As I was sorting through the nymphs and larvae I found at Elk Run on Friday, I made a note that I'd better check this one with care. I suspected that it might be a Nemourid stonefly (see the entry above from 1/30/11 -- "The Super Hatches of Spring"), and with the help of the microscope I now know I was right.
This Nemourid is genus Nemoura -- not the genus we normally find. The Nemourid that shows up in large numbers in samples done in the spring is genus Amphinemura, and you may recall that it is very distinct because of its "cervical" gills. Here's a reminder.
No frilly gills sticking out from both sides of the neck on the Nemourid I found on Friday. Still my first thought when I saw the nymph at the top of the page was "It's a large winter stonefly." These stonefly families can be confused because the wingpads on mature nymphs are shaped exactly the same.
But, this stonefly lacks the "mottled" green/yellow colors on the wingpads and pronotum that we find on the large winter genus Strophopteryx; it also lacks the distinct light stripe that runs the length of the body on the large winter genus Taeniopteryx -- more importantly, it has no "coxal" gills as we would find on the latter genus.
But could it be some other genus of large winter stone, one that I haven't seen? The question for which we need an answer is -- is there some way to distinguish Nemourid stoneflies from large winter stoneflies? The answer is "Yes," and for this we must look at the tarsus. The tarsus is the third part of the leg out from the body (the order is: femur, tibia, tarsus, tarsal claw), and the tarsus divides into three segments. Here are two pictures of the tarsus of the stonefly whose identity is in question.
This is a challenge for microscope, camera, and reader...but, I hope you can see that "segment 1" is larger/longer than "segment 2". Because that's the end of the story: that means that this is a Nemourid.
On a large winter stonefly, tarsal segments 1 and 2 are equal in size.
The next question to ask is how do we know that this particular Nemourid is genus Nemoura? Well, we've already established that it's not genus Amphinemura since it lacks "cervical gills". The Nemoura identification is made based on the nature and shape of the pronotum. Let's have a look.
I'm not sure this is clear in the blog photo, but there are short, spiny hairs on the sides of the pronotum, but none on the bottom. Also, the pronotum corners are "rounded": this gives us Nemoura.
I have personally seen very few Nemoura Nemourids in our streams, and I think that they're rare: in addition to this one, I found two at Long Island Creek in Fluvanna County last winter and one in the Lynch River. Also, these seem to appear and mature earlier than the Amphinemura Nemourids that we see in the spring -- in large numbers. The two I found in Long Island Creek were found on March 12th, and they were mature. Our "frilly neck" Amphinemuras tend to mature in late April and May. One final suggestion: Nemoura Nemourids seem to prefer small, narrow streams (like Elk Run and Long Island Creek). (If someone knows that I'm wrong about that, please let me know.)
One more look at a Nemoura Nemourid, this one from Long Island Creek. And then a look at the labium of the Nemourid: note how similar this is to that of the large and small winter stoneflies (see the entry on "Looking under the hood," 2/8/11).