Saturday, May 25, 2013

Is it a new Epeorus flatheaded mayfly -- "Epeorus fragilis"?

I went to South River this morning where -- you may recall -- I find lots of Epeorus pleuralis flatheaded mayflies every winter: they quite literally cover the bottoms of rocks.  E. pleuralis nymphs ought to be pretty well gone, so I was expecting to see their summer replacements, Epeorus vitreus.   I did see some, but none that would make very good photos.  But I continued to look.

Then I found the nymph in the photo above, which just looked a little bit odd: for one thing, the color didn't seem to be right for either E. pleuralis or E. vitreus, both of which tend to be brown or olive brown.  And on this nymph, terga 4-6 and 10 are clearly paler than the rest.

Remember that E. vitreus nymphs have four obvious pale spots on the front edge of the head.  They're easy to see in this photo.

Those are missing on Epeorus pleuralis, but on Epeorus pleuralis, there is a faint "V" at the front middle edge of the head.

But note that with both of these species, the space between the eyes at the back of the head is quite wide: i.e. the eyes hide very little of the posterior edge of the head.

Now have another look at our nymph.

And let's take a closer look at that head.

The eyes are much closer together at the rear of the head; there is a very distinct "V" at the front of the head; and the edge of the head is largely pale.

Let's see what Steven Beaty says about Epeorus fragilis (which is part of the E. pleuralis group).

(E. fragilis) -- mature nymphs < 6.7 mm; lateral margins of head capsule with abrupt transition near outer anterior corners of eyes, leaving eyes to hide much of the posterolateral margin of the head; ratio of head width to distance between antennal pedicels 2.11 - 2.45 (median 2.21); head with anterior margin mostly pale with a distinct "V" medially; abdominal terga 3-7 without small, paired, dark submedial spots.  Recorded from Deep Creek, GSMNP (DeWalt 2004).  ("The Ephemeroptera of North Carolina," p. 17)

I think the head of our nymph is a pretty good fit for that description.  What about the ratio of head width to the space between the antennae?  The ratio I found was 2.2.  That's right on the money.  And, there are no submedial spots on the terga.  One other thing, remember the size.  The nymph in our photos was 6 mm.  Both E. pleuralis and E. vitreus tend to be larger than that.  And, it's not that we're looking at an immature nymph: notice that the wing pads are already quite long.

So I wonder if I've found something new.  I'll want to run this one by Steven Beaty, and he might point something out that I'm missing.  Maybe it's just a "regular" E. vitreus or E. pleuralis.  But I like E. fragilis for the ID at the moment.   This is one on which I'll continue to work.

Other photos:

1. A surprise: Isoperla nr. holochlora.  This differs from I. holochlora -- which is very common right now -- in that the pale area at the front of the head does not fully blend into the yellow line at the front of the head, and the abdominal "stripes" on I. nr. holochlora are not easy to see; they're indistinct.

2. I found two fully mature Peltoperlids (Roach-like stoneflies): deep, rich colors and black-tipped wing pads.  Beautiful!

3. And I saw quite a few Remenus, Perlodid stoneflies.  This was the largest one of the group.


But the question of the day is -- is this Epeorus fragilis?


Note:  I'm now pretty sure that this is, indeed, Epeorus fragilis.  Note Donald Chandler's photo of Epeorus fragilis at:  It's a match.  Common name of adult: Pale evening olive.

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