Quills were originally developed by Theo Gordon (Quill Gordon) to fish the catskills.  Quills are one of the first Eastern mayfly hatches come spring time.  The flies hatch on the bottom, rise as adult, and skate the surface as their wings dry before they take flight.  It's at this stage where they are the most vunerable, and trout will target the adult that's not quite ready to take flight.