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River Bluet



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Enallagma anna

Williamson, 1900


Order Odonata
Suborder Zygoptera
Superfamily Coenagrionoidea
Family Coenagrionidae
Genus Enallagma
Species anna Williamson, 1900 [Enallagma]
Syn Enallagma culicinorum Byers, 1927


Identification

This is a large blue western species. The face and thorax of the male are blue. The middorsal thoracic stripe is black and half the width of the mesepisterna. The pale blue antehumeral stripe is not quite half as wide as the middorsal stripe and the black humeral stripe is distinctly narrower than the antehumera l stripe. The legs are blue with a black stripe and the tarsi pale or dark. The abdomen is bright blue dorsally becoming paler laterally and ventrally. Segment 1 is black on the basal half dorsally. Segment 2 has a black subapical spot and there is a hastate stripe on the apical 1/2 - 3/4 of segments 3-6. Segment 7 is black dorsally. Segments 8-9 are blue with ventrolateral streaks or spots. Segment 10 is black dorsally. The cerci are black and distintly protrude beyond the abdomen. Females are blue or tan with triangular-shaped mesostigmal plates and middle lobe of the pronotum lacking distinct pits. Segment 8 is variably marked with black, ranging from a hairline stripe on the basal half to a broad stripe extending the full-length of the segment. Segment 9 and 10 are nearly all black dorsally.

Size

Total length: 30-36 mm; abdomen: 23-28 mm; hindwing: 18-22 mm.

Similar Species

Male Arroyo Bluets (E. praevarum ) are shorter, not as robust, and overall daker. The black dorsal markings on segments 3-5 cover at least half the length of each segment in Arroyo Bluet and these areas are restricted to 1/4 to 1/2 of each segment in River Bluet. Boreal Bluet (E. boreale ) is stockier and the black rings on the middle abdominal segments are truncate. The postoccular spots in Northern Bluet (E. cyathigerum ) are larger and segment 10 is entirely black, not just dorsally. The cerci in Familiar Bluet (E. civile), Alkali Bluet (E. clausum ) and Tule Bluet (E. carrunculatum ) do not protrude significantly (greater than the length of segment 10, beyond the abdomen. Female River Bluets are very difficult to tell apart from other species. Size, distribution and association with the males will help, but they can only reliably be distinguished by their mesostigmal plates.

Habitat

Slow streams and rivers, often associated with warm springs.

Discussion

Within our area this species is only found in far northwestern New Mexico at the southern limits of its range. Females lay eggs in tandem may submerge below water to lay eggs. In Utah, this species is common at elevations between 4,200 and 7,000 feet.

Distribution

Southern Canada and western U.S. west of the Rocky Mountains.