Saturday, June 22, 2013

Second sighting of the "uncommon" small minnow mayfly "Iswaeon anoka": a new stretch of South River

(Note: See the next entry.  This identification was wrong.)

I was hoping to find some small minnow mayflies today -- and I did -- but I didn't expect to see this one.  Iswaeon anoka.   I've seen this species only one time before -- at the Rivanna River on 6/24/12 (see the posting on 6/25).  Today I went to South River up in Greene county, but I was wading 4-5 miles downstream from the site in the mountains to which I normally go.

Beaty notes, on the genus Iswaeon, that it's "primarily a mountain taxon, usually collected in the summer": I. anoka, he adds, is "uncommon." (Beaty, "The Ephemeroptera of North Carolina," p. 10)
Let's review (citing Beaty's description):

I. anoka -- nymphs ~ 4 mm; a small, sometimes indistinct, dark dot at each tibial-femoral junction; large, a distinct dark spot laterally above the meso-and metanotal coxae; large median dark spots sometimes present on terga 2 and 6; midventral dark spots sometimes present on abdominal sterna 2-8 or 9; dorsum of abdomen often with a pale median longitudinal stripe set against a darker background, sometimes reduced to small pale spots and interrupted on middle segments; caudal filaments with dark medial band.

I should add that the defining feature of the genus Iswaeon is: "tibia narrow at base and distinctly widened medially to apically."  That we can see in the photo below.

Also clear in this photo is the small dot at the "tibial-femoral connection."   In this next photo, we can see the dark dots at the meso- and metanotal coxae.

The "large median dark spots" on terga 2 and 6 are clear on 2 on this nymph; it's a bit of a stretch to see one on 6.  That we can see in this photo on which I've also noted the pale median longitudinal stripe.

Here's a good look at the dark medial band on the tails (caudal filaments).

Very cool.  I'm so glad to find this one again.  I've been planning to look for it in the Rivanna, but the Rivanna is high and muddy, and likely to stay so for sometime to come.

This nymph was close to 5 mm.

The other small minnow mayflies this morning?  One tiny Baetis intercalaris, probably the most common small minnow we see in the summer, and a couple of Acentrella turbidas.

B. intercalaris:

A. turbida -- a real beauty:

And a second A. turbida alongside the I. anoka:


And I found quite a few Drunella cornutella spiny crawlers.  I guess there a lot more common than I had assumed.

Perlesta common stoneflies galore!


And here are the riffles in which I was looking.  Pretty spot: I'll be back.

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