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Stream Bluet



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Enallagma exsulans

Hagen, 1861


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


Identification

The face of the male is dark and the top of the head is mostly black with only a narrow strip of blue visible. There are two small tear drop-shaped pale postocular spots narrowly separated or confluent with the pale occipital bar. The hind margin of the posterior pronotal lobe has a blue medial tubercle, bearing setae, and short stripes laterally. The lateral portions of the prothorax are pale blue. The middorsal carina and stripe of the pterothorax are generally black. The former is sometimes tan posteriorly with the latter as much as 2/3 the width of the mesepisterna. The pale blue antehumeral stripe is approximately a third the width of the middorsal stripe. The broad black humeral stripe is equal to or as much as 3 times the width of the antehumeral stripe. The abbreviated black li ne on the interpleural suture is confluent with the humeral suture. The rest of the thorax, aside from a dark spot on the metapleural fossa, is blue fading ventrally. The wings are clear or with only slight smoky cast. The legs are pale with a sometimes interrupted black stripe on the femora and tibiae and the tarsi are pale and armed with black spurs. The abdomen is blue with a largely black dorsum. Segment 1 is entirely black dorsally except for a pale apical ring. There is a black stripe extending the full-length of segment 2, dorsally. Segments 3-6 each have a full-length black stripe narrowing basally to interrupt a pale blue ring. The apical fifth of this stripe is expanded laterally on each segment. Segment 7 is either as the previous segments or with black extending over entire dorsum. Segment 8 has a full-length black dorsal stripe or sometimes only a basal triangle extending for more than 1/2 the segment. Segment 9 is entirely blue with only a small basal black triangle. Segm ent 10 is entirely black dorsally and strongly emarginated laterally. Sometimes the emargination envelope pale blue spots laterally. The dark cerci are distinctly forked, when viewed laterally; both lobes directed posteriorly, but the lower lobe noticeably longer than the upper one and approximately half the length of segment 10. The pale paraprocts are slightly shorter than the upper lobe of the cerci and the dark apices are directed posterodorsally. The female is generally more green than blue. The head pattern is like that of the male. The prothorax lacks any distinct pits on the middle lobe and the hind margin of the posterior lobe has a median tubercle bearing numerous setae. The middorsal carina is distinctly tan and bordered by a narrower black middorsal stripe that is generally not quite half as wide as the mesepisterna. The pale antehumeral stripe is 1/2-2/3 the width of the middorsal stripe. The black humeral stripe is generally less than twice the width of the antehumeral st ripe and nearly always divided longitudinally; sometimes with the brown replacing the black entirely. The metapleural stripe is not confluent with the humeral stripe. The rest of the pterothorax is as in the male. The mesostigmal plates are more or less triangular with a distinct concavity just posterior to each plate. The abdominal pattern is similar to the male, but with segment 8 bearing a narrow pale apical ring. Segment 9 has two confluent basal, black spots or triangles, emarginated with blue medially. Segment 10 may be entirely blue or occasionally a small black dorsal triangle is present.

Size

Total length: 31-37 mm; abdomen: 24-30 mm; hindwing: 17-21 mm

Similar Species

Dorsally, abdominal segment 8 in Turquoise (E. divagans ) and Skimming (E. geminatum ) Bluet is pale blue, while the posterior portion of segment 7 is blue in Azure Bluet (E. aspersum). Skimming Bluet also lacks a pale occipital bar. Female Mexican Wedgetails (Acanthagrion quadratum ) are similar, but the postocular spots are not contiguous in that species.

Habitat

Common along shores of slow moving streams, rivers and occasionally lakes.

Discussion

Stream Bluet is widespread throughout central Texas but is more frequently encountered to the east, where it can be abundant. They are often sparse in the earlier parts of the day but seem to become more numerous in the late afternoon as temperatures start to cool. In a study of the reproductive behavior of an Indiana population, mating lasted an average of 76 minutes but was witnessed to last as long as 2 hours. Females submerge themselves to oviposit, sometimes while still in tandem with males and other times with the male breaking away after contact with the water. Females remain submerged for 15-31 minutes, while the longest recorded time for a male was 9 minutes. Females have been observed probing backwards down stems of Potamogeton in a deliberate and repetitio us manner, depositing eggs. One author noted capturing a female to whose thorax was attached the last abdominal segments of the male; a true testimony to the secure coupling mechanism of Stream Bluet!

Distribution

Eastern U.S. from Georgia north to Maine and southern Canada, west to Michigan and south to Texas and Mexico.