Thursday, October 25, 2012

Sky-Diving Dragonflies

Dragonflies are known for their impressive aerobatic skills.  They can turn on a dime, hover with precision, plow into the water to create a splash-dunk, and even spin-dry at 1,000 rpm in mid flight.  They're amazing flying machines.

A couple days ago, Betsy and I were watching some late-season darners at Cranberry Lake when one made an impressive maneuver and caught a fly right in front of us.  The darner was flying straight and level at about eye level.  It suddenly noticed a fly just above and slightly behind it, and it came to an abrupt stop.  The darner hovered with its body almost vertical, and as it hovered it rotated and repositioned itself slightly so that it was on a direct line with the fly.  Once in position it accelerated and captured the fly in the blink of an eye.  What a display of precision, patience, and power.

All of this is well and good, but did you know that dragonflies can sky dive?  Well, at least that's what I call it.  What they do is bend their abdomen upward, raise their wings up over their body, and then drop briefly in free fall.  They look like a skydiver with arms and legs pointing upward as they plummet toward Earth.  Here are a few photos of a sky-diving darner taken from a slow-motion video:

A darner (probably a male Paddle-tailed Darner) just before doing a sky dive.

The sky dive is beginning, as the darner raises its abdomen.

Now abdomen and wings are raised upward.  At this point the darner drops briefly in free fall.

The same darner is getting ready for a second sky dive.

There goes the abdomen pointing upward.

Now the wings follow the abdomen in pointing upward, and the darner drops downward.

Here the darner is pulling out of his brief free fall.

Here's the video from which these screen captures were taken.

video

So what's going on with sky diving – why are they doing it?  It's hard to know for sure.  At first I wondered if they might be losing altitude quickly, like whiffling geese that sometimes even turn upside down as they drop down to land.  A good link for photos of whiffling geese is the following:

http://clives.shawwebspace.ca/photos/view/whiffling_geese_/

With more observations this explanation seems less and less likely – the darners are able to increase or decrease altitude much more quickly than they do while gliding in free-fall sky diving.

It may be that they are cleaning their wings or abdomen.  Sometimes a perched damselfly will be seen to arch it's abdomen upward – it appears to be rubbing the abdomen against the wings in a cleaning action.  Darners may be doing the same sort of thing, only in mid flight, as they do so many things in their life.  Sky diving isn't very common, so it may be a while before we know more about its purpose.

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