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Arroyo Bluet



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Enallagma praevarum

Hagen, 1861


Order Odonata
Suborder Zygoptera
Superfamily Coenagrionoidea
Family Coenagrionidae
Genus Enallagma
Species praevarum (Hagen, 1861) [Agrion]


Identification

The male's face of this western species is blue. The top of the head is black except for pale occipital bar and tear-drop shaped postocular spots. The middorsal thoracic carina is blue and the middorsal stripe is black and half the width of the mesepisterna. The antehumeral stripe is blue and usually slightly less than half the width of the middorsal stripe. The black humeral stripe is slightly narrower than the antehumeral stripe. The rest of the pterothorax is blue becoming paler laterally and ventrally. The legs are blue or tan with a black stripe on the femora and tibiae. The tarsi are pale. The abdomen is largely blue marked with balck in the following ways. Segment 1 is black on the basal half. The dorsum of segment 2 may be half to entirely black. The black on segment 3 is limited to small apical spot or a full-length stripe dorsally. Segments 4-5 have a dorsal spot on the apical forth of each segment, sometimes extending into a stripe. There black for at least 3/4 of segment 6 and nearly all of 7. Segment 10 is black dorsally. The cerci are 1/2-3/4 the length of segment 10. The paraprocts are 2/3 the length of the cerci and slightly upturned. The female is very similar to the male, but may either be tan or blue. The postocular spots are generally larger and confluent with the occipital bar. The legs are generally paler than in the male. The mesostigmal plates are strongly divergent anteriorly. The middle lobe of the prothorax lacks distinct pits. Abdominal segments 3-7 each have a broad black stripe dorsally that narrows in the basal 1/5 of each segment. Segment 8 has a black triangle and segments 9-10 each have a full-length black stripe, usually narrowing at its middle on 10.

Size

Total length: 26-35 mm; abdomen: 20-27 mm; hindwing: 15-21 mm.

Similar Species

Familiar Bluet (E. civile ) is similar, but the black on the abdomen of Arroyo Bluet is generally more extensive giving it an overall darker appearance. The other similar western bluets, including Boreal (E. boreale), Northern (E. cyathigerum ) and Alkali (E. clausum), have a more reduced dark spot dorsally on segment 2 that does not appear diamond-shaped.

Habitat

Common in ponds and slow reaches of streams.

Discussion

Arroyo Bluet prefers high, rather dry regions. It exhibits polymorphism in head color pattern where individuals range from lacking an occipital bar to having it confluent with postocular spots. One study showed this variation along with a dimorphic prothoracic color pattern differs in frequency between the sexes. Another study in southeastern Arizona found females were in greater abundance than males. The higher numbers of females is offset by a greater mating expectancy among males.

Distribution

Western U.S. from North Dakota south to Texas and west to California; also south through Mexico to Guatemala and Belize.