Tuesday, September 20, 2011

Genus ID of Stoneflies Found in Montana

Another rainy day; another good day to do some microscope work.

While fishing in Montana last month, I found two "common stoneflies" (Perlidae) that I had never seen in the East.  I got live photos of only one -- the one pictured above -- which the folks at Bugguide.net identified for me as Claassenia sabulosa.  With time on my hands today, I thought I might work through the genus ID of these two beautiful stoneflies.

For this purpose, the dichotomous key that I normally use for genus ID  -- Barbara Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America -- isn't sufficient: I'm looking at insects from Northwestern North America that, as I suspected, are not genera that we find in the East.  So, we have to use the "big book" -- R.W. Merritt, K.W. Cummins, and M.B. Berg, ed., An Introduction to the Aquatic Insects of North America, pp. 325-327.   The lastest edition, the Fourth Edition, published in 2008, has a mere 1058 pages of detail for the identification nymphs, larvae, and adults.

Let's get started.  Claassenia turned out to be very easy.  There are three features to look for and they're right at the start of the key.  Two microscope photos are all that we need.  And in our key, we turn to p. 325.

51 Occiput with transverse row of regularly spaced spinules
51' Occiput without spinules, except possibly laterally near the eyes, or with a sinuate, irregularly spaced spinule row

There is a clearly defined row of "regularly spaced" spinules.

52 Two ocelli
52' Three ocelli

There are three.

53 Ab [abdominal] terga with more than 5 intercalary bristles ...... Claassenia
53' Ab terga with no more than 4 intercalary bristles

There are "lots" of intercalary bristles, those little dark brown spikes on the top of each abdominal segment.  So, this is clearly a Claassenia nymph.  Another look (and note the tiny Perlodid stonefly taking a ride on the Claassenia's cerci!)

ID of our second nymph proved to be more complicated, involving a number of steps.  But let's begin with a photo of the bug.

I am so sad that I have to use a photo of the preserved insect.  I found this one at the end of a day of fishing the Blackfoot River: I had a vial to preserve it -- I did not have my camera with me!  So, we have to make do.

Again, let's begin with a close-up view of the head.

So remember our options are, 51, an "occiput with [a] transerse row of regularly spaced spinules," or, 51', no spinules, "or with a sinuate, irregularly spaced spinule row."  Bingo!  That's what we have.

From 51' we move to 55 and 55', where we have to decide between "no spinules" or the "sinuate, irregularly spaced row."  Since we have the "sinuate row," we move to 56.

56 Ab terga with fewer than 5 or no intercalary bristles......Hesperoperla
56' Ab terga with more than 5 intercalary bristles

Again, there are a lot more than 5.  We move to 57 and 57'.

57 ...posterior fringe of Ab terga mostly of short setae whose length is about one-fourth the length of Ab segments
57' ...posterior fringe of Ab terga with numerous long setae whose length is three-fourths or more the length of Ab segments....Attaneuria

The setae are pointed out in the photo above, and they are clearly no more than one-fourth the length of the segments.  We move to 58.

58 Cerci without a dorsal fringe of long silky hairs
58' Cerci with prominent dorsal fringe of long silky hairs

You'd have to say that fringe is pretty prominent!  We move to 59.

59 ...Ab7 sternum [the back side of abdominal segment 7] usually with incomplete posterior fringe
59' ...Ab7 sternum usually with a complete posterior fringe....Calineuria

We flip our nymph onto its back.

Segment 7 does have a "complete" fringe; segment 6 does not.  Our stonefly is genus Calineuria.  Now if I only had a "live" photo.  Have to wait for next year's trip back to Missoula!

Below.  Fishing on the Blackfoot River, north of Missoula, Montana.

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