Sunday, April 14, 2013

Isoperla davisi: a new species -- and I almost missed it!

In the first form of yesterday's entry, I called this photo one of the best pix I had gotten of an Isoperla namata Perodid stonefly.   It is a good picture: the nymph in question is not Isoperla namata.   It's Isoperla davisi, a species I have not seen before.

This is Isoperla namata.

The color's the same, and on both there are three dark stripes that run the length of the abdominal segments -- hence the reason I made the mistake.    At the moment, almost all of our streams are loaded with I. namatas, so what you expect to see can keep you from seeing what you're actually seeing!  But take a close look at the abdomen on I. namata.

The center "band" is composed of a series of lines and dots, and below the lateral stripes there are two dots on each abdominal tergite (for a total of 6 on each segment).  Now look at the abdomen of our new insect.

No dots -- just three solid stripes that are subequal in width.  How about the head patterns?

I. namata:

A dark transverse band on the head, extending back to the rear ocelli and then beyond.  What do we find on our new insect?

The head is dominated by a dark area that reaches out in all four directions, in which there are two pale areas that are enclosed.

It took me awhile to figure this out, but I finally found the description in Steven Beaty's "The Plecoptera of North Carolina," p. 24 under "I. transmarina group."  Let's see what he says.

I. transmarina gr. -- consists of both I. davisi and I. transmarina.  Nymphs 10-14 mm; apex of lacinia narrower than base and constricted medially, with row of 5-6 stout setae below subapical tooth; pale area anterior to median ocellus enclosed (davisi) or not enclosed and open in front (transmarina); width of triangular pale spot in ocellar triangle about one half the distance between the lateral ocelli, possibly open behind (transmarina) or closed behind (davisi); longitudinal abdominal stripes subequal in width, with narrow pale borders.  (I have underlined those portions that deal specifically with I. davisi.)

Unfortunately -- or fortunately, in terms of the gene pool! -- I did not keep this nymph, so I cannot verify the size, nor can I provide a photo of the lacinia.  However, the lacinia as described does not vary a lot from that of I. dicala which you can see in yesterday's entry.  But Beaty's descriptions of the head pattern and the abdominal stripes are dead on for those of our nymph.  It's I. davisi.

There is something else that supports this conclusion.  Beaty mentions that there are pictures of the nymph and adult of I. davisi in a book by Pescador.  The book in question is: Manuel L. Pescador, Andrew K. Rasmussen, and Baron A. Richard, A Guide to the Stoneflies (Plecoptera) of Florida, which was published in 2000 by the State of Florida's Department of Environmental Protection.  There is a downloadable pdf. version of that text on line at:  On p. 77, there are pictures of the head and abdomen of I. davisi (a nymph): they are an exact match for the nymph that I found yesterday at Buck Mt. Creek.

Cool!  There is, by the way, a photo of I. transmarina available on line at ("Discover Life," Donald Chandler) if you want to compare the two.

I. davisi -- common name, "Alabama Stripetail" -- is attested in South Carolina, Florida, and Alabama: Beaty implies that it has been found in North Carolina as well.  I'm not sure if it has been found before in Virginia.

Additional photos:

Always fun to find a new species.  Now, of course, I don't understand how I confused this with Isoperla namata!

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