Formerly known as Enallagma cyathigerum and its range believed to to be circumpolar, recent research indicates that the North American and Eurasian lines of this bluet diverged 250,000 years ago and the North American stock now goes by the epithet Enallagma annexum.
Northern and Boreal Bluets cannot be reliably distinguished in the field by sight and it's necessary to examine the male's terminal appendages or the females mesostigmal plates. A tandem pair of Northern/Boreal Bluets – both species have large postocular spots and tapering humeral (shoulder) stripes.
A dorsal view of a typical Northern/Boreal Bluet female – the paired blue spots on S8 may be fused in some individuals. The females can be either blue or tan.
The cerci of the male Northern Bluet are sharply upturned at their lower tips.
Terminal appendages of the Boreal Bluet – the cerci are rounded and angled downward.
The Vernal Bluet (Enallagma vernale) must also be examined in the hand to separate it from the Northern Bluet, the cerci of the male Vernal Bluet have a ridge. There's no way my camera will pick up that kind of detail but it should be visible with a 20× loupe, and thus far I have found no evidence of Vernal Bluets flying in my area.
As of this writing I'm uncertain whether the Vernal Bluet is considered a subspecies of the Northern Bluet (according to my field guide there is evidence of hybridization) or has been elevated to species level.
