With no known sense of hearing and a limited smelling capacity, a dragonfly's greatest assets are its eyes. Dragonflies are believed to have the largest compound eyes of all insects, giving them an incredible hunting advantage. Adults have a visual field of almost 360 degrees and extremely mobile heads, allowing them to see practically everywhere but directly behind—an important piece of information for dragonfly collectors and hunters alike. Dragonfly eyes each contain as many as 30,000 ommatidia, hexagonal facets that collect views from all different directions, which overlap to create a mosaic-like image.
In many ways, a dragonfly's sight has become more evolved than ours. Humans have three opsins, or light-sensitive proteins, that each absorb a specific color of light such as red, green, or blue. Dragonflies can have up to five opsins, which allow them to see ultraviolet light in addition to a wide range of color. Their advanced eyes can also detect the polarization of light and see fast-moving images more clearly than we can. They can see light flicker at 80 times a second (twice that of humans), an ability that is thought to aid them in identifying the patterns on rapidly beating wings.
Of the many dark spots on a dragonfly's eye, the largest is called the pseudopupil, where dense clusters of ommatidia create fovea that allow for minimal blurring while the dragonfly is traveling at high speeds. Bigger spots on the eyes indicate better vision in that direction. Daytime hunting dragonflies have bigger spots on the upper parts of their eyes. These spots have facets that detect blue and ultraviolet light best, making the sky seem much brighter than the ground—an effect that allows dragonflies to easily spot their prey against a bright background when swooping up from underneath.