Pseudagrion pruinosum (Burmeister, 1839)

During a field trip, as I was scanning a fast-flowing stream with my binoculars, in search of odonata - not birds, I spotted 15 meters away, in a pool, a wheel pair of Pseudagrion pruinosum.
After insemination the pair flew off in tandem, landed on an aquatic plant. The female plunged her abdomen below the water surface. After a while the male released his grasp and the female completely submerged herself while the male perched above on guard. My first sighting of a submerged oviposition !
The female descended 10 centimeters below the surface. I watched the scene 30-35 minutes and left. I would have liked to see the female reappear, but there were so many things to discover around...
Normally, on her reappearance, the pair should reform in tandem.

With his orange face, reddish eyes and pterostigma, greenish abdomen (except the last segments), gray and black thorax, the male is unmistakable.

Considering that odonates abandon their water-breathing gills when they emerge, how do the adults get their oxygen when they return to the depths? According to literature, a thin envelope of air clings to the body and wings which gives them a silvery appearance while they’re under water. Body movements likely force the air in and out of tracheal openings facilitating respiration.

According to Corbet in Dragonflies: Behavior and Ecology of Odonata (1999), this  behavior is almost completely restricted to damselflies (suborder Zygoptera) which all oviposit endophytically- inserting their eggs into plant material instead of just dropping them from the end of the abdomen. The duration of uninterrupted submerged oviposition is often at least 30 minutes and frequently close to an hour. Apparently the record goes to the Marsh Bluet (Enallagma ebrium), a North American species, which is known to go submerged for an incredible five hours! Descending to at least 10 centimeters is common, but some are known to go as far as a meter below the surface!
Some advantages of underwater oviposition include exemption from harassment by males looking to copulate and protecting the eggs from desiccation.

 The male in tandem with the female, just after copulation, searching an oviposition site

In damselflies (suborder Zygoptera) the male clasp the prothorax of the female. In dragonflies (suborder Anisoptera) the male clasp the female back of the head. Those appendages, like lock and key, will only fit into the same species female; that's why they have a great taxonomic value. 

The female crawling down a plant stem to get under the surface, few seconds
before the male release his grasp. 

A completely submerged oviposition. Note the silvery appearance.
Not crystal clear water, but I was a bit responsible I think ;)

Pseudagrion pruinosum has a wide distribution throughout Asia from China to Laos, Cambodia, Vietnam, Thailand, Singapore, Borneo, and throughout Peninsular Malaysia.
It can be found in upland and mountain areas, near open streams or ponds.

In the Hanoi's area, it is a quite common sight along slow-flowing portions (almost still water) of open swift streams where aquatic vegetation can develop.

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