Asahina (1997) described the subspecies Protosticta khaosoidaoensis satoi based on some differences of color pattern of thorax and abdomen compared to the nominotypical subspecies from Thailand.
Later, van Tol (2008) raised this subspecies to species rank, and described two forms of P. satoi, a normal form and a dark form. He then stated it was possible that the dark form of Protosticta beaumonti Wilson, 1997 figured by Wilson and Reels (2003) from Guangxi may be conspecific with the dark form of this taxon, and the normal form may be conspecific with prototypical beaumonti from Guangdong. If so, P. satoi is a junior synonym of P. beaumonti. This taxonomic issue is not yet clarified.
This slender and delicate Platystictid damselfly live low in the dense forest undergrowth, making very short hunting flights. It is a tame fellow which likes to pose for the pictures. Often its curiosity will bring it close to you. Many times I saw it hovering just in front of my nose!
Labium whitish. Pronotum with anterior and posterior lobes blackish, median lobe whitish.
Dorsal synthorax dark-greenish with metallic sheen. Sides with pale brown mesepimeron, dirty yellow metepisternum and metepimeron with a fine stripe over metapleural suture.
Abdomen long and thin, but S8-9 obviously inflated. Pale blue marking on S9 in both sexes.
The shot above is the best I could do with showing the appendages (rubbish, I know). Superior appendages strongly curved with complicated protuberances. With a macro lens, impossible to get detailed close-ups of so tiny parts of the body.
Female is similar to the male in general maculation pattern, but abdomen more stout.
P. satoi dark form can immediately be distinguished by its almost wholly black synthorax with a triangular posterior marking on the metepimeron.
The coloration of the pronotum of the males shows significant variation, the median lobe may be either fully pale colored or heavily smudged dark.
P. satoi is only known from northern Vietnam. It is not an uncommon sight in lowland and montane forest streams at Tam Dao, Cuc Phuong, Ba Vi National Parks... Sometimes, I found it along streams in heavily degraded forest, but always with dense marginal vegetation. The dark form is rarer.
Further studies should reveal whether P. satoi is a junior synonym of P. beaumonti, or whether both represent distinct species.
More information about genus Protosticta in Vietnam here.