The ant and the dove

An ant went to the bank of a river to quench its thirst, and being carried away by the rush of the stream, was on the point of drowning. A dove sitting on a tree overhanging the water plucked a leaf and let it fall into the stream close to her. The ant climbed onto it and floated in safety to the bank. Shortly afterwards a bird catcher came and stood under the tree, and laid his lime-twigs for the dove, which sat in the branches. The ant, perceiving his design, stung him in the foot. In pain the bird catcher threw down the twigs and the noise made the dove take wing.

Moral lesson: One good turn deserves another

Ants
Ant{| class="section_hdr" Ant, common name for members of a family of about 11,000 species of insects that live in highly organized societies called colonies. Ant colonies have elaborate social structures in which the various activities necessary for the feeding, shelter, and reproduction of the colony are divided among specially adapted individuals.
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Ants belong to an order of insects called the Hymenoptera, a group that also includes bees, wasps, and sawflies. Some species of wasps and bees resemble ants in that they live in colonies and are therefore said to be social, but ants are the only hymenopterans in which every species is social. Ants are distinguished from other hymenopterans in that they have bent, or elbowed, antennae and an indented abdomen that forms a narrow waist.

Ant colonies range in size from a few members to many millions of members. Members of an ant colony typically fall into categories known as castes, each with a different role. The majority of colony members are female worker ants that are unable to mate. Worker ants do not have wings and perform most of the work of the colony, including searching for food, nursing young, and defending the colony against ants from other colonies. Queens are larger than worker ants and are the only females of the colony capable of mating. Queens are born with wings, which they break off after mating. They mate with winged male ants, later using the sperm from the mating to produce fertilized eggs, which hatch to produce more worker ants and a new generation of queens. Aside from mating with the queens, males play no social role in colony life and die soon after mating.

Ants live on landmasses all over the world, except for the permanently frozen Arctic and Antarctic, the coldest mountaintops, and a few islands. They flourish in soil, rotting wood, leaf litter, dead trees, and living trees in such varied habitats as mountains, deserts, swamps, and human homes. Ants are most abundant in the tropical regions. In the rain forests of the Amazon, for instance, ants are so numerous that their total weight is about four times the weight of all the area’s mammals, birds, reptiles, and amphibians combined.

Ants play crucial roles in the ecosystems in which they live. Many species dig underground nests that have numerous openings and tunnels. Air and water pass into the soil through these passageways, making oxygen and moisture available to the roots of plants. Seed-eating ants remove seeds from plants and transfer them to underground storage chambers within their nests. This activity disperses the seeds, so that some of them can sprout in areas that are distant from the parent plants. Ants of many species feed on other insects, which may be either living or dead. In this way, ants reduce the size of some other insect populations and recycle organic matter. In turn, ants are a source of food for other animals, such as spiders, other insects, woodpeckers, and blue jays; toads, salamanders, and turtles; and anteaters, armadillos, and aardvarks. A few ant species are considered pests because they sting, invade human houses and yards, or damage wooden buildings. Most ants are between 2 and 10 mm (0.08 and 0.4 in) long. Some ants, however, are a mere 0.7 mm (0.03 in) in length, and others are nearly 3 cm (1.2 in) long. Like other insects, ants have bodies that contain three major segments: the head, the thorax, and the abdomen. Unlike other insects, however, the first segment of the ant’s abdomen is fused to the back of the thorax. For this reason, scientists use other terms to designate the second two segments of an ant’s body—alitrunk corresponds to the thorax, and gaster corresponds to the abdomen. The shape of the ant head varies among different species; it may be spherical, triangular, egg-shaped, or rectangular. In all ants, at the back of the head is an opening through which nerves, the beginning part of the digestive tract, and blood pass into the alitrunk. At the front of the head is the mouth, which is associated with three appendages, or mouthparts. The mandibles, sometimes described as jaws, are long and broad, and they are serrated, or toothed, along their inner sides. Used for digging, carrying, collecting food, building nests, fighting, and cutting, the mandibles are probably the most important work tool that ants possess. The other mouth appendages are the maxillae, or lower jaws, used for chewing foods to extract liquids, and a tongue for sucking up liquid food. The mouthparts also have two pairs of slender palpi, segmented structures resembling small antennae, which play a role in tasting food.

