Parasites usually have some pretty simple goals: Find hosts, infect them, and then jump to however many other hosts they need to complete their life cycle. To that end, many of them have evolved some cool ways to alter the anatomy, physiology and even behavior of their hosts to help them along on their journeys. Toxoplasma gondii, for example, diminishes its mouse hosts’ fear of cats, making it more likely they’ll become prey and let the parasite jump to a feline host, where it reproduces. 

Other parasites make their way into new hosts not through violence, but love—or at least sex. IIV-6/CrIV is an insect-infecting virus that’s sexually transmitted, and ensures its spread by inducing its hosts to mate more readily and faster, all while castrating them to ensure more bodily resources for itself.

The parasitic fungus Entomophthora muscae pulls a similar trick on its housefly hosts, but with a macabre twist. After killing one fly, it makes the corpse irresistible to others, and infects any fly that attempts to mate with the body.

Evolutionary biologist Anders Pape Møller uncovered the bizarre behavior in the early 1990s while studying flies at Danish dairy farms. Presenting live male flies with a choice between two dead females—one infected with the fungus, and the other not—he found the males were more interested in the infected bodies, and approached them first in 80 percent to 100 percent of the trials in each run of experiments. They didn’t just give the bodies a curious glance, either. “Domestic flies were attracted to dead, infected individuals and attempted to copulate with these,” Møller wrote. The flies’ forays into necrophilia cost them, and most of the males that came in contact with an infected body showed signs of infection themselves within a week. 

The fungus, Møller figured, was changing some characteristic of its hosts to make them more attractive to the opposite sex, even after death. That change, he found, has to do with their bellies. A big abdomen is a signal of fertility in flies, and may be more attractive to potential mates—and an E. muscae infection causes a fly’s abdomen to swell as the fungus grows inside it.

In a second experiment that wouldn’t feel out of place in the insect version of Hannibal, Møller cut the abdomens off of infected and uninfected female flies, swapped them, and again tested the preferences of the males. An uninfected fly with a swollen, fungus-laden donor abdomen attracted more males than either an infected fly with a “normal” abdomen stuck on it, or an unaltered, uninfected fly. That backed up Møller’s suspicion that a swollen abdomen makes an infected fly more attractive. But an infected fly with its own abdomen still attached attracted more males than any of the others, which suggests that the fungus has some other way of luring males to infected bodies in addition to bloated bellies. 

Whatever else the fungus has in its bag of tricks, Møller thinks that its strategy may be a dead end. There’s pressure on its hosts to defend against it, but also pressure on potential hosts to avoid the infected. If uninfected flies could overcome their attraction to infected ones or better distinguish dead flies from live ones and spare the deceased their sexual advances, than the fungus’ transmission method would fail. As it is, he wrote, the fungus’ spread is potentially “evolutionarily unstable.” 

[h/t Tom Houslay]