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Rambur's Forktail



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Ischnura ramburii

Selys, 1850


Order Odonata
Suborder Zygoptera
Superfamily Coenagrionoidea
Family Coenagrionidae
Genus Ischnura
Species ramburii (Selys, 1850) [Agrion]


Identification

The males have a dark head with small pale green postocular spots. The thorax is green with broad black middorsal and humeral stripes that surround a thin pale antehumeral stripe. The abdomen is dark dorsally and pale laterally with blue on all of abdominal segment 8 and anterolaterally on 9. The dorsoapical projection on segment 10 is slight. The cerci are short and blunt. The paraprocts are twice as long as the cerci, unforked and project posteriorly. Females are found in three color forms, including an andromorphic, male-like form. Gynomorphic forms inclue an orange-red form, where the thorax and abdominal segments 1-2 are entirely orange-red except for a prominent black mid-basal stripe on the former. The rest of the abdomen is dark. A second gynomorphic form is olive-green and patterned like the orange-red form. Because young individuals of the green form look like the red form, it is unclear whether a true genetically determined "red form" exists. A prominent vulvar spine on segment 8 is present. The mesostigmal plates are subtriangular with a low, pale ridge forming a continuous posterior border.

Size

Total length: 27-36 mm; abdomen: 21-29 mm; hindwing: 15-19 mm.

Similar Species

Desert Forktail (I. barberi ) can be distinguished from Rambur's Forktail by the continuous black stripe dorsally on the abdomen and the absence of blue dorsally on segment 9. Only abdominal segment 9 is blue in male Furtive Forktails (I. prognata ) and only segments 1-2, never 3 and 4, are pale in young female Citrine (I. hastata ) and Furtive Forktails.

Habitat

Heavily vegetated ponds, lakes, marshes and slow reaches of streams exposed to sunlight including brackish waters.

Discussion

Rambur's Forktail is the most widespread forktail of the New World, ranging as far north as Maine, southward to southern California, Mexico, Central and South America. It occurs year round in the southern parts of its range. It also inhabits the Hawaiian Islands, where it was introduced in 1973, with possible subsequent introductions. As widespread as this species is, surprisingly little has been written about its biology. Both sexes will remain close to the water and although males are not territorial, females are known to be highly predaceous and often cannibalistic. Males often do not release females from the wheel position for several hours, and sometimes as many as seven, to secure their genetic contribution. Red females will sometimes attack males, but more often curl th eir abdomen downward while fluttering their wings in a refusal display. Females often lay eggs late in the afternoon, unattended by males, on the underside of floating vegetation or debris, by curling the abdomens. There is an apparent lack of color preference by males, but there are selective advantages and disadvantages of various color forms in populations.

Distribution

Florida to Maine west to Illinois and south to Texas through Mexico to Chile; also Hawaii.