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Boreal Bluet



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Enallagma boreale

Selys, 1875


Order Odonata
Suborder Zygoptera
Superfamily Coenagrionoidea
Family Coenagrionidae
Genus Enallagma
Species boreale Selys, 1875 [Enallagma]
Syn Enallagma calverti Morse, 1895


Identification

The male's face is blue with a black bar across the top of the frons. The top of the head is broadly black with two pale more or less oval postocular spots that aren't confluent with the compound eyes. The middorsal thoracic carina and stripe are black. The antehumeral stripe is pale blue and roughly 2/3 as wide as the middorsal stripe. The jagged black humeral stripe is subequal in width to the antehumeral stripe at its upper and lower ends but is constricted towards the middle. The rest of the pterothorax is blue with an abbreviated black stripe on the upper forth of the interpleural suture. The legs are pale with heavy dark stripes on the outer surface of th e femora and the tibiae and tarsi are pale. The first abdominal segment is nearly all blue with a small lateral black spot apically. Segment 2 is blue with a distinctive dorsal black crescent on the apical 2/3 and a black apical ring. Segments 3-5 are blue with a broad black apical ring for a quarter of its length. Segments 6-7 have a black band extending as much as 3/4 the segment's length, respectively. Segments 8-9 are entirely blue and segment 10 is black dorsally. The cerci are dark and rounded apically, extending to approximately a third the length of segment 10. There is a distinct medial tubercle visible when viewed dorsally. The paraprocts are much longer, approximately 1.5 times the cerci, and curved slightly upwards. The head and thorax of the female are similarly colored to the male, with the pale areas more extensive. There are no distinct pits on the middle lobe of the pronotum. The hind margin of the mesostigmal plates is indistinct medially. The abdominal color pattern is similar to the male, but with a dark basal spot on the dorsum of segment 1. Segment 2 has a dark apical ring and a middorsal black stripe that extends the full-length of the segment and is distinctly expanded apically. Segment 3 bears a basally tapering middorsal stripe that is confluent with a broad apical ring extending approximately 1/4 of the segment's length. Segments 4-7 have a broad basally pointed dark stripe dorsally. Segment 8 bears a dark apical ring extending 1/3 its length and a narrow middorsal stripe that is broadly confluent with the ring. Segments 9-10 are entirely black.

Size

Total length: 28-36 mm; abdomen: 22-29 mm; hindwing: 17-22 mm.

Similar Species

The pale blue areas on the abdomen of Familiar Bluet (E. civile ) are more extensive. In Tule (E. carunculatum ) and Northern (E. annexum ) Bluets the black laterally on the abdomen is more extensive than in Boreal Bluet, but this is variable. The black spot dorsally on abdominal segment 2 in Alkali Bluet (E. clausum ) is larger extending to nearly half the segment's length. Critical examination with a hand lens or microscope may be necessary to tell these species apart at least initially.

Habitat

Fishless ponds, lakes, slow moving streams and occasionally saline waters.

Discussion

This species is widespread, but presently is only known from New Mexico within the south-central United States. Boreal Bluet occurrs in a variety of pond and lake habitats, but generally only when fish are lacking. Pairing begins shortly after emergence and continues throughout the summer. The female sometimes lays eggs unaccompanied by the male, but more often this is done in tandem. Emergent aquatic vegetation seems to be preferred and egg laying generally occurs just above the water level. One Canadian study of larval Boreal Bluets, found one occurring with the predatory fish, the northern pike (Esox lucius), and a second that did not. The population occurring with the pike adopted antipredator behavior in response to chemical stimuli from injured conspecifics and from chemical stimuli given off by the pike themselves. The study found that individuals that were previously unexposed and unresponsive to stimuli from the pike learned to recognize these stimuli after a single exposure.

Distribution

Northern Canada and U.S. south to California, New Mexico and Mexico.