Acetyl choline (ACh) is the nerve transmitter substance released by motor neurones (at a site called the neuromuscular junction) to stimulate muscle contraction. The muscles relax (stop contracting) when the ACh is removed from the neuromuscular junction (NMJ) by the action of acetylcholinesterase. By inhibiting the cholinesterase the insect can no longer break down ACh in the NMJ and so its muscles lock up in a state of tetany (continuous contraction) making flying and respiration impossible, and the insect then dies of asphyxiation.