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.
__________________