Thursday, 23 July 2015

Nesoxenia lineata - new for Vietnam

July 23 I received a message from James Holden. He is currently surveying Odonata in Cat Tien National Park. He alerted me to a small Lathrecista asiatica-like dragonfly that was not just smaller, but had very different venation. Could I help?

When I checked my earlier post on Lathrecista, I noticed to my excitement and shame that I had included there a photo of exactly the same species James had now found, caught by me on August 5 last year.

Superficially the two species are very similar, but the venation is very different. The smaller species (James measured 27mm total length) has two rows of cells between the cubital space and the wing margin in the hind wing. It has a free triangle in the hind wing, but in the fore wing the triangle is 2-celled and the sub triangle 3-celled. There are two bridge cross veins in both wings. And amazingly, there is no anal loop in the hind wing. These characters fit very well Nesoxenia lineata, but that species has distinctive thoracic markings. I have found no pictures with the extensive pruinosity of both mine and James' specimen. And N. lineata is apparently somewhat larger.

Rory Dow commented that the venation indeed points to N. lineata. He also mentioned that he would not be surprised if the heavy pruinosity is either the result of age or of geographical variation. And that the size is close enough, given the likely small sample on which the size of the species is probably based in the literature.

Yesterday I received another photo from James, this time of a female. This female too seems lightly pruinose, but the underlying pattern of the thorax is clearly visible and fits N. lineata like a glove.

N. lineata has been recorded in Thailand, but has never been recorded from Vietnam. But judging from both my own and James' observations, it is regular at Cat Tien. Because both our males and the female display heavier pruinosity than usual, this may be a peculiarity of the Cat Tien population.

The possible Nesoxenia caught by James. Note the venation characters. Red extends onto S9. 

This is Lahtrecista asiatica. Much larger, distinct anal loop, 4 rows of cells between cubital space and wing margin in hind wing, only 1 bridge cross vein
My specimen of last year August, showing obscured thoracic pattern.

Blow-up of hamule, strongly hooked
Photo by James, including his hand, of female N. lineata, showing the typical thoracic pattern of the species

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