Mayfly

(hexagenia limbata)

They emerge and mate in huge masses. See this report from 2010 http://www.treehugger.com/corporate-resp...

This is a mayfly (Ephemeroptera http://en.wikipedia.org/wiki/Mayfly ). Their group -order- name comes from the fact that adults (like this one) have a very short lifetime. I have no idea what family or genus this is. There are many emerging in early June in Wisconsin http://www.wiflyfisher.com/wisconsin-fly...