Green Darner Dragonfly

2010 August 7

I found this large green dragonfly resting undamaged by the side of the road as I was pushing my bike up the hill on June 16. It wasn’t dead, but it was obviously dying of unspecified causes[1], as it let me pick it up with no more response than a slow motion of the legs and a slight vibration of the wings.

Look at the wingspan on that monster! The larger grid squares on the graph paper are 1 cm, so it has about an 11 cm wingspan. It looks like a female Green Darner, Anax junius. Green darners are, of course, green, and one of their distinguishing features is that they have a black spot on the forehead just in front of the eyes. The mature females have a reddish-brown to green abdomen, while the mature males have blue abdomens.

I was able to get some pictures of the head, eyes, and mandibles, and the last one shows the mandibles open slightly. So, we can see how the mandibles are basically shears useful for cutting open the exoskeleton of its prey so that it can get to the meat inside[2]

There are also some fairly aggressive hooks on the tips of the front legs, which probably help to hang on to the prey item.

The tip of the abdomen is also consistent with it being female, the pictures I’ve seen on BugGuide have that same structure with two petal-like appendages.

Dragonflies have a very deep thorax to hold their wing muscles, which is one of the things that makes them very powerful fliers.

The wings are pretty constrained in their motion – they can move up and down, but they can’t be folded over the back like most other insects do. Looking at the roots of the wings, I think that the wings have two attachment points for the muscles, which would make it possible to quickly produce significant warping of the wings[3] to give very fine flight control. Which is what gives dragonflies the ability to swoop, hover, and even fly backwards with such accuracy, allowing them to snatch most other insects right out of the air.

A couple of weeks ago, S_ first noticed an interesting dragonfly behavior by either this species, or a close relative. Our house has lighted doorbell buttons, and every evening when the sun goes down, a large dragonfly will hover a few inches in front of each button for a period of a minute or so at a time. At first we thought that the dragonflies were drawn by the light, but there were two problems with that: (1) They hover for a while, then dash off, and then come back less than a minute later. They don’t look like they are mesmerised by the light; and (2) There is exactly one dragonfly patrolling each doorbell. But, when we look closely, there is a small swarm of midges and tiny moths, each of which is too small to see easily, hovering around the doorbells. What we think is happening is that the doorbell lights are attracting the tiny insects, and the dragonflies have figured out that they can come and easily graze on them all night long. And the reason why there is one big dragonfly per doorbell, is that if any other smaller dragonflies try this trick the resident big one will either chase it off, or eat it.

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[1] It might have had an encounter with a car that caused internal injuries, but it certainly wasn’t struck directly. I suspect, though, that it was dying of some other cause (disease or parasites) and it was just coincidental that it was on the side of the road.

[2] Some years ago, when S_ and I went on a “Circle Tour” around Lake Superior, we found that the north shore of the lake is prime habitat for dragonflies. There are relatively few migratory insect-eating songbirds there, because they don’t like to migrate across the lake, so the dragonflies have the niche of daylight predator of airborne insects almost all to themselves. There were dragonflies flying around us constantly, evidently snatching up the various biting flies that were trying to drink our blood. On one occasion, a dragonfly grabbed a large horsefly that had been plaguing us, landed in front of us with it, and then neatly sheared off the horsefly’s head before flying off with the body. I suppose that a fly’s head doesn’t have much nutritional value, what with being mostly eyes.

[3] The amount of lift and direction of thrust you get from a flapping wing depends pretty strongly on the wing’s curvature, so by warping their wings the dragonflies can quickly make tiny adjustments to their trim and direction of motion. This makes it possible for them to not just hover, but to hover as if they were somehow nailed to a spot in the air, and then to immediately zoom off as soon as those huge eyes of theirs spot a prey item.

5 Responses
  1. August 7, 2010

    Beautiful shots yet again. I’m loving the new-and-improved camera and flash setup!

  2. August 10, 2010

    Thanks, Andy. One point I’ve been running into that I should mention for other people trying to photograph bugs: I’m finding that for the really big ones, where you have to back off quite a distance in order to fit the whole insect in the frame, the aperture needs to be opened up quite a bit (to around f/8 or so) to get enough light, even with the light concentrator cone. This is OK, because the depth-of-field is less of a problem as you get further away from the subject, I just have to remember to do it. In these pictures, some of them could have done with a wider aperture, they came out a bit dark.

  3. August 11, 2010

    I love dragonflies. I’m jealous of the prehistoric world where they had 3′ long ones. Wouldn’t that be totally cool? Dragonflies the size of a Great Dane zomming all over the place?

    I also like the fact that the wings are limited and yet they are so maneuverable. When evolution gets something right, it stops. Sort of the like the alligator.

    And awesome photos, too!

  4. August 30, 2010

    Great photos. I’m going to have to keep some graph paper around and start checking my doorbell lights!

  5. Della3 permalink
    August 31, 2010

    I’ve witnessed a number of dragonflies beheading various other flying insects on my late night windows in years past. It seems to be their preferred method of dispatching prey. You say you think the wings have only two attachment points for muscular control. As I look at the photos, I’m wondering if I see 2 or 3 more such points that are much weaker. Sort of like the human pinkie and ring fingers, they might be adding just a little bit of additional control. We think our 4th and 5th fingers are much less significant, but we’d have a harder time catching a ball without them, and a baseball pitcher would especially miss them. It would make more sense if the wings had more “fingers”, considering their maneuverability. Birds also have much more control over the shape of their wings in flight than would be guessed without the slow motion films we have today. And bats also have great, turn on a dime, movements. If I recall correctly, the “wings” of bats are imbued with 4 long, finger-like structures that give them great control. I also suspect the double-wing technology has something to do with the dragonfies’ hovering and backward movement abilities. Do we have any aeronautical engineers in the audience?

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