Hirvenoja, 1973
Body
Medium sized larvae up to 8 mm long. The anterior parapods are separate, the middle claws with at most 5 toothlets. The two posterior parapods bear rows of claws. Procercus is present. Two pairs of anal tubules present. On the abdominal segments 1 - 6 the L4 bundle consists of < 20 seta and is shorter than the segment Cric (I) sylv body seta. On segment 7, L4 consists of at least 3 setae (Hirvenoja, 1973 ).
Head
The head is dark (Hirvenoja, 1973).
Antenna
The antenna has a normal length and consists of five segments. One of the small ringorgans on the antenna is situated in the middle part of the first antennal segment Cric laetus antenna.
Mentum
The mentum has one median tooth and 6 pairs of lateral teeth. Median tooth < 2x the first lateral tooth. The second lateral tooth is reduced and partly fused with the first lateral Cric sylvestris agg mentum.
Labrum
The pecten epipharyngis consists of one compound scale Cric (Iso) pecten epiphary. SI is bifid. The premandible with 2 apical teeth Cric tibialis premandible (Hirvenoja, 1973).
Mandible
The mandible with a smooth inner side and the apical tooth is shorter than the joined inner teeth.
Differential characteristics
The setal tufts of L4 Cric (C) 14 Cric (I) sylv body seta on the abdominal segments are distinctive for Cricotopus (and Orthocladius holsatus). C. tremulus and C. lygropus have no setal tufts however.
C. laetus can be separated from all other Isocladius (except C. laetus) by:
the L4 bundle shorter than the segment, consisting of less than 20 seta
L4 on segment 7: 3 setae. C. laetus can be separated from C. ornatus by the claws of anterior parapods with appr. 5 toothlets (C. ornatus 5-10); First antennal segment with one round spots near the ring organ and the other in the middle of the segment Cric laetus antenna while in C. ornatus both spots near the ring organ Cric ornatus antenna. The thoracic horn of C. laetus is transparent, while C. ornatus has a brown thoracic horn.