Allomerus decemarticulatus
Encyclopedia
The Amazonian
Amazon Rainforest
The Amazon Rainforest , also known in English as Amazonia or the Amazon Jungle, is a moist broadleaf forest that covers most of the Amazon Basin of South America...

 ant
Ant
Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

 species
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...

 Allomerus decemarticulatus is a social insect
Eusociality
Eusociality is a term used for the highest level of social organization in a hierarchical classification....

 found in the tropics of South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

. They were documented by Alain Dejean et al. (2001) in French Guiana
French Guiana
French Guiana is an overseas region of France, consisting of a single overseas department located on the northern Atlantic coast of South America. It has borders with two nations, Brazil to the east and south, and Suriname to the west...

, a small French territory on the coast of northeast South America just north of Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

. This species is most notable for the workers’ complex and extreme predatory behavior
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

 which involves a symbiosis
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 with both a plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

 and fungal
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...

 species.
They live in leaf pockets of a host plant species, Hirtella physophora
Hirtella physophora
Hirtella physophora is a species of plant in the Chrysobalanaceae family. It forms an association with the ant species, Allomerus decemarticulatus. The ants live in honeycombed cylinders they attach to the plant's stems. The plant nearly always has these ants associated with it....

, which is also native to the Amazon. These leaf pockets are areas inside of the plant between the leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

 and the stem
Stem
-Science:* Plant stem, the above ground structures that have vascular tissue and that support leaves and flowers   Also see similar:**Stipe **Stipe * Stem cell...

. Each colony will inhabit a tree, with about 1200 individual workers per tree and 40 workers per leaf. Their diet primarily consists of large insects that are captured on the plant, but the ants also eat parts of the plant such as nectar and some kinds of food bodies
Beltian body
A Beltian body is a structure found on the leaves of some species of Acacia. Beltian bodies, named after Thomas Belt, are found on the tips of each leaflet and are rich in lipids and proteins and often red in colour. They are believed to have evolved in a symbiotic relationship with ants...

 produced by the plant. They are able to capture their prey, which are much larger than themselves, by constructing a platform that acts as a trap for the unsuspecting prey. The ants hide in the trap and attack when any 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...

 lands on it. This technique is an example of ambush predation
Ambush predator
Ambush predators or sit-and-wait predators are carnivorous animals that capture prey by stealth or cunning, not by speed or necessarily by strength. These organisms usually hide motionless and wait for prey to come within striking distance. They are often camouflaged, and may be solitary...

.

Physical Characteristics

The worker is about 2 mm long, with a head width and length at about .6 mm. The queen
Queen ant
A queen ant is an adult, reproducing female ant in an ant colony; generally she will be the mother of all the other ants in that colony. Some female ants do not need to mate to produce offspring, reproducing through asexual parthenogenesis or cloning and all of those offspring will be female.Ant...

 is approximately double that size, with a head width of 1.05 mm, head length of .98 mm, and a total length of 5.53 mm. The workers have 10-segmented antennae, from where they get their name. They also have abundant hair, with long hairs on their head (greater than .13 mm) and shorter hairs on their abdomen (less than .07 mm). They have brownish yellow colored bodies and whitish hair.

Habitat

A. decemarticulatus areneotropical, so they are located in the Amazonian tropics of South America. They seem to prefer living in H. physophora, which range
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...

 is limited to Brazil and French Guiana, so it seems that their own range is limited by the range of the plant, because the ants are only found where the plant lives. They live and hunt on H. physophora, living in leaf pouches and patrolling the plant’s stem for prey.

Plant Mutualism

The relationship between A. decemarticulatus and H. physophora is very specialized. H. physophora is a small tree in the Amazonian rainforest
Rainforest
Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm...

 that resides in the undergrowth
Undergrowth
Undergrowth usually refers to the vegetation in a forest, which can obstruct passage through the forest. The height of undergrowth is usually considered to be 0.3 – 3 m . Undergrowth can also refer all vegetation in a forest, which isn't in the canopy....

. It is also an example of a myrmecophyte
Myrmecophyte
Myrmecophyte is a plant that lives in a mutualistic association with a colony of ants. There are over 100 different genera of myrmecophytes. These plants possess structural adaptations that provide ants with food and/or shelter. These specialized structures include domatia, food bodies, and...

, because it contains ant-domatia
Domatia
Domatia are tiny chambers produced by plants that house arthropods.Domatia differ from galls in that they are produced by the plant rather than being induced by their inhabitants...

, leaf pouches and hollow chambers where the ants nest permanently. These leaf pouches occur when the edge of a leaf adjacent to the petiole
Petiole (botany)
In botany, the petiole is the stalk attaching the leaf blade to the stem. The petiole usually has the same internal structure as the stem. Outgrowths appearing on each side of the petiole are called stipules. Leaves lacking a petiole are called sessile, or clasping when they partly surround the...

 curls under. The result is two hollow spheres on both sides of the stem of the leaf. The ants live inside of these domatia. There is only one colony per tree, but that colony divides up into many different domatia. Also located in the leaf pouches are extrafloral nectaries, which provide the ants with nectar made by the plant, and also some food bodies that provide the ants with further nutrition as they mature to become foragers and insect predators. This is the first known instance of extrafloral nectaries being observed inside of ant-domatia.

