I'm perplexed, and I have a question for the professional entomologists who follow this blog: do Clioperla and Isoperla nymphs differ enough to justify the existence of two different genera? I am an amateur at this, so I have no authority whatsoever to question established classifications. But let me make my case: I think there's good reason to shift Clioperla clio (there's only one species of Clioperla in North America -- i.e. clio) into the Isoperla camp, making it Isoperla clio.
Here is the case. There are three anatomical features used to distinguish genera of Perlodid stoneflies: 1) the presence or absence of submental gills; 2) the shape/form of the mesosternal ridge; and 3) the shape/form of the lacinia.
1) Neither Clioperla nor Isoperla has "submental gills" -- these:
2) Clioperla and Isoperla nymphs have the very same type of mesosternal ridge. Reading from Peckarsky (Freshwater Macroinvertebrates, p. 71), "Posterior ends of arms of mesosternal ridge meet posteriorly, forming a forked- Y pattern." It looks like this:
3) The laciniae on Clioperla and Isoperla nymphs look exactly the same. From Peckarsky (p.71), "Lacinia rounded from 2nd tooth to base." They look like this:
The only distinction between the genera is the pattern formed on the abdominal terga. Again, let me read from Peckarsky: "
45a. Dorsal abdominal segments with alternating transverse or longitudinal light and dark stripes or bands...Isoperla
45b. Dorsal abdominal segments uniform, brownish except for a few small light spots (which may be longitudianl rows) ... Clioperla
Here's an Isoperla namata on which we can clearly see the longitudinal stripes:
And here's a Clioperla clio, on which we can see the "light spots" which also form longitudinal rows:
Here's the problem: the so-called "stripes" on Isoperla nymphs are often formed by a combination of "dots"! On the I. namata nymph pictured above, the central stripe is made up of a series of crosses, which are actually two dots with a line down the center. That's clearer in the following photo (click to enlarge):
Now look at the terga on this fully mature Clioperla clio nymph:
True, these are "light" spots and a line, not "dark" spots and a line -- but does that minor difference warrant a separate genus?
And, what do we do with the center "stripes" on Isoperla similis nymphs (and the lateral "stripes" are simply "dots" as they are on the C. clio) -- they're light, not dark!
Again, the "stripe" is really a combination of a line and a couple of dots.
I really don't see why Clioperla clio is in a class of its own. Should it not be just another species of Isoperla? Counter arguments are clearly invited.