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
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:
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:
