I hang my head in shame, since I once again -- in my posting of 1/18 -- identified an insect before I carefully checked the ID with my microscope. The Perlodid stonefly in the photo above, which I found at the Rapidan River on Wednesday, is not Isogenoides hansoni -- it's Malirekus hastatus, a Perlodid that I've not seen before.
The first clue that I was wrong with the Isogenoides ID came when I looked at the mesosternal ridge.
This is what that looks like on an Isogenoides nymph, where median ridge of the "Y" intersects the transverse ridge at the top--
Here is the ridge of our Rapidan nymph.
No median ridge, no transverse ridge. Oh dear. So, let's start our quest for genus ID with Barbara Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America (p. 71). And we begin with couplet 37.
37a. Submental gills at least twice as long as greatest width...38
37b. Submental gills less than twice as long as greatest width, or absent...40
I did not see any submental gills when I first looked at the chin. But, that led me nowhere, so I looked a little bit closer, and
sure enough there they were, I've just never seen gills that short and that wide at the base. That means, by the way, that we opt for 37b. in the couplet and move on to 40.
40a. Lacinia of maxilla terminating in a single tooth lacking spinules or hairs on the mesal margin...Remenus
40b. Lacinia with a shorter spine mesal to major spine and commonly with additional spinules or hairs...41
41a. Lacinia with a sharp angle just below second, smaller tooth sometimes in the form of a knob with tufts or spinules of hairs...42
41b. Lacinia without a knob...43
There is indeed a "knob" below the smaller tooth and it does have tufts of hairs on it. So we go on to 42.
42a. Submental gills present; outer ventral lacinial surface with basal patch of about 50 dark clothing hairs...Malirekus
42b. Submental gills absent or greatly reduced; ventral lacinial surface without dark clothing hairs, but may have smaller patch of setae...42.5
Here's a close-up of the ventral lacinial surface.
This is a tough call, since I can't really tell if there are 50 hairs on the surface, but, since the option of 42.5 leads us to a choice between Yugus and Diura -- neither of which match the nymph in our picture -- I would tentatively conclude that this is a Malirekus. (For pictures of a Yugus nymph, see the entry for 3/25/11. The body is bright yellow. And, Diura is primarily a Western Perlodid genus.)
But, let's seek confirmation from two other sources, beginning with Steven Beaty's description of Malirekus in "The Plecoptera of North Carolina," p. 26.
Genus diagnosis: Nymphs 15-19mm [this one was 15mm, fairly large]; conical submental gills [great description!]; triangular lacinia with low marginal knob bearing a tuft of setae and ventral surface with a cluster of approximately 50 clothing hairs near base; single curved row of spinules on back of head, obsolete [i.e. missing] near midline.
In his "Notes," he adds some things that are important to us. "Malirekus hastatus is the only species to occur in the southeastern United States. Separate Malirekus from Yugus by the single row of spinules on the occiput. The patch of clothing hairs on the lacinia a traditional diagnostic feature for separation of these two genera, may be difficult to see (setae can be dark or clear) or is not always present as the hairs can fall off."
So, we should not be overly concerned about not finding "50" clothing hairs on the ventral surface of the lacinia, and, we should be able to see a "single row of spinules on the occiput" (i.e. the back of the head).
And there is a row spinules with a gap in the middle.
I think the Malirekus ID is pretty secure, and "conical submental gills" describes the gills on our nymph perfectly.
But, let's check in one final source, Stewart and Stark's Nymphs of North American Stonefly Genera (Plecoptera), pp. 416-417 (Malirekus hastatus is illustrated on 417). Here we find a more complete description than we've found so far of the body and head.
"Nymph Morphology: Body brown with dark brown markings; antennae, legs, cerci light brown. Head mostly brown with light M mark forward of anterior ocellus; two pale, ovate spots lateral to ocellar triangle, and pale interocellar spot; two large ovate occipital spots, broken by reticulate brown lines and bordered behind by row of short, blunt setae."
Let's take a close look at the head, remembering that we've already seen the "row of short blunt setae" at the back of the head in the previous picture.
Perfect! That's enough for me -- I no longer have doubts. This nymph is genus Malirekus, and, following Steven Beaty, Malirekus hastatus. The Rapidan is a river that's rich in Perlodids. I've now found the following: Isogenoides hansoni, Yugus, Malirekus hastatus, Helopicus subvarians, Isoperla namata, Isoperla holochlora, and Isoperla nr. holochlora. Will I find something new on my next trip?
That would be great!