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This blue has a complete row of orange spots on the hindwing underside, extending onto the forewing (although sometimes faint here). Other species with a full row of hindwing orange spots (Icaricia acmon, I. shasta, Euphilotes ancilla) never have the orange spots extending onto the forewing. This species can be confused with the Melissa Blue (L. melissa) where the ranges overlap in the southern foothills. Idas has narrower orange spots, capped with a black crescent. In Melissa, the orange spots are wider and the black crescents capping the orange spots are flatter, resulting in a more banded appearance. The upperside of idas females can vary from solid brown to mostly purple-blue, and the extent of orange spots varies from none to well-developed. Melissa females always have a solid band of orange, while the strongest-marked Northern females have a row of orange crescents rather than a solid band. It is best to appreciate these differences by looking at more than one individual; uncertain specimens must be dissected for positive identification (see Guppy & Shepard 2001). The specimens illustrated in "Alberta Butterflies" (Bird et al. 1995) as melissa are actually idas. This group of species has a very muddled taxonomic history in North America; there are likely several species involved under what has been referred to as "idas". In Alberta, southern Mountain populations are subspecies atrapraetextus, while subspecies scudderii supposedly inhabits the central/northern mountain and boreal region (Guppy & Shepard 2001). Balint and Johnson (1997) in their revision of polyomatine blues place Lycaeides as a subgenus within the larger genus Plebejus.

The early stages of the Northern Blue in North America are undescribed. In Wisconsin, larvae hatch in May after overwintering as an egg, and pass through 3 or 4 instars, pupating in mid June (Wolf 1993). Adult males often form large congregations at moist spots along mountain trails. In the boreal region, Northern Blues are found in jack pine sandhills, close to patches of blueberries (Vaccinium spp.) on which the larvae feed (Wolf 1993, Nielsen 1999).