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With Uneven Wings, How Does New Fly Species Soar? |
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John Pickrell in England for National Geographic News |
| January 13, 2004 |
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Human engineers strive to create airplane wings matching in shape and size, and for good reason. When wings are mis-matched in nature, flight—if possible at all—can be dangerously unpredictable. That's why scientists were shocked to spot an insect with wings that seem to defy the forces of nature. One tiny predatory fly, new to science and discovered in Arizona, has left and right wings that are consistently different in shape and size, or asymmetrical. The size discrepancy, alone, is six percent or more. According to bug scientists behind the find, this is the greatest degree of wing asymmetry yet seen in any insect, bird, or other flying animal. The oddly-paired wings, seen only in males of the new species Erebomyia exalloptera, might be an unusual type of ornament for wooing females, researchers wrote in the science journal Biology Letters. "Six percent is a pretty significant difference," said entomologist and study author Justin Runyon. "At first we thought maybe they wouldn't be able to fly at all, or that they might fly in circles." Flying in Circles Though a few other insects, including a species of butterfly and a damselfly, have been shown statistically to have minute differences in wing size before, the variances have been too miniscule for the human eye to detect. Runyon, a doctoral student presently at Pennsylvania State University, and supervisor Richard Hurley of Montana State University, first discovered the animal on a field trip to Arizona in 2001. The pair were collecting flies of the family Dolichopodidae, all long-legged carnivorous species that feast on other insects. "They use their [powerful] mouth parts to suck the juice out of little bugs," said Runyon. Runyon spotted a dolichopodid he didn't recognize and later analysis in the lab suggested that the unknown flier had wings that didn't match up. "I thought it was just a malformed individual, but then I found another, and another," said Runyon. A subsequent trip to the same Arizona sites in spring 2003 revealed more of the same odd-winged Erebomyia. This time the researchers observed the animal airborne, proving that the wings are functional. Runyon and Hurley considered that environmental contamination could explain the deformity, but they found the same mis-matched wings on flies at two locations hundreds of miles apart. In addition, both sites appear to be pristine and pollution free. Communication on the Wing During their 2003 field trip, however, the scientists noted an unusual mating behavior that might explain how such a potentially harmful physical characteristic like asymmetrical wings is able to persist in the fly. Typically, damaging traits that serve no useful purpose are rapidly weeded out of a population by natural selection. Mis-matched wings can spell doom, because animals with compromised flying ability are less likely to successfully feed themselves, find mates, and avoid predators. When approaching females to mate, male Erebomyia were observed settling a few millimeters away and frantically fanning their wings. It could be that "the males are attempting to communicate," with buzzing sounds undetectable to the human ear, said Runyon. Females might use the quality of those sounds to detect differences between a male's right and left wings. Though the idea is mostly speculation at this stage, Runyon believes that females could be selecting those mates that have the most mis-matched wings. Erebomyia's mis-matched wings are "indeed unusual and interesting," commented entomologist Richard Mankin with the United States Department of Agriculture's Agricultural Research Service in Gainesville, Florida. "There is enough difference in the wing area that, at close range, the female could detect a difference in the sound level of the buzzing from the right and left wings," he agreed. "Some species of symmetrically-winged Caribbean fruit flies produce well-characterized sounds in a similar manner," said Runyon, and evidence exists that females select mates dependent on the intensity of the wing-fanning noises. Furthermore, other known relatives of Erobomyia have long hairs on their wings that are thought to be ornaments for wooing females. Handicap Theory But why should females select mates that have the most exaggerated deformity? One idea, known as the handicap theory and proposed by evolutionary biologists in the 1970s, suggests that those mates with the most inconvenient, large, or gaudy ornaments are often the most attractive to females. This is because those males have faced a greater challenge in life not to be eaten by predators, or perish by some other means, and therefore wear an honest badge of their genetic quality. The male's brilliant tail plumage in some species of bird of paradise (family Paradisaeidae) is a good example, said Runyon. As it increases in length, the birds cannot fly as well, or escape as easily from predators, but they become more attractive to females. The sexual display theory is a plausible explanation for the evolution of this Erebomyia's weird mis-matched wings, added Makin. However, with further research, "no one would be surprised" if new theories of how this seemingly harmful character can persist come to light. Nationalgeographic.com Bug Resources News Stories and Features Was This Earth's First Predator? Bugs Fighting Back in Evolutionary War on Humans? Study of Wasps Imported to Hawaii Shows Risks of "Biocontrol" Hornets From Hell" Offer Real-Life Fright "Killer Bee" Touted as Economic Lifesaver in S. 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