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Leafcutter ants, a non-generic name, are any of 47
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
of leaf-chewing
ant Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of ...
s belonging to the two
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
'' Atta'' and ''
Acromyrmex ''Acromyrmex'' is a genus of New World ants of the subfamily Myrmicinae. This genus is found in South America and parts of Central America and the Caribbean Islands, and contains 33 known species. Commonly known as "leafcutter ants" they compris ...
''. These species of
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
,
fungus-growing ants Fungus-growing ants (tribe Attini) comprise all the known fungus-growing ant species participating in ant–fungus mutualism. They are known for cutting grasses and leaves, carrying them to their colonies' nests, and using them to grow fungus o ...
are all
endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...
to
South South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþa ...
and
Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...
,
Mexico Mexico (Spanish language, Spanish: México), officially the United Mexican States, is a List of sovereign states, country in the southern portion of North America. It is borders of Mexico, bordered to the north by the United States; to the so ...
, and parts of the
southern United States The Southern United States (sometimes Dixie, also referred to as the Southern States, the American South, the Southland, or simply the South) is a geographic and cultural region of the United States of America. It is between the Atlantic Ocean ...
.. Leafcutter ants can carry twenty times their body weight and cut and process fresh vegetation (leaves, flowers, and grasses) to serve as the nutritional substrate for their fungal cultivates. ''Acromyrmex'' and ''Atta'' ants have much in common anatomically; however, the two can be identified by their external differences. ''Atta'' ants have three pairs of spines and a smooth
exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of the ...
on the upper surface of the
thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the c ...
, while ''Acromyrmex'' ants have four pairs and a rough exoskeleton. The exoskeleton itself is covered in a thin layer of mineral coating, composed of rhombohedral crystals that are generated by the ants. Next to humans, leafcutter ants form some of the largest and most complex animal societies on Earth. In a few years, the central mound of their underground nests can grow to more than across, with smaller radiating mounds extending out to a radius of , taking up and containing eight million individuals.


The lifecycle of a leafcutter ant colony


Reproduction and colony founding

Winged females and males leave their respective nests ''en masse'' and engage in a
nuptial flight Nuptial flight is an important phase in the reproduction of most ant, termite, and some bee species. It is also observed in some fly species, such as '' Rhamphomyia longicauda''. During the flight, virgin queens mate with males and then land t ...
known as the ''revoada'' (Portuguese) or ''vuelo nupcial'' (Spanish). Each female mates with multiple males to collect the 300 million sperm she needs to set up a
colony In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state' ...
.. Once on the ground, the female loses her wings and searches for a suitable underground lair in which to found her colony. The success rate of these young queens is very low, and only 2.5% will go on to establish a long-lived colony. To start her own fungus garden, the queen stores bits of the parental fungus garden
mycelium Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...
in her infrabuccal pocket, which is located within her oral cavity.. Colonies are generally founded by individual queens
haplometrosis This is a glossary of terms used in the descriptions of ants. A B D E F G H M N O P Q R S T U W See also ...
. Because colonies with multiple queens over the lifespan of the colony have been found by a large number of investigators by Weber 1937, Jonkman 1977, Huber 1907, Moser & Lewis 1981, Mariconi & Zamith 1963, Moser 1963, and Walter et al 1938 it is believable that some colonies have multiple foundresses termed
pleometrosis Pleometrosis is a behavior observed in social insects where colony formation is initiated by multiple queens primarily by the same species of insect. This type of behavior has been mainly studied in ants but also occurs in wasps, bees, and termite ...
. Pleometrosis ''is'' confirmed only for ''
Atta texana The Texas leafcutter ant (''Atta texana'') is a species of fungus-farming ant in the subfamily Myrmicinae. It is found in Texas, Louisiana, and north-eastern Mexico. Other common names include town ant, parasol ant, fungus ant, cut ant, and nigh ...
'' by Vinson 1985.