Most ants have two compound eyes, which are made up of light-sensitive compartments called ommatidia. These compartments work together to generate an image in the ant’s brain. Some types of ants have three simple eyes, called ocelli, at the top of their heads. Ocelli can detect light, but they do not form images. Different species of ants vary in their ability to see: Some have well-developed sight, but others are entirely blind. Sight is of little importance to those ants that spend all or much of their lives underground.

Attached to the front of the head is a pair of flexible, segmented appendages called antennae, which contain organs of taste, smell, and touch. Each antenna is shaped like a human arm that is bent at the elbow. This antennae shape is an identifying feature of ants. Antennae are an ant’s main source of information about the world. When an ant is active, its antennae are in nearly constant motion—tapping the ground or vegetation, other ants, and food sources, or sampling odors from the air. An ant’s head connects to the alitrunk. This middle part of the body bears three pairs of jointed legs, each of which ends in a claw. The legs are used not only for walking or running but also for more dexterous tasks, such as handling food. Each of an ant’s front legs contains two combs that the ant uses to clean its other legs and its antennae.

In males and young queens, the alitrunk contains two pairs of wings: a larger front pair and a smaller rear pair. During flight, the hind pair of wings is hooked to the rear edge of the front pair so that the two pairs function as a single unit.

Behind the alitrunk is the narrow petiole, a one- or two-segmented section that forms what appears like a waist. This slender body part helps ants to bend while passing through twisting underground tunnels. In all ants, the alitrunk contains a structure called the metapleural gland, which secretes an antiseptic chemical that destroys bacteria and fungi. Ants rely on this chemical to keep their moist underground nests free from microorganisms that might destroy eggs, larvae, pupae, or stored food supplies. The hindmost body section of the ant, called the gaster, contains the heart, most of the digestive system, the excretory system, and the reproductive system. The segments of the gaster form a series of rings of different sizes, with the largest rings in the middle. When an ant’s digestive system is filled with food, the gaster expands by spreading out, or telescoping, these rings. At other times, the smaller rings of the gaster fit compactly inside of the larger ones.

The workers of many ant species carry a stinger within the hind end of the gaster. These ants use the stinger to defend against their enemies. In some species, worker ants lack a stinger but use the tip of their gaster to squirt or dab poison at other small animals and when fighting battles with other ants, fending off predators, or killing insects or other animals that they use as food. Ants have a rigid, external skeleton called an exoskeleton that gives the soft, inner body its form. The brains of ants process information from various sense organs via a nerve cord that runs along the lower side of the body. They also possess a tube-like heart, which contains muscles that contract to force ants’ colorless blood through their bodies. Instead of lungs, ants have a set of branching, air-filled tubes called tracheae that provide body tissues with oxygen. Adult ants digest only liquid foods. Some ants obtain nutrients from solid food, such as seeds, but they must turn this food into a liquid before swallowing it. These ants mix digestive juices into the food to help dissolve it. They then use their tongues to lap up the resulting juices and semiliquid bits of food. Once inside the mouth, partially solid food enters a chamber beneath the mouth opening, where a filter prevents solid particles from entering the digestive tract. These particles are pressed into a solid pellet that the ant removes from its mouth.

Food passes from the mouth into an organ called the crop, an expandable sac in which liquids can be stored for long periods without being digested. In many species, ants share the food stored in the crop with other ants, reflecting their social way of life. A valve called the proventriculus at the inner end of the crop allows a trickle of food to pass into the ant’s midgut, where it is digested. The reproductive organs of ants are located inside the gaster. Males have a pair of sperm-producing organs called testes, which are connected by ducts to an organ called the aedeagus. During mating, the male inserts his aedeagus into an opening in the queen’s body, called the vulva, to deposit sperm. Queen ants have a pair of egg-producing organs called ovaries, a genital chamber through which an egg passes after leaving the ovary, a sperm-storing structure called the spermatheca, and an egg-laying organ known as the ovipositor.

