Friday, October 28, 2011

Sorting Paddle-tailed and Shadow Darners Out-of-Hand, Part 1


Paddle-tailed Darner (Aeshna palmata) and Shadow Darner (A. umbrosa) are both rather common and widespread—nearly ubiquitous, I’d say—across the Northwest during the latter part of summer and fall. Not only are they superficially quite similar in appearance, but they are also frequently found flying together. Add to this situation the fact that more and more people are relying on photographs and binoculars instead of nets to identify odonates, and it becomes evident that “new” field marks are needed to sort out these bugs. In this post I’ll summarize the various differences between these two species and explain which in particular seem to be the most useful, at least at this time, for “out-of-hand” identification. Some of these have been well-understood for a long time and some have only recently come to light.

The "Classic" Field Marks

When I started identifying odonates in the mid-90s, separating these two species involved the relative thickness of the thoracic stripes, the relative size of the dorsal blue spots on the abdomen, whether or not there were blue spots on the dorsum of S10 (the posterior-most abdominal segment), whether or not there were pale spots on the underside of the abdomen, and whether or not there was a dark face line. The latter three characters work well much of the time, but the others are not only variable, but also quite subjective and difficult to assess without experience and comparative material. You also hear about Shadow Darners frequently having an extension or “flag” at the top of the anterior lateral thoracic stripe. It is more often well-developed on Shadow Darners, but I find it variable enough on both species to be unreliable.

Ruling Out Other Species First

Before you can figure out whether your subject is a Paddle-tailed or Shadow Darner, you have to first know that it is one of those species. These two species (including males and females) have a pair of pale, narrow to moderately wide, relatively straight stripes on the side of the thorax. These are often mostly yellow, but transition to pale blue or greenish-blue at the tops (occasionally more pale blue or greenish-blue overall), and they are pale enough and contrast with the otherwise dark brown thorax enough to be conspicuous from a distance.

Male Paddle-tailed Darner abdominal
appendages, lateral view. You see two
spines because one of those is on the
opposite cercus.
Males have cerci (upper abdominal appendages) which are somewhat flattened, but twisted so they appear relatively wide from the side. Each cercus also has a spine near the tip below the rounded apex which is small, but easily seen through binoculars if you’re close enough, and in good quality photographs (it’s just barely visible in the photo above). I prefer to label this type of Aeshna cercus as “spined”, but you will also see some authors describe it as “wedge” or “paddle” in shape. Males of two other species in this region have similar appendages—Walker’s Darner (Aeshna walkeri) and Lance-tipped Darner (A. constricta), but these are more restricted in range and habitat, and are not frequently encountered in the Northwest. I may discuss those species in a later post.

Females of some other species are very similar to Paddle-tailed Darner and they can be problematic to differentiate. In particular, female Variable Darner (Aeshna interrupta) can be extremely similar when they have relatively bold thoracic stripes. In that case, small structural differences are the only way to confidently identify them and those are difficult to see out-of-hand. Sometimes you just have to accept that not everything is identifiable.

Caveats

I started to go into a long discussion of a couple of issues, then I thought it best to restrict it to some brief statements for this post. 1) Remember variation. Practically every character varies to some degree or another—sometimes a little, sometimes a lot—and the extent of variation of some characters is not yet fully understood. 2) Because of variation, don’t rely on any single field mark whenever possible. These issues really deserve a much more thorough discussion, but let’s keep it there for now and move on...

What to Look For

Differentiating Paddle-tailed and Shadow Darners depends, in part, on whether your subject is a male or female, although some characters work for both. Below is a simple table to guide you. The characteristics are roughly in order of usefulness as far as I have determined, with the most useful at the top. The first three work for both males and females. This ranking is pretty subjective, and is subject to change if better information comes along. In other words, I could easily change mind mind if additional observations warrant it. Some of these criteria I consider tentative and particularly in need of further assessment (indicated with *), but at this point they appear to be helpful (remember the caveats about variation and relying on a single character).


