A "Cheat Sheet" for the flatheaded mayflies (Heptageniidae)

While genus ID for most EPT taxa requires microscope work, there are a number of families where the genus can be nailed down by volunteers that monitor streams, certainly if they're using a loupe.  That is the case for the Heptageniidae -- flatheaded mayflies -- so I've worked up a cheat sheet for those who might want give this a try.  Just be advised, that this only applies to the flatheaded genera I've seen.  The two I've not yet encountered are Macdunnoa and Stenonema.  Both are type 1.

                                        Cheat sheet for flatheaded mayflies

 1.  with thin, single filament gills on segment 7

a. Maccaffertium (most are mottled with banded legs); gills truncate

b. Stenacron (uniform in color – gray/dark brown, though some have stripes on abdomen); gills pointed, abdomen long and thin

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2. with "fan-like" gills that overlap ("suction cup" gills)

a. 2 tails – Epeorus

b. 3 tails with fibrilliform on top of gills – Rhithrogena

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3.  gills stick out to the sides, don't overlap

a. fibrilliform present on all gills – Heptagenia

b. no fibrilliform on final gill, head very wide; small mayfly: Leucrocuta (common)

c. no fibrilliform on final gill, intersegmental setae present on cerci (probably need microscope): Nixe (uncommon)
d. maxillary palps visible on sides of head -- Cinygmula

Let's give it a try.

1. with thin, single filament gills on segment 7

a. Maccaffertium

b. Stenacron

One caveat: there is one Maccaffertium species -- modestum -- that does not fit into the "mottled" mold and could be confused with Stenacron (I've made the mistake).  But, the color differs markedly  from the Stenacron nymphs that we see.

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2. with "fan-like" gills that overlap ("suction cup" gills)

a. Epeorus.  If there are only two tails this call's a slam dunk.

b. Rhithrogena

The "fan" of the gills is not so prominent on these nymphs, but the "fibrilliform" (frilly filaments) on the tops of the gills is easy to see.  Both Epeorus and Rhithrogena have "suction cup" gills.  You can see this with a loupe (though this is a microscope view.)

Epeorus and Rhithrogena are "clingers" that use those gills to hold on to the rocks in fast water.

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3.  gills stick out to the sides, don't overlap

a. Heptagenia.  You can see those "frilly filaments" behind each of the gills.

b. Leucrocuta.  There are strands of filaments (nothing like the fibrilliform on the Heptagenia nymphs) behind the gills on segments 1-6.  They're absent on 7.

Leucrocuta nymphs are pretty darn small, and as you can see they have very wide heads.  (That's also Leucrocuta at the top of the page.)

c. Nixe.  This genus is uncommon.   I've only seen it in the Pacific Northwest, but according to Beaty they are found here on occasion.

Nixe cannot be distinguished from Leucrocuta out in the field.  It takes a microscope to see the "intercalary setae" on the cerci (tails), present on Nixe, absent on Leucrocuta. 

 Still, it's a safe bet that the "fatheaded" flatheads you find are genus Leucrocuta.

d. Cinygmula.  

I can tell this one at sight -- others might need a loupe.  Look for two bumps on the sides of the head, the maxillary palps.

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Not so tough after all, right?  And it might add to your enjoyment of picking bugs off the net!   Which genus are you most likely to see?  Maccaffertium, no doubt about it.  These nymphs are in our streams year round, the other genera are seasonal, generally speaking in spring and summer.