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
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.
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.
I would like to thank Betsy Walker and Dennis Paulson for helpful discussions and comments.
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.
The original, unedited version of this article can be found at the following link:
Red Saddlebags gliding in tandem, and then putting on the air brakes:
Another air brake video:
Normal egg laying in Red Saddlebags:
A second male Red Saddlebags attempts to intercept a female: