What role do ant soldiers do?

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In most species of ants the soldiers do not do much work like feeding the larvae or building the nest, or even storing food as the repletes of honeypot ants do. Soldiers of many species of ants don't even have jaws that permit them to feed themselves at all; they have to beg food from their nest-mates.

What is more, in some species of ants, like some species of Pheidole, in which the "soldiers" have large heads and big jaws, the soldiers are of no obvious use at all, fleeing danger while the workers do the fighting,

However, in most species of ants that have a "soldier caste", those soldiers do most of the fighting when there is some creature such as a large animal or a swarm of enemy ants attacking the nest. Soldiers of some species of ants have stings or squirts of formic acid or other nasty chemicals that they use for fighting the enemy, and nearly all soldier ants bite too. Some other kinds of soldier ants just bite. This they do while the workers grab as many larvae and eggs as they can and carry them to safety if possible.

The soldiers of at least some kinds of "army ants" join the workers when they go out hunting for animals to eat. They have huge jaws and not only help killing prey animals, but in tearing or cutting them up.

Some kinds of soldier ants have big heads with very heavy armour, with which they completely block the entrances to the nests, so that even if they don't do any actual fighting, they stop all sorts of enemies from getting in. This is a useful thing to do, because it also helps in keeping out small species of thief ants and other small parasites that otherwise could sneak in and out between the ants' legs without being noticed.