This is an insert designed to make a simple but effective fly trap. Remove the top from an empty drink bottle and keep it somewhere safe. The insert fits the standard smaller size bottle necks such as coke. Place some small pieces of meat scraps inside the bottle then fill the bottle with about 2 or 3 inches of water. Drop the insert into the neck of the bottle and place the whole thing somewhere in a warm spot in your garden.
After a couple of days the meat will go bad and begin attracting flies which will fall into the water and drown. When the bottle gets too full of dead flies you can either remove the insert and replace the bottle top or simply screw the top on over the insert if you don't fancy touching it. You can then throw the whole thing away or if you want to reuse the bottle, tip the contents out into a hole in the ground after waiting for the last of the flies to die (not easy, there will be a lot of flies!). I strongly suggest you wear gloves when handling the bottle once it has done it's job! This trap will kill hundreds of flies and clear a good area around your home for a while.