Large larvae, up to 12 mm long.
Frontal apotome and clypeus narrowly separate. Labral sclerites SI and SII are present.
Antenna with 4 segments, each segment consecutively shorter. Ring organ on basal 1/2 of segment 1. Blade shorter than segment 2. Lauterborn organs absent. Style present.
Labrum with all S setae simple. Labral lamellae consisting of 2 small, well separate, apically separate lobes. Pecten epipharyngis consisting of 3 broad triangular scales. Chaetulae laterales simple; chaetulae basales apparently absent. Ungula broadly U-shaped; basal sclerite apparently absent. Premandible broad, with 2 apical teeth; brush absent.
Apical tooth shorter than combined width of 4 inner teeth. Seta subdentalis long, apically pointed. Seta interna present.
Mentum with 1 median and 6 pairs of lateral teeth present; first lateral teeth larger than remaining teeth. Ventromental plate apparently absent; beard absent.
Palpiger and galea apparently without chaetulae or lamellae; pecten galearis absent. Setae maxillaris simple; one longer than the other.
Body with separate parapods, each bearing an apical crown of claws. Procercus small, bearing 4 anal setae. Anal tubules sub-globular about as long as posterior parapods.
Eurycnemus is known from two species. Eurycnemus crassipes is reported from Austria, Belgium, Bosnia, France, Germany, Hungary, Ireland, Netherlands, Russia, Slovenia, Spain, Sweden, Switzerland and United Kingdom (England, Scotland) (Ashe et al., 2000) and Eurycnemus nozakii, from Honshu, Japan (Kobayashi, 1998).
The two described species of Eurycnemus are parasitic on Trichoptera. Eurycnemus crassipes (Panzer) is reported from Hydropsyche siltali (Hydropsychidae) as a parasite of prepupae/pupae, at least in the 3rd and 4th instars (Ashe et al., 2000). The Japanese species Eurycnemus nozakii, is ectoparasitic on Goera japonica (Goeridae) in running waters of Honshu, Japan (Kobayashi, 1993; 1998).