Saturday, February 26, 2011
Omissions: Additional Stonefly and Caddisfly Genera
In describing and commenting on stonefly and caddisfly genera in previous entries -- specifically, in "Common Stoneflies" (12/27/10), "Perlodid Stoneflies" (12/26/10), and "The 'Common Netspinner Caddisfly'" (1/5/11) -- I have omitted genera that I've personally never seen in our rivers and streams -- with one exception. There is one genus of "common stonefly" I have found in our streams that I simply failed to mention. But there are two Perlodid stoneflies and one "common netspinner" caddis not mentioned above that may in fact be in our streams waiting to be discovered (at least they're included as "northeastern genera" in Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America.) So let me comment on these.
1. Common stonefly (family: Perlidae): genus Agnetina. First, the common stonefly genus I forgot to mention: Agnetina (pictured above). I found two of these last summer in the main stem of the Rivanna. They just don't belong there! I think they took a wrong turn. This is a stonefly with a tolerance value of "0" -- what is it doing in what is, at best, mediocre water?! This is why we (I!) have to learn to be less rigid in my expectations.
How do we know this nymph is genus Agnetina? There are two features that we have look for, the first being an "occipital ridge" (a distinct line across the back of the head) that is made up of closely set spinules. And, it's certainly there.
Now, this feature is also present on genus Paragnetina (covered in the previous entry on common stoneflies) -- but Paragnetina nymphs lack "subanal gills," Agnetina nymphs have them, and this nymph has them. Hence, Agnetina.
2. Perlodid stonefly: genus Isogenoides. This is a genus I hope to run into some day in a high elevation, very clean stream: it has a tolerance value of "0". (Oh, well, then it might be in the Rivanna!)
The only place I've seen them so far is Montana -- and the pictures below are of one of those nymphs.
Peckarsky, et.al. (Freshwater Macroinvertebrates, p. 71) note two features for identifying this particular genus: 1) "submental gills at least twice as long as greatest width," and 2) "median ridge of mesosternum extends anteriorly beyond fork of Y to transverse ridge." The microscope pictures tell all (note: the "submentum" is essentially the bottom part of the chin, and the "mesosternum" is the middle part of the chest).
First, the "submental gills":
And then the "median ridge of the mesosternum":
Isogenoides is a Perlodid genus that has been found in Virgina -- some of you may have seen it.
3. Perlodid stonefly: genus Arcynopteryx. This is a genus that may not be found in Virginia -- I'm not really sure. It is found in the states of New England. Again, my sample was brought from Montana.
You'll love the defining feature on this one! Again I'll quote from Peckarsky, et.al. (p. 69): "Anterior ends of arms of Y ridge of mesosternum meet anterior corners of furcal pits." Thank God that the microscope can pick all of this up.
If anyone sees this genus in the state of Virginia, please let me know. Oh, and this genus also has the long, thin, submental gills as we find on Isogenoides.
4. "Common netspinner" (family: Hydropsychidae): genus Arctopsyche. Wiggins notes one species of this genus that's present in the "southeast," so, we might keep an eye out for this. (Until then, the only place that I've found them is Montana.)
To identify this netspinner genus we have to look at the chin where the "gula [is] narrowed posteriorly" (Peckarsky, et.al., Freshwater Macroinvertebrates, p. 101).
The "gula" is the sclerite that separates the two sides of the chin -- called the "genae". In most common netspinners, the genae touch at some point. Here the "gula" completely separates the two, and as you can see, does indeed "narrow" as it moves from top to bottom.