Wednesday, February 15, 2012

Isoperla namata: Comments on a Number of Issues

This picture was taken on 3/24/11 at the Rapidan River in Madison County.  It's the Perlodid stonefly, Isoperla namata, which is the most common Isoperla species we see in our streams -- more, it's the most common Perlodid stonefly we see in our streams.  But there are a number of issues surrounding the ID call of "Isoperla namata" that I think we need to discuss.

1. Let me begin by noting that Isoperla namata is a species that is primarily found in the Ozarks -- Missouri and Arkansas (for the distribution of I. namata, look at:  (Note that "npwrc" stands for the National Prairie Wildlife Research Center).   In fact, the common name for I. namata is "Ozark Stripetail."
For this reason, Steven Beaty ("The Plecoptera of North Carolina," p. 24) refers to the Perlodid nymphs that are found in NC that "appear" to be I. namata as "Isoperla nr. namata" -- i.e. they look like I. namatas, but I. namata shouldn't occur here (!).  In Virginia, we don't have this problem.  That's because Stewart and Stark, in their Nymphs of North American Stonefly Genera (Plecoptera), p. 412, include Virginia among the states where I. namata is found (AR, IN, KY, ME, MO, OH, PA, VA, and WV.)  

2. There is another species of Isoperla which, in appearance at least, is exactly the same as I. namata, but the geographical distribution is different.  This is Isoperla montana.  This is supposedly found, primarily, in the northeast: CT, DE, ME, MN, NH, NS, NY, ON, PA, and PQ (Stewart and Stark, p. 412).  (Also see,  Note that the species ID, "montana," has nothing to do with the state of Montana, it refers to that fact that these nymphs are primarily found in mountain streams.   (The common name is "Montaine Stripetail").   Now, if you look up "Isoperla montana" on, you'll see that these nymphs look exactly like I. namata.  In fact, some have been moved into I. montana from I. namata, for, what appears to be nothing more than geographical reasons.  I.e. I. namata is only found  -- supposedly -- in the Ozarks and the Mid-west (!).

3. Let's get closer to home and look at some of the nymphs that I've found this season and last.  But let me begin with Beaty's description of Isoperla nr. namata (Beaty, "The Plecoptera of North Carolina, p. 24).

"North Carolina specimens have a dark transverse band on head usually without backward extensions to lateral ocelli but sometimes with extensions and with lighter brown areas behind ecdysial line (emphasis added by Henricks): abdominal longitudinal lines narrow; abdominal tergites with a transverse row of 6-8 sometimes faint dots."  He adds, "Isoperla namata is supposedly geographically restricted to the Ozarks and the midwest..."

4. As I have looked through my Isoperla photos from last year and this year, I've found that most match what Beaty calls Isoperla nr. namata, but, with most of the nymphs that I've seen, the dark transverse band does extend to the lateral ocelli.  E.g., in addition to the nymph pictured at the top of the page,

a. 3/28/11, Powells Creek:

b. 4/23/11, Powells Creek:

However, I've also found some nymphs where the dark band does not extend to the lateral ocelli.  For example, this nymph also found at Powells Creek on 3/28:

And, this one -- though I'm not sure where it was found and when:

5.  Now, let's have a look at what I've found so far this season -- noting the dates and locations -- but I'll save the photos of the nymphs that I've found in Sugar Hollow for last.   I've called all of these nymphs "Isoperla namata".   Note how the head pattern becomes clearer with time.

a. 11/19/11 -- Rapidan River.

b. 12/13/11 -- Rapidan River

c. 12/21/11-- Upper Doyles River, the first one that is "clearly" Isoperla namata".

d. 1/4/12 -- South River (Greene County):

e. 1/18/12 -- Rapidan River:


Now, let's have a look at the nymphs I've found in Sugar Hollow, in the small, mountain streams that feed into the Moormans.

a. 1/14/12:

b. 2/2/12:

c. And finally, the nymph that I found on Monday, 2/13/12:

I never saw it until I looked at all of these photos this morning.  The dark spots/lines -- whatever you want to call them -- behind the rear ocelli, and behind the ecdysial suture (that's the line on which the head will split open when the adult emerges) are missing.  True, this could be because those dark spots have not yet developed.  But, note that they are very clear on the young nymphs found on 1/4 in South River and on 1/18 at the Rapidan River.

I'm not sure if this is important -- i.e. if this is a morphological feature that sets the Sugar Hollow Isoperlas apart.  Still, it's significant to me that I've seen this only on the nymphs that I'm finding in these "pristine" streams in the Blue Ridge, in Sugar Hollow.

So now you have some idea of the mysteries entomologists have to work out when dealing with the "Isoperla namata," or "Isoperla montana," or "Isoperla nr. namata" nymphs that we find in our streams.
Still lots of work to be done.

Below:  a mature I. namata -- April 18, 2011: Long Island Creek, Fluvanna County.

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