it is the mayfly(adult)

Mayflies are very sensitive to pollution, and as such are usually only found at high quality, minimally polluted sites. Along with caddisflies and stoneflies, they are one of the three most commonly used indices of aquatic ecosystem health.

more info:

http://www.cirrusimage.com/ephemeroptera_mayflies.htm

http://www.maine.gov/dep/water/monitoring/biomonitoring/sampling/bugs/mayflies.htm

http://en.wikipedia.org/wiki/Mayfly