bodies, known as a bivouac. Bivouacs tend to be found in tree trunks or in burrows dug by the ants. The members of the bivouac hold onto each other's legs and so build a sort of ball, which may look unstructured to a layman's eyes, but is actually a well-organized structure. The older female workers are located on the exterior; in the interior are the younger female workers. At the smallest disturbance, soldiers gather on the top surface of the bivouac, ready to defend the nest with powerful mandibles and (in the case of the Aenictinae and Ecitoninae) stingers. Inside the nest, there are numerous passages that have 'chambers' of food, larvae, eggs, and most importantly, the queen. Symbionts Many species of army ants are widely considered to be keystone species due to their important ecological role as arthropod predators and due to their large number of vertebrate and invertebrate associates that rely on army ant colonies for nutrition or protection. During their hunt, many surface-raiding army ants are accompanied by various birds, such as antbirds, thrushes, ovenbirds and wrens, which devour the insects that are flushed out by the ants, a behavior known as kleptoparasitism. A wide variety of arthropods including staphylinid beetles, histerid beetles, spiders, silverfish, isopods, and mites also follow colonies. While some guests follow the colony emigrations on foot, many others are phoretically transported, for example by attaching themseles on army ant workers such as the histerid beetle Nymphister kronaueri. The Neotropical army ant Eciton burchellii has an estimated 350 to 500 animal associates, the most of any one species known to science. Taxonomy Historically, "army ant" in the broad sense referred to various members of five different ant subfamilies. In two of these cases, the Ponerinae and Myrmicinae, only a few species and genera exhibit legionary behavior; in the other three lineages, Ecitoninae, Dorylinae, and Leptanillinae, all of the constituent species were considered to be legionary. More recently, ant classifications now recognize an additional New World subfamily, Leptanilloidinae, which also consists of obligate legionary species, so is another group now included among the army ants. A 2003 study of thirty species (by Sean Brady of Cornell University) indicates that army ants of subfamilies Ecitoninae (South America), Dorylinae (Africa) and Aenictinae (Asia) together formed a monophyletic group, based on data from three molecular genes and one mitochondrial gene. Brady concluded that these groups are, therefore, a