Wednesday, May 25, 2016

And it's a new Rhithrogena at the Rapidan River. Species? Could be R. manifesta

Maybe the most colorful mayfly nymph that I've ever seen.  Yellows, browns, oranges -- just spectacular.  I knew when I was taking these photos that this was something new, and I suspected a flatheaded mayfly, but I wasn't sure of the species until I had it under my scope.  Rhithrogena.  "Abdominal gills 1-7 enlarged and meet ventrally forming a ventral disk; three caudal filaments; color variable on mid-sized specimens." (Beaty, "The Ephemeroptera of North Carolina," p. 22)

Unfortunately this is one of the genera where Beaty urges caution, noting "A fair amount of color variation and overlap of characters make Rhithrogena species determination difficult.  Leave small, immature specimens at genus."  Were I to wager a guess, I'd go with R. manifesta: "nymphs 7 mm; abdomen dark brown with tergites 8-9 bright yellow; sternites 2-8 with dark median rectangular patch containing a narrow, dark, transverse line near posterior margin. ... Recorded from SC, TN, and VA."  This nymph was 6-7 mm, and I could be persuaded that those are "dark median rectangular patches" on sterna 2-8.  But we'd better not jump on that ID since this nymph is still immature.  (Be in touch if I hear something from Beaty.)

Gorgeous.  Without any question, one of the most colorful insects I've found.

The Rapidan is still high and fast after the 2-3 inches of rain we had over the weekend.  Still, I found lots of insects.  One other beauty, a fairly mature common stonefly Agnetina capitata.  I'd guess that this one will be hatching come July.

But what a treat finding that Rhithrogena.

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