Brandeis Magazine

By David Levin

Professor Paul Garrity (Photo Credit: Gaelen Morse)

Blood is thicker than water, the saying goes. This reminder might be useful for humans, but for mosquitoes, choosing between the two is a nonstarter.

Access to both fluids is critical to mosquitoes’ survival — without the nutrients and amino acids in blood, the insects would be unable to make their eggs; without water, they’d have no place to lay them. Their very existence hinges on finding a hot, tender spot on an animal’s body and a nice pond or puddle to use as a nursery for their aquatic young.

Biology professor Paul Garrity studies how these insects sense the world around them — from their sensory organs and neurons to their genes. Exactly how mosquitoes find water isn’t clear, but his lab is starting to chip away at the mystery. According to a recent paper published in the journal PNAS, Garrity’s team has identified the exact spots on mosquito antennae that can sense humidity, and mapped the genes that create them.

This discovery could have real-world implications. According to the World Health Organization, mosquito-borne diseases — like dengue, malaria, and yellow fever — infect more than 300 million people each year, and kill hundreds of thousands. Understanding exactly how the insects’ moisture-sensing organs function could lead to the development of a pesticide that stops the organs from working, robbing the insects of their ability to locate water and produce offspring. As a result, the population of affected bugs could plummet.

Unlike many of the mosquito’s sensory cells, which extend to the tips of feathery hairs called sensilla, humidity-sensing organs are buried deep inside the antennae, tucked away in tiny air-filled chambers that are mostly sealed off from the outside world.

“Their openings basically look like mini-golf holes,” Garrity says.

Within each tiny chamber, a handful of neurons lie within a vase-shaped protrusion called the sensilla ampullacea. Because the neurons are completely enclosed, Garrity says, mosquitoes likely don’t sense water vapor by physically touching it. Instead, changes in air humidity may alter the physical properties of the chamber that surrounds the neurons, squeezing or stretching them in the process.

“We think the neurons might work like pressure sensors,” he says. “The sensilla would swell and shrink in response to changes in humidity, and the neurons could sense that.”

To test the importance of these neurons to the mosquito, Garrity’s lab used gene-editing tools to disable water vapor-sensing organs in a group of mosquitoes, then compared the insects to their unaltered cousins. The difference was immediately apparent. When presented with a dish of water, normal female mosquitoes swarmed to it and laid their eggs right above the water line. The mutant mosquitoes, by contrast, seemed completely lost and laid no eggs there at all.

“That was one of the most dramatic results we’ve ever seen,” says Garrity. “It’s incredible — you get rid of this one moisture-sensing pathway, and they just can’t find water to lay eggs.”

Although his lab is still working to understand the physical, chemical, and genetic mechanisms at work when a mosquito senses water, Garrity’s optimistic they’re on to something. He hopes their research will eventually lead to mosquito-control techniques that are effective on a large scale — protecting millions of people worldwide from deadly diseases.