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Mexican Forktail



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Ischnura demorsa

Hagen, 1861


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


Identification

The head and thorax of the male are blue-green and heavily marked with black. The antehumeral stripe is present and complete. Laterally, the abdomen is blue-green proximally, changing to lighter yellow-green distally. Segments 8-9 are blue, each with a prominent lateral black stripe extending more distally on segment 8. Also, there is often a black dorsal stripe basally on 8. Segment 10 is black dorsally and blue laterally with a distinct dorsoapical prominence that is deeply forked, extends well above the segment and is easily viewed in hand. The cerci are strongly curved downward. The upper arm of the deeply bifurcated paraprocts extends beyond both the lower arm and the cerci. The andromorphic females are uncommon and differ from males in having the entire dorsum of abdominal segment 10 blue. The more common gynomorphic females are pale orange or tan. The antehumeral stripe is sometimes reduced to a hairline. The abdomen is nearly all black dorsally, sometimes with segments 8-10 like the andromorphic form. The mesostigmal plates are rather quadrate in shape with only a moderate flange on the anterior border and the distal tubercle on the posteromedial corner. The females may become heavily pruinose after only a few days and this pruinosity may entirely obstruct the thoracic color pattern.

Size

Total length: 21-26 mm; abdomen: 17-21 mm; hindwing: 11-15 mm.

Similar Species

The postocular eyespots are much larger in Lilypad Forktail (I. kellicotti). In Mexican Forktail, the black ventrolateral stripe on segment 9 generally does not extend beyond half the length of that segment and in Western Forktail (I. perparva ) it is often longer. Eastern Forktail (I. verticalis ) is similar but the dorsoapical projection in males is reduced considerably such that it appears absent in the hand.

Habitat

Creeks, streams, springs and slow reaches of rivers with moderate vegetation.

Discussion

A detailed study on the genetics of polymorphism in this species found the same process of natural selection operating as discussed in Plains Forktail (I. damula). Gynomorphic forms are more cryptically colored and therefore less likely to suffer predation and live longer than the andromorphic forms.

Distribution

Utah and Kansas south to Texas, New Mexico and Arizona, southward into Mexico.