Tuesday, July 12, 2011
Pursuing Dragons and Damsels: Off to the Rivanna
I've been hoping to find -- and photograph -- more dragonfly and damselfly nymphs, so this morning I went to a place on the Rivanna where I've found a lot of them before. I managed to find two "Narrow-winged" damselflies (family: Coenagrionidae) and another Hagenius clubtail dragonfly nymph.
The narrow-winged damsel in the photo above is one of the prettiest specimens I've ever seen. For beginners -- damselfly nymphs have three "tails" like most mayfly families and genera, but the "oar-like" or "paddle-like" shape of the tails makes them easy to distinguish. And, in reality, these "tails" are not "tails" at all; they're "caudal filaments" -- essentially gills, i.e. they use them to breathe.
Narrow-winged damselflies are easy to ID at the streams, at least when they're as big as the one in the photo above (3/4" - 1" in length). Flip them over and look at the "chin" (as it were), the "prementum".
It should look like this.
It's wide at the top and narrow at the bottom. The nymphs that I found today were Argia in terms of the genus. To determine this, we have to look at the palpal lobes, on which we find in Peckarsky (Freshwater Macroinvertebates, p. 46): "Distal margin of each palpal lobe produced into 3 pointed hooks, middle one shorter than end hook and usually about 1/2 as long as movable hook." Have a look.
Narrow-winged damselflies are "tolerant" insects. In the new list of tolerance values used in North Carolina, Coenagrionids, those with tolerance values that have been determined, run from 8.3 to 9.5. But it's good for us to remember that finding "tolerant" insects in rivers and streams does not necessarily mean that that body of water is somehow "polluted" (through siltation, sewage, etc.). Insects that are mature or maturing during the summer in this part of the country have to be tolerant of reduced levels of oxygen in the water because of high water temperatures. These are insects that have ways to adjust to the lower oxygen levels -- e.g. they're smaller in size than insects we find the rest of the year, or, like common netspinners, they have numerous gills so they can suck up all the oxygen that they can find.
The dragonfly nymph that I found was an Hagenius nymph. I have posted photos of this genus before.
It's a "clubtail" dragonfly (a Gomphid --look at the size and shape of the antennae) that's distinguished by its flat, circular body. This one was about the size of a penny -- maybe a little big bigger. (If you click on the photo to enlarge it, you can see the tiny 4th segments of the antennae.)
Looks nothing at all like the clubtail we saw last week that I found in the Buffalo/Roach River.
What else did I find? Lots and lots of common netspinners. On rocks that were covered with the tangled grass (algae?) that grows here in the summer, they were simply crawling all over the place. It's like there has been a population explosion of netspinners and fingernets in all of our streams, but the Rivanna in summer has extremely high numbers of both.
I also saw flatheaded mayflies, brushlegged mayflies, and small minnow mayflies. (The Rivanna in summer also has large numbers of small minnow mayflies.) Here are a couple of photos (two different nymphs).
Both two-tailers, so in terms of genus they're either Acentrella, Heterocloeon, or Plauditus. The gills on the first nymph seem to have pigment at the center, so I decided to have a closer look with my microscope. And, it turns out that these Baetids are not only Heterocloeons, they're Heterocloeon curiosum! That's because they have "procoxal gills" -- little finger-like projections that stick out from the base of the front legs. Here are pictures of the pigment in the abdominal gills and then one of the procoxal gill.
(Photo below -- the other narrow-winged damselfly that I found this morning.)