Euphaea guerini Rambur, 1842

Formerly, together with E. masoni, this species was known as subspecies taxon level but now it is believed to belong into a distinct species. Tol 1995, proposed distributional evident of them in Vietnam and in his papers two species were separated in geography. However two species have been reported at the same stream of a secondary forest in Quang Binh Province so they must be also distinguished in ecology and behaviors.

A medium damselfly, this species has dark body and wings in lateral view. The head is black; the thorax of male is dust black, and dark brown with yellow strips markings in female. The legs are short and stout with a row of teeth on front edge of femurs. The wings of male are dark and on upper side and in the sunlight, the hind wings have green/blue flash (the color changes up to position of view) on basal anal area. In female, wings are hyaline with basal orange-yellowish area. Male abdomen is black with a stuff of hairs on lower apical corner of segment tenth in lateral view.

The mature forms do not fly very far from their breeding habitat; they live at clear streams in good forests. The males are not easy recognized in shade but in sunshine they show their hind wings with green metallic flash for territorial and courtship behavior. 

Male with blue hind wing, photographed by Cuong Do, Quang Binh Province

Male with green flash hind wing, photographed by Cuong Do, Quang Binh Province

Blue hind wing male, photographed by Cuong Do, Hon Ba, Khanh Hoa Province