There is exciting news for those of us intent on identifying the Isoperla Perlodids we find in our streams. Steven Beaty will soon put online a summary of the work he has done on Isoperlas, detailed notes on where things stand at the moment. The title -- "A morass of Isoperla nymphs (Plecoptera: Perlodidae) in North Carolina: a photographic guide to their identification," (Department of Environment and Natural Resources, Division of Water Resources, Biological Assessment Branch, Raleigh). It features photos of preserved nymphs, and photos on which are noted key features used in species ID. I'll let you know when this comes out and where it can be found. But since he has shared with me a copy of this in advance, I thought I might sum up the data that is important to us.
1. Photos and descriptions of the following nymphs are included.
Isoperla cf. fauschi
Isoperla holochlora -- light form
Isoperla holochlora -- dark form
Isoperla kirchneri complex: I. kirchneri, I. montana, I. siouan, and I. tutelo
Isoperla similis/pseudosimilis Groups: I. bellona, I. cherokee, I. pauli, I. pseudosimilis, I. reesi, I. starki, I. stewarti
Isoperla "Collins Cr" n.sp.
Isoperla "Mayo R" n.sp.
Isoperla nr. holochlora
Isoperla nr. transmarina
Isoperla sp. 10
Appended to the work at the end, photos and a description of Isoperla sp. VA.
2. Of special interest, the nymphs we've been calling Isoperla montana, Isoperla montana/kirchneri, and Isoperla montana group -- these (and the one at the top of the page)
are now labeled "Isoperla kirchneri complex." This complex includes four different species -- kirchneri, montana, siouan, and tutelo -- but distinct species descriptions have not yet been made. (Beaty notes, however, that "preliminary morphological differences between species are unproven but promising," p. 23.) Nymphs of this complex are described in the following way: "head with irregular transverse M-type medial band, extensions back towards posterior ocelli variable, anterior frontoclypeus with 2 pair of small brown markings, some specimens with markings narrowly connected by brown lines or even widely coalesced into a larger brown area." (p. 23)
3. Also relevant to recent entries I've posted -- the I. holochloras have been renamed: "Isoperla holochlora -- light form", and "Isoperla holochlora -- dark form".
Beaty confirms what I have found -- that the "light form" is fairly widespread and found in the mountains and piedmont while the "dark form" is confined to the mountains. He also notes, on the dark form, that the dark form "co-occurs with light form of holochlora but emerges 1-2 months earlier and when both forms are collected together, the dark form specimens will be much larger." (p. 21) That sums it up.
Related to I. holochlora is I. powhatan ("part of the holochora complex," p. 31). The front of the head looks much the same. However, the abdomen of powhatan is fairly dark, and the "ocellar spot" resembles an inverted U. I can't say for sure that I've seen this, but I am intrigued by this photo I took on 5/31/11.
On "light form" nymphs, that ocellar spot is normally "sub-triangular to diamond-shaped" (p. 19). I.e. like this
4. One more -- Isoperla similis. We are now presented with two different groups: Isoperla similis Group, and Isoperla pseudosimilis Group. In the former, we have three species -- I. bellona, I. cherokee, and I. starki; in the latter, four species -- I. pauli, I. pseudosimilis, I. reesi, and I. stewarti. But no way is provided to tell them apart at the level of nymphs. (Again, "preliminary morphological differences between at least some species are unproven but promising." (p. 33) Still, Beaty does confirm that "species in these groups appear to be restricted to small, cold, high elevation streams." That's where we find them.
small headwater stream in Sugar Hollow in April
Entry Run in Greene County
5. So what are the species that we've found, to date, in our streams (Albemarle, Greene, and Madison Counties)? Well certainly four of those listed by Beaty: I. kirchneri complex, I. holochlora -- light form, I. holochlora -- dark form, and I. similis. Possibly I. powhatan, but I'm far from sure about that. What else? There are five more we can note.
1) Isoperla davisi
Beaty: "widespread but more common in Piedmont and Inner Coastal Plain." (p. 12) Both of these nymphs were found in Buck Mt. Creek, the only place that I've found this species so far. I think -- but don't know for sure -- that the spread wingpads on the second nymph are a sign that the nymph is getting ready to hatch.
2) Isoperla dicala
Beaty: "occur in small, cold, higher elevation streams of good or excellent water quality; uncommonly collected." (p. 13) I've found them in Buck Mt. Creek and the Rapidan River.
3) Isoperla lata/pseudolata
Beaty: "found in high, cold, excellent waters (so far). ... relatively rare. ... may be pattern differences between lata/pseudolata but are, as of yet, unproven." (p. 25) I've only seen one -- at the Rapidan River.
4) Isoperla orata
Beaty: "High quality, small to medium mid-elevation, cold water streams." (p. 27) The first two nymphs in the photos above were found in the Rapidan River, the third was in Buck Mt. Creek. I still think that nymph number one differs from nymphs two and three -- see the posts of 5/15, 5/21, and 8/26 -- but no distinction is made in this study.
5) Isoperla sp. VA
While this species has not yet been found in North Carolina, as Beaty says, it is "likely to occur." (p. 48) He notes, "nymphs so far collected from small headwater streams from VA; along Blue Ridge Parkway (Bedford and Botetourt Co., D. Lenat), White Rocks campground on WV/VA border (Buchanon Co., V. Holland), and in central VA from the northern Piedmont (Albemarle Co., R. Henricks)."
In closing, I should note that Beaty adds a "Disclaimer" at the very beginning: "This manual is unpublished material. The information contained herein is provisional and is intended only to provide a starting point for the identification of Isoperla within North Carolina. While many of the species treated here can be found in other eastern and southeastern states, caution is advised when attempting to identify Isoperla outside of the study area. Revised and corrected versions are likely to follow."
But it is a place to begin and gives us -- as amateurs -- a great source to use as we continue to search for these beautiful stoneflies. Anxious for the new year to begin! Three species -- frisoni, slossonae, and nr. holochlora -- should occur in the types of streams that I visit ("high quality, cold water streams"). I'll be looking.