Small larvae, up to 4 mm long.
Antenna with 4 or 5 segments; segment 3 entire to weakly subdivided to clearly divided into 2 parts (i.e. 5 segmented); segments consecutively shorter in 5 segmented antenna or segment 3 longer than either segment 2 or 4 in the 4 segmented antenna; segment 2 usually apically broadened. Ring organ near base of segment 1. Blade long, often extending far beyond apex of flagellum. Lauterborn organs distinct. Style long, but much shorter than segment 3.
Labrum with all S setae simple. Labral lamellae absent. Chaetulae and spinulae reduced, weak. Pecten epipharyngis consisting of 3 large scales. Chaetulae laterales simple, covered by pecten epipharyngis; chaetulae basales with forked apices. Ungula U-shaped with elongate basal sclerite. Premandible simple, inner margin of apex weakly concave; brush absent.
Mandible with apical tooth shorter than combined width of 3 inner teeth. Seta subdentalis long, apically pointed and slightly curved. Seta interna absent. One strong spine on mola.
Mentum triangular with 2 median teeth on apex and 4-5 pairs of lateral teeth; first lateral teeth sometimes small and appressed to median teeth. Ventromental plate elongate; beard long, with branches radiating from a common area; apices of branches plumose.
Maxilla with palpiger and galea apparently without lamellae or chaetulae. Pecten galearis absent. Seta maxillaris simple.
Body with anterior and posterior parapods separate, each bearing an apical crown of claws. Procercus about as long as wide; 5-7 anal setae present. Anal tubules longer than length of posterior parapods. Body segments 7-10 each with alternating simple and plumose setae, 2 of each setal type on each side of each segment.
The larvae of Synorthocladius and Parorthocladius are similar. The mentum of Synorthocladius has 2 teeth on its apex and 2 simple setae and 2 plumose setae alternating on each side of all but the last abdominal segment. Parorthocladius has 3 teeth on the apex of the mentum and at most only 1 plumose setae on each side of each abdominal segment. The segmentation of the antenna of Synorthocladius ranges from distinctly 4-segmented to an incompletely divided segment 3 to distinctly 5-segmented.
The larvae of Synorthocladius inhabit springs and small to large bodies of standing and flowing water.
The genus is reported from the Holarctic and Afrotropical regions. There are undescribed Nearctic species.