Animals see the world around them like a continuous video. But in reality, they piece together images sent from the eyes to the brain in distinct flashes a set number of times per second. Humans average 60 flashes per second, turtles 15, and flies 250.

Flies' eyes evolved to pick up light with a series of tiny string-like structures that lie horizontal to the path that light travels through the eye. These structures react to light mechanically whereas vertebrates have long tube-like cells facing the light, with chemicals that react to light at the base. It's more sensitive in terms of being able to give a large signal to the tiniest amount of light and it can also respond faster than the rods and cones in the vertebrate eye.

http://www.bbc.com/news/science-environment-41284065