An egg travels from the queen’s ovary to the genital chamber on its way to being laid by the ovipositor. During this journey, the egg passes near a duct that leads out of the sperm-storing spermatheca. The duct is surrounded by a muscle that, when contracted, opens the duct. If the duct is open, one or more sperm may leave the spermatheca and fertilize the egg. Fertilized eggs give rise to either sterile female workers with no wings or fertile queens—females that are larger than workers and have wings. If the duct is closed, the egg is released without being fertilized. Unfertilized eggs develop into males, which always have wings and are equipped with reproductive organs. Ants are among those insects that undergo complete metamorphosis—that is, an ant’s body structures dramatically transform, not just enlarge, during the progression from birth to maturity. An ant begins life as an egg, which hatches to become a larva—a legless, helpless creature that, in most ant species, cannot move about and does nothing but eat and grow. Eventually, the larva reaches the size it will have as an adult ant. At this point, depending on the species, it may spin a silk cocoon around itself in preparation for entering the pupal stage. As a pupa, it acquires adult body structures such as legs and antennae. In some species, young ants pass through the pupal stage without forming a cocoon. Ants differ from other insects in that an egg will give rise to one of three types of adults—a queen, a worker, or a male. These ant forms are critical for the creation of social communities with members that perform specialized roles.

The life span of adult ants is highly variable. Queens of many species may live a decade or more. Queens usually live much longer than their workers. For instance, a fire ant queen lives about seven years, but her workers live only for about 50 to 150 days. Large workers usually live longer than small workers. In many species that live in the Earth’s temperate regions—areas between the warm tropics and the freezing polar circles—workers live for one to two years. Male ants usually die within a few weeks, typically shortly after they mate. In the great majority of ants, colonies are families or groups of related families. There are always at least two generations present at one time. In the simplest case, each colony consists of the queen and her daughters—the workers. These workers are full sisters to each other because they all have the same mother and father. Some colonies may consist of full sisters and workers with different fathers, or half sisters. In still others, a colony may contain more than one queen, resulting in a complicated family structure.

The needs of an ant colony shape every aspect of the lives of its members. Ants create nests for their colony that provide shelter and contain sites where worker ants perform specialized tasks, such as removing the outer husks from seeds. Ants use a variety of sophisticated forms of communication to ensure that all of the work needed to sustain colony life is carried out. For many ant species, colony life includes producing individuals who will leave the colony, venturing out to create a new colony. Life in an ant colony typically centers on a nest, which provides protection and shelter for the queen and the developing young. Depending on the species, ant nests may serve a variety of additional functions, such as providing chambers for storing food and maintaining a temperature that is cooler or warmer than the outside environment. Unlike social species of wasps and bees, whose home building tends to follow a rigid plan, ants build a wide variety of nests, and even closely related species may differ greatly in their nesting habits. Ants often build nests fairly quickly, taking advantage of the materials available in a given location. Such ants do not invest a great deal of energy and materials in nest building, so they can abandon a nest and move to a new site if necessary.

Many ants build their nests in soil. These nests may be just below the surface or as deep as 10.6 m (35 ft) below the surface. Underground nests typically consist of horizontal chambers connected by vertical tunnels. Some species build nests above the ground, creating chambered mounds of soil or leaf litter. Other species construct nests from chewed plant fiber, called carton. Weaver ants fasten leaves together to form nests, and some ants, including acacia ants, nest in hollow thorns or special cavities in plants. A few species have no permanent nests. In these species, worker ants link their legs together to form sheets of living ants that serve as temporary shelters for their queen and the developing young.

Many species nest in hollow acorns, fallen branches, spaces between rocks, or other cavities that they did not create themselves. Climate influences the location of ant nests. In cool climates, ants often build their nests under stones, where the nests will be warmed when the stones are heated by the sun. In warm climates, especially in the humid tropics, many ant species nest in tree cavities, such as holes made by other insects or openings created by rotting wood.

Within most nests, the queen and the developing young are often in the core of the nest. Worker ants commonly move the eggs, larvae, and pupae into whatever part of a nest is warmest at a particular time. Older workers are most often found in outlying parts of the nest or foraging for food outside. A crucial function of each colony is to produce individuals who will give rise to new colonies. This task can only be performed by sexual individuals, males and new queens, which are called reproductives. Reproductives have wings and are hatched only during short periods each year. After they emerge from the pupal stage and become adults, the reproductives typically remain in the nest for one to several weeks. Under highly specific conditions of weather and time of day, the reproductives leave the nest and fly high into the air, where they join reproductives from other colonies in what is called a nuptial or wedding flight. The reproductives usually mate with partners from a different colony that they encounter during the wedding flight. They may mate while they are flying or after they have landed on nearby vegetation. A queen may mate with one male or with several males, but in all cases, she stores the sperm in her spermatheca. The queen will never mate again, and this is her lifetime supply of sperm.

