Tuesday, August 13, 2013

An Instance of Mistaken Identity

Stream Bluet (Enallagma exsulans), female – this specimen was so aberrantly marked and colored compared to the norm for this area that I mistook it for a female Double-striped Bluet. The postocular spots are very small, the paler stripes within the middorsal carina and humeral stripe are quite pronounced, and the "W" on S9 is relatively elongated.

The lines on the middorsal carina and humeral stripes of local female Stream Bluets I've encountered in the past few years are barely visible. And the "W"-shaped spots on their ninth abdominal segments generally look exactly like a "W"; the next two images illustrate a couple of variations.

However, were this a Double-striped Bluet it would be hundreds of kilometers from home. Not impossible, as I recently found a group of Slender Bluets breeding in Stoco Lake. But the habitat is also be wrong – I found this damselfly along the Moira River where the water runs fast and the bottom is rocky, ideal Stream Bluet territory. Double-striped Bluets prefer ponds and lakes, or the slower portions of streams and rivers. And having researched images of other female Stream Bluets on the Internet and obtained some second opinions (at BugGuide.Net) the conclusion is – yes, this is indeed a Stream Bluet.

Alhough it would have been exciting to have stumbled upon another odonate species outside of its historical range, I would never in any event accept this as a Double-striped Bluet sighting without checking the mesostigmal plates or seeing and/or photographing a male or a mating pair.