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A fairly large (5.0 - 6.2 cm wingspan) brownish grey moth with darker markings. The forewings are grey with darker lines and spots, and with the veins lightly lined with dark scales. The postmedian line is doubled and the interspace is white, forming a contrasting toothed band. The hindwing is pale grey-brown with a faint darker median line and veins in the male. The female is similar but darker, especially the hindwings which are sooty brown. The antennae of both sexes are simple. A. dactylina is similar, but is lighter powdery grey, with fewer markings and in particular lacking the doubled, white-filled postmedian band on the forewing, the median line on the hindwings, and the dark scales along the veins.

The larvae are solitary defoliators on various deciduous trees, in particular maple and birch. They are covered in long white or pale yellow hairs, with several tufts of longer, black hairs. There is a single brood each year, which overwinter as pupae. They are never common enough in Alberta to be considered a pest. Most likely to be found associated with urban and shelterbelt plantings of Manitoba maple. The adults are usually attracted to light. The American Dagger-moth is primarily a species of the eastern hardwood forests. The southern ssp. eldora is paler with crisper markings and Alberta specimens from Medicine Hat have been assigned to this form by Bowman (1951). Bowman also listed May as the adult flight period, apparently in error as all specimens in his collection are labeled as collected in June-July.

A species of the eastern hardwood forest, recorded west to southeastern Alberta, and south to UT, CO, TX, KS and GA. In Alberta, it has been collected mainly in the eastern aspen parklands and grasslands regions, north to Lloydminster. Reports of American dagger moths from further west in Alberta (Peace River and foothills regions) in Prentice (1962) are questionable and need verification.