Common Whitetail Dragonflies, Male and Female

2011 October 22

I caught these on July 22, 2011[1]. There were a number of them hanging out on the west (sunny) side of our barn in the afternoon, and it was pretty easy to sneak up on them with an insect net and just clap it over them as they sat there. At first I thought that they were two different species, one with a white abdomen and big dark spot on each wing, and a smaller dark spot at the base of the wings:

and the other with a dark abdomen with small yellow spots on the sides, and two smaller dark spots on each wing along with the dark wing base:

But then I looked them up, and realized that what I had on my hand was just one species, the Common Whitetail, Plathemis lydia. They are obviously strongly sexually dimorphic. The males don’t develop the chalky blue-white abdomen until they are sexually mature, and they evidently flash their abdomens at each other when they fight over females. I suspect that what was going on at the side of our barn is that the females were hanging out and watching while the males duked it out in the air, with the males landing occasionally to rest between combats.

One place the sexual dimorphism didn’t extend was the head. Unlike a lot of other kinds of insects where the males have bigger eyes than the females, for these dragonflies both the males and females had similarly huge eyes that covered nearly the entire head.

In fact, looking at the back of the male’s head here, it looks like the eyes continue out past the rest of the head, making it kind of hollow in there. Depending on how one looks at it, you could say that his eyes are bigger than his head.

Common whitetails are relatively early-season dragonflies, being most common in early summer. And sometimes they are extremely common. One year in June, Sandy and I drove around Lake Superior, and on the northern shore (in Canada), these dragonflies were all over the place. At the same time, we noticed that there were practically no small birds. I think what happens is that smaller migratory birds don’t bother to fly across the lake, and the lake is too long east-to-west for them to go around. As a result, birds like swallows and flycatchers that are normally the apex predators for flying insects simply aren’t present on the north shore. So, the dragonflies have taken over the “top aerial predator of flying insects” niche without any competition from (or predation by) the birds, and have become extremely numerous. I think that if one wants to know what the airborne insect life was like back in the Carboniferous period, with the giant dragonflies and no birds, the north shore of Lake Superior is probably pretty close.

[1] I wasn’t the person who first spotted them. We were hosting a picnic in our yard for a large group of friends that day, and Mary Lynn (who has a particular interest in dragonflies) was the first to notice them. She’s a professional artist and, among her many, many, many interests she likes to use insects as artistic subjects. So she asked if I had anything we could use to catch one for her, which is when the insect nets came out. And I figured that while I was about it, I might as well catch a couple to photograph myself.

7 Responses
  1. October 23, 2011

    Nice macro work on the eyes!

  2. October 24, 2011

    Thanks, Andy.

  3. October 24, 2011

    Great post. Awesome photos. A link is on the way.

    I’ve always loved dragonflies. I’m jealous of prehistoric times when dragonflies were huge. Of course, I wouldn’t want to have been running around out in the open with them flying overhead, but I’d love to have seen them in action.

    Do you suppose their diet would have been different as their enormous size would have allowed them a more sophisticated digestive system?

  4. Shannon C. permalink
    October 25, 2011

    For all that I think I know about insects, I really had no idea that there used to be giant dragonflies. A quick Google search revealed this interesting article (not spam, I swear!):

  5. October 26, 2011

    Shannon: While I’ve known about the giant Carboniferous dragonflies since I was a kid, I have to admit that the first I heard about them was from watching “Monster on the Campus” on “Shock Theater”. In the movie, the first inkling they got that the irradiated coelacanth blood would cause animals to “regress to an early evolutionary form” was when a dragonfly got a bit of it and turned into a monster with a 3-foot wingspan. I can’t remember whether this was before or after the main character accidentally got some of the blood in his pipe and turned into a murderous subhuman.

  6. Della3 permalink
    November 7, 2011

    Giant dragonflies? Wow! That would have been amazing to see. If, according to the article Shannon refers us to, the insects were larger because of the higher oxygen content in the prehistoric air, then there must have been plenty of large mosquitos and flies for them to eat. I wonder if they kicked up a duststorm when taking off and if they sounded like helicopters when they were flying around. The cacophony of sound from all the large creatures that existed back then must have been very loud.

  7. Clare permalink
    July 18, 2014

    Thanks for posting your ideas about the north shore of Lake Superior. I’m headed there in a few days and am excited to see all the dragonflies you mention! I think I’ll need to buy a field guide. Your idea about the “top ariel predator” niche makes a lot of sense to me.

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