The two wings on each side hook together during flight. Flapping up and down alone does not permit flight; but driving forces arise from propeller-like twists given to each wing during up and down strokes. Large nerve centers called ganglia in the ventral nerve cord, control the wings. Wings hinge by their narrowed bases to the thorax and are free to move up and down, but flight requires forward and backward motions as well as twisting or partial rotation of wings on their long axes. Thoracic muscles provide power, but most of these muscles attach not to the wings themselves, but to movable parts of the thorax that move the wings indirectly.

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