Saturday, November 19, 2016
The Moormans River begins at the base of the dam on the Sugar Hollow Reservoir. Flowing into the lake are the North and South Forks. The North Fork is a boulder-strewn, cold water stream that's popular with fly fishermen, and in it we find high quality insects. Took these photos this morning.
1. Common stonefly, Acroneuria carolinensis (pictured above)
2. Common stonefly, Acroneuria abnormis
3. The "strong casemaker" (Odontoceridae), Psilotreta labida
4. And the giant stonefly, Pteronarcys biloba, tolerance value -- 0.0
Oh, and clinging to our giant stonefly, the large winter stonefly, Taeniopteryx burksi
Friday, November 18, 2016
In reviewing additional photos of Acroneuria carolinensis last night, I noticed two things that differ from the points that I made yesterday. 1) On some specimens, the dark areas on the femora look just the same as those on Acroneuria lycorias. I.e., they're extensive and extend from the proximal to the distal ends on the anterior edge. And 2) the dark dots on the terga are anteromedial, not posteromedial, and reach back into the pale anterior band.
Thursday, November 17, 2016
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.
Sunday, November 13, 2016
Yesterday, I hit the Rivanna at Crofton: today, it was a small stream in Sugar Hollow. And, at both streams I found much the same thing: common stoneflies, Acroneuria abnormis in both cases, and flatheaded mayflies -- but not the same species. In the photo above, the "brown" form on A. abnormis. So, so broad, a very unusual shape. But it made for some very good photos.
And the flatheads I found in the Rivanna were -- like those in Buck Mt. Creek -- Maccaffertium modestum.
Different habitat, different results. In Sugar Hollow today, it was the "patterned" form of A. abnormis.
And the flatheads out there were Maccaffertium pudicum, a species with a much different tolerance value (2.1 vs. the 5.7 of M. modestum).
And there were Goera larvae (weighted case-makers) in our small mountain stream.
But the story at the moment seems to be Acroneuria stones and Maccaffertium flatheads. Things will be changing real soon.
Monday, November 7, 2016
I've been told by the experts that there are streams in Virginia where the "patterned" and "brown" forms of Acroneuria abnormis co-occur (see the entry of 9/23) -- so I'm looking. This morning my friends and I checked out the Moormans in Sugar Hollow but no luck. Still, I found two beautiful Perlids, A. carolinensis (in the photo above) and A. abnormis. I thought I'd post a few photos and note how we tell them apart.
In most cases, as we see here, color is good clue: carolinensis nymphs are lighter in color, more yellow. But there are two anatomical features that seal the deal. 1) The tergal banding is reversed. On carolinensis nymphs the posterior margins are dark, on abnormis nymphs they're light. And 2) there are dark bars on the leading edges of the femora on abnormis: the femora on carolinensis are entirely pale save for dark spots both distal and proximal.
The carolinensis nymph flipped on its back for one of my photos and gave me a good look at its terminalia. The size -- thickness -- of the terminalia is the thing we use to determine the gender. Those of the female are larger than those of the male. I'd guess that this was a male -- but don't hold me to that! (For an illustration of the terminalia of Acroneuria nymphs, see Stewart and Stark, Nymphs of North American Stonefly Genera, p. 315.)
Friday, November 4, 2016
Not really the stonefly I was expecting to see this morning. While this species is common in the Rivanna at this time of year (see the entry from 9/24), this is only the second (third?) time that I've seen it at Buck Mt. Creek. Agnetina annulipes, the Southern Stone.
The key features are easy to see in this photo. Agnetina nymphs have a complete setal row on the occiput, and anal gills are present. And on annulipes nymphs -- vs. what we find on flavescens and capitata -- tergite 10 is completely dark.
It's one of the smaller stonefly nymphs that I see. This one measured just a little over 5 mm. Not sure how much it will grow as it matures.
Another surprise this morning -- and like A. annulipes, a nymph that's fairly common in the Rivanna but uncommon here -- the giant stonefly, Pteronarcys dorsata. This one measured 21 mm.
Distinguishing features? The lateral corners of the pronotum are "produced," those in front almost hook-like. Also distinctive are the "dots" on the abdominal terga. While only 4 rows are visible in this photo, there were actually 5.
P. dorsata, you may recall, is the most tolerant of the "giant" stoneflies with a TV of 2.4. (For more on P. dorsata, see Steve Beaty's "The Plecoptera of North Carolina," vers. 4.0, p. 77).
And while we're on "tolerant" insects, the most common insect I found today was the flatheaded mayfly, Maccaffertium modestum. (TV, 5.7). It may be the only Maccaffertium species that I see in this stream -- but I'll have to check that to be sure. Here are photos of three of the nymphs that I found.
There are two different patterns -- both dorsal and ventral -- on nymphs of this species. On this type, the armature (dark bands) on the femora is very pronounced while ventrally, we find "anteromedial bars" on segments 8 and 9, that on 9 in the form of an inverted "U". (See Beaty, "The Ephemeroptera of North Carolina," vers. 4.0, p. 35, also Bednarik and McCafferty's "Biosystematic Revision of the Genus Stenonema," Canadian Bulletin of Fisheries and Aquatic Sciences, 201, 1979), p. 68 figure 67) The venter looks like this.
I was expecting to see lots of small winter stoneflies in the leafpacks today -- and I did find a few. Unfortunately, when I went to take photos, they had disappeared from my bowl. Alas!