Aquatic Insects of Central Virginia: The Return of Our "Winter" Small Minnow Mayfly: Heterocloeon amplum

My destination this morning was the upper Doyles River. But I decided to take a quick look at the "lower" Doyles River -- just outside of White Hall -- before I moved on. That's as far as I got. The bottoms of rocks and the leaf packs were just loaded with insects. Stoneflies: I found some common stoneflies -- Acroneuria and Eccoptura -- but there were lots of Perlodids, Clioperla, Diploperla, Helopicus, and for the first time, Isoperla namata. And, I found both large and small winter stoneflies. For mayflies: I saw a few, small, flatheaded mayflies, Epeorus pleuralis, and a surprisingly large number of spiny crawlers (!), Ephemerella dorothea. Everything seems to be showing up early this year.

But I was most excited by finding quite a few small minnow mayflies -- the same species I saw here (and at Buck Mt. Creek and the Lynch River) last year in the winter. I'm quite sure this is Heterocloeon amplum, and I'll make the case for that in a moment. But first, a couple more pictures.

As I found last year, some of these nymphs are olive in color, others a mixture of orange and brown (see the photos posted on 3/2/11, for example).

And here's a nice double with the two colors together.

These were the largest small minnow nymphs I saw all of last year (7-9 mm), and the most prolific. That will come as no surprise to the fly fishermen: the "blue-winged olive" hatch -- adult small minnow mayflies -- is one of the earliest hatches we see every spring.

Now, how do we determine that this is Heterocloeon amplum? There's a short way and a long way. The short way is through a process of elimination. These nymphs have two tails, so the genus is either Acentrella or Heterocloeon. Acentrella is out of the question because no Acentrella reaches a length of 7 mm (see the descriptions of Acentrella species in Steven Beaty's "The Ephemeroptera of North Carolina," p. 4). So we move to Heterocloeon. Almost all species of Heterocloeon small minnow mayflies have "procoxal gills," gills at the base of the front legs. Those look like this on Heterocloeon curiosum.

The nymphs that I found this morning do not have procoxal gills, and the only species of Heterocloeon lacking those gills is Heterocloeon amplum.

That takes care of the "short" method of identifying our nymphs. But let's try to establish the species ID in a more rigorous way. Here's what Beaty has to say on H. amplum ("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. Found in streams to rivers in the Mountains and Piedmont. Mainly collected during the cooler months.

He also notes that, on this species, the "hind wing pads [are] usually small but sometimes absent."

The smallest nymph in the photos above was 7mm, the largest were 8mm. The "suboval" shape of the gills and the "rudimentary tracheation" can be seen in our pictures (there's a thin line of pigment in the center of each gill). Or, better yet, look at this photo (click to enlarge).

For the shortened leg setae (hair), we need a microscope shot.

The same is true if we want to see the wide medial band on the tarsi and the way in which the apex slightly expands.

On the hind wing pads -- they are present on the nymphs that I found, and they are indeed small.

Finally, let's look at the labial palps.

I'd say they are "parallel sided," at least the penultimate segment. So, I'm pretty satisfied with the call of H. amplum. And I suspect I'll be seeing a lot more of them through March into April.
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Other photos -- it was a good morning for pictures.

1. Perlodid stonefly, Helopicus subvarians. This was big one -- around 1" -- and note how the wing pads are flaring out from the body (= pretty mature).