Species 5x.  C. acerbus Hirvenoja 1962

Adult:

Male (Fig. a).  (Note - this terminology predates Sæther (1980)).  AR 3.6 (3.53-3.76).
Ground colour of thorax brownish black.  Preepisternum between the Lmb so brightened that Prsc and Lmb barely standout.  Shoulders sometimes significantly lightened.  Sc, Pn and mesosternum uniformly brownish black.
Halteres only slightly darkened, almost white.
Chaetotaxy of thorax: dm 15.0 (10-22), dl 36.5 (27-48), pa 8.0 (7-11), sa 1.5 (1-5), sc 45.4 (38-54).
Wing length 3.7 (3.5-4.0) mm.
Legs blackish brown, and the joints often somewhat darkened.
Leg segment lengths: Fe 1.19 (1.04-1.27), Ta1 1.53 (1.45-1.59), Ta2 0.81 (0.78-0.86), Ta3 0.67 (0.63-0.71), Ta4 0.52 (0.49-0.56), Ta5 0.23 (0.20-0.26); BR 2.01 (1.35-2.57).
Abdominal tergites blackish brown, brighter than thorax.  The anal borders of segments VII-VIII with a narrow light border.
Hypopygium (Fig. c): SVo (fig. d) of E-type of Strenzke (1950) (length: maximum width about 4.51, with 1 - 2 setae in the middle).  Gonostylus relatively broad.  Parts of IX tergite with a long striation almost similar to reticulation (From figure this would seem to refer to the numerous setae (perhaps 25-30), each apparently in a single pale area).  Dorsal lamella of the Pran and the base narrowed, am 9.0 (1-14), sti 10.4 (8-11).The anal borders of segments VII-VIII with a narrow light border. Hypopygium (Fig. c): SVo (fig. d) of E-type of Strenzke (1950) (length: maximum width about 4.51, with 1 – 2 setae in the middle.  Gonostylus relatively broad.  Parts of IX tergite with a long striation almost similar to reticulation (From figure this would seem to refer to the numerous setae (perhaps 25-30), each apparently in a single pale area).  Dorsal lamella of the Pran and the base narrowed, am 9.0 (1-14), sti 10.4 (8-11).

a. Male; b. Female; c. Hypopygium; d. SVo.  From Hirvenoja 1962

Female (Fig. b):  Color charateristics very similar to those of the male.  The shoulders are however more pronounced, and the abdominal tergites have a narrow light border.
Chaetotaxy of the thorax: dm 17.5 (13-26), dl 50.8 (45-62), pa 10.9 (8-13), sa 1.2 (1-3), sc 48.9 (40-69).
Wing length 3.6 (3.5-3.7) mm.
Leg segment lengths: Fe 1.26 (1.17-1.40), Ta1 1.66 (1.55-1.87), Ta2 0.84 (0.65-0.92), Ta3 0.65 (0.58-0.67), Ta4 0.53 (0.51-0.60), Ta5 0.25 (0.23-0.28). BR 1.93 (1.7-2.0)

The colour features of C. acerbus correspond somewhat to those of C. lugubris Zett (see Lindeberg 1959). Particularly the colour of the abdominal segments is very similar. According to the identification table of Strenzke (op.c.) the relationship of C. acerbus lies with plumosus L., salinarius Kieff., dorsalis Meig., and crassimanus Str.  Quite peculiar however are the setae in the middle of the Claspette, which speaks against a very close relationship.
Studied were 10 males and 10 females, from a population in a melt-water lake in the forest, in Lake Posolampi.
Types Nr. 8420 and 8421 in the collection of the Entomologischen Abteilung des Zoologischen Museums of the University of Helsinki, Finnland.