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Ants are one of the most successful groups of insects in the animal kingdom. They are of particular interest because they are a social insect and form highly organized colonies or nests which sometimes consist of millions of individuals. Colonies of invasive ant species will sometimes work together and form super-colonies, spanning a very wide area of land. Ant colonies are sometimes described as superorganisms because they appear to operate as a single entity.

Ants have colonized almost every landmass on Earth. They can constitute up to 15% of the total animal biomass of a tropical rainforest; in the Amazon the combined weight of the ants is said to be four times larger than that of the tetrapods in the same area. As of 2006, there are 11,880 known ant species, most of which reside in hot climates.

Ancestry

Ants are classified as a family, Formicidae, belonging to the order Hymenoptera which also includes sawflies, bees and wasps. Ants are a lineage derived from within the vespoid wasps. Unlike bees and more advanced wasps, ants have never evolved the ability to build comb cells, which is often an advantage if the colony is under attack and the offspring needs to be moved into safety. Also, the rise of army ants would probably be a lot more difficult if they depended on cells. Molecular clock analysis indicates that ants evolved from other groups of wasps in the Cretaceous period about 140 to 168 million years ago to become the Leptanillinae After the rise of angiosperms about 100 million years ago, ant evolution also showed rapid change, and about 60 million years ago they really started to achieve ecological dominance. [http://www.sciencemag.org/cgi/content/short/312/5770/15h. Several fossils from the Cretaceous are intermediate in form between wasps and ants, further confirming the wasp ancestry of ants. Like other Hymenoptera, the genetic system employed by ants is Haplodiploidy.

In 1967 Wilson et al. obtained the first remains of an ant from the cretaceous age (Sphecomyrma freyi). The specimen was more than 80 million years old. This species provided the link between modern ants and the early non-social wasps. Cretaceous ants shared a couple of wasp-like traits together with modern ant-like characteristics. Further specimens found allowed the assembly of all of the cretaceous formicoids into a single subfamily, the Sphecomyrminae, and into at most two genera (Sphecomyrma and Cretomyrma).

During the Cretaceous times, representatives of only a few species of primitive ants ranged widely on what was the super-continent Laurasia (the northern hemisphere). They were scarce in comparison to other insects (about only 1%). It was the adaptions due to radiation which gave ants the dominance at the beginning of the Tertiary Period. Of the species existent in the Cretaceous and Eocene eras, only 1 of about 10 genera is now extinct. 56% of the genera represented on the Baltic amber fossils (early Oligocene), and 96% of the genera represented in the Dominican amber fossils (apparently early Miocene) still survive today.

Morphology

Ants are distinguished from other insects by the following traits: elbowed antennae; a strongly constricted second abdominal segment forming a distinct node-like petiole; the petiole can be formed by one or two "parts" or segments (only the second, or the second and third abdominal segments can form it). Ants have a wingless worker caste; the presence of a metapleural gland is also distinctive.

Ant bodies, like other insects, have an exoskeleton, meaning their skeleton is on the outside - not covered by skin and tissue like humans. Ants do not have lungs. Oxygen passes through tiny pores, the spiracles, in their exoskeleton - the same holes through which carbon dioxide leaves their body. Nor do they have a heart; a colorless blood, the hemolymph, runs from their head to rear and back again along a long tube. Their nervous system is much like a human spinal cord in that it is a continuous cord, the ventral nerve cord, from head to rear with branches into each extremity.

There are three main divisions to an ant's anatomy: the head, thorax and metasoma:
The head of an ant has several important parts. Ant eyes are compound eyes, similar to fly eyes: they have many smaller eyes attached together which enables them to see movement very well. Also attached to the head of an ant are two feelers. The feelers are special smelling organs that help ants communicate. Ants release pheromone (chemicals that have different smells) to communicate with each other and the feelers pick these smells as signals. The head also has two strong pinchers, the mandibles, which are used to carry food, to dig, and to defend. There is also a small pocket inside the mouth where ants can store food and give to others in need.

The thorax of the ant is where all six legs are attached. At the end of each leg is a sharp claw that helps ants climb and hang onto things. Most queens and male ants have wings, which they drop after the nuptial flight; however wingless queens (ergatoids) and males can occur.

The metasoma of the ant has a poison sack. Ants have stingers and can inflict a very painful sting. This is a useful way to defend against the many predators ants have.

Development

The life of an ant starts with an egg. If the egg is fertilized, the ant will be female; if not, it will be male. Ants are holometabolous, and develop by complete metamorphosis, passing through larval and pupal stages before they become adults. The larval stage is particularly helpless – for instance it lacks legs entirely – and cannot care for itself. The difference between queens and workers (which are both female), and between different castes of workers when they exist, is determined by feeding in the larval stage. Food is given to the larvae by a process called trophallaxis in which an ant regurgitates food previously held in its crop for communal storage. This is also how adults distribute food amongst themselves. Larvae and pupae need to be kept at fairly constant temperatures to ensure proper development, and so are often moved around various brood chambers within the colony.

