Friday, June 10, 2011

northern bluet ~ 06/10/11 ~ Pinnacles

northern bluet
Enallagma annexum (formerly E. cyathigerum)

So much for looking at wing position to distinguish bluets (Enallagma spp.) from dancers (Agria spp.). Supposedly bluets have wings held close to the body, whereas dancers hold their wings up and away from their abdomens. Here's the second time where my pictures show the exact opposite of this. I've used the advice given by Jim Johnson at Northwest Dragonflier and Odonata.Bogfoot.net from my previous vivid dancer post to look at the color of the last (i.e., 10th) abdominal segment, at least in western damselflies. If it's mostly black on top, it's a bluet, and if it's all blue, it's a dancer. Again, another one of those naming things that doesn't mesh with what I now know to look for.

There are 4 Enallagma spp. reportedly found at Pinnacles: E. annexum, E. civile (familiar bluet), E. carunculatum (tule bluet), and E. praevarum (arroyo bluet). I ruled out the tule and arroyo bluets, because their abdominal segments are mostly black. I then ruled out the familiar bluet, because it has smaller eyespots and a triangular "fin-shaped" appendage at the tip of the abdomen. Finally, there is the possibility this could be an almost identical boreal bluet (E. boreale), but the distinction is in the tiny cerci (compare b with e) and not anything I can tackle with my point-and-shoot. Now, as before, I could be totally wrong with this ID. Jim?

The second picture above is a cast exoskeleton (aka exuvia) from a damselfly naiad. I love how they hug the tule/bulrush, something I'm not going to try to ID. What would damselflies do if there wasn't any tule around to hug?

3 comments:

Imperfect and tense said...

Well, that's made my day! A damselfly post. Thanks, Katie.

There are 11 species of blue/black damsels in the UK and sorting them out can be tricky, too. We tend to rely on several cues:

1. The pattern on segment 2 of the abdomen.
2. The amount of blue on segments 8, 9 and 10 of the abdomen.
3. Thorax markings (width of stripes).

Additionally, 2 species have red eyes and 1 species has white legs.

I am not familiar with any of the species you mentioned, but sympathise greatly with your predicament. Good Luck!

Jim Johnson said...

Hi Katie,

I can't argue with anything you said—not that I'm looking for an argument! E. annexum and boreale are tough to differentiate in photos unless the detail at the rear end is really good and the angle is right.

E. boreale is recorded from San Benito County (http://odonatacentral.org/index.php/MapAction.distributionViewerPane/location_type/State/taxon_id/42973/location_id/25/linked/1), so we can't rule that out by range. You may have to be content with calling them annexum/boreale until you look at one in-hand.

Jim

Katie (Nature ID) said...

You're welcome, Graeme. I forgot you and the Admiral are into odonates. I noticed a while ago your URL is named after the azure hawker. When you get a moment, check out Jim's sites; they're impressive.

Awesome, Jim. Thank you very much. I wonder which species is more common. E. boreale is not on the official Pinnacles list.