I actually heard once that if you spray them with something, they lay their eggs just as they are dying & the eggs are immune to whatever you sprayed them with. So eventually they are mutants that just won't die!
We were told a similiar thing with Redbacks when we had the place sprayed for them. The chemicals kill the actual Redback, but not the eggs, which don't hatch till it's safe and then you have an infestation of the buggers again! We were told to book a follow up appointment a few months later to get any eggs that would hatch after the pest man had been.