The importance of laciniae in Perlodid stonefly genus ID: focus, Isoperlas

(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.

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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.

There is an apical tooth and a subapical tooth; the base is wider than the apex; below the subapical tooth, there are usually 4-6 spine-like setae with finer setae often continuing on to the base.  And, Isoperla laciniae are often medially constricted.

Here is the description we find in Merritt, Cummins and Berg, p. 332.

88' Apical lacinial tooth much shorter than rest of lacinia

89  Inner lacinial margin with row of at least 4-5 long seta; no prominent knob bearing pegs below subapical lacinial tooth....Isoperla  (Remember that Malirekus has that "prominent knob.")

And in Peckarsky (p.71)

41b. Lacinia without a knob, rounded or tapering from the smaller spine to base

 Beaty's description is more detailed ("The Plecoptera of North Carolina," p. 23)

Lacinia bidentate with apical tooth shorter than rest of lacinia and typically without low rounded knob; lacinial margin with row or tuft of at least 4-5 long setae, some species with row approaching base."

This description holds true for almost all of the Isoperla species I've found.  This lacinia -- along with the longitudinal abdominal bands -- will help you ID the following species: Isoperla dicala, Isoperla holochlora, Isoperla nr. holochlora (which actually lacks the longitudinal bands), Isoperla montana/kirchneri, Isoperla similis, and Isoperla davisi.  But there are some exceptions that need to noted.

1. Isoperla orata.  

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.

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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:

http://portal.ncdenr.org/web/wq/taxonmanual.
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Isoperla nr. holochlora.  An Isoperla species on which the longitudinal stripes on the abdomen -- usually three dark bands against a lighter background -- are indistinct.