Atta sexdens
Encyclopedia
Atta sexdens is a species
of leafcutter ant
belonging to the tribe Attini. Atta species are native to the New World
, from Southern United States to Northern Argentina
in the South. They are absent from Chile. They cut leaves to provide a substrate for the fungus
farms which are their principal source of food. Their societies are among the most complex found in social insects. A. sexdens is an ecologically important species, but also an agricultural pest. Other Atta species, like Atta texana, Atta cephalotes
and others, have similar behavior and ecology.
farms and waste chambers. The farms contain the fungal culture which sustains the colony and also hosts larva
e and pupa
e. The waste chambers are located at the rim of the colony and are significantly larger. They are used to dispose used fungal culture and dead ants. A mature colony contains 5–8 million workers.
The colony often forms the centerpiece of a large jungle clearing. At the top of the mound there are structures resembling sand castles surrounding hundreds of openings to the colony. Positioning the openings on top of these structures minimises the amount of rainwater flowing into the colony. The openings also have an important role in air conditioning
. As the ant activity and fungal metabolism
heat up the colony, hot air rises through the central passageways. Simultaneously fresh air is drawn in from the openings at the rim of the colony.
s moving the well-developed mandibles.
In addition to the queen the colony contains four castes of sterile
female workers:
Soldiers are the physically largest caste, with a head width of 3 mm and well-developed sharp mandibles. They are completely dedicated to defence, and do not participate in the running of the colony. In the event of attack by a predator, the soldiers storm out of the nest and attempt to overpower the aggressor. Their bite can easily penetrate human skin. The soldier caste is expensive to maintain, and they are absent from young colonies with less than 100,000 workers.
Forager-excavators are the second-largest caste, with an average head width of 2.2 mm. They are responsible for venturing out to cut pieces of leaves and carrying them back to the colony to feed the fungus. They also excavate new chambers to the colony.
Within-nest specialists have an average head width of 1.4 mm. They usually remain inside the colony where they process the leaf material brought in by the foragers to a smaller form. They also dispose waste, help the gardener-nurse caste to take care of larger larvae, and attend to the queen.
Gardener-nurses have an average head width of 1 mm. They are the physically smallest caste, being several hundred times less massive than the larger, more robust soldiers. Their tasks are to attend to the fungal culture and to the developing eggs
, larvae and pupae.
The queen is the largest ant in the colony, outweighing the smaller worker by a factor of 700. She is also the only member of the colony capable of reproducing. The queen resides in a special chamber where she is continually fed, cleaned and protected by workers. The queens have a life span of 10–20 years.
There is considerable variation in size within the castes, some of which may be related to the division of labour. Some researchers separate seven castes, while others dismiss the idea of caste altogether. The division of labour also depends on the age of the worker. Foraging expeditions are hazardous and therefore handled by older, and thus more expendable, members of the caste.
, larva, pupa, and adult. They are, like all hymenopterans, haplo-diploid, meaning males are haploid
(one of each chromosome
) and females, including workers, are diploid (a pair of each chromosome). Sex is determined by the type of egg that is laid. Unfertilized eggs
will turn out to be males, which are strictly used for mating and are short-lived. Fertilized eggs produce females. The caste of the resulting adult depends on environmental cues.
Three to four weeks after being laid, the egg hatches and a larva emerges. In Atta ants, the larvae are fed by secretions from gardener-nurse ants and trophic egg
s (unfertilized eggs that are used as food). The larva
e, after an additional 3–4 weeks, spin cocoons around themselves and pupate. After 3–4 more weeks, the pupae hatch into adult workers.
A. sexdens larvae grow embedded in the fungal garden. Despite being surrounded by food, they are incapable of feeding themselves. Their adult sisters constantly feed and clean them. This is not a primitive feature. On the contrary only the most advanced ant societies can expend such lavish care to their offspring as required by Atta larvae.
