Monday, May 27, 2013
Getting a start on the Small Minnow mayflies of Summer: Powells Creek
I think of summer as "Baetidae" season -- the season of the small minnow mayflies. Of the 11 small minnow species I've located so far, only 3 -- Baetis tricaudatus, Heterocloeon amplum, and Plauditus dubius -- show up in the winter, the rest are around from late spring through the fall. The "rest" are Acentrella nadineae, Acentrella turbida, Baetis flavistriga, Baetis intercalaris, Baetis pluto, Heterocloeon curiosum, Heterocloeon petersi and Iswaeon anoka. Together, they produce those wonderful "Blue-winged olive" hatches that occur on cloudy days in late summer and fall.
So far this spring, I've only seen a few small minnow nymphs, all of them B. intercalaris, one of the most common Baetids we find in our streams. Normally, I see quite a few in April and May, so where have they been? The high water might be a problem since they can't "cling" to the rocks like the flatheaded mayflies, or maybe I've been spending too much of my time focussing on the Perlodids. But today I was determined to find some small minnow nymphs, so I went to Powells Creek in Crozet where I've found a lot of them in the past.
And I found some; in fact I found three different species -- Acentrella nadineae (that's a small one, 3 mm, in the photo at the top of the page), Acentrella turbida (also small, 3 mm), and a small Baetis intercalaris nymph. So, the season begins.
1. Acentrella nadineae. I found quite a few, and they were various sizes: the smallest, 3 mm, one that was about 5 mm and one that measured just about 6. Mature nymphs run 5 - 5.8 mm. (Beaty, "The Ephemeroptera of North Carolina," p. 4.) The orange/red markings on the thorax and terga makes it easy to identify this one.
It was another morning when the sun disappeared just as I got to the stream, so my photos sure aren't the best. Here's a good A. nadineae photo that I took in the fall of last year.
2. Acentrella turbida. I found 2 or 3, all of them small. This one was 3 mm. The A. turbida thorax is broad, and when they swim around in my tray they often flip up the back of the abdomen and the tails. Not easy to photograph this one.
3. Baetis intercalaris. Only 5 mm, but they're only 5-6 mm when they're fully mature.
Note the dark bands at the base, mid-point, and tips of the cerci (the tails). They show up even better on this mature nymph that I found in September last year.
I always enjoy the small minnow season. The Rivanna River is a good place to find various species.
Just have to wait for the water levels to drop.
And yes -- they're here in big numbers: very, very noisy! Have to get some photos since I can't be sure I'll be here to see them again in 17 years. Cicadas.