Saturday, June 23, 2012
The "Fawn Darner" Dragonfly: Boyeria vinosa
This photo was taken on June 7th at the Lynch River, and in the entry posted that day, I pointed out that this Darner dragonfly was genus Boyeria based on the fact that the tips of the paraprocts curve inward (for Boyeria genus ID, see Barbara Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America, p. 27). In this entry, I'll try to get this down to species ID: this appears to be Boyeria vinosa, the common name for which is the "Fawn Darner."
We can make this ID using a resource that I found just yesterday: "Odonata Larvae of Michigan: Keys for, and notes on, the dragon and damselfly larvae found in the State of Michigan." You can find this online at: http://insects.ummz.lsa.umich.edu/michodo/mol/Home.htm, and for Boyeria species ID, the relevant keys are found at: http://insects.ummz.lsa.umich.edu/michodo/mol/Boyeria.htm. There we find the following words:
"Prementum width about 0.66x of its length, folded labium clear short of the posterior margin of mesocoxae; Ab5-9 with lateral spines; epiproct often cleft, clearly shorter than paraprocts. B. vinosa"
Fortunately, I had taken a photo of the prementum with the nymph on its back, and that photo shows us two of the things that we need to see. This is the picture.
Since I did not preserve the nymph, I can't get an "actual" measurement of the width and length of the prementum. But, I can determine the ratio from a physical print. If I've measured correctly, the width of the prementum (top edge) is 0.657x of its length. That's right on the money. But we can also see from this photo that the folded labium (the prementum and postmentum combined) does indeed end short of the "posterior margin of the mesocoxae (where the middle legs attach to the body)".
Without the actual insect, I cannot check for the "spines" on the abdominal segments, but we can see the paraprocts and the epiproct, and the epiproct is "cleft," and slightly shorter than the curved tips of the paraprocts.
I'd call that nymph Boyeria vinosa -- a Fawn Darner. The B. vinosa tolerance value is high: 5.8.
I think it's likely that the Darner I found yesterday at the Moormans was also a "Fawn," but I can't tell for sure since I did not photograph the prementum. But we can see the tail end from this photo.
A closer look:
From now on, I'll be taking prementum photos! And I do have a Darner dragonfly in my reference collection that might be a different species -- Boyeria grafiana. With B. grafiana nymphs, the "Prementum width [is] about 0.60x of its length, [and the] folded labium extends to [the] posterior margin of [the] mesocoxae, or beyond." The measurement here is 0.603 -- again, right on the money -- but I can't say that the labium extends all the way to the posterior edge of the mesocoxae. Darn close.
(For photos of adult B. vinosa dragonflies, go to: http://bugguide.net/node/view/4301/bgimage?from=24.)
Note: In using the "keys" today, I have struggled to understand the difference between the "prementum" and the "labium," in Odonata. My understanding at the moment -- and remember, I'm an amateur -- is that the "labium" includes the "prementum" and the "postmentum," and it is hinged (I think the "hinge" is the pale line at the end of the labium). It is the prementum that extends from the hinge to pick up food in the act of eating. If I find out that I'm wrong, I'll let you know. I wish the keys made this clear.