Basically it was very similar to a species I had seen earlier at Cat Tien National Park, Nychogomphus duaricus. Although that is much further south, this species has also been recorded from Yunnan in China. But checking Chao it dawned on me that this was in fact Nychogomphus flavicaudus, a species thought to be endemic to Hainan.
Having said that, these species are extremely similar, so I will not be surprised if DNA-analysis would indicate they are in fact the same species, or subspecies. The most noticeable differences are 1) the antehumeral pattern (a humeral spot and a separated humeral line or series of small spots forming a broken humeral line in N. duaricus and only an antehumeral spot in N. flavicaudus) and the size (abdomen around 36mm and hindwing around 27mm in N. duaricus, abdomen around 40mm (total body length 54mm) and hindwing 33mm in N. flavicaudus). It must be noted that Asahina (1986) points out that specimens of N. duaricus from Malaysia also miss the antehumeral line. But if we follow current practice, then the specimens from Cao Bang must be considered N. flavicaudus.
This brings the number of Nychogomphus species in Vietnam to three. The other is N. lui (see Sebastien's site. I have not been lucky enough to bump into this species that inhabits large rivers and occurs in the suburbs of Hanoi).
|Male Nychogomphus flavicaudus. Note the antehumeral spot, but complete lack of antehumeral stripe.|
|Appendages in dorsal view|
|Appendages in lateral view|
|Facial pattern similar to N. duaricus, but extent of yellow patches somewhat reduced|