With spring knocking at my door (well, maybe pulling up the driveway if not quite yet knocking at my door), I thought I’d do a little summary of the Pacific Northwest’s forktails (Ischnura). Most of our species appear relatively early in the year, so this is a good time to review them and make sure you’re familiar with the species around here.
I’m going to focus on the males in this post. Since they vary little and are more distinctly patterned than females usually are, it’s easier to get a handle on them. Females—of some species anyway, are quite variable depending on their maturity and whether they are gynochromatic (“female-colored”) or androchromatic (“male-colored”). It’s not that females are necessarily more difficult to identify in the field—well, sometimes they are; they are definitely more complicated and deserve their own discussion. Here’s a post I wrote last year on female Western Forktails (Ischnura perparva) and the changes they go through as they mature, but this is certainly not characteristic of our other species.
Forktails, as a group, are frequently encountered across the Northwest at marshes, well-vegetated ponds and lakes, and sluggish streams. Just about any fresh water habitat you visit in the region (aside from fast-flowing, sparsely vegetated streams) is bound to host at least one species of Ischnura, and it’s not unusual to find two (occasionally three) species flying together.
What puts the fork in forktail?
|The abdomen tip on a male Pacific Forktail (I.|
cervula). Image by Steve Valley, taken at the
Oregon State Arthropod Collection, Corvallis,
I should add that, while Ischnura is a cosmopolitan genus, the name “forktail” is only used by English speakers in the Americas. European and Australian species are known as “bluetails” in English, or in the case of the British species, Blue-tailed Damselfly and Scarce Blue-tailed Damselfly. I’m not sure that the American name is any better since some species don’t have the fork at all and seeing it requires a very close look when it is there.
How do you know a forktail when you see one?
Remember that I’m only talking about males in this post, although some of this will work for androchromatic females which can look just like the males. The Pacific Northwest species are generally pretty small like many of our damselflies. In color, they are blue and black (sometimes with a green thorax) with the top of the abdomen mostly black except for a patch of blue near the tip (mostly on segments 8 and 9—the second and third segments from the tip). The black on the abdomen may be broken by very narrow pale rings at the abdominal joints, but otherwise the abdomen is all black on the top of those middle segments. This difference doesn’t work in eastern parts of the continent where some bluets (Enallagma) have a similar abdominal pattern, but it’s very useful in this region.
|At top, the abdomen of a male Western Forktail (I. perparva); below, the abdomen of a male Tule Bluet (Enallagma carunculatum). Notice that the abdomen of the forktail is nearly all black along the top except for the mostly blue segments 8 and 9 (second and third from the tip). In this case, there are narrow pale rings which interrupt the black on the middle segments, but some species lack those. The bluet's abdomen has less black, interrupted by obvious blue bands, and some other species have less than that.|
Narrowing down the choices
If you know you have a forktail, identifying it to species starts with the pattern on the thorax. In the area composed of southern British Columbia, Washington, Oregon, Idaho, and western Montana, the thorax pattern will narrow down your choices to one or two species; further south and east you’ll have other possibilities to consider.
In the Northwest, a striped thorax limits your choices to two species: Western Forktail (I. perparva) which is very common over the entire region, and Swift Forktail (I. erratica) which is mostly limited to areas west of the Cascades. If you’re east of the Rockies, you’ll need to consider the Eastern Forktail (I. verticalis) which is extremely similar to Western. Toward the southwestern US are more striped species, with the Desert Forktail (I. barberi) the one that gets closest to the Northwest (a single individual has been found in southeastern Oregon).
A spotted thorax (you can also think about those spots as reduced stripes with only the ends remaining) means that you have the Pacific Forktail (I. cervula)—another very common and widespread species across the region. East of the Rockies is the very similar Plains Forktail (I. damula); the size of the forked process at the tip of the abdomen is helpful in separating these species where they overlap.
The unmarked thorax is limited to the Black-fronted Forktail (I.denticollis), a primarily southwestern species which gets into southern Oregon and Idaho. This species has a twin, the San Francisco Forktail (I. gemina), which is limited to, of course, the San Francisco Bay area, and requires an in-hand view to differentiate.
The Northwest species
Below are the four common forktails of the Northwest (the ones that are common in parts of the region, at least). Click the image to see the species’ range at OdonataCentral.org.
Now you know all you need in order to go out and identify forktails in the Northwest (the males, at least). Check some wetlands in your area when you get a stretch of nice sunny weather and see what you can find—hopefully I won’t have to wait much longer in my neighborhood!