22 February 2010

Tau Emerald Dragonfly - Hemicordulia tau

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Tau Emerald Dragonfly - Hemicordulia tau

Hemicordulia tau belongs to the Hemicorduliidae family, which has two species in the Esperance region, both very similar and difficult to separate without careful investigation. These dragonflies are known as vagrants, which means they are wanderers and not permanent in any location, however they are both common and also can be found in all Eastern States including Tasmania. They are excellent fliers and seldom rest, but when they do, they usually hang vertically with wings extended at right angles. When on patrol along creeks, swamps and pools, they often hover with their body motionless whilst investigating any intruder and are known to chase off other less agile dragonfly species, besides rival males.

The Tau Emerald Dragonfly is more widespread than the similar Australian Emerald Hemicordulia australiae and successfully occupies the dry interior and Nullarbor where water is limited and often temporary. Other differences include the markings on the frons (nose), which on the Tau Emerald is yellow with a black T marking, whereas is darkly coloured without markings on the Australian Emerald. The extent of the blue/green color is another distinguishing feature that relates to the abdominal segments, of which there are nine (excluding the thorax and anal appendages). With the Tau Emerald the seventh segment (third from the anal appendage end) is largely darkly coloured on top, but the Australian Emerald’s is strongly constricted at the top of the seventh segment and the one below (eighth).

Hemicordulia tau is around 5 cm (2”) in head/body length and locally likely to be encountered during the warmer months from January to March. The metallic blue/green markings on the otherwise yellow abdomen are quite striking, particularly when viewed in reflective sunlight, making this species one not to miss.

A very useful and relatively inexpensive publication permitting identification to species level is The Complete Field Guide to Dragonflies of Australia by G.Theischinger and J.Hawking.

Update 21/03/2010
Two photographs exchanged.
Update 23/05/2010

Photo of black facial T marking added.