Saturday, February 25, 2017

Those Nemourids were genus Prostoia -- possibly Prostoia similis

The Nemourids I found yesterday at Buck Mt. Creek key out to genus Prostoia.  They're either P. completa or P. similis  At the moment I favor the latter and I'll show you why.  But, to be sure of my ID, I'm sending the nymphs to Steven Beaty for confirmation.

Let me make the case for genus Prostoia.  From Steven Beaty's "The Plecoptera of North Carolina," p. 16 (version 4.0, 2015):  Nymphs small, 5-6 mm, and stout. ... rounded pronotum with marginal fringe of short, small, irregularly spaced spines, anterior thoracic gills absent; fore tibiae with dorsal fringe of fine hairs, usually complete but sparse; ventral (inner) tibial fringe of silky setae absent; outer marginal setae of fore tibia thick, conspicuous, in two rows."

1) Two of the nymphs that I found were 5mm, one was 6.

2) The pronotum was indeed "rounded," and you can see the short, marginal spines in these pictures.

You can also note from these photos that the dorsum of the pronotum is covered with short spines (tiny black dots), something we can see in the illustration of the Prostoia pronotum in Peckarsky (Freshwater Macroinvertebrates of Northeastern North America, p. 68).

3. Anterior thoracic gills (elsewhere called cervical gills)-- these --

are absent.  Such gills are only found on Amphinemura and Zapada nymphs.   Here's a close-up of our nymph.

4. There is indeed a setal fringe on the fore tibia, as well as two rows -- marginal and inner -- of thick setae/spines.


When I got to this point yesterday, I thought I had been right some time ago when I came up with Prostoia completa for the ID.  On P. completa Beaty notes: "This is a common species in the east and possibly in North Carolina.  It occurs in small streams to large rivers."  But the key to the P. completa ID is "tibiae with a well-developed dorsal fringe of setae; cerci with short intercalary hairs present on several middle and distal segments."  The fringe of setae is well-developed, no problem there.  But what about the cerci?

  I've looked and looked but I can't see any hairs between those joints (intercalary setae).  So is there a species on which those setae are absent?  The answer is yes: Prostoia similis.  "similis -- cerci with short intercalary hairs present only on distal segments or absent altogether."  (Beaty, p. 16)

That's where things stand at the moment.  Maybe Beaty will see something with his superior equipment.  If so, I'll let you know.  Until then, my money's on P. similis, a species that has been attested in Virginia, and in SC, TN, and WV as well.

Some more photos from yesterday.

Oh!  And I almost forgot.  There's a good chance that the adult that I photographed yesterday was P. similis as well.  It's an exact match for the P. similis adult pictured on Discover Life (

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