Friday, August 23, 2013

What's that little blue thing? (The Northwest Bluets)

Oh, the little damselfly that has a blue thorax with black stripes and an alternating blue-and-black pattern on the abdomen? That’s a bluet. Which one? Umm...

A generic Pacific Northwest bluet (Enallagma). Note the abdomen
with an alternating blue-and-black pattern, the thorax with blue
and black stripes, and blue postocular spots on top of the head.
Eight species of bluets (Enallagma) are recorded in the region composed of Oregon, Washington, Idaho, and British Columbia and, superficially anyway, they all look alike with their alternating blue and black abdominal pattern and blue and black striped thorax. They really all look alike if you don’t know what to look for, but I’m here to help. I’m not going to explain how to identify each of those eight species in this post—there are field guides and manuals for that, but I’ll give you a few tips to help you narrow down the choices.

The males are separated easily enough if you net them and take a close in-hand look under magnification, but I realize not everyone wants to do that (I encourage it, however). The females are often quite a bit tougher and most of them require close scrutiny for confident identification, so I’m going to leave them out of this discussion for now. There are additional “Bluets” in the genus Coenagrion which are usually identifiable based on pattern and coloration, and I’m going to leave those out too. I don’t want to overwhelm you, but I may already be doing that even without these other bluets.

Is it a Bluet?: First of all, it’s important to know that you are actually looking at bluets. Some of the dancers (Argia) are similar in color and pattern to the bluets, so you need to eliminate those as possibilities. Look closely at the black humeral or “shoulder” stripe on the thorax (the black stripe on either side of the top middle stripe): On bluets that stripe tapers more-or-less gradually from front to back (it may thicken a little bit at the posterior end, but not significantly); on dancers that stripe either narrows abruptly near the middle or it is forked. Also check out abdominal segment 10 (the last one at the rear end): On bluets that segment is black on top, so the last three segments are blue–blue–black; on the blue dancers which occur in the Northwest, all three of those segments are blue.

Also check out the eyes—each eye on a male bluet has a black “cap” and typically a dark, horizontal line below that; male dancers (the blue ones in the Pacific Northwest, at least) have more uniformly blue eyes—they may darken a little toward the top, but they lack a the black cap and horizontal line. There is also a difference in wing “attitude” while perching: dancers usually hold their wings higher above the abdomen while bluets frequently hold their wings lower so that the hind margins of the wings are at or below the abdomen. If you look at enough of these things you’ll see some overlap, and it may vary based on a variety of factors like temperature, perching orientation, and alertness. It is still something to look for. If you want to get geeky, compare the length of their tibial spines relative to the intervening spaces in good macro photos or in-hand.

Top, the head and thorax (left) and end of abdomen (right) of a bluet (Enallagma); bottom, the
corresponding parts of a dancer (Argia). Note the differences in eye color, black "shoulder" stripe
shape, and color of the last (S10) abdominal segment.

There are two important things to look at on male bluets which can be seen with close-focusing binoculars or in close, sharp photographs (assuming the angle is right): 1) The ratio of black to blue on the middle abdominal segments, and 2) the length of the cerci (upper abdominal appendages) relative to the paraprocts (lower abdominal appendages). The combination of these two things, when seen well, will identify many (but not all) of the Northwest species with relative (but not total) confidence.

Top, a bluet (Enallagma) with a small amount of black on
the third abdominal segment (S3); bottom, a bluet with a
large amount of black on the same segment.
Amount of black: To judge the amount of black, look at the top surface of abdominal segment 3 (S3). The third abdominal segment is the relatively long segment that is nearest to the thorax (S1 is tiny and difficult to see except in-hand, and S2 is easier to see, but short). If the black on S3 is limited to a narrow ring at the posterior end, it would be considered a small amount; if the black extends forward so that it is half or more of the segment length, that would be a lot; if it’s somewhere in-between, let’s call it intermediate. In the latter case, the leading edge of the black area is often stretched into a short point on top of the segment.

Length of cerci: Yes, this means looking at tiny little things on tiny little insects, but given a good side view with close-focusing binoculars or in a sharp macro image, it can be done (but if you really want to be sure, net them and take a look with a magnifier). For starters, just judge the length of the cerci (upper appendages) relative to the paraprocts (lower appendages): Are the cerci longer than the paraprocts, shorter, or about the same?