After mating, the queen most commonly prepares to found a new colony. She begins by searching for a suitable nesting site, which may be under a rock, beneath bark, or inside a hole she digs herself. She then seals herself in and begins to lay her fertilized eggs. Drawing on the large reserves of fat and protein in her body, she rears the resulting larvae, feeding them on her secretions and on special eggs laid specifically to be eaten. After the larvae have pupated, they emerge as small adult workers, the smallest in the life cycle of the colony. The workers open the sealed nest to the outside and take over the work of caring for the queen’s later offspring, searching for food and maintaining the nest so that the queen’s only remaining task is egg-laying. The colony grows as the workers rear more workers, which are usually larger than the workers born in the first generation. When the colony has reached a certain size, the queen lays eggs that hatch to become males and new queens. During certain periods of the year, these reproductives leave the nest to mate and form new colonies.

In some species, a queen does not start a new colony after mating. Instead, she returns to her home nest, joining the queens already there. At a later time, several of these queens, together with some of the colony’s workers, may start a new nest some distance away. This behavior, called nest-fission or splitting, may produce two nearly equal-sized nests, or very unequal ones. This type of colony reproduction sometimes results in the formation of groups of related colonies called supercolonies. The individual colonies that make up a supercolony often freely exchange workers and queens. The largest supercolonies extend over 2.5 sq km (1 sq mi), contain 45,000 interconnected nests, over 300 million workers, and more than 1 million queens. In both large and small colonies, ants divide up work tasks within the colony, such as digging tunnels, rearing the brood, collecting food, and laying eggs. This division of labor means that each ant is a specialist, performing some but not all of the various tasks needed for colony life. While different ant species use varied ways to divide up the tasks performed by workers in a colony, one crucial division of labor is common to all ant species: Only queens lay eggs, while workers raise the young. In species that have single queens and large colonies, the reproductive output of queens is astounding. A fire ant queen may lay 100 eggs per hour, around the clock, and an African driver ant queen lays 3 million to 4 million eggs per month. Worker ants typically do not reproduce. Some workers may have small ovaries and may release eggs, but no workers have a sperm-storage organ, so they never produce fertilized eggs.

The tasks that any individual worker ant performs are influenced both by its age and by its physical characteristics. Young workers typically take care of the larvae and pupae. Middle-aged workers take care of the nest, repairing breaks in underground tunnels, transporting food, or carrying discarded materials from the nest. Only the oldest workers leave the nest to search for food. They usually do not survive long at this dangerous task because it places them at risk of being eaten by larger animals.

Males play a small but crucial role in colony life. Their sole function is to mate with young queens so that the queens can produce fertile eggs. Males carry out this function shortly after they mature. After mating, males wander away from the colony and die. Ants use sophisticated communication to coordinate their specialized tasks. A key form of ant communication is the chemical signal called a pheromone. Many species emit alarm pheromones to alert nestmates to danger and attract them to the site of a disturbance. Some ants spread streaks of so-called trail pheromones on the ground to guide nestmates to food, areas needing defense, or new nest sites. The long lines of ants sometimes seen streaming from a nest are following these signals. Pheromones are also used to mark territory boundaries in some species. Intruders that enter a nest are attacked because they have a different odor than the inhabitants. Workers use chemical signals on the larvae’s body surfaces to identify and feed young in the dark interior of a nest. Workers groom one another in response to odors, and ants even rely on odor to learn whether a colony member has died. When a dead ant is discovered in the nest, it is carried outside.

The queen emits a large number of pheromones, which serve various purposes in the colony. Some of a queen’s pheromones attract workers to groom and feed her. Many pheromones given off by a queen affect the physiology, rather than the behavior, of other ants. For example, certain pheromones released from a queen prevent the ovaries of adult workers from maturing, and other pheromones from queen ants may determine the caste of the developing larvae.

Some ant species use sounds to communicate alarm. Carpenter ants drum their heads on the floor of their chambers, and leafcutter ants and harvester ants make squeaking sounds if their nest caves in. Nestmates follow these sounds to find and rescue the trapped ants.

Ants also use touch, particularly using their antennae, to communicate. Ants that have discovered a large food source may attract the attention of other ants by striking them with their antennae, legs, or heads. Some primitive ants engage in duels that involve hitting one another with their antennae. These duels determine which of the workers will be dominant and remain near the brood.

