"The two huge and often colorful compound eyes are perhaps the dragonfly's most striking feature.
Each eye is made of thousands of individual visual units (or emmatidia), complete with its own lens
and light-receiving cells. Not limited to light and dark, these eyes can perceive color, as well
as ultraviolet and polarized light. Because of the large number of closely spaced lenses in each
eye, dragonflies are especially adept at detecting motion - an important ability for an aerial predator.
Note how the eyes curve around the head, allowing the dragonfly to see upwards, downwards, forward
and to the rear. If you are very close, you may see three tiny eyes, or ocelli, atop the head,
generally assumed to be organs that measure light intensity."
From the book, "Dragonflies & Damselflies of the Southwest" by Robert Behrstock.