Here's an article I'm submitting for the next issue of Argia, the journal for the Dragonfly Society of the Americas.
As part of our dragonfly watching routine over the past several
years, my wife Betsy and I have studied the splash-dunk/spin-dry suite of
behaviors (Walker, 2014a). We enjoyed doing so again this year. The purpose of
this paper is to update the results of our observations that now cover a total
of 602 splash-dunk events and 13 spin-dry videos.
As a reminder, recall that splash-dunks are events where
a dragonfly slams into the water at full speed to bathe (Walker, 2011), and a
spin-dry is the subsequent 1,000 rpm spinning motion in mid flight to shed
the water (Walker, 2014b). This is illustrated in Figure 1, which shows a typical 3 splash-dunk event, followed by a
spin-dry. The dragonfly in this illustration is the Paddle-tailed Darner (Aeshna palmata), which is the species
most commonly seen doing this behavior.
Figure 1 A typical splash-dunk/spin-dry
event. The drawing is by Sabine Deviche (devichedesigns.com).
The 2016 season was memorable in a couple different ways.
First, we noticed much less dragonfly activity than normal at our usual
dragonfly locations in Anacortes, WA. For example, no American Emeralds (Cordulia shurtleffii) were seen at
Cranberry Lake this year, though we usually see at least a few. In addition, there
were fewer Four-spotted Skimmers (Libellula
quadrimaculata) and Eight-spotted Simmers (Libellula forensis) than in previous years.
On a more positive note, the other interesting occurrence
this season was a particularly impressive spin-dry performed by an
Eight-spotted Skimmer, like the one shown in Figure 2. This occurred in mid summer, when we were observing
dragonflies in eastern Washington at the Quincy Lakes complex of lakes and
beaver ponds near Quincy, WA. At one point we were looking down from a hillside
at a small beaver pond. The water was dark, and we immediately saw a brilliant
Eight-spotted Skimmer take flight from the shoreline. It flew out over the
water, did a series of three splash-dunks, gained a bit of altitude, and then
did a spectacular head-over-heels spin-dry with its flashy black-and-white
wings spinning rapidly. It was quite a sight to behold, especially when
compared to the much more common, but less showy, spin-dry of a darner with its
clear wings.
Figure 2 A male Eight-spotted Skimmer
showing off its flashy wings.
The Number of
Splash-Dunks per Event
Whenever we see a dragonfly initiate a splash-dunk event,
we count the number of splash-dunks it performs before it rises to do a
spin-dry. This is often just a single splash-dunk, but in many cases the event
extends to a series of several splash-dunks in a row. The maximum number of
splash-dunks we’ve seen in any one event is 8, as described in the case of the
constipated dragonfly (Walker, 2013).
Figure 3 shows the results of our
observations for the six dragonfly seasons from 2011 to 2016. In a normal year
we see an average of 115 events, but this year, with its low activity level, we
saw only 25 events. Still, the total number of events represented in Figure 3
is 602. Notice the almost exponential falloff as the number of splash-dunks
increases, with a noticeable “shoulder” at 3 splash-dunks. This feature has
persisted for several years now (Walker, 2014a), indicating an unexpected
preference for events with this number of splash-dunks.
Figure 3 Cumulative splash-dunk data
for the 602 splash-dunk events observed during the years 2011 to 2016.
The average number of splash-dunks per event is 2.31.
This result is unchanged over the last three years—a significant finding,
considering that the statistics are derived from hundreds of observations.
Temporal
Distribution of Splash-Dunking
Another important aspect of splash-dunk behavior is the
time of year in which it occurs. We record the date of all the splash-dunks we
record, and the month-by-month result is shown in Figure 4. Notice the large peak in September, when more than half
of all events are observed.
Figure 4 Temporal distribution of 602 splash-dunk
events from 2011-2016.
Part of the reason for the September peak is that this is
also the peak month for the flight season of the Paddle-tailed Darner, which is
shown in Figure 5. Notice the
similarity between the chances of seeing a Paddle-tailed Darner and the chances
of seeing a splash-dunk. The main discrepancy is that fewer splash-dunks are
seen in July and August than one might expect on the basis of the flight
season.
Figure 5 Flight season for the
Paddle-tailed Darner.
The flight season of the Shadow Darner (Aeshna umbrosa) is shown in Figure 6. Again, the flight-season and
splash-dunk distributions are similar, but notice that more splash-dunks would
be expected in October and November if the Shadow Darner were the primary
splash-dunker. It seems that the actual splash-dunk distribution is roughly an
average of what one might expect from these two darners.
Figure 6 Flight season for the Shadow
Darner.
One final comparison is shown in Figure 7. This is the flight season of the Blue-eyed Darner (Rhionaeschna multicolor), which is also
seen to splash-dunk on occasion. Notice the very different temporal
distribution for this species compared with the distribution of splash-dunks.
Figure 7 Flight season of the Blue-eyed
Darner.