Much of the work in ant colonies is performed without direct communication. In these instances, ants take cues directly from the tasks themselves. For example, the collapse of a passageway will induce certain ants to repair parts of the damage, while other workers respond to later cues to step in to complete the job. The ability of ants to respond directly to tasks avoids attracting huge numbers of workers to participate in a task that requires only a few workers. Ants display a rich diversity of behaviors in obtaining food, promoting the survival of their colonies, and defending against predators. The various behaviors of ants are influenced by their senses and by information stored within their nervous systems. Ants seem to be capable of a certain amount of learning. For example, foraging ants rely on memory—a form of learning—to recognize landmarks in the area where they search for food. Ants feed in a variety of ways. Some hunt other insects, some collect and feed on insect corpses and other edible debris, and many cultivate certain insects that the ants “milk” to obtain a sweet liquid called honeydew. Still others collect seeds for food, and perhaps most remarkably, some ants grow and feed upon massive gardens of fungi.

In a behavior called trophallaxis, worker ants that have found liquid food outside of the nest share it with their nestmates whose work responsibilities may make them unable to leave the nest to search for food. The foraging ants store liquid food in their crop. When a hungry ant encounters a nestmate that has returned from foraging for food outside the nest, the hungry ant strokes the forager ant’s head with its antennae. The two ants then position themselves so that their mouths are touching. The forager ant forces a drop of food from its crop into its mouth and then into the mouth of the other ant. Trophallaxis enables food to flow quickly within a colony. Worker ants provide constant care to young ants, including feeding the larvae and transporting both larvae and pupae to sites within the nest that have an optimal temperature for development. Workers constantly lick the larvae and pupae, both to clean them and to coat them with chemicals to prevent the growth of disease-causing bacteria.

A queen ant is typically surrounded by a multitude of workers, who lick her to keep her clean and feed her through trophallaxis. Worker ants also continually lick and feed one another. Workers will carry one another on certain occasions, such as when an inexperienced worker must be transported to a new nest. All species of ants keep their nests in good repair and keep them clean by removing debris, such as empty cocoons. They place this refuse outside the nest or in a special chamber. Different species of ants that occupy the same area usually compete for food, nest sites, or space. Some species defend territories against invasion by workers from other colonies of their own species, and sometimes colonies of other species as well. Some ant species defend only their food finds and nests, while the most timid ants defend only their nests, relying on stealth, luck, and speed to get food.

Some species of ants respond to an invasion by rolling themselves into balls and remaining motionless. This response makes it hard for other ants to distinguish them from particles of soil or sand. Many ants respond more aggressively to danger. Ants commonly fight by biting each other or by gripping enemy ants while dismembering them. Some ants use their mandibles to injure an enemy and then squirt poison into the wounds.

Battles among ants may involve a huge number of individuals. Many species use trail pheromones to attract workers to the site of skirmishes. Among European wood ants, battles may last for weeks and result in thousands of dead ants each day. These huge casualties of war help the victorious colonies because battles typically occur in the spring, a time of protein shortage, and the surviving colony members eat the protein-rich dead ants. Scientists recognize about 11,000 species of ants. These species display a remarkable diversity in the environments they live in, the foods they eat, and their strategies for survival. Some scientists group the many species of ants according to these survival strategies, which can be compared to certain human activities, such as harvesting, weaving, and gardening. The fire ant was accidentally brought to Alabama from southern Brazil around 1940. It was spread further in nursery plants and sod, and now occurs in most of the southeastern United States. The fire ant depends on human disturbance of natural landscapes; it constructs its earthen mounds in lawns, roadsides, and pastures. Generally regarded as a pest, the fire ant can be detrimental to some forms of agriculture, but beneficial to others. Moreover, it is an energetic stinger and may cause medical problems in people sensitive to its sting.

Fire ant colonies contain up to about 300,000 workers. They feed primarily on insects, honeydew, and various scavenged foods. Each colony defends a territory of up to about 186 sq m (roughly 2,000 sq ft) against its neighbors, limiting the number of colonies that can share a piece of land. The workers travel to all parts of their territory in underground tunnels that radiate out from the central mound. Almost all traffic moves in these tunnels; it is only within the last few centimeters of their destination that foraging workers emerge from the tunnels.

