Monday, 10 April 2017

Orthetrum internum headaches

Last Thursday I was in Sa Pa and as usual this time of the year Orthetrum internum was already out in good numbers. Wen-Chi Yeh had responded to my posting of the species in 2014, but I had overlooked the significance of his comment. Basically he said that my photos looked different from what he was used to for O. internum. I had noticed that lack of pruinosity on the  dorsal side of the thorax, but in my posting mentioned this as one of the characteristics of O. internum. How wrong can you be?

Yesterday I visited Pia Oac in Cao Bang and ran into O. internum is several places. This in itself is strange, because I had never seen it there before. But even more strange was the fact that they were clearly pruinose and much more black-and-yellow. These were true O. internum! But if so, then what is the species in Sa Pa?

Structurally the two are identical. I am not sure if there are structural differences visible to us between O. internum and O. japonicum. These two were considered subspecies of O. japonicum in the past, until DNA sequencing made it clear they were in fact separate species. So, the lack of structural differences does not necessarily mean that the Sa Pa species is O. internum, nor does it support the other option, of course. But in coloration it is very different. The labium and labrum are pale orange, not black. The frons lacks black edges dorsally. The venter is not nearly as black, with the hamulus pale brown with only the tooth darker brown, but it is black-and-white in O. internum. And the thorax ground colour is black or dark brown in O. internum, with clear yellow on the prothorax (especially posterior lobe). In the Sa Pa species the ground colour if brown with pale markings, and there is no yellow on the prothorax, just pale areas. And most interestingly, there never is any pruinosity on the dorsum of the thorax!

From Sa Pa, note orange labium and labrum, brown color of thorax, no pruinosity, pale thoracic markings. Photo taken in 2014
Last Thursday. Note some characteristics. 
But this is a very black O. internum. Note also black labium and labrum, black on top of frons, pruinosity on the thorax, from Pia Oac
In dorsal view
A slightly less black male from Pia Oac. Note also the clarity and extent of the yellow markings on the thorax.
Note the yellow posterior lobe of the prothorax.



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