I can't profess to the exact size of the eastern March Brown, as I've only fished it a few times, but the Western March Brown is generally a size 12-14. It's irrelevant anyway, as you pointed out, they are two different species. The Western March Brown, Latin name Rhithrogena (Morrissoni is the subspecies found on the Yakima River), has three tails according to the sources I could find. "Rhithrogena Western March Browns have 3 tails. Gills on belly side shaped into suction disc. Slight notch on front of head. Body length 5-12mm." (
http://home.earthlink.net/~troutbugs/html/insect_identification.html)
The term BWO or Blue Wing Olive, encompasses a massive variance of subspecies of Baetis all lumped together. I'm not sure what it's like here in Washington, but the state of Wisconsin has 17 identified subspecies lumped under the term BWO, ranging from a size 24 to a 18. I've fished the BWO hatch across the US, and have seen many different types, I've even seen two different subspecies hatching at once. On the South Platte River one day it took me four hours to figure out that the fish were keying on the size 22 Baetis with a black head, not the size 18 Baetis I saw most prevelent on the water. By then, of course, the hatch was over.
To quote Al Caucci, a recognized expert on Mayflies, "The tiny blue-winged olive duns of Pseudo-cloeon are easy to recognize in spite of their small sizes. They have no hind wings and only two tails..." Also, to quote another website, "Baetis - Blue-Winged Olive. Antennae long, 2 or more times longer than width of head. Some species have 2 tails. Plate-like gills on abdominal segments 1-7. Body length 3-12mm." (citing,
http://home.earthlink.net/~troutbugs/html/insect_identification.html)
This bug has none of the identifying features of a Western March Brown, three tails and a mottled wing, and has all of the indentifying features of Baetis, two tails, blueish-green body, no hind wing, and a dun colored fore-wing.
worldanglr
http://www.worldanglr.com/
Calling Fly Fishing a hobby is like calling Brain Surgery a job.
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