Saturday, July 7, 2012

Hooray, Hooray, Acentrella Nadineae! Another Rewarding Trip to the Rapidan River

I was thrilled to find Acentrella nadineae small minnow mayflies this morning at the Rapidan River -- but can I not, not start with this photo?!  What a stunning common stonefly -- Paragnetina immarginata!  This was 25-30 mm long (1 1/4" -- 1 1/2" long).   And I saw at least six nymphs of this size.  I also saw shucks where many had already hatched.  This one is back in the river and will get its chance very soon.  Another look.

And now for the find of the day: Acentrella nadineae.

This is a small minnow species that was first recognized as a species in 2009 in an article by McCafferty, Waltz, and Webb (see the entry I posted on 8/6/11).   They noted that it had been found in southern and eastern Ohio and in parts of North Carolina.  But I found them in a number of our streams last summer, so I've been anxious to see them again.   With the orange and red splotches on their abdomens and thoraxes, they are easily distinguished from other species of Acentrella.

More photos.

And one that was a bit more mature.

Let's do the usual rundown on species identification, quoting Steven Beaty's description ("The Ephemeroptera of North Carolina," p. 4).

A. nadineae -- large, nymphs 5 - 5.8 mm but usually larger than co-occurring A. turbida; hind wing pads strap-like and greatly reduced or absent; foretibia plus foretarsus length subequal to length of forefemur; row of long setae on dorsal margins of tibia and tarsi; gills elongate, asymmetrical and with basomedial pigmentation splotches; distinct abdominal color pattern often tinged with red.

1) The small nymph I found was about 5 mm long; the more mature one was closer to 5.5.

2) The hind wing pads were visible -- but very small.

3) In this photo you can compare the forefemur with the foretibia and tarsus and also see the long setae on the tibia, and even a little bit on the tarsus (click on the photo to enlarge).

4) Here, the shape of the gills is very clear, as is the baso-medial splotch of pigmentation.  A. nadineae gills are very distinct.

5) Finally, on the orange/red patterns on the abdominal tergites -- I think they're clear from the photos above, but just in case....

So, I have at last found the summer small minnow mayfly that I've been after.  That alone made the trip to this river worthwhile.

The dominant taxon today, by the way, was the "humpless" case-maker -- Brachycentridae.  I saw hundreds of cases on the rocks: I'm not sure I lifted a single rock without seeing at least 5-6.  Every Brachycentrid we see in the Rapidan is, in terms of the species, Brachycentrus appalachia.  The key to species ID is the five black stripes on the head (see the second photo below).

Oh, and yes, there were M. ithaca flatheaded mayflies in this stream.  The trout up here will be happy -- and so will the fly fishermen!


I'll be back to the Rapidan again soon.  It's such a pleasure looking for insects in this one.

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