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You think flies hunt for gaps? Actually, they're not looking for gaps at all. A hairline crack under the door, and it gets in. Doors and windows wide open, and it keeps banging into the glass. How can the same fly be so clever, then so clueless? Because coming in and getting out, it uses two different tricks.
First, the way in. The food in your kitchen and the trash can keep letting smells into the air. Air from inside leaks out through the cracks around doors and windows. The smell leaks out with it.
So outside that crack trails an invisible ribbon of smell. Scientists have filmed fruit flies doing this: catch a whiff, and they turn and rush straight into the wind. Lose the smell, and they zigzag side to side until they catch it again.
Smell it, rush. Lose it, zigzag. Keep doing that, and you end up right at the crack. It didn't find the gap. The gap brought the smell to it. Now, the way out.
Inside, the smell is everywhere, and no ribbon points to the door. The smelling trick is useless now. So it falls back on its other trick: fly toward the brightest spot. Outdoors, the brightest side is the sky. Flying toward light means flying out.
But indoors, the brightest thing is often a window. And in that window is a sheet of glass it can't see. That open door is often dimmer than the window. So it never heads that way. It hits the glass, but that side is still the brightest. So it hits it again.
So the fly isn't smart one minute and dumb the next. It follows smell to get in, and light to get out. Indoors, the light just fools it. What other bugs have you seen that get in but can't get out? Tell me in the comments.
It didn't get dumber.
Coming in and getting out,
it uses two different tricks.
The food in your kitchen and the trash can keep letting smells into the air. Indoor air leaks out through the cracks around doors and windows, and the smell leaks out with it.
So outside the crack
trails a patchy,
invisible ribbon of smell.
Scientists used cameras in a wind tunnel to track fruit flies chasing a smell. Catch a whiff, and they turn and rush into the wind. Lose it, and they zigzag side to side until they catch it again.
Rush, zigzag, rush, zigzag, and you end up right at the crack. That study used fruit flies; here they stand in for house flies.
It didn't find the gap.
The gap brought
the smell to it.
Inside, the smell is everywhere, and no ribbon points to the door. The smelling trick is useless now, so the fly falls back on its other trick: fly toward the brightest spot.
Outdoors, the brightest side is the sky. Flying toward light means flying out. That trick usually works great.
But indoors, the brightest thing is often a window, with a sheet of glass the fly can't see. If the open door leads to a hallway, it's usually dimmer than the window, so the fly never heads that way.
It hits the glass.
That side is still brightest.
So it hits it again.
Scientists have also found that flying insects keep their backs turned toward the brightest side to stay upright. Outdoors that's the sky and it works fine; near a lamp, it makes them circle the light and get trapped.
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