Friday, September 30, 2011

Acentrella turbida: Our "Fall" Small Minnow Mayfly

This is another "Fall" insect I was hoping to find, and sure enough there they were in the Lynch River this morning.  Small minnow mayfly (Baetidae) -- Acentrella turbida.   I have only seen this small minnow species in three of our streams -- the Lynch River, the Doyles River, and the Moormans -- and I've only seen it in the fall.  While I can't say for sure that they aren't around any other time of the year (I'm suspicious of a couple of nymphs I found in early summer in the Rapidan and the Moormans), when I want to see them, I look in September, October, and early November.  Of the nymphs that I found this morning, one was already mature -- dark in color with long, black wing pads.

How do we know these small minnow nymphs were Acentrella turbida in terms of genus and species?
Well, they have only two tails, so they're either Acentrella or Heterocloeon in terms of the genus.  But, most species of Heterocloeon have visible rear wing pads and forecoxal gills.  These nymphs do not have forecoxal gills, and hind wing pads are absent.

There is one species of Heterocloeon (Heterocloeon amplum) that does not have the forecoxal gills and hind wing pads -- but it's a fairly large nymph, 7-9mm.  The two nymphs pictured above were 4mm and 5mm, exactly what they should be if they're A. turbida nymphs.

We have Acentrella.  And now for the species.  A. turbida nymphs have two distinct features: 1) the thorax is broad, wide.  Clearly the case with these nymphs.  When you see them swimming around in a tray, they lack the "streamlined" look we expect to see with a small minnow mayfly.  They can be, and have been, confused with flatheaded mayflies.  2) The other feature requires a microscope photo.  The leg segments -- femora, tibiae, and tarsi -- are covered with long dense setae (hairs).  Have a look.

This dense setae also shows up in a photo I took in late October last year.  (Note the very broad thorax.)

So there we have it: Acentrella turbida.  I'll turn my attention to finding some small square-gill mayflies (Caenidae) and one or two "rolled-winged" stoneflies (Leuctridae) which are also around in the Fall.

Some more views of the A. turbida nymphs from today.

Oh.  The tolerance value of A. turbida is 2.0 (NCDWQ): look for them in "quality" streams.

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