(Walker, 1856)
Body
Small to medium sized larvae up to 5.5 mm long. Living larvae often with pale yellowish or greenish abdomen. The thorax with purple or green lateral stains. The anterior and posterior parapods are separate and bear rows of claws Parapha cf. impensus ant pp 8. The anal segment is placed ventrally on the preanal segment, thus forming a more or less right angle with the body axis Parapha impensus anal segm 8. The procercus is very small, two pairs of anal tubules are about as long as the posterior parapods Paraph impen analtub 8 (Wiederholm, 1983; Moller Pillot, 1984b; Saether and Wang, 1995).
Head
The head length is 0.25 - 0.33 mm. The smaller eye is placed ventrally of the larger one instead of distally Paraphaenocladius eye.
Antenna
The antenna consists of five segments, segment 3 shorter than segment 4. The blade is distinctly longer than the flagellum. Lauterborn organs distinct Parapha impensus antenna 8.
Mentum
The mentum has one median tooth and 5 pairs of lateral teeth. The median tooth may have a central depression but remains single. The ventromental plates are lobed and do not cover teeth of the mentum Parapha impensus mentum9. The beard is absent. The setae submenti are close to the mentum.
Labrum
The pecten epipharyngis consists of three short spines, SI is plumose and the labral lamellae are fused to form a plate with a smooth of serrated tip. The premandible has three apical teeth (Wiederholm, 1983; Moller Pillot, 1984b).
Mandible
The mandible has an apical tooth and 3 inner teeth (Wiederholm, 1983).
Differential characteristics
The combination of two eyes on top of each other and the more or less right angle between the anal segment and the body axis are distinctive.
P. impensus can be separated from the other Paraphaenocladius by the short anal tubules. Also the antennal blade distinctly longer than the flagellum is distinctive.