|Enallagma annexum |
common name northern bluet, common bluet, common ble damselfly
Ponds, lakes, marshes, and slow moving streams. Not limited to fish-free waterbodies in Alberta.
Adults fly early June through August in Alberta. Flight season extends in more temperate regions.
A large bluet, with adults ranging from 29-40 mm in total length and 17-24 mm in hindwing length. Males are blue (andromorphic), while females are polymorphic and may be either blue or brown (Bolts et al. 2007). Males have a narrow humeral stripes and a wide median stripe on dorsal surface of thorax. Black abdominal banding is present in both sexes. In males, the first three black bands occurring on abdominal segments 3 to 5 are short and similar in length, segments 6 and 7 are mostly black dorsally and ventrally, segments 8 and 9 are mostly blue, and segment 10 is mostly black on the dorsal surface. In males, the subapical bar on abdominal segment 2 is black and often crescent-shaped. In females, the abdomen is mostly black on the dorsal surface, with pale basal rings on segments 3 through 8. Abdominal segment 8 in females may have more blue or light brown than other segments, while segments 9 and 10 are generally black. Male eyes are dorsally black and ventrally blue, while female eyes are dorsally dark brown and ventrally light brown. Postocular spots are large and may form dumbbell shape in both sexes. The northern bluet is virtually indistinguishable from the boreal bluet (Enallagma boreale) except when viewed under magnification. Male northern bluets and boreal bluets differ in clasper morphology (McPeek et al. 2011), as shown in image. Female northern bluets may be differentiated from female boreal bluets by the complete rear margin on the mesostigmal laminae (i.e. “shoulder pads”; located on the upper front edge of the pterothorax), or by the triangularly shaped front margin on the black marking on the dorsal surface of abdominal segment 8. Hagen’s bluets (Enallagma hageni) and marsh bluets (Enallagma ebrium) are very similar in colouration to northern bluets, but both are notably smaller and may have smaller postocular spots. Familiar bluets (Enallagma civile) are also similar in appearance, however the cerci are longer than the paraprocts in familiar bluets, while the opposite is true for northern bluets (Acorn 2004; Paulsen 2009).
As with most bluets, males congregate in large numbers around water bodies and will perch on a variety of vascular plants, alert to passing females (Paulson 2009). Perching sites at habitat edges seem to be preferred to those located in dense vegetation (Paulson 2009). Similar to Enallagma cyathigerum, unreceptive females tend to avoid water bodies frequented by males to limit male harassment, leading to male-biased sex ratios over and around suitable mating grounds (Bolts et al. 2007). Males will compete for receptive females appearing near the water (Bolts et al. 2007). After a male has successfully courted a female, copulation lasts 10 to 27 minutes (Paulson 2009). While still in tandem with a male, the female will oviposit on floating vegetation and not on living emergent water plants. Males and females may break tandem for submerged oviposition, which may last up to 90 minutes (Paulsen 2009). After eggs are laid, nymphs hatch relatively quickly and will remain aquatic throughout the winter (Acorn 2004). Nymphs are typically ambush predators, feeding on mobile prey (Koperski 1997). In early June and throughout 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).
Not currently of concern. Northern bluets are common throughout their range.
Adults feed on a variety of small soft-bodied winged insects (e.g., mosquitoes, mayflies, small moths) and pick small insects (e.g., aphids) from vegetation. Nymphs feed on a broad diversity of aquatic invertebrates (e.g., mosquito and other dipteran larvae, mayfly larvae) (Koperski 1997; Lung and Sommer 2001).
Northern bluets are common throughout north western and central North America. The species range extends along the entire west coast from the southernmost tip of California to northern Alaska. The range of the northern bluet also extends to the northeast, bordering Newfoundland, and south to Indiana and West Virginia (Paulson 2009).
The northern bluet (E. annexum) is restricted to North America and was split from the Eurasian species Enallagma cyathigerum on a genetic basis in 2005 (Stoks et al. 2003; Paulsen 2009; Cannings 2014). Both E. annexum and E. cyathigerum are commonly referred to as northern bluets in their respective continents. Northern bluets, together with boreal bluets (E. boreale) and Eurasian bluets (Coenagrion concinnum), are among the most northerly of all damselfly species, and have been recorded as far north as 68° in the Yukon, Canada (Acorn 2004). Northern bluets may inhabit very acidic and saline waters, with conductivity up to 8,000 micromhos/cm and a pH of 3-4 (Acorn 2004). McPeek et al. (2011) suggest that the morphology of cerci is a primary means of species recognition used by northern bluets to differentiate conspecific individuals from heterospecifics when mating. Cerci shape acts to maintain species boundaries between similar bluet species, such as northern and boreal bluets, thus making it a viable diagnostic character. Male northern bluets play an important role in using their claspers to rescue females caught in water’s surface tension after submerged oviposition (Acorn 2004). The popular television show, Acorn, The Nature Nut, features footage of this rescue phenomenon in their Odonate episode.
|species page author||MacDonald, Z. G.||2016 |