Insects are the only group of invertebrates that have evolved wings and flight. Large insects such as dragonflies have flight muscles attached directly to the wings. Smaller insects have the flight muscles attached to the thorax, making it oscillate and indirectly causing the wings to beat. Some very small insects make use not of steady-state aerodynamics but of the Weis-Fogh clap and fling mechanism, generating large lift forces at the expense of wear and tear on the wings.Understanding of the aerodynamics of flexible, flapping wings and how insects fly is imperfect. One application of this research is in the engineering of extremely small micro air vehicles with low Reynolds numbers.