Mosquitoes are a buzzing and biting nuisance, but are also the deadliest animals to humans, due to the transmission of different viruses and parasites. Some species of these flying killing machines feed exclusively on humans, but to be such a successful feeder, they must have evolved precise targeting mechanisms to distinguish between human and animal odor. Now, researchers are finally figuring out how they do this and a new study published in Nature might answer the question: What are mosquitoes detecting and how do they detect it?.

“We sort of dove into the brain of the mosquito and asked, ‘What can you smell? What lights up your brain? What’s activating your neurons? And how is your brain activated differently when you smell human odor versus animal odor?’” Carolyn “Lindy” McBride, an assistant professor of ecology and evolutionary biology and neuroscience, said in a statement.

The team created genetically engineered Aedes aegypti mosquitoes — vectors of Zika virus, dengue virus, yellow fever virus, and chikungunya virus – using CRISPR-Cas9. These transgenic insects had brains that lit up when active, allowing the scientists to image the brains at a high resolution. The researchers then delivered human and animal-flavored air to said mosquitoes through a wind tunnel, to determine what caught the insect’s fancy.

Human odor is made up of many different compounds, and these same compounds are also present in most mammal odors but in different ratios. Past research has found that the compounds on their own are not attractive to mosquitoes, so one challenge is to determine the exact alluring compound ratios.

The team used odor from 16 humans, two rats, two guinea pigs, two quail, one sheep, and four dogs to tickle the mosquito’s appetite. How they collected these samples was pretty interesting. For sheep, they had a farm donate several fleeces, and for dogs, they visited a grooming salon and gathered trimmed hairs from the loveable pooches.

And for humans… well… “For the human samples, we had a bunch of great volunteers,” study author Jessica Zung said. “We had them not shower for a few days, then strip down naked and lie down in a Teflon bag.”

Now you may be asking, "why did the brave human volunteers have to be naked?" Well, other clothing fibers could distort the data as they carry their own smell.

Once they had retrieved all of these smells, they designed a clever system to puff odor at the genetically-engineered mosquitoes in the imaging setup area. The mosquito brain has 60 nerve centers called glomeruli, and the team originally hypothesized that most of these centers would be involved in helping the mosquito find their next meal and distinguish human smells from animals, but it turned out to be the opposite.

“When I first saw the brain activity, I couldn’t believe it – just two glomeruli were involved,” said Zhilei Zhao, a research team member. “That contradicted everything we expected, so I repeated the experiment several times, with more humans, more animals. I just couldn’t believe it. It’s so simple.”

Through the experiments, it was determined that mosquitoes detect two chemicals (decanal and undecanal), which are enriched in human odor and probably originate from unique human skin lipids rather than sweat.

Overall, this exciting collaborative research may help the development of new repellents. Allowing us all to enjoy the nice outside air without the fear of getting our blood stolen by these nasty insects.