Complex sentence: Fly sprays kill insects by interrupting the nerve signals from the motor neurons in the insect, this in turn causes them to go into a permanent contraction which makes flying and respiration impossible, the insects then die of asphyxiation.
Split into simpler sentences: Fly sprays kill insects by interrupting the nerve signals from the motor neurons in the insect. This in turn causes them to go into a permanent contraction which makes flying and respiration impossible, and the insects then die of asphyxiation.