Genus ID of a Small Minnow Mayfly: Heterocloeon

In the entry I posted yesterday, I said that I would ID the small minnow nymph in the photo above as genus Heterocloeon.   I thought I might add some notes on this issue, just to show readers how miserable genus ID can be for an amateur with a minimum of professional training -- especially when we're working on a small minnow mayfly.

First, here are two more photos of the nymph in question.

and,

Now, let's work through our two dichotomous keys -- 1) Barbara L. Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America, p. 33, and 2) R.W. Merritt, K.W. Cummins, and M.B. Berg, ed., An Introduction to the Aquatic Insects of North America, Fourth Edition, p. 192.

Peckarsky:
38a.  Two well-developed cerci; median caudal filament absent or less than 1/8 as long as cerci....39
38b. Three well-developed caudal filaments; although median caudal filament may be shorter and thinner than cerci, it is much longer than 1/8 length of cerci....41

Cerci mean "tails" to us; the "median caudal filament" would be a third, middle tail.  Since there is no median caudal filament visible, we clearly select 38a. and go to 39.

39a. Center region of gills with a large pigmented area (Fig. 69); procoxae with a single filamentous gill on inner margin....Heterocloeon
39b. Center region of gills without a large pigmented area; procoxae lacking a gill....40

"40" would take us to a choice between Acentrella and one type of Baetis (two-tailers).  But as I noted yesterday, the "center region of the gills" do indeed seem to have a large pigmented area.  Here's a good look at the photo.

But what about the second requirement listed -- the "procoxal gill"?    No picture of this is provided.
So, I have to guess what to look for and where to look.  But this ought to mean that there is some sort of gill showing on the inner edge of the front leg where it's attached to the body.  Here's what I found with the microscope.  (You may want to click on the photo to getter a closer view.)

I can't say for sure that I've found the "gill" -- but I'm willing to call this Heterocloeon based on the pigmented gills.

But let's look at our other key: Merritt, Cummins, and Berg, starting with couplet 36.

36. Terminal filament well developed; hind wing pads present...Baetis (in part)
36'. Terminal filament reduced; hind wing pads absent or present....37

Well, we know the terminal filament (= median caudal filament) is reduced, so we move on to the next couplet.

37.  Tarsal claws with two rows of denticles or a row of denticles and a secondary ridge that may be serrate or not (these claw characters require viewing at 400X): terminal filament minute to a visible stub not longer than tenth tergite; procoxae with or without gills....Heterocloeon
37'. Tarsal claws with a single row of denticles; terminal filament reduced, but longer than tenth tergite; procoxae without gills.

Now, I can't magnify to "400X" with my microscope (my maximum is "90X"), so I have no way to determine anything about the "rows of denticles" on the tarsal claws.  Also, note that in this key, "procoxal gills" are not a requirement for making a call of Heterocloeon!  (It is not unusual for keys to contradict one another.)  So, all I can do is look at the "terminal filament" (i.e. what would be a "middle tail").

The "10th tergite" is the top of the last abdominal segment, and I'd say the "terminal filament" is, indeed, not longer than the 10th tergite.

Ah joy!  Well, we do the best we can when we try to determine genus ID.  I think I'll play golf for a couple of days.