Fire ants exist in two social forms with very different ways of life. Colonies of the single-queen form are simple families that are territorial, reproduce by sending out winged queens to start new colonies, and have workers that vary greatly in size. In contrast, each mound of the multiple-queen form contains many unrelated families, is not territorial, and reaches much higher populations. The ecological impact of the multiple queen form is much greater than that of the single-queen form, reducing the diversity of native ants and other insects. Army ants create nomadic hunting colonies, many of which live entirely underground. One of the best known army ants is the Burchell’s army ant, found in Central and South America. In this species, colonies of about 500,000 workers, all of whom are blind, send out long raiding columns each day to collect prey, which are commonly other insects or spiders. The columns of hunting ants form a swarm that covers the forest floor, flushing prey from hiding. They then use their mandibles to kill and dismember their victims. These huge colonies consume so much of the available prey in a given area that they need to be constantly on the move to find more food.

In place of permanent nest structures, army ant workers grasp one another’s legs to form hanging sheets of ants, called bivouacs. Within these living nests, army ants protect and feed the queen and her young. The colony moves to a new bivouac site every night until the larvae pupate and no longer need feeding. Then the colony stays in one nest site for about three weeks while the queen lays about 100,000 to 300,000 eggs in just a few days. After these eggs hatch, the colony resumes its nomadic life.

Army ants found in Africa are called driver ants. Driver ants form huge colonies, which in some cases consist of between 10 million and 20 million workers. Driver ants have extremely powerful mandibles. Their massive swarms are capable of immobilizing and killing large prey. Many species of ants harvest seeds. These so-called harvester ants, which are probably the ants noted in the Bible for their industriousness, live in arid or semiarid parts of the world. They ensure a continuous food supply by collecting seeds and storing them in special underground chambers for use when other types of food are scarce.

Harvester ants are abundant in the deserts of North America, where the combined weight of all harvester ants in the area may equal that of seed-eating rodents. These two types of animals compete with one another for the desert’s seeds.

In harvester ant colonies, worker ants use their mandibles to collect seeds and carry them into the nest. Other workers, located in chambers that are close to the surface of the ground, remove the seed’s outer husk and carry it outside. Next, workers transport the inner, nutritious part of the seed to a deeper chamber that is reserved for storing seeds. If a seed is damp and at risk for rotting, the ants carry it to the surface and wait for it to dry before bringing it to the storage chamber. A large nest of harvester ants may contain enough seeds to fill several cups.

To eat plant seeds, ants use their saliva and the mechanical action of their mouthparts to reduce the seeds to a pasty substance called ant bread. Ant bread may be fed to larvae, which are the only ants capable of digesting solid food, or it may be chewed further to become a liquid that can be swallowed by adult ants. Weaver ants are tree-dwelling ants that live in the warm parts of Asia, Africa, and Australia. These ants construct nests by pulling leaves together and binding them in place with thousands of strands of silk. This silk is obtained from a colony’s larvae, which have enlarged silk glands that would otherwise be used to spin cocoons for pupation. If the leaves are too far apart, the workers form living chains to pull the leaves together. A single colony may occupy dozens of leaf nests in several trees. Workers mark the colony’s territory in the trees with a pheromone, and they defend their trees fiercely against intruders from other colonies. Certain agricultural ants cultivate a fungus that they use for food. The most widely known fungus-gardeners are the leafcutter ants of Central and South America, whose enormous nests may contain millions of workers.

Leafcutter ants plant their fungus gardens on a surface of chewed-up leaves. Workers clear trails that radiate from the nest into the surrounding forest for 100 m (330 ft) or more. As they move along the trails, columns of workers climb trees to cut pieces of leaves and transport them back to the nest. Smaller workers cut the leaves into very small bits and place them in special chambers in the nest that house sponge-like fungus gardens. The tiniest workers plant gardens with fungal filaments and cultivate them until the fungus can be harvested and eaten. Larvae are placed within the gardens to feed. When a young queen starts a new colony, she carries in her mouth a pellet that contains strands of fungi from her most recent meal. She then fertilizes the pellet with wastes from her own body, and the fungus multiplies.

Leafcutter ants are among the largest consumers of leaves and grass in the American tropics, consuming far more vegetation than all vertebrates together. As a result, they are frequent pests of tropical agriculture. Numerous species of ants collect a sweet substance called honeydew that is excreted by various tiny insects, including aphids, mealy bugs, and scale insects. The insects most commonly used for this purpose are aphids, which pierce plant tissues to suck up juices from a plant. Among some of these ants, workers leave their nests regularly to watch over groups of aphids and protect them from predators. In some instances, these ants construct shelters out of soil or carton to shield the aphids from the environment.

