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Superorganism extraordinaire-a full expose on army ants.

By JW Dowey - 08 Aug 2016 14:40:0 GMT
Superorganism extraordinaire-a full expose on army ants.

The only good army ant---. These magnificent warriors are only ants after all, and only formidable in the gigantic numbers of a Dorylus army in Africa. The flying males, “sausage flies,” that are eaten by some Africans were not associated with the armies till quite recently. People thought they were a different species. Army ant image; Credit: © Marek Borowiec

Marek Lech Borowiec (University of California in Davis, US)has provided us with a streamlined review of the entire driver ant group, the infamous Dorylinae, in Zookeys. It is an impressive monograph. The splitting of Pangaea must account for some of the vast diversity and geographical range of a group of 680 species that may have begun as predators of their own kind, but have achieved a fame beyond the insect world for sheer predatory success!

Some have become omnivorous over the eons in which they have lived, but even now, we haven’t studied exactly how they operate, apart from many species creating a remarkable bivouacking nest, formed from living workers. The connecting link has always been the giant insects known as sausage flies, which are simply the males of all the dorylines. However, the sad news is that genetically, the 2 groups, from the New World and the Old are in fact separate! There is an American clade and an Old World clade plus several minor groups which don’t fit with the dorylines perfectly.

Termite hunters such as Acanthostichus in the Americas seem related to the terrifying Eciton group. The features that Old World ants have in common, are all evolved independently according to extensive genomic studies. Aenictus separated from Dorylus there (possibly during the Cretaceous, about 50-150 mya).) The Aenictus spp. Are found throughout tropical Asia and Africa, with a small number of species (around a dozen) in Australasia. They all prey on other ants and their brood, while many drivers use insects such as wasps for food, as well as all invertebrates (and small vertebrates) that cross their path. To fit this life-style, workers, including soldiers, have reduced or absent eyes. The giant soldiers are called major workers in the current terminology. Why would they need them!!!? Some tree-living species, especially, have larger eyes, but remember that many little-known species have adopted the typical ant habitat- underground. Stings are best known in the South American drivers, but they are present in Dorylus and its relatives. Some Central American species seem to be predating on large underground invertebrates and even snakes.

The much famed and feared Dorylus species are found in Africa and in Asia, although some species are not obviously army ants. The classic armies take non-arthropod prey being generalist feeders. Social insects are safer with these African swarms! One Dorylus, Dorylus orientalis, has even adopted the herbivorous habit, becoming an agricultural pest. One of their attributes is the possession of other invertebrates in their entourage. Instead of avoiding them like the plague, beetles and many others have become guests, although that is not always their role. Even antbirds have evolved adaptations like those of American driver ant followers, while chimpanzees use sticks to eat up the ants themselves!

In South America, Eciton spp are accompanied by phorid flies, staphylinid (rove) beetles, silverfish, mites and many more, according to many studies. Unlike the Dorylus species, these ants have very fixed nomadic phases, depending on their brood’s state of development. Their stationary phase is spent above ground in some species, with workers forming the bivouac with their bodies, and underground in others. Foraging during the nomadic phase can involve some arboreal activity in some Eciton. Even the top speed has been recorded in the best known driver species, at 20m/hour. Every day the column sets out at a slightly different direction, clearing the whole area of prey. During emigration though, the procession is slower with workers and brood, followed by up to hundreds of species of their myrmecophile guests, and then the royal progress, penetrating as far as 450m from the original nest in Eciton hamatum. You can imagine little resistance from prey, but the ants Atta and an Azteca succeed quite well, while Azteca chartifex and one other species actually attack the army ants!

While these American ants specialise particularly in other ant broods, predation by driver ants is shown to differ by Dr. Borowiec. Many other species are included in his interesting and voluminous paper, although many species have not been observed, unfortunately. He aims to help to ID the ants, which many will find a valuable aid, with photographs of special characteristics. The general reader could be put off by the science, but the attraction is in finding more about one of the most fascinating super-organisms ever evolved and some insight into their largely unknown habits and well as the most spectacular and awesome. Another species with an awful reputation is the jumping jack, also with formidable jaws! Here is that story, direct from Down Under.