Mayfly for sure. I live about 1 block from the mississippi river so these are really no big deal. They only live for like 48 hours and then they die, something like that. They have to attemt to reproduce in that amount of time so that they will exist next year at this time. I'm not positive on that info but I think that is how it is. Sometimes they get so bad on the bridge that they have to get out the snowplows and shovel them off the bridge. They get extremely slippery otherwise.
Amdskip-don't quote me on above info