Tuesday, September 30, 2014

Six Splash-Dunks and a Spin-Dry

Here's a recent video from Cranberry Lake in Anacortes.  It was taken on September 28, 2014, and shows a series of six splash-dunks, followed by a spin-dry that sheds a lot of water drops.

The dragonfly in this video is definitely a darner, though it could be either a Paddle-tailed Darner or a Shadow Darner – these are the two darners flying at the lake now.

We saw 15 splash-dunk events that day, in about an hour of observing, with the number of splash-dunks ranging from 1 to 8.  This is the peak season for splash-dunking, but soon the activity will start to taper off.  Here's a plot of the splash-dunk activity per month for the last three years.

We haven't seen any sticking events yet, where the dragonfly gets stuck in the water.  As the temperature drops below 65˚ F we expect to see these events start to occur.

Just to fill in the details on the splash-dunk/spin-dry suite of behaviors, we present some of the key features below.

First, the basic idea is of splash-dunking is illustrated schematically below:

Here we see a dragonfly plowing into the water a number of times (six for the above video), for the purposes of cleaning its body.  In a splash-dunk, the dragonfly completely immerses itself in the water, and comes to rest for about half a second.  This is in contrast to getting some water to drink, where the dragonfly just barely touches the water, and keeps flying at normal speed.

After a series of splash-dunks, the dragonfly gains some altitude and performs a spin-dry motion at 1,000 rpm to shed the water – which often comes off in a visible halo of water droplets.  So just what is a spin-dry?  Briefly, it's a tumbling, head-over-heels motion, like a somersault, or a diver spinning on the way to the water.

There is a lot of confusion on this point, so to be specific, let's look at a dragonfly and its three principle axes of rotation:

The spin-dry occurs about the dragonfly's "pitch" axis of rotation – that is, the axis that passes along the length of the wings.  This gives the head-over-heels motion mentioned above.  A sketch of a spinning dragonfly is shown next:

Finally, it was mentioned that the dragonfly does its spin-dry at 1,000 rpm.  That figure is obtained by counting the number of frames of high-speed video needed for a rotation, and converting the result to revolutions per minute (rpm).  Here's a plot of data from a number of spin-dry videos:

The red line shows the average spin rate, which is just above the 1,000 rpm mark.  This spin rate results in a centripetal acceleration of about 10g, which is more than enough to throw off any clinging water.

These are the key features of splash-dunking and spin-drying.  With a keen eye, these events can be seen in realtime at your local lake or pond.

No comments:

Post a Comment