How insects fly has been a mystery to scientists for many years. How can, say, a big heavy bumblebee beat its tiny wings and fly? According to the conventional laws of aerodynamics, insects cannot fly. Studies of animal flight have attained a new level of detail over the last decade, due to a tremendous progress in measurement techniques and heightened interest in flapping flight among the academic, military and industrial spheres. This interest is, for the most part, inspired by the very high manoeuvrability seen in flapping flight compared with conventional propulsion systems seen in fixed-wing aircraft flight.
Scientists recently discovered that when insects beat their wings they take advantage of a leading edge vortex. As air rushes past the beating wings, a tiny tornado-like vortex forms on top of the wing, at its very front edge. This swirl of rushing air decreases the pressure above the wing which produces lift. But for mosquitoes things are not straightforward. They have abnormally long, narrow wings, which they flap back and forth 800 times each second — far faster than any other insect of comparable size. To compensate for these rapid oscillations, their stroke amplitude (the angle through which the wing sweeps) is a very shallow 40 degrees, which is less than half that of any other insect measured to date.
Such a shallow angle shouldn’t be enough to provide lift. This unusual flapping pattern of short, fast sweeps means that mosquitoes cannot rely on the conventional aerodynamic mechanisms that most insects (and helicopters) use. So how does the mosquito do it?
Through careful, slow motion photography scientists discovered that mosquitoes use two additional aerodynamic features: a so-called trailing-edge vortex, and a type of lift mechanism generated by the rotation of the wing.
Most insects generate lift during each wing’s downstroke. The research team discovered that mosquitoes also generate lift by rotating their wings at the end of the up and down strokes. This helps them capture the wind from the previous wing stroke and lets them generate lift along the entire length of their wings.
When the wing reverses direction, a second vortex is formed at the back of the wing from the air that rushed over it during the previous stroke. The mosquitoes angle their wings to take advantage of this trailing edge vortex to generate lift in a process known as ‘wake capture’. Of the three independent mechanisms for flight, two of them are unique to mosquitoes.
In addition to this, once a mosquito has taken a blood meal, it must exert forces sufficiently high to take off when carrying a load roughly equal to its body weight, while simultaneously avoiding detection by minimizing tactile signals exerted on the host’s skin. Researchers discovered that by carrying out a precisely controlled extension of their relatively long legs throughout the push-off phase, mosquitoes spread push-off forces over a longer time window than insects with short legs, thereby reducing detection.
All modern technology can do is measure these extraordinary movements. It is unable to produce anything like a flying insect. And yet we are expected to believe that all of this complex motion — wings beating at 800 times per second, wing joints that reverse the stroke to create extra lift and specialised long wings to take advantage of a whole of wing lift — evolved to this kind of detail by blind chance.
Furthermore, female mosquitoes have a very intricate set of tools by which they can extract blood from mammals. The mosquito’s mouth, also called a proboscis, isn’t just one tiny spear. It is a sophisticated system of six thin, needle-like mouthparts that scientists call stylets, each of which pierces the skin and finds blood vessels.
Mosquitoes have more than 150 receptors— proteins on their antennae and proboscis, that help them find victims, or figure out if a particular puddle of water has enough nutrients to support mosquito larvae.
Unfortunately, we humans leave an alluring trail. When malaria-causing Anopheles mosquitoes, for example, come out at night to look for blood, they track the carbon dioxide we exhale as we sleep. As they get closer to us, they detect body heat and substances called volatile fatty acids that waft up from our skin.
When the female mosquito pierces the skin, a flexible liplike sheath scrolls up and stays outside as the insect pushes in its six drill bits. Another set of needles, the mandibles, hold tissues apart while the mosquito saws into the skin. Then a fifth needle, called the labrum, pierces a blood vessel.
Mosquitoes do not find the blood vessel randomly. Instead, scientists have recently discovered that receptors on the tip of the labrum respond to chemicals in our blood that drift up through the tissue like a ‘bouquet of smells’ to help guide the way to a likely vessel. Once the labrum gets into the blood vessel, it also serves as a straw.
As a female mosquito’s gut fills up with blood, she filters the nutritious red blood cells from the fluid and excretes the water. The red blood cells provide a large protein component, so eliminating the water lets her take in five to ten times more blood than she otherwise could.
The mosquito’s sixth needle—called the hypopharynx—drips mosquito saliva from the bug into us. That saliva contains substances that keep our blood flowing. It is powerful stuff. It makes our blood vessels dilate, blocks our immune response and lubricates the insect’s proboscis. It causes us to develop itchy welts and can serve as a conduit for dangerous viruses and parasites.
The female mosquito needs blood to grow her eggs. If she is evolving these intricate, specialised drilling tools over millions of years, then how did the first batch of eggs survive if she was incapable of obtaining blood at the beginning of her evolutionary cycle? Everything about this little creature shouts “Design!”