(Walker, 1856)
Body
Medium sized to large larvae up to 7.1 mm long. The anterior parapods are separate. The two posterior parapods bear rows of claws. Procercus and two pairs of anal tubules present (Saether, 1975; Wiederholm, 1983; Moller Pillot, 1984b).
Head
The head length is 0.35-0.47 mm and postmentum is dark and contrasting with the rest of the gula Heterotr marcidus head ventr (Saether, 1975).
Antenna
The antenna consists of seven segments Heterotr marcidus antenna Segment 3 much shorter than segment 4, The blade is shorter than the flagellum (Saether, 1975). Lauterborn organs reduced or absent (Wiederholm, 1983; Moller Pillot, 1984b).
Mentum
The mentum has two median teeth and 5 pairs of lateral teeth. The ventromental plates are large. The beard is absent Heterotr marcidus mentum.
Labrum
The pecten epipharyngis consists of three serrated spines, SI is plumose and the premandible has two indistinct teeth (Wiederholm, 1983).
Differential characteristics
H. marcidus can be separated from the other Heterotrissocladius species by
a paired mendial mentum teeth and dark postmentum Heterotr marcidus head ventr.