N80587 maneuvered around the observation tower at Shark Valley at an altitude of 600 feet. The flight then departed the area climbing toward the southeast at 100 knots. N1063F was performing slow flight with full flaps extended at approximately 800 feet and a southwesterly heading. As the radar targets of both airplanes crossed, N1063F turned toward the west and began to fly an erratic path. N80587 continued to climb normally and returned to the Tamiami Airport for landing. N1063F was lost from radar one minute later. A tire mark on the upper inboard area of the right wing of N1063F was found which best matches the left main tire of N80587. The rear wing spar of N1063F failed in the area of the tire mark and the airplane experienced inflight separation of the wings. The pilot of N80587 did not recall colliding with another airplane or any event that would suggest this. He passed a polygraph test to that effect.