Thursday, June 26, 2014

A real mix at the Rapidan River today -- but a beautiful Stenacron interpunctatum

It's the flatheaded mayfly that I don't see all that often: today it's the first insect I saw.  The fleatheaded mayfly, Stenacron interpunctatum.  On the bottom of a rock-- like almost all flatheaded mayflies -- close to shore.  The Rapidan's still fast and high, so I had to stick to the edge of the current.

This type of flathead is easy to spot because the body is long and thin.  Beaty describes this species this way: "nymphs 8-11 mm; 7-10 spines on maxillary crown; white streaks, often in H-pattern on tergites 8-9; caudal filaments with alternating banding pattern.  A spring and summer species.  The most common and tolerant Stenacron species in NC" ("The Ephemeroptera of North Carolina," p. 22). This nymph was 10 mm.  The white streaks on the abdomen are obvious.  We can see the "banding" on the caudal filaments (tails) by looking closer as well as shallow "H" on tergite 9.

I wish this nymph had kept all 6 of its legs, but if you look twice at a flathead, it sheds legs and gills!


On the very same rock, I found one of the things I was expecting to see -- the cased caddis, Pycnopsyche scabripennis.

The Rapidan always has them at this time of year, and this case construction is common -- kind of resembles a hot dog that's covered with ketchup and relish.  These are big larvae.  This one was 22 mm, the case just a little bit longer.  I found three of these larvae this morning, all with the very same case, though one had added a long stick to one side as an extension.

But there were other case-makers around.  I also found the humpless casemaker, Brachycentrus appalachia, and the strong casemaker, Psilotreta labida.  I was expecting to see lots of B. appalachia larvae -- but I only found one, and it was still very small.

And at long last, I'm starting to see some of our "summer" small minnow mayflies.  Today it was Baetis pluto (female)...

and Baetis intercalaris (male), the one with the parentheses marks -- ( ) -- on the terga.

The B. pluto nymph was fully mature -- ready to pop -- as we can see from the dark color and the black wing pads.  Normally, tergite 5 is very pale compared to the rest.  But there are other features that give it away, such as the dark, medial banding on the caudal filaments (tails), and the length of the middle tail ("3-4 to subequal to lateral filaments").  (See Beaty, "The Ephemeroptera of North Carolina," p. 6)


Still tough to find good streams to explore.  I count on the Rapidan and the Rivanna for interesting insects in the summer.  The Rivanna remains unwadeable, so I may be heading to Madison county a lot.


Oh.  A Perlesta "common stonefly," of course.

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