Apparently, the Americas black widow spider is genetically identical to the Australian redback spider. And just as poisonous. The spiderlings get carried across the Pacific Ocean frozen, at great height (50-90,00 feet), by the prevailing winds.
It's rare to see them here without the red stripe on their back (females, the males are almost microscopically small), whereas the red back stripe is rarely seen on the Americas variety.
All very interesting.