Tuesday, August 9, 2011

Netspinners and Small Minnow Mayflies: Back to the Rivanna

I was busy all morning running some errands.  But I was close to Darden Towe Park in C'ville and stopped by to see what I could find in the Rivanna River.  The entry title is right -- "netspinners" and small minnow mayflies, all of the small minnows Heterocloeon curiosum like the one in the photo above.  There were a lot of them crawling around on the rocks and algae today.  Quite a few were "mature," like the one in this picture, but some were still young with very small wing pads.  The color difference between young and mature is quite striking.  Have a look.



But through the microscope, both immature and mature reveal the defining characteristics of this particular species: grey pigment in the center of each gill, and "procoxal gills."


I picked up over a dozen of these H. curiosums, but that doesn't compare to the hundreds of netspinners I saw.  There are so many of them crawling around in the algae at the moment that it really gives me the creeps!  And in the Rivanna, they're to a one, a drab olive in color -- just not very appealing.

Still, there were other things to be found.  I picked up another "Trico," a "Little Stout Crawler" mayfly.  It was not the prettiest Trico that I've ever seen -- it was filthy!  They seem to like to root around in the silt on the rocks.

Still, it was easy to pick out its "triangular operculate gills," the large "gill plate" that covers the actual functioning gills.

I also found, to my surprise, several Heptagenia flatheaded mayflies in addition to the expected lot of Maccaffertiums.   So I've now found this genus in the Moormans, Buck Mt. Creek, and now the Rivanna itself.   The photo is of a small one, but it's fully intact: it has all of its legs and its tails!  (If you've never picked up a flatheaded mayfly with tweezers, you won't know how hard it is to keep all of their body parts on!)   Two photos: in the second one, the Heptagenia nymph is joined by an adult Riffle beetle.

Finally, I got a good shot of a young common stonefly today, genus Acroneuria.  I'm seeing a lot of them now, just about everywhere that I go.  This is the genus that will have the yellow "M" on its head when it's mature.  If you look closely (enlarge the photo), you can start to see it already.

Next spring this stonefly will look more like this.

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