One strategy odonates employ to avoid excessive heat is to simply get out of the sun. On extremely hot days, you may find congregations of odonates of multiple species enjoying the shady side of a tree. Typically it has to be really hot for odonates to resort to that behavior, however. In general, odonates (males in particular) don’t want to miss out on reproductive opportunities at their favorite haunts, and time spent avoiding the sun is time spent not reproducing.
|Obelisk of Senusret I in |
Heliopolis [Public domain],
via Wikimedia Commons
How does obelisking help odonates prevent overheating? By aiming their abdomen toward the sun, they reduce the amount of body surface area that receives direct sunlight. Consider the obelisk at right. When the sun is directly overhead, the surfaces that are receiving the highest density of sun rays are the facets at the top (a relatively small percentage of the total surface area); the sides are nearly parallel to the sun rays so they are receiving a much lower density of solar energy per unit of surface. When the sun is low in the sky, it is the long, vertical surface (a relatively large percentage of the total surface area) that is nearly perpendicular to the solar rays, and it receives the greatest density of solar energy.
|Male White-faced Meadowhawk (Sympetrum obtrusum), Jackson County, Oregon.|
|Female Band-winged Dragonlet (Erythrodiplax umbrata), Los Ríos Province, Ecuador.|
Another way to think about it is in terms of the size of the shadow cast by the obelisk. When the sun is high in the sky, the obelisk’s shadow is pretty small. This means that the obelisk is absorbing a relatively small slice of the solar energy that would otherwise hit the ground if the obelisk wasn’t there. When the sun is low in the sky, the obelisk’s shadow is very long and, at this time, the obelisk is absorbing a larger percentage of the sun’s energy. The more solar energy something absorbs, the more heat that is produced.
So, for a odonate on a hot, sunny day, obelisking is all about making its profile (from the sun’s point of view), and therefore its shadow, as small as it can in order to minimize the amount of solar radiation that is absorbed by its body. This allows them to regulate their temperature without having to abandon the action at their local pond or stream (as long as it isn’t so hot that they have to seek shade instead).
|Male Gray Sanddragon (Progomphus borealis), Graham County, Arizona.|
|Male White-belted Ringtail (Erpetogomphus compositus), Wheeler County, Oregon.|
Not all odonates employ this technique for staying cool. Among dragonflies you see it most regularly in the clubtails (Gomphidae) and skimmers (Libellulidae), and within those families particular species are frequent practitioners while others don’t seem to do it at all. The Blue Dasher (Pachydiplax longipennis)—like the one in this blog’s title banner, is one of those frequent practitioners. The tropical Micrathyria also frequently obelisk, and three species are shown here.
|Male Spot-tailed Dasher (Micrathyria aequalis), El Oro Province, Ecuador.|
|Male Micrathyria ocellata, El Oro Province, Ecuador.|
|Male Micrathyria pseudeximia, Manabí Province, Ecuador.|
I’ve read that the impressive Dragonhunter (Hagenius brevistylus)—a huge clubtail of eastern North America, has its own variation which is to point its abdomen down, away from the sun, which has essentially the same effect. This is a big, beefy species and I imagine that it takes a lot of energy to hold that abdomen up for any length of time! Perhaps other odonates which typically perch by “hanging” (with the abdomen down) obelisk in this manner too, but it isn’t nearly as obvious as the sunward style.
Some damselflies obelisk as well, although apparently not as often as some of the dragonflies, at least in my experience. In particular the broad-winged damsels (Calopterygidae) and some other groups like the dancers (Argia) are known to use this method of thermoregulation on occasion.
|Female Beaverpond Clubtail (Gomphus borealis), Somerset County, Maine.|
|Female Paiute Dancer (Argia alberta), Harney County, Oregon.|
But wait, there’s more! Something I realized only recently while reading Philip Corbet’s monumental Dragonflies: Behavior and Ecology of Odonata (1999)—it’s really impossible to open up this book and not learn something new—is that obelisking isn’t strictly about keeping cool. When the sun is low and the air temperature is relatively cool, an odonate may obelisk in order to warm up. This time the abdomen is not pointed toward the sun, but is instead held up with the long axis perpendicular to the solar rays in order to maximize its exposure. I’m not certain that I have observed obelisking in this context, but I’ll be watching for it.