Ants are social insects of the family Formicidae and, together with wasps and bees, form the order Hymenoptera. There are an estimated 22,000 species, of which more than 12,500 have been classified. They are easily distinguished by their articulated antennae and the characteristic structure of their body, as its segments are joined by small joints and their waist is particularly thin. Ants form colonies ranging in size from a few dozen predators living in small natural cavities to highly organized societies covering large areas and consisting of millions of members. The largest colonies are composed mainly of sterile wingless females that form "worker" classes, "soldier" classes, or other specialized groups. Almost all ant colonies have some fertile males called drones and one or more fertile females called queens. Colonies are sometimes described as superorganisms because the ants seem to behave as a single entity, working collectively to support the colony. Ants thrive in almost all ecosystems, and constitute perhaps 15-25% of the biomass of land animals. Their success in so many environments has been attributed to their social organization and their ability to modify habitats, exploit resources, and defend themselves. Their long-term parallel evolutionary history with other species has created mimicry, symbiotic, parasitic, and mutualistic relationships.