I think there may be a common misconception here that mayflies are the same as black flies. Mayflies are not all that common in NH (and do not bite), whereas black flies are the bane of all who need to spend time outdoors this time of year.
Here are some sites for more information:
This link provides links for a distribution map of mayflies in NH
http://www.npwrc.usgs.gov/resource/d...fly/nh/toc.htm
This link has drawings of the mayfly:
http://ohioline.osu.edu/hyg-fact/2000/2166.html
This link has information of the Black Fly:
http://www.ultimate.com/washington/wla/blackfly/