MalesFemales
1presence/absence of ventral abdominal spots
2lateral pale mark on S1
3face line (if either bold and black, or absent—not if thin)
4S10 with/without dorsal pale markingsshape of dorsal abdominal spots*
5epiproct color* (regional?)presence/absence of cerci—not reliable, but worth noting
6shape of anterior thoracic stripes*
7shape of pale markings on dorsum of S2*

 
1. Presence/Absence of Ventral Abdominal Spots (Males and Females)

Male Shadow Darner abdomen, middle segments, ventral surface.
 
Abdomen of female Shadow Darner on
left; Paddle-tailed on right. Ventro-lateral
view. Red arrows point to the pale spots.
Shadow Darner has paired pale spots on the underside of the abdomen, most prominently on S4–S6 (blue on males—as in image above, blue or tan on females) and Paddle-tailed lacks them being more-or-less uniform in tone/color. Those spots are not so obvious on some female Shadow Darners, but you should see them with a pretty good view. The image at right includes a ventro-lateral view of the abdomen of a pretty dull female Shadow Darner on the left and a female Paddle-tailed on the right. The pale spots don’t stand out so well on this female Shadow Darner and require a close look.

This is by far the best field mark when you can see it—the trouble is that it is often hard to see in the field. Look back at my post Fun With Darners! which has photos of three perched darners. All of the darners I saw that day were perching with their bellies toward the vegetation and it was tricky, at best, to get a view of the underside of the abdomen without scaring them off. If you can get a side view of your subject you may just be able to see the spots if they are there.

Note that a few other Northwest species also have pale ventral abdominal spots, but they are not otherwise especially similar to Shadow Darner and they are more restricted in range and habitat preference.

 
2. Lateral S1 Pale Mark (Males and Females)

Left to right: the thorax and first two abdominal segments of a male Shadow Darner—the first abdominal segment (S1) is spot-lighted; Shadow Darner S1, lateral view; Paddle-tailed Darner S1, lateral view.
 
Female Paddle-tailed Darner. The white
arrow identifies the S1 lateral pale mark
adjacent to the black posterior margin.
Paddle-tailed Darners have a vertical, irregularly-shaped (usually thicker toward the bottom) pale mark on the side of S1, adjacent to the posterior black border; on Shadow Darner there is no pale mark adjacent to the black border, but there is an extremely thin pale streak within the black border (sometimes there are only minute traces of that pale streak, and I assume it can be totally lacking on some individuals). In each species I see virtually no variation of this character that approaches the appearance of the other species, and it works for both males and females.

You don’t even need a perfectly lateral view to see that pale S1 mark on Paddle-tailed Darners; even with an oblique view—with the wings obscuring the base of the abdomen, that mark is often visible like in the photo at right. Incidentally, of all of the “spined” darners that occur in the Northwest, the Paddle-tailed is unique in having that irregular pale mark adjacent to the black border. On the other hand, other species which have females that are similar to Paddle-tailed Darner—especially Variable Darner, have that mark too, so just be aware.

 
3. Face Line—If Either Bold and Black, or Absent (Males and Females)

Left to right: male Paddle-tailed Darner with a bold black face line; female Shadow Darner lacking a black face line; male Shadow Darner with a very thin (not useful) face line.
 
This character is variable on both species, but it’s a good field mark if that line is rather bold (indicating Paddle-tailed Darner) or if it is absent (indicating Shadow Darner; obviously it takes a pretty good look to see that it is absent). If it is there but very thin it isn’t of any help. I find that male Shadow Darners frequently have a thin dark face line—more so than females. It is important to realize that there is a suture across the face where that line is (when present) and because of the angle of lighting, it can appear dark even when there is no dark pigment. Even in that case it typically appears rather thin and usually brown/gray rather than black, and falls in the category of “not useful”.

 

 
This topic is continued at Sorting Paddle-tailed and Shadow Darners Out-of-Hand, Part 2.



4 comments:

  1. Excellent presentation, expressed in terms even a neo-odontophile can follow and appreciate. Thanks, Len
    p.s. "mind mind" is still digesting the finer points. ("What to look for" paragraph)

    ReplyDelete
  2. Jim,

    Let me know when you decide to do a book. I'll be first in line.

    ReplyDelete
  3. @Blubird Thanks very much, Len. I appreciate that.

    ReplyDelete
  4. @George Sims George, you are too kind! Thanks for the comment!

    ReplyDelete