Molecular model of the organophosphate insecticide chlorpyrifos (C9.H11.Cl3.N.O3.P.S) over a background of oranges. Chlorpyrifos is widely used in agriculture, it kills insects by acting on the insect nervous system. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), nitrogen (blue), oxygen (red), sulphur (light yellow), phosphorus (yellow) and chlorine (green).