I've been looking for this one since September: the Perlodid stonefly, Isogenoides hansoni. The Rapidan River is the only stream in which I've seen this particular species. I found two small ones last year -- in September and October -- and one mature one the previous year on 3/25 (on which see the entry posted on 5/14/12). This is an insect that is "relatively rare" (Steven Beaty, "The Trichoptera of North Carolina," p. 22).
It's worth reviewing how we identify this beautiful stonefly -- both genus and species. Let's follow Beaty's descriptions, but I'll interrupt with pictures to verify the points that he makes.
Genus Diagnosis: Lacinia triangular, bidentate with a distinct knob below the subapical tooth and with a row of marginal hairs approaching base. Picture.
He continues: mesosternum with median longitudinal suture joining fork of mesosternal grooves to transverse anterior suture. Picture (in which I've called the "median longitudinal suture" the "median ridge" and the "transverse anterior suture" the "transverse ridge."
prominent submental gills, projecting about three times their basal width. Yes.
cerci with a dorsal setal fringe (I don't have a photo of that); body light brown with contrasting darker brown pattern, covered with clothing hairs. The colors of the body are clear from the pictures, though I don't think the clothing hairs show up very well.
On to the species ID, again using Beaty.
I. hansoni -- nymphs 16-24 mm; large denticles on the ventral mandibular tooth. (No photo of those; size of this immature nymph was ~ 10 mm.)
conspicuous, sharply delineated M-shaped pale mark anterior to median ocellus; ocellar triangle bordered by dark but with pale central spot; dark transverse bands on anterior third to half of terga 1-9 and a dark, transverse band along each posterior margin. Relatively rare.
All of those features are visible on this photo of the nymph from today.
Finding an I. hansoni nymph made the trip worthwhile on its own. But, as always, there were lots of beautiful insects in the Rapidan River.
1. A brushlegged mayfly, genus Isonychia. I think they're prettier now than they are when they're fully mature. Great close-up of the head and front legs in the second photo.
2. The spiny crawler mayfly, Ephemerella subvaria. I saw a lot of them.
3. Large winter stoneflies, Taeniopteryx burksi/maura. One of the most numerous insects I saw.
4. Glossosomatids -- Saddle-case maker caddisfly larvae. Also quite common.
5. Pronggilled mayflies, genus Paraleptophlebia. Very common.
6. A free-living caddisfly larva -- Rhyacophila fuscula.
7. A common netspinner -- Ceratopsyche alhedra. ("Prefers clean streams and fast current. Mountains. Uncommon. Very intolerant." Beaty, "The Trichoptera of North Carolina," p. 72.)
8. And a Lepidostomatid case-maker. Odd case. Normally the building blocks are evenly clipped pieces of bark and/or leaves.
I'll stop there -- but there was more. It's a very rich stream with really good insects. Isogenoides hansoni.