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 a

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.