Vision is an integral part of the lives of many animals, and insects are no exception. Although insects may often be seen as "simple" creatures, many possess sophisticated vision that helps them orient their bodies, carry out complex navigational moves, locate and capture food, find mates, and avoid predators. Despite the superior acuity of the human eye, our own visual system would not suit the needs of a butterfly, dragonfly, or bumble bee. What these and many other insects have instead is a pair of multifaceted compound eyes, which can detect the wavelengths of the visible spectrum of light as well as ultraviolet light. Each compound eye is composed of individual structures called ommatidia, which create the characteristic "facets" of the insect eye. Each ommatidium contains a light-focusing lens, light-sensitive visual cells, and pigment cells that help keep each ommatidium separate from its neighbors. Insects also have up to three small simple eyes, called ocelli that detect light and shadow.

The compound eyes of different insects can have different numbers of ommatidia, ranging from a single ommatidium per eye in some ants, lice, and fleas, to more than 30,000 ommatidia per eye in some dragonflies. Differences in compound eye complexity and sensitivity help insects make a living within their specific environment. Bees can be attracted to yellow and red flowers by the pattern of ultraviolet light that the flowers reflect, for example, and some species of butterflies have UV-reflecting scales on their wings that aid in mate recognition. Increasing numbers of ommatidia provide corresponding increases in depth perception, detection of movement, and visual acuity. Fast-flying and predatory insects generally have more ommatidia, and so it comes as no surprise that dragonflies, the fastest-flying predators of them all, are at the top when it comes to their numbers of ommatidia.

With about 30,000 ommatidia each (one actual count of a Common Green Darner eye found 29,247), a dragonfly's compound eyes are so massive they cover almost the entire head. Each ommatidium points in a slightly different direction; together, these miniature lenses create a mosaic image formed from smaller, partially overlapping images. As hunters of other speedy insects, dragonflies' enormous multifaceted eyes work in concert with coordinated wing and neuronal actions to allow them to determine distance, direction of movement, and speed of their prey to make a quick meal in flight. Their big bulging eyes also give dragonflies an almost 360 degree field of vision, allowing them to see in every direction except directly behind their heads-a good approach for the eager dragonflier to remember!

If you look closely at a dragonfly's eyes, you may see dark bands or spots. These aren't just reflections of the light--they are "acute zones" formed by clusters or bands of larger ommatidia that provide better detection of movement. The centers of these acute zones are visible as striking dark spots, also called pseudopupils. The position of these acute zones differs in different dragonflies, based on their foraging behavior and habitat type. These differences allow a dragonfly in a forest to discern the horizon above the line of tall trees, or to perceive a small moving prey item against the backdrop of a bright sky.

We know that dragonflies' spectacular eyes enable them to detect and capture prey with great efficiency, but do they also play a role in their long-distance migration? Does their excellent vision help dragonflies hug the coastlines of the Atlantic and Pacific Oceans, follow the shorelines of the Great Lakes, track visual landmarks such as the horizon and mountain ranges, and detect suitable habitat to feed or perch for the night? It has been noted that some of the large migratory dragonflies, such as North America's most famous migrant, the Common Green Darner (Anax junius), have impressive high-resolution acute zones comprised of a band of enlarged ommatidia that encircle the top of the eye, allowing them to take advantage of a greater amount of light coming from the sky above in order to catch fast moving prey as they are flying to their next destination. So the next time you are face to face with a dragonfly, gaze deeply into its eyes and ponder the beautiful complexity of vision in these "simple" insects.