Splash-Dunk/Spin-Dry Analysis For 2011-2016

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 13^th slow-motion spin-dry video to my collection, giving just that much more specific information on the behavior.

Figure 9 shows this 13^th 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, 5^th edition.  Pearson Addison-Wesley.

Wing Drooping in Red Saddlebags

The following is an article I've submitted for publication to Argia, the scientific journal of the Dragonfly Society of the Americas.  It reports an interesting type of wing behavior recently observed in red Saddlebags.

Wing Drooping in Red Saddlebags

James S. Walker

Anacortes, Washington

<jswphys@aol.com>

In a recent article (Walker, 2015), I discussed the behavior I refer to as wing whacking.  This type of wing-related behavior was in addition to previously described wing behaviors like wing grabbing (Walker, 2013a), sky diving (Walker, 2013b), and spin-drying (Walker, 2011, 2014).

When I wrote the wing whacking article, I didn’t expect to be reporting on a different modality of wing behavior anytime soon – but I was wrong.  In the last couple months I’ve encountered another type of wing use in which a dragonfly depresses, or “droops,” its hindwings.  The details of “wing drooping,” and its associated behaviors, are the subject of this article.

The Wing Droop

This dragonfly season has been notable in a number of significant ways.  For one, our backyard pond in Mesa, Arizona has played host to two new species this year.  Starting in March, we began to see Red Saddlebags (Tramea onusta) and Red-tailed Pennants (Brachymesia furcata) at our pond.  These new species were in addition to our usual early-season residents, which include Mexican Amberwings (Perithemis intensa), Familiar Bluets (Enallagma civile), and Rambur’s Forktails (Ischnura ramburii).

The Red Saddlebags were of particular interest because they were actively laying eggs, giving many opportunities to observe the detach-and-reattach procedure that is typical of saddlebags.  I was able to obtain several slow-motion videos of their egg-laying behavior, some of which can be viewed on YouTube at the links given at the end of this article.  One video shows a nice example of the typical egg-laying process, while another shows a pair separating so the female can lay eggs, only for her to be intercepted by a second male trying to make off with her as a mate of his own.

I also obtained videos showing pairs of Red Saddlebags gliding serenely in tandem between egg-laying events.  It was in one of these videos that I first observed wing drooping – the second significant development of this dragonfly season.  Once I noticed the drooping, I found that it was happening in virtually all of my videos.  In fact, wing drooping had also occurred – though unnoticed at the time – in videos I took of Black Saddlebags (Tramae lacerata) years ago. 

It’s funny how a new behavior, once properly identified, turns out to have been present and visible all along.  As Sherlock Holmes said in The Hound of the Baskervilles, “The world is full of obvious things, which nobody by any chance ever observes.”  In fact, it turned out that wing drooping was also occurring in still photos I’ve taken of perched Red Saddlebags.  I’ll begin by describing wing drooping in a perched individual, because it’s easier to see the “droop” in that case.

In Figure 1, we see a male Red Saddlebags perched in its normal fashion, near the tip of a twig.  Notice that the plane of the hindwings is more or less parallel to the long axis of the abdomen.

 Figure 1  A male Red Saddlebags perched normally.

In Figure 2, we see the same individual doing a quick wing droop.  In this case, the hindwings are depressed downward below the abdomen, which shows off the saddle patches to good effect.  A moment later the hindwings were returned to their normal position.  The droop and return to normal position is usually completed in a fraction of a second, and can be hard to see in real time – unless you’re looking for it.

 Figure 2  The same male Red Saddlebags “drooping” its hindwings.

Drooping the hindwings like this seems to serve at least a couple different purposes for the dragonfly.  These uses are explored below.

Wing Drooping to Brake

As mentioned earlier, I first noticed wing drooping in a slow-motion video of a pair of Red Saddlebags gliding in tandem.  They were progressing smoothly, slowly gaining altitude in a slight headwind.  Then, suddenly, the male drooped his hindwings, effectively deploying his “air brakes.”  The pair immediately slowed almost to a stop, and descended 10 to 15 centimeters.  The male then returned his hindwings to their “upright and locked position,” and the pair continued flying at a lower altitude.

I decided to digitize the frames of the video over the time span of the wing drooping.  In all, I digitized 120 frames, shot at 240 frames per second, for a total real-time span of 0.5 s.  For each frame I recorded the horizontal and vertical positions of the dragonflies relative to a nearby fixed object.  Figure 3 shows the results, where for clarity I have plotted data points for every fifth frame.  Thus the elapsed time between successive points in Figure 3 is about 0.02 s.

 Figure 3  Position of a pair of Red Saddlebags during the process of wing drooping.

