From releasing a chemical that makes a colony of ants turn on itself, to manipulating insects to commit suicide, some parasites have developed some brutal ways to hack their hosts’ brain. Now researchers are trying to understand just how they do this, and hope it could shed light on how animals behave in general.

The field of neuro-parasitology is a relatively new one, as researchers begin to understand just how many parasites are able to invade their hosts and control their brains. By revealing how the body snatchers manage this, it could give an insight into how the animals themselves make these decisions.

“Parasites have evolved, through years of co-evolution with their host, a significant 'understanding' of their hosts' neuro-chemical systems,” explained Professor Frederic Libersat, who co-authored the study published in Frontiers of Psychology. “Exploring these highly specific mechanisms could reveal more about neural control of animal behavior.”

There is massive diversity in parasites that have evolved to manipulate their hosts’ brains, including viruses, fungi, and even insects themselves. Most of the interactions described to date involve insect hosts, but this is unsurprising considering that most animals described are insects to begin with.

The researchers document the most common ways in which these insect hosts are exploited by their mind-controlling parasites.

One of the most well-known examples is that of the fungi Cordyceps. These infect the brains of insects, commonly ants, and highjack their navigational system in a particularly peculiar way that eventually ends with the host behaving in a completely unfamiliar way.

At first, the fungi infects the insect's body by sending out thread-like mycelia that feed off the insect's least important organs, leaving those that are needed most to function. The fungi then makes the host climb to a high point and grip on with all its strength, before bursting out of its body and raining down spores onto the ground below, increasing the chance that another host will become infected.

Other parasites use hosts not only to feed their offspring but to defend their young even if the host is directly feeding them with their own bodies. One of these gruesome tales begins with the parasitic wasps of the Glyptapanteles species, the females of which inject their eggs into a caterpillar host. These develop into around 80 larvae by feeding off the insides of the caterpillar, before all but two will burst out of the body.

The ones that exit build a cocoon to finish their development, but the remaining ones do something truly creepy. They take control of what remains of the caterpillar, and by some as yet unknown mechanism make the half-eaten remains thrash back and forth to deter predators from eating their siblings.

Working out how these parasites achieve their macabre results will help researchers understand how the animals control their own behavior in the first place.