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A smaller bluet, with adults ranging from 27-33 mm in total length and 15-21 mm in hindwing length. Males are predominantly blue (andromorphic), while females are polymorphic and may be range from light blue to yellow-green to light brown. Thoracic stripes are typical of most bluets in both sexes. Black abdominal banding is present in both sexes. In males, the black bands occurring on abdominal segments 3 to 5 increase in width towards the end of the abdomen on the dorsal surface. Male abdominal segments 6, 7 and 10 are mostly black on the dorsal surface, and segments 8 and 9 are predominantly blue. Males have a prominent apical spot on the dorsal surface of abdominal segment 2. In females, the abdomen is predominantly black due to widening of abdominal bands on the dorsal surface. Male eyes are dorsally black and ventrally blue, while female eyes are dorsally brown and ventrally tan or or pale green. Postocular spots are prominent in both sexes, forming a dumbbell shape. Over much of its range, Hagen’s bluets are virtually indistinguishable from marsh bluets (Enallagma ebrium) except when viewed under magnification; however, in certain populations, abdominal banding may be narrower in Hagen’s bluets than in marsh bluets. Male Hagen’s bluets and marsh bluets differ in morphology of cerci. The large size of the mesostigmal plates, or “shoulder pads”, of female marsh bluets differentiate them from all other female bluets except Hagen’s bluets; however, the mesostigmal plates of female Hagen’s bluets are raised above the thorax, while the mesostigmal plates of female marsh bluets lie flat towards their posterior end. Female Hagen’s bluets may also resemble female taiga bluets (Coenagrion resolutum), but may be differentiated by the presence of a spine on the ventral side of abdominal segment 8 in Hagen’s bluets. Northern bluets (Enallagma annexum), boreal bluets (Enallagma boreale), and familiar bluets (Enallagma civile) may be similar in colouration in both sexes, but all 3 are notably larger (Acorn 2004; Paulsen 2009).

Males congregate near the edges of water bodies, perching on emergent and shoreline vegetation or algal matts (Paulson 2009). Male Hagen’s bluets are not territorial and engage in scramble competition for mates (Fincke 1982). Mating pairs form either adjacent to or away from water bodies (the only Enallagma species known to do so), generally around midday and into the evening (Fincke 1982; Acorn 2004; Paulsen 2009). Copulation lasts an average 22 minutes, and pairs stay in tandem for an average of 58 minutes before oviposition (Paulsen 2009). Females may oviposit on floating or, more commonly, submerged vegetation. Submerged oviposition lasts up to 30 minutes (Paulsen 2009). Males may hover over submerged females, waiting to grab them upon resurfacing to mate with, leading to a second round of oviposition (Paulsen 2009). Thus, males have two alternative strategies for finding mates; i) searching or waiting at water body edges for unmated females, or ii) waiting at submerged oviposition sites for females to resurface after oviposition (Fincke 1985). The latter strategy is more common in the afternoon, as fewer unmated females are present at water edges. Fincke (1982) found that females average 6 days between oviposition bouts. Similar to marsh bluets, it may be assumed that nymphs hatch anywhere from a few days to a few weeks after oviposition and continue to grow through the late summer and fall (Kormondy and Gower 1965). Hagen’s bluets are entirely aquatic during their larval stage. Nymphs enter diapause in early winter and remain relatively inactive until spring when growth resumes (Ingram 1975). In early June through July, nymphs will climb out of the water onto a favorable perch—usually vegetative matter emerging from the water’s surface. Here they will undergo their final molt, and the adult winged form will emerge (Acorn 2004). Adult Hagen’s bluets are relatively short lived, with an estimated lifespan of 12-13 days. (Fincke 1985).

Similar to marsh bluets, it may be assumed that adult Hagen’s bluets feed on a diverse assortment of small soft-bodied winged insects (e.g., mosquitoes, other dipterans, mayflies, small moths). Nymphs likely feed on a variety of mobile prey, primarily aquatic invertebrates (e.g., mosquito and other dipteran larvae, mayfly larvae) (Lung and Sommer 2001, Koperski 1997).

Hagen’s bluets are common throughout north-central North America, ranging from central British Columbia to the Atlantic coastline of Newfoundland. Can be found as far south as Indiana and Maryland. Does not extend as far north as the Yukon, Northwest Territories, or Nunavut (Acorn 2004; Paulsen 2009).