This is a simple idea which actually works!
The principle is horseflies are attracted to warm bodies so they can have a nice nibble and drink of your blood.
The black painted ball becomes the hottest body around, so attracts the flies.
They bite the ball and realise their mistake (and here's the clever bit)
The flies' response to being stupid is to fly up and away, they fly into the funnel and end up trapped in the bottle.
You come along later and kill them as revenge for all the little buggers who weren't caught and bit you anyway!
The most important part is the black ball - I found some fishing floats on the beach and painted them, these have the advantage of being quite heavy so the trap doesn't blow about too much.
The funnel is made of polythene but you could use any material you have, maybe polythene helps warm up the ball.
The funnel is kept rigid at the base by a hoop.
Step 1: Paint Your Balls
Your balls need to be black to absorb all that nice solar radiation - while they are drying move on to the next step
Step 2: Make a Hoop and Funnel
The hoop has to be a bit bigger than the ball. I used a bit of 8mm copper tube, forming the circle around a barrel and fastening it together with a straight pipe fitting.
I'm sure there is a clever way to cut out a circle of polythene to turn into a cone but I just made it a bit bigger than the hoop - once you cut out the section and work round the hoop with gaffer / duct tape you find there is plenty of wiggle room. I made sure all the joints were well sealed with duct tape so there were no holes for the critters to escape.
I suppose you could use any material / sailcloth etc and sew it if you were clever - I'm not and plastic keeps the rain off so perhaps the flies also use it as an umbrella... I don't imagine it will last very long but I can easily remake it next summer.
Step 3: Fasten It Together
The ball needs to hang just below the funnel. I found it easiest to hang the whole thing from the same rope and used an old drinks bottle as the prison. The top of the funnel could do with a hoop so it suspends a bit better. I'm sure there are lots of better ideas to make it more beautiful and efficient but this was quick easy and cheap - and works!