Colony hierarchy

In a mature leafcutter colony, ants are divided into
castes Caste is a form of social stratification characterised by endogamy, hereditary transmission of a style of life which often includes an occupation, ritual status in a hierarchy, and customary social interaction and exclusion based on cultura ...
, based mostly on size, that perform different functions. ''Acromyrmex'' and ''Atta'' exhibit a high degree of biological polymorphism, four castes being present in established colonies—minims, minors, mediae, and majors. Majors are also known as soldiers or dinergates. ''Atta'' ants are more polymorphic than ''Acromyrmex'', meaning comparatively less difference occurs in size from the smallest to largest types of ''Acromyrmex''. * Minims are the smallest workers, and tend to the growing brood or care for the fungus gardens. Head width is less than 1 mm. * Minors are slightly larger than minima workers, and are present in large numbers in and around foraging columns. These ants are the first line of defense and continuously patrol the surrounding terrain and vigorously attack any enemies that threaten the foraging lines. Head width is around 1.8–2.2 mm. * Mediae are the generalized foragers, which cut leaves and bring the leaf fragments back to the nest. * Majors, the largest worker ants, act as soldiers, defending the nest from intruders, although recent evidence indicates majors participate in other activities, such as clearing the main foraging trails of large debris and carrying bulky items back to the nest. The largest soldiers (''
Atta laevigata ''Atta laevigata'' ( Smith, 1858) is one of about a dozen species of leafcutter ants in the genus '' Atta'', found from Venezuela and south to Paraguay. This species is one of the largest leafcutter species, and can be recognized by the smooth ...
'') may have total body lengths up to 16 mm and head widths of 7 mm.


Ant–fungus mutualism

Their societies are based on an
ant–fungus mutualism The ant–fungus mutualism is a symbiosis seen between certain ant and fungal species, in which ants actively cultivate fungus much like humans farm crops as a food source. There is only evidence of two instances in which this form of agricultur ...
, and different species of ants use different species of fungus, but all of the fungi the ants use are members of the family Lepiotaceae. The ants actively cultivate their fungus, feeding it with freshly cut plant material and keeping it free from
pests PESTS was an anonymous American activist group formed in 1986 to critique racism, tokenism, and exclusion in the art world. PESTS produced newsletters, posters, and other print material highlighting examples of discrimination in gallery represent ...
and
mold A mold () or mould () is one of the structures certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi. Not ...
s. This mutualistic relationship is further augmented by another
symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or para ...
partner, a bacterium that grows on the ants and secretes chemicals; essentially, the ants use portable
antimicrobial An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ar ...
s. Leaf cutter ants are sensitive enough to adapt to the fungi's reaction to different plant material, apparently detecting chemical signals from the fungus. If a particular type of leaf is toxic to the fungus, the colony will no longer collect it. The only two other groups of insects to use fungus-based agriculture are ambrosia beetles and
termites Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...
. The fungus cultivated by the adults is used to feed the ant larvae, and the adult ants feed on leaf sap. The fungus needs the ants to stay alive, and the larvae need the fungus to stay alive, so the mutualism is obligatory. The fungi used by the higher attine ants no longer produce spores. These ants fully domesticated their fungal partner 15 million years ago, a process that took 30 million years to complete. Their fungi produce nutritious and swollen
hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...
l tips (
gongylidia Gongylidia (singular gongylidium) are hyphal swellings of fungus cultivated by higher-attine genera of fungus-growing ants. This fungus no longer exists naturally outside the ant colonies. Gongylidia are fed to the developing larvae and distribu ...
) that grow in bundles called staphylae, to specifically feed the ants. '' Leucoagaricus gongylophorus'' is the most commonly documented fungi farmed by higher attine ant species.


