(Perlodid stonefly, genus Skwala. Oregon, October, 2013.)
For those of you who, like me, like to work at the level of genus ID, with Perlodid stoneflies, one of the first things you need to examine is the laciniae ("teeth" used for tearing) of your nymph -- shape and constitution. If you look at the keys -- Peckarsky's Freshwater Macroinvertebrates of Northeastern North America; Merritt, Cummins, and Berg, Aquatic Insects of North America -- you'll see that laciniae can differ markedly.
For example, each lacinia of the genus Remenus has only one tooth.
Diploperlas have two (apical and subapical).
The laciniae of Malirekus stones are more elaborate: two teeth, a knob with a tuft of setae, and dark clothing hairs along the base.
In general, the shape and make-up of the lacinia can help us get to the level of genus when we're working through keys. But Isoperla Perlodids can present a bit of a problem. Here is the lacinia of Isoperla dicala which represents very well the general Isoperla description we find in the keys.
Here is the description we find in Merritt, Cummins and Berg, p. 332.
Beaty's description is more detailed ("The Plecoptera of North Carolina," p. 23)
The apical tooth of the lacinia is very long, as long as the rest of the lacinia. And there is a bit of a knob below the subapical tooth on which we find exactly 3 setae.
2. A second exception -- the nymph I've been calling Isoperla nr. orata -- in part because the laciniae are so much alike.
The lacinia differs in one important way: there appear to be 5-6 setae on the knob below the subapical tooth.
3. And for the strangest one I've seen so far, Isoperla lata.
This lacinia looks nothing like the other Isoperlas we've seen.
Beaty's description - "lacinia distinct, broad, apex as wide as base and covered with a dense brush of setae." Wierd, and it looks like there's only one tooth.
I guess the point to be made is use your keys with caution: they don't always get you to the right place. And with Isoperlas, I'd encourage you to go right to Beaty's descriptions. Remember that his "The Plecoptera of North Carolina" is available on line. You can download it and print it. Go to: