Help & opinions here

[Rocketfish]

http://www.hatchesmagazine.com/page/january2006/86

My guess is some type of the Maccaffertium.

Wing mottling and number of tails would be wrong for Epeorus.

Isn't it mid-fall in Argentina? Are there any flies here outside of the various small types of BWO's that hatch in the fall?

Steven...thanks. And I don't know, I mean I am m not in Patagonia at this time (trout region) but near Buenos Aires. So I'm not fishing; the mayfly appeared at home. I also checked March Browns in this area. Bigger and darker than this relatively pale mayfly. Yes, it's beginning of fall.
However, I think it is very interesting to compare the bugs (in this case a mayfly) that you have in the North America with our insects and see (if any) the differences.
This one is very usual in Patagonian lakesMayfly Traful.jpg.