Saturday, May 21, 2011

The Perlodid stonefly, genus Isoperla: Can we pin down some of the species?

In the entries I've posted throughout the winter and spring, I've focussed a lot on Perlodid stoneflies, in part because I've found large numbers of them, and in part because they're beautiful insects.  My intention in writing this blog was to focus on identification of family and genus, and I've been able to identify five different Perlodid genera: Clioperla, Diploperla, Isoperla, Helopicus, and Yugus.  At the outset of my work, I had only seen one Perlodid genus when sampling our streams -- Isoperla -- and in particular the type of Isoperla that's pictured at the top of this page.

However, I now know that we have, in Virginia, at least 12 species of Isoperla Perlodids -- this based on the species distribution list published by Stewart and Stark (Nymphs of North American Stonefly Genera (Plecoptera), pp. 412-13).   They are: I. bilineata, I. dicala, I. holochlora, I. lata, I. major, I. marlynia, I. namata, I. orata, I. signata, I. similis, I. slossonae, and I. transmarina.

To date, I have found at least 5 -- and possibly 6 -- of these species.  Using photos in, and with the help of comments from some of the experts at, I think I can identify to the level of species 3 out of the 6.  I hope that eventually I'll be able to identify the rest -- but that will clearly require assistance from qualified specialists.

In the meantime, I thought I'd pull together all of my data so readers can see the species that I've found so far.  Let me begin with a review of the key features that identify the genus Isoperla, illustrating those traits with microscope photos.  (All quotes are from Barbara Peckarsky,, Freshwater Macroinvertebrates of Northeastern North America, pp. 71-73.)

1) "Anterior ends of arms of Y ridge of mesosternum meet posterior corners of furcal pits" (i.e., the forked points of the "Y" point up.)

2) Submental gills are absent.

3) "Lacinia with a shorter spine mesal to major spine and commonly with additional spinules or hairs."

and 4) "Dorsal abdominal segments with alternating transverse or longitudinal light and dark stripes or bands."   This trait is clearly visible in the photo at the top of the page.  However, Stewart and Stark (Nymphs of North American Stonefly Genera, pp. 410-11) qualify this.  They caution that "several Isoperla species lack the typical abdominal pigment pattern."  This is worth keeping in mind.

I will now show you photos of the nymphs I have found -- two for each of the species -- followed by a microscope shot of the head, since the head pattern seems to be a relevant factor in determining species.

1. Isoperla namata (I think -- using photos in  I originally thought this was I. montana, but that species is not attested in the state of Virginia.)

2. Isoperla similis (I think -- using photos in

3. Isoperla holochlora (identified by experts at

4. Isoperla sp. (Note: This may also be Isoperla holochlora: some experts say "yes"; others say "no".  It does not have alternating light and dark abdominal stripes.  But, this speciment is still immature, and the head pattern is certainly similar to I. holochlora.)

5. Isoperla sp.

6. Isoperla sp.

Any help from professional entomologists with species identification of the Isoperla nymphs pictured above will be greatly appreciated.

One final note on "tolerance values" for at least some of the Isoperla genera supposedly found in Virginia (the values given are those used in North Carolina).

I. bilineata          5.5
I. dicala              2.2
I. holochlora       0.0
I. namata            1.8
I. orata                0.0
I. similis               0.7
I. slossonae         2.6
I. transmarina     5.6

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