Cockroaches vs Slugs

In terms of survival; the cockroach is said to be able to withstand anything from being stood on, to a nuclear blast and the ensuing fallout. A slug, in contrast, is killed by a few sprinkles of good old sodium chloride. How on earth did this bizarre weakness evolve? A lethal susceptibility to such a common compound surely cannot be good for the species. Cockroaches take the lead, for now at least.