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Plains Forktail



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Ischnura damula

Calvert, 1902


Order Odonata
Suborder Zygoptera
Superfamily Coenagrionoidea
Family Coenagrionidae
Genus Ischnura
Species damula Calvert, 1902 [Ischnura]


Identification

Males have a dark thorax dorsally, with the antehumeral stripe reduced to a pair of small pale spots. The remaining areas of the thorax are blue. The abdomen is largely black with segments 8-9 blue except for an abbreviated basal lateral stripe. The dorsoapical prominence on abdominal segment 10 is not forked and is about a forth the height of the rest of the segment. The caudal appendages are distinct with the cerci bearing a prominent posteroventral process and the paraprocts are upcurved with bluntly pointed apices. The female is one of only two species in the region with a prominent nipple-like process on each side of the pronotum. Andromorphic females are common and nearly identical to males with a reduced antehumeral stripe. Gynomorphic females have a complete antehumeral stripe and the pale abdominal colors are orange or tan with the occasional blue markings laterally on segments 1-2, the apex of 7, and 8-10.

Size

Total length: 23-34 mm; abdomen: 18-27 mm; hindwing: 11-19 mm.

Similar Species

Male Black-fronted Forktails (I. denticollis ) have the entire front of the thorax black. Pacific Forktail (I. cervula ) is similar with the antehumeral stripe separated into a distinct posterior and anterior pale spot, but the dorsoapical projection on abdominal segment 10 does not reach as high above the segment in Plains Forktail as it does in that species. Close in-hand examination of the female mesostigmal plates will be necessary where these two species overlap.

Habitat

Ponds, springs and slow moving streams with heavy marginal vegetation.

Discussion

Egg laying may occur unaccompanied by the male or in tandem, usually in emergent vegetation or algal mats. One study explored the genetic basis for the female polymorphism and its relationship to natural selection in this species. It found that the male-like andromorhpic females were more vulnerable to predation than the crypically colored and thus longer-lived gynomorphic forms. These gynomorphic forms, however, were found to engage in interspecific mating that ultimately lowered their reproductiv e potential. These selective pressures explain the obscured higher frequencies of andromorhpic females in populations.

Distribution

Eastern slope of Rockies and Great Plains from Texas to North Dakota and Wyoming south to Arizona.