Tuesday, October 25, 2011

Now Playing in a Stream Near You: "Small Winter Stoneflies" (Capniidae)!

I spent the morning at the Doyles River in Doylesville, where I found the first -- for me at least -- small winter stoneflies of the season.  At this point they are immature and very tiny -- 3-4mm long, .5mm wide -- but we will be able to watch them grow and mature in November and December: most will hatch and be gone by the end of that month.  Before they hatch, they'll look something like this.  (Picture taken 12/4/2010.)

If you're out sampling right now, there are a couple of things that will help you identify this stonefly to family level -- though you should never make that call in the field unless you really know what you're doing.  1) The color at this time of year is pale yellow -- small ones are almost translucent.  2) With the long thin shape, they are "thread-like," and in some ways resemble large midges.  However, they never wiggle and squirm on the net or in a tray to the degree that we see with midges.  And 3) as is the case in the photo above, small winter stoneflies tend to "float" on top of the water in trays, not "sink."  Another photo in which the nymph is floating on top of the water.

But to be "certain" of your identification, you have to look at these in the lab.  There are three features that you want to find.  1) "Look under the hood" at the labium.  There should be no "notch" between the glossae.  Even with these tiny nymphs, we can clearly see that the glossae (g) directly parallel one another.

2) And this is a tough one, running along the outer edge of abdominal segments 1-9 is a dark line -- a "fold." This is called the "pleural fold" or "ventrolateral fold," or "membranous fold," and that it extends from segments 1-9 helps us to distinguish Capniid nymphs from Leuctrid (Rolled-winged) nymphs, where the pleural fold only runs from segments 1-4.

3) The hind wing pads on Capniid nymphs are stubby and short (truncated) while the front wing pads are slender and long.  On more mature nymphs, by the way, this is a feature you can see with the naked eye -- or at least with a loupe.

On a mature nymph they look like this:

So there you have it, we're off and running -- the winter stoneflies are here!  And here's what they look like when they're really tiny.

But I found a second sign that the winter season is here.  I found two young Perlodid stoneflies -- also light in color.

Can we determine genus identification?  We can certainly narrow it down.  They're either Isoperlas or Clioperlas, and we know that because, 1) the tops of the mesosternal "Y" ridge point up, they don't go up and then point back down.

And 2) the lacinia of the maxillae have two large spines at the top with additional spines/hairs to follow, and they're rounded from the top to the base.

That brings us to Isoperla or Clioperla, and the key thing then becomes colors and patterns.  It's simply too early to "see" what we have to see to decide between those two genera.  But my money's on Clioperla.  And that's because we see Clioperla much sooner than we see Isoperla, and the key feature for Clioperla ID is that most of the head, from the eyes up to the dark tip at the end, is a light colored yellow.  Like this.

I think that color pattern is already starting to show on the two nymphs that I found today.

I found a lot of flatheaded mayflies today, and a fair number of crane flies.  But the last thing of interest that I found was again some small minnow mayflies.  I found  one small Acentrella turbida nymph, the first that I've seen in this river.  But the surprise of the day -- two more, beautiful Baetis pluto nymphs!
I'll end with those pictures.

Acentrella turbida

Baetis pluto

Oh, and I saw a number of brushlegged mayflies.

It's great to see a new season beginning.


The Doyles River at Doylesville.

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