Thursday, April 12, 2012

Isoperla nr. holochlora ("nearly" holochlora): the Rapidan River, Where Else?!

This is the insect I was hoping to see today at the Rapidan River, and I found it as I was thinking about heading home.  Perlodid stonefly, Isoperla nr. holochlora (going by Steven Beaty), which means it's similar to Isoperla holochlora, but not quite the same.  How do they differ? The main difference is that on I. nr. holochlora the large yellow spot in the middle/front of the head comes to a point anteriorly but does not merge with the yellow line crossing the labrum; on I. holochora nymphs it does.

Isoperla holochlora:

Isoperla nr. holochlora:

You might also notice that the three abdominal stripes that ID Isoperla are easier to see on I. holochlora than they are on I. nr. holochlora.  But, they were very easy to see on this immature I. nr. holochlora that I found today in the same set of riffles.

Isoperla nr. holochlora is a Perlodid stonefly that I've only found in the Rapidan River.  Not only that, I've only found it in what I call "location 2," a set of riffles that I explore that are .3 miles downstream from where the Rapidan flows out of the National Park.  I found this "teenager" last year on May 4th in this same set of riffles.

Isoperla holochora has a tolerance value of 0.7; Isoperla nr. holochlora is 0.0.

I found a wealth of insects today -- as I always do when I go to this stream -- including a lot of Green stoneflies (Chloroperlids) that are now maturing into full hatching colors.  I took photos of two of them.
As you can see, the second is a bit further along than the first given the black "U" shape that has appeared where the wing pads diverge.



Beautiful insects.  And there were more...

1. A very colorful brushlegged mayfly (Isonychia bicolor): these mayflies mature in mid summer.

2. Spiny crawler mayflies -- all E. invaria.  The stream was loaded with them.  One of the features we use to key out this species is -- quoting Beaty ("The Ephemeroptera of North Carolina, p. 27) -- the "pale transverse stripe between [the] eyes, [which] may be interrupted medially."  I've pointed that out in the first picture.

3. Perlodid stonefly, Isoperla namata.  There were still a lot of these nymphs in the leaf packs -- but I also saw a lot of adults flying around.

4. Common stonefly, Paragnetina immarginata.  Since this is the only place that I find this particular species, I always like to get one or two photos.

5. And a small minnow mayfly, Acentrella turbida (male).

I also saw large numbers of Giant stoneflies (next year's class), Epeorus pleuralis flatheaded mayflies, Roach-like stoneflies (Peltoperlids), and freeliving caddisfly larvae (R. fuscula).

(Below: looking downstream at "location 2" on the Rapidan River.)

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