Friday, August 12, 2011
Same Old, Same Old -- With One Important Exception: Back to the Rivanna at Crofton
Let's start with the exception. This is a common stonefly -- family Perlidae -- but this is the first time I've seen this genus since I started this blog. Genus Agnetina. I've seen these stoneflies only one place in our watershed and only one time before -- in the Rivanna last summer. I was delighted to see this one today. It was a small nymph, 1/4" to 1/2" in length, but note how fully colored it is, and the wing pads are starting to bow.
How do we recognize this Perlid genus? There are two things that we have to see. 1) There must be an occipital ridge (line at the back edge of the head) with "a closely set regular row of spinules" (Peckarsky, Freshwater Macroinvertebrates, p. 69). Can we see that using a microscope?
For sure. (Actually, this "ridge" is visible on the photo at the top of the page if you double-click on the photo to enlarge it.) 2) There must be "branched subanal gills present," and the "basal cercal segments [are] without an inner fringe of long hairs" (same source, same page). (Also, enlarge the picture at the top of the page to see how they "branch".)
There are "spiky hairs" at the base of the tails (cercal segments), but no "fringe of long hairs". So, genus Agnetina. This is, by the way, a relatively "intolerant" Perlid genus (0 to 2, depending on species) -- so what's it doing in the Rivanna?
The other insects I found today are all things I've found here (Crofton) before. Still, I got some very nice photos. I'll just post the photos with some general comments.
1. Calopterygidae (Broad-winged damselfly) -- a beauty!
2. Coenagrionidae (Narrow-winged damselfly) --also a beauty.
3. And now -- a double! The broad-winged and narrow-winged side-by-side in the tray. Photos like this should help monitors to tell the two apart. The differences are rather dramatic.
4. An Emerald dragonfly (Corduliidae). I saw so many of these today that I started to brush them off the rocks so I could focus on other things. And they're getting big!
5. A gorgeous flatheaded mayfly, genus Maccaffertium; very unusual colors (has it recently molted?). I've not yet worked on species ID for this genus of flathead: something to keep for the future.
6. The "not unexpected" small minnow mayfly, Heterocloeon curiosum.
7. And last but not least, I found a number of Tricos (Little Stout Crawler mayflies). My goal this summer was simply to find one: my goal now is to find one that isn't covered in silt!
Actually, I preserved this one, and the microscope photo reveals more detail -- and a good look at the "triangular operculate gills" -- than the live shot.
Below -- a picture of the Rivanna River below Crofton bridge in normal, summer flow.