A new worker spends the first few days of its adult life caring for the queen and young. After that it graduates to digging and other nest work, and then to foraging and defense of the nest. These changes are fairly abrupt and define what are called temporal castes. One theory of why this occurs is because foraging has a high death rate, so ants only participate in it when they are older and closer to death anyway. In a few ants there are also physical castes – workers come in a spectrum of sizes, called minor, median, and major workers, the latter beginning foraging sooner. Often the larger ants will have disproportionately larger heads, and so stronger mandibles. Such individuals are sometimes called "soldier" ants because their stronger mandibles make them more effective in fighting other creatures, although they are still in fact worker ants and their "duties" typically do not vary greatly from the minor or median workers. In a few species the median workers have disappeared, creating a sharp divide and clear physical difference between the minors and majors.

Most of the common ant species breed in the same way. Only the Queen and breeding females have the ability to mate. Contrary to popular belief, some ant nests have multiple queens. The male ants, called drones, along with the breeding females are born with wings, and do nothing throughout their life except eat, at least until the time for mating comes. At this time, all the breeding ants in the colony are carried outside (save for the queen) where other colonies of similar species are doing the same. Then, all the winged breeding ants take flight. Mating occurs in flight and the males die shortly afterward. The females that survive land and seek a suitable place to begin a colony. There, they break off their own wings and begin to lay eggs, which they care for. Sperm obtained during their nuptial flight is stored and used to fertilise all future eggs produced. The first workers to hatch are weak and smaller than later workers, but they begin to serve the colony immediately. They enlarge the nest, forage for food and care for the other eggs. This is how most new colonies start. A few species that have multiple queens can start a new colony as a queen from the old nest takes a number of workers to a new site and founds a colony there.

Communication and behavior

Ant communication is accomplished primarily through chemicals called pheromones. Because most ants spend their time in direct contact with the ground, these chemical messages are more developed than in other Hymenopterans. So for instance, when a forager finds food, she will leave a pheromone trail along the ground on her way home. Home is typically located through the use of remembered landmarks and the position of the sun as detected with compound eyes and also by means of special sky polarization-detecting fibers within the eyes.

In a short time other ants will follow this pheromone trail. Returning home, they reinforce this same trail which in turn attracts more ants until the food is exhausted, after which the trail is no longer reinforced and so slowly dissipates.

This elementary behavior explains how ants adapt to changes in their environment. When an established path to a food source is blocked by a new obstacle, the foragers leave the path to explore new routes. If successful, the returning ant leaves a new trail marking the shortest route. Since each ant prefers to follow a path richer in pheromone rather than poorer, the resulting route is also the shortest available.

Ants make use of pheromones for other purposes as well. A crushed ant, for example, will emit an alarm pheromone which in high concentration sends nearby ants into an attack frenzy; and in lower concentration, merely attracts them. To confuse their enemies, several ant species even use what are termed propaganda pheromones.

Like other insects, ants smell with their antennae, which are long and thin. These are fairly mobile, having a distinct elbow joint after an elongated first segment; and since they come in pairs--rather like binocular vision or stereophonic sound equipment--they provide information about direction as well as intensity. Pheromones are also exchanged as compounds mixed with food and passed in trophallaxis, giving the ants information about one another's health and nutrition. Ants can also detect what task group (e.g. foraging or nest maintenance) to which other ants belong. Of special note, the queen produces a certain pheromone without which the workers would begin raising new queens.

Ants attack and defend themselves by biting and in many species, stinging, often injecting chemicals like formic acid.

While many types of animals can learn behaviors by imitating other animals, ants may be the only group of animals besides primates and some other mammals in which interactive teaching behavior has been observed. Knowledgeable forager ants of the species Temnothorax albipennis directly lead naive nest-mates to newly discovered food sources by the excruciatingly slow (and time-costly) process of "tandem running". The follower thereby obtains knowledge that it would not have, had it not been tutored, and this is at the expense of its nest-mate teacher. Both leader and follower are acutely sensitive to the progress of their partner. For example, the leader slows down when the follower lags too far behind, and speeds up when the follower gets too close, while the follower does the opposite (Franks and Richardson, 2006).

Locomotion

Ants usually lose, or never develop, their wings. Therefore, unlike their wasp ancestors, most ants travel by walking. Some tend to develop literal paths--the tiny equivalent of deer trails--or create unseen paths using chemical hints left for others to smell.

The more cooperative species of ants sometimes form chains to bridge gaps, whether that be over water, underground, or through spaces in arboreal paths. Among their reproductive members, most species of ant do retain wings beyond their mating flight; most females remove their own wings when returning to the ground to lay eggs, while the males almost invariably die after that maiden flight.

Some ants are even capable of leaping. A particularly notable species is Jerdon's Jumping ant (Harpegnathos saltator).