All eggs are identical when laid. The caste of the resulting worker is determined by the conditions, which are in turn regulated by adult workers. The most important factor is the quantity of food. The largest amount of food results in virgin queens while slightly less food creates soldiers. A shortage of one caste causes the workers to produce more ants of that caste. A drastic reduction in the work force may cause the colony to revert to the caste structure of a young colony, which does not have soldiers.
s after they discover a source of suitable plant material. Other workers follow this trail to the leaves. They cut the plant material to pieces suitable for an individual ant to carry back to the colony.
Smaller workers sometimes ride on the leaf pieces while the foragers carry them. The causes of this behavior is not known for certain, but the purpose may be to protect the exposed foragers from attacks by parasitic insects. These insects include phorid flies
of the family Phoridae which lay their eggs on foragers. The resulting larvae eat the ant alive.
Basidiomycota
. Leaves and other soft plant material brought into the nest by the foragers is chewed into a pulp and fertilized
with faeces. A small piece of fungus is placed on this substrate. The gardener-nurse caste takes care of the cultivation, transplanting fungus onto fresh substrate and weeding out wrong species of fungus, such as a parasitic Escovopsis, which sometimes can contaminate nests. They also use secretions from their salivary gland
s and help form antibiotic
-producing Streptomyces
bacteria
to keep their fungal gardens a strict monoculture
.
The gardener-nurses also cut pieces of mycelium
for the other castes to eat. In addition to the fungus, the A. sexdens feed on plant sap
. They are the only source of nutrition for the ants, apart from the trophic egg
s laid by the queen when the colony is young.
The identity of the fungus remains a mystery. It is known that they are a species of the basidiomycete family lepiotaceae. Some researchers believe that all gardening ants cultivate just one species, Leucocoprinus gongylophorus. The fungus produces special structures, called gongylidia, which have evolved
to be eaten by ants.
secretions from their metapleural gland
s on their bodies. Dirt collected by licking is stored in the workers' infrabuccal cavities, which are special recesses in their mouths. The purpose of these activities is to avoid infections, especially by parasitic fungi. Several fungus species, such as the Cordyceps
fungi, specially infect and kill ants.
All waste in the nest, including infrabuccal pellets, spent substrate from the gardens and dead ants are carried in the waste chambers in order to avoid contamination. Feces
, however, is not carried away but used to fertilise the fungal gardens. The waste chambers are larger than the human head and located at the rim of the colony. There the waste can decompose without endangering the ants. Due to the high concentration of various nutrient
s in the decomposing waste, a thick net of plant root
s usually permeates the compost
.
young queens visit the colony's fungal gardens and place a small piece of fungal mycelium
in their infrabuccal cavities. During their nuptial flight the queens mate with several males, who die shortly after. The queen retains the sperm
in a special organ for the rest of her life.
The mated queen lands on the ground and tears off her now-unnecessary wings
. Then she digs a vertical tunnel to a depth of approximately 30 centimeters. At the end of the tunnel she excavates a small chamber. The queen then starts a fungal garden using the piece brought from her birth colony. Usually the garden is fertilized only with faeces, but sometimes it is necessary for the queen to forage a small quantity of plant material. The queen also lays a few eggs.
The queen then tends to the fungal garden and the developing larvae and pupae, feeding them fungus and trophic egg
s. The queen herself eats nothing during this period, sustaining herself on fat deposits and her shrinking flight muscle
s. After the first brood of workers is ready, they take over the running of the fledgling colony and the queen becomes strictly an egg-laying machine. This process takes 40–60 days.
At first the colony grows slowly, but after 2–3 years the growth becomes faster and faster. One possible reason for this is that before the colony is established enough to sustain any large soldier-caste workers, it is useful to remain unnoticed. Soldiers usually appear when the colony has a population of about 100 000. After the number of workers reaches 5–8 million, the colony stops expanding and diverts resources into producing winged virgin queens and males.
