Where there’s standing water, there’re mosquitoes: Bloodsucking adults land on the stagnant pools of water to lay their eggs just underneath the surface. Now, researchers have figured out how small semi-aqueous insects like water striders and pesky mosquitoes are able to walk so effortlessly on water: Their six legs generate an upward force that supports their body weight, and it’s all thanks to one super-flexible leg segment called the tarsus. The findings were published in AIP Advances this week.
A mosquito leg consists of three segments that are covered in microscopic, water-repelling scales: The femur sticks out from the abdomen and connects to the tibia, which connects to the tarsus. The tarsus is very long, thin, and flexible, unlike the femur and the tibia, which are both shorter, pretty thick, and stiff. And the tarsus is the only segment that actually comes in contact with water. Previous work on how water surfaces support insects have focused on whole legs, not individual segments.
So, a team led by Jianlin Liu from China University of Petroleum wanted to examine the forces that leg segments generate against a water surface. They captured Culex pipiens pallens mosquitoes in Jinzhou and Shenyang, China. Then to measure the force produced by the flexible tarsus, they stuck a mosquito leg to a steel needle and adjusted the angle and force between the leg and the surface of the water, all while taking readings with a microscope and a camera.
The mosquito’s ability to walk on water, they found, is thanks entirely to the tarsus's buoyant horizontal contact with the water’s surface. When its ultra-flexible tarsus conforms to the surface, its six legs generate an upward force that’s 20 times its own body weight.
"This finding overthrows the classical viewpoint that the longer the mosquito leg, the more efficiently it produces buoyant force," Liu says in a news release. Rather, by reducing the amount of leg that’s actually in contact with water, the water’s adhesive force on the insect is dramatically reduced—which helps with takeoff.
In the top row (a-c), you can see the typical sequence of the bottom half of the tarsus depressing the water surface progressively. The tarsus is flexible enough to deform into a curve that conforms to the free surface of the water. For comparison, the whole hind leg is used in the second row (d-f), and it’s not able to support the weight of the body.
How the structural ability of the tarsus to achieve such a large supporting force per unit length, however, remains a mystery for now. [Via Science]
Images: shutterstock.com (top), 2014 X.Q. Kong et al. (middle), Jianlin Liu/China University of Petroleum (bottom)