Africanized Honey Bee (Apis mellifera scutellata)
South and Central America, the American Southwest
Why you must fear it:
You know how you can spot one of these? You can't. There is no physical way to determine the difference between an Africanized bee and a common European bee. None whatsoever.
You can, however, easily tell the difference based on their behavior. Regular bees will give you about nine seconds of being too close to the hive before deciding you're a threat and then attacking you. So it's pretty easy to just walk past them without any screams. And if you do get them after you, they'll consider you to be 'chased off' after about 300 feet.
Africanized bees do not roll this way. They give you half a second of being too close before they decide it is time to completely **** your shit up and empty the entire hive--tens, maybe hundreds of thousands of angry, angry bees. When you run, flailing and crying and soiling yourself while screaming "JESUS CHRIST I'M COVERED IN BEES," they will chase you for over half a mile.
More scary shit:
Africanized bees owe their existence to science. Warwick E. Kerr created them in Brazil during the 1950s by crossing a European bee with an African bee. He wanted a bee that could live in the jungle. He got a bee that swarms by the hundreds of millions, is insanely territorial, mindlessly aggressive, has killed anywhere from a few dozen to a few thousand people. And, can live in the jungle.
And after they escaped and swarmed northward, it turned out they were a-OK with deserts, too. They'll be in Montana by 2010.