So far this looks like simple parasitism
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

 by the ants, as if they simply take advantage of the plant for food and shelter. However, the plant also benefits greatly from this relationship. With the help of their traps and predation, the ants defend the plant from other insects and parasitic plants. Any insect that may kill or eat the plant is quickly captured and killed by the ants, which is also to their own benefit.

The specialized structures of the plant reveal a very interesting coevolution and symbiosis between these two species. A recent study performed by Céline Leroy et al. (2008) revealed many characteristics that the plant has adapted to support the ants. First, the domatia are located next to the stems that the ants use for hunting. Second, they contain extrafloral nectaries and food bodies to feed the ants if they do not have other sources of food. Third, there are less chloroplasts
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

 found inside of the domatia, which means that it has a lower photosynthetic
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 capacity. Fourth, more stomata
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

 were found inside of the domatia, apparently there to capture the carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 from the ants’ respiration
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

 taking place inside. Finally, there was a greater deposit of cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 found in the domatia, which would result in a thicker cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

 and a more rigid surface to support the weight of the ants. These findings show that the areas destined to become domatia are inherited and a result of coevolution; they were solely produced to support this specific species of ant.

Fungal Symbiosis

There are many different species of fungus that grow alongside the domatia where the ants live. In fact, when the founding queen first starts laying eggs in the domatia of a new H. physophora, the hyphae
Hypha
A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...

 of multiple different species of fungi will entirely cover the entrance to the domatia. When the worker ants mature, they actually have to dig their way through the fungal covering to get to the outside of the plant. However, almost even more remarkable, out of all of these fungal species, A. decemarticulatus will only cultivate one specific species. This fungus is a sooty mold
Mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...

 which, according to one study by Mario X. Ruiz-Gonzalez et al. (2010), is characterized by closely related haplotypes
Haplotype
A haplotype in genetics is a combination of alleles at adjacent locations on the chromosome that are transmitted together...

 (genetic markers) in the order Chaetothyriales
Chaetothyriales
Chaetothyriales is an order of ascomycetous fungi within the class Eurotiomycetes and within the subphylum Pezizomycotina.-References:*C.J. Alexopolous, Charles W. Mims, M. Blackwell, Introductory Mycology, 4th ed. ISBN 0-471-52229-5...

. The ants use this mold to construct their traps.

Another noteworthy observation is that, unlike typical mutualisms between ant and fungus, these ants do not receive any nutrition from the fungus. They strictly manipulate the physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 of the fungus to construct a mortar for a trap that is able to catch much larger prey.

Trap-making

Manufacturing this trap is an incredible act in itself. The workers construct the traps on the stems of the plant, basically by making a raised and hollow platform on one section of the stem. The structure itself simply looks like a part of the plant, as if it grew in width slightly. The ants will also make small holes in this platform which are slightly wider than their own bodies. The individual workers will hide in these holes underneath the surface, invisible to their prey from the outside. They will position their heads outwards from the plant with mandibles
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

 open, waiting for prey.

The actual production of the trap occurs by first cutting plant hairs (trichomes
Trichome
Trichomes are fine outgrowths or appendages on plants and certain protists. These are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae.- Algal trichomes :...

) from a narrow vertical stretch of the stem outside of the domatia. The ants will then arrange these hairs to outline the structure of the trap and regurgitate the mold that acts as a paste and holds the trichomes together. They use their fungal relationship to gather this mold, by collecting the 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...

 from the fungus grown on the plant. This mold will continue to grow in between the trichomes and around the holes to fill out and reinforce the structure.

Predatory Behavior

Normally, there are only a small number of ants in any given area on the plant stem. There are about 40 workers per leaf, but only around two will be patrolling outside. Once an insect lands on the plant, the closest ant will immediately surface and grab on to a leg
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...

, antenna
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

, wing
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...

, or some other appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...

 of the prey. This ant will pull the prey, which will in return try to escape by pulling the opposite direction. However, the ant will rarely ever let go of its grip. The ant immobilizes the prey and, using an extreme feat of strength, will hold the much larger prey to the plant as more ants from around that area arrive. This ability to hold the prey is extremely important because the ants move and congregate fairly slowly.

After the first ant has immobilized the prey, it releases pheromones to call other ants to the area. The first few ants to arrive will each grab a different leg and pull in opposite directions, “spread-eagling” the prey. This is basically identical to the medieval torture tactic of “rack torture
Rack (torture)
The rack is a torture device consisting of a rectangular, usually wooden frame, slightly raised from the ground, with a roller at one, or both, ends, having at one end a fixed bar to which the legs were fastened, and at the other a movable bar to which the hands were tied...

.” Anywhere between six and 16 ants will participate in this spread-eagling. As the prey lies motionless and helpless, worker ants will congregate and help either hold the prey down further if that is necessary or start stinging and biting the prey. In doing this, they use a venom that they produce to paralyze and kill the prey. Afterwards, the group of workers will carry the body back to the colony, where it is dismembered and cut up into little pieces to be eaten by the group.

