If you look first at the behavior of the fly on the right, you can see that whenever it reaches the midpoint of the chamber where the two odors meet, it marches straight through, as it did before.
右側のハエの行動を 最初に見ると ハエが部屋の真ん中  二つの匂いが混じっているところに たどり着くと 今までと同じように まっすぐ進みます