Sunday, January 30, 2011
"Dragons and Damsels": Odonata, Pt. I.
Dragonflies and damselflies belong to the order Odonata. Dragonflies form the suborder Anisoptera; damselflies form the suborder Zygoptera. I thought I'd address identification of the dragonflies we find in our streams in this entry, then do the same with the damsels in "Odonata, Part II.".
I have seen nymphs of five dragonfly families in our watershed streams: 1) Clubtail dragonflies (Gomphidae), 2) Darner dragonflies (Aeshnidae), 3) Emerald dragonflies (Corduliidae), 4) Skimmer dragonflies (Libellulidae), and 5) Spiketail dragonflies (Cordulegastridae). I have not attempted to identify the genera of all of these nymphs: I do know that we have at least three different genera of the Gomphidae (Clubtail) family: Ophiogomphus (the Snaketail dragonfly), Progomphus (the Common Sanddragon dragonfly), and Hagenius (the Dragonhunter dragonfly -- also called the "Black Clubtail").
I. Clubtail dragonflies (Gomphidae)
Virtually every Gomphid we see, I think, is genus Ophiogomphus (the Snaketail dragonfly). I have only seen two that were not. So, let's begin by noting the defining characteristics of this particular genus. They are: 1) as with all members of the Gomphidae family, the antennae have four segments, and the third segment (away from the head) is huge in comparison with all of the rest: the fourth segment is small; it may not be visible. And 2) the wingpads diverge from the main axis of the length of the body. Let me show you three pictures.
While the wingpads are too small to see in the first photo, and very tiny in the second, we can clearly see how they diverge from the main axis in the third.
The two Clubtails I've found that do not belong to this genus, are both nymphs that I found on my own. One is genus Hagenius (the "Dragonhunter" or "Black Clubtail").
The third antennal segment is still very large, while the fourth is hard to see. But the wingpads here are "parallel" to the main axis of the length of the body, and the body is flat, long, and wide. The ruler shows this nymph to be about 1 1/2" long. I had no idea what I had found when I came upon this in the Doyles River last summer!
Finally, I have found one Clubtail that's genus Progomphus (the Common sanddraggon dragonfly). This is a strange looking insect.
This nymph was in a handful of sand I picked up in the Mechums River while doing a geomorphology study with other members of StreamWatch. With this Gomphid, again the wingpads diverge from the main body axis, and the third antennal segment is clearly the largest, but the fourth antennal segment is really quite large and very visible. Below is a ventral view of the antennae.
In the DEQ and StreamWatch reckoning of tolerance values, all Gomphids are given a "1". However,
in the EPA listing of tolerance values by region, North Carolina gives the genus Progomphus a value of "8.7," and Hagenius a value of "4". StreamWatch finds most of its Gomphids (Ophiogomphus) in 1) its reference sites, 2) Buck Mt. Creek, 3) Cunningham Creek, 4) Long Island Creek, and 5) two sites on the main stem of the Rivanna (Crofton and Rivanna Mills).
II. Darner dragonflies (Aeshnidae)
They're ugly! Any questions? Darner dragonflies have long, slim bodies, they're uniformly black or dark brown, and they have short, thin, antennae with six or seven segments. But the key feature is the labium (lower lip) which is flat (looked at from the side), and wider at the top than it is at the bottom (when looked at from underneath). This is clear in the following photo.
Darner dragonflies have a TV of "3," and few are found in our watershed streams -- if we can trust the StreamWatch data. When I go out to streams on my own, however, I frequently find them. I think the problem is that they do not live in the "riffles" where most samples are taken: they prefer the slow, weed-choked water by the sides of the streams. I always find them close to the shore.
III. Emerald dragonflies (Corduliidae)
This is an insect we find only rarely. A few have been found in the North Fork of the Rivanna, and I've found some in the Rivanna itself. This has a tolerance value of "5". This dragonfly is closely related to the "Skimmer dragonfly" (Libellulidae); in fact, some keys do not try to distinguish between them. However, if we go to the "Bible" of keys -- Merritt, Cummins, and Berg, An Introduction to the Aquatic Insects of North America, Fourth Edition, pp. 246-249 -- the two are distinguished by the depth of the "crenulations" (= notches) on the distal margins of the palpal lobes. On Corduliidae nymphs the notches are deep; on Libellulidae nymphs the notches are shallow.
The "palpal lobes" are the upper, hinged parts of the labium (lower lip), seen here in ventral view. These notches would be deep in the eyes of Merritt, Cummins, and Berg. So, this is Corduliidae.
(Please note that I do not have a Libellulidae nymph in my collection: I wish that I did.)
IV. Spiketail dragonflies (Cordulegastridae)
This is another dragonfly we find only rarely. StreamWatch records one found in the Rivanna River at Milton, and my group found one at the StreamWatch reference site for Albemarle County in the fall of 2009. It was a big one, and I have a photo of it as it was preserved.
This was about 1 1/2" long. I have one more in my own reference collection -- but I don't know where I found it. Still, we can use it to see what is used in defining this family.
The feature on which we need to focus is the lower lip (labium) which in this case has dropped down in front of the head. (Death was a "jaw-dropping" experience! Sorry.) The two "lobes" at the end of the lip are, again, the "palpal lobes," and it's the distal margins of those palpal lobes that are important for us. Here's a close-up view.
Note the "jagged" edges of those palpal lobes. That makes this a "Spiketailed" dragonfly.