Friday, July 15, 2011

ID Challenge #2 Answer

I intended to post an answer to ID Challenge #2 much earlier than this, but it just didn’t work out. I’ve been on a road trip to/from Fort Collins, Colorado (for the Dragonfly Society of the Americas annual meeting) and I didn’t have as much down time as I thought I would. During the down time I did have, I was, well… …down. Anyway, better late than never.



Click image for a larger version.
What we have is a rather plain-looking odonate with not a lot of obvious patterning. Let’s work this from the top down. Is it a dragonfly or a damselfly? We can’t tell how big this thing is, but it has a relatively long, slender build, the wings are slender and petiolate (narrowed at the base), and the wings are more-or-less parallel to the abdomen while in the closed position. This all indicates that we are looking at a damselfly. Even during those few times when dragonflies close their wings while perched (like during or right after emergence) the angle of their “hinges” puts the wings almost perpendicular to the abdomen, not parallel to it.

We can also see that this is a female since there is a curved ovipositor underneath the abdomen near the tip. All female damselflies have this sort of ovipositor (used to insert eggs into plant or other material), but only certain families of dragonflies have this kind of equipment.

We can narrow down the choices further if we take a close look at some of the little bits. There is a well-developed vulvar spine just in front of the ovipositor (“S” in the close-up at right). In the Pacific Northwest this eliminates genera like Argia (dancers) and Coeanagrion (Eurasian bluets). This is variable among the Ischnura (forktails)—even within some species.

Now let’s look at the legs. The spines on the tibiae (“T” in the close-up at right) are relatively short. Some genera like Argia (dancers) and Nehalennia (sprites) as well as species in Lestidae (spreadwings) have longer tibial spines—about twice as long as the spaces between the spines, so this feature eliminates those.

What’s left? Well, there are the Enallagma (American bluets), of which several species occur in the Northwest. Females of these species are more boldly patterned with black on the abdomen and thorax and they also tend to have conspicuously pale postocular spots on top of the head. But what if it’s an immature that isn’t displaying typical mature color/patterning? I always like to use structural differences whenever possible to be certain. We also haven’t eliminated the Ischnura (forktails) completely.

Look at that last image again, at the underside of the thorax. There’s an obvious bump there with coarse hairs on it. This points us directly to one genus that we haven’t even mentioned yet: Amphiagrion (red damsels). The vulvar spine and short tibial spines are consistent with this genus too. Amphiagrion is what we have.

The Pacific Northwest species is A. abbreviatum (Western Red Damsel), and females are quite variable both in color and patterning. Some are very dull like this individual, some are more brown or orangish, and some are male-like with a bright red abdomen and a mostly black thorax; some have less black patterning on the thorax, but more obvious black spotting on the abdomen. Whatever their color and patterning, all have that bump on the underside of the thorax which is very helpful.

Several commenters got this one right and I’m sure they spotted that bump. I think it’s important to know the little structural details which are far less variable than coloration and can help you identify confusing individuals like this.

3 comments:

  1. Hi Jim. Can you confirm a bluet for me? http://natureid.blogspot.com/2011/06/northern-bluet-061011-pinnacles.html

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  2. Thank you so much for taking the time to go over the ID step by step. It was very helpful.

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