Another reason for a lot of splash-dunking in the Fall
may be that this is also the season when spider webs carrying young spiders are
frequently drifting through the air. It is not uncommon to see darners speeding
by in September with a spider web trailing from their abdomen. This is quite possibly
the reason for many of the splash-dunks we see.
Splash-Dunk
Species
As mentioned above, most of the splash-dunks we observe
are performed by Paddle-tailed Darners. We also see Shadow Darners doing
splash-dunks, though they seem to hit the water with less force than do the
Paddle-tailed Darners. A comparison between these two species is presented in Figure 8. The two darners on the left
are Shadow Darners, and the one on the right is a Paddle-tailed Darner.
Figure 8 A comparison between Shadow
Darners (the two on the left) and a Paddle-tailed Darner. These are “free
range” dragonflies that have been “whispered” onto my fingers.
We occasionally see splash-dunks performed by the
following darners as well:
California Darner (Rhionaeschna
californica)
Blue-eyed Darner (Rhionaeschna
multicolor)
Common Green Darner (Anax
junius)
Species from other families of dragonflies have also been
observed to splash-dunk—like the Eight-spotted Skimmer mentioned above—though
usually just in isolated incidents. These species are as follows:
Four-spotted Skimmer (Libellula
quadrimaculata)
Eight-spotted Skimmer (Libellula forensis)
Western Pondhawk (Erythemis
collocata)
Autumn Meadowhawk (Sympetrum
vicinum)
Blue Dasher (Pachydiplax
longipennis)
The Autumn Meadowhawk is notable on this list for being
the only species we have observed so far to do a spin-dry while attached in
tandem. Their tandem spin-dry was very slow, and lasted for only a couple
rotations.
Spin-Dry
Statistics—The Fastest Rotating Animal
A fitting end to a series of splash-dunks is an
invigorating spin-dry to shed the water. Data is harder to obtain for a
spin-dry than for a series of splash-dunks because the spins happen so quickly.
When I get a good slow-motion video of a spin-dry, however, I can then analyze
it in detail. This season I added a 13th slow-motion spin-dry video
to my collection, giving just that much more specific information on the
behavior.
Figure 9 shows this 13th darner
performing a splash-dunk—one of 6 it did in this event. It then gained some
altitude (perhaps 3 to 5 feet) and did the spin-dry shown in Figure 10. This video was detailed enough that I could count the number of
frames (filmed at 240 frames per second) corresponding to each individual
rotation. As a result, we know that the spin-dry consisted of 7 revolutions, lasted
0.458 seconds, and had a maximum spin rate of 1,200 rpm.
Figure 9 A darner approaches the surface
of Cranberry Lake (left), and then plows into it (center), sending up a plume
of droplets as it comes to a complete stop. It then emerges from the water to
do it all over again (right).
Figure 10 The darner from Figure 9 in
the process of doing a spin-dry. It is surrounded by a fine halo of water
droplets shed by the 1,200-rpm rotation.
Figure 11 collects the results from 13
slow-motion videos of the spin-dry behavior. It shows the spin rate for each
event, along with a red line indicating the average value. As can be seen,
1,000 rpm is a good round-figure to characterize spin-drying in dragonflies—the
fastest known rotational motion of any animal.
Figure 11 Spin rate for 13 different
darners doing a spin-dry. The red line indicates an average value just more
than 1,000 rpm.
To be specific, the data from these 13 videos gives the
following numerical results:
Number of rotations in a spin-dry = 5.85 ± 1.18
revolutions
Time spent spinning = 0.443 ± 0.062 seconds
Maximum spin rate = 1,060 ± 207 rpm
Finally, the centripetal acceleration associated with a
spin-dry is quite large—certainly more than enough to shed water. The angular
speed is w = 1,200 rpm = 111 rad/s, and the corresponding
centripetal acceleration is rw2, where r is the
radius of the spinning object in meters and w is the angular speed in rad/s (Walker,
2016). It’s hard to estimate r, but a
reasonable value for a 70-mm darner is somewhere between r = 0.01 m and r =
0.03 m, giving an acceleration of 120 m/s2 to 370 m/s2.
Thus, the spin-dry produces an acceleration ranging from about 10g to 40g, where g = 9.81 m/s2
is the acceleration due to gravity. This is quite an impressive “g force” for any organism to endure—and
they don’t even get dizzy.
Future observations may yield videos of other types of
dragonflies doing a spin-dry. When this occurs, the comparison with the
spin-dry of darners will be of great interest.
Acknowledgements
I would like to thank Betsy
Walker for help collecting the data presented here.
Literature Cited
Walker, J.
S. 2011. Splash-Dunk Analysis,
2011. Argia 23(4): 29-30.
Walker, J.
S. 2013. The Strange Case of the
Constipated Darner. Argia 25(3): 29-30.
Walker, J.
S. 2014a. Splash-Dunk Analysis for
2011-2014. Argia 26(4): 32-33.
Walker, J.
S. 2014b. Life at 1,000 RPM. Argia
26(2): 11-13.
Walker, J.
S. 2016. Physics, 5th edition.
Pearson Addison-Wesley.
