Most species of termites have soldiers.  This is a special caste within the colony that takes on certain roles. Soldiers tend to have large heads.  It is very common for their mandibles to be greatly enlarged. In many forms the soldier cannot feed itself and so needs constant tending, just like a juvenile.  Soldiers also have relatively tough bodies.  They are usually the last to die when the colony is poisoned, starved, heated, cooled or desiccated.

It appears that the specialised defenses of soldiers are primarily useful against attacking ants.

1. Very large, sabre-shaped, often pointed mandibles (eg. Coptotermes) attached to large muscles in the head which can pierce an ants' exoskeleton

1. Long  asymetrical mandibles that latch and release with force (like snapping your fingers) which can knock an enemy senseless. See photo on right from Quah.
    
2. Chemical attack from liquid stored in the frontal gland, either oozing from a hole in the head (fontanelle) or sprayed from the tip of an especially pointy head (rostrum).  The Rhinotermitidae tend to use latex-like sticky irritants that ozze from their heads and get pickup up on the jaws and antennae of attacking ants.  The Nasutitermitinae use a clear irritant sprayed from the rostrum.
    
3. A thick head.  The drywood termites often have soldiers with a blocky (phragmotic) head that is used to simply plug tunnel openings (especially the kick-holes).