Acromyrmex species' hard outer covering, the exoskeleton or cuticle, functions as armour, protection against dangerous solar waves, an attachment base for internal muscles, and to prevent water loss. It is divided into three main parts; the head, thorax, and abdomen. A small segment between the thorax and abdomen, the petiole, is split into two nodes in Acromyrmex species.
Diagram of an ant's anatomy
The antennae are the most important sense organs Acromyrmex species possess, and are jointed so the ant can extend them forward to investigate an object. It can retract them back over its head when in a dangerous situation, for example, a fight. Acromyrmex species have eyes, but their eyesight is very poor. Like all insects, the eye is compound, meaning it is made up of many eyelets called ommatidia, with the number of these eyelets varying according to species. Male ants tend to have more ommatidia than other castes. The ocelli, which are generally found on top of the heads of queens, are thought to aid aerial navigation by sunlight.
Acromyrmex is dark red in colour. In addition to the standard ant anatomy, the back of the thorax has a series of spines which help it manoeuvre material such as leaf fragments on its back.
Acromyrmex can be distinguished from the closely related leafcutter ant genus Atta by having four pairs of spines and a rough exoskeleton on the upper surface of the thorax compared to three pairs of spines and a smooth exoskeleton in Atta.
Much of the inside of the Acromyrmex head is occupied by the muscles that close the jaws; the muscles that open the jaws are much smaller. The brain, though tiny, is a very complex organ, and allows Acromyrmex to learn and react to its surroundings. It can remember colony odour, navigation, and where it has placed a certain object.
The heart is a long, tubular organ running the entire length of the body, from the brain to the tip of the abdomen. It has valves within it that prevent blood from flowing the wrong way. The fluids bathing the internal organs is circulated by the heart; these fluids then filter through the organs and tissues. The pharynx, which is part of the gut, controlled by six muscles, pumps food into the oesophagus. Debris in the food, such as soil, is filtered before it enters the oesophagus and is collected in a tiny trap, the infrabuccal pocket. When this pocket becomes full, the Acromyrmex ant empties it into an area within or outside the nest designated as a waste-products area.
Several glands in the head secrete various substances, such as those responsible for the digestion of food. Another gland within the head produces digestive and, in some species, alarm chemicals; these chemicals are used to alert nearby ants of impending danger, and any ant that detects this alarm will automatically go into "battle mode". If an ant is crushed, a huge blast of this chemical is released, causing the entire colony to go into "battle mode".
The thorax contains muscles to operate the legs and wings and the nerve cells to co-ordinate their movements; also contained in this part of the body is the heart and oesophagus.
The abdomen contains the stomachs, poison glands, ovaries in the queen, and the Dufour's gland, among other things. Acromyrmex ants have two "stomachs", including a dry, social stomach in which they can store food and later regurgitate to larvae, the queen and other ants. This is separated from the stomach proper by a small valve; once food enters the second stomach, it becomes contaminated with gastric juices and cannot be regurgitated. The exact function of the Dufour's gland is unknown, but is thought to be involved in the release of the chemicals used in the production of odour trails, which the ants use to recruit nest mates to a food source. It may also produce sex-attractant chemicals.
Ecology
A. balzani worker carrying a leaf
Reproduction
Winged females and males leave their respective nests en masse and engage in a nuptial flight known as the revoada. Each female mates with multiple males to collect the 300 million sperm she needs to set up a colony.[2]
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. Before leaving their parent colonies, winged females take a small section of fungus into their infrabuccal pouches to 'seed' the fungus gardens of incipient colonies, cutting and collecting the first few sections of leaf themselves.
Colony hierarchy
A mature leafcutter colony can contain more than 8 million ants (the maximum size of the colony varies between species), mostly sterile female workers. They are divided into castes, based mostly on size, that perform different functions. Acromyrmex ants exhibit a high degree of biological polymorphism, four castes being present in established colonies - minims (or "garden ants"), minors, mediae, and majors. Majors are also known as soldiers or dinergates. Each caste has a specific function within the colony. Acromyrmex ants are less polymorphic than the other genus of leafcutter ants Atta, meaning comparatively less difference in size exists from the smallest to largest types of Acromymex. The high degree of polymorphism in this genus is also suggestive of its high degree of advancement.
