Monday, May 28, 2012

And the First Small Minnow Mayfly to Show Up in Large Numbers Is....Baetis intercalaris

Well, at least at Buck Mt. Creek.  I went looking for small minnow mayflies this morning, and I found them, and I found a fair number.  They were all the same species, Baetis intercalaris.  This is a small minnow species I saw in a lot of our streams all summer last year, and well into the fall.  It's a species that's easy for us to identify right on the spot -- with some form of magnification -- because of it has two features that are easy to see: 1) on just about every tergite there are pale, submedian "parenthese marks,"
( ), and 2) the banding on the tails is unique.  There are three gray bands on the tails, the first right at the base of the cerci.  (Click to enlarge)

This is a live photo, seen through a macro lens.  Needless to say, the parentheses marks show up a lot better in a microscope view.

Baetis intercalaris is a tolerant insect, with a TV of 5.0.  It is also commonly found in our streams: Beaty calls it "ubiquitous" (Steven Beaty, "The Ephemeroptera of North Carolina," p. 6).

Since it is so common, I decided to see what Knopp and Cormier (Mayflies, pp. 46-47) had to say about this "Blue-winged Olive" nymph (to fly fishermen).   I was a little surprised.

"Baetis tricaudatus of the first mayflies to hatch across the East-Midwest region and usually appears during early April, or when water temperatures have climbed to the low 40's.  B. intercalaris is the second East-Midwest species of this complex to emerge, doing so about two to three weeks after the first of the B. tricaudatus hatches."

Hmm....  So, why am I just starting to find them?  And, the nymphs that I'm finding are small.  I find this perplexing.  They give a hatch range of mid-March to mid-June for this species.  Yet, most of the mature nymphs I saw last year, I saw in August and September -- like this one, I found on September the 12th.

However,  I do have to note that the very first B. intercalaris nymph that I found last year, I found on May 11th (Powells Creek), and it was fairly mature  This one.

I guess it's possible that B. intercalaris is bi-brooded: hatching in spring, laying their eggs, which as nymphs grow up over the summer, and then those nymphs hatch in late summer and fall.  If I find out more about this, I'll let you know.  (But Knopp and Cormier give no indication that that is so.)

I found one other insect of interest this morning, a flatheaded mayfly which I mistook -- as I've done before -- for a Heptagenia marginalis.  This is the nymph in question.

Since I was not really certain of my ID, I preserved this nymph and took a close look at home with my microscope.  And what did it turn out to be?  Cinygmula subaequalis, the same flatheaded species that I found, for the first time, at South river on 4/27!  What gives this away, you may recall, is the fact that the maxillary palps are actually visible sticking out to the sides of the head when you look at the head from above.  Like this:

(Sorry that I could not get a good shot showing the palps on both sides of the head.  Better photo below taken on 4/27.)


Other photos from today's trip.

1) Flatheaded mayfly, Epeorus vitreus -- with a head that was surprisingly narrow, or a thorax that was abnormally wide.

And 2) a Flatheaded mayfly, genus Leucrocuta, a very small one.  (We might think of this as the "fatheaded" flathead!)


But best to finish with some more shots of one of the Baetis intercalaris nymphs.

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