Air intake underneath the car

The engine air intake is behind the side glass window, not underneath. There is cooling for the radiators underneath the car from the front behind the front wings. So, this is not unlike a conventional car. There are also small cooling radiators at the rear in the wing for the engine oil and the gearbox.

On the spiders, the engine air intake is at a different location. It's at the rear spoiler. I'm guessing it's because the top cover has to open and close which makes a tight fitting tube/channel hard to do mechanically.

 

Correct.

What is interesting is that the 458's air requirements are met by the relatively small intakes. I believe there is a slight ram effect at high speeds, good for about 5 hp as I recall.

 

But there is an air intake, below the car (underside) immediately in front of the rear wheels. It is cooling air for the engine bay, as opposed to combustion air. A high pressure area exists immediately in front of the rear tires at speed. Ferrari takes advantage of this high pressure zone to siphon off some air.

 

Go to the 458 section of Ferrari's web side and read the bit under aerodynamics and watch the video clip. You can see the vents in the clip, albeit it is an illustration as opposed to a photo.