Tuesday, September 13, 2011

Some Surprises at the Rapidan River

It's one of those days when I'm not really sure where to begin.  I went to the Rapidan River in search of "Strong Case-maker" caddisfly larvae (Odontoceridae) since I found them here at this time last year.  Did not find a one.  But, I had a couple of real nice surprises that made the trip very worthwhile.

Above -- a very small Perlodid stonefly, genus Isogenoides.  This is the first time I've seen this genus here in the East (I've seen it before in Montana).  Very exciting.  It seems very early to find a Perlodid stonefly, but perhaps this is a genus that hatches sometime in the winter.  I'll have to read up.  Keys to identification: 1) this one has "submental gills"; and 2) the "median ridge of the mesosternum extends anteriorly beyond the fork of the Y to the transverse ridge" (Peckarsky, Freshwater Macroinvertebrates, p. 71).  Here are the microscope pictures for confirmation.

What a treat!  Here's another look (at what I thought was just another "tiny" common stonefly!)  Beautiful colors.

The other bug that surprised me was this tiny spiny crawler nymph, genus Ephemerella.

That's the edge of a "humpless case-maker" caddisfly on the left -- to give you some idea of how small this is.  But look at the colors (click on the photo to enlarge).  They may show up better still in the microscope photo I took when I got home.

Those colors are very, very unusual, and it's very odd to see even minute spiny nymphs at this time of year.  But, let's go back and remember the spiny crawlers I found here -- in the Rapidan River -- in the winter, on January 19th of this year, to be exact.  And I continued to find this colorful species here through mid to late March.  This is apparently Ephemerella subvaria.

I would not be surprised to find out that the nymph that I found today is the same species (E. subvaria) as this beauty I found in early winter.  Again, this is something I have to research when I have time.

That's all I have for "surprises," but I did get some other nice photos of some of the insects that I normally find when I come to this stream: 1) fingernet caddisfly larva, genus Dolophilodes; 2) common stonefly, genus Paragnetina; and 3) a humpless case-maker caddis (Brachycentridae), genus Brachycentrus.



Brachycentrus Brachycentridae

The Brachycentrids I find in this river make beautiful cases -- always richly colored.  And, this Brachycentrid actually came far enough out of its case to let us see why it's a Brachycentrus.
Brachycentrus nymphs have four sclerites (brown spots) on their fleshy mesonotums.  In the photo below, the two on the side might not be easy to see; the two in the middle are very clear.  (Please click on the photo to enlarge it.)

For an overview of the insects I found today in the stream -- 1) The common stonefly was probably the dominant taxon, genus Acroneuria and genus Paragnetina, all different sizes.  2) I found the odd common netspinner and 2-3 fingernet caddis.   3) I saw a fair number of flatheaded mayflies -- Epeorus, Maccaffertium, and Heptagenia, lots of them were very tiny.  4) I only found 3-4 humpless case-makers.  5) I saw many, many tiny insects (mayfly nymphs, I think) that were so small there is no way I could pick them up with my tweezers.  And 6) in addition to the "surprise" spiny crawler, I found 2-3 small minnow mayflies, all Baetis intercalaris, as in the photo below (key = the "parentheses" marks at the top of each abdominal segment).

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