Friday, May 15, 2015

Chasing Isoperlas at the Rapidan River

It's one of my favorites: it's "uncommon" with a tolerance value of 0.0.  The Perlodid stonefly, Isoperla orata, which, to date, I've only found at the Rapidan River.

I only found one.  On the other hand, I found a significant number of the nymphs I've been calling Isoperla nr. orata (it's similar to I. orata but not quite the same).   Lots of these in the leaf packs.

I've already noted the ways in which these two species differ.  Still, I had a good chance yesterday to compare them, and I did note another difference that I had missed.   The "inset" at the front of the head -- the pale area anterior to the median ocellus -- is deep and straight across at the bottom on Isoperla nr. orata,

on Isoperla orata, it's shallow and rounded.

Well, I did see one other thing: note how the pronotums differ in color and pattern.

What do they share in common?  The shape and structure of the lacinia.  On I. nr. orata, there seem to be 5-6 setae on the knob below the subapical tooth; on I. orata, that number is 3.

I need Beaty to study those setae when he has time.

I found one other insect that required microscope work: a Eurylophella spiny crawler.  (Note the large gill on abdominal segment 4.)

Beaty describes 12 species of this type of nymph, divided into 4 different groups (Beaty, "The Ephemeroptera of North Carolina," pp. 28-30).   We think we've found 3 species so far in local streams (Albemarle County).  The one that I've seen in Buck Mt. Creek is E. verisimilis, which Beaty claims is "the most common Eurylophella in the Piedmont and Mountains" in North Carolina.   And as it turned out, that's what these nymphs turned out to be.  Key features: E. verisimilis has occipital tubercles,

check, and the "submedian tubercles on terga 1-4 [are] relatively short, stout and blunt, while the turbercles on terga 5-7 are dark (and long and pointed).  Check.  (I've taken a close-up of 4-7.)

Eurylophella verisimilis, size of this one was 7 mm.


Two additional insects.

1.  A very nice Isoperla nr. holochlora, and it was a bruiser -- 13 mm!


and 2.  another Pycnopsyche scabripennis.  Look at the size of that case!  (Petri dish is 3" in diameter.)


Nice spring day at the Rapidan River.  These are the two sites that I explored.


(Oh.  By the way, yesterday I didn't find a single Isoperla montana/whatever (photo below): they were all over the place, HUGE numbers, when I was here on 4/24.  Must have been quite a hatch!)

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