Th e taxonomy and nomenclature of the Nearctic species of Enargia Hübner shows a long history of misunderstood species concepts and misapplied names, and the group is revised here. Enargia infumata (Grote, 1874) is a senior synonym of what has been referred to as E. mephisto Franclemont, 1939 for the past 70 years. Late summer moths from boreal areas of Canada and northeastern United previously identifi ed as E. infumata have no available name and are here described as E. fausta sp. n. A

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... pe for Orthosia infumata Grote is designated. Adults and genitalia of the three North American species (E. infumata, E. fausta, and E. decolor) are illustrated, and a diagnostic key is provided. A peer-reviewed open-access journal B. Christian Schmidt / ZooKeys 39: 205-223 (2010) 206 two European species in detail. In the Nearctic three species were revised by Franclemont (1939) . As such, Enargia is a Holarctic genus primarily of temperate zones. A diagnosis of the genus is given by Fibiger and Hacker (2007) , who note the following autapomorphies: 1) peniculus (lobe-like extension of the dorsal tegumen) extremely large and consisting of two lobes: a stout lobe at the base of the uncus and a long, rounded-triangular lobe on the subdorsal tegumen, and 2) vesica with two broadbased cornuti, with the apex directed toward the base of the vesica. In North America, Enargia decolor (Walker) has a transcontinental distribution and can reach population levels high enough to cause extensive defoliation of aspen (Populus tremuloides Michx.) stands in boreal forests (Wong and Melvin 1976, and references therein). Th e other two Nearctic species, which have previously gone under the names E. infumata (Grote) and E. mephisto (Franclemont), apparently do not reach outbreak population levels, although records could possibly be confounded by the diffi culty in identifying adults and particularly larvae of all three species, and given the taxonomic issues discussed below. All three species can be quite variable in wing markings, and prior to Franclemont's revision all North American taxa were treated as a single species, although Dod (1905; 1913) recognized that E. infumata was distinct from E. decolor. Due to the variability in wing pattern and colour, and inadequate study of diagnostic morphology traits, the two closely-related species previously known as E. mephisto and E. infumata have been misunderstood. Enargia infumata (including E. mephisto) is actually more widespread and variable than previously defi ned, whereas E. infumata (of authors) is much more conservative in colour variation and has a narrower more northerly distribution. Th e confusion of these two taxa is compounded by the fact that the type specimen of E. infumata (Grote) is the same species as E. mephisto (Franclemont), so E. infumata (Grote) is a senior synonym of E. mephisto, and E. infumata (of authors) has no available name. Th e purpose of this paper is to rectify this taxonomic issue, and to provide a diagnostic review of the North American Enargia.