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 31 known species. Commonly known
as "leafcutter ants" they comprise one of the two genera of advanced
attines within the tribe Attini, along with Atta.
2.2 Colony hierarchy
2.3 Ant-fungus mutualism
2.4 Waste management
2.5 Foraging behaviour
3 Interactions with humans
5 See also
7 External links
Profile view of an A. balzani worker
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
Diagram of an ant's anatomy
The antennae are the most important sense organs
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 varies according to species. Male ants
tend to have more ommatadia 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 color. In addition to the standard ant
anatomy, the back of the thorax has a series of spines which help it
maneuver material such as leaf fragments on its back.
Acromyrmex can be identified from the closely related Atta genus of
leafcutter ants by their 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.
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
A. balzani worker carrying a leaf
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.
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 infrabucchal
pouches to 'seed' the fungus gardens of incipient colonies, cutting
and collecting the first few sections of leaf themselves.
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.
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.
In addition to feeding the fungal garden with foraged food, mainly
consisting of leaves, it is protected from
Escovopsis by the
antibiotic secretions of
Actinobacteria (genus Pseudonocardia). This
mutualistic microorganism lives in the metapleural glands of the
Actinobacteria are responsible for producing the majority of
the world's antibiotics today.
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.
Leafcutter ants in Costa Rica
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. 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.
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.
Acromyrmex contains 32 species:
Acromyrmex ambiguus Emery, 1888
Acromyrmex ameliae De Souza, Soares & Della Lucia, 2007
Acromyrmex aspersus F. Smith, 1858
Acromyrmex balzani Emery, 1890
Acromyrmex biscutatus Fabricius, 1775
Acromyrmex coronatus Fabricius, 1804
Acromyrmex crassispinus Forel, 1909
Acromyrmex diasi Gonçalves, 1983
Acromyrmex disciger Mayr, 1887
Acromyrmex echinatior Forel, 1899
Acromyrmex evenkul Bolton, 1995
Acromyrmex fracticornis Forel, 1909
Acromyrmex heyeri Forel, 1899
Acromyrmex hispidus Santschi, 1925
Acromyrmex hystrix Latreille, 1802
Acromyrmex insinuator Schultz, Bekkevold & Boomsma, 1998
Acromyrmex landolti Forel, 1885
Acromyrmex laticeps Emery, 1905
Acromyrmex lobicornis Emery, 1888
Acromyrmex lundii Guérin-Méneville, 1838
Acromyrmex niger F. Smith, 1858
Acromyrmex nigrosetosus Forel, 1908
Acromyrmex nobilis Santschi, 1939
Acromyrmex octospinosus Reich, 1793
Acromyrmex pubescens Emery, 1905
Acromyrmex pulvereus Santschi, 1919
Acromyrmex rugosus F. Smith, 1858
Acromyrmex silvestrii Emery, 1905
Acromyrmex striatus Roger, 1863
Acromyrmex subterraneus Forel, 1893
Acromyrmex versicolor Pergande, 1894
Acromyrmex volcanus Wheeler, 1937
List of leafcutter ants
^ a b Bolton, B. (2014). "Acromyrmex". AntCat. Retrieved 20 July
^ a b Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of
Curious and Unusual Animals, Greenwood Press, p. 298,
ISBN 0-313-33922-8 .
^ "Archived copy". Archived from the original on 2012-02-02. Retrieved
^ 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 .
^ 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 .
Media related to
Acromyrmex at Wikimedia Commons