The Army Ant exists in only a few levels in Darwinia, although the levels they are in, they are there in spades. Army Ants are as their name suggests, ants that attack in large hordes that spew forth from large ant hills dotted about certain places in a map. They have a flooding wave like mentality, and are not put off by any of your attacks, although they will be distracted by dropped souls. Their attacks are melee based, and they must come into contact with a unit of yours to cause it damage, however they always choose picking up souls over attacking.
In fact, Army Ants in Darwinia are highly attracted to souls, so much so that any batch of souls is sure to draw a horrendous tide of Army Ants if anywhere near their nests. This can keep the Army Ants occupied and your forces free from attack while the ants collect the souls, but will end up producing additional ants when brought back to the hill.
The key to fighting Army Ants successfully in Darwinia is to cut a swathe through them with high explosives, while heading straight for the Ant Hill. As soon as you have a Squad within range, or a turret manned that can fire upon the hill, focus on doing so.
The hills take a number of shots to go down, but once they do the source for the ant’s respawning is cut off. This typically causes hordes of ants from nearby hills to come and collect the masses of souls often left behind from a destroyed hill. Continue taking them out until you wipe out a hill with no other ones nearby, and collect yourself a mass of souls to bolster your forces with.