Worker ants stroke the aphids with their antennae to induce them to release drops of honeydew. The ants then transfer this honeydew through trophallaxis to another group of workers, who carry it back to the nest and share it with nest workers. Individual workers may spend days or weeks among the same group of aphids. Certain species of ants produce individuals who serve as living food containers, called honeypots. During seasons when the colony has access to abundant nectar and honeydew from plants, aphids, or other insects, worker ants feed these liquids to young adult workers called repletes. Over time, the crops and gasters of the repletes become so full that these ants are unable to walk. The repletes become living storage tanks and simply hang from the roof of specially designed storerooms. When food is scarce, the repletes regurgitate their stored liquids back to nestmates through trophallaxis. A replete is typically stretched so full with liquid that if it falls from its hanging place it may burst open and die.

Honeypot ants occur most often in semi-arid zones where some method of food storage is needed to sustain colonies during times of the year when food is scarce. Ants differ from their hive-making relatives, the honeybees, in that they cannot create waxy structures for storing liquid foods, so they use their bodies for this purpose. Many types of ants have evolved mutually beneficial relationships with particular types of plants. An example is the relationship between acacia ants of the American tropics and some species of acacia trees. The trees provide hollow thorns that the ants use as nests. In addition, these trees have nectar glands that provide the ants with sugary food, and their young leaves have nutritious attachments that the ants feed to their larvae. In return, these aggressively stinging ants protect the acacia from insect and vertebrate enemies. Many species of ants exploit the labor or resources of other species of ants. Slave-making ants steal pupae from the nests of neighboring ants of a different species. When the pupae emerge as adult workers in the slave-makers’ nest, they regard that nest as their own and embark on a life of labor for the ants that captured them.

Some species of ants, commonly called Amazon ants, cannot survive without slaves. These ants have sharp mandibles that are better adapted for puncturing enemies than for nursing larvae or obtaining food. Amazon ants rely on slaves to forage for food, dig and maintain the nests, tend the young, and feed adult members of the colony through trophallaxis. Experiments show that when Amazon ants are separated from their slaves, they starve to death even if food is present.

Some ants are considered temporary social parasites. A newly mated queen of one of these species may enter the nest of a different species, kill the resident queen, and use the worker force to rear her own offspring. The new queen’s eggs develop into workers that gradually replace the host workers. Ants evolved from a wasp ancestor that hunted insect prey to feed its larvae in a nest. An important difference between wasps and the earliest ants was the presence of several generations in the same nest, which allowed cooperation to evolve. Increasing cooperation essentially resulted in a wingless worker caste and winged queens.

The oldest known fossil ants are 92 million years old. Preserved in amber, these fossil ants were discovered in 1998 in New Jersey. These ants, which lived during the Cretaceous Period (138 million to 65 million years before present) when dinosaurs flourished, had characteristics that are intermediate between wasps and today’s ants. One body structure that marks them as ants, however, is the presence of a metapleural gland in the alitrunk.

Ants gradually emerged as one of the Earth’s dominant insects. During the Cretaceous Period, ants made up only a small part of the insect life. By the Oligocene Epoch, which spanned from about 38 million to 24 million years ago, ants were far more abundant. By this time, about half of the modern groups of ants were already present.

The social evolution of ants is still in progress. Today, some species of ants show only little coordination within colonies, whereas others create highly cooperative societies that function as smoothly as a single organism. This diversity of social characteristics makes ants fascinating to study or observe. Ants perform many essential functions in ecosystems. They turn soil, move organic matter and soil nutrients, reduce insect populations, serve as food for other animals, disperse seeds, and sometimes pollinate flowers. In agriculture, they are the most important predators of insects, helping to keep pests under control.

In certain parts of the world, people use ants for highly specialized purposes. Native Americans of the southwestern United States sometimes eat honeypot ants as a sweet treat or a form of medicine. In Europe, various species of ants are intentionally introduced to timber forests, where they prey on caterpillars, preventing them from eating the leaves on trees. People in the American tropics sometimes welcome army ants into their homes. These people temporarily move out of their homes while the ants sweep through and consume or drive out any pests that may have infested the home.

The activities of ants occasionally bring them into conflict with humans. A number of ants have become pests, particularly those that have been accidentally transported outside of their natural range. One example is the fire ant. Another ant pest widely transported by humans is the pharaoh’s ant, which is native to North Africa but has spread throughout much of the world. The pharaoh’s ant cannot tolerate cold temperatures, and so in cold climates it lives in human houses and in other large, heated buildings, such as hospitals and factories. Perhaps the most widespread ant pest is the Argentine ant. Carpenter ants, native to North America, live in timber instead of soil. Once inside a human home, they make their nests within wooden structures, which they hollow out with their sharp mandibles.

