Archibasis viola, another Archibasis in Cat Tien

November 16 Kameliya and I went to the Birdlake area in Cat Tien. Close to where the trail starts off the main track there are a few clean ponds with a lot of floating and emergent vegetation. This is where I noticed a purple damselfly. Identification was straightforward. Archibasis viola, a species that has, according to the IUCN website, been reported from Vietnam, although this is based on non-published information. A. viola occurs all over South-East Asia in wet low-land forest and swamps, so it is not surprising it also occurs in Cat Tien. Interestingly we also bumped into the only male and female of Archibasis oscillans of the trip not very far from this spot, bit at a small stream inside the forest. Both the male and the female were sitting there on a bush. Of A. viola I saw several tandems, but could not get photos of the female. For A. oscillans I could, a nice addition to the material from August.

The male of A. oscillans. The flash makes it appear less bright blue than it is in reality.

Archibasis oscillans, female, after release (hence the smudges on the wings). Note the bite out of the antehumeral, just like in the male.

Male A. viola, less purple under the flash, but still obviously purple.

Close-up of a different male. Note purple labrum and anteclypeus, lack of purple lines on occipital ridge, straight antehumerals. On S2 the typical Archibasis square spot.

Lateral view doing more justice to its purple colour.

A. viola appendages in ventro-lateral view

Same in dorsal view

Same in lateral view

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of the genus, occurring in Indonesia, the Philippines, Malaysia and Singapore and, according to the IUCN redlist, with unpublished records in Thailand and Vietnam.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