Taxonomy
| Morphology
| Origins | Why
ants are so successful | Mating
| Gene flow |
Communication
| Division of Labor
| Foraging |
Symbiotic relationships
Taxonomy
The classification of ants is as follows:
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Genus: Approximately 290 known genera
Species: Approximately 8800 known species
Morphology
Ants have three major body parts: head, thorax, and gastor (abdomen).
The thorax can be broken down into two major parts: the alitrunk which
contains the legs and wings, and the petiole which is found directly
anterior to the gastor and is found only in ants. Ants have mandibles
(jaws) which are of varied structures. These varied structures provide
for a plethora of functions ranging from grasping, tearing, cutting
and other special tasks. Most ants have a stinger at the end of the
gastor. Some ants can release this stinger in a similar fashion to
honeybees. The stinger is only found in female ants and is a modified
ovipositor (egg laying organ). Ants have compound eyes which have
not been shown to effect their behavior, although some ants seem to
be able to detect movemen t. They have very sensitive antennae that
are used for a wide array of communication.
Nearly all ants have a unique gland found on the petiole called the
metapleural gland. Most importantly, this gland has been shown to
contain antibacterial and antifungal chemicals which are essential
for survival in the humid, dark nests in the ground or rotting vegetation.
This gland secretes an antiseptic substance that at times acts as
a repellent to attacking organisms. It is also thought that the metapleural
gland releases pheromones for communication.
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Origins
The oldest fossil remains of modern ants is preserved in amber and
has been dated by radioactive isotopes to be approximately 80 million
years old. It has many of the characteristics of modern ants while
also presenting many characteristics of the ant's distant relative,
the nonsocial aculeate wasp. Like modern ants, the fossil contains
the metapleural gland, petiole turning down towards the gastor, and
a relatively small thorax without wings.
The fossil contains the following wasp like characteristics: short
mandibles with only two teeth, stinger that can be extruded, unconstricted
gastor, and double tibial spurs. The fossil antennae have a short
first segment and a long flexible segmented structure for the rest
of the antennae. This antennae combines both wasp and ant features.
An early Cretaceous fossil that may be an ant has been discovered.
Unfortunately, the gastor has been folded back over the thorax, thereby
restricting the view of the petiole. It is a less well preserved specimen,
and, therefore, scientists are uncertain as to its' morphology.
Why
ants are so successful
It appears that ants were the first, and remain the only,
social insect predators to utilize the moist, dark dirt and rotting
vegetation for nesting. Since ants shed their wings after mating,
they can crawl into a much smaller space than their relatives, the
wasp. The wasp has wings and a relatively large thorax which limits
where it can nest. The ant retains some of the weapons of the wasp,
i.e. the stinger and other chemical weapons, which make the ant a
mean and efficient predator.
One of the greatest advantages for ants is their social behavior.
Working as a colony with specialized duties, they are more efficient
than non-social insects in getting necessary jobs done. For example,
when you have a number of individuals solely responsible for feeding
larvae, there is a higher likelihood that the larvae will be fed by
at least one individual. Other ants in the colony are responsible
for bringing food to the feeders. Others are responsible for bringing
food into the colony, etc. The feeders are specialized to complete
their single task, and, therefore, do not have to succeed at a great
number of tasks to get the food to the larvae.
Their varied mandibles are an irreplaceable tool for accomplishing
the jobs necessary for the multiple behaviors displayed by various
individuals of colonies.
The metapleural gland excretes antifungal and antibacterial materials
that ants spread throughout their colonies through their wanderings.
This protects their brood and their food supplies in the humid underground
environment.
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Mating
Most all ants mate in one of two ways. The first is known
as the male-aggregation syndrome. At a time usually triggered by environmental
events such as a large rainfall, the males and unfertilized queens,
which have been patiently waiting underground, suddenly take to the
air and swarm in what is commonly called the nuptial flight. This
swarming often appears as a column of flying ants rising off the ground.
The queen is usually inseminated in mid-air and then flies off to
a locale where she breaks off her wings and begins to dig a hole either
in the ground or in plant material, which will become her nest. She
lays her eggs and remains with them until they hatch, living off of
her own body reserves. Eventually they will hatch and become the first
of her workers that will take care of her and the colony as it increases
in size and numbers.
