Speaking of variation...the small minnow mayfly, Acentrella nadineae

It's one of my favorite small minnow mayflies.  We see it in the summer, and I hope to see some tomorrow at the Lynch River.  Acentrella nadineae.  The orange/red markings are very distinctive, as are the gills with that pigmentation.  "Gills elongate, asymmetrical, and with basomedial pigmentation splotches; distinct abdominal color pattern often tinged with red."  (Beaty, "The Ephemeroptera of North Carolina," p. 4)

And it's a nymph that I've seen in all shapes and sizes, from very small,

to fully mature.

Even one that hatched out in my bowl.

before

 and after

Most of the nymphs that I've found have looked much the same: dark brown body with spots of orange on the terga, the "shoulders," and even the "neck."  Here are some photos.

But on occasion I find variation, and these two really stick out.

1) Number one is a nymph that I found -- well, I found several that day -- in the Lynch River on 7/2/11.

From the gills and the orange areas on the terga, we can clearly see that it's nadineae, but it's unusually green.

2) The other "odd man out" is this nymph that I found at the Doyles River on 7/26/12.  And again, I had others like it that day.

What is striking with this one is the pale strip that goes up through the center.  The center of every tergite is pale, but orange posteromedially.

If you look at the other nymphs in our photos, that pattern is not characteristic.  This is more common.

Terga 1, 5, 8, 9, and 10 are pale with the orange, but 2-3 and 6-7 are dark.

Now, this could be a matter of development and maturation -- gills tend to darken on nymphs as they mature.  Still, if you look at the "tiny" insects in the photos above, you can see that even there, terga 2-3 and 6-7 are much darker than all of the rest.

Habitat?  Water chemistry?  Types of nutrients in the water?  Maybe someday we'll know what explains these variations.
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The question of habitat and species variation

I've been saving this one for a rainy day.  It hasn't been raining -- but there's not a lot more to do at the moment.

I've written a number of entries this year on a number of species where we're finding variant patterns.
True of Isoperla orata (5/21), Isoperla holochlora (5/16) and Isoperla montana (1/25 and 4/17).  Here's a reminder of the patterns we've found and where we have found them.
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1. The "standard" pattern for I. orata -- the one that matches the species descriptions we find in the keys -- is this one:

This is a nymph that I have only seen at the Rapidan River.  But there is a variant form.  This.

It differs from the standard nymph in the following ways.

This is a pattern I also see at the Rapidan River, but I see it elsewhere as well: the upper Doyles River and Buck Mt. Creek, streams in which I do not find the "standard" form of the nymph.
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2. Isoperla holochlora.  The normal form of this insect can be found in a lot of our streams.  Buck Mt. Creek, Powell's Creek, the small, first order streams in Sugar Hollow, South River and the Rapidan River.  It looks like this.

But there is a variant form that I've only seen in two of our streams: the Rapidan River and its tributary Staunton Run.  It's a larger nymph, with a different pattern on the front of the head, and one on which the abdominal stripes are hard to distinguish.

Nonetheless, according to Beaty, it hatches out as the same adult of the normal form of the insect.

3. And then there's Isoperla montana for which we find a number of patterns.  But this is the most common pattern I see.

Again, this pattern in "common."  A lot of our streams are simply loaded with these nymphs in the spring (go in March and April).  The Doyles River, Buck Mt. Creek, Powell's Creek, the Rapidan River, South River and so on.  But there is a variant form that I only see in the small streams in Sugar Hollow, and in those streams it's the only form there.

Note the absence of the dark bars behind the lateral ocelli at the back of the head.

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What's going on?  Why do certain patterns occur in one stream but not another, variations of the very same species?    It could certainly have something to do with stream location and size.  The Rapidan River is a mountain stream, but it's a fair sized mountain stream as it comes out of the Shenandoah National Park.

Buck Mt. Creek is a mid-sized stream in the rolling hills east of the Blue Ridge which is 8+ miles away.  Where I go to look for insects, it has already passed through some farms.

The upper Doyles River is also at the base of the mountains, but it does not have the size of the Rapidan, nor is the vegetation as dense.

And the head water streams in Sugar Hollow are also in the mountains, but they're very small.

As entomologists start to work on these species variations surely the location and size of the stream will be a relevant factor, but there may be other factors as well.

In discussing variations in A. evoluta, Steven Beaty had this to say on variation in pattern.  (These are simply musings by Beaty, and should not be quoted as some kind of final position.)  "Patterns can be influenced by maturity level, temperature, habitat, region, preservation method, UV light, diet, and who knows what else (pollution?). "  Sure will be fun to see what they find.

The "best of the best": looking at photos

I've been putting together PowerPoint presentations of the mayflies, stoneflies, and caddisflies that I've found, trying to use the best photos I've taken of every species.  They're organized by order, family, and species.  Yesterday, I updated those files, and I was once again struck by a handful of pictures of which I'm especially proud.  The colors are vibrant, the patterns distinct, and the detail is focussed and sharp.  While all of those photos and more have been posted on Flickr (https://www.flickr.com/photos/aquaticinsects_of_central_virginia/albums, "Personal Favorites"), I thought I might pick out the best of the best for this entry.
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1. In the photo at the top of the page, the large winter stonefly, Taeniopteryx burksi/maura.  They're common in most of our streams in December, January, and February.  This one, as I recall, was found in South River up in Greene County.

2. Another large winter stonefly, Taenionema atlanticum.

This is an intolerant insect that we only find in very good streams, for the most part streams that are small.

3.  A Perlodid stonefly, one of my favorites, Isogenoides hansoni.

This is a species I've only seen at the Rapidan River.  Fairly uncommon.  What a gorgeous nymph!
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4. Some small minnow mayflies, starting with one that is especially colorful: Acentrella nadineae.

We know it's a male by the large eyes.

5. From the late summer/early fall -- Heterocloen petersi.  Rarely seen: in the Rivanna.

6. One from the winter -- Heterocloen amplum.

7. And another H. amplum, a male that's fully mature.

8.  A spiny crawler nymph -- Ephemerella subvaria.  Another species that's fairly uncommon, and another species that I've only seen at the Rapidan River.  Two photos.

9. And a different mayfly, the pronggilled mayfly, genus Leptophlebia.  Found this one at my upper site on the Doyles.

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10. Some case-makers, beginning with the "weighted case-maker," Goera calcarata.

Most Goerids make cases with two large stones on each side, not the four we see with this larva.

11.  Two case-makers stacked up together.  On the top, the "humpless case-maker," Brachycentrus appalachia; on the bottom the "little mountain case-maker," Apatania incerta.

Provenance -- the Rapidan River.

12. And a "northern case-maker," Pycnopsyche scabripennis.  Also found at the Rapidan.

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13.  I'll  finish up with some more stoneflies.  This is a small winter stonefly, Allocapnia pygmaea.

They're prolific in the winter in Buck Mt. Creek.  They're tiny, and it's not easy to get a photo that's sharp.

14.  The Perlodid stonefly, Isoperla holochlora, "I. holochlora -- dark form."

15. The Perlodid, Isoperla kirchneri complex (kirchneri, montana, siouan, or tutelo).

16. Another Perlodid, Malirekus hastatus.  These are fairly common in the winter and spring in the head water streams in Sugar Hollow.

17.  And the common stonefly (Perlidae) -- a large one -- Paragnetina immarginata.  Rapidan River.

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 Warm and sunny this weekend with tolerable humidity levels.  Tomorrow, I hope to look at Entry Run and South River.