Sunday, October 30, 2011

Sorting Paddle-tailed and Shadow Darners Out-of-Hand, Part 2

This is the continuation of Sorting Paddle-tailed and Shadow Darners Out-of-Hand, Part 1. Read that post first if you haven’t already...

 

 
4m. S10 With/Without Pale Dorsal Markings (Males Only)

Left to right: typical male Shadow Darner with black S10
male Paddle-tailed Darner with blue on S10; atypical
male Shadow Darner with blue on S10.
This character is often pretty easy to see and assess on perching individuals: the top of S10 (the posterior-most abdominal segment where the cerci and epiproct are attached) is either all black (Shadow Darner) or clearly has blue spots—often partially fused (Paddle-tailed Darner).

I have some male Shadow Darner specimens with small, very obscure pale areas on S10 which may not even be visible in the field. However, I have photos of one exceptionally colorful (more blue than normal) individual which had obvious pale markings on S10, although they were less well-defined than they are on Paddle-tailed. If it wasn’t for that individual, I might have ranked this character a little bit higher in usefulness. On the other hand I have yet to see the opposite variation (lack of, or very small and obscure spots) on male Paddle-tailed Darners, although it’s not impossible. I suppose the rule of thumb here is that if S10 clearly lacks any pale markings, Shadow Darner is a safe bet; the presence of obvious, sharply-defined pale markings very likely indicates Paddle-tailed, but consider the possibility of odd-ball individuals. Again, I remind you to not rely on any single character if possible.

Female Shadow Darner on left; female
Paddle-tailed on right. S9, S10, and
bases of broken cerci. On both, note
the small pale areas on the posterior
edge of S10 where the cerci attach.
This character actually works pretty well for females too, but because of some complicating factors I decided to make it a “male only” character. First, on female Paddle-tailed those pale S10 spots tend to be smaller and less well-defined compared with males, so a closer look is often required to evaluate them. Another complication is that small areas on the posterior edge of S10 adjacent to the cerci are usually conspicuously pale on females of both species. On Paddle-tailed Darner, these don’t stand out as separate spots from the usual bigger pale spots unless you get a very close look; on Shadow Darner you may see those pale areas on the posterior edge and not realize that they aren’t the spots (see the comparison at right). Those images show a big difference on S9 too (the uppermost segment), but that is quite variable on Shadow Darner.

 
4f. Shape of Dorsal Abdominal Spots (Females Only)

Female Shadow Darner on left; female
Paddle-tailed on right. Middle abdominal
(part of S3 to S7) segments.
This is one of those tentative characters which appear to be helpful at this time, but that could change with additional observations. There are two things to look at on the middle abdominal segments (primarily S4–S6): the pair of spots at the posterior end of each segment and the smaller, roughly triangular spots just anterior of middle (I’ll refer to them as “middle spots” for brevity).

On female Shadow Darners, the posterior spots are usually narrow bars with a more-or-less flat (can be slightly convex or slightly concave) anterior edge; the middle spots are vary narrow and only slightly triangular with the inner ends just a little wider than the outer. If they are more clearly triangular, the inner side is less than half the the length of the anterior side.

On female Paddle-tailed Darners, the posterior posts are more oval with a clearly rounded anterior edge (there may be small concave “divets” along that edge); the middle spots are more obviously triangular with the inner edge more than half the length of the anterior edge.

 
5m. Epiproct Color (Males Only)

I started noticing in my photos that the epiproct (the lower abdominal appendage) on male Shadow Darners, when viewed dorsally, is mostly very pale off-white with a black border. The epiproct on most male Paddle-tailed Darners is all dark (medium brown in the middle if you get a close look), however there seems to be some regional variation in this. You can see the differences in images in section 4m above—it’s pretty obvious when you compare the two Shadow Darners (left and right) with the Paddle-tailed (middle). The epiproct is the shorter appendage between the cerci.

This difference appears to hold up pretty well over most of the Northwest, however I have some specimens of Paddle-tailed Darner from the very arid Alvord Basin in southeast Oregon (part of the Great Basin) which have pale epiprocts very similar to Shadow Darners. I’m going to keep an eye on this character, but it seems to be useful in the more humid, cooler parts of the region, at least.

 
5f. Presence/Absence of Cerci (Females Only—NOT Reliable)

End of abdomen of Shadow Darner on
the left; Paddle-tailed on the right.

This is not a reliable identification character, but it’s interesting enough to merit mention. It is not unusual to come across mature female darners that are missing their cerci. Presumably they break off during daily activities like ovipositing, or maybe during copulation. Female Shadow Darners are a little different in that they lose their cerci at such a high rate that it seems to be routine for them. I don’t have a single photo or specimen of a female Shadow Darner with her cerci intact, but I do have many examples of other species in possession of them. I’d say that any obviously mature female Aeshna with intact cerci during the latter end of the season (well after most have emerged) is very likely not a Shadow Darner, but definitely don’t rely on this alone.