Ant-fungus mutualism
Like Atta, Acromyrmex societies are based on an ant-fungus mutualism, and different species use different species of fungus, but all of the fungi the ants use are members of the genus Leucocoprinus. The ants actively cultivate their fungus on a medium of masticated leaf tissue. This is the sole food of the queen and other colony members that remain in the nest. The mediae also gain subsistence from plant sap they ingest whilst physically cutting out sections of leaf from a variety of plants.
This mutualistic relationship is further augmented by another symbiotic partner, a bacterium that grows on the ants and secretes chemicals; essentially, the ants use portable antimicrobials. Leafcutter ants are sensitive enough to adapt to the fungus' reaction to different plant material, apparently detecting chemical signals from it. 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 that have evolved fungus-based agriculture are ambrosia beetles and termites. The fungus cultivated by the adults is used to feed the ant larvae and the adult ants feed on the leaf sap. The fungus needs the ants to stay alive, and the larvae need the fungus to stay alive.[3]
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 (genus Pseudonocardia). This mutualistic microorganism lives in the metapleural glands of the ants.[4] Actinomycetota are responsible for producing the majority of the world's antibiotics today.
Waste management
Leafcutter ants have very specific roles for taking care of the fungal garden and dumping the refuse. Waste management is a key role for each colony's longevity. The necrotrophic parasite Escovopsis of the fungal cultivar threatens the ants' food source, and is a constant danger to the ants. The waste transporters and waste-heap workers are the older, more dispensable ants, ensuring the healthier and younger leafcutter ants can work on the fungal garden. Waste transporters take the waste, which consists of used substrate and discarded fungus, to the waste heap. Once dropped off at the refuse dump, heap workers organise the waste and constantly shuffle it around to aid decomposition.
Acromyrmex has evolved to change food plants constantly, preventing a colony from completely stripping off leaves and thereby killing trees, thus avoiding negative biological feedback on account of their sheer numbers. However, this does not diminish the huge quantities of foliage they harvest. Once foraging workers locate a resource in their environment, they lay down a pheromone trail as they return to the colony. Other workers then follow the pheromone trail to the resource. As more workers return to the nest, laying down pheremones, the stronger the trail becomes. The strength to which workers adhere to the trail (trail fidelity) depends mostly on environmental factors, such as the quality of the resource.
Interactions with humans
In some parts of their range, Acromyrmex species can be quite a nuisance to humans, defoliating crops and damaging roads and farmland with their nest-making activities.[2] For example, Acromyrmex octospinosus ants harvest huge quantities of foliage, so they have become agricultural pests on the various Caribbean islands where they have been introduced, such as Guadeloupe.
In Central America, leafcutter ants are referred to as "wee wee" ants, though not based on their size. They are one of the largest ants in Central America.[citation needed]
Deterring the leafcutter ant Acromyrmex lobicornis from defoliating crops has been found to be simpler than first expected. 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.[5]
Atta is a genus of New World ants of the subfamily Myrmicinae. It contains at least 17 known species.
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 and Central America, Mexico, and parts of the southern United States. Leafcutter ants can carry twenty times their body weight and cut and process fresh vegetation to serve as the nutritional substrate for their fungal cultivates.
Fungus-growing ants 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 on which they later feed.
Atta sexdens is a species of leafcutter ant belonging to the tribe Attini, native to the New World, from the southern United States (Texas) to northern Argentina. 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, such as Atta texana, Atta cephalotes and others, have similar behavior and ecology.
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 agriculture evolved in ants resulting in a dependence on fungi for food. These instances were the attine ants and some ants that are part of the Megalomyrmex genus. In some species, the ants and fungi are dependent on each other for survival. This type of codependency is prevalent among herbivores who rely on plant material for nutrition. The microbes’ ability to convert the plant material into a food source accessible to their host makes them the ideal partner. The leafcutter ant is a well-known example of this symbiosis. Leafcutter ants species can be found in southern South America up to the United States. However, ants are not the only ground-dwelling arthropods which have developed symbioses with fungi. A mutualism with fungi is also noted in some species of termites in Africa.
