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Fragile Forktail



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Ischnura posita

Hagen, 1861


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


Identification

Males are generally yellow-green and mature females are blue. Both sexes are recognizable by a conspicuous division o f the pale antehumeral stripe into an exclamation mark set against a nearly all dark abdomen. The rest of body is metallic black. This species is unique among forktails with the pterostigma in all wings similar and the fewest postquadrangular antenodal cells. The dorsoapical projection on segment 10 in males is forked, but short. The cerci and paraprocts are short and subequal in length. Only andromorphic females are known, but with age they become dark blue with heavy pruinosity. Females lack vulvar spine on segment 8 and have subtriangular mesostigmal plates with raised lateral corners.

Size

Total length: 21-29 mm; abdomen: 16-22 mm; hindwing: 10-16 mm.

Similar Species

The antehumeral stripe in Plains (I. damula ) and Western (I. perparva ) Forktails is divided into subequal anterior and posterior spots and abdominal segments 8 and 9 of males are blue. Citrine Forktail (I. hastata ) females are similar, but generally paler in color, have a complete (though often obscured ) antehumeral stripe and may have a vulvar spine on segment 8.

Habitat

Heavily vegetated ponds, marshes and slow moving waters.

Discussion

This common widespread species is found in every county in Arkansas and in all but eight parishes in Louisiana. It was introduced to Oahu in 1936 and is now found on all but one of the major Hawaiian islands and as far north as Newfoundland and south into Mexico. A study on the roosting behavior of a north-central Texas population found that unlike most odonates, both sexes were regularly encountered at ponds during the day. Both sexes roosted at night significantly higher on the same branches where they perched earlier in the day. At night the body was found to be at a right angle to the stem, possibly allowing for a quicker escape from predation and more efficiency in warming.

Distribution

Eastern U.S. from Florida north to Newfoundland west to North Dakota and south to Texas through Mexico to Belize and Guatemala; also Hawaii.