The virgin queens are very rarely successful. Assuming that the number of colonies in an area remains constant over time, on average only one queen among the many thousands sent out by a colony successfully founds a new one. However, a mature colony of several million workers faces very few dangers. No known predator, except the mostly subterranean army ant Nomamyrmex esenbeckii, actively attacks the nests and even other highly aggressive army ant
s show a healthy respect for an A. sexdens colony. If spared from floods and human activity, the colony is usually destroyed only when the queen dies of old age, giving a successful colony a lifespan of 10–20 years. In that time the colony will have sent out numerous males and virgin queens to found new colonies.
. This has two important results: The soil becomes aerated with the excavation of tunnels and chambers, and enriched with nutrient
s as the ants bury their waste products. This represents a major way in which nutrients are recycled in their environment.
A. sexdens and other leafcutter ants are also important herbivore
s, consuming 12–17 % of the leaf mass produced in neotropical rainforest
s. One colony's consumption of plant material is comparable to that of a large mammalian herbivore, such as a cow.
A. sexdens along with a related species, A. cephalotes are the principal insect
pests where they are found, destroying billions of dollars worth of crops with their ability to quickly defoliate and strip crops of anything useful to the ants. In fact, Atta ants are considered the number one herbivorous pest in many areas where they are found.
It is speculated that if A. sexdens would spread into tropical Africa, results would be devastating. As the local plants have not developed defensive compounds against leafcutters and Africa does not have parasites evolved to infect them, the results for both the ecosystem
and agriculture
would be devastating.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of leafcutter ant
Leafcutter ant
Leafcutter ants, a non-generic name, are any of 47 species of leaf-chewing ants belonging to the two genera Atta and Acromyrmex.These species of tropical, fungus-growing ants are all endemic to South, Central America, Mexico and parts of the southern United States.The Acromyrmex and Atta ants have...
belonging to the tribe Attini. Atta species are native to the New World
New World
The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...
, from Southern United States to Northern Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
in the South. They are absent from Chile. They cut leaves to provide a substrate for the fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
farms which are their principal source of food. Their societies are among the most complex found in social insects. A. sexdens is an ecologically important species, but also an agricultural pest. Other Atta species, like Atta texana, Atta cephalotes
Atta cephalotes
Atta cephalotes is one of 41 species of leafcutter ant. This species is part of the Attini tribe . A single colony of ants can contain up to 5 million members, and each colony has one queen that can live more than 15 years...
and others, have similar behavior and ecology.
Colony architecture
A. sexdens colonies are primarily subterranean with a mound of excavated material on the surface. The diameter of the colony may reach 10 meters with a depth of 6 meters. The colonies contain up to 2000 chambers with a combined volume of more than 20 cubic meters. There are two basic types of chambers: FungusFungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
farms and waste chambers. The farms contain the fungal culture which sustains the colony and also hosts larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e and pupa
Pupa
A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago...
e. The waste chambers are located at the rim of the colony and are significantly larger. They are used to dispose used fungal culture and dead ants. A mature colony contains 5–8 million workers.
The colony often forms the centerpiece of a large jungle clearing. At the top of the mound there are structures resembling sand castles surrounding hundreds of openings to the colony. Positioning the openings on top of these structures minimises the amount of rainwater flowing into the colony. The openings also have an important role in air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
. As the ant activity and fungal metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
heat up the colony, hot air rises through the central passageways. Simultaneously fresh air is drawn in from the openings at the rim of the colony.
Anatomy
A. sexdens follows the basic body plan of ants fairly closely. They have sharp spikes or hooks rising from their heads and midsections to deter predators. Another distinguishing feature is a relatively large, two-lobed head. Its purpose is to accommodate the large muscleMuscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s moving the well-developed mandibles.
In addition to the queen the colony contains four castes of sterile
Infertility
Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...
female workers:
Soldiers are the physically largest caste, with a head width of 3 mm and well-developed sharp mandibles. They are completely dedicated to defence, and do not participate in the running of the colony. In the event of attack by a predator, the soldiers storm out of the nest and attempt to overpower the aggressor. Their bite can easily penetrate human skin. The soldier caste is expensive to maintain, and they are absent from young colonies with less than 100,000 workers.
