Sunday, February 16, 2014

Heterocloeon amplum: the small minnow mayfly we'll see when we can get back into the streams

It's been a tough winter, and the 16 inches of snow Wednesday night sure didn't help.  It's melting. But at the moment I have no place to park near the streams.  I'll see if I can get out tomorrow.

In the spring and summer I see a lot of different small minnow mayflies: in the winter, I normally see only two: Baetis tricaudatus -- in small, very clean mountain streams -- and Heterocloeon amplum in somewhat larger streams further away from the mountains (Doyles River, Buck Mt. Creek, Lynch River, etc.).  I thought I might review the identification of H. amplum nymphs for those of you who will soon return to sampling.

The colors of H. amplum nymphs are normally gender specific.  Females are olive --

males tend to be olive/brown.

Both of these photos, by the way, were taken on February 6th, 2012 -- so you can see what we're missing!

While the colors may vary, morphologically males and females are exactly the same.  They differ from the other Heterocloeons in that they lack procoxal gills, and unlike H. curiosum, they do not have a dark spot in the central part of the gills.  I.e. they don't look like this:

Here is Beaty's description ("The Ephemeroptera of North Carolina, p. 9):

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.

1. palpi parallel sided

2. shortened leg setae

3. dilated tarsi with pale medial band

The gill tracheation is a little bit tricky.  In normal light, it certain is simple: a thin line in the center with 1-2 finer branches.

However, if the sun shines through the gills, we can see very fine tracheal branches.


So we know what to look for in microscope views.  But can we recognize H. amplum nymphs at the stream?  I think that we can.  For one thing, the only other small minnow mayfly we're likely to see -- Baetis tricaudatus -- has three tails ("three" caudal filaments), H. amplum nymphs have only two.

But the thing that strikes me as really unique is the color of tergites 4 and 5: both tergites are orange on the males, yellow/orange on the females, with paired dark medial spots.

Terga 6-8 are dark; 9 and 10 are light.  This color scheme is easy to spot even on very small nymphs like the one I found on 1/20 this year.


Only one word of caution.  Late in the season (late March, early April) you might see some H. amplums that look a little bit different --

they're totally black.  The reason?  I suspect it's because these nymphs are completely mature and ready to pop.  I've seen the same thing with brushlegged mayflies.

Even so, if you look closely  at the H. amplum nymph, it's clear that terga 4 and 5 are lighter that 6-8.

(Oh.  And for the fly fishermen -- H. amplum is the biggest small minnow mayfly we see.  I'd use a size 14 hook -- maybe even a 12 -- when the Blue Winged Olives hatch in the spring.)

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