The Acromyrmex and Atta ants have much in common anatomically; however, the two can be identified by their external differences. Atta ants have 3 pairs of spines and a smooth exoskeleton on the upper surface of the thorax while Acromyrmex have 4 pairs and a rough exoskeleton. Next to humans, leafcutter ants form the largest and most complex animal societies on Earth. In a few years, the central mound of their underground nests can grow to more than across with smaller, radiating mounds extending out to a radius of , taking up and containing eight million individuals.