Saturday, September 24, 2016

Acroneuria evoluta: the Rivanna River at Crofton

These brown stoneflies were fairly common today at the Rivanna River at Crofton.  I picked up about 10 to look at for photos, and while I can't say for sure that they were all the same species, all three that I photographed were Acroneuria evoluta.   Here's Beaty's description.  "dorsum of head with interrupted M-shaped head pattern, appearing as a transverse row of 3 light spots in front of anterior ocellus; abdomen not banded; anal gills present.  Uncommon. ... Nymphs more common during summer thru late fall." ("The Plecoptera of North Carolina," version 3.3 [2011], p. 14)  The assigned TV is 1.7.  (For more on this ID, see the entries posted on 6/12/15 and 6/20/15.)

You can see the lack of abdominal banding in the photo above: and you can almost make out the 3 dots on the top of the head.  For the anal gills, let's draw in a little bit closer.

Another example

Actually, the 3 dots on the head might show up better on this very small nymph that I found (9 mm vs. 15 mm for the nymph at the very top of the page).

And the gills,

A. evoluta can be confused with the "brown" form of A. abnormis, the nymph that we looked at last week,

and I'm pretty sure that I've seen both nymphs at this site.  Both lack banding on the abdominal terga; both are brown.  But, remember that A. abnormis "brown" nymphs lack anal gills, and there is no "M-pattern" on the top of the head, not even light dots.

The Rivanna River at Crofton is the only site where I've found this species of common stone (Perlidae).  I've not seen it at Darden Towe Park in Charlottesville -- not yet anyway -- where the brown A. abnormis is common.    Moreover, it's a species that is not attested in the state of Virginia according to "NatureServe Explore" (  I've sent them my data, but so far I've been ignored.  Alas, the fate of the amateur entomologist!

I didn't see much else on the rocks in the Rivanna this morning, though I did see a few of these nymphs, Agnetina annulipes,

and there are still some damselfly nymphs kicking around

This nymph is a "narrow-winged" damselfly -- Coenagrionidae, genus Argia.

And this is what the Rivanna looks like at Crofton.


  1. Boris Kondratieff and I are current working on updating the checklist of Virginia stoneflies and I have looked at many Acroneuria in this effort and I have not seen any A. evoluta from Virginia. Perhaps A. arenosa as you have considered? We have often collected A. abnormis and A. arenosa together. Those Acroneuria are an amazing group. Do you ever collect adults? I have found that females with eggs are best for species ID's. Of course a series of both sexes is even better.

    Chris Verdone