Monday, July 23, 2012
Looking for Insects in Roaring Creek
I spent the weekend in Pennsylvania -- it was my wife's high school reunion -- and we visited friends who own a cottage on the banks of Roaring Creek. Roaring Creek is a medium-sized stream that flows into the Susquehanna between the towns of Catawissa (my hometown) and Danville (my wife's hometown). On Saturday morning, we had pretty good sunshine so I waded into the stream to see what I could find. For the most part, I saw the same insects I've been seeing in central VA -- flatheaded mayflies, common netspinners, a few common stoneflies, and a handful of small minnow mayflies (Baetis intercalaris and Acentrella nadineae). But there were some surprises.
The most interesting find was the common stonefly (Perlid) in the photo above. I could tell by the colors and the angular shape of the head that this was a genus or species I had not seen before. It turned out to be a new species of Paragnetina -- Paragnetina media. The only Paragnetina I've seen in our streams -- and only in the Rapidan River -- is Paragnetina immarginata, which looks like this.
You might wonder how these nymphs can be the same genus, but genus ID is based on anatomical features. In the case of the two nymphs in our photos, what they have in common are 1) an uninterrupted row of spinules across the back of the head (the occipital ridge), and 2) an absence of subanal gills. These are both obvious in close-ups of the live insect: no microscope photos are needed.
Once I knew this was a Paragnetina common stone, I turned to Steven Beaty's "The Plecoptera of North Carolina" (p. 18) to determine the species ID. In Beaty, the Perlid that I found in PA is identified as Paragnetina ichusa. He notes, however, that P. ichusa and P. media are "inseparable, and since Stewart and Stark (Nymphs of North American Stonefly Genera, p. 353) attest the existence of P. media in Pennsylvania but not P. ichusa (only in NC, SC, and TN), I've decided to use P. media for our identification. (Beaty sometimes uses P. ichusa/media for the species ID.)
For our ID, then, we must look at Beaty's description of P. ichusa.
P. ichusa [or P. media] -- nymphs 15-23 mm; head brown with small, light areas around each ocellus and tentoria; femora brown; abdomen brown sometimes with small light mesal areas on tergites 7-9; terga 4-9 without short, thick intercalary setae; anal gills absent.
Another look: this nymph was ~ 16 mm and is still immature.
Yes, the femora are brown, and yes, the abdomen is brown: on this nymph I do not see "light mesal areas on tergites 7-9." The "small, light areas around each ocellus" are very clear in this close-up of the head.
"Tentoria" normally refers to "an internal skeletal framework in the head of an insect" (online definition) -- which wouldn't be relevant here. I'll have to check with Beaty to see how he's using the word in this context. But, there are certainly a lot of "pale areas" on the head, so we ought to be covered.
Yes, anal gills are absent. Of the missing short, intercalary setae on tergites 7-9, I have not been able to get a usable picture.
I have used two other sources to confirm this identification. One is the online site Troutnut.com on which we can find photos of a P. media nymph that match the one that we've found (see the photos at http://www.troutnut.com/specimen/654); the other is the description of P. media found in the web page "Aquatic Insects of Michigan" (go to: http://insects.ummz.lsa.umich.edu/~ethanbr/aim/Keys/Plecoptera/id_pom_perlidae.html). There we read: "pronotum fringed laterally and posteriorly with short stout bristles interspersed with a few long bristles."
The short and long bristles are visible in this microscope photo of the left rear edge of the pronotum.
So, another new Perlid for me: Paragnetina media. This is a species that is attested in Virginia as well as PA, so we may run into one in the future. Tolerance value in North Carolina -- 0.2.
And now for this odd-looking Diptera larva.
This is another inhabitant of Roaring Creek in northeastern PA -- an "aquatic snipe fly" or "watersnipe fly," family: Athericidae, genus: Atherix. This is only the second time I've found one of these larvae. This is a fairly intolerant insect with a tolerance value of 2. This larva was about 8 mm.
There is good information on watersnipe flies on the website, "Digital Key to Aquatic Insects of North Dakota" (see page: http://www.waterbugkey.vcsu.edu/php/familydetail.php?idnum=7&f=Athericidae&ls=larvae). There we read as follows: "Distinguishing characteristics -- The larvae of the watersnipe flies have abdominal segments containing ventral pairs of prolegs bearing crochets and terminal processes longer than the terminal prolegs. These terminal processes have distinctive hairs on them."
There are pairs of prolegs on the first 7 abdominal segments, but only one proleg on segment 8. Those are pointed out in the following photos.
Even better, a photo in which we can see both of the prolegs.
Finally, in this photo we can see the "fringed" terminal processes and the "crochets" (velcro like hooks) on some of the prolegs.
Let me return to the "Digital Key to Aquatic Insects of North Dakota" for additional information. "Genus Atherix: There is one genus of Athericidae, Atherix, found in North Dakota (and most of North America). The larvae are predators, eating other fly and mayfly larvae. They can be found in the riffle areas of vegetation of rivers and streams. Adults do not bite humans or animals."
From the reading I've done, I can tell you that the larva in the photos above is either A. lantha or A. variegata, but that's as far as I go. I have not located a key with which I could identify this to the level of species.
I found one other insect of interest in Roaring Creek: a common netspinner. It turned out to be Hydropsyche in genus. But it's a little bit different than the Hydropsychyes that I normally see, so I'm hoping to ID this one to the level of species. I'll let you know if I'm successful.