I can’t help wondering if female Shadows remove their cerci intentionally as some kind of signal—that they are mature enough to copulate, or that they have already copulated. I don’t know, but I can’t imagine why else they would lose their cerci so much more often than other female darners. A couple of times I caught a female Shadow Darner and, as soon as I pulled her out of the net, the end of her abdomen curled toward the mandibles and “snip, snip”—both cerci fall away. In those cases it was certainly self-inflicted, and the fact that both cerci were bitten off—not just one of them, suggests to me that it wasn’t an “accidental”, random event. We may never know for sure, however.

 
6m. Shape of Anterior Thoracic Stripes (Males Only)

Left to right: male Shadow Darner with continually widening anterior thoracic
stripes; male Paddle-tailed Darner with constricted stripes; male Paddle-tailed
Darner with broken stripes.

Most male Shadow Darners have relatively heavy anterior thoracic stripes which continually widen from front to back (wedge-shaped). On most male Paddle-tailed Darners those stripes are usually either constricted or completely broken just before the terminal expansion.

There is some overlap in this character: occasional male Paddle-tailed Darners have anterior thoracic stripes which appear parallel-sided before the terminal expansion, and some male Shadow Darners have a very slight constriction before the terminal expansion. For now it appears that this character is useful if it is either wedge-shaped or obviously constricted (or broken), and anything in-between is not useful. As always, don’t rely on any single character whenever possible.

 
7m. S2 Mid-Dorsal Stripe (Males Only)

The dorsal surface of S2. Two male Shadow Darners on the left; three Paddle-tailed on the right.
 
This one certainly needs further assessment, but it appears that the mid-dorsal stripe on male Shadow Darners is typically complete and expands (either at a spot or into a funnel-shaped terminus) anterior of the perpendicular stripes on either side; on male Paddle-tailed Darners the mid-dorsal stripe on S2 is complete or broken and often without any obvious expansion; if it does expand at a spot, the expansion is at the level of the perpendicular lateral stripes.

Something I just noticed while looking at these is that Paddle-tailed Darner has a wide black posterior margin on S2, while Shadow Darner has an extra thin blue line within the black margin. That extra blue line is not always well-defined, but it is present on all of the male Shadow Darners that I have photographed. I also see some differences in the adjacent areas of S3—the extent of blue on the sides and whether they connect across the anterior end of the segment, although that is obviously variable. I’ll look into those some more too.


Conclusion

I’ve always thought of Shadow Darner as darker overall than Paddle-tailed Darner since the lateral thoracic stripes and dorsal abdominal spots average smaller (and maybe the name has a psychological effect too). I have come to realize, however, that Shadow Darner makes up for the limited color in some places with added color in other places—the anterior thoracic stripes and the top of S2 on males, for example. And, of course, they have those ventral pale spots on the abdomen too. You just have to know where to look. Out-of-hand identification really boils down to understanding where Paddle-tailed Darner has more color, and where Shadow Darner has more color.

Over time, I’m sure, some of these characters may prove to be too variable to be very useful, while other potential field marks will come to light. It’s always interesting to see how geographic variation plays a role too—as it appears to do so with male Paddle-tailed Darner epiproct color.

As always, I appreciate feedback, especially if your observations differ from mine.

6 comments:

  1. Great series! I'm with George - I think you should be doing books. But since we're overrun with great field guides, you need a new niche. Something like Kenn Kaufman's Advanced Guide to Birding - something to take it to the next level. And I keep thinking how your gift (I know, it's probably 99% just effort, not to diminish that!) for portraying complex things clearly could add value to technical manuals - to a new larval manual, or an update of other in-hand type books. Your posts here kind of bridge the gap between a field guide and the details we're used to seeing only in-hand.

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  2. @Chris Hill Thanks, Chris. I appreciate that. Actually, I have been coauthoring a field guide/manual on the odonates of the Northwest off-and-on for several years. The project evolves periodically, and admittedly it's been off far more than on!

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  3. Jim,

    I've said it before--you are really GREAT at explaining detailed information simply and understandably. I'll even skip buying food for the kids to save up enough to buy ANY book you write.

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  4. @George Sims Thanks again, George. Don't hold back on the kids' food, though. (not yet, anyway)

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  5. Excellent! I especially appreciate the attention to differentiating the females- Besides the fact that males are more commonly noticed(with their bright colors)and have more opportunity to be seen (as they hang out more by the water), females tend to be more difficult to differentiate; Many, (including me) all too often give up too fast- For me, that's partially due to not having as many resources to help and guide me to a good ID. I'm learning to be more patient in coming to an answer- but just as important, if not more--- Thank-you for adding to that resource! :)

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  6. @Sandra Hunt Thanks very much, Sandra. I'm glad you find it helpful!

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