Hungry mosquitoes use an arsenal of sensory tools to hunt you down. They sniff out the carbon dioxide you exhale; they home in on your heat signature. But a previously under-appreciated tool in the mosquito's kit is the same one you use just before slapping at it in horror: vision. At Caltech, Floris van Breugel put mosquitoes in a wind tunnel to tease apart how they find their meals. He used Aedes aegypti, a tropical species that spreads yellow fever and other diseases. The insects were all female, since female mosquitoes are the bloodsuckers. The mosquitoes flew in a wind tunnel 1.2 meters long. Close to the upwind end was a dark spot on the floor—to a mosquito, this could be the shape of a person standing below, or some other animal worth checking out. Then van Breugel pumped a plume of carbon dioxide into the tunnel. A whiff of this gas tells mosquitoes that somewhere nearby, an animal is exhaling. As the insects roamed around, he tracked their movements in three dimensions, producing maps of more than 20,000 individual flights. While the air was clean, the mosquitoes ignored the dark spot on the ground. But as soon as they sensed CO2, the mosquitoes became very interested in the dark spot. Over the next three hours, while van Breugel continued to send carbon dioxide into the tunnel, the mosquitoes spent much of their time hovering about three centimeters from the target. That close to the floor, the insects couldn't smell the CO2 anymore. But it didn't seem to matter: once they'd sniffed carbon dioxide, they started using their eyes to search for victims. In another experiment, van Breugel put two glass objects into the wind tunnel. They could be either dark-colored or transparent, and either room-temperature or heated to around human body temperature. He saw that the mosquitoes preferred a warm dark object to a cool dark one. But they were able to hunt down a warm object even when it was transparent. This means the insects use both heat and visual cues to look for food. Finally, van Breugel added damp cloths to the glass objects, so that they gave off a little water vapor just like an exhaling animal would. Now the mosquitoes noticed the warm objects from even higher in the air. Searching for water vapor rather than just heat, van Breugel and his coauthors point out, could keep the insects from wasting their time on warm but nonliving objects such as dark rocks. So here's how a mosquito finds you. First, if she smells carbon dioxide, she accelerates upwind, zigzagging across the path of the gas. Somewhere between 5 and 15 meters away, she spots you with her eyes. Then she makes her final approach, perhaps using heat, humidity, and the scents on your skin to decide whether (and where) to land. Simply following the trail of CO2 would lead the insect straight into your mouth, where neither of you wants her to be. Looking only for heat would limit the mosquito to targets that are already close by. But by deploying a string of sensory tools, the mosquito can find all her juicy options. "It's not that visual cues are more important than temperature," van Breugel says, "but that they can be detected from further away." Previously, the researchers had seen the same "odor-induced visual attraction" in fruit flies. Unfortunately, the scientists say, this means there's not much hope for those of us trying to avoid mosquito bites. Even if you could hold your breath indefinitely, any other humans nearby will create a plume of CO2 that leads mosquitoes close enough to see you. "The best is probably to convince your friend to wear high-contrast clothing," van Breugel says, such as something with black-and-white stripes or spots, "while you wear light, muted colors that blend in with the surroundings." Good luck with that.
Image: by Geoffrey Froment (via Flickr)
van Breugel, F., Riffell, J., Fairhall, A., & Dickinson, M. (2015). Mosquitoes Use Vision to Associate Odor Plumes with Thermal Targets Current Biology, 25 (16), 2123-2129 DOI: 10.1016/j.cub.2015.06.046