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.
|Close-up of the left forewing tip with the cell |
adjacent to the posterior edge of the pterostigma
colored in green.
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.|
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 . . .