Found this mayfly in the cab of my pickup today, on the way back from Darlington Spring Ditch, down near Three Forks on the Madison River, Montana. March 4, 2012. First mayfly of the year for me. I put him a fly box and then transferred him to my fly tying vise once I got home--which is surrounded by a light tent. Managed to squeeze off two frames and then she flew. Haven't seen her since. This was a big mayfly. Roughly the size of a Pale Morning Dun. Is this a Baetis of some kind? Sure looks like it. Some giant sub-species? Or what? Without the bulging salmon-colored eyes, this is probably a female. Regardless species. The exaggeratedly thick (and high bulging) thorax is new to me. Ah. Perhaps a Western March Brown.