|Click image for a larger version.|
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).
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.
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|
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...