A fresh look at the Ameletidae (Painted minnow mayfly) that live in our streams

While I've been waiting for rain (and yes, we finally got some!), I was browsing through some of my photos and decided to take a fresh look a the issue of species ID for the Ameletidae nymphs that I've found over the years.  I'm more convinced than ever that I've found at two different species, possibly three.  And while I doubted the Ameletus cryptostimulus ID for the that nymph at the top of the page for awhile, that's no longer true.  Let's have a look.

1. Ameletus cryptostimulus

Small mountain streams, and they're fairly common in March and April in the Moormans tribs that I visit in Sugar Hollow.  Let me work through the description found in Beaty's "The Ephemeroptera of North Carolina," p. 1.  I will rely on dorsal features only, I don't have any specimens at the moment so I can't check things like mandibles, and ventral patterns.

cryptostimulus -- labrum tan with darker medial subtriangular mark; tarsi with a dark apical band only.

Check, and note that the apical bands on the tarsi are visible in all of our photos (though it just looks like a dark tip).

posterior spinules present on abdominal terga 5 or 6-10.

On 6-10 for sure and probably on 5.

each abdominal tergum with two pairs of pale spots; one submedian pair and a pair anterolateral to those, also with a small pair of dark brown submedial spots on each segment; abdominal tergum 2, and sometimes 7, mostly pale, terga 6-9 with an additional single, medial pale spot removed anteriorly.

Yep.  One more thing.

caudal filaments basally brown and with a dark brown medial band followed by a pale band and tipped finally with brown.  Recorded from GSMNP and small mountain streams.

This is the feature that gave me pause a while back.  Are the caudal filaments (tails) "basally brown"?  I now think they are, just not dramatically so in immature nymphs.  But look at them on the one mature nymph that I've found.

No doubt about it, though, true, it's only the lateral tails that are fully brown.  Note that we can also see on this nymph that terga 2 and 7 are paler than the rest.

Ameletus cryptostimulus -- I'm a believer.  Tolerance value -- not enough data to reach a conclusion.
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2. Ameletus lineatus

tarsi with both dark basal and apical bands; posterior margin of abdominal terga 1 or 2-10 with spinules; abdomen dorsally with 2 large pale sublateral spots on 2-7 and with segments 2-3 and 6-7 with a submedian pale spot; terga 8-10 mostly dark with some pale etchings; segments 1-9 with dark comma-like submedian markings visible particularly when pale spots are partially fused.

The pale spots mentioned in the description are all visible in this photo.

 For the spinules, here's a close-up of the abdomen of a second nymph.

Have to look closely, but you can actually see them on segments 2-10.   We can see the "comma-like submedian markings" clearly on a younger nymph that I found in the spring in the Moormans river itself.

For the caudal filaments, Beaty says this.  "caudal filaments with dark median band interrupted every four segments by very narrow pale bands.  Common in the Mountains and Piedmont.  A parthenogenetic species. "  Bingo.  Ameletus lineatus.

One note of caution.  There is another species -- Ameletus ludens -- for which Beaty says "dorsal and sternal abdominal coloration similar to A. lineatus."  To determine the species for sure, we'd have to see the sternal pattern.  But those differences are small enough for Beaty to conclude that A. ludens is "possibly a northern variant of A. lineatus.  We find much the same in our new key.   On A. lineatus: "This species might be equivalent to A. ludens."  (Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species, p. 40)  If I find these nymphs in the spring, I'll be sure to look at the venter, but it sounds like we won't really come up with anything firm.

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.

A "Cheat Sheet" for pronggilled mayfly (Leptophlebiidae) genera

One more, and it's the easiest one to do.  With very little help -- this guide and a loupe -- monitors and volunteers should be able to determine pronggilled mayfly genera right at the stream.  Only one thing to check: the gills.  I've found four genera, and four types of gills.

A "Cheat sheet" for pronggilled mayfly (Leptophlebiidae) genera

 1. gills forked with tracheal branching at base: Neopleptophlebia

2. gills forked without tracheal branching at base: Paraleptophlebia

3. gills forked, each branch ending in a cluster of filaments normally 3 (look like pitchforks): Habrophlebia

4. each gill consisting of two petals (bilamellate) which end in a pointed filament: Leptophlebia
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1. gills forked with tracheal branching at base: Neopleptophlebia

They look like this.

And the gills are forked with tracheal branching.

While I can't say this for sure, I suspect that this is the genus that monitors most often see.  They're in our streams from late fall through much of spring (December - April?).  For fly fishermen, they hatch as "Blue Quills."  I see tons of them at the Rapidan River: look in the leaf packs.

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2. gills forked without tracheal branching at base: Paraleptophlebia

Less common, typically in pretty small streams (though I find them as well at the Rapidan River).

(Also the nymph at the top of the page.)  Easy to see that the gills lack the "tracheal branching" we see on Neoleptophebia.

I've identified three species for this genera.  The one pictured here -- P. guttata -- is the most common.  This genera is one that I typically see from late spring into summer.

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3. gills forked, each branch ending in a cluster of filaments normally 3 (look like pitchforks): Habrophlebia

Really cool gills.  No mistaking this genus, and the gills are easy to see with a loupe (actually, you can see them with the naked eye.)  But, you'll have to be lucky to see one.  They're "uncommon": this is the only one that I've seen (Rapidan River).

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4. each gill consisting of two petals (bilamellate) which end in a pointed filament: Leptophlebia

Beaty says this genus is "widespread" ("The Ephemeroptera of North Carolina," p. 78), but to date this is the only one that I've seen (Doyles River at Brown's Cove).

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Easy-peasy.  Just have to find them.