Friday, May 4, 2012

A Shocker and a Mystery at Buck Mt. Creek

And I do mean a "shocker"!  The flatheaded mayfly Heptagenia marginalis.  This is a insect I expect to see in the summer -- July and August -- not in May.  I saw the first one last year on July 25!  True, everything's showing up early this year because of the mild conditions we've had in the winter and spring -- but this one took me by surprise.

When I first saw it I thought it must be a genus Leucrocuta -- but the head is too small (see the entry for Tuesday).  And, the yellow on tergite 8 is characteristic of Heptagenia nymphs.  But I wasn't sure about the ID until I downloaded the pictures.  Remember that there is fibrilliform (small, feathery filaments) behind the gill on tergite 7 on Heptagenia nymphs: that fibrilliform is missing on Leucrocuta.  Look closely at the final gill on this nymph in the photo below and you can actually see the fibrilliform there -- a small dark thread.  (Click on the photo to enlarge it.)

If that doesn't work, here is a microscope view.

This is one of my favorite flatheaded mayflies, and it's nice to see them early this year.  Later, they'll look something like this.

As for the mystery...  I was hoping to see some small minnow mayflies today: our summer species (B. pluto, A. nadineae, B. intercalaris, and H. curiosum) should show up anytime now.  And I did find two small minnow mayflies -- but I'm not sure what they are!  They were very small (3-4 mm) and different colors (male and female?), and at first I thought they were Plauditus dubius nymphs.  But the abdominal colors don't match that at all.  And they are not genus Heterocloeon -- no forecoxal gills.  Acentrella turbida?  Possibly.  But I need to do more microscope work to see if I can figure this out.  In any event, here are some photos -- not the best since the nymphs were so small, and the light conditions today were less than ideal.

(Note added 5/5/12:  They were indeed Acentrella turbida.)

The other insects I saw were not unexpected.  I saw lots of Epeorus vitreus flatheaded mayflies, which are getting fairly mature, and I again ran into a number of the large spiny crawlers Drunella tuberculata.

What I did not see were the hordes of the common spiny crawler -- E. dorothea -- that I've been running into of late.  But, May is hatch time for that species, and I think the nymphs are disappearing now from our streams.

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