Tuesday, May 10, 2011

Another Return to Buck Mt. Creek

I found some very unusual things today at Buck Mt. Creek -- and we can begin with this really neat Rhithrogena (a genus of flatheaded mayfly).  I first reported on finding this genus at Buck Mt. Creek in my post of 5/1, noting that the color is normally yellow.  But look at this!  The head is white, as are two segments at the end of the abdomen.  I've never seen anything like it.  Is this a matter of species?  Or is it just the coloring of one individual nymph?  Questions I can't really answer.

On 4/25 I reported on another find I had made at this stream: a genus Drunella spiny crawler mayfly.
I've not seen them in a lot of our streams, and the only time I saw a good number at the same time was in a StreamWatch sample we took at the Moormans River last spring.  Sure enough, the first three insects I found on the rocks today were all Drunellas!  One was a real beauty.  (Remember that these differ from the spinys we normally see by having "muscular" fore legs with "tubercles" on them.)

A second Drunella nymph was unusually large -- but the colors were drab and it was covered with silt.  I took a few pictures of it anyway, not intending to use them.  When I looked at the photos at home I discovered that this one has a really odd looking head: the corners of the head are pointed like horns.  Take a look.

Wierd, wierd!  I'm going to send this photo to the folks at Bugguide.net to see if they know the species.

Now, for the main reason I went back to Buck Mt. Creek so early (I was just here on May 1).  In my post of May 1, I noted finding some small, small minnow mayflies of whose genus ID I was not really sure.  I was split between Baetis and Acentrella.  Did they or did they not have metathoracic wing pads (they show on Baetis nymphs but not Acentrella.)  So I was hunting today for some small minnows.  I found four, took some live pictures, and examined them closely when I got home.  Acentrella.  No sign of those wing pads at all.  And, at the stream I was already pretty sure of this call given the shape of the nymphs.  They had very "broad shoulders", i.e. the thorax is wider than anything I've seen on a Baetis.  Here's one of my shots.

This is very interesting.  In the winter, every small minnow I found in this stream was genus Baetis.  Then on 4/25, I reported on finding a Heterocloeon.  Now we have Acentrella.    Could it be that the winter small minnows are always Baetis, while in the spring we find both Heterocloeon and Acentrella replacing the Baetis?  Stay tuned.  This is a matter that I will explore as the spring and summer continue.

But I was in for yet another surprise in my visit this morning.   I picked up a stonefly that I thought was another Perlodid, probably a species of Isoperla.  But I couldn't be sure since it was small and the colors were poorly developed.  I preserved it and brought it home.  I readily saw that I was wrong.  It was a very small Perlid (common stonefly), and the genus is one that I've seen just a few times -- genus Perlesta.  Here's a live shot that I took at the stream of the nymph in question.

It's easy to see from the photo that this is a Perlid -- it has "hairy armpits" (Rose Brown's expression), frilly gills that stick out from the body behind each pair of legs.   But how do we determine the genus?
The first thing we have to look at is the back of the head.

Now, here are out options (I'll be quoting from Peckarsky, et.al., Freshwater Macroinvertebrates, p. 69):

1) Occipital ridge present; a closely set regular row of spinules completely across back of head inserted on occipital ridge...
2) Occipital ridge absent; spinules on back of head present mainly near eyes, or arranged in a transverse row of varying completeness, but always at least a little wavy or irregular...

I've highlighted my choice.  That eventually takes us to another couplet:

1) Branched subanal gills present; head patterned....Perlesta
2) Branched subanal gills absent; head monochromatic brown....Attaneuria

The head is clearly patterned, and, yes, it does have subanal gills.

It's genus Perlesta.  It's worth noting that this is the first time I've found tiny, immature Perlids since I started this project last fall.  The eggs by terrestrial Perlids last fall probably wintered over as tiny nymphs in the substrate and are just starting to grow and mature in the warmer waters of spring.  Perlids typically hatch and mate in July and August.

I took two other good photos that are worth showing in closing.  The first is a Perlodid stonefly, genus Isoperla (possibly Isoperla montana), the second is a tiny flatheaded mayfly, genus Leucrocuta.

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