We have seriously entered dragonfly season north of the ridge, and the last two days have produced two of my favorites, the Pennants. There are four species of Pennants recorded from Pennsylvania. Of these, two are found in our mountains: the Halloween Pennant (Celithemis eponina) and the Calico Pennant (Celithemis elisa). 1
These are striking, elegant insects that appear every year about the end of June. They are well known for their habit of perching at the top of a weed or piece of grass and fluttering in the wind, rotating like a windsock as the wind changes. I suspect that is why they are called “pennants.”
I once came across Pennants in an article about the temperatures at which various dragonflies fly. I looked on the internet, but I couldn’t find the whole article. I did find the abstract, which I here quote in part:
“Thoracic temperatures … of “flier” and “percher” dragonflies were monitored at the onset of flight….Spontaneous takeoff by perchers in shade was dependent on [ambient air temperature] and usually occurred at approximately 7 C above the minimum [thoracic temperature] required for flight. Small perchers (100-200 mg) remained perched until [thoracic temperatures] = 16 C [60.8 F] or more, while larger perchers (300-400 mg) did not fly until [thoracic temperature] increased to at least 19 C [66.2]. Perchers flew earlier in the day and had higher [thoracic temperatures] at takeoff when in sun versus shade. …The data also indicate that these perchers from a temperate climate are able to fly at [thoracic temperatures] lower than those reported for perchers from warmer climates.” 2
I sorta figure that a Pennant is a percher. 3 However, I had no idea how much one weighed—until I found a nice chart about wing-loading in dragonflies; C. eponina weighs in at about 130 mg. 4
So, Pennants should fly early in the day—I have noticed that—and they should begin to fly when the temperature reaches, or exceeds, about 60 degrees—I never noticed that, but I don’t carry a thermometer into the field.
Now the internet had me by the throat—what else might I learn from Dr. Beckmeyer? Well, quite a bit, it turns out, and if I understood aerodynamics, I believe I could learn exponentially more. 5
One thing will suffice, a little insight as to the difference between fluttery dragonflies—like Pennants—and powerful dragonflies that fly constantly—like Darners. It took me some long, hard head-scratching, but I think I finally sorted out Dr. Beckmeyer’s Figure 2: Speed Required to Fly at Different Wing Loadings and Lift Coefficients.6 From Figure 1, we see that a Halloween Pennant has a wing load of about 1.4 N/m2.7 Now, if I follow Dr. Beckmeyer’s logic, using Figure 2, at 1.4 N/m2, it appears to me that the wings of a Pennant would be about three and a half times more efficient than a Green Darners in producing lift. If I’m right (a very large if), windspeeds a darner would scarcely notice could blow a pennant around like dandelion fluff. The implications of that for when the Pennants hunt, where they hunt, what they hunt, and what hunts them are almost endless.
If I’m wrong, well, that’s nothing new. I hope someone will tell me.
- According to Odonata Central
- Vogt, Daniel F. and Bernd Heinrich. “Thoracic Temperature Variations in the onset of Flight in Dragonflies (Odonata: Anisoptera),” Physiological Zoology, Apr 1983. v. 56 (2).
- Sadly, because I could not access anything beyond the Abstract, I am not certain in which category—percher or flier—the Pennants were placed by the authors, nor do I remember. Pennants certainly perch more than they fly.
- Beckmeyer, Roy J. “Aerobatic Anisoptera and Zooming Zygoptera: Dragonfly flight from A to Z,” WindsofKansas.com, see Figure 1: Wing Loading versus Weight for various wing areas. http://www.windsofkansas.com/figure1R2z.JPG [Accessed 30 June 2012].
- Sorry Dr. Costa Campos; you tried.
- Ibid. http://www.windsofkansas.com/figure2z.jpg .
- That’s Newtons per square meter—I have no idea what that means.