I really hate flies. More than ants. More even than mosquitoes. A single fly buzzing around my face is enough to drive me crazy. And everything we ever hear about flies is how dirty they are, how they carry diseases, live in manure piles, leave fly specks on the walls, and generally create havoc, disease and destruction wherever they’re found. And they’re found everywhere.
But listening to that drone as a house fly whizzes around, zigzagging, pivoting, flying upside down, backwards and forwards, brushing my eyelashes, I wonder how the fly can do that. And, yes, I even think wouldn’t it be awesome to fly like that?
Flight in the animal world has evolved four separate times – pterosaurs (an ancient flying reptile), birds (evolved from theropod dinosaurs), bats and insects. These four unrelated lines evolved flight independent of each other. Our common housefly (Musca domestica) is one of the most accomplished of modern aerial acrobats. But how do they do that? Some of the answer seems simple enough. They have wings. They flap them 200 times per second. That ought to get them off the ground. And yet, scientists have been puzzled because for a long time their theories and equations said that a fly could not fly because it could not generate enough lift to keep its weight in the air. It took high-speed photography and robots suspended in oil to figure it out. The key, they discovered, was something called leading edge vortex. This vortex of spinning, swirling air is created at the leading edge of a wing—an airplane, a helicopter, a hummingbird, a fly. This vortex creates lift. On a fixed-wing airplane, this vortex dissipates rather quickly. On a helicopter, with a wing that is spinning, the leading edge keeps changing direction, and that keeps creating leading edge vortices. A fly takes this one step farther. It flaps its wings by rotating them backward and forward, flipping the wing over each time it changes direction. This rotary motion creates leading edge vortices as the wing changes shape and direction, and this gives the fly the lift it needs to stay in the air.
OK, it’s aerodynamics, and it’s complicated. But how do they move so fast and so precisely? This question, too, has fascinated scientists and people with fly swatters forever. Flies, it seems, have evolved a type of muscle that activates and contracts when it is stretched. This is very different from the way our muscles work. These muscles control the flapping of the fly’s wings. They’re called asynchronous muscles because, unlike our muscles, these muscles don’t coordinate with the nerve impulses. Instead, when the muscles that send the wing through the first half of its flap cycle contract, they stretch the muscles that bring the wing forward through the second half of the flap cycle. Stretching activates the muscle, so they go back and forth stimulating each other without direct signals from the fly’s brain. They can do this really fast. Meanwhile the fly has other tiny muscles that act on four tiny skeletal elements that, when moved, distort or change the shape of the wings. These muscles are responding to signals from the fly’s brain, and this allows the fly to make very rapid changes in speed and direction by changing the shape of the wings.
Hummingbirds are also very acrobatic fliers and can flap their wings 50 to 75 times per second. But whereas the fly has 10 neurons to control its wings, the hummingbird has thousands of them.
To put this amazing apparatus to work, a fly needs to know where it is in the world, where that fly swatter is, and where to find food. Several sensory input strategies coordinate in that tiny fly brain. Although its eyes see at the level of a 25- by 25 pixel display, they form the fastest visual response system known—ten times faster than a human’s visual response. They can see that fly swatter almost before you reach for it. The fly also has two light-receptor eyes on top of its head. These eyes detect the horizon and body rotation, both of which are important in knowing which way is up.
Flies, like many other insects, can navigate by the sun. They also have special receptors that allow them to see and to navigate by the polarized light in the sky. They can choose and maintain a heading in relation to these visual cues and can fly long distances on the same heading.
Flies have extraordinarily well-developed olfactory senses. Using their antennae like a dog’s nose, they can smell, track down, and locate the sources of odor plumes which will bring them to food or my picnic beer. The antennae also sense air movement and wind speed.
Sensors on the edges and base of each wing give a fly input on wing shape and also serve to augment the fly’s taste and smell.
Most insects have four wings. Flies, however, have two wings. Long ago, the hindwings evolved into two club-shaped organs called halteres. The name comes from the Greek name for club-like handheld weights that Greek athletes used to give an impetus to their jumps. Located at the base of the functional wings, these organs beat back and forth in time with the wings. They serve two vital purposes. The first is acting like a metronome to keep a steady beat with the wings. This keeps the fly’s wings properly timed to flip over at just the right millisecond in their rotation. That timing, then, is critical to maintaining the leading-edge vortices that provide the lift the fly needs to stay in the air. The second function of the halteres is to serve as a gyroscope, detecting the forces that occur when the fly rotates during flight. With the gyroscope to keep it steady, the fly can and does employ its agile, acrobatic flight to avoid that fly swatter.
OK, so I still don’t like flies as individuals. However, as marvels of engineering, fascinating creatures and important contributors to the ecosystem, I respect them. Just keep them out of my house!
Photo from Wikimedia.org by USDA.gov. Alt text: Common Housefly at rest against a bright background. Large compound eyes, and the animal’s major distinguishing characteristic: it has only two wings