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Throughout 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).

Females 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.

Not much is known of C. ater specifically. It seems, however, that C. ater is an unusually active and robust flier: White et al. (1985) studied the dispersal of various Chrysops species in northern New York State, and found that C. ater dispersed the farthest: some individuals were recaptured up to 7 km from the site of release. The minimum dispersal rate for the species was calculated to be 1.5 km/h, easily the highest among all the Chrysops studied. Chrysops ater is a facultatively autogenic species, that is, females may or may not require a blood meal in order to generate a brood. Lake and Burger (1980) found that females of C. ater in northern New England were autogenous (not requiring a blood meal for egg production), whereas Magnarelli (1976) found that in eastern New York State, 94% of females were anautogenous (requiring a blood meal for egg production). Combined with the observation of Thomas (1972) that autogenic females of several Chrysops spp. have more than three times the amount of lipids as a percent of dry body weight than anautogenous females, the inference is that autogenic egg production is dependent upon the amount of nutrients the females are able to store from their larval stage: when nutrient reserves are low, the females are forced to take a blood meal in order to produce a brood (Lake and Burger, 1980).