Just as ants sometimes cause harm to humans, humans are a source of harm to ants. Little information is available about endangered species of ants, but scientists suspect many species of ants may be in danger of extinction. The most vulnerable ant species are those that live only in a small geographic area and require a specific type of habitat. An example is a type of leafcutter ant that lives in coastal forests in Brazil. Army ants are increasingly threatened by humans. Army ants live in tropical forests and cannot survive in areas where these forests have been destroyed.

Scientific classification: Ants make up the family Formicidae in the order Hymenoptera.

Doves
Pigeon, common name for members of a family of birds; smaller species are commonly known as doves, but sizes of pigeons and doves overlap. The birds, almost worldwide in distribution, are most abundant in warm regions.

Pigeons have small heads, short necks, stout bodies with short legs, and sleek plumage and have a fleshy or waxy protuberance, the cere, at the base of the bill. They dwell in trees or on the ground and feed on seeds, fruit, acorns and other nuts, and insects. Pigeons fly rapidly and are noted for their cooing call. They build loose, almost flat, nests of twigs, bark, straw, and weeds; the female lays one or two tan or white eggs.

The best-known species is the common pigeon, whose wild ancestor, native to Europe and Asia, is called the rock dove. It is about 33 cm (about 13 in) long, bluish gray above, with black markings on the wings and a whitish rump; below, it is purplish on the breast and bluish on the abdomen. The sides of the neck, especially in males, are iridescent. The more than 200 domestic breeds as well as the variably colored street or feral pigeons are derived from this species. Homing pigeons, which also vary in color, are bred for their navigational abilities and not for plumage characters. Among the other domestic breeds are the carrier pigeon, a tall, erect form with large wattles around the eyes and the base of the bill, whose name is commonly misapplied to the homing pigeon; the frills, characterized by forward curvature of the tips of the feathers of the neck and body, giving a ruffled appearance; the pouters, which can dilate the crop region into a swollen globe; and the jacobin (a favorite pet of Queen Victoria), whose elongated neck feathers form a hood over the head.

The appropriately named white-crowned pigeon, a Caribbean species that extends north to southern Florida, is the only wild member of its genus found in the eastern United States. The larger band-tailed pigeon, about 39 cm (about 15 in) long, has a range extending from coastal British Columbia and the Rocky Mountains south to Argentina. The passenger pigeon, once common throughout the United States, has been extinct since 1914.

Well-known pigeons outside the Americas include the crowned pigeons, which inhabit New Guinea and adjacent islands and are characterized by an erect crest of modified feathers; the Australian bronze-wings, which are characterized by bronze spots on the wings; the Nicobar pigeon, of islands in the East Indies, characterized by long, iridescent, dark-green hackle feathers of the neck that hang down over the back and shoulders; and the large (up to 50 cm/20 in long), fruit-eating imperial pigeons, with about 37 species in Asia and the Pacific islands, varying from multicolored to pure white.

Among the pigeons called doves, many are widely distributed in Eurasia and Africa. An Asian species, the spotted dove, has been successfully introduced in many parts of the world, including southern California and Hawaii. A domestic form of uncertain wild origin, the ringed turtledove, has feral populations in California and Florida; usually buffy with a black ring on the hind neck, a pure white variety is a popular addition to movie scenes of large weddings. The most common North American dove is the mourning dove, named for its plaintive call. It is about 30 cm (about 12 in) long, with a brown body, bluish-gray wings, and a long, white-tipped tail. Once found chiefly in open countryside, this species has become a familiar sight in urban residential areas. Smallest of the doves are the sparrow-sized ground doves of the southern United States and the New World tropics.

Scientific classification: Pigeons make up the family Columbidae of the order Columbiformes. The common pigeon is classified as Columba livia, the white-crowned pigeon as Columba leucocephala, and the band-tailed pigeon as Columba fasciata. The passenger pigeon is classified as Ectopistes migratorius. Crowned pigeons make up the genus Goura; Australian bronze-wings, the genus Phaps; and imperial pigeons, the genus Ducula. The Nicobar pigeon is classified as Caloenas nicobarica, the spotted dove as Streptopelia chinensis, the ringed turtledove as Streptopelia risoria, and the mourning dove as Zenaida macroura. Ground doves make up the genus Columbina