A Guide to Genus ID of small minnow mayflies (Baetidae)

Small minnow mayflies (Baetidae) are not easily identified to the level of genus by the casual observer.  But I can give you some key things to look for that will help you narrow it down.   The season is also a factor, for while small minnow mayflies are in our streams all year, genera tend to take turns.   One other thing, I can only describe the 6 genera I've seen.  Still, you ought to know that Beaty lists 18 small minnow mayfly genera, and our new key says there are 22 genera in the Southeast.  Some of these other genera are rare, uncommon, or found in habitats that I don't explore (lentic: ponds, lakes, large slow-moving rivers); others, with luck, I'll run into some day.

Key features to keep in mind: 1) how many tails?  2) are the tails banded or unbanded? 3) are there distinct patterns on the terga?
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           Baetidae (Small minnow mayflies): A Guide to Genus ID

 I.  three tails

1. tails usually banded, middle tail of varying lengths; antennae 2-3X longer than head; antennal scape normal:  Baetis

2. tails banded and close to equal in length; antennal scape with notch/distal lobe: Labiobaetis

Note: three tails almost always = Baetis

II. two tails

A. tails not banded

1. abdoman with paired submedian dots of equal size; hairy legs: Acentrella

2. forecoxal gills present: summer Heterocloeon (H. curiosum with pigment splotches present in gills)

3. forecoxal gills absent; tarsi dilated apically with wide pale medial band: winter Heterocloeon (big nymph)

B. tails banded

1. no hair on tibia and tarsi: Plauditus

2. small nymph; tibia narrow at base, widens medially to apically: Iswaeon (uncommon)

Note: two tails banded almost always = Plauditus

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Seasons

1. Baetis: spring through fall

2. Labiobaetis: summer

3. Acentrella: 1) turbida, year-round; 2) nadineae, mid-spring to mid-fall

4. Heterocloen: 1) curiosum and petersi, summer; 2) amplum, winter – early spring

5. Plauditus: year-round

6. Iswaeon: summer

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I. Three tails

1. Baetis.   Here are pictures of two of the species I see, the nymph in the photo at the top of the page is a third.

The fourth is one that has unbanded tails, one that's only found in quality mountain streams -- Baetis tricaudatus.

The long antennae are visible in all of these photos.  The antennal scape (the segment touching the head) is normal, that means there is no "notch" at the front.

To be fair, and to be sure of our ID, we should check the labial palps since the shape of the labial palps is important for all small minnow genera.  Here's what they look like on Baetis nymphs.

But my goal here is to attempt a genus ID without the use of microscope views.

Seasons?  Baetis tricaudatus is around in the winter, all of the other species I've found -- I think I'm right in saying this -- are spring-fall taxa.

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2. Labiobaetis.  There is a light form and a dark form.

As you can see, the tails are banded and fairly equal in length.  But Labiobaetis differs from Baetis by having a "notch" at the front of the basal antennal segment.

Of course, that notch might be real tough to see even with the use of a loupe.  But here's the thing, this is a genus that I've only seen once, down at the Rivanna River (at Crofton).  So it's a safe bet, that if you see a small minnow mayfly with three tails -- that nymph is a Baetis!

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II. Two Tails: 

A. unbanded

1. Acentrella.  I've found two species so far: A. turbida seems to be a year-round taxon, the A. nadineae season runs from mid-April to mid-September (roughly speaking).  A. turbida nymphs have a very broad thorax; A. nadineae nymphs have splashes of red and orange.

A. turbida

A. nadineae

The most striking feature that they share in common is the paired dots on the abdominal terga.

A microscope helps us to see the other thing that they share, long silky setae on the femora and tibiae.

But the thick thorax on A. turbida and the orange/red markings on A. nadineae pretty much seal the deal. 

2. Heterocloen.  I go summer and winter on these.  The summer species I see are H. curiosum (with pigment splotches in the gills) and H. petersi.

H. curiosum (male then female)

H. petersi

Both species have forecoxal gills, and those gills are easy to see with a loupe.  If you see a two-tailer in the summer, turn it over to check for those gills.

The winter species I see, H. amplum, is entirely different.  No forecoxal gills, no pigment splotches, and it's much bigger than its summer cousins.

The color patterns are set, the female is a drab olive green, the male is more of an orange/brown.  But note that with both genders, terga 4-5, and 9-10 are light in color, 6-8 are dark.  Perhaps the key feature to look for is the pale medial band on the tarsus, and note that the tarsus is lightly dilated apically. 

Heterocloeon amplum is the most common small minnow mayfly I see in our mountain streams in the winter. 

By the way, if you have a microscope, have a look at the denticles on the tarsal claws: they get progressively longer on Heterocloeon nymphs.

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B. tails banded

1. Plauditus.  The defining character for genus ID is the shape of the labial palps -- which I've never been able to see clearly (the nymphs are quite small).  I know what this is from the species description provided by Beaty ("The Ephemeroptera of North Carolina," p. 22) -- these key out to Plauditus dubius.

(pics: female then male)

However, the banded tails, and the fact that Plauditus is very common in the streams that I visit are  pretty good indications.  The only other two-tailer I've seen with banded tails, Iswaeon anoka, is uncommon, and I've only seen one in all of the years I've been looking.  One trait you can see on Plauditus nymphs with a microscope, is the lack of setae on the tibia and tarsus.

2. Iswaeon.  And this is Iswaeon anoka.  

And if you get lucky, you might one day see one.  This one was in the Rivanna River at Darden Towe Park.  It does have a distinguishing feature that you might see with a loupe: the tibiae widen apically.

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So where does that leave us?  I'd say this for the streams that I visit in central Virginia...

1. If you have a small minnow mayfly with three tails -- it's almost certainly genus Baetis.

2. If you have a nymph with two tails, and there's no sign of banding on the tails, and it's summer, then I'd look to see if it has forecoxal gills; if it does, it's Heterocloeon (curiosum or petersi): if it doesn't, and the thorax is very broad, it's Acentrella (turbida), but if there are splashes of red and orange on the terga, it's Acentrella nadineae.

3. If, on the other hand, it's winter, and that two-tailer without any banding is big, and terga 4-5 and 9-10 are markedly lighter than terga 6-8, it's Heterocloen (but amplum).  (But you'll still see Acentrella turbida in our streams at that time of year.)

4. And if the two tails are banded, it's almost certainly genus Plauditus (dubius).
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Best I can do at the moment.  I hope that folks that monitor streams in central Virginia will look closely at their small minnow mayflies this fall and see if they can determine the genus ID.  There are lots of Baetidae around, and this will add to the fun of taking those samples.