As annoying as mosquitoes can be, most biologists harbour a healthy respect for them. Just think of the adaptations a mosquito must possess to steal blood from a large, powerful mammal or bird: carbon dioxide and infrared sensors for stalking a host, mouthparts for stealthily piercing skin and probing for a capillary, and injected anaesthetics and anticoagulants for avoiding detection and keeping the blood flowing.
It's not surprising then that most research on mosquito biology has focused on their blood-sucking and associated disease-spreading abilities. But new research from Chengwei Wu and colleagues suggests that mosquitoes possess adaptations on their legs and feet that are also worthy of our study. Flies and geckos have received a lot of attention lately because of their ability to walk on vertical or even overhanging surfaces. The mechanisms by which they do this is an active area of research and has already inspired the development of novel dry adhesives.
Mosquitoes don't tend to walk or run on vertical surfaces like flies and geckos, but they have no trouble landing on a smooth vertical surface like a wall, even after doubling their body weight with a blood meal. Wu and colleagues wondered if mosquitoes make use of the same tricks that flies and geckos use to stay stuck. By examining the fine structure of mosquito feet with a scanning electron microscope, the researchers found that the feet possess hundreds of hockey-stick-shaped setae that are indeed similar to the tiny bristles used by both geckos and flies.
Clinging to walls and ceilings is not all that mosquitoes can do with their legs. After a female mosquito has successfully acquired a blood meal and converted it into eggs, she sets off to find a pool of standing water where she can lay them. Laying eggs in water is a tricky business for an insect– the eggs must be placed in the water without the animal getting stuck and drowning. Damselflies avoid this problem by standing on nearby vegetation and dipping only their ovipositors into the water. Mosquitoes, however, land directly on the water surface and remain afloat in a posture that is reminiscent of how water striders `walk' on water – with only their legs touching the surface. When the researchers examined the fine structure of mosquito legs, they found that they possess tiny scales that are somewhat similar to the microsetae found on the legs of water striders. In water striders, these spindly projections result in a much higher effective hydrophobicity, or water repellency, than could be accomplished by simply coating them with a hydrophobic compound such as wax.
To test whether mosquito legs possess similar super-hydrophobicity, the researchers measured the contact angle of water droplets that they placed on the legs, which provides a quantitative measure of the affinity of a water droplet for a surface. On a hydrophobic surface, a water droplet beads up into an almost perfect sphere, which translates into a high contact angle between the droplet and the surface. Hydrophilic surfaces are better at overcoming surface tension and tend to spread a water droplet out, resulting in a lower contact angle. Wu and colleagues found the contact angle of droplets on mosquito legs to be about 153°, close to the value obtained for water strider legs. They also measured the force that a single mosquito leg could bear before it penetrated the water's surface and found it to be not as high as for water striders' legs but still 23 times larger than the animal's body weight. The ability of mosquito legs to support these kinds of loads explains how female mosquitoes can so readily land on and take off from water.
Although water striders don't stick to walls, and flies usually drown when they land on water, this study demonstrates that wall-clinging and water-walking are not mutually exclusive adaptations in insects.