When it came to insect penises, Charles Darwin had it right. The famed naturalist suspected that insect genitalia, which are frequently festooned with bizarre combinations of hooks, spines, and knobs, essentially functioned like peacock tails. That is, they helped males beat out their rivals for females. Now, researchers have confirmed this hypothesis by zapping fly penises with a laser.

Darwin's hypothesis relies on something called preinsemination sexual selection. Basically, the idea holds that the male with the most effective strategy for getting a female to mate with him--attractive plumage, for instance--is most likely to pass on his genes to the next generation. But since then, studies in various insects have suggested that sexual selection can happen during or even after mating. Researchers noticed, for example, that certain flies engage in courtship displays only after copulation has begun, perhaps as a way to get the female to favor a male's sperm over that of his competitors. (Female flies typically mate with multiple males over a short time period.) They also reasoned that the complicated penis ornaments might help sweep away other males' sperm during mating. But there was no direct way to test this.

Enter laser beams. Evolutionary ecologist Michal Polak of the University of Cincinnati in Ohio and entomologist Arash Rashed now of the University of California, Berkeley, modified a laser commonly used to cut very small things, like the nerve cells of nematodes, so that it could zap off the hooks on fruit fly penises. "We can cut the tiniest of structures with the highest of precision," says Polak, all without harming the fly.

The duo then placed cut and uncut males in vials alone with a female. Cut males vigorously attempted to mate, but--unlike the uncut males--most slid off the females, unable to copulate. When cut males did manage to hang on long enough, they proved just as fertile as uncut males, showing that spines had little to do with postinsemination sexual selection. Instead, the genital spines give an advantage before insemination by fastening the genitalia together like Velcro fasteners.

The work, published online in the Proceedings of the Royal Society B, gives Darwin the win, says Polak. But he notes that it doesn't rule out postinsemination sexual selection in other species. And although Darryl Gwynne, a behavioral ecologist at the University of Toronto, Mississauga, in Canada, praises the study, he says postinsemination sexual selection could still be taking place in fruit flies. Females inseminated by a spineless male may allot fewer nutrients to his offspring, he says, perhaps because she views that male as less healthy than his competitors.