When I was at the Rivanna on Monday (7/2/12), I saw at least six of these dragonfly nymphs: look for them in that gnarly, fibrous vegetation that covers the rocks in the summer. We've seen this one before: it's a member of the Corduliidae family, the "Emerald Dragonfly," they're actually pretty common in the Rivanna at Crofton.
This summer, I thought I'd pay more attention to genus and species ID of our "dragons and damsels."
For the genus ID, we can use Peckarsky's Freshwater Macroinvertebrates of Northeastern North America. The following microscope photo is crucial for both genus and species ID.
We turn to p. 51 in our source.
33a. Abdomen with a middorsal hook, spine, or knob on at least segments 6 or 7.....34 Absolutely. We can see "knobs" on segments 2-9, so we can move to 34 and not bother with 33b.
34a. A middorsal hook, spine, or knob on abdominal segment 9.....35 Yes. We move on.
35a. Lateral spines of abdominal segment 9 reaching almost to tip of epiproct or beyond...Corduliidae .....37 They reach well beyond the epiproct tip. This establishes that our nymph is a Corduliidae and not of the Libellulidae family. We go to 37.
37a. No lateral spines on segment 8...Williamsonia
37b. Distinct lateral spines on segment 8 ... ..38 The spines on 8 are very distinct so we proceed to couplet 38.
38a. Middorsal hooks knoblike, with apices blunt and rounded; crenulations on distal margin of palpal lobe very deep, each crenula 2 or more times as deep as wide..... Neurocordulia
38b. Middorsal hooks spinelike, with apices acuminate; crenulations on distal margin of palpal lobe relatively shallow, each crenula at most as deep as wide ... 39
The middorsal hooks are knoblike, and they are blunt and rounded. Here's a view of the knobs from the side.
And the crenulations on the palpal lobes are indeed very deep, at least 2 times as deep as they are wide. I have marked the width and depth of one of the crenula and noted the depth on some others.
The genus of our "Emerald Dragonfly" is Neurocordulia. Now, what of the species. For this, we use a source that's online -- a "Key to the Mature Larvae of Neurocordulia." (web page: http://insects.ummz.lsa.umich.edu/michodo/mol/Neurocor.htm) Our source distinguishes two different species: N. yamaskanensis and N. obsoleta.
1a. Lateral spine of abdominal segment 9 does not surpass tips of the epiproct and paraprocts; mid-dorsal abdominal spines pointing rearward -- N. yamaskanensis
1b. Lateral spine of abdominal segment 9 extending past the tips of epiproct and paraprocts; mid-dorsal abdominal spines pointing upright -- N. obsoleta
Look back at our first diagnostic photo above. The lateral spines on segment 9 clearly extend past the tips of the epiproct and the paraprocts. For the mid-dorsal spines/knobs, look again at our second microscope photo. The knobs, all in all, point upright. While those towards the rear might appear to point back, that's mostly due to the slope of the body.
So there we have it. Our dragonfly nymph is Neurocordulia obsoleta: common name, the "Umber Shadowdragon."
The other dragonfly nymph I found in the Rivanna on Monday is also one that we've seen before. It's a strange looking Gomphid (Clubtail dragonfly), and we already know the species ID -- Hagenius brevistylus (common name: Dragonhunter).
Not the prettiest insect we find in our streams. Previous nymphs that I've found have been more colorful -- orange/red with stripes. But most of our insects get darker as they mature, and this one was clearly getting ready to hatch. It was big. I'd say at least an inch long and about 3/4 inch wide.
We recognize Clubtail dragonflies by their "clublike" antennae. You may remember that these antennae have 4 segments, the largest of which is segment 3. Segment 4 is tiny and may or may not be visible.
I took a close-up shot of the antennae of this particular nymph just to see if we could see that final segment. We can, but look how tiny it is! You'll have to click on the photo to see it.
Below, an Umber Shadowdragon that I found at Crofton last year and a younger Dragonhunter clubtail.