How do flies know when you pick up a fly swatter?

Question

How do flies know when you pick up a fly swatter?

Answer 1

It's the location and angle of their eyes.  They don't sense the fly swatter but see it coming.

Flys also take off in reverse instead of forwards which makes them harder to smack.

To sneak up on a fly you must be slow, and at level with it's upper wing tips.  and make your angle of smacking approach from down and behind, not front.

Answer 2

They have good vision.

Answer 3

Fly's posses a compound eye which is made up of multiple Lenses ( the part that allows you to focus light) which gives them a 360 degree view of the world. So it doesn't know what you picked up all it knows is that your larger than it is and coming towards it so it needs to fly away.

Answer 4

telepathy?

or maybe they detect subtle changes in The Force.???

Answer 5

with all those eyes they can see you.