Insects don′t have that complex nerve-cardiovascular system like mammals have. In insects most parts of the body have a mostly autonomous nerval system and so parts of the body can funtion on their own for quite a while until the lack of body fluids finally will end all movment in these parts too.

Well, about 4 minutes ago I thought I killed an ant. It was kind of big, had red legs and a huge back. Well, I just came back from upstairs to sweep up the ant I thought I'd killed and I see it moving. The middle part and the head are unattached and I still see the antennas moving and the legs.. It was very disturbing but I could not stop looking. Does anyone know why it's still moving even when all the parts are unattached?