Why am I seeing worms?

Kat Arney put this to fellow Naked Scientist Chris Smith...

Chris - I think this is Scheerer's phenomenon. It's the blue entoptic phenomenon. When you look at a bright blue sky, you are seeing the white blood cells crawling through your capillaries on the back of your eye. The white cells actually, because of the blue dominance of the light, they actually reflect the white light back at you and as a result, you see the incidental migration of a cell through the network of capillaries. And that's why it looks like a worm crawling because the white cells are few and far between in number compared with the red cells which soak up the light and you are adapted to the red light so you don't see them. So, I think that's the reason.