Ant

it's a nice spotting - you also might find this one helpful to identify - http://www.projectnoah.org/spottings/645...

i rather say this is some sort of Cephalotes Species - this ants have special workers with flat heads and the task to close and open the entrances to their hive: http://en.wikipedia.org/wiki/Cephalotes

This looks like a species of Spiny Ant. It is very interesting (I never knew spiny ants existed before this!). It's possible that it's Genus is Polyrhachis.
You can compare it to these.
http://web.mac.com/ajaynarendra/Ants/For...