Wednesday, December 12, 2012

Here in force: the large winter stonefly Strophopteryx fasciata

I said it wouldn't be long -- and I was right.  I saw 10-12 tiny, S. fasciata large winter stoneflies in the very first leaf pack I picked out of the stream this morning at Buck Mt. Creek.  They did not look like the nymph in this photo -- this  photo was taken on February 6!  This is what I was finding today: toddlers.

I found them in the decomposing leaves, but I also found them crawling on the bottoms of rocks.  Size: 3-5 mm.   That's very small for a nymph that will reach up to 10 mm before it hatches later on in the winter.

Strophopteryx large winters, unlike Taeniopteryx large winters, do not have coxal gills.  But they do have a distinctive morphological feature: a triangular ventro-apical plate (i.e. a projection at the tip of the underside of the body) -- this.

And even though the nymphs I was finding today were small, the ventro-apical plate was still easy to see in a microscope view.

Another S. fasciata anatomical feature that is easy to see even on very small nymphs is the mottled appearance of the head and the nota (the pronotum and the wing pads).

There is also very clear banding on the abdominal terga.  But this is much more pronounced on nymphs that are more mature.

How do we get to the species ID, Strophopteryx fasciata?  Actually the distinctive features can only be seen on nymphs that are close to being fully mature.  Let's look at Seven Beaty's description ("The Plecoptera of North Carolina," p.7):

S. fasciata -- nymphs 9.5-10 mm; abdominal terga yellow with uneven, dark brown transverse bands on anterior half of each tergite; median row of transverse dark dots on each tergite.

The banding and dots are visible on the nymph in the picture above, but I'll point them out.  (I've also noted the dots on the first photo of the ventro-apical plate.)

So, two of our large winter stoneflies are already here -- Taeniopteryx burksi/maura and Strophopteryx fasciata.  I suspect that the third species is already present as well.  But we only find that one -- Taenionema atlanticum -- in small, very clean, mountain streams.  I won't be surprised to see some tomorrow at South River up in Greene county.

I didn't do a lot of exploring this morning since I immediately found what I was after.  But I did find a fair number of T. burksi/maura large winters, along with quite a few Uenoids and Glossosomatids.  There was no need for microscope photos to see that the Uenoid I photographed was Neophylax oligius.  Very distinctive of N. oligius is the yellow stripe on its head.

ventral views:

dorsal views:

And the T. burksi/maura nymphs continue to grow and spread out those wing pads.

But the find of the day was the large winter stonefly, Strophopteryx fasciata.  I easily saw 50-60 nymphs, so they are coming on strong.

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