Friday, April 22, 2011

A One Species Day, 22 April 2011

With a weather forecast of sunny skies and a high in the lower 60s, I thought I’d get out to a couple of wetlands in the area of Battle Ground, Washington, that I’ve never visited before and see what they were like. I expected to see Pacific Forktail (Ischnura cervula), and I hoped to scrounge up something else—but didn’t. With this being a late spring, I can imagine all sorts of odonate nymphs just itching to pop out. They need more time obviously.

Here are a couple of Pacific Forktail images from today. None were completely mature, but a couple of the males were close—only the eye color wasn’t quite there yet. Note that the eyes are brown and gray instead of black and green.

One interesting male had the smallest dorsal thoracic spots that I ever recall seeing. They were so small that I first thought the top of the thorax was entirely black. Here’s a close-up to compare with the male above:

Technically it was a two species day since I flipped over a piece of lumber that was floating in a pond and I found this little darner (Aeshnidae) nymph. The little guy was only about a half-inch long, so it has a lot of growing to do before it emerges.

You can see how short the wing pads are (on top of the thorax just ahead of the base of the abdomen)—when it’s close to emerging the length of those will be at least the width of the head. It’s probably an Aeshna, the common species in the area being Paddle-tailed Darner (A. palmata) and Shadow Darner (A. umbrosa). I don’t expect to see adults of those species until July at the earliest.

It’s supposed to be warmer tomorrow (70°F!) and maybe I can get out again and find something else flying.


  1. The nymph in my pond that I accidentally pull up all have wing pads ~50% of abdomen length - I think they are just waiting until warmer weather. But a friend's pond had Cardinal Meadowhawks emerging on a coolish day, and having to overnite emerged, with wings spread, or still held over their backs, and it rained 1/2" that nite. Amazingly, they were able to complete their emergence and fly off the next day. Anything you can post to teach us about this stuff? I thought their wings would have been ruined....

  2. @Kathy Biggs I can't shed a lot of light on that—I'm trying to learn more about collecting and identifying nymphs which I haven't done a whole lot of so far. I think there is a lot to learn about how nymphs know when to emerge. I think they can control the timing to some degree when the conditions are not favorable, but I'd think at some point they just can't put it off any longer. It's an interesting subject.

  3. What triggers nymphs to emerge? Is it day length? Is it water temperature? Is it something else?

  4. @Ann Hi Ann. I just realized that I never responded to your questions. In most cases, I'd guess that it's a combination of factors like day length and temperature, like you suggest. Probably one factor or another is dominant in some species. For example, some species that have relatively short seasons as flying adults may be triggered more by day length. Some species have very long flight seasons (with emergence occurring throughout much of it) and maybe temperature is the dominant factor. It's an interesting subject that surely could use some more research.