Ants are social insects that evolved from ancestors in the Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. Today, more than 12,000 species are classified with upper estimates of about 14,000 species.
Ants form colonies that range in size from a few tens of predatory individuals living in small natural cavities to highly organized colonies which may occupy large territories and consist of millions of individuals that are mostly sterile females forming castes of workers, soldiers, or other specialised groups. The colonies are sometimes described as superorganisms because ants appear to operate as a unified entity, collectively working together to support the colony.
Ant societies have division of labour, communication between individuals, and an ability to solve complex problems. These parallels with human societies have long been an inspiration and subject of study. Many human cultures make use of ants in cuisine, medication, and rituals. Some species are valued in their role as biological pest control agents. However, their ability to exploit resources brings ants into conflict with humans as they can damage crops and invade buildings. Some species, such as the red fire ant, are regarded as invasive species, since they can spread rapidly into new areas.
Ants communicate with each other using pheromones. These chemical signals are more developed in ants than in other insect groups. They perceive smells with their long, thin and mobile antennae. The paired antennae provide information about the direction and intensity of scents. Since most ants live on the ground, they use the soil surface to leave pheromone trails that can be followed by other ants.
In species that forage in groups, a forager that finds food marks a trail on the way back to the colony. This trail is followed by other ants, and these ants then reinforce the trail when they head back with food to the colony. When the food source is exhausted, no new trails are marked by returning ants and the scent slowly dissipates. This behaviour helps ants deal with changes in their environment. For instance, when an established path to a food source is blocked by an obstacle, the foragers leave the path to explore new routes. If an ant is successful, it leaves a new trail marking the shortest route on its return. Successful trails are followed by more ants, reinforcing better routes and gradually finding the best path. But ants use pheromones for more than just making trails. A crushed ant emits an alarm pheromone that sends nearby ants into an attack frenzy and attracts more ants from further away. Several ant species even use propaganda pheromones to confuse enemy ants and make them fight among themselves.
Pheromones are also exchanged mixed with food and passed among members of the community, transferring information within the colony. This allows other ants to detect what task group other colony members belong to. In ant species with queen castes, workers begin to raise new queens in the colony when the dominant queen stops producing a specific pheromone.
Many animals can learn behaviours by imitation but ants may be the only group apart from mammals where interactive teaching has been observed. One species of ant leads a nest mate to newly discovered food by the excruciatingly slow process of tandem running. The follower obtains knowledge through its leading tutor. Both leader and follower are acutely sensitive to the progress of their partner with the leader slowing down when the follower lags, and speeding up when the follower gets too close. Controlled experiments with some colonies suggest that individuals may choose nest roles based on their previous experience.
An entire generation of identical workers was divided into two groups whose outcome in food foraging was controlled. One group was continually rewarded with prey, while it was made certain that the other failed. As a result, members of the successful group intensified their foraging attempts while the unsuccessful group ventured out less and less. A month later, the successful foragers continued in their role while the others moved to specialise in brood care.
Ants perform many ecological roles that are beneficial to humans, including the suppression of pest populations and aeration of the soil. The use of weaver ants in citrus cultivation in southern China is considered one of the oldest known applications of biological control. On the other hand, ants can become nuisances when they invade buildings or cause economic losses.
In some parts of the world, large ants are used as surgical sutures. The wound is pressed together and ants are applied along it. The ant seizes the edges of the wound in its mandibles and locks in place. The body is then cut off and the head and mandibles remain in place to close the wound. Some ants have toxic venom and are of medical importance.
In South Africa, ants are used to help harvest rooibos, which are small seeds used to make an herbal tea. The plant disperses its seeds widely, making manual collection difficult. Black ants collect and store these and other seeds in their nest, where humans can gather them. Up to half a pound of seeds can be collected from one ant mound.