An insect sees the world very differently from a human, for its eye is built on a different plan. Whether housefly, butterfly or beetle, all insects have compound eyes made up of separate units. Some species also have simple eyes. Certain moths and dragonflies have as many as 30,000 units in each eye. Ants may have as few as 6. Each unit has its own lens or facet, so that the insect sees a mosaic image, something like the dots of a greatly magnified newspaper photograph. Because the facets of an insect’s eye have a fixed focus and cannot be adjusted for distance, insects see shapes poorly. On the other hand, compound eyes are excellent for detecting motion and thus avoiding predators or tracking down prey.
With eyes that cover most of their heads, flies and dragonflies have almost 360 degree vision; they can detect predators coming at them from behind, above and below. Ants, which spend most of their time underground, get along with rudimentary eyes, and some species are blind.
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