Characteristics & Similar Species
As the name “ater” suggests, Clusiodes ater is a darkly-pigmented species, but most Clusiodes other than the pale species in the C. melanostomus complex are also quite dark, so body colour alone is insufficient to characterize the species.

Clusiodes ater differs from C. orbitalis and C. nitidus in having a wing that is only darkened anterodistally (rather than clouded along the entire costal margin), and it differs from C. caestus in having a white shoulder stripe (usually absent in C. caestus) and different facial pigmentation (see key).

Clusiodes terminalis (the putative sister species of C. ater) has nearly identical patterning, but the male face and fore legs are yellow (light yellow to dark brown in C. ater) and the thin anterodistal process of the surstylus is somewhat smaller. These intangible differences appear to be weak justification for recognizing two species but more specimens and further study are required before either combining the species or confirming their separate status.

Distribution
Compared to most other North American Clusiidae, Clusiodes ater is a widespread species currently known from both eastern and western North America. In the east it is known from northwestern Illinois to eastern Ontario, Nova Scotia and Cape Cod, and in the west it is found along the mountains from New Mexico to western Alaska. We suspect that this apparent east-west disjunction is just a collection artifact, but more field work in central North America is needed to flesh out this and other fly distribution patterns.

Biology
Male agonistic behaviour, typical of other lekking Clusiidae, has been observed in Clusiodes ater (Caloren & Marshall, 1998): males in sunspots were found to engage in ambush attacks and head to head confrontations with potential rivals. Mated females were later found to oviposit in newly-fallen Acer trees - an interesting observation considering that most reports of oviposition and larval habitat suggest that clusiids prefer trees in a more advanced state of decay. Similar observations of male agonistic behaviour have been made for other species in the Clusiodes geomyzinus species group, including those in the C. melanostomus complex.
Large numbers of this species have been collected on bleeding Acer stumps (Caloren & Marshall, 1998).