*Matti Hamalainen sent me a photo of female Megalogomphus sommeri and pointed out that the shape of the abdomen was not consistent with that species. I have to concur. He also suggested it might be the rare Macrogomphus rivularis and yes, the long S9 seems to suggest that genus. All the more reason to try to relocate it.
** Well, we now know that it is another genus with very long S9, but new to Vietnam, Labrogomphus
|Labrogomphus torvus, female|