What a Butterfly Sees (Image 3)
The color pattern of the orange sulphur (Colias eurytheme) butterfly. The upper images are of the male and the lower images of the female. The left image shows how the butterflies appear to humans in daylight. The images on the right have been made using only the ultraviolet (UV) light reflected off the wings and show the high UV reflectance of the male. The UV reflectance is directional and in this image is seen only on the male's left wings because the light source was to the right of the specimens. These butterflies are common throughout North America and have a wingspan of about 4 centimeters. Ron Rutowski, an entomologist and behavioral ecologist at Arizona State University, is studying butterfly vision.
More about this image
Butterflies are very nearsighted. While they can spot color and conspecifics, they cannot recognize patterns. In addition, their vision is monocular, not binocular like humans, making them unable to assess depth or distance in the same way we can. However, nature has made up for this in other ways. Some species of butterflies, like the empress Leilia (Asterocampa leilia), have a visual field of about 344 degrees on the horizontal plane--only 16 degrees short of seeing all the way around its body. And vertically it is almost a full 360 degrees. The average human has a visual field of only 190 degrees. This wide field of vision helps protect them from predators, particularly birds. Most butterfly species have visual fields that are equally impressive.
Although butterflies cannot see detailed patterns, some species do rely on colors, particularly colors that are not visible to human eyes, to find and consider a mate. Rutowski found that virgin females of the orange sulfur species, are able to visually assess the relative age and fitness of competing males using ultraviolet (UV) scales on the males' wings, and receptors in the female's eyes. These receptors allow the females to see UV wavelengths. Missing scales indicate aging and reduce a male's ability to seduce a mate. This research was supported in part by the National Science Foundation.
To learn more about this research, see "An Eye for Survival," in the ASU Research Magazine. (Date of Image: 2002) [One of 10 related images. See Next Image.]
Credit: Ron Rutowski, Ph.D.
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