Compound eye (noun, “KAHM-pownd ahy”)

A compound eye is a type of eye that is made up of many smaller visual units. Each unit is called an ommatidium. A single ommatidium works sort of like a single human eye, collecting and processing incoming light. This allows the ommatidium to capture one small view of the world. All those small views from a compound eye combine like the pixels of an image to create one larger view.

Lots of creatures have compound eyes. That includes many insects. Think flies, bees, ants, beetles and butterflies. Crustaceans sport compound eyes, too. The mantis shrimp is one example. Clams and sea stars are members of the compound eye club as well. In fact, the oldest known fossil of an eye was a compound eye. It belonged to a trilobite that lived some 500 million years ago.

Different species have different numbers of ommatidia per eye. Some ants have only a few. Meanwhile, some dragonflies have tens of thousands. The more units a compound eye contains, the sharper an image it creates. This is similar to the way higher resolution images made up of more pixels look crisper and clearer. But even tens of thousands of ommatidia, or pixels, per eye is not very many. The resolution of human vision is estimated to be millions of pixels.

So compound eyes are not known for their sharp vision. But they do offer animals very wide fields of view. And they are generally quite good at detecting movement.

In a sentence

Flies and bees may have compound eyes, but other creepy-crawlies such as spiders have eyes more similar to our own.

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