Forager-excavators are the second-largest caste, with an average head width of 2.2 mm. They are responsible for venturing out to cut pieces of leaves and carrying them back to the colony to feed the fungus. They also excavate new chambers to the colony.
Within-nest specialists have an average head width of 1.4 mm. They usually remain inside the colony where they process the leaf material brought in by the foragers to a smaller form. They also dispose waste, help the gardener-nurse caste to take care of larger larvae, and attend to the queen.
Gardener-nurses have an average head width of 1 mm. They are the physically smallest caste, being several hundred times less massive than the larger, more robust soldiers. Their tasks are to attend to the fungal culture and to the developing eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
, larvae and pupae.
The queen is the largest ant in the colony, outweighing the smaller worker by a factor of 700. She is also the only member of the colony capable of reproducing. The queen resides in a special chamber where she is continually fed, cleaned and protected by workers. The queens have a life span of 10–20 years.
There is considerable variation in size within the castes, some of which may be related to the division of labour. Some researchers separate seven castes, while others dismiss the idea of caste altogether. The division of labour also depends on the age of the worker. Foraging expeditions are hazardous and therefore handled by older, and thus more expendable, members of the caste.
Development
A. sexdens like all ants have four stages of development: eggEgg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
, larva, pupa, and adult. They are, like all hymenopterans, haplo-diploid, meaning males are haploid
Ploidy
Ploidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
(one of each chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
) and females, including workers, are diploid (a pair of each chromosome). Sex is determined by the type of egg that is laid. Unfertilized eggs
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
will turn out to be males, which are strictly used for mating and are short-lived. Fertilized eggs produce females. The caste of the resulting adult depends on environmental cues.
Three to four weeks after being laid, the egg hatches and a larva emerges. In Atta ants, the larvae are fed by secretions from gardener-nurse ants and trophic egg
Trophic egg
A trophic egg is an egg which is not laid for reproduction but for nutrition, often for offspring hatched from regular eggs. Trophic eggs are usually unfertilised....
s (unfertilized eggs that are used as food). The larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e, after an additional 3–4 weeks, spin cocoons around themselves and pupate. After 3–4 more weeks, the pupae hatch into adult workers.
A. sexdens larvae grow embedded in the fungal garden. Despite being surrounded by food, they are incapable of feeding themselves. Their adult sisters constantly feed and clean them. This is not a primitive feature. On the contrary only the most advanced ant societies can expend such lavish care to their offspring as required by Atta larvae.
All eggs are identical when laid. The caste of the resulting worker is determined by the conditions, which are in turn regulated by adult workers. The most important factor is the quantity of food. The largest amount of food results in virgin queens while slightly less food creates soldiers. A shortage of one caste causes the workers to produce more ants of that caste. A drastic reduction in the work force may cause the colony to revert to the caste structure of a young colony, which does not have soldiers.
Foraging
The A. sexdens workers forage leaves up to range of 60 meters from the colony. The scouting workers leave behind a trail of pheromonePheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
s after they discover a source of suitable plant material. Other workers follow this trail to the leaves. They cut the plant material to pieces suitable for an individual ant to carry back to the colony.
Smaller workers sometimes ride on the leaf pieces while the foragers carry them. The causes of this behavior is not known for certain, but the purpose may be to protect the exposed foragers from attacks by parasitic insects. These insects include phorid flies
Phorid fly
Phoridae is a family of small, hump-backed flies resembling fruit flies. Phorid flies can often be identified by their escape habit of running rapidly across a surface rather than taking to the wing. This behaviour is a source of one of their alternate names: scuttle fly. They are a diverse and...
of the family Phoridae which lay their eggs on foragers. The resulting larvae eat the ant alive.
Fungus cultivation
A. sexdens, like all leaf cutter ants, are mycophagic (fungus-eaters). They live in a symbiotic relationship with a fungus belonging to the subphylumSubphylum
In life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
Basidiomycota
Basidiomycota
Basidiomycota is one of two large phyla that, together with the Ascomycota, comprise the subkingdom Dikarya within the Kingdom Fungi...
