The Spider will crawl to the edge of the ceiling, then down the
wall toward a symmetric point on the floor, and across the floor
to the fly. The path is symmetric about the midline of the wall.
View the room from the north and "unfold" it by cutting
the lateral surface along the south side somewhere and folding
out the ceiling and the floor at the points where the spider's
path crosses. The minimum distance should be when the spider,
the midpoint of north line on the wall, and the fly are collinear.
Why is it not?