Little and Large by danob ()
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The eyes of butterflies are large spherical structures. These are compound eyes consisting of thousands of hexagonal shaped omatidea. Each omatidea, or miniscule sensor, is directed at a slightly different angle from the others. Collectively they are directed in every direction -- up, down, forwards, backwards, left and right. Because of this, butterflies are able to see in virtually every direction simultaneously.
There is a price to pay for having an exceedingly small brain and omnivision. Butterflies cannot focus their vision as what they see is at best a blur. Furthermore, they are sensitive to only the three most basic features of vision which would be light, colour and motion. Butterflies can distinguish night from day. They might distinguish colour along a very narrow band of the light spectrum. Hence a butterfly might see and feed from the nectar of a purple flower but be oblivious to the red flower nearby. Butterflies are also sensitive to movement. When you try to sneak up to photograph a butterfly it won't think, "Here comes Danny boy again". It would however be aware that something big out there is moving, getting closer and it's time to fly.