Crematogaster is an ecologically diverse genus of ants found worldwide, which are characterised by a distinctive heart-shaped gaster (abdomen), which gives them one of their common names, the Saint Valentine ant. Members of this genus are also known as cocktail ants because of their habit of raising their abdomens when alarmed. Most species are arboreal (tree-dwelling). These ants are sometimes known as acrobat ants.
Sericomyrmex is a genus of fungus-growing ants in the subfamily Myrmicinae.
Tetraponera is a genus of ants in the subfamily Pseudomyrmecinae that are commonly known as slender ants and are characterized by their arboreal nature and slender bodies. The 96 described species of Tetraponera all of which live in hollow structures of plants and trees, such as thorns or branches; these hosts are known as myrmecophytes. Tetraponera species are closely related to the New World genus of ants Pseudomyrmex, but differ in their relationships with host plants.
Acromyrmex fracticornis 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 within the tribe Attini. It is found in the wild naturally in southern Brazil, Paraguay and northern Argentina.
Acromyrmex heyeri is a species of leaf-cutter ant, a New World ant of the subfamily Myrmicinae of the genus Acromyrmex.
Acromyrmex hystrix is a species of leaf-cutter ant, a New World ant of the subfamily Myrmicinae of the genus Acromyrmex.
Acromyrmex insinuator is a social parasite of the closely related Acromyrmex echinatior. This specific parasite is of particular interest as it is an opportunity to study the development of social parasitism in the Attini tribe, and provides further evidence for Emery's rule, which theorizes social parasites among insects tend to be parasites of species or genera to which they are closely related to.
Acromyrmex rugosus 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 within the tribe Attini.
Acromyrmex striatus is a species of the leaf-cutter ants found in the Neotropics.
Acromyrmex versicolor is known as the desert leafcutter ant. A. versicolor is found during the summer months in the Colorado and Sonoran deserts when there is precipitation. They form large, distinctive nest craters that are covered with leaf fragments. Living and dead leaves are collected by workers and used to cultivate fungus gardens. Each colony can have multiple queens, if they do this is a practice called polygyny, and each queen has her own batch of “starter” fungus. This species does not sting.
Acromyrmex evenkul 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 within the tribe Attini.
Acromyrmex pubescens is a species of New World ants of the subfamily Myrmicinae of the genus Acromyrmex. They are native to Paraguay.
Cyphomyrmex is a genus of fungus-growing ants found primarily in South and Central America. However, some species do come up to the southern portion of North America. They grow a variety of fungi in the tribe Leucocoprineae. Most fungal gardens are grown in small nodules, some species to cultivate entire mycelium, though. Colonies are monogynous and are relatively small with about 100 workers on average.
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.
Leucoagaricus gongylophorus is a fungus in the family Agaricaceae which is cultivated by certain leafcutter ants. Like other species of fungi cultivated by ants, L. gongylophorus produces gongylidia, nutrient-rich hyphal swellings upon which the ants feed. Production of mushrooms occurs only once ants abandon the nest. L. gongylophorus is farmed by leaf cutter ant species belonging to the genera Atta and Acromyrmex, amongst others.
↑ Zhang, M. M.; Poulsen, M. & Currie, C. R. (2007), "Symbiont recognition of mutualistic bacteria by Acromyrmex leaf-cutting ants", The ISME Journal, 1 (4): 313–320, doi:10.1038/ismej.2007.41, PMID18043642 .
↑ Ballari, S. A. & Farji-Brener, A. G. (2006), "Refuse dumps of the leaf-cutting ants as a deterrent for ant herbivory: does refuse age matter?", The Netherlands Entomological Society, 121 (3): 215–219, doi:10.1111/j.1570-8703.2006.00475.x, S2CID85410418 .
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
