Archibasis oscillans, a new species for Vietnam

Archibasis is a genus of medium-sized damsels occurring in South-East Asia and Australia, most abundant in New Guinea and Indonesia. 8 species are recognized at the moment, but information on the identification features of several species is scarce. Two species occur close to Vietnam. The first is the blueish Archibasis oscillans, that has been recorded from India to Thailand and Laos (and likely Cambodia, see Kosterin). The other is Archibasis viola, a purplish member of the genus, occurring in Indonesia, the Philippines, Malaysia and Singapore and, according to the IUCN redlist, with unpublished records in Thailand and Vietnam.
During my visit to Cat Tien in early August I saw a blue Archibasis on a regular basis, both inside the forest and outside near the fish ponds (but under a tree). I was able to collect several males.
Lieftinck (1949) published drawings of the appendages of several Archibasis species, also of Archibasis oscillans. The appendages are similar, but not identical. The inferiors miss the cleft visible in the drawings. In that respect they are more close to those of A. mimetes of Australia and New Guinea. That species however looks rather different. In appearance my males and females are similar to all published photos of A. oscillans that I have been able to find and the description otherwise also fits well. I cannot preclude that there may be an as yet undescribed species involved, but for the moment it seems best to consider this A. oscillans. Its occurrence is certainly not unexpected. It is a species of lowland forest occurring in the same climatic zone in nearby countries and thus fits the pattern of other species found in Cat Tien that occur in neighboring countries and have been found for the first time in Vietnam during recent fieldwork in Cat Tien, like Aciagrion tillyardi, Ceriagrion calamineum, Coeliccia kazukoae, Gynacantha basiguttata, Nychogomphus duaricus, Brachydiplax sobrina and others.

As field photos turned out bad due to foggy lenses, I took this picture in the lodge. A typical Archibasis.

The dorsum of head and thorax. Note large postocular spots, markings on prothorax, dents in humeral stripe and just visible, blue spot on center of dorsal side of S2.

Dorsal view of the appendages, note the "flag" at the tip of the superiors.

Dorsolateral view of the appendages

Lateral view, with inferiors 2/3rd of superiors, digiform, and in lateral view small cleft in superiors.

Ventrolateral view of appendages