It seems to me that more bugs would collect on the back of the prop because of angle of attack. But some will obviously collect on the front as the blade is still moving with the front of the blade forward. The bug is traveling at an angle that is somewhat less than 90 degrees to the blade cord line. (minus angle of attack) But the bug has inertia and can not move out of the way of the prop, so it strikes the front. With a wing the bugs are traveling relatively paralell with the wing, so no bugs strike on the top or bottom of the wing.