The onset of wing drooping and loss of altitude is clear in Figure 3.  The descent lasts for only about 0.2 s, and hence the whole process is over in the blink of an eye.  Once you know what to look for, however, it can be seen.  The flash of the “saddles” at the start of the descent is particularly easy to spot.  It would be much harder to study this behavior if the saddle patches were not present – perhaps other families of dragonflies are drooping their wings to air brake while in tandem also, but just haven’t been noticed yet because their wings are clear.

It’s not surprising that wing drooping can be used for braking.  This is quite evident from Figure 2, where the Red Saddlebags looks just like an airplane with its flaps lowered.  It turns out, however, that there is at least one other use for wing drooping.  We consider that use next.

Wing Drooping to Signal

After noticing the wing droop in the gliding videos, I looked more carefully at the videos showing the detach-and-reattach egg-laying process.  It turned out the male was wing drooping there as well.

In a typical egg-laying video, a pair hovers over a spot for several seconds – perhaps half a minute or more – before finally deciding to lay eggs at that location.  Careful inspection of the slow-motion videos shows that the male quickly droops his hindwings just before releasing the female.  She then detaches, and the male returns his hindwings to their normal position.  The female descends, dips her abdomen into the water, and rises as the male reattaches.

So far, in all the cases I’ve seen in my videos, the release of the female is immediately preceded by a brief wing droop.  Is the droop a visual signal to the female?  After all, with such large wing patches the droop is a distinctly visible action.  Does it indicate that the male intends to detach?  Alternatively, could the wing droop simply be a side effect of the action necessary for the male to release his grip on the female?  Further study should clarify the matter, but at first glance it appears the male is sending a signal to the female.

Using wing patches as a signaling device is nothing new, of course.  Such behavior is well known in species like the River Jewelwing (Calopteryx aequabilis), for example, where various types of fluttering wing movements are used by the male to show off its wing patches for the benefit of the female (Paulson, 2009).  In the case of the Red Saddlebags the display is very brief, and much harder to see and recognize – at least it’s harder to see with the human eye.

Conclusions

Analysis of slow-motion video shows that wing drooping is a stereotypical motion in which a dragonfly depresses its hindwings.  The drooping can have the effect of an air brake, slowing the dragonfly and decreasing its altitude.  It appears that the drooping can also serve as a signal between the male and female, especially in species with prominent wing patches, as in the case of saddlebags.  Perched individuals also exhibit wing drooping, and in such cases the possibility of a signal to nearby rivals must be entertained.

Acknowledgements

I would like to thank Betsy Walker and Dennis Paulson for helpful discussions and comments.

Literature Cited

Walker, J. S. 2013a.  Attaching in Tandem: The Role of “Wing Grabbing” and “Wing Pulling.”  Argia 25(4): 28-29.

Walker, J. S. 2013b.  Skydiving Dragonflies.  Argia 25(1): 20-21.

Walker, J. S. 2011.  Spin-Dry Dragonflies.  Argia 23(3): 29-31.

Walker, J. S. 2014.  Life at 1,000 RPM.  Argia 26(2): 11-13.

Walker, J. S. 2015.  Wing Whacking.  Argia 27(1): 28-29.

Paulson, D. R. 2009.  Dragonflies and Damselflies of the West, page 42.  Princeton University Press, Princeton, New Jersey.

Online Material

The original, unedited version of this article can be found at the following link:

http://thedragonflywhisperer.blogspot.com/2015/06/wing-drooping-in-red-saddlebags.html

Red Saddlebags gliding in tandem, and then putting on the air brakes:

https://www.youtube.com/watch?v=O8wygjeVYrc

Another air brake video:

https://www.youtube.com/watch?v=tNQpLQNKVrA

Normal egg laying in Red Saddlebags:

https://www.youtube.com/watch?v=CSvPZzgNCbU

A second male Red Saddlebags attempts to intercept a female:

https://www.youtube.com/watch?v=AS6sWAbzRCo

Splash-Dunking Gone Bad: The Sticking Frequency

(This post is a version of an article I published in Argia 24(1): 19-22, the official journal of the Dragonfly Society of the Americas.)

There are many aspects of the recently described splash-dunk/spin-dry behavior in dragonflies (Walker, 2011) that are of interest. In this paper I concentrate on what happens when splash-dunking goes awry and a dragonfly gets stuck in the water. To put this phenomenon in context, I start with a brief overview of some of the general features of splash-dunking and spin-drying.

The Frequency of Splash-Dunking

Splash-dunking is a fairly common event at Cranberry Lake in Anacortes, WA, where my wife Betsy and I do most of our observing. Though the rate of splash-dunking varies from day to day, as one might expect, on a typical day a splash-dunk event is observed every 5 to 10 minutes.

