A bull ant "showing off" at its burrow's entrance.
Info on bull ants:
Picked from the above link:
"Bull ants have a fearsome reputation, and deservedly so. They are large with long, straight, powerful jaws and a potent venom-loaded sting. They attack intruders of any size that come too close to their nest. Bull ants also have well-developed vision and will follow or even chase an intruder a good distance from the nest. Usually the sight of large aggressive ants streaming out of the nest is enough to prompt a hasty retreat. If not, the ants deliver painful stings by gripping the intruder with their mandibles (jaws), curling their abdomen to reveal the sting and injecting the victim with venom. Often multiple stings are delivered".
Personnaly, I don't find them agressive (sure, better avoid their sting!!). They're supposed to be primitive (in "ants standards"!), but react to encounters, see them form "far" and move accordingly (also backward, facing you)...