Thursday, November 17, 2016
Stoneflies at Buck Mt. Creek: one I was hoping to see and some that were fully expected
This is the one I was hoping to see: common stonefly (Perlidae), Acroneuria lycorias in part because I continue to work on distinguishing this species from Acroneuria carolinensis. This is a problem I've noted before. Entomologists think the two can be confused, and it's true that they look much the same. Compare our nymph from today with the A. carolinensis nymph that we noted just 10 days ago.
Pretty darn close. However, with A. lycorias nymphs "anal gills [are] usually present" (Beaty, "The Plecoptera of North Carolina," vers. 4.0, p. 38) while on A. carolinensis they're typically absent. But according to Steven Beaty, this is not a hard and fast rule. He says, "...the distribution maps for A. carolinensis and A. lycorias suggest that these two species have historically been confused with each other. There are some populations of Acroneuira carolinesis that have either sparse anal gills, anal gills only on one paraproct, or missing the gills entirely (particularly smaller specimens)." (p. 38)
Obviously, the evidence I have isn't extensive. Still, it has been consistent and I hope that others will consider the factors I've noted. What I've found is that with the nymphs that lack anal gills, there are small, dark, posteromedial extensions on the terga, and the dark spots on the femora are restricted to the distal and proximal locations. This.
Also, I've only found them in cold mountain streams (Upper Doyles River, Entry Run in Greene County, Rapidan River). By contrast, the nymphs that have anal gills 1) lack any posteromedial extensions, and 2) the spots on the femora are extensive and may even extend across the anterior edges.
This is a nymph that I have -- so far -- only found in Buck Mt. Creek (not in the mountains), but I'm sure it occurs elsewhere in other streams.
In the end, the only way to be sure that these distinctions are valid is to find some corresponding adults. Not sure that's something I'll ever do.
More pics of this nymph from this morning.
What are the stoneflies I was expecting to see? Small winters -- Allocapnia sp. (pretty sure they're all pygmaea).
Even saw one that was fully mature.
Unfortunately for this little nymph, our A. lycorias nymph saw it as well --
and inhaled it!
(Note the rear end and tails sticking out.) Alas.