Tuesday, September 27, 2011

ID Challenge #3 Answer

Click image for a larger version.
All you have to work with in ID Challenge #3 is a pair of wings, but they are patterned very distinctively. A scan through any of a number of field guides to North American dragonflies (of continental or regional scope) would narrow down your choices to two, or maybe three species. That’s the easy part. The challenge is figuring out which one of those is the owner of our pair of wings.

So let’s start at the top: is it a dragonfly or a damselfly? The hind wing is noticeably more broad than the fore wing—particularly at the base, so it’s a dragonfly. You also won’t find any damselfly with a wing pattern like that anywhere in North America (or anywhere in the world as far as I’m aware, but I could be wrong about that).

Now let’s figure out the family. In ID Challenge #1 Answer I talked about using the anal loop in the hind wing to figure out that mystery dragonfly’s family. In this case too, it is more-or-less foot shaped with something that you could call a “heel” and something that you could call a “toe” (see the close-up at right). This puts it in the skimmer family—Libellulidae. [...in the limited sense. This is one of those places where you can get caught in the crossfire of dueling authorities—it depends on who you read. According to some, Libellulidae includes the subfamilies Macromiinae, Corduliinae, and Libellulinae, in which case the foot-shaped anal loop would indicate the subfamily Libellulinae; according to others Libellulidae has no subfamilies, with Macromiidae and Corduliidae being families on their own, and this is the taxonomy that I go with until I see something resembling consensus.]

One of the possible contenders that you might have picked out in a North American field guide is the Prince Baskettail (Epitheca princeps) which has a superficially similar wing pattern, although it can be ruled out on the basis of my statement that our subject occurs in the Pacific Northwest. The Prince Baskettail doesn’t occur west of the Rockies, but let’s consider it anyway just to make sure. It’s in the family Corduliidae [or subfamily Corduliinae] and like other members of that family, its anal loop is more club-shaped with an irregular expansion that isn’t clearly foot-shaped. Something else that rules out the Prince Baskettail is the shape of the dark spot at the base of the hind wing: bar-shaped on our subject; distinctly triangular on the baskettail.

That leaves two choices in the skimmer family, Libellulidae [or subfamily Libellulinae], with wings like ours: Twelve-spotted Skimmer (Libellula pulchella) and female Common Whitetail (Plathemis lydia)—the male of the latter species has a very different wing pattern. [Another taxonomic sidenote is that the Whitetails are sometimes placed in the genus Libellula, but these days most authors place them in Plathemis.] These are both widespread species across most of the continental US and southernmost Canada. You can view distribution maps for Libellula pulchella and Plathemis lydia at OdonataCentral.org.

There are some pretty consistent differences between the Twelve-spotted Skimmer and the female Common Whitetail in the shape and extent of the dark spots on the wings, but it’s helpful to know a couple of venational landmarks to see exactly how they differ. Take a look at the wings on the right (here I’m using the wings of the Western Pondhawk, Erythemis collocata, since they are completely clear and all of the veins are visible): the shaded red spaces are the “triangles”, and the blue dots mark a major junction known as the “middle fork” in each wing. Notice how the middle fork is at the inner corner of a roughly triangular space which is highlighted in green. These are venational features that all dragonflies have in both the fore and hind wings and are easily recognizable once you’re familiar with them. The foot-shaped anal loop in the hind wing should stick out like a sore thumb by now, so you know which family contains Erythemis...

So, take another look at our subject. Let’s start with the dark bar at the base of the fore wing—it extends across the base of the fore wing triangle, but not any further, and the bar does not extend into the triangle itself. The basal bar in the hind wing similarly doesn’t extend much past the triangle (which is hidden within the dark bar). Both basal bars have neat, clean edges along their posterior or trailing edges (the side that is “down” on your screen). Next we’ll move on to the middle spot in each wing which have a very jagged inner edge where they extend inward to the middle fork and filling the triangular space which is highlighted in green above. These features point us clearly to the female Common Whitetail, Plathemis lydia.

On a Twelve-spotted Skimmer (Libellula pulchella), the basal bars extend noticeably beyond the triangles and “ooze” toward the trailing edge along a pretty sloppy edge—especially into the fore wing triangle, and the middle spot does not extend inward as far toward the middle fork, leaving much of that adjacent triangular space clear. So, Twelve-spotted Skimmer has more extensive basal bars and more limited middle spots, the opposite of what you see on female Common Whitetails. Another more subtle difference is that Twelve-spotted Skimmer wings are a tiny bit narrower relative to their length, and Common Whitetail wings are a little wider relative to their length which makes them look stockier.

Here are full-width images of a female Twelve-spotted Skimmer followed by our subject again which we now know to be a female Common Whitetail for comparison. Pick out the triangles and middle forks in each example even if they are obscured by dark spots.
Twelve-spotted Skimmer (Libellula pulchella)

Common Whitetail (Plathemis lydia), female
Of course, you have to watch out for variation which can be especially significant when you’re dealing with the extent of pigments. If you search hard enough I’m sure you can find one or two individuals of either species that don’t fit the mold, and that’s why it’s important to use multiple characteristics when identifying similar species. The differences that I discussed above should make for pretty good rules of thumb, however. More than likely you’ll have the rest of the bug to examine anyway, but I thought this would be a fun and interesting exercise.

Most of those who commented on ID Challenge #3 got it right, so congratulations to them! Something like the wings of female Common Whitetail may be something that you “know”, but you may not be exactly sure how you know what you know. Whether you knew what they were or not, I hope that this explanation was helpful. Until next time...


  1. Awesome diagrams showing the landmarks in the wing. Those don't stick in my head - I have to go relearn them over and over in the manuals, but if I had diagrams like the ones you made, it would at least make looking them up a lot quicker and more pleasant.