The small minnow mayfly Heterocloen amplum: the "odd man out" of the genus

I went to the Doyles yesterday looking for the nymph in the photo above, but all I found was water that was starting to freeze right down to the bottom!  22º -- I should have known better.  But this photo will do.  This H. amplum nymph was found in the Doyles on February 6th of last year.  You may recall that I found a small H. amplum here last month on 12/21.  This one:

Heterocloeon amplum is a small minnow mayfly that I associate with the winter: I see them in a number of streams from late December into the beginning of April.  They show up in good numbers in Buck Mt. Creek and the Doyles, but I see them as well at the Lynch River, the Rapidan River, and even in the North Fork of the Moormans.   It's the biggest small minnow mayfly I've seen, measuring 7-9 mm, so fly fishermen should enjoy this early hatch of the "Blue-winged Olives."

If you look back at some of my earliest postings (early 2011), you'll see that I originally identified this as genus Baetis -- then I thought it might be Acentrella.  It took me awhile to identify this one correctly.  But in my defense, Steven Beaty ends his discussion of H. amplum with the following note: "Known previously as both Baetis ampla and Acentrella ampla."  ("The Ephemeroptera of North Carolina," p. 9.)

The truth is, it just doesn't fit well into this genus -- Heterocloen.   Of the two-tailed Baetids described in Barbara Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America, pp. 33-36),  Heterocloen is defined by using the following features: "Center region of gills with a large pigmented area; procoxae with a single filamentous gill on inner margin."
Neither of those features is found on H. amplum nymphs!  In fact, those features are true of only one of the species that we find in this genus -- Heterocloeon curiosum.  The Peckarsky book is somewhat out of date and must be used with caution.

Let's take a closer look at this genus, using, as I normally do, the descriptions given by Steven Beaty (p. 9).  First, Beaty notes that the genus is divided into two different groups: Heterocloeon (Heterocloen) and Heterocloeon (Jubilatum).   Beaty notes four species that belong to the first group: H. berneri, H. curiosum, H. frivolum, and H. petersi.  All of the species that belong in this sub-genus have something in common: they all have "forecoxal" or "procoxal" gills.  These gills are tiny, finger-like gills that stick out from the base of the front legs.  They look like this:

Only two members of this sub-genus have been found in North Carolina, H. curiosum and H. petersi, species that I've found in our streams as well.   (H. berneri is reported from South Carolina and H. frivolum from Tennessee).

H. curiosum is common: I see these nymphs in large numbers in the Rivanna River each summer.  And as I noted above, they're a perfect fit for the description that Peckarsky provides for the genus as a whole: they have gills with pigmentation in the center, and they have procoxal gills.  They are also sexually dimorphic -- i.e. males and females have different colors and patterns.  The gill pigmentation is clear in all of these pictures.

males:

females:

And if you get lucky and get a close-up of a nymph turned upside-down, you might see the forecoxal/procoxal gills!

H. petersi is not as common as H. curiosum: to date, I have only found one nymph -- that too was in the Rivanna.

H. petersi nymphs also have forecoxal gills, but the abdominal gills do not have center pigmentation.  Rather, Beaty describes them in the following way: "gills grey or grey-brown with light margin."  Beaty also notes that H. petersi is distinctive in having "no dorsal pattern" on the abdominal terga.  You can see both of those features in the following photo.

The nymph in these photos is clearly a male -- note the large eyes.  But since I've not yet found a female, I can't tell you if this species too is "sexually dimorphic."  That is something that's true of Heterocloen amplum.

So let's look at this "odd" one.  H. amplum is the only member of the sub-genus Jubilatum that Beaty describes.   It has very little in common with other members of this particular genus.  Like all Heterocloeons, H. amplum nymphs have two caudal filaments (tails) and normally hind wing pads are present -- but that's about it.   They do not have forecoxal gills, and they do not have gill pigmentation.  Even the denticles on the claws differ from those that Beaty cites for all genus members: "claw usually with 2 rows of denticles with 1 row being long denticles and the second row being small, squarish pegs (400X magnification required)."  H. amplum claws do not fit this mold: "1 row of denticles becoming progressively longer and a secondary ridge without denticles."

H. amplum nymphs like H. curiosum (and possibly H. petersi) are sexually dimorphic, so perhaps that's a feature in common.

males:

females:

But let's read the full description of the species H. amplum.

H. amplum -- nymphs 7-9 mm; unique labial palpi parallel sided; shortened leg setae; femora, tibiae and tarsi relatively shortened, tarsi slightly dilated apically with wide, pale medial band; gills large, suboval, with rudimentary trachea.  (Beaty, p. 9)

1) labial palpi, parallel sided:

2) shortened leg setae:

3) femora, tibiae, and tarsi relatively short (clear from the photos above):

4) tarsi dilated apically with a wide, pale band:

5) gills suboval with rudimentary trachea. (Actually, in the sunlight we can see a thin central stem that runs through the gill from which very fine filaments branch out to the edges).

And we can also see -- without a 400X microscope -- the denticles that are characteristic of the sub-genus Jubilatum.

They do, indeed, become "progressively longer."
_______________

Heterocloen amplum, especially when we look at those claws and note the lack of procoxal gills, neatly fits into the sub-genus (Jubilatum) ID.  So, I guess the question I have is what is it that ties the two sub-genera together?  I can't see what it is, but I guess the entomologists know.  When I look at the three species of Heterocloen we find in our streams, I can cleary find common ground between H. curiosum and H. petersi, but for me H. amplum sort of stands on its own as the "odd man out" in this genus.

Here's Beaty's full description of the genus Heterocloeon.

Genus Diagnosis: Forecoxae usually with a single filamentous gill or protuberance (osmobranchia); tergal scales absent; claw usually with 2 rows of denticles with 1 row being long denticles and the second row being small, squarish pegs (400X magnification required); hind wingpads usually present; two caudal filaments.

Features that are italicized in this description are considered to be crucial.

Maybe more should be made in the keys of the common shape of the gills.  Were the grey pigment removed from the gills of H. curiosum, the gills of our three species would look much the same: "gills large, oval, with rudimentary trachea."