(Edwards, 1929)
Body
Small sized larvae up to 4.5 mm. The body with stout setae about as long as the segment. The laterals in pairs. The anterior and posterior parapods are separate and bear rows of claws. Procercus well sclerotized and two pairs of anal tubules present Tvet discoloripes anal segm (Wiederholm, 1983; Moller Pillot, 1984b; Schmid, 1993).
Head
The head length is 0.310 - 0.422 mm (Schmid, 1993).
Antenna
The antenna consists of five segments. The blade reaches the tip of the 4th antennal segment. Lauterborn organs are conspicuous.
Mentum
The mentum has two median teeth and 5 pairs of lateral teeth. The mentum has alternating lighter and darker longitudinal bands due to varying thickness of the chitin. The ventromental plates are weak. The beard is absent Tvet discoloripes mentum.
Labrum
The pecten epipharyngis consists of three scales, SI is branched Tvet. calv S1 pict and the premandible has one apical tooth (Wiederholm, 1983; Moller Pillot, 1984b; Schmid, 1993).
Mandible
The mandible has an apical tooth and 5 pair of inner teeth Tvet calvescens mandible. The mola has 2 - 3 simpel spines (Schmid, 1993).
Differential characteristics
Tvetenia can be identified by the stout body setae Tvet discoloripes body setae, mentum with 5 pairs of lateral teeth Tvet discoloripes mentum, weak ventromental plates and SI plumose. Tvetenia differs from Eukiefferiella by having long stout body setae.
Tvetenia calvescens can be separated from the other Tvetenia species by:
Two median mental teeth, molar with spines and L4 about as long as the segment.