Thursday, October 4, 2012

Should "Clioperla clio" be "Isoperla" clio?

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.

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