Due to Shane's inquiry about Isonychia
on another thread, I thought I'd bump this one back up. 'Tis the season, and I missed this thread when it was first posted.
Just to try to confirm a few things from the earlier posts:
often has a fairly flexible emergence. Sometimes you will see most of them crawl out to emerge; other times most will emerge instream; sometimes they will emerge both ways. Weather and water conditions probably play a role. Low water sometimes seems to promote crawling out, and high water seems to promote instream emergence, but I'm not sure there are really hard and fast rules about this. (In some streams, low water seems to trigger instream emergence.) Under "normal" conditions, I often see a mix of both, slightly favoring crawling out.
Paul Weamer is right, and the "green" color is not exclusive to the Upper Delaware. I have often seen greenish or olive-bodied Iso duns. In my experience they have always been females, so I have a bit of a hunch about the green color. (Strictly a hunch--as soon as I see a green male, I'll have to revise my thinking.)
The color of the underlying eggs often influences the body color of female mayflies. This is especially true of (usually) lighter-colored species like Epeorus vitreus
(the females are sometimes called "Pink Ladies" due to the underlying egg color) or some Stenacron
species (the females are sometimes called "Salmon Spinners"). The egg color could also show through on recently emerged ("teneral") duns of normally darker-colored species.
The egg color of Isonychia bicolor
spinners (probably the most common species in much of the East and upper Midwest) is usually greenish. (Some other Iso species have orange or yellowish eggs, and I'm not at all sure that egg color is always constant within a species.) I think that the greenish color of recently emerged (female) bicolor
duns might be due to the color of the underlying eggs. Just a thought.
Although I can't provide the photo of a green Iso dun that Matt requested at the beginning of this thread, Jason does have two photos that show somewhat "olive" (female) Iso duns that might have darkened from a more greenish color:
It's a shame that we didn't get to see Tonyz's photo--perhaps it was a male. :)