According to the book "The Chemistry of Organophosphorus Pesticides", the insecticide Chlormephos (S-(chloromethyl) O,O-diethyl phosphorodithioate) has an oral LD50 in rats of 7 mg/k. According to that same book, the oral LD50 in rats for parathion is 6.4 mg/kg, making chlormephos about on par with parathion in terms of toxicity. Both of them work primarily by inhibition acetylcholinesterase. Both of them would presumably require metabolic activation due to the presence of a P=S bond. Despite their similar toxicities, their leaving groups differ significantly. So, my question is: How do the structures of chlormephos and parathion and their leaving groups in particular influence their toxicity, and why are their toxicities similar?