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.