Argentine ants (Iridomyrmex humilis) are spanning over Melbourne! It's no joke. These tiny ants have formed a 100 km (60 miles) supercolony under Melbourne, according these Reuters and AP reports.
In Argentina, the ants' native country, these ants often display strong clannish and aggressive behaviour, fighting with Argentine ants from other colonies, thus keeping populations in check. But they're not always aggressive. Sometimes they practice inter-colony cooperation, especially against other insects.
Unfortunately, in Australia, these ants are taking to the cooperative mission a tad too seriously. Or maybe they are just assimilating! :)