Thursday, September 5, 2013

Buck Mt. Creek Continues to Amaze

My destination this morning was the Rivanna River at Crofton where I found some very cool insects in September last year.  I went to the river, and I looked for insects, but I found very little.  The Rivanna is still abnormally high; the boulders that provide me with "tables" on which I can scrutinze rocks are all underwater.

But the morning sun was too good to waste, so I headed off to Buck Mt. Creek, my expectations low I might add.  Surprise, surprise -- I found some very nice insects including this young Giant stonefly, Pteronarcys biloba, tolerance value, 0.0.  If you look back to my posting of 6/20, you'll see another Giant that I found in this creek, one that appears to have been Pteronarcys dorsata (on which see the posting of 8/16).  Two Giant species in the same stream?  That's the way it looks at the moment.  Note the long tails, the "hooks" forming on the anterior corners of the pronotum, and, critically, the sharp lateral projections on the abdominal segments.

Now I'm intrigued.  Do P. biloba and P. dorsata co-exist?  You can be sure that I'll be carefully checking every Giant I see in BMC from now on.

P. dorsata? on 6/20.


But another pleasant surprise.  I found three different species of flatheaded mayflies in the short while I was wading the stream: Maccaffertium ithaca (the "Light Cahill"), Heptagenia marginalis, and a very small Epeorus vitreus (so there are still some around).

1. M. ithaca.  I didn't take diagnostic photos -- though I saw what I needed to see in my microscope study.  "Sterna 5-6 or 7-8 with transverse sternal bands with anterolateral extensions" [and] "no denticles on tarsal claws."  (Steven Beaty, "The Ephemeroptera of North Carolina," p. 19)  Note that the thorax seems to be splitting in preparation for hatching.

2. Heptagenia marginalis.  This is one that we normally see in late summer -- August and September -- though this is a small one that will probably hatch in October.  Remember that this genus differs from Leucrocuta in having a fibrilliform portion on gill 7 (Leucrocuta does not), and that the species ID is determined by the oblique black lines on the sides of the abdominal segments.  (Beaty, "The Ephemeroptera of North Carolina," p. 18)

3. And the very small Epeorus vitreus, tiny, tiny wing pads.


One more surprise -- though I've seen a few of them in here before: common netspinner, Macrostemum.   You'll recall that these larvae are common in the North Fork of the Rivanna below the bridge at Advance Mills.   In some of my photos, the "carina," the rim that forms the edge of the head, is easy to see.


It was a quick trip to Buck Mt. Creek, but one that I'm glad that I made.

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