Wednesday, March 23, 2016

Just a beautiful insect: fully mature, Perlodid stonefly, Helopicus subvarians

This morning I was off to Buck Mt. Creek where the water was still pretty high.  Still, it's the time when we find Perlodid stoneflies, and Buck Mt. Creek has proven very productive.

This is Helocipus subvarians, a stonefly that I first identified on 4/2/11.  It's one of the largest Perlodid stoneflies that we find in local streams: this nymph measured 22 mm.  I think I'm right in saying that I've only seen it in Buck Mt. Creek and the Doyles River.  Tolerance value is 1.2.   This nymph is very mature, in fact, I thought it might hatch in my tray while I was taking these photos.
Most striking, how dark the head is both in front of and behind the black transverse band.

On less mature nymphs, even those with dark wingpads, the front of the head is usually orange.  I.e.,

Nice find.  Such rich colors.

My other discovery today was a very small Perlodid stonefly -- ~ 5 mm -- the one we ID at the moment as a variant form of Isoperla orata.

As I've noted before, the patterns of the two (main form and variant form) are quite distinct.  On the variant form of Isoperla orata, there two dark spots behind the ecdysial suture, and the bottom of the notch at the front of the head is straight across.

But on the main form of the species, the dark spots are missing, and the bottom of the notch at the front of the head is rounded.

Also, Isoperla orata is uncommon and only found in very clean mountain streams.  I've never seen it in Buck Mt. Creek, only at the Rapidan River.  And while the variant form can be found side-by-side with the main form, it is also found in streams like Buck Mt. Creek and the upper Doyles River, good streams, but not the equal of the Rapidan River.   For more on this point, see the entry on habitat and species variation posted on 8/26/15.


Hope to get up to Entry Run in Greene County this weekend.  Next week, it's off to Montana!

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