The reason that flies (mosquitoes, horseflies, gnats, etc.) are so hard to catch is their unique wing structure. Most insects have four wings in two pairs. Flies use just their front pair for powering flight. Their back wings have evolved into little, club-shaped structures (halteres). The halteres help stabilize the fly by acting as tiny gyroscopes. If the fly changes its direction or orientation, the vibration of the halteres resists the change, sending information to sense organs at the base of each haltere. The sense organs provide feedback to the muscles that are controlling the wings, allowing for rapid adjustments during flight. The front wings are doing their part too, flapping as fast as 1,000 times per second. A fly has a bulging chest (thorax) to contain the powerful flight muscles. Some flies are even faster, thanks to an adaptation that gets their whole thorax vibrating to power the wings.

Insects are the only animals without backbones (invertebrates) that can fly. Wings of insects are flat and paper-thin, supported by a network of veins. They flap their wings at incredible rates, up to about two hundred times per second. Most insects have two sets of wings that work in tandem because they are coupled by a fold, a hook, or other structure that catches the back wing as the front wing beats. Wings are made of two layers of cuticle for strength. The front wing is often hardened to serve as a protective cover, leaving the back wing to provide most of the flying power. Many insects walk around instead of, or in addition to, flying. The good walkers tend to touch three legs to the ground at a time, alternating with other sets of three (tripedal gait). The stable triangles formed by the legs allows them to move quickly without falling over. Some insects "walk" on water or swim.