. Leaves and other soft plant material brought into the nest by the foragers is chewed into a pulp and fertilized
Fertilizer
Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...
with faeces. A small piece of fungus is placed on this substrate. The gardener-nurse caste takes care of the cultivation, transplanting fungus onto fresh substrate and weeding out wrong species of fungus, such as a parasitic Escovopsis, which sometimes can contaminate nests. They also use secretions from their salivary gland
Salivary gland
The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose...
s and help form antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
-producing Streptomyces
Streptomyces
Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...
bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
to keep their fungal gardens a strict monoculture
Monoculture
Monoculture is the agricultural practice of producing or growing one single crop over a wide area. It is also known as a way of farming practice of growing large stands of a single species. It is widely used in modern industrial agriculture and its implementation has allowed for large harvests from...
.
The gardener-nurses also cut pieces of mycelium
Mycelium
thumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or within many other...
for the other castes to eat. In addition to the fungus, the A. sexdens feed on plant sap
Sap
Sap may refer to:* Plant sap, the fluid transported in xylem cells or phloem sieve tube elements of a plant* Sap , a village in the Dunajská Streda District of Slovakia...
. They are the only source of nutrition for the ants, apart from the trophic egg
Trophic egg
A trophic egg is an egg which is not laid for reproduction but for nutrition, often for offspring hatched from regular eggs. Trophic eggs are usually unfertilised....
s laid by the queen when the colony is young.
The identity of the fungus remains a mystery. It is known that they are a species of the basidiomycete family lepiotaceae. Some researchers believe that all gardening ants cultivate just one species, Leucocoprinus gongylophorus. The fungus produces special structures, called gongylidia, which have evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
to be eaten by ants.
Cleaning
A. sexdens workers spend a considerable time cleaning each other and the queen. They also spread antibioticAntibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
secretions from their metapleural gland
Metapleural gland
Metapleural glands are secretory glands that are unique to ants and basal in the evolutionary history of ants. They are responsible for the production of an antibiotic fluid that then collects in in a reservoir on the posterior of the ant's alitrunk...
s on their bodies. Dirt collected by licking is stored in the workers' infrabuccal cavities, which are special recesses in their mouths. The purpose of these activities is to avoid infections, especially by parasitic fungi. Several fungus species, such as the Cordyceps
Cordyceps
Cordyceps is a genus of ascomycete fungi that includes about 400 described species. All Cordyceps species are endoparasitoids, mainly on insects and other arthropods ; a few are parasitic on other fungi. The best known species of the genus is Cordyceps sinensis, first recorded as yartsa gunbu in...
fungi, specially infect and kill ants.
All waste in the nest, including infrabuccal pellets, spent substrate from the gardens and dead ants are carried in the waste chambers in order to avoid contamination. Feces
Feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
, however, is not carried away but used to fertilise the fungal gardens. The waste chambers are larger than the human head and located at the rim of the colony. There the waste can decompose without endangering the ants. Due to the high concentration of various nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
s in the decomposing waste, a thick net of plant root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s usually permeates the compost
Compost
Compost is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Compost is a key ingredient in organic farming. At its most essential, the process of composting requires simply piling up waste outdoors and waiting for the materials to break down from anywhere...
.
Reproduction
From late October to mid-December, the A. sexdens colonies produce winged virgin queens and males. Before the nuptial flightNuptial flight
Nuptial flight is an important phase in the reproduction of most ant, termite and some bee species. During the flight, virgin queens mate with males and then land to start a new colony, or, in the case of honey bees, continue the planned succession of an existing hived colony.- Before the flight :A...
young queens visit the colony's fungal gardens and place a small piece of fungal mycelium
Mycelium
thumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or within many other...
in their infrabuccal cavities. During their nuptial flight the queens mate with several males, who die shortly after. The queen retains the sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
in a special organ for the rest of her life.
