This nymph was among the insects I found yesterday at the Rapidan River. It's clearly a Pronggilled mayfly (family: Leptophlebiidae; genus: Paraleptophlebia) -- the "Blue Quill" to fly fishermen. If you click on the photo to enlarge it, the "pronged" gills are easy to see. But this is unlike any nymph of this family that I've seen before.
Having looked at photos of adult, terrestrial pronggilleds on Troutnut.com last night (http://www.troutnut.com/specimen/683), I quickly realized that I had found a nymph that was very close to "making the change". The head is already the head of the terrestrial form; so too is the abdomen -- with the exception of the few gills that still stick to the sides. But, the wings have not yet fully emerged; they're still encased in the long, black wing pads of the pre-hatching nymph. It's possible that this nymph had moved to the surface, or was on its way to the surface to fully emerge, when I picked it up. In my tray, it stayed on top of the water -- it never went down to the bottom.
One further point: note the VERY large eyes. This is a sure sign that this is a male. Male mayfly adults have large eyes to help them see the females for mating as the imagoes swarm over the stream. Another look, with a good look at those eyes.
Having reached this realization, I remembered that I had found and photographed pronggilled nymphs on my two previous trips to this river this winter. So, let me show you the photos I took on those visits.
Once again we can see how two things occur as a mayfly nymph grows and matures: 1) the wing pads get longer and darker, and 2) the nymph gets darker in color, and any patterns show up in greater detail.
2/23: Pronggilled mayfly at the Rapidan River
3/24: Pronggilled mayfly at the Rapidan River
I have one other photo of a pronggilled nymph that I took at an even earlier date. This specimen is preserved, and it was found at Long Island Creek in Fluvanna County on 1/5.
P.S. I feel more confident that the Limnephilid I found yesterday is genus Pycnopsyche. For the key features of this particular genus, look back to my entry of 4/22. On our specimen from yesterday, the "metanotal SA1 sclerites" are clearly present and not fused, and I think I can see the posterior sclerites on the lateral humps. Here is a full view of the larva out of its case. It is exactly 1 inch long!
On the genus of the Perlodid stoneflies I found -- I still feel uneasy with the call of Isoperla. Nonetheless, the critical features for identification -- the "Y" shape of the mesosternal ridge, and the shape of the lacinia and the setae on the lacinia -- key it out to Isoperla or Clioperla. The head pattern rules out Clioperla.