A fungus that grows inside female flies, controls their behavior, then explodes out of their abdomen has somehow been found to be worse than everyone thought, thanks to new research.

Entomophthora muscae – somewhat satisfyingly translating as "fly destroyer" – is a fungus that infects houseflies and fruit flies, among other species. After a fly has picked up the fungus' spores, it behaves as it normally would for a few days – though those days are now numbered. The fungus is growing inside it, feeding on its innards, and taking over its nervous system.

Four or five days after the spores make their entrance, the fly begins to behave oddly. It climbs to a high spot in what is called "summitting" behavior, gluing itself in place with liquid from its mouth. Once perched, it begins to twitch. 

The fly finally extends its wings upwards before dying and remaining in this odd position. This is when the fungus begins to make its way out of the fly's abdomen. For some reason, the odd fungus, more than living up to its moniker, only kills at dusk. 

By getting up high, the spores can spread – but the wings being in an upright position also has its advantage. Male flies will mate with the dead body – lured in by chemical signals known as sesquiterpenes – getting themselves infested in the process.

“I think the fat females are especially attractive for the males,” retired UC Riverside entomologist Brad Mullens told KQED's Deep Look, describing the whole ordeal as a nightmare for the flies. "If their little brains could comprehend it, they would live in fear."

So, how do these creatures get any worse, you ask? Well, new research from the University of Copenhagen has found that this chemical-induced necrophilia gets more irresistible as the corpse rots. 

The distressing study compared the behavior of male flies around infected and non-infected female cadavers. The males were more likely to mate with the rotting females, which of course had been arranged by the fungus to appear to be receptive to mating. This only got worse the longer the females had been dead.

"We see that the longer a female fly has been dead, the more alluring it becomes to males. This is because the number of fungal spores increases with time, which enhances the seductive fragrances," lead author Henrik H. De Fine Licht explained in a press release.

In the study, 15 percent of males mated with corpses of the infected females 3-8 hours after death. This rose to 73 percent 25-30 hours after death. Apart from the obvious value of the study (learning about grotesque fungus), the research has practical applications.

"Flies are quite unhygienic and can sicken humans and animals by spreading coli bacteria and any diseases that they are carrying. So, there is an incentive to limit housefly populations, in areas where food is being produced for example," Licht said.

"This is where the Entomophthora muscae fungus may prove useful. It might be possible for us to use these same fungal fragrances as a biological pest control that attracts healthy males to a fly trap instead of a corpse."