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Comment by scotty79 | Hacker News Reader

I had a really weird observation. Do you know how in the summer flies tend to enter rooms and circle the middle of it (where usually the turned off light fixture is)? And they seemingly can't or don't want to find open window to leave the room?

At one point for unrelated reasons I replaced light fixture in my room with more than 200W worth of strongest E27 Philips LED lightbulbs I could find (20*1521lumen).

Few flies gathered in the middle of the room as usual during the summer day. Then I turned on this light just out of curiosity. The flies dispersed within seconds, suddenly perfectly capable of not flying in circles. I don't think it was the heat or the discomfortably bright light just scaring them off. Even this amount wasn't as bright as sunny day. I think more light and more distributed light (not just comming from the direction of the window) fixed their navigational abilities somehow.

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I have a few theories, these are my own speculations, so I could be wrong.

I speculate that the flies rely on a phenomenon called "optic flow" for navigating around obstacles. Basically the rate of feature movement perceived on each side of their head/compound eyes determines proximity to objects. Their vision is very blurred, so features need to be quite large, or at least contrasting. If you have a dimly lit room, with white walls, the whole space will appear mostly featureless to the fly, until they get close enough to the wall, and thus fly around in circles.

Perhaps they also use a fixed contrasting object as a reference point, like the light fixture (turned off) and return to it if they veer off too far into the "featureless void". If you turn on the light, the perceived illumination in the space is inverted. The light makes previously invisible objects around the room more contrasting for them to navigate about and to fly closer, even land.

This actually squares with the study, no? I mean, when they are inside - and it's my experience as well - they might take the window as the main source of light and think that's the sky, so they start to fly in circles.

That's probably it. When the "sky" doesn't align with gravity they get confused.

The observable effect is true, but I don't think it squares. Circles below the ceiling in the middle of the room aren't perpendicular to the window as a light source.

I see this mostly with one particular type of fly that just loves going around light fixtures, maybe they're sensitive to the EM spectrum rather than visible light?

I think flies might use both sky and gravity for navigation and when sky is not up it messes with them and results in circles.

I don't think those circles are exactly perpendicular to the vertical direction. I think most of them are skewed.

I notice that clouds of gnats will gather at the boundaries between light and shade. Other insects may adopt similar behaviors.

I was under the impression they confuse lights for stars and it messes with them. So if you simulate the sun the probably don’t get confused. You can but red spectrum lights that also are supposed to be bug friendly

I like bright lights as much as anyone else, could you explain how you got a 20x fixture set up in your room? I've heard of 500 watt E38 mogul fixtures, but not 20x mini ones

I just screwed 20 cheapest lightbulb sockets to a board and connected them with wires.

Flies easily fly through the window if it's substantially brighter than the room.

Not in my experience. The walls were dark. And the window was bright occupying half of the surface of one wall. They still flew in circles.

I think it might be what other commenters are pointing out. That flies consider the brightest direction to be the sky, but if it's to the side (with respect to gravity) it messes up their navigation.

I wonder now, do flies ever venture inside of caves?

I’m curious: how are those 20 bulbs installed? You got a single fixture with 20 bulbs??

Fixture adapter like this maybe? https://a.co/d/iOgDKZf

Just screwed 20 lightbulb sockets to a board and connected them with wire.

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