Not long ago, it was hip to be "fly."

It meant you had style. You looked good. There were “fly girls” and “fly guys,” dressed to the nines in their “fly dresses” and “fly suits.” No one who used this cool urban slang from the 1980s to about 2000 knew who the truly original “fly guy” was. Well, it was a fly. A male fruit fly, to be exact.

A new study from Sweden shows that female fruit flies choose their mates based on how good-looking their wings are. Males with the most colorful wings gets to copulate and make more flies.

It is surface. It is shallow. It has nothing to do with brains.

“Our experiment shows that this newly discovered trait is important in female choice in fruit flies and is the first evidence that wing interference patterns have a biological signaling function between the sexes during sexual selection,” said Jessica K. Abbott, a biologist at Lund University of Sweden and the study's lead author.

Wing color display is a secret known only to fruit flies. A male fruit fly’s wing appears colorless to the human eye, which is usually focused on flies only when a person is trying to swat one. Researchers who study the flies discovered years ago that against a dark background the wings get psychedelic.

Until now it wasn’t known that female fruit flies were checking out the flappers and sizing up males based on how mesmerizing they are. The research paper’s findings were published this week in the Proceedings of the National Academy of Sciences.

Wing interference color is determined by a wing’s thickness, measured in nanometers, thinner than a dime. It is found in other tiny insects — wasps, for example, the research says.

“In, the researchers...investigated whether the colors influence choice of partner, [and] to what extent females use these colors to select a mate. The results show the colors are important,” Lund University said a statement.

The research doesn’t reveal whether male fruit flies have to work as hard as other colorful showoffs in the animal kingdom, strutting like peacocks, moon-walking like the manakin or dancing a jig like the blue-footed booby. As far as the study shows, fruit fly dudes play it cool and let their wings do all the talking.

Clearly, "more vivid wings were more attractive for females compared with males with dull wings," the study said. During fruit fly courtship, males sometimes beat a tune with their wings, and that could also play a role in selection. Even when females had many choices — flies everywhere — males with pretty wings got all the attention.

Maybe, said Lund University professor Erik Svensson, the study’s findings will lead to “more research on wing interference patterns in other species and increase interest in the role that the light environment plays in mate choice.”

Why would anyone care?

Type “fruit flies” into an Internet search engine and a question is the first thing to pop up: “How do I get rid of them?” The bugs are tiny, active — another way of saying they are annoying — and tend to swarm in the kitchen, particularly in the summer, when some careless knucklehead leaves half-eaten fruit on the counter.

But scientists love them. Fruit flies have been studied to death as genetic research subjects in labs for nearly a century. The fruit fly s considered an ideal species for genetic tests because it breeds like crazy, lays hundreds of eggs and has multiple chromosomes.

Scientists have watched females carefully select mates but had no idea about their motivation. Was it brains? Brawn? Personality?

No one took time to realize that fruit flies were so… fly.

"It is quite remarkable that one of the most studied organisms on this planet has still so many surprises to offer, and that it is not until now that somebody have shown a clear mate choice trait under sexual selection," Svensson said in an e-mail from South Africa.

"I think people have suspected that wing movements were important," he said, "but they did not consider the wing interference patterns on those wings, simply because they were not known."