Tuesday, April 19, 2011

ID Challenge #1 Answer

You can see the original challenge with full-size image here. Let’s take a top-down approach to figuring out this thing. To me this means first determining whether it’s a dragonfly (suborder Anisoptera) or a damselfly (suborder Zygoptera), then determining the family, and so on.


The identification of this odonate comes down to wing venation (obviously!), but first let’s look at the overall shape of the wings. Is either the forewing or the hindwing noticeably wider than the other? Yes, the hindwing is distinctly broader than the forewing, particularly near the base. This means that we are looking at a dragonfly, not a damselfly. The fore- and hindwings of damselflies are relatively narrow and virtually identical in shape to each other. The fact that the wings are held almost straight out to the sides also supports our conclusion that this is a dragonfly rather than a damselfly. Most damselflies perch with the wings closed, and those which tend to keep their wings open don’t typically hold them all the way out to the sides like this (not in North America, anyway).

Close-up of the wing bases with the anal loops
colored in green.
Next, let’s look at a feature of the hindwing venation known as the anal loop. It is partially obscured on both hindwings, but we can see more than enough to help us out. On this individual, the anal loop is clearly foot-shaped with a “heel” at the bend and ending at a “toe” near the hind margin of the wing. This foot-shaped anal loop points us directly to the family Libellulidae, a.k.a. skimmers. Dragonflies in other families have anal loops which are more semicircular or elongated (but not distinctly foot-shaped), or they may have no obvious anal loop at all. Conversely, there are some libellulids which don’t have the classic foot-shaped anal loop, but these are not typical of the family. You can assume that any dragonfly with an obviously foot-shaped anal loop is in the family Libellulidae and that’s what we have. [Geek speak: This assumes the taxonomy which has Macromiidae and Corduliidae at the family level separate from Libellulidae. Some authors place them within Libellulidae as subfamilies, Macromiinae and Corduliinae, alongside Libellulinae. In the latter taxonomy, the foot-shaped anal loop would indicate the subfamily Libellulinae.]

Close-up of the left forewing tip with the cell
adjacent to the posterior edge of the pterostigma
colored in green.
So where do we go from here? The pterostigma (the large darkened cell near the tip of each wing) is an obvious feature, but it’s pretty average looking (not very short, nor very long relative to its width), so that doesn’t really help a lot. It does rule out certain genera like the whitefaces (Leucorrhinia) which have relatively short pterostigmas (length to width ratio of about 2:1), but this doesn’t narrow down our choices very much.

Now look at the row of cells adjacent to the posterior edge of the pterostigma. This is unusual. Nearly all dragonflies have two or more crossveins adjacent to the pterostigma in the row right behind it, but not this one. There is only one crossvein adjacent to the pterostigma which is near the outer end, and there isn’t another crossvein for a little ways toward the inner part of the wing. That produces quite a long cell—about 50% longer or more than the pterostigma. This feature alone points us to the genus Pachydiplax which is handy because it contains only one species—P. longipennis or the Blue Dasher, a widespread species over much of the US and southernmost Canada.

So let’s lift the veil and take a look at the original unaltered image . . .


Male Blue Dasher (Pachydiplax longipennis) at Ice House Lake, Washington.
Now we can see that the abdomen is almost totally covered with pale blue pruinescence, there are short dark streaks enveloped in amber patches at the wing bases, and the eyes are bluish-green—all features of the Blue Dasher. Some individuals also have the diffuse amber patches on the outer half of the wing like this one does (most apparent on the left forewing). The identification of this guy (and it is a male) would have been a lot easier with the whole picture, but it becomes easy to forget about the little details which make identification straightforward in less than optimum circumstances.

Because of its continental range, its local abundance, and its approachability, the Blue Dasher is one of the most photographed odonates among nature photographers. I’ve seen a lot of photos of this species that were backlit (for the aesthetic touch), but the photographer didn’t know what species it was. In those conditions the wing venation is often very easy to discern. More than once I have found severed Blue Dasher wings (presumably left behind by a predator) which were quite easy to identify because of the features mentioned above. 

I hope you found this challenge fun and informative. Above all else, this was really a demonstration of what can be done with venation alone. Learning odonate venation isn’t very easy—especially remembering the names (and there’s more than one naming convention too!), but I think it is interesting and rewarding. I recommend that you start with the more obvious features (like pterostigma, nodus, triangles, anal loop) and branch out (pun intended) from there. Any good field guide or manual to the odonates will have illustrations of the wings that identify the important veins, cell groups, and other features.

I consider this a test run of additional ID challenges that I plan to post on a regular basis. I will vary the difficulty level—some easier, some tougher—but it can be difficult to judge how challenging something like this will be for others (most of whom I’ve never even met). Using the submitted comments as a gauge, I’d have to say that this was a difficult challenge. I heard from four people, all of whom were correct, but I think that’s low compared to the total number of page views. I would like to engage more readers with these challenges and I hope I am more successful next time. Until then . . .

9 comments:

  1. Hi Jim,

    I would have been the fifth commenter/guesser except my browser froze mid-comment and I never got back to it. I would have been wrong, though - I thought that back wing was broad and started running through Gliders and Saddlebags, but wasn't finding any matches...

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  2. I was short on time and couldn't find my Aquatic Insects of California which I had planned to use as my informant! ;-)
    It's great to have you teaching us Jim!

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  3. This was a great quiz! Although I am not familiar with NW Odonata, I find this solution to the answer very useful. Thanks a lot!

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  4. @Chris Hill Hey Chris, don't let a frozen browser stop you next time!

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  5. @KATHY BIGGS Kathy, that was one of my first odonate references which was pretty useful in Oregon too. I have fond memories of using that one!

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  6. @Ani Thanks for the comment--I'm glad you enjoyed the post!

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  7. I really liked this post and how you color-coded what to look at. Considering my blog is essentially ID challenges all the time for me, I doubt I'll participate. I will look forward to reading your answers.

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  8. @Nature ID (Katie) I can understand that, Katie, but one of these times there will be something that you know and you'll just have to participate.

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  9. This was a GREAT ID challenge, and I especially appreciate the great detail with which you explained the identification process. How about some of those sneaky little blue damselflies in the future?

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