Angua1 says... It's the reflection of light off the back of their eyes. When I walk the dogs or run early in the morning I wear a cap with led lights on the brim - exactly the right spot for this kind of reflection to shoot back so I can see it - different angle than if I was holding a flashlight. Anyway, with this light I noticed the reflection off deer eyes is greenish, more muted with rabbits, very bright with raccoons and foxes. And then one day I thought i noticed some bright tin foil or something sparkly on the ground - nope spider eyeballs! In the fall the sparkles on the ground are amazing. I highly recommend experimenting with using a led flashlight in the fall. It's very cool to find critters, including spiders, this way.