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.