Grants two Flies that orbit Isaac, blocking enemy shots that hit them. These kill other flies that touch them, but do not damage other enemies.
Can stack with a Pretty Fly/Cube Of Meat/Guardian Angel/Itself up to a maximum of three. Flies have the lesser priority regarding orbitals and are replaced by other orbitals when all three "orbital slots" are taken. Flies cannot replace other orbitals and nothing happens if picked up when you already have three orbitals.
If Isaac picks up a Pretty Fly while all three orbital are already filled, the fly will not appear visually nor provide extra blocking. However, if the player later picks up a cube of meat that would turn a level 2 cube of meat into a level 3 cube of meat (and thus vacating an orbital spot) the pretty fly from earlier will replace the cube of meat as the 3rd orbital.
Guardian Angel - The potential to block shots is greatly inceased.
This item is probably a reference to Alice Cooper's song "Halo Of Flies" from the album "Killer".