A dragonfly—often a male Paddle-tailed Darner—plows headfirst into the water (the splash-dunk) a number of times to bathe. When satisfied that it is sufficiently clean, it gains altitude and spins head-over-heels at 1,000 rpm to dry off. This is the spin-dry. The dragonfly spins about a horizontal axis during the spin-dry, like a bicycle wheel, as indicated in the following illustration:
We first started taking data on this behavior in 2011, and since that time we've observed 762 separate splash-dunks events. For each event, we record the number of splash-dunks, which ranges from 1 to 8. The distribution of splash-dunk numbers in an event is shown below:
Notice that roughly 40% of the events consist of a single splash-dunk, many have 2 or 3 splash-dunks, and a couple events have had 8 splash-dunks. The average number of splash-dunks per event is 2.32, which is unchanged to three significant figures over the last three years.
The temporal distribution of splash-dunks has also been stable over the last few years. Here are the results for the number of splash-dunk events seen per month from 2011 to 2018:
The peak of the season is September, which also—not coincidentally—is the peak of the flight season for the Paddle-tailed Darner.
We also keep track of the number of spin-drys seen at the end of an event. Here are the results:
In almost 30% of the events, a spin-dry isn't seen, either because the dragonfly didn't do a spin-dry, or—more likely—because it did a spin-dry out of view. A single spin-dry is seen in about 65% of the events. Sometimes a dragonfly does a spin-dry, and then immediately does another. This is seen in roughly 5% of the events. In addition, a couple events results in 3 spin-drys.
With so many events observed over the last 8 years, the results have become very stable and reproducible from year to year.
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