Thursday, February 3, 2011

The "Ameletid Minnow" Mayfly: family: Ameletidae

This is a mayfly I was not sure I'd find before I finished this blog -- but I lucked out and found two this morning.  With a tolerance value of "1," it finds the clean, cold water that it desires in a limited number of streams that we sample.  StreamWatch data reports Ameletids in its Albemarle county reference stream (it's a tiny stream that comes out of the mountains), in the Doyles River where it flows out of the National Park, and -- perhaps surpisingly -- in the Whippoorwill Branch of the Mechums River.  That's it.

The Ameletid mayfly is easily confused with the brushlegged mayfly (family: Isonychiidae) when we see it on rocks and on nets.  They have a similar shape, and they both flop around like fish pulled out of the water when they're exposed to the air.  They're also both very fast swimmers.  The Ameletid, like the brushlegged, has brushy tails that it can pull together and use like a paddle, and it's common to see a dark band across the tails in both of the nymphs.

But up close, they look quite a bit different.  Let's look closely again at the brushlegged mayfly.

Let's focus on three distinct features.  1) On the brushlegged mayfly, the abdominal gills set up horizontally when the nymph is at rest, on the Ameletid mayfly, they point vertically up and down.  2)
The antennae on the brushlegged mayfly are about twice as long as the head is wide; on the Ameletid mayfly, the antennae are less than 1/2 the width of the head (this may show up better in the photo below.  And 3) -- and most importantly -- the Ameletid nymph has no hair on its forelegs: the legs are not "brushy".

This is a nymph that we find in the streams in the winter: January, February, and March: it hatches in early spring.  When it's fully mature (this one isn't), it is a beautiful, beautiful creature.  Mature colors are a mix of oranges and tans and browns.  If you have a copy of J. Reese Voshell's A Guide to Common Freshwater Invertebrates of North America, take a look at p. 145.  This illustration is right on the mark.

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