The ends of two bluet abdomens. Left, the cerci (C) are shorter than the paraprocts (P); right, the cerci
are longer than the paraprocts.

Put them together: So, with these two bits of data you can start narrowing down your choices. Here’s a nifty character map that illustrates the various combinations of these two characters . . .

This diagram illustrates how you can use our two characters to narrow down your choices among the eight species of Northwest bluets. Notice, for example, that Tule Bluet (E. carunculatum) has long cerci (longer than the paraprocts) and a lot of black on the third abdominal segment. Familiar Bluet (E. civile) also has long cerci, but only a small amount of black on S3. Boreal Bluet (E. boreale) has short cerci and a small amount of black on S3.

This diagram also gives you a sense of the typical amount of variation that you can expect to encounter. Notice River Bluet (E. anna)—it’s in the long cerci part of the diagram and mostly in the intermediate part of the S3 black spectrum, but it also extends down into the “Narrow Ring” end of the S3 black spectrum where it overlaps with Familiar Bluet. So the black on S3 is somewhat variable on that species. The same is true of Alkali Bluet (E. clausum), but notice that it also has little “fingers” that extend into the short cerci part of the diagram. This doesn’t mean that the cerci length is variable, but I did this because the cerci on Alkali Bluet can appear to be shorter than the paraprocts depending on their “attitude” (more erect or angled down). This diagram does not take into account more extreme variation which can be exhibited by any of the species. For example, I have heard of Tule Bluets with a small amount of black on S3, but it’s quite rare—at least I’ve never observed that myself. Northern and Boreal Bluets (E. annexum and E. boreale, respectively) are primarily in the short cerci/limited black region of the diagram, but they sometimes have extra black (at least in some regions), so they have a long finger extending up into that area.

Now consider range: Three of these species are found throughout much of the region and must always be considered when identifying bluets: Tule (E. carunculatum), Northern (E. annexum, formerly E. cyathigerum), and Boreal (E. boreale). Other species are more limited in range and may be included or excluded on that basis—with one important caveat: what we know about the range of species changes constantly, and you may find one in an area where it was previously unrecorded. Marsh and Hagen’s Bluets (E. ebrium and E. hageni, respectively) are primarily northern: Marsh Bluet makes it into British Columbia, northern Idaho and northeastern Washington, and Hagen’s is found in the interior of British Columbia. River Bluet (E. anna) is found east of the Cascade Mountains in eastern Oregon and southern Idaho, while Alkali (E. clausum) is more widespread, if patchy, east of the Cascades from British Columbia to Oregon and Idaho. Familiar Bluet (E. civile) is a widespread species across the eastern and southern United States, but in the Northwest it is only known from southern Oregon (as well as an outlying record in the interior of British Columbia).

The "NOBO Bluet": The real trouble makers in this mix are Northern and Boreal Bluet. These species are so similar that photos of one or the other (where they are found together) are often simply labeled “NOBO Bluet”—NOBO being a contraction of NOrthern and BOreal. There are no consistent color/pattern differences between males of the two species—at least none that anyone has discovered so far; both have cerci that are shorter than the paraprocts and are similar enough in shape that they require a really good look to separate. Both occur all over the Northwest, often together at the same places. So, with these two species it really takes in-hand examination of the cerci to be certain of their identity—or really, really sharp macro photos at just the right angle.

The infamous "NOBO Bluet"—either Northern or Boreal Bluet. I know which one it is because I can make out the cerci in the original image. Can you tell?

This is only the start: Like I said at the beginning, these pointers will only help you limit the number of candidate species. Consult regional field guides for additional field marks which may be useful in some cases and for illustrations of the males’ cerci and paraprocts. And remember, if you really want to be sure of the species, examine it in-hand.


  1. This is a great post Jim, just not for beginners!!
    It's a little different in Calif, but for the most part this will work there. And the part about s10 on Bluets vs Dancers is actually quite good for beginners.

  2. Thank you Jim .... This will be very helpful.

  3. Tremendously helpful Jim! Thanks.
    greg haworth
    se portland