Animals must wage a never-ending war against parasites, constantly evolving new ways of resisting these threats. Resistance comes in many forms, including genes that allow their owners to shrug off infections. But one species of fly has developed a far more radical solution – it has formed a partnership with a bacterium that lives in its body and defends it against a parasitic worm. So successful is this microscopic bodyguard that it’s spreading like wildfire across America’s besieged flies.
The fly Drosophila neotestacea is plagued by a nematode worm called Howardula. Around a quarter of adults are infected and they don’t fare well. The worm produces thousands of young in the body of its hapless host, and the little worms make their way into the outside world via the fly’s ovaries. Not only does this severely slash the fly’s lifespan, it also always sterilises her. But according to John Jaenike from the University of Rochester, the fly is fighting back.
If you looked at the penis of a Drosophila fly under a microscope (for reasons best known only to yourself), you’d see an array of wince-inducing hooks and spines. These spines are present in all Drosophila and they’re so varied that a trained biologist could use them to identify the species of the owner.
What’s the purpose of these spines? Are they intended to actually wound the female during mating? Do they help the male fly to scrape out the sperm of his rivals? Do they actually pierce the walls of the female’s genital tract, allowing the male to bypass any barriers to his sperm, as other insects do? All of these explanations have been put forward, and it seems that all of them are wrong.
The spines are nothing more than biological Velcro. During sex, they help the male fly to clasp onto his mate from the inside so he can’t be easily dislodged. We know this thanks to Michal Polak and Arash Rashed, who shaved male flies with a laser to see if their sexual performance would be affected.