Marching Caterpillars

Its a grayish start to the day but that hasn't deterred the annual march of the caterpillars. These caterpillars seem to live in the pine tree close to the house in a sort of fist size cocoon thing and then when the sun begins to warm the world in spring they come out to march. They follow each other nose to tail down the tree and off they go. I have no idea where they go or even what they may turn into eventually or how they get up the tree but when they come down.... Hey there must be over 100 in each line and they are everywhere. Almost like a plague, you can't help but run them over as you drive in and out as they stretch right across the road. The locals here call them processionaires and strongly advise us not to touch or pick them up. In fact they go out of their way to kill them, sometimes resorting to pouring petrol over them and setting them alight! Evidently the little hairs on their bodies are extremely irritating and can bring you out in a huge painful burning rash and your arm or whatever touched swells to huge proportions and if you are really allergic to them can cause you to go into a state of shock and that means hospital! No wonder they aren't very popular but why does such a small insect have to have such a powerful defence mechanism? They look so harmless I really would like to know what they eventually turn into just incase 1. It is also as dangerous and I can avoid it or 2. It turns out to be a beautiful moth/ butterfly now that would be the better selection. I will see if the natural history museum in London by the power of email can enlighten me......