Coping with the heat, or not?

Insects need heat to be active, some more than others. But they can also overheat. One way that dragonflies (odes) may cope with excessive heat is to obelisk. What?!

A Blue Dasher obelisking.

If you know that the Washington Monument is an obelisk, then you may be able to understand why this behavior (above) is called obelisking. The dragonfly sticks its abdomen straight up into the air. Why does it do this? Well, the thought is that the dragonfly points its abdomen at the sun exposing the least amount of surface area of its abdomen to the sun’s hot rays.

The photo above was taken at just about high noon with the sun directly overhead. As you can see, there is very little surface area of the abdomen exposed to solar radiation. The odd thing though, is that not all species obelisk. Of course, not all dragonflies perch in the open sunlight and some are on the wing much of the time, so it may not be necessary for those species to thermoregulate in this manner. Blue Dashers perch often, and in direct sunlight.

Is the color of the dragonfly a player in whether or not the ode obelisks? I don’t recall seeing a Common Whitetail obelisking. Whitetails perch flat out on the pavement. They have white (or very light blue) abdomens which would seem to reflect the sun’s rays, no need to obelisk.

I don’t recall seeing Slaty Skimmers obelisking either. They have very dark abdomens which would absorb more heat than the abdomens of both Whitetails and Blue Dashers. Slaty Skimmers seem likely candidates to partake in obelisking, but I don’t see them engaging in that behavior. So, what’s the deal?

Common Whitetail (left) and Slaty Skimmer.

It’s probably not the color.

Same individual as above with abdomen not completely elevated (it just landed).

You can see in the above photo that the sun is shining on the abdomen of the dasher which has not yet completely raised its abdomen. If the ode stayed in this position very long it may overheat.

But is that really what this dragonfly is doing, trying to stay cool? It’s been suggested that the raising of the abdomen may also be a way of displaying to other males entering their territory. And, some species habitually perch with their abdomens raised, whether pointing towards the sun or away from it.

swift setwing
A Swift Setwing in typical posture (photo taken at Occoneechee Mountain State Natural Area, Hillsborough, NC).

It’s probably a combination of many things that cause a dragonfly to raise its abdomen skyward while perched, thermoregulation being one reason, especially in dragonflies that perch out in the open sun. Most of the species that have been observed obelisking are odes that regularly perch in direct sunlight. And, it has been shown that Blue Dashers, captive Blue Dashers, will raise their abdomens when exposed to a 250 watt lamp in the lab. So it seems that the posture does perform as a thermoregulatory tool, at least to some degree in some species.

How do you thermoregulate, drink a cool beverage? stay inside and read a book? or visit…

…Into the Mist!

Stay cool, daddy-o!

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