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.