EPOW - Ecology Picture of the Week

Dragonfly Spectacle in the Jungle

Skimmer Dragonfly (Hadrothemis coacta), Family Libellulidae
Salonga National Park, Democratic Republic of the Congo, Africa

Explanation:  On a long day's trek through the swamp forests of remote Salonga National Park, in the heart of the Congo River Basin of tropical Africa, we came upon a small pool likely created by forest elephants and maintained by elephants and wild forest hogs.  In the darkness of the jungle understory, I heard an unusual sound repeated over and over, like crinkling cellophane or grasshopper wings vibrating in flight.

It was a large insect making endless loops over this small pond, flying so quickly that it was a mere blur in this dim light.  Only by positioning myself nearby and taking flash photographs was I able to stop the motion and even determine what it was:  a female skimmer dragonfly.

And apparently she was laying eggs, or ovipositing, while in flight, making grand, two-meter (six-foot) tall circles over the water.  

According to odonate (dragonfly) expert Dennis Paulson, females dragonflies that have flanges on segment 8 of their abdomen, as does this species, tend to flick their eggs into the air where they drop either on the water or on the adjacent shore.  This behavior might serve to thwart predators who then cannot zero in onto any one area of egg concentration because the eggs are dispersed.  And flicking the eggs onto land avoids egg predators in the water (e.g., highly carnivorous mosquito larvae).  

However, why she was breaking the water surface with every circle was not clear; perhaps the water aided in egg emergence or made them easier to flick.  

As she approached the water surface, her body was 
horizontal ... but in a split second ... 

... she suddenly struck the water surface and tucked 
her abdomen underneath her, apparently flicking an egg 
in the process.  This happened too quickly to follow with your eye!
 

The resulting disturbance in the water surface took on an
unusual pear-shaped depression that lasted only for an instant,
and could only be really appreciated with a high-speed flash photo.

 
This species -- I am not aware of any common name, so I will dub it the Jungle Skimmer -- is a fairly common species ranging from Uganda through the Congo to Liberia.  
 

3-frame animation of Hadrothemis coacta -- the "Jungle Skimmer" --
showing the bottom part of the loop path that was continuously flown.
(Click for larger version.  © Bruce G. Marcot)

Caught in flight, notice how the forewings are in the upstroke position while the hindwings are in the downstroke position.  They alternate like this, somehow forming aerodynamic lift and amazing maneuverability!

Acknowledgments:
     My thanks to Dennis Paulson and Dutch odonate biologist K.D.B. Dijkstra for species identification and ecological information.