Waste management

Leafcutter ants have very specific roles in taking care of the fungal garden and dumping the refuse. Waste management is a key role for each colony's longevity. The necrotrophic parasitic fungus ''
Escovopsis ''Escovopsis'' is a genus of seven formally acknowledged parasitic microfungus species that rely on other fungi to be their hosts. This genus formally circumscribed with a single identified species in 1990; in 2013 three other species were added ...
'' threatens the ants' food source and thus is a constant danger to the ants. The waste transporters and waste-heap workers are the older, more dispensable leafcutter ants, ensuring the healthier and younger ants can work on the fungal garden. The '' Atta colombica'' species, unusually for the Attine tribe, have an external waste heap. Waste transporters take the waste, which consists of used substrate and discarded fungus, to the waste heap. Once dropped off at the refuse dump, the heap workers organise the waste and constantly shuffle it around to aid decomposition. A compelling observation of ''A. colombica'' was the dead ants placed around the perimeter of the waste heap. In addition to feeding the fungal garden with foraged food, mainly consisting of leaves, it is protected from ''Escovopsis'' by the antibiotic secretions of
Actinomycetota The ''Actinomycetota'' (or ''Actinobacteria'') are a phylum of all gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to s ...
(genus ''
Pseudonocardia ''Pseudonocardia'' is the type genus of the bacteria family Pseudonocardiaceae. Members of this genus have been found living mutualistically on the cuticle of the leafcutter ants because the bacteria has antibiotic properties that protect the fu ...
''). This mutualistic micro-organism lives in the metapleural glands of the ant. Actinomycetota are responsible for producing the majority of the world's
antibiotics An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and preventio ...
today.


Parasitism

When the ants are out collecting leaves, they are at risk of attack by some species of phorid flies,
parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
s that lay eggs into the crevices of the worker ants' heads. Often, a minim will sit on a worker ant and ward off any attack. Also, the wrong type of fungus can grow during cultivation. ''Escovopsis'', a highly virulent fungus, has the potential to devastate an ant garden, as it is horizontally transmitted. ''Escovopsis'' was cultured, during colony foundation, in 6.6% of colonies. However, in one- to two-year-old colonies, almost 60% had ''Escovopsis'' growing in the fungal garden. Nevertheless, leafcutter ants have many adaptive mechanisms to recognize and control infections by ''Escovopsis'' and other micro-organisms. The most common known behaviors rely on workers reducing the number of fungal spores by grooming, or removing an infected piece of the fungus garden and throwing it away at the waste dump (described as weeding).


Communication

Leafcutter ants use stridulation (substrate-borne vibrations) to communicate with each other.


Prey plants

Leafcutter ants prefer
disturbed habitat In ecology, a disturbance is a temporary change in environmental conditions that causes a pronounced change in an ecosystem. Disturbances often act quickly and with great effect, to alter the physical structure or arrangement of biotic and abiotic ...
s, likely due to higher concentrations of pioneer plant species. These are more attractive food sources because pioneer plants have lower levels of
secondary metabolite Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norma ...
s and higher
nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excre ...
concentrations than the shade-tolerant species that will come later.


Interactions with humans

In some parts of their range, leafcutter ants can be a serious agricultural pest, defoliating crops and damaging roads and farmland with their nest-making activities. For example, some ''Atta'' species are capable of defoliating an entire citrus tree in less than 24 hours. A promising approach to deterring attacks of the leafcutter ant ''
Acromyrmex lobicornis ''Acromyrmex lobicornis'' is a species of leaf-cutter ant, a New World ant of the subfamily Myrmicinae of the genus ''Acromyrmex''. This species is from one of the two genera of advanced attines (fungus-growing ants) within the tribe Attini. ...
'' on crops has been demonstrated. Collecting the refuse from the nest and placing it over seedlings or around crops resulted in a deterrent effect over a period of 30 days..


See also

*
List of leafcutter ants This is a list of leafcutter ants, comprising 42 species from two genera: ''Atta'' and ''Acromyrmex ''Acromyrmex'' is a genus of New World ants of the subfamily Myrmicinae. This genus is found in South America and parts of Central America and t ...
* '' Atta sexdens'' * Lepiotaceae


References


External links

* *
The Lurker's Guide to Leafcutter Ants
* {{DEFAULTSORT:Leafcutter Ant Ants Atta (genus) Symbiosis Folivores Acromyrmex