ELi5 Why can flys see you coming a mile off when you try to swat them…but they can never seem to find their way back out of an open door?
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while they may “see you coming” it is actually more that they can sense it.
flies can feel your hand moving toward them by the rush of air it creates. this is the main reason fly swatters are perforated. Rolled up newspapers and magazines can work as well, as they do not act like a paddle pushing a wall of air toward the fly.
They just don’t have much brain to work with. Dodging in a random direction when something big comes their way is dead simple. Figuring out what a door is might as well be rocket science.
Doors don’t exist in nature. They never evolved to have to understand and deal with doors. Things trying to kill them have always existed, and they had to evolve to stay alive.
This is also why the best way to swat a fly is not to approach slowly and then swat at the last second. The best approach is to quickly move in from across the room and swat with no hesitation. This doesn’t give the fly enough time to sense you and dodge in a random direction.
As others have said to a fly an open door, a window, a lightbulb, a TV that’s on, etc might all look very similar if the fly is just looking for daylight, making it very difficult to navigate an indoor space to find their way outside with such a tiny brain.
Flies have evolved with what amounts to a built in reflex to avoid incoming objects. It’s automatic for the fly and works well.
In fact, basically everything is automatic for flies. They can’t really think or adapt to changes. When they are trying to get somewhere and they hit an obstacle their ‘program’ has them move a few inches in a random direction and try again. This works well enough for getting around plants or rocks in the wild, but it’s a really bad way to get though an open window.
1. Flies don’t see why they should head out your door. They’re attracted to smells, and if there aren’t smells drawing them out they’re not going.
2. Flies can’t see very far. Like, a few meters at most, even though their eyesight is better than most insects (your average insect has a resolution like a computer screen from the 80s).
Their eyes don’t “see” like ours. They are just light sensitive, there’s no picture. They are super-receptive to sudden changes in light (like the shadow of your arm) but the change in light around an open door only tells them something is there and not there, they don’t know which side is out.
So there is a trick with flys, especially if you have quick fingers. Put your hand into a flicking position ( middle finger being held by thumb). Slowly let your hand approach the landed fly and release your finger. No fly can move that fast and it’s devastating to it.
Also they don’t want to leave, they want to land find nourishment and lay maggots. It’s what they do.
Flies are just socially awkward, they don’t want to leave through the open door because they don’t want to seem rude.
They have built in mechanisms to fly away backwards when they see a shadow. Doors are a resent invention as far as their evolution has gone and they will probably never adapt to it.
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