Thursday, September 29, 2011
Back to the Streams: the Doyles and the Moormans
Hallelujah! The sun actually shined today in central Virginia: there is hope. I should have been mowing my lawn -- but who could resist seeing what's going on in the streams. So I went off to the Doyles River to the north of White Hall and then up to the Moormans in Sugar Hollow.
In the Doyles, the dominant taxon was the common stonefly (Perlidae). The leaf packs were literally crawling with them -- I didn't see a single insect on the bottoms of rocks.
Now, at this point in the game, I hate to say that I'm stymied, in terms of genus and species, by a couple of common stoneflies -- but I am. I must have seen 20-30 Perlids in the leaf packs at the Doyles -- most of them, I think, genus Acroneuria (the characteristic brown color with the yellow "M" on the head) -- but I only photographed and preserved the two in the picture above. They had unusual colors. Let's take a closer look.
1) I think this might be an immature Acroneuria carolinensis:
This keys out as genus Acroneuria -- there are not "spinules" on the occipital ridge, and there is clearly a "fringe" of silky setae at the base of the cercal segments.
And, since the abdominal segments are "light" at the top and dark at the bottom, I'd move towards A. carolinensis. Still, I'm bothered by two things that don't seem to fit: 1) the "M" on the head, doesn't look like the "M" that we normally see -- the sides are too rectangular, and the entire thing should be anterior to the median ocellus, here it encloses the median ocellus. (???)
2) I'm also bothered by the overall color -- it's too yellow. But, that might be because it's immature; it may darken as it ages.
2) The second nymph might be genus Beloneuria:
Beloneuria nymphs key out the same as Acroneuria nymphs -- with one exception: Acroneuria nymphs have a fringe of silky setae on the basal cercal segments; Beloneuria nymphs do not. This nymph has subanal gills (Acroneuria nymphs rarely do) and whorls of spines on the cercal segments -- but no silky fringe.
So, what's my hesitation in declaring this Beloneuria? Well, I've never seen this genus before, and Stewart and Stark (Nymphs of North American Stonefly Genera, p. 326) do not list Virginia as a state where the presence of this genus has been attested.
So, for the moment I'm stymied. If I resolve my doubts, I'll let you know. The other things of interest I found in the Doyles were a couple of "tiny" crane fly larvae. At this stage in the game -- the autumn -- these larvae are not the fat, gray, chubbys, that stream monitors commonly see. Have a look.
This larva was about 5-7mm long and 2-3mm wide. Still, the main features we look for on this type of larva are already easy to see. There are well-defined "welts" on the abdomen, and we can also see the unique "spiracular plate" at the tail end.
Off to the Moormans. The dominant taxon here was the small minnow mayfly, though I also saw some netspinners and fingernet caddisflies, a few small flatheaded mayflies, snails (Pleurocerids, Physids, and Planorbids), and a few common stoneflies (genus Acroneuria). The small minnows were the same two species that I've been seeing here for the last couple of months -- Baetis intercalaris and Acentrella nadineae. I got a good "double" on two of the B. intercalaris nymphs.
The surprise of the day was this "Water Scavenger Beetle" (Hydrophilidae).
We really don't see them that often, and I'd expect them in the Rivanna -- not in the Moormans! This beetle is predaceous -- so handle with care!
Back to "work" tomorrow -- I think I'll go up to the Lynch. But I have to leave time for mowing the lawn!