Friday, September 30, 2011

Is the Acentrella Turbida Small Minnow Mayfly a "Bi-brooded" Species?

(If you have not already done so, be sure to read the previous entry -- "Acentrella Turbida: Our Fall Small Minnow Mayfly") before reading this one.)

The picture above was not taken this morning: it was taken on May 9th at the Rapidan River.  (Double click on the photo to have a clear view of the "long, dense setae".)

In the entry I posted this morning,  I mentioned in passing that I wasn't entirely sure that some of the small minnow mayflies I found at the end of the spring/start of the summer were not A. turbida nymphs -- the nymph that I expect to see in the fall.  Let me show you additional photos -- in addition to the one at the top of the page -- of the nymphs that I had in mind.

5/9, Rapidan River

5/9, Rapidan River

6/6 Rapidan River

6/24, Moormans River

6/24, Moormans River

Note that the nymph found on 6/24 is fully mature and ready to hatch.   Now, I know there is an A. turbida hatch in the Moormans late in the year: I found a lot of A. turbida nymphs in the Moormans last year at the end of October.  So, if the nymph in the photo above is A. turbida, there are clearly at least two generations of this particular species in at least one of our streams.

The similarity between the nymphs you see on this page and those in the previous entry, the nymphs that I found this morning, is striking.  Both have the broad thorax typical of A. turbida nymphs, and of course, both have only two tails.   Unfortunately, I did not preserve any of the nymphs pictured above, so I cannot be sure that they were A. turbida nymphs.  I know at least some of them were Acentrella -- no visible rear wing pads.  But I know I did not look for the long, dense setae on their legs: I did not expect to see A. turbida nymphs in May and June!

Knopp and Cormier -- Mayflies: An Angler's Study of Trout Water Ephemeroptera, pp. 63-64 -- put Acentrella turbida into a group about which they say, "Nymphs of this complex are generally bi-brooded, usually producing concentrated hatches in June-July and again during September-October, with occasional sporadic hatches in between."  And they add, "...under ideal water and phototropic conditions, the eastern hatches of Acentrella turbida may produce three broods with peak hatches timed to May, August, and October."

Well, well.  I've been hoping to see a "second generation" of at least one small minnow species, and maybe we've got one.  Clearly, in October and early November, I'll be looking for A. turbida nymphs in the Moormans and Rapidan Rivers.  But I'll also have to document the small minnow mayflies I find next year with greater care.

Another one: 6/15, Moormans River in Sugar Hollow

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