Friday, April 17, 2015

Still a long way to go to sort out the various species collectively known as Isoperla montana

As you know, at the moment a lot of our streams are crammed with these nymphs commonly known as Isoperla montana, nymphs that will be hatching anytime now as "Yellow Sallies."  And if you're a regular reader, you know that for the last year or so, I've been calling these critters I. montana/kirchneri, since I. kirchneri has been distinguished from I. montana, at least as an adult.

I've just contacted Steve Beaty in North Carolina to see if anyone has been able to describe the I. kirchneri nymph: the answer is "yes" and "no"!  Some features have been distinguished in a tentative way, but more testing needs to be done.  Not only that, he mentioned in passing that at least two other species of Isoperla share features in common with Isoperla montana and Isoperla kirchneri.  (He mentioned two species I've never heard of before: I. tutelo and I. siouan.)

Uncle!  As an amateur I can't do much with this: the pros will have to figure it out.  That requires rearing of nymphs, and -- I should think -- DNA testing.  Until then, I intend to use "I. montana group" to cover the various types of nymphs that look so much alike.  It could very well be, that the various types of I. montana that make up this group are actually different species.  Here are the differences that I have been noting.

1. The "paradigm," for me.

This is the nymph that I see most often, and I see it in a great many streams.  Key features: 1) the dark transverse line on the head extends down to the ocelli, creating "bars," and 2) there are also dark bars behind the ocelli, i.e. beyond the ecdysial suture.

2. Type 2.  The dark transverse line does not extend to the ocelli, i.e. there are no bars in front of the ecdysial suture.

3. Type 3.  There are no bars behind the ecdysial suture -- but there may be small "spots."

4. Type 4.  This one also lacks bars behind the ecdysial suture, but, look at the abdominal pattern.  The light spots on the terga are almost completely enclosed creating "cells" that are reminiscent of Isoperla orata.

This is Isoperla orata.

5. And then we have, a) one with very thin bars in front, a nymph that is very light in color,

b) and one with very thick bars at the back of the head,

c) even one where there are no bars at the back of the head but sizeable spots.


In the end, species distinctions depend on anatomical differences, not differences in the features we see on the head or on the abdominal terga.  So, the features herein distinguished may or may not be important.  In any event, from now on, all of these nymphs, for me, will be part of the "Isoperla montana group."

Off to Sugar Hollow tomorrow.

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