Mandibles ("jaws") are crucial tool for many insects, including ants. Ants use their mandibles for several activities, for examples, transporting of food, decapitating of food, hunting of prey, defense, and of course, in this case, fighting.

This pair of ants was seen locking their mandibles. They hardly moved a centimeter during my 10 minutes stayed with them.

Nikon D200 with Sigma 150mm Macro 1/200sec at f14 +0.7EV ISO 200, Flash fired.