Other than THE HEX I doubt that a single hatch exists that is more widely known or better loved in Michigan than the Hendrickson. The thick, grey clouds of dancing bugs you'll see before a Hendrickson spinner fall is something you'll always remember. Some years there can be amazing numbers of individuals in the stream: a single square foot of gravel in Michigan's Au Sable River yielded over 1,200 nymphs of Ephemerella invaria and subvaria. Multiply that by the number of square feet of gravel in a stream, and you can get an idea of how a hatch can cover the water with duns. The peak emergence in Michigan is from May 5th to May 25th making it the first and most eagerly anticipated "heavy" hatch of the season.
So let's say you're already aware that the glassy-winged, early evening Hennie with her bright yellow egg sac is the spinner stage of that dusky colored upright winged dun you watched emerge that same early afternoon. And let's say you also know that the dun (Hendrickson, a.k.a. Beaverkill) and the spinner (Adams Female, a.k.a. Lady Beaverkill) are indeed females. But did you know that the male of the species, imitated with the ruddy-ringed Red Quill is a different and maybe an even more important fly? As a matter of fact if fly-fishers would retire the Females Adams and focus instead on their Red Quill and Hendrickson dries, their hook-ups would be much more numerous at day's end.
And let's also say you knew the Hendrickson was first tied by Roy Steenrod of Liberty, New York in the spring of 1916 and it was patterned after the natural Ephemerella subvaria mayfly. Since Steenrod didn't know the scientific name for the insect, he named it in honor of Mr. Albert Hendrickson. So popular and effective did the pattern become, that today we refer to the natural mayfly as the Hendrickson.