The mated queen lands on the ground and tears off her now-unnecessary wings
Insect wing
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...
. Then she digs a vertical tunnel to a depth of approximately 30 centimeters. At the end of the tunnel she excavates a small chamber. The queen then starts a fungal garden using the piece brought from her birth colony. Usually the garden is fertilized only with faeces, but sometimes it is necessary for the queen to forage a small quantity of plant material. The queen also lays a few eggs.
The queen then tends to the fungal garden and the developing larvae and pupae, feeding them fungus and trophic egg
Trophic egg
A trophic egg is an egg which is not laid for reproduction but for nutrition, often for offspring hatched from regular eggs. Trophic eggs are usually unfertilised....
s. The queen herself eats nothing during this period, sustaining herself on fat deposits and her shrinking flight muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s. After the first brood of workers is ready, they take over the running of the fledgling colony and the queen becomes strictly an egg-laying machine. This process takes 40–60 days.
At first the colony grows slowly, but after 2–3 years the growth becomes faster and faster. One possible reason for this is that before the colony is established enough to sustain any large soldier-caste workers, it is useful to remain unnoticed. Soldiers usually appear when the colony has a population of about 100 000. After the number of workers reaches 5–8 million, the colony stops expanding and diverts resources into producing winged virgin queens and males.
The virgin queens are very rarely successful. Assuming that the number of colonies in an area remains constant over time, on average only one queen among the many thousands sent out by a colony successfully founds a new one. However, a mature colony of several million workers faces very few dangers. No known predator, except the mostly subterranean army ant Nomamyrmex esenbeckii, actively attacks the nests and even other highly aggressive army ant
Army ant
The name army ant is applied to over 200 ant species, in different lineages, due to their aggressive predatory foraging groups, known as "raids", in which huge numbers of ants forage simultaneously over a certain area, attacking prey en masse.Another shared feature is that, unlike most ant...
s show a healthy respect for an A. sexdens colony. If spared from floods and human activity, the colony is usually destroyed only when the queen dies of old age, giving a successful colony a lifespan of 10–20 years. In that time the colony will have sent out numerous males and virgin queens to found new colonies.
Ecological impact
At the end of its life span an A. sexdens colony has moved up to 40,000 kg of soilSoil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
. This has two important results: The soil becomes aerated with the excavation of tunnels and chambers, and enriched with nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
s as the ants bury their waste products. This represents a major way in which nutrients are recycled in their environment.
A. sexdens and other leafcutter ants are also important herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
s, consuming 12–17 % of the leaf mass produced in neotropical rainforest
Rainforest
Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm...
s. One colony's consumption of plant material is comparable to that of a large mammalian herbivore, such as a cow.
A. sexdens along with a related species, A. cephalotes are the principal insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
pests where they are found, destroying billions of dollars worth of crops with their ability to quickly defoliate and strip crops of anything useful to the ants. In fact, Atta ants are considered the number one herbivorous pest in many areas where they are found.
It is speculated that if A. sexdens would spread into tropical Africa, results would be devastating. As the local plants have not developed defensive compounds against leafcutters and Africa does not have parasites evolved to infect them, the results for both the ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
and agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
would be devastating.
Human-Atta comparisons
Due to the vast differences between humans and A. sexdens it is impossible to make accurate comparison between them. However, to give some idea of the scale of A. sexdens endeavors, the following would be the achievements of these ants if they were scaled up in size to approximately human proportions:- The length of their foraging trails would be approximately 15 km.
- The foraging ants would travel at 26 km per hour while carrying a 300 kg load.
- The colony would be about one km deep.
- A mature A. sexdens superorganismSuperorganismA superorganism is an organism consisting of many organisms. This is usually meant to be a social unit of eusocial animals, where division of labour is highly specialised and where individuals are not able to survive by themselves for extended periods of time. Ants are the best-known example of...
would have 3 trillion neuronNeuronA neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s, outnumbering the neurons in human brainBrainThe brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
by a factor of 30.