Eight Miles Apart, Two Different Species: Acroneuria carolinensis, Acroneuria abnormis

In the photo above, the "common stonefly" (Perlid), Acroneuria carolinensis.  In the photo below, the "common stonefly" (Perlid), Acroneuria abnormis.

I've been itching to get up to the upper Doyles River, but I've been waiting for the water rise.  It has, after significant rain on Monday and Tuesday.  But I can't say that I found a lot of insects: my common complaint at this time of the year.  Things will pick up by the end of this month.  I did find a few "smallish" Giant stoneflies, and a few "smallish" flatheaded mayflies, along with a handful of brushlegged mayflies -- also on the small side at this time of the year.  But I did see significant numbers of common stoneflies almost all of them Acroneuria carolinesis.

So I moved further downstream, making my way back to Whitehall, where I found much the same insects, though there are no Giant stones in this part of the river.  Again, there were common stoneflies in the leaves and on the rocks, but the nymphs that I saw -- like the one in the second photo above -- were all A. abnormis.

The most common, "common stonefly" I see in our streams is Acroneuria abnormis: A. carolinensis, by contrast, isn't common at all.  It's a species I only see in very good streams, or in the best part of a stream, as I found with the Doyles River today.  Acroneuria abnormis has a tolerance value of 2.1; Acroneuria carolinensis has a tolerance value of 1.2.

How do they differ?  Well, let's begin by noting how they're the same.  What makes them Acroneuria nymphs?  This is a genus that's defined by three features: 1) there is, almost always, a pale "M" on the top of the head; 2) there is no setal row on the occiput -- i.e. at the back of the head -- rather, setae is only found behind the eyes; and 3) there is a basal fringe of "silky setae" on the cerci (tails), but no anal gills (though there are some exceptions to this).  We can note those features on both of our nymphs without recourse to microscope views.

A. carolinensis:

A. abnormis:

But how do they differ?  Well the "M" pattern on A. abnormis nymphs is sometimes "interrupted," as it is here.  But the main difference is the abdominal banding: on A. abnormis the posterior margins of the terga are light, anterior margins are dark: with A. carolinesis, just the reverse is true.  (Also note, on the A. abnormis nymph, that the dark tergal bands are irregular in shape, not uniformly "thick." This is key to the species ID.)

Both of these nymphs were still immature, they will continue to grow over the winter, hatching sometime next summer.  Still, these specimens were fairly large: the A. abnormis nymph was about 14 mm; A. carolinesis, about 17 mm.  But both were fully colored and patterned, and it was a treat to get some good photos and put them back into the stream.

A. carolinesis:

A. abnormis:

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For full descriptions of Acroneuria nymphs, be sure to look at Steven Beaty's "The Plecoptera of North Carolina," p. 14.  He gives detail on nine different species.