Dragonfly sighting!

I was planning on discussing the differences between adult dragonflies and damselflies in this post, but I can’t reist talking about a dragonfly I saw on Saturday instead.  This lovely little dragonfly is Pachydiplax longipennis, also known as the blue dasher:

Pachydiplax longipennis

Blue dasher (Pachydiplax longipennis)

Isn’t he gorgeous?!  Pachydiplax longipennis is a very common dragonfly in the Tucson area, medium sized (a couple of inches long) and usually found perching on vegetation near ponds.  As you can see, the males have a sort of whitish waxy coating over blue bodies (they are pruninose) and they have bright green eyes.  The extent of the pruinosity varies a bit from place to place, but here in Tucson it covers most of their bodies.

This particular male was perching on the bush next to my fiancee’s car (more about why he was near the car in a moment).  Dragonflies (not damselflies – see my next post to learn how to tell the two apart!) are generally split into two groups based on their behaviors.  Perchers, like Pachydiplax longipennis, sit on vegetation near water.  From their perches, they are able to protect their territories from other males, see females that come into their territories, and spot insects and other animals they might want to eat.  When disturbed for any of these reasons, they’ll dart off their perch and fight, mate, or grab their prey, but they spend a majority of their time sitting on their perch watching and waiting for things to happen.

The other group of dragonflies is made up of the fliers.  These dragonflies fly almost constantly, patrolling their territories for mates, aggressors, or prey on the wing.  They rarely stop moving unless they are mating or wrestling with a particularly large piece of food.  You would be hard pressed to get a photo like the one above if this dragonfly were a flier instead of a percher because they rarely sit still that long.

Back to why this dragonfly was near the car.  Dragonflies have amazing vision which they use to find territories, mates, and food.  See those giant eyes that take up almost the entire head of the dasher pictured above?  These are clearly very visual animals.  Dragonfly nymphs are aquatic, so their eggs need to be laid either in or near water.  So, dragonflies need to be able to find the water.  They do this by looking for certain patterns of polarized light as they fly overhead.  If a male finds the right pattern of light, the one that screams “Water!” at him, he’ll set up a territory and protect his patch of the water from other males in the area.  If he’s a percher, he’ll find a nice place to rest nearby and survey his territory from his perch.  Why, then, was this dragonfly near a car?  The clear coating on many cars gives off the same pattern of light as water in ponds.  As far as this dragonfly is concerned, the car is a pond!  He thinks he must protect it from other dragonflies that might be in the area, so he was perched in the bush nearby and watching the area around the car.

This dragonfly exhibited one other behavior while I watched.  This picture is unfortunately not as clear as it could be because he was having a hard time staying still, but you’ll get the idea:

Pachydiplax longipennis obelisk

Blue dasher (Pachydiplax longipennis), obelisk position

This dragonfly is in what is called the obelisk position, sitting on the antenna of my fiancee’s car.  Have any idea why they might rest in this position?  I’ll give you a hint: this photo was taken around noon on a hot and sunny day.  Dragonflies actually use this position as a way to help cool their bodies down when they are overheated.  Dragonflies, like all insects, are cold blooded (=exothermic) and their body temperature closely tracks the temperature of the air.  When it is very hot or very sunny, their body temperatures get too high and they must use a variety of behaviors to cool down.  Dragonflies are pretty unique in using the obelisk position.  Here’s how it works.  Imagine you are out under a very hot sun.  You have two choices of positions: standing upright or lying flat on your back.  Which position should you use to minimize the amount of sun that hits your body?  If you chose standing upright, you’re correct!  By standing upright, the summer noon sun will only hit your head and shoulders, leaving the rest of your body shaded.  If you laid on the ground, half of your body would be hit by the sun!  Dragonflies use the oblesik position in exactly the same way.  By extending their abdomens upwards, instead of holding it horizontally as they usually do (shown in the first photo), they minimize the amount of sun hitting their bodies.  The sun only hits the tip of the abdomen, part of the thorax and head, and part of the wings rather than the entire top surface of the dragonfly.  So, this dragonfly is in this position because he is hot and needs to cool down.  If the obelisk position isn’t enough, he’ll fly into a shaded area, even if it means abandoning his territory for a while.  Defending a territory isn’t worth risking death by overheating.

This dragonfly flew away shortly after I took this photo.  I disturbed him enough that he gave up on defending his “pond” and flew away to look for a place that had fewer dangerous large mammals.  I hope he found a backyard pond instead of another car!  I always find the car thing a little depressing.

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Text and images copyright © 2009 DragonflyWoman.wordpress.com

 

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