Closeup of a fly's compound eyes

Word of the Day: Ommatidium

Closeup of a fly's compound eyes
This fly (Order Diptera) has compound eyes composed of thousands of ommatidia, or photoreceptors. July 21, 2012.

Unlike human eyes, the eyes of insects are compound, which means they’re composed of thousands of light-receiving units called ommatidia (singular ommatidium). These “eye units” are arranged on a curved, convex surface, so they all point in slightly different directions. Each ommatidium is limited in resolution, so in order to see “well,” insects must have many ommatidia composing each eye.

But they do have an advantage over the simple eyes of mammals like humans: fantastic angle of view, which is why it’s so difficult to sneak up on insects, particularly those like flies and dragonflies with very large compound eyes containing many ommatidia.

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