WD40's powerful lethal effect on insects why?

I have sprayed large flies with commercial insecticides. They die...eventually. They take maybe 5-8 minutes to expire, buzzing and spazzing all the while. I get it. The toxin, whatever it is, is working on their central nervous systems and takes time to do it's job. But WD40 OTOH, kills INSTANTLY...like it turns the fly into a lead pellet or something. It simply drops out of the air dead and falls to the floor. Why??Does it infiltrate the insects body cavities and displace all of it's fluids? Does it instantly shut down all electrical activity? It's got to be something like that I'm guessing.