Saturday, November 12, 2011

The First "Isoperla" Perlodid Stonefly of the New Season

Today I got to explore yet another -- this is the fourth -- tributary to the Moormans in Sugar Hollow.  These are streams that flow through private land so they will be nameless.

Perlodid stoneflies -- the little ones that will hatch out next spring and summer -- first show up in our streams in late October and early November.  If you look back to the entries of 10/25, 11/3, and 11/8, you will see that I've already found three different genera in local streams: Clioperla, Helopicus, and Diploperla.  Further away, at the Rapidan River, I found a small Isogenoides nymph as early as September 19.

Today I found two tiny nymphs that I knew were Perlodids, but it took microscope work to determine the genus -- and species.  The Perlodid nymph in the picture above is Isoperla similis.  How do we know?   This is information I've already covered in earlier entries, but repetition can't hurt anything.

The features that show this to be Isoperla are: 1) the anterior ends of the mesosternal "Y" ridge point up, not down, and 2) the lacinia is rounded, with two prominent spines at the tip followed by numerous hairs.  Here are the photos I took of this nymph.

Now, Clioperla nymphs have these features as well, but the head pattern's all wrong for Clioperla.  And it's the head pattern that brings us to I. similis.  On the head of I. similis nymphs there are 1) two pale spots close to the labral suture (front edge of the head), 2) a pale "M" in front of the anterior ocellus, and 3) pale spots lateral to, and in front of the lateral ocelli.   I've pointed out those features in the photo below.

Isoperla similis is a very intolerant species with a TV of 0.8 (NC DWQ), and the Moomans tribs are the only streams -- to date -- in which I've seen it.  Here's a fairly mature nymph that I found back in April in another trib on the other side of the valley.  (See the entry for April 14.)

And here's another look at the small nymph that I found this morning.


There were three insects that dominated my sample today (well, four, if I count the crane fly larvae which are prevalent now in the leaf packs): green stoneflies (Chloroperlidae), small winter stoneflies (Capniidae), and giant stoneflies (Pteronarcyidae).  I also found a number of fingernet caddisfly larvae, genus Dolophilodes (not the common genus Chimarra).  The sunlight today was great for getting good photos.

1. Green stoneflies, genus Sweltsa

The wing pads on this last nymph are starting to form the "V" on the posterior edge that is distinctive of Sweltsa.

2. Small winter stoneflies -- getting bigger, and this is a nice one.

Note the long tails vs. what we see on the green stoneflies.

3. A Giant stonefly.  There were a lot of them in this stream, and this was the biggest.  That's a Roach-like stonefly taking a ride on its abdomen.

4. And, here is one of the fingernet caddis larvae that were also crawling around in the leaves.  Remember that the "frontoclypeal apotome" -- leading edge of the head -- is asymmetrical on the genus Dolophilodes: on Chimarra nymphs it's notched.  Dolophilodes larvae also seem to have yellow bodies, vs. the more common orange that we see with Chimarra.


Below: another good reason to go out looking for streams at this time of year in Virginia.

No comments:

Post a Comment