Friday, December 2, 2011
Early Winter at Powells Creek in Crozet
Mirroring my findings from yesterday at Long Island Creek, my findings today included a lot of small winter stoneflies (again the dominant taxon, and again, not very mature), quite a few large winter stoneflies (though not so mature as those found yesterday), a lot of common stoneflies, genus Eccoptura, and some BIG Maccaffertium flatheaded mayflies -- but they were not M. vicarium.
But the treasure today was the beautiful Perlodid stonefly in the photo above. And today I had my vial with me (!), so I could preserve this little beauty, and identify it without any problem. The genus is Diploperla, species, duplicata. (Duplicata is the only species of Diploperla that does not have submental gills -- S.R. Beaty, "The Plecoptera of North Carolina," p. 21). The defining characteristics, you may recall, are 1) an "incomplete" "Y" ridge, where the "arms" of the Y meet the bottom edge of the mesosternum separately, and 2) a lacinia on which the terminal spine is 1/2 as long as the lacinia itself. And here are the photos of both of those features.
Now, the interesting thing is that I found a second Perlodid today -- much smaller than this one -- which I assumed was another D. duplicata. But I was wrong. Here's the nymph in question.
This turned out to be Isoperla, but which species of Isoperla? it's just too early to say. To determine this is Isoperla and not Diploperla, we look at the very same features we looked at before: the mesosternum and the lacinia. Here, the mesosternum has a well-defined "Y" ridge (it looks like a "Y"), and the terminal lacinial spine is short, and it's followed by numerous smaller spines -- not just one.
Take a look.
Which species of Isoperla? I need to see the head pattern more fully developed. But, the only species of Isoperla I saw in this stream last year was the common Isoperla nr. namata (Beaty's terminology: exact ID is still not certain since I. namata is not supposed to occur in our region.) Here is an I. nr. namata I found in the Powells on March 28th of this year.
For the rest, I'll just show some photos.
1. Flatheaded mayfly, genus Maccaffertium (possibly M. pudicum). At the stream, I was sure this was M. vicarium, until I looked at the ventral side: note that there are no dark bands on the posterior edges of the sternal terga.
2. One of the large winter stoneflies -- followed by a look at its coxal gills.
3. One of the small winter stoneflies
4. And, finally, two more views of our beautiful Diploperla duplicata.