Ants are eusocial insects of the family Formicidae and, along with the related families of wasps and bees, belong to the order Hymenoptera. They are a diverse group of more than 12,000 species, with a higher diversity in the tropics. They are known for their highly organized colonies and nests, which sometimes consist of millions of individuals. Individuals are divided into sub-fertile, and more commonly sterile, females (“workers”, “soldiers”, and other castes), fertile males (“drones”), and fertile females (“queens”). Colonies can occupy and use a wide area of land to support themselves. Ant colonies are sometimes described as superorganisms because the colony appears to operate as a unified entity.
Ants have colonized almost every landmass on Earth. The only places lacking indigenous ant species are Antarctica, Greenland, Iceland, parts of Polynesia, the Hawaiian Islands, and other remote or inhospitable islands. When all their individual contributions are added up, they may constitute up to 15 to 25% of the total terrestrial animal biomass.
Termites, sometimes called white ants, are not closely related to ants, although they have similar social structures. Velvet ants, although resembling large ants, are wingless female wasps.
The Formicidae family belongs to the order Hymenoptera, which also includes sawflies, bees and wasps. Ants are a lineage derived from within the vespoid wasps.
Phylogenetic analysis indicates that ants evolved from vespoids in the mid-Cretaceous period about 120 to 170 million years ago. After the rise of angiosperm plants about 100 million years ago, they diversified and assumed ecological dominance about 60 million years ago. Several fossils from the Cretaceous are intermediate in form between wasps and ants, adding further evidence for wasp ancestry. Like other Hymenoptera, the genetic system found in ants is haplodiploidy.
In 1966 E. O. Wilson, et al. obtained the first amber fossil remains of an ant (Sphecomyrma freyi) from the Cretaceous era. The specimen was trapped in amber from New Jersey and is more than 80 million years old. This species provides the clearest evidence of a link between modern ants and non-social wasps. Cretaceous ants shared both wasp-like and modern ant-like characteristics.
During the Cretaceous era, only a few species of primitive ants ranged widely on the super-continent Laurasia (the northern hemisphere). They were scarce in comparison to other insects (about only 1%). Ants became dominant after adaptive radiation at the beginning of the Tertiary Period. Of the species extant in the Cretaceous and Eocene eras, only 1 of approximately 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.
Ants are distinct in their morphology from other insects by having elbowed antennae, metapleural glands, and by having the second abdominal segment strongly constricted into a distinct node-like petiole, forming a narrow waist between their mesosoma (thorax plus the first abdominal segment, which is fused to it) and gaster (abdomen less the abdominal segments in the petiole). The petiole can be formed by one or two nodes (only the second, or the second and third abdominal segments can form it).
Ant bodies, like other insects, have an exoskeleton, an external covering that provides a protective casing around the body and a place to attach muscles, in contrast to the internal skeletal framework of humans and other vertebrates. Insects do not have lungs, but oxygen and other gases like carbon dioxide pass through their exoskeleton through tiny valves called spiracles. Insects also lack closed blood vessels but have a long, thin, perforated tube along the top of the body (called the “dorsal aorta”) that functions like a heart in that it pumps hemolymph towards the head, thus creating some circulation of the internal fluids. Their nervous system consists of a ventral nerve cord running the length of the body, with several ganglia and branches along the way into each extremity.
The three main divisions of the ant body are the head, mesosoma and metasoma or gaster.
The head of an ant has many sensory organs. Ants, like most insects, have compound eyes with numerous tiny lenses attached together enabling them to detect movement very well. They also have three small ocelli (simple eyes) on the top of the head, which detect light levels and polarization. Most ants have poor to mediocre eyesight and others are blind altogether. Some ants have exceptional vision though, including Australia’s bulldog ant. Also attached are two antennae (“feelers”) which are special organs that help ants detect chemicals. The antennae are used in communication, detecting pheromones released by other ants. The antennae are also used as feelers, aiding in their sensory input about what is in front of them. The head also has two strong jaws, the mandibles, used to carry food, manipulate objects, construct nests, and for defense. In some species a small pocket inside the mouth holds food for passing to other ants or their developing larvae.
The thorax of the ant is where all six legs are attached. At the end of each leg is a hooked claw that helps ants climb and hang onto things. Most queens and male ants have wings; queens shed the wings after the nuptial flight leaving visible stubs, a distinguishing feature of queens. Wingless queens (ergatoids) and males can also occur.
The metasoma (the “abdomen”) of the ant houses many important internal organs, including the reproductive organs. Many species of ants have stingers used for subduing prey and defending their nests.
The life of an ant starts with an egg. If the egg is fertilized, the ant will be female (diploid); if not, it will be male (haploid). Ants are holometabolous, and develop by complete metamorphosis, passing through larval and pupal stages (with the pupae being exarate) 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 the 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 the 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 correspondingly 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 emerge from pupation with wings (although some species, like army ants, do not produce winged queens), and do nothing throughout their life except eat and mate. At this time, all breeding ants, excluding the queen, are carried outside 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 fertilize 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.
Ant colonies can be long-lived. The queens themselves can live for up to 30 years, while workers live from 1 to 3 years. Males, however, are more transitory, surviving only a few weeks. Thus ants are more K-selected than most insects. Ant queens are estimated to live 100 times longer than solitary insects of a similar size.
Ants survive the winter by going into a state of dormancy or inactivity. The forms of inactivity are varied and some temperate species have larvae that go into diapause while in others the adults alone pass the winter in a state of reduced activity. This does not happen in the tropics.
Ants show a wide range of morphological differences between the castes. While in some species, these differences are small, they are large in others. In some ant species there can be several size variants within the worker castes. Workers cannot mate; however, because of the haplodiploid sex-determination system in ants, workers of a number of species are able to lay unfertilized eggs leading to fully functional haploid males. The role of workers may change with their age and in some species, young workers are fed until their gasters are distended, and play a role in food storage. These workers with a storage role are termed repletes.