Thursday, August 15, 2013

An unusual find at South River

I wasn't sure where to go this morning since the Rivanna, once again, is high and muddy from recent rains.  But I thought I'd see what I could find in South River up in Greene County even though there's not much going on in the small mountain streams in the summer: the wonderful insects that we find in those streams in the winter and spring are very small at the moment, many of them still calling the substrate home.

For the most part, I found what I expected to find: lots of small Peltoperlids (Roach-like stoneflies); a number of "tiny" Giants (P. proteus); the odd Epeorus vitreus flatheaded mayfly; and common netspinners -- Ceratopsyche sparna and Ceratopsyche alhedra.  But the very first insect I saw was this Brush-legged mayfly, Isonychia sp.  Why is it unusual?  It's fully mature: in fact, I think it's in the process of hatching!

Evidence.  1) It was lethargic and easy to pick up with my tweezers.  Normally, brushlegged mayflies flip and squirm and bounce off the rocks when you try to get near them.  2) The thorax was bulging, as it would be if the wings were about to emerge.  And 3) the eyes seemed a bit glassy, something I've noticed before on nymphs in the process of hatching, and if you look closely, I think you can see that the thorax is, in fact, splitting open.

One other feature -- in this side view, you can see that the wing pads themselves are pushing up from the body.

Very cool.   Brushlegged mayflies normally hatch at this time of year -- late summer -- and I've been seeing "shucks" on the rocks in the Rapidan River.


A few other photos.

1. Common netspinner, Ceratopsyche sparna. (Bronze head with two anterolateral pale spots.)

2. Common netspinner, Ceratopsyche alhedra. (Head is totally black.)

3. And a common stonefly (Perlid), Acroneuria carolinesis.  (Note that the tergites are banded, light at the front and dark at the back.)


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