The official factory designations are 458 Italia for the coupe and 458 spider for the convertible. That's why you don't hear it referred to as the 458 Italia spider. The same applied to the 360 with the coupe being the 360 Modena and the convertible the 360 spider. The badge on the dash is where you find the official factory designation.
Current: F50, 246 GT (x 2), 246 GTS
Past: 360M, F430 spider, 430 Scuderia, California (x 2), 458 Italia (x 2), 458 spider, 246 GT