Figure 1 shows data recorded at Cranberry Lake during the 2011 dragonfly season. The upper set of data points shows the clear decrease in temperature during the season. The lower set of data points show the splash-dunk rate in dunks per hour. The average dunk rate is 6 dunks per hour, and the maximum rate is 12 dunks per hour. Though the temperature drops about 20 ˚F during the observation period, the average dunk rate is essentially unchanged.

Figure 1  Temperature (upper data) and dunk rate (lower data) versus date of observation. The straight lines show the trends in the data; namely, a clear decrease for the temperature and no significant change for the dunk rate.

The Dragon Splash

When people see one of my slow-motion videos of darners slamming into the water during a splash-dunk (Walker, YouTube), they invariably remark on the intensity of the splash, and wonder how the wings survive such an impact. The fact that the wings hit the water with some force is illustrated by the shape and size of the splash that is produced.

Figure 2 shows a head-on view of a splash produced by a splash-dunking darner. The darners flying when this picture was taken were primarily Paddle-tailed Darners (Aeshna palmata), though a few Shadow Darners (A. umbrosa) were seen as well. When viewed from this angle the shape and symmetry of the splash becomes apparent. The image shown here is a frame capture from a slow-motion video, and hence of low resolution. Still, it shows the key features of what I like to call the “dragon splash.” Notice the tri-lobed structure of the dragon splash, with a central component produced by the impact of the body, and symmetric side splashes from the wings impacting the water.

Figure 2  The tri-lobed “dragon splash” produced by a darner impacting the water.  Dragonflies typically splash-dunk 1 to 6 times in succession, each time producing an impressive splash.

Dunk Time

When darners perform a splash-dunk, they don’t dillydally in the water. They generally pop right back out in less than half a second. The number of dunks observed for a variety of time intervals is shown in Figure 3. The bar labeled “0.325” corresponds to times between 0.325 s and 0.349 s, the bar labeled “0.350” corresponds to times between 0.350 s and 0.374 s, and so on for the other bars. The average time it takes for a dragonfly to emerge from the water after a splash-dunk is 0.383 s.

Figure 3  Number of dunks versus time spent in the water.  The first bar is for times from 0.325 s to 0.349 s, the second bar for times between 0.350 s and 0.374 s, and so on.

Spin-Dry Parameters

After doing 1 to 6 splash-dunks, a dragonfly rises well above the water and does a spin-dry, which usually consists of 5 rotations and lasts about 0.44 s. Rotation rates have been observed as low as 760 rpm and as high as 1,600 rpm. The average rotation rate for our observations is 1,014 rpm.

There’s a good reason extended spins with many more than 5 rotations are not observed. A complex object in three dimensions – like the body of a dragonfly – has three independent axes of rotation, each of which has its own moment of inertia. Rotation about the axes with the maximum and minimum moments of inertia is stable, but rotation about the axis with the intermediate moment of inertia is not stable. In the case of a dragonfly, the axis of rotation through the wings – which is the axis of the spin-dry motion – is the one with the intermediate moment of inertia. As a result, the spin-dry motion is inherently unstable. In fact, dragonflies pulling out of their spin-dry are often observed to be “wobbling” as they complete their last spin, a sign that the instability is affecting their rotation.

When Splash-Dunking Goes Wrong

Life doesn’t always work out as planned. For splash-dunking dragonflies, this means that sometimes they don’t make it back out of the water. If they can’t become airborne again in half a second or less, they just aren’t going to make it at all. The result is generally death by drowning, though predation may play a small role as well.

Figure 4 shows a male Paddle-tailed Darner (A. palmata) that hit the water about 50 yards from shore and promptly became stuck. We refer to this as a “sticking event.” All sticking events we observed occurred on the first splash-dunk, all happened with air temperatures below 65 ˚F, and all were irreversible.

Figure 4  A male Paddle-tailed Darner (A. palmata) struggling to escape the water after a splash-dunk that didn’t go well.  At this point the darner is close to shore, after struggling for several minutes, and its wing beats are weak.  Just after getting stuck its struggles were much more vigorous, several times getting the dragonfly to the verge of escape.


It’s difficult to watch these gutsy animals struggling to free themselves from the water after getting stuck. They try so hard, come so close to escaping, and continue to struggle for such a long time. The individual shown in Figure 4 struggled for several minutes until – surprisingly – it “paddled” its way to shore right in front of me. I took the opportunity to rescue it and place it on a bush in the sun. After several minutes of drying out it took wing, apparently no worse for wear. I couldn’t help wondering if it would splash-dunk again.