Gliding ants
There are several species of gliding ant. In fact this may be a common trait among most arboreal ants. They use their flattened heads and legs rather like a modern parafoil to steer in a controlled fall until they again contact the trunk of the tree from which they fell, or leapt to avoid predation.

Ant cooperation and competition

Some species of ant are known for attacking and taking over the colonies of others ant species. Others are less expansionist but nonetheless just as aggressive; they attack colonies to steal eggs or larvae, which they either eat or raise as workers. Some ants, such as the Amazon Ants, are incapable of feeding themselves, but must rely on captured worker ants to care for them.

Types

There is a great diversity among ants and their behaviors. They range in size from 2 to about 25 millimetres (about 0.08 to 1 inch). Their colour may vary, most are red or black, but other colours can also be seen. A few types, such as the genus Pheidole of North America, have a metallic lustre.

See list of ant genera (alphabetical) for an alphabetical compendium of worldwide ant genera, and antbase.org/Hymenoptera Name Serverfor a complete catalogue of all the currently known ant species of the world and their synonyms. Antbase.org/Hymenoptera Name Server is providing an up to date record of the actual number of species, and allows to follow the description of new taxa.

Of special note:

  • Bullet Ants (Genus Paraponera), located in South America, are the most venomous insect. Stings are incredibly painful, though usually non-fatal.
  • Jack Jumpers, also located in Australia.
  • Siafu Ants, located in Africa, these will eat people. All three of these ants mentioned here were seen on the Discovery Channel's show Killer Ants. All three of these have killed people but the Siafu Ant will kill and ''eat people.
  • Some of the more advanced ants are the army ants and driver ants, from South America and Africa respectively. Unlike most species which have permanent nests, army and driver ants do not form permanent nests, but instead alternate between nomadic stages and stages where the workers form a temporary nest (bivouac) out of their own bodies. Colonies reproduce either through nuptial flights as described above, or by fission, where a group of workers simply dig a new hole and raise new queens. Colony members are distinguished by smell, and other intruders are usually attacked, with notable exceptions.
  • Some ants will raid the colonies of other ants, taking the pupae with them, which once hatched act as workers in the raider's colonies despite not being genetically related to the queen. A few species, such as the Amazon ants (e.g. Polyergus rufescens), have become utterly dependent on such slaves, to the point of being otherwise unable to feed themselves.
  • Some ants, called honeypot ants, have special workers called repletes who simply store food for the rest of the colony, generally becoming immobile with greatly enlarged abdomens. In hot, dry places, even deserts, in Africa, North America and Australia where they live, they are considered a great delicacy.
  • Weaver ants (Oecophylla) build nests in trees by attaching leaves together, first pulling them together with bridges of workers and then sewing them together by pressing silk-producing larvae against them in alternation.
  • Leafcutter ants (Atta and Acromyrmex) feed exclusively on a special fungus that lives only within their colonies. They continually collect leaves which they cut into tiny pieces for the fungus to grow on. These ants have several differently sized castes especially for cutting up the pieces they are supplied with into even smaller pieces. Leaf cutter ants are sensitive enough to adapt to the fungi's reaction to different plant material, apparently detecting chemical signals from the fungus. If a particular type of leaf is toxic to the fungus the colony will no longer collect it. The ants grow the fungus because it produces special structures called gongylidia which are fed on by the ants.
  • Fire ants are unique by having a poison sac where the contents consists largely of piperidine alkaloids.
  • Silver ants navigate by using their eyes instead of pheromones to find their way back home.
  • Scientists recently discovered that Sahara desert ants have a internal pedometer that keeps tracks of how many steps they take, and use it to find their way back to the nest. *
  • Some ants are equipped with mandibles called trap-jaws. This snap-jaw mechanism, or catapult mechanism, is possible because energy is stored in the large closing muscles. The blow is incredibly fast, about 0.5 ms in the genus Mystrium. Before the strike, the mandibles opens wide and are locked in the open position by the labrum, which functions as a latch. The attack is triggered by stimulation of sensory hairs at the side of the mandibles. The mandibles are also able to function as a tool for more finely adjusted tasks. Two similar groups are Odontomachus and Dacetini - examples of convergent evolution.
  • Australian green ants are eaten by the aboriginals. Their abdomen tastes like lemon sherbet, are high in vitamin C and have antibiotic properties. Squashed green ants mashed in water makes up an excellent lemon-lime flavored drink. The Amazon is also said to have lemon ants.*
  • The Australian bulldog ant Myrmecia pilosula has only a single pair of chromosomes. Males have just one chromosome since they, like all male Hymenopterans, are haploid. The Australian bulldog ants are also among the biggest and most primitive. Each individual hunts alone, using its large eyes instead of their chemical senses to find prey. Like all ants they are social, but their social behavior is poorly developed compared to more advanced species.
  • Scientists recently discovered that Polyrhachis sokolova, a species of ant, can swim and live underwater.

Relationships between ants and other species

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Ant".

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