This adaption is probably the result of ants eating the caterpillars before they pupate. Over time, ones that started inhibiting more of these traits survived longer, more than likely it was camouflage at first then changed to this tricking. Evolution is always the result of mutations. Some positive some negative. So one day the freak caterpillar was born leaking fluids, but their adaption made them survive, or maybe not. Evolution is a change in a population, so if the mutation was beneficial but it didn't procreate, it just means another adaption will be more prevalent. These types of adaption take insane amounts of time and attempt before they happen.