Small larvae, up to 4 mm long. Head small relative to body size, shorter than broad.
Antenna short, less than 1/3 as long as mandible, lying in concavity on head capsule, 5 segmented; broad segment 1 not much longer than segment 2, and segment 5 longer than segment 4. Ring organ not discernible. Blade extending about level of segment 3. Lauterborn organs not discernible. Style about as long as segment 3.
Labrum with all S setae simple. Labral lamellae, chaetae, and spinulae absent. Pecten epipharyngis and cheatulae laterales consisting of a group of serrated scales. Ungula not discernible. Premandible narrow, more or less straight and not apically incurved; brush absent.
Mandible with apical tooth slightly longer than combined width of 4 inner teeth; inner teeth spine-like. Seta subdentalis spine-like, larger than an inner tooth. Seta interna absent.
Mentum with a median toothless concavity flanked on either side by a conical projection; group of 4-5 spine-like lateral teeth arising on each side of mentum, well separated from conical projections. Ventromental plate broad; beard absent.
Maxilla reduced; with palp shorter than wide, palpiger and galea apparently without chaetulae or lamellae, and lacinia with few chaetulae.
Body with anterior parapods fused, divided apically and each with an apical crown of claws. Anal end truncated with separate but short posterior parapods, each with a circle of claws. Procercus absent; tuft of 8 very short anal setae present. Anal tubules absent.
Larvae of Symbiocladius are distinctive, by virtue of their very small head, fused anterior parapods, lack of a procercus and their short posterior parapods each bearing a circle of claws. The mentum with median concavity flanked on each side by a conical projection and the grouping of spine-like lateral teeth is also distinctive.
Larval Symbiocladius are parasitic on mayfly nymphs (Ephemeroptera) and are usually attached under the wing pads. Gilka et al. (2007) review and illustrate the biology of S. rhithrogenae in central Europe.
The genus is Australian (1 species), Neotropical (3 species, Gonser and Spies, 1997) and Holarctic, where larvae of 2 species are known from the Nearctic and 1 from the western Palaearctic.
The status of subgenus Acletius Roback for S. (A.) wygodzinsky living on Thrauliodes (Leptophlebiidae) nymphs in southernmost Argentina (Ushaia) is uncertain.