The Big Compound Eye in the Sky

Color vision in humans depends upon three light-sensitive proteins, called opsins, that are present in our retinas. Each type of opsin absorbs one color of light in the spectrum. In humans, the colors absorbed by these opsins are red, green or blue. The many wavelengths of light reflected into our eyes from the surfaces of objects around us mix together to provide a rich palette of color. Yet, despite our visual color range, there is a creature with even greater scope; the dragonfly.

Plathemis lydia (Libellulidae), also known as the Common Whitetail, is a day-flying dragonfly species. [Dennis Paulson photo.]

Unlike humans, day-flying species of dragonflies have four or five opsins, allowing them to see colors that are beyond human visual capabilities, such as ultraviolet (UV) light. Their opsins are grouped in specific ways within small hexagonal packages, called facets. Together, these facets comprise the insect's compound eye. "They are segregated in the compound eye so that the upwards facing eye has only blue and UV receptors, and the downwards facing eye has receptors for longer wavelengths, [such as] green and orange," says Robert Olberg, dragonfly vision expert and professor of biology at Union College in Schenectady.

This patterned concentration of opsin types, particularly those sensitive to blue and UV, gives special advantages to hunting dragonflies. For example, it is thought that the sky appears to be very bright to a dragonfly, thereby providing a clear background against which small moving prey can be easily detected, according to Dennis Paulson, dragonfly expert and director of the Slater Museum of Natural History at the University of Puget Sound, in Tacoma.

Are there color-blind dragonflies? "We don't know," replies Paulson. "There are some [species] that tend to fly only at dusk; perhaps some of them have limited color vision."

Dusk-active dragonflies have sacrificed most of their color vision in favor of increased light-collecting capacity by having fewer, larger facets in their eyes. They also lack all color sensitive opsins except green, which provides the broadest range of light sensitivity for any opsin. As a result, these dragonfly species probably also have a corresponding decrease in overall color perception.

Gomphus lynnae (Gomphidae), also known as the Columbia Clubtail, is another day-flying species. The Clubtails are often difficult to find. (Incidentally, this species is named for Lynn Erckmann, its discoverer. Lynn -- who likes to refer to herself as "Dracula's Mother" -- is the manager for the lab where I completed my graduate degree!) [Dennis Paulson photo.]

Dragonflies (and bees) have the largest compound eyes of any insect; each containing up to 30,000 facets, and the eyes cover most of the insect's head, resembling a motorcycle helmet. In contrast to a human eye, each facet within the compound eye points in a slightly different direction and perceives light emanating from only one particular direction in space, creating a mosaic of partially overlapping images.

Does this mean that dragonflies have 30,000 eyes? "No," replies Olberg. "It's more like a human having 10,000 to 30,000 photoreceptors spread out across the retina -- but better than that because each facet has several spectral types of receptors."

Dragonflies can also detect the plane of polarization of light, which humans cannot do without the aid of sunglasses. The advantages of this capability are unknown for dragonflies, but other insects are known to use polarized light as a sort of "sky compass" by which they navigate.

Another visual advantage of the multifaceted eye is a dragonfly's acute sensitivity to movement, as anyone who has tried to catch one can tell you. "Dragonflies can see in all directions at the same time. That's one of many advantages of a compound eye; you can wrap it around your head," explains Olberg. "The spherical field of vision means that dragonflies are still watching you after they have flown by. However, the backward-looking part of the eye has rather low resolution. So, if you want to catch a dragonfly, let it go by you and then swing your net like a baseball bat from behind. If you swing at them while they are approaching they'll usually see the net coming and easily avoid it. They are awfully good at what they do." Olberg concludes.

Many thanks to Robert Olberg, a PhD graduate of the UW's Zoology Department, Dennis Paulson and David O'Carroll for allowing me to interview them for this story. Thanks to Dennis Paulson for generously providing the beautiful dragonfly photographs that you see here. Thanks also to George at Hanslope for the dragonfly wallpaper.