This amazingly gruesome predatory behavior is a great example of a collaboration between solitary and cooperative predation. The first ant will initially act alone in his predation, but after that ant is holding the prey, other ants will come join him in the hunt, and still other ants will travel back to the domatia and recruit even more ants to aid in holding down and eventually dismembering the prey. This also demonstrates how much more effective a group of individuals can be than an individual working on its own. A single ant could never be able to kill such large prey; they are only able to do so by incorporating communication and cooperation.

This predatory behavior is very effective against such large prey that the ants consume. Dejean and other researchers (2001) tested the limits of its effectiveness by exposing the ants to termites
Termite
Termites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...

 and grasshoppers
Grasshopper
The grasshopper is an insect of the suborder Caelifera in the order Orthoptera. To distinguish it from bush crickets or katydids, it is sometimes referred to as the short-horned grasshopper...

, which are about 40 and 142.2 times the size of the worker ants, respectively. So a single Allomerus decemarticulatus worker restraining a grasshopper is about the equivalence of a 175 pound person holding down a struggling 25,000 pound object that is actively trying to escape. When the termites were exposed, each was captured by the ants; none got away. However, with grasshoppers, all were seized by at least one leg, but out of the 20 introduced, five were captured completely, 12 escaped but lost their leg that was clasped on to by the worker, and seven jumped away.

Any prey that has the ability to jump or fly away will always be more effective in escaping than non-flying insects like the termites. However, many times those that do escape will still lose a leg or other appendage in the process, like 60% of the grasshoppers in this experiment. Because the prey is so much larger than the ants, even just getting this one appendage as a food source is a victory. For example, a hind leg of a grasshopper is still about 12.4 times the size of a single ant and a great resource in itself.

Other Animal Interactions

If the multiple relationships of A. decemarticulatus were not complex enough, they also commonly interact with an assassin bug, Zelus annulosus
Zelus annulosus
Zelus annulosus is an assassin bug found in South America. It frequently associates with Hirtella physophora , a plant that houses colonies of plant-ants Allomerus decemarticulatus and provides the ants with nectar....

, which often resides on H. physophora plants. However, these bugs have adapted physiological and behavioral characteristics that allow them to avoid the predation of A. decemarticulatus, while also maintaining a mutualistic relationship with the plant.

Similar to the ants, Z. annulosus normally lives on younger H. physophora individuals, where the females lay eggs on the stem. As they begin to develop, the young bugs will live among the trichomes of the stem and hunt on the leaves of the plant. The relationship between the assassin bug and the plant acts independently from that between the ants and the plant. One difference between the two relationships, however, is that the assassin bugs do not take any food source from the plant like the ants do.

Z. annulosus has basically adapted to live and hunt around A. decemarticulatus. The assassin bug species uses this particular plant to raise their nymphs
Nymph (biology)
In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...

 because the trichomes of the plant deter larger ant species that may kill the young developing bugs. Also, the bugs secrete a sticky substance that allows them to walk on top of these trichomes, thus avoiding the traps of A. decemarticulatus. So in the relationship between Z. annulosus and H. physophora, the bug receives shelter from potential large predator ants, and the plant received a second line of defense against herbivores. The assassin bug also cohabitates the plant peacefully with the ants. They hunt in similar areas on the plant, but the assassin bugs are suspected to actively avoid any of the ants because they are much quicker than the ants.

Relevance to Sociobiology

By simply observing the stem of one H. physophora individual, we see the complex interactions between 4 different species: an ant, a plant, a fungus, and an assassin bug, as well as all of the prey that lands on the plant. The plant gives a little of its own resources to the predators, but it gets two forms of defense against herbivores and parasitic plants. To the plant, this defense is worth much more than the loss of some food. The plant has maximized its own fitness by establishing two separate and independent relationships that are not very costly but very rewarding. The insects both benefit as well, given a safe habitat and a steady food supply of other insects that land on the prey. In the perspective of A. decemarticulatus, they do not actively give any of their own resources to the plant or to the assassin bug. They simply live among them, use their resources, and hunt.

In regards to the predatory behavior or A. decemarticulatus, Similar behaviors have been observed in other ant species, such as symbioses with plants (like in Pseudomyrmex ferruginea
Pseudomyrmex ferruginea
The acacia ant is a species of ant of the genus Pseudomyrmex. These arboreal, wasp-like ants have a orange-brown body around 3 cm in length and very large eyes...

), cultivating a fungus (like in leafcutter ants
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...

), and sneaking up to and ambushing larger prey (like in Azteca Andreae
Azteca andreae
Azteca andreae is an arboreal ant species found in the tropics of South America, most notably in French Guiana. They are most notable for their predatory skills and strength. They are ambush predators that are able to capture and eat other insects much greater than their own size.-Physical...

, another species studied by Dejean). However, most remarkably, Allomerus decemarticulatus seems to incorporate each of these advanced behaviors to make a powerful apparatus for tricking impressively large prey.

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