Context:
Inside this fruit fly reproductive tract, a battle is taking place – a battle of sperm. In turquoise are the sperm of
Drosophila simulans; in orangey-red are those of the closely related
Drosophila mauritiana.
John Belote of Syracuse University in New York and colleagues took this incredible snap to begin unravelling the secrets of
post-sex sperm competition.
Females of promiscuous species – like the fly pictured here and many others,
including our own– often mate with more than one male. As a result, the ejaculate of different males mix in the reproductive tract and the sperm cells compete to fertilise the female. But she doesn't leave it up to them alone. She can exercise what's known as
cryptic female choice, giving some sperm an advantage over others.
More:
http://www.newscientist.com/article/dn24290-glowing-sperm-go-head-to-head-in-fight-to-be-the-daddy.html