This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive ID.
Calloway on Jul 9, 2008July 9th, 2008, 7:32 am EDT
Got some guys wanting to say the following is a Hex Limbata. I'm not sure what it is but a little light in color for the Hex's I am familar with. Would like to say golden Drake but only two tails.
This is Hexagenia limbata. That species displays tremendous variation in appearance over its transcontinental range. Sometimes the observed differences are due to this variation, sometimes they are due to the difference in coloration between male and female, and sometimes they are attributable to capturing the insect at various stages of darkening following a molt. Teneral (freshly emerged) insects will be much paler and will have less distinct markings than those that have had time to age and darken. This is true of nymphs, duns, and spinners. Your mayfly is a female imago (spinner) that has yet to express her egg packets.