You Have No Idea How Good Mosquitoes Are at Smelling Us

Nothing gets a female mosquito going quite like the stench of human BO. The chase can begin from more than 100 feet away, with a plume of breath that wafts carbon dioxide onto the nubby sensory organ atop the insect’s mouth. Her senses snared, she flies person-ward, until her antennae start to buzz with the pungent perfume of skin. Lured closer still, she homes in on her host’s body heat, then touches down on a landing pad of flesh that she can taste with her legs. She punctures her victim with her spear-like stylet and slurps the iron-rich blood within.

The entire ritual is intricate and obsessive—and nearly impossible to disrupt. Of more than 3,500 mosquito species that skulk about the planet, fewer than 10 percent (and only the females, at that) enjoy nibbling on humans. But once they’re on the prowl for people, neither rain nor zappers nor citronella candles will deter them. From the tips of their antennae to the bottoms of their little insect feet, these human-loving mosquitoes bristle with human-sensing accouterment, says Leslie Vosshall, a neurobiologist at Rockefeller University. “They really are in the business of finding us.”

Even aggressive genetic interventions aren’t enough to deflect a mosquito’s bite. The genome of a species called Aedes aegypti—a striped skeeter that prefers to feed on humans and can ferry viruses such as dengue, Zika, yellow fever, and chikungunya into our blood—encodes more than 300 distinct types of chemical sensors that help the insects navigate their world. Researchers have managed to introduce tweaks that futz with more than 100 of those genes at once, and yet those mutant mosquitoes “still find and bite humans, which just blows my mind,” says Meg Younger, a neurobiologist at Boston University. The most progress scientists have made through these techniques is cutting the insects’ attraction to us roughly in half, says Joshua Raji, a sensory biologist at Johns Hopkins University.

The reason is, frankly, depressing, as Vosshall, Younger, and their colleagues have found. Their recent work shows that mosquitoes’ odor-detecting systems are, unlike many other animals’, patchwork, chaotic, and riddled with fail-safes that make the insects’ sense of smell extraordinarily difficult to stump. It’s an essential adaptation for a creature that is hyper-focused on us: “They are finding a way to survive,” Raji told me. The insects are literally coded with backup plan after backup plan for stalking us.

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For years, scientists were sure that mosquitoes’ odor detection didn’t work in such complicated ways. In the 1990s, researchers performed a set of experiments that suggested that animals across the tree of life, including us humans, subscribed to a pretty standard smelling MO: To deduce distinct scents, creatures manufacture many, many types of olfactory nerve cells, each of them sensitive to exactly one specific type of odor. When complex fragrances filter in, their individual components nestle into receptors atop distinct neurons, like plugs fitting into sockets. The revved-up neurons then shuttle signals to the brain on parallel, independent tracks—keeping their intel separate until a central hub in the animal’s noggin collapses it all together, says Margo Herre, a neurobiologist who trained with Vosshall. It’s an additive system of switches that, coded correctly, yields precision in spades: Tripping Neuron A might mean there’s something hazelnutty nearby. But add Neuron B and Neuron C to the mix, and that could suggest it’s actually Nutella. Scientists called this the “one receptor, one neuron” rule, and for decades, Raji told me, it’s what everyone figured they would find in just about any creature that possessed a sense of smell.

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