The interesting bit (to me) isn't so much that they're black, but that you can see the compound eye at work. Each lens has a very narrow field of view and only

The interesting bit (to me) isn't so much that they're black, but that you can see the compound eye at work. Each lens has a very narrow field of view and only appears black when the observer is within it, while a human pupil appears black from pretty much any angle because it's always using the one lens and aperture.