Thursday, March 1, 2012

The Northwest Forktails (Ischnura)

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,
Oregon, USA.
The reason why the Ischnura are called “forktails” is not obvious to the casual observer since it requires a close, magnified view. Males of most species have a raised projection on the top of the last abdominal segment, and if you look at that from the rear or from the front, the top is forked. At right is the end of the abdomen of a male Pacific Forktail (Ischnura cervula) and that process at the top is the “fork”. The relative height, angle, and forkedness of that projection depends on the species and some don’t really have anything there at all.

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.
All male forktails also have blue postocular spots on top of the head (see photos below). Most of our coenagrionid damsels have these, including the bluets, but it is something that eliminates a couple of other species which have mostly black abdomens. The eyes are “capped” with black over the top part—another characteristic shared by bluets. The lower parts of the eyes are often grass green, but they can be blue as well (I’m not sure if this is individual variation or age-related variation). None of our other damsels ever have green eyes.

Something else that you might notice on some Ischnura in the hand, or through binoculars, or in macro photos, is that the fore wing pterostigmas (the darkened cells near the tips of the wings) can be different in size, shape, and/or color from hind wing pterostigmas. At right is a close up of the wing tips of the male Swift Forktail (I. erratica) in the image at the top of this post. Notice how the two fore wing pterostigmas (at top) are bigger and darker than the two hind wing pterostigmas (closer to the abdomen). Pterostigma differentiation varies by species—more obvious on some like this one, not so obvious on others.

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.

So, look at the top of the thorax. Are there pale stripes within the black area, only spots within the black area, or is the black area unmarked?

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

Pacific Forktail (Ischnura cervula): The four isolated spots on the top of the thorax make this one of the most easily identified damselflies in the Northwest. It is also one of the most widespread species, occurring at practically any well-vegetated marsh or pond. If you build a pond in your backyard, this will probably be the first odonate to colonize it.

Western Forktail (Ischnura perparva): A striped species which can be either green or blue on the thorax. This is another very widespread species in the Northwest, sometimes occurring with great abundance in places. This one is also found on small streams more often than our other forktails.

Swift Forktail (Ischnura erratica): This is another striped species, larger and more robust than our other forktails, and always blue on the thorax (never green). The blue patch near the end of the abdomen is usually longer than it is on the other species, extending partially onto segment 7, but it doesn't extend as far down the sides. This is one of its most obvious field characters. Males have conspicuous, spiky paraprocts (lower abdominal appendages) which are obvious in macro photos—you can see them at the posterior (left) end of this guy. Also look for the extra black stripe on the side of the thorax; on other species this is limited to a spot or partial streak. Found mostly at well-vegetated ponds west of the Cascades (frequently beaver ponds), but it has also been found in the Blue Mountains in recent years.

Black-fronted Forktail (Ischnura denticollis): This is a small species with either a green or blue thorax, which lacks obvious pale markings within the black area. The blue patch near the end of the abdomen is usually shorter than it is on the other species, not reaching the front end of segment 8. Also look at the fore wing pterostigmas which are conspicuously white at the outer corner. This is a primarily southwestern species which reaches its northern extent in southern Oregon and Idaho. It’s a very common species at alkaline hot springs, but it is also found at other wetlands with dense vegetation (usually sedges).

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!


  1. Sandra Hunt-von ArbMarch 3, 2012 at 10:20 AM

    Great summary of the forktail males. As usual, you have been able to get to the important characteristics and made it look easy! The comparative pictures are great too! I'll be printing this one out and adding it to my "personal field guide". Thanks!

    1. Thanks, Sandra. I'm glad you found it helpful.