I've just finished about 20 months of work creating an online
encylopedia of American fly hatches, illustrated with thousands of
closeup bug pics, mostly of mayflies.

If you'll forgive the promotion (it's quite on-topic!) here's the link:
http://www.troutnut.com

For those familiar with the old version of the site: the new one has
about four times more pictures and much better information about all
the hatches.