Species 5x.  C. acerbus Hirvenoja 1962

Adult:

Male (Fig. a).  (Note - some 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.  Scutal vittae sometimes significantly lightened.  Scutellum, postnotum 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.
Anterior 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) i.e. no beard.
Abdominal tergites blackish brown, brighter than thorax.  The anal borders of segments VII-VIII with a narrow light border.
Hypopygium (Fig. c): 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).  Anal point narrowed at base and coming almost to a point.  Superior Volsella (fig. d) an E-type, in one figure closest to h-type of Strenzke 1959 (length: maximum width about 4.51, with 1 - 2 setae in the middle).  Inferior Volsella reaching beyond the end of the anal point to about the mid-point of the gonostylus, setae shown as simple.  Gonostylus relatively swollen and narrows fairly rapidly over the posterior third.

Male hypopygium; and female antenna of C. aversa  From Roback 1962

Female: 4.8 mm long (variation 4.8-5.1); antennal segments (fig. 23) in ratio 28 : 23 : 21 : 23 : 40 (i.e. AR 0.26, A5/A1 1.71); frontal tubercles 0.035 x 0.015; thorax like male; area between vittae slightly darker, legs like male; leg ratio 1.75; scutellum slightly darker than in male; spermathecae (2) ovoid, 0.09 x 0.07 mm; cerci light brown.