SeasonalityThroughout most of its largely boreal range, C. ater is found from mid May through early July, with June being the month of peak activity (Teskey 1990); in northern New York State, C. ater can be collected from late May through early July, with June being the month of peak activity (White et al. 1985); in southwestern Quebec, C. ater can be found only from late May through mid June (Leprince et al. 1983).
IdentificationFemales are predominantly black. Teskey (1990) describes the characters as follows: on the head, the frontal callus and a large area around the ocelli are glossy black; the antennae are slender, and the scape and a portion of the pedicel are yellowish; the clypeus has a median grey pruinose (powdery) stripe half its length; the palpi are black. On the thorax, the the hairs are largely pale and the scutum has a single, broad, gray longitudinal stripe; in keeping with the body, the wings are darkly pigmented in all but the apical quarter of the basal cells, and in the crossband. The abdomen has white hairs on the sternites and most of the tergites; tergite 1 has an inverted "V" of black hair, and tergites 2-3 have lateral patches of black hairs.
Males are similar to females, except smaller and the hairs on the head, thorax, and abdominal tergites 1-3 are black at the base.
Teskey (1990) elevated C. ater from a subspecies of C. carbonarius to full species rank on the basis of several consistent morphological differences, the principal adult difference being the the less well-defined dark crossband of the wing in C. ater as compared to C. carbonarius: in C. ater the band does not reach the posterior margin of the wing, or only does so narrowly.