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Double-striped Bluet



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Enallagma basidens

Calvert, 1902


Order Odonata
Suborder Zygoptera
Superfamily Coenagrionoidea
Family Coenagrionidae
Genus Enallagma
Species basidens Calvert, 1902 [Enallagma]


Identification

The front of the male's head is blue with a broad black stripe. The top is large is largely black with a pale occipi tal bar and small postoccipital spots. The middorsal thoracic carina is pale blue and bisects the broad black middorsal stripe that is approximately half the width of the mesepisternum. The humeral stripe is black and subequal in width to the antehumeral stripe, but narrowed basally, and divided for nearly its entire length by a thin pale blue line. The femora are pale blue or cream with dark stripe on their outer surface. The tibiae and tarsi are generally pale and lack dark markings. The abdomen is largely bright blue, becoming paler ventrally. The entire top of segment 1 is black except for a narrow blue apical band. A black stripe runs the entire length of segments 2 and 3 dorsally and extends laterally to the last quarter of each segment. There is a black hastate stripe on the apical 1/2-2/3 of segments 4-6 dorsally. The entire dorsum of segment 7 is black, except for pale basal and apical rings. Segments 8-9 are generally all blue and segment 10 has an irregular black stripe dors ally. The cerci are black and extend for approximately half the length of segment 10. They are sharply truncate, when viewed laterally and have a distinct basal ventrally directed lower lobe. The paraprocts are pale becoming darker apically, curving upwards and extending slightly more than half the length of the cerci. The head and thorax of the female closely resemble the male, but with the pale colors more extensive. Females occur in three different color forms, with pale colors either blue, green or brown. Generally there are small pale spots anterior to each ocellus. The pale stripe dividing the humeral stripe is most often confluent anteriorly and posteriorly with the rest of the pterothorax. There is a distinct anterior high ridge towards the middle of each depressed mesostigmal plate and these plates have strong prominences at each posteromedial corner. The abdominal segments are similar to the male, but with the dorsum of segments 3-6 entirely black except for a narrow, basal p ale ring. Segment 8 is black dorsally with only a narrow apical pale ring. Segment 9 is black dorsally with a large blue spot in the basal 1/3-2/3 of each segment that narrows and is distinctly emarginate apically. This spot is sometimes divided into two separate triangles. The top of segment 10 is entirely blue.

Size

Total length: 21-28 mm; abdomen: 17-22 mm; hindwing: 10-15 mm.

Similar Species

Double-striped Bluet is the smallest of our bluets. The thin pale stripe that longitudinally divides the dark humeral stripe (and gives the species its name ) is distinctive among all dancers (Argia ) and bluets in our area.

Habitat

Various permanent and semipermanent ponds, lakes and reservoirs as well as slow reaches of streams and rivers.

Discussion

Double-striped Bluet, originally described from Texas, has expanded its range westward and northward. This expansion is probably a result of extensive irrigation affording new suitable breeding localities. It has also expanded its range eastward this century, now reaching as far as Florida and the Carolinas, north to New York and Michigan. It was first reported from Canada in southwestern Ontario collections taken in 1985. Females are often only observed around water while in tandem. Egg laying occurs in tandem where floating masses of filamentous algae and other vegetation are preferred. Double-striped Bluet perches over water most often from 10:00am to 4:00pm.

Distribution

Florida to Ontario west to Colorado and California; south to Texas and Mexico.