They have two types of flight muscle: so-called power muscle, which is stretch-activated, which means that it activates itself and does not need to be controlled on a contraction-by-contraction basis by the nervous system.
ハエには2種類の飛行筋があるんだ 力筋と呼ばれるものは　伸展活性型なんだけど それ自体が活性型なので、 伸縮の度に神経による制御を必要としない