<a name=top>Devorah A. N. Bennu, GrrlScientist</a>

Devorah A. N. Bennu, GrrlScientist - Science Kit

Devorah A. N. Bennu, GrrlScientist
Devorah A. N. Bennu, PhD. Chapman Postdoctoral Fellow. Independent Scholar. American Museum of Natural History, New York City

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.

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.

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.

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Why are Bluebirds Blue? The Physics of Structural Colors in Bird Feathers [Schemochromes]

Most avian colors are the result of different types of pigments that are deposited into the developing feather. However, pigments alone do not produce all avian feather colors. Blues and whites typically result from small changes in feather structure that alters their light reflective properties. These fundamental modifications cause blue light to be selectively reflected from the feather surface in the case of blue feathers, while white feathers reflect all visible light. In short, blues and whites are structural colors, or schemochromes.

Visible light is composed of many colors of light, each with distinct wavelengths. Red light, for example, has a long wavelength (~700nm) while blue light has a much shorter wavelength (~400nm). When visible light encounters particles with the same diameter or larger than its component wavelengths, those specific light photons are reflected. Such reflected light photons are collected and seen by the observer's eye, thereby imparting color to the perceived image. Because blue light has a very short wavelength, it is selectively reflected more easily than other colors of light with longer wavelengths. This was first understood in 1869, when scientist John Tyndall noted that miniscule particles in earth's atmosphere preferentially scattered blue light resulting in the familiar "sky blue" of a clear summer day. Shortly afterward, Rayleigh demonstrated that Tyndall's "fine particles" are gases in our atmosphere, specifically, nitrogen and oxygen. Tyndall's contribution is widely recognized by describing this phenomenon as "Tyndall scattering" and referring to structural blue colors as "Tyndall Blues."

Tyndall scattering can be demonstrated at home using a simple experiment to produce a pale Tyndall blue color. First, mix one or two drops of milk into a glass of water then place this glass in a dark room and focus a flashlight upon it, and the fluid will appear bluish. This bluish color results from blue light bouncing off milk particles suspended in the water while other, longer, light wavelengths pass through the fluid, unobstructed. Of course, milk has some larger diameter particles in it that reflect other wavelengths of light that slightly longer than blue, thereby contaminating the pure "Tyndall" blue color.

Blue coloring in most bird species results from preferential scattering of blue light by the feather structure. When a blue feather is observed under a powerful microscope, the surface layer of keratin appears cloudy or milky due to the presence of small air cavities. A cross-section of the feather reveals an underlying layer of melanin granules and tiny air pockets in the middle of the feather barb. These small air cavities act like tiny particles because they selectively scatter blue light while the melanin granules absorb longer wavelengths of light, intensifying the blue. Structural differences are immediately obvious when a red feather, which derives its color from pigments, is viewed under the same microscope. The surface of the red feather is transparent and colorless while the underlying structures are filled with red pigment granules that reflect only red light.

The differences between structural and pigment colors can be demonstrated using several simple experiments. Because blue color is entirely dependent upon the reflective structure of the feather, it turns dark when ground up into a powder. However, a red or yellow feather retain their original color when subjected to the same treatment because pigments are not damaged when the feather structure is ruined. Pigments can also be removed from the feather without damaging its structure. When a red or yellow feather is placed into an appropriate solvent, the pigment granules will dissolve into the solvent, leaving behind a colorless feather. Blue feathers can also lose their blue coloring when placed into a liquid with a particular optical density, such as balsam, that fills the air cavities in the feather structure, thereby preventing reflection of blue light. Thus, such a feather appears dark when it is wet, but its lovely blue color returns after it has dried.

The physical phenomena that generate structural blue colors are similar, but not identical to, those that produce iridescent colors, such as those seen in purple martins and magpies. For example, iridescent feathers often appear to be very bright when compared to a structural blue feather viewed under the same light. Unlike iridescent feathers, blue feathers remain blue to the observer when the feather is rotated in relation to the light source whereas the coloring of iridescent feathers will vary and then become black as the angle of the light shifts. Similar to an iridescent feather, a structural blue feather will appear dark when it is placed directly between the light source and the observer because light cannot be reflected from the feather surface into the observer's eye.

White also is also a structural feather color that relies upon the same principles described for blue feathers, except that white is produced when all wavelengths of light are reflected. A white feather also shows comparable structural characteristics. When a white feather is observed under a powerful microscope, the surface structure appears crystalline, resembling cut glass or snow, clearly capable of reflecting all visible light. White feathers also contain many air cavities in the feather barbs that increase the total reflection of all spectrums of visible light. As previously described, both melanin granules and air pockets are found in the middle of blue feathers; however, white feathers lack melanin but contain many more air cavities. This lack of underlying melanin granules can be easily demonstrated because a lustrous white feather becomes transparent when it is immersed in balsam.

Touracos (Family: Musophagidae) are unique among birds because they alone produce their own green (and blue) pigments. However, all other birds make green feathers using a combination of both structural and pigment colors. Basically, the feather retains its blue-reflecting structures but embedded within its keratin structure are either yellow carotenoids (producing pure bright greens) or melanins (producing darker olive greens). Thus, it is possible to produce either blue or yellow birds from green parents, through the loss of either a yellow pigment or blue-reflecting feather structures -- a fact that has provided thousands of bird breeders with many decades of pleasure.

Surprisingly, despite humanity's deep appreciation for colors, there remain many questions yet to be answered about colors in animals. For example, why do birds rarely produce blue pigments? Why do they instead rely upon structural changes within their plumage to provide the lovely blue color that so many humans associate with "the bluebird of happiness?"

Related Products: Explore concepts presented in the above article with these Science Kit Products.

Feather Dissection Kit
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30,000 Facets Give Dragonflies a Different Perspective: The Big Compound Eye in the Sky

Why are Bluebirds Blue? The Physics of Structural Colors in Bird Feathers [Schemochromes]

Catching Dinner on the Fly ...The Night is Alive With the Sound of Echoes

It's All in the Olfactory Pits: Going Home Makes Scents to a Salmon

Take a Peek at Nature's Website: Spiders Spin Steely Silken Splendor