Maybe they suffer from the barber pole problem. Since there is only an edge, and no corners, the flies cannot localise themselves when they are too close. Witho

Maybe they suffer from the barber pole problem. Since there is only an edge, and no corners, the flies cannot localise themselves when they are too close. Without corners, they wont be able to visually tell whether or not they get closer to the target, not if they move along the edge. If the stripes are non-homogeneous. They wont be able to tell neither how far they moved across them either.