India, April 26 -- Tropical fire ants were dangerous enough when they were restricted to their native habitats in Central and South America. The voracious eaters will attack anything with living tissue. Their venom is so powerful that they have been known to cause fatal reactions in humans. They were suitably respected, feared and avoided by the tribes of the Americas. Then the first Spanish ships arrived, in the 16th century. They began to dig up dirt and load it onto ships as ballast; then dump some of this dirt, as needed, at various pit stops; and sail home with the rest. The earth often contained this hardy, fast-spreading omnivore. Around the world, these insects now threaten tortoise species and iguanas; eat butterfly eggs, contribut...