We observed the first sticking event on September 19, 2011, after having observed 90 successful splash-dunks starting back on the 4th of July. As the season progressed, and the temperature dropped, the sticking frequency increased to higher and higher levels. At the end of the season, when the temperature had dropped into the upper 40s, the sticking frequency was a full 25% – one in four splash-dunks resulted in death. The close inverse correlation between temperature and sticking frequency is shown in Figure 5.

Figure 5  Temperature (upper data) and sticking frequency (lower data) as a function of the date of observation.  The inverse correlation between temperature and sticking frequency is evident.

The same kind of behavior was seen in the fall of 2010, before we started collecting data. I remember going to Cranberry Lake one day in late October 2010 when the temperature was below 50 ˚F. I would say as many as 10 darners were stuck in the water and trying to escape at any one time. It was depressing to see them struggling, knowing their efforts were futile.

In Figure 6 we plot sticking frequency as a function of temperature. Notice the nice fit to an exponential decay with increasing temperature. Another way to state this is that as temperature is decreased, the rate of increase in the sticking frequency is roughly proportional to the value of the sticking frequency. In this sense, the sharp rise in sticking frequency seen in Figure 5 is an indication of the dragonflies “hitting the wall” when it comes to their low-temperature flight capabilities.

Figure 6  Sticking frequency as a function of temperature.  The drop-off with increasing temperature is roughly exponential.

Minimum Flight Temperature

Our observations at Cranberry Lake show that dragonflies like the Paddle-tailed Darner (A. palmata) can fly at ambient temperatures as low as 44 ˚F. This is in sharp contrast to a minimum flight temperature of 57.2 ˚F reported for aeshnids (including A. palmata) in Alaska (Sformo and Doak, 2006). In any case, it’s clear that flight at such low temperatures is pushing the envelope when it comes to a dragonfly maintaining the elevated thoracic temperature necessary for the flight muscles to operate efficiently. Sformo and Doak report thoracic operating temperatures in A. palmata of about 97 ˚F.

It’s difficult enough for a dragonfly to maintain the necessary high thoracic temperature when the surrounding air temperature is below 50 ˚F, but the situation becomes much worse when the dragonfly splash-dunks into water. Even though the water temperature was the same as the air temperature at Cranberry Lake (within ±1 ˚F), water drains thermal energy away from a dragonfly at a much higher rate than air. Specifically, Newton’s law of cooling states that the rate of transfer of thermal energy is proportional to both the temperature difference and the thermal conductivity (Walker, 2010). Noting that the thermal conductivity of water is about 23 times greater than that of air, it’s clear that a dragonfly will lose thermal energy rapidly when it is in contact with cool water. Though a number of studies have addressed thermoregulation in dragonflies at high-temperature extremes (May, 1976; May, 1995), less attention has been paid to low-temperature performance, and so far none seem to consider cooling due to contact with water.

Finally, one might wonder whether the increase in sticking frequency with decreasing temperature could be caused by an increase in the surface tension of water, making it harder for the dragonfly to escape (Kuntz, 2012). While this may be a contributing factor, the surface tension increases uniformly by only about 2% over the same temperature range where the sticking frequency increases sharply by over 20%. It seems the most important factor determining the sticking frequency is maintaining the thorax at operating temperature.

Summary

The sticking frequency of splash-dunking dragonflies shows a strong inverse correlation to ambient temperature. In fact, dragonflies engaging in the splash-dunk/spin-dry behavior when temperatures are less than 50 ˚F are at significant risk of becoming stuck in the water – which is lethal.

Literature Cited

Sformo, T. and Doak, P. 2006, Thermal ecology of Interior Alaska dragonflies (Odonata: Anisoptera). Functional Ecology 20: 114-123.

Kuntz, R. 2012. Private communication.

May, M. L. 1976. Thermoregulation and adaptation in dragonflies (Odonata: Anisoptera). Ecological Monographs, 46(1): 1–32.

May, M. L. 1995. Dependence of flight behavior and heat production on air temperature in the green darner dragonfly Anax junis (Odonata: Aeshnidae). Experimental Biology 198: 2385-2392.

Walker, J. S. 2010. Physics. Addison-Wesley, 4th edition, pg 555-559.

Walker, J. S. 2011. Spin-Dry Dragonflies. Argia 23(3): 29-31.

Walker, J. S. 2011. Splash-Dunk Analysis, 2011. Argia 23(4): 29-30.

Walker, J. S. Slow-motion videos of splash-dunking and spin-drying can be found on the YouTube channel "thedragonflyguy".