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Nocturnal mosquitoes navigate in the dark without crashing into surfaces. When they land on humans or other animals to feed, they do it very gently in order to remain stealthy; being noticed could spell disaster. Since these nocturnal mosquitoes cannot see what they are doing with their eyes, they use a different sensory mode – mechanosensing.Mosquitoes, and other flying animals, fly by accelerating the air around them, creating fast jets beneath each flapping wing. These jets change shape in the presence of obstacles such as the ground or walls.Thanks to an exquisitely sensitive array of receptors at the base of the antennae on mosquitoes’ heads, called the Johnston’s organ, the mosquito is capable of detecting these changes in airflow patterns. The researchers called this “aerodynamic imaging”: it gives the mosquito a picture of the world around them even in the dark and when they cannot feel surfaces by physical contact.The team used computational fluid dynamics simulations, based on high-speed recordings of mosquito flight, to investigate the effects of the ground and walls on airflows around the body. They discovered a trend: the Johnston’s organs on the antennae detect airflow changes very easily at low altitude, with the response diminishing at higher altitudes, until the threshold for detection is not met.They were surprised to see that one of the locations with the largest differences in airflow patterns occurs above the head, which means that the insects’ antennae were optimally positioned to sense these changes despite being farthest away from the ground.Aeroplane and helicopter pilots will be familiar with a phenomenon called ‘ground effect’ which tends to come into play when very close to the ground, usually noticeable at an altitude lower than two wing lengths.Using their new data, the researchers predicted the maximum distance at which the Culex mosquito can detect surfaces: more than 20 wing lengths, which is far larger than the expected distance for detection based on existing aerodynamic models.
