The ants, one of the most successful groups of insects, are of particular interest because they form advanced colonies, and can constitute up to 15 percent of the total animal biomass of a tropical rainforest. They belong to the order Hymenoptera and are close relatives of the vespoid wasps.
Ants appear in amber, found in central New Jersey, believed to be from the Cretaceous period. It is thought that they evolved from the wasps that had appeared during the Jurassic period. They are morphologically distinguished mainly by having six legs, sharply elbowed antennae, wingless worker caste, the first abdominal segment being fused with the thorax (= alitrunk or mesosoma), a wasptail like constriction between the first and second abdominal segment, and by having one to two bead-like round to scale-shaped pedicel formed from the second and third abdominal segments respectively, which in wasps are joined to the gaster. Ants are also the only animals that possess the metapleural gland. They can “hear” with organs on the legs, antenna, thorax and head which can detect sound vibrations moving through the ground. Also, they “talk” with chemicals, having at least 10″“20 chemical “words”. Most queens and males of ants have wings, which they lose after nuptial flight; however wingless queens (ergatoids) and males can occur. The currently known 11,836 (Aug. 29, 2005) ant species occur worldwide but are especially common in hot climates. 22,000 ant species are expected to live on planet Earth.
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 – because it does not need to care for itself. The difference between queens and workers, 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. 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 thus have 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. All ants in the colonies are females to begin with, but only the Queen and breeding females have the ability to mate. 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. 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 a new colony starts.
Communication and behavior
Ant communication is 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 on its way home (found typically through remembered landmarks and the position of the sun), it will leave a trail along the ground, which in a short time other ants will follow. When they return home they will reinforce the trail, bringing other ants, until the food is exhausted, after which the trail is not reinforced and begins to slowly dissipate. A crushed ant will emit an alarm pheromone, that in high concentration sends other ants nearby into an attack frenzy, and in lower concentration attracts them, while a few ants use what are called propaganda pheromones to confuse their enemies.
Ant trails have no intrinsic polarity; that is to say, an ant walking on a straight non-branching trail cannot tell whether it is walking to or from the nest. Trails always divide as a “Y”, with the two secondary trails at a 60 degree angle to each other, and it is the geometry at trail junctions that give ant trails polarity.
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 they provide information about direction as well as intensity. Pheromones are also exchanged as compounds mixed in with the food interchanged 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) other ants belong to. Of special note, the queen produces a special pheromone without which the workers will begin raising new queens.
Ants attack and defend themselves by biting, and in many species, stinging, in both cases sometimes injecting chemicals into the target.
There is a great diversity among ants and their behaviors. They range in size from about 0.08 to 1 inch (2 to about 25 millimeters). Their color may vary; most are yellow, brown, red, or black, but other colors can also be seen. A few types, such as the genus Pheidole of North America, have a metallic luster.
Of special note:
- Some of the more advanced ants are the army ants (from South America) and driver ants (from Africa). 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, 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. The ants and the fungus can somehow communicate with each others, which makes it possible for the fungus to “tell” the ants if they feed it with some food which is not healthy for it.
- 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.
- 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. Other groups other than Mystrium are Odontomachus and Dacetini, examples on convergent evolution.
- Australian green ants are eaten by the aboriginals. Their abdomen tastes like lemon sherbet, and is high in vitamin C as well as antibiotic properties. Squashed green ants mashed in water makes up an excellent lemon-lime flavored drink.
- 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.
Symbiotic relationships with ants
- Aphids secrete a sweet liquid called honeydew. Normally this is allowed to fall to the ground, but around ants it is kept for them to collect. The ants in turn keep predators away and will move the aphids around to better feeding locations.
- Myrmecophilous or ant-loving caterpillars (blues, coppers, or hairstreaks) are herded by the ants, led to feeding areas in the daytime and brought inside the ants nest at night. The caterpillars have a gland which secretes honeydew if the ants massage them.
- Some myrmecophagous (ant-eating) caterpillars secrete a pheromone which makes the ants think the larva is one of their own. The caterpillars will then be taken into the ants’ nest where they can feed on the ant larvae.
- Allomerus decemarticulatus has developed a tripartite association with their host plant and a fungus in order to ambush their prey.
Humans and ants
Ants are useful for clearing out insect pests and aerating the soil. On the other hand, they can become annoyances when they invade homes, yards, gardens and fields. Carpenter ants damage wood by hollowing it out for nesting. Nests may be destroyed by tracing the ants’ trails back to the nest, then pouring boiling water into it to kill the queen. (Killing individual ants is less than effective due to the secretion of pheromones mentioned above). Ordinary chalk can be used to keep ants at bay; drawing a line or circle around the protected area may prevent them from entering.
Some species, called killer ants, have a tendency to attack much larger animals during foraging or in defending their nests. Human attacks are rare, but the stings and bites can be quite painful and in large enough numbers can be disabling.
Ants have often been used in fables and children’s stories to represent industriousness and cooperative effort, as well as aggressiveness and vindictiveness. In parts of Africa, ants are the messengers of the gods. Ant bites are often said to have curative properties. Some Native American religions, such as Hopi mythology, recognize ants as the very first people. Others use ant bites in initiation ceremonies as a test of endurance.
Termites, sometimes called “white ants,” are in fact not closely related to ants, though they have a somewhat similar social structure. They comprise the order Isoptera.
PHOTO CAPTION: Photograph of fire ants by Scott Bauer. Widely disliked for their venomous, painful stings, fire ants have spread across much of the southern United States.