Induced Movement

No flies on Rama: The flying bugs illusion
These two bugs fly clockwise along circular orbits of the same size, in all 3 movies. They are in counter phase; one is at 6 o’clock when the other is at 12 o’clock.

Now the right orbit looks twice as big as the left orbit, because the CW moving background is in phase with the LH orbit but in counter-phase with the RH orbit, which it enhances.

The orbits look elliptical, wide on the left and tall on the right.  The background moves CCW and is in counter-phase with the horizontal components of the left fly but the vertical components of the right fly.