Tuesday, October 11, 2011

Heterocloeon amplum: Our "Winter" Small Minnow Mayflies?

I found some beautiful small minnow mayflies (Baetidae) over the winter, from mid-February through March -- and a few in early April (the photo above was taken on April 1st).  The nymphs that I found in the Doyles River, the Lynch River, and Buck Mt. Creek appeared to me to be the same species (same size, same color pattern, same morphological features).  The nymph above was found in the Doyles.  The following nymphs were found in 1) Buck Mt. Creek, and 2) the Lynch River:

If you check my spring entries, you'll see that I initially thought these were Baetis nymphs in terms of the genus.  But then I found out that we have no two-tailed Baetis nymphs in the southeast: I was told by a pro that these were either Acentrella or Heterocloeon.  Since they did not have "pro-coxal gills" -- and most species of Heterocloeon do -- in early summer I made up my mind that they must have been Acentrella.

I now have better sources available to me, and I feel strongly that our winter small minnow mayflies were Heterocoeon amplum.  Let me lay out the evidence that has moved me in this direction.

1.  My winter nymphs -- and I preserved 6 specimens, all fairly mature -- are 7-9mm long (that's fairly large for a small minnow mayfly).  No Acentrella nymph gets that big: the largest are 4-6mm.  H. amplum, on the other hand, are the largest of the Heterocloeon nymphs, measuring 7-9mm.

2. The nymphs I collected, as I noted above, did not have pro-coxal gills.  H. amplum is the only species of Heterocloeon that lacks pro-coxal gills.

3. H. amplum nymphs are largely collected in winter.

4. And, the key anatomical features that are used to identify this particular species are as follows.

a) H. amplum nymphs may or may not have hind wing pads.   If they have them, they're small.
Take a look.

This hind wing pad is long, but it's thread-like, not wide like a hind wing pad should be.

b) H. amplum nymphs have unique labial palpi: the sides are parallel.

I would say that these are.

c) The "denticles" (spine-like projections) on the tarsal claws are increasingly longer as they move towards the tip.

No doubt about it.

d) Finally, the tarsi have a wide, pale medial band, and the gill tracheation is minimal.  These features do not require us to use a microscope view.

So I think we can make a pretty good case for identifying the small minnow mayflies I found in the winter as Heterocloeon amplum.  Without any question, these were the largest small minnows that I saw all year, and, by the way, I have not seen them again.  I would have to conclude that this species only produces one brood a year.

But, let me finish with my standard caveat.  I'm an amateur entomologist, and my conclusions could be wrong.  Still, all of the evidence available to me at this moment points in this direction.   And, I think the logic rather demands it.
1) It has to be Acentrella or Heterocloeon.
2) It cannot be Acentrella since no Acentrella species is this big.
3) There is only one Heterocloeon species that does not have pro-coxal gills, but is this size = Heterocloeon amplum.  Pretty sound.

(Below -- H. amplum small minnow mayfly, March 1st, Buck Mt. Creek.)

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