misidentified and belongs to an undescribed species, and possible undescribed genus. The pupa also does not agree with the generic concept of Stempellina. On the other hand, the associated larvae, which are not part of a reared series, have many important characters in common with other Stempellina species, although differing in lacking knob-like processes on the dorsal side of the head and in having a simple distal spur in addition to a palmate expansion on the antennal pedestal. It seems likely, therefore, that this larval type is wrongly associated with the pupa and adult. Although the larva almost certainly is correctly ascribed to Stempellina it remains to be seen whether it is truly the larva of S. montivaga.
Brundin (1948a) described several species and gave an incomplete key. To the species dealt with by Brundin must be added several from North American including an unnamed form from n.w. Canada.
Species groups of Stempellina
- 1. Dorsal surface of head capsule without prominent knob-like processes. Antennal pedestal with simple distal spur and con spicuous palmate process mesally ... montivaga group (montivaga Goetghebuer)
- -- Dorsal surface of head capsule with at least 2, sometimes about 12 conspicuous knob like processes. Antennal pedestal with palmate process mesally but lacking simple apical spur ... 2
- 2. Dorsal surface of head with 2 knob-like pro cesses posteriorly ... bausei group (bausei Kieffer, subglabripennis Brundin
- -- Dorsal surface of head with about 12 prom inent, knob-like processes ... johannseni group (johannseni Bause)
Larvae of Stempellina are eurytopic, occurring in springs, streams, larger rivers, lakes, brackish water, moorland pools and thermal springs. Larvae construct long, curved, tapered, transportable cases.
The genus has a worldwide distribution. At least 14 species are known, including some unnamed larval types.