The female-calling syndrome has queens aggregating at the surface
of colonies and calling males to them via pheromones. There are many
exceptions to this basic description of mating. For example, some
females are inseminated on the ground and then fly off to find their
nests. With another species, the males will be swarming close to the
ground waiting for females to emerge from their colonies. When she
appears, they surround her in a rolling, writhing ball of males until
the newly inseminated female breaks out of the ball after about 20-30
seconds and flies away to find a nest site.
Gene
Flow
It is often argued whether ants and other social insects
are altruistic in their contribution to the colony rather than trying
to reproduce and directly pass on their own genes to their offspring.
Yes, they seem to be altruistic, in they are helping their sisters
at their own expense without directly passing on their genes. No,
they aren't altruistic, in that they are passing on their own genes
by allowing the colony to survive and expand to new colonies. This
type of natural selection is called Kin Selection. Genes are selected
in nature according to their ability to contribute to the success
of the species. If an altruistic gene helps a colony to survive, then
that gene will be passed onto kin which will in turn have the altruistic
gene that will help that colony survive. If the gene for altruism
works against the success of the colony then the individuals with
the gene will not be successful and the gene will be selected against.
One possible explanation for kin selection arises from the fact that
all workers are females. It turns out that the workers are more closely
related to each other than they would be to their own offspring. This
means that they are more successful in making more genetically similar
individuals by helping the colony than by having their own offspring.
Specifically, males arise from unfertilized eggs so they have only
a half complement of genes, all from their mother. Female workers
arise from fertilized eggs and therefore have a full complement of
genes of which half come from their mother and half come from their
father. Workers always get the exact same half of their gene complement
from their father since he only has a half to begin with. Workers
are, therefore, at least fifty percent related to each other since
half of their genes always come from their father. Workers can end
up with either half of their mother's genes which means that they
will be on average 75 percent related to each other. If the workers
did have the opportunity to reproduce, (which they don't), the best
they can do is to contribute 50 percent of their genes to their offspring.
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Communication
Ants need to be able to communicate for an array of reasons. The following
is a list of the 12 major reasons for communication that researchers
have identified (Holldobler and Wilson, p. 227).
- Alarm
- Simple attraction
- Recruitment, as to a new food source or nest site
- Grooming, including assistance at molting
- Trophallaxis (the exchange of oral and anal liquid)
- Exchange of solid food particles
- Group effect: either facilitating or inhibiting a given activity
- Recognition, of both nestmates and members of particular castes,
including (broadly) discrimination of injured and dead individuals.
- Caste determination, either by inhibition or by stimulation.
- Control of competing reproductives
- Territorial and home range signals and nest markers
- Sexual communication, including species recognition, sex recognition,
synchronization of sexual activity, and assessment during sexual
competition.
The majority of communication seems to be chemical. Ants also tap
each other, feel each other out with their antennae, straddle each
other to give certain messages, and grasp and stroke each other as
well.
One gland, the pygidial gland on the gastor, is used to lay down trails
for the same individual or for others to follow in the same track.
It also seems to be used to warn colony members of danger and as a
pheromone to attract fellow foragers to food sites. In fire ants,
the Dufour's gland is the source of trail- laying chemicals. Some
trail pheromones can last several days. In leaf cutter ants, they
may create a main trunk trail leading away from the colony only to
branch out in several directions a short distance later. They continue
to branch out like arteries to capillaries until single ants are foraging
for leaf material. When they obtain their leaf fragment, they turn
around follow their pheromone trail back along the path to the main
trunk and then the colony.
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Division
of Labor
Ant colonies are grossly divided into queens, males, and workers.
The job of the queen is to lay eggs. The males generally do nothing
for the colony. They wander around accepting food from the workers
until the time comes for mating. They die almost immediately after
mating. Workers are generally sterile females. They have a variety
of tasks to perform for the colony ranging from foragers, defenders,
brood feeders, food preparers, caretakers of the queen and nest construction,
among other things. The size of the ant in the colony is often different
according to the task the ant needs to perform. The queen is often
the largest; brood keepers are the smallest. Soldiers are large, and
foragers are smaller.
When ants specialize, they help the colony survive in a number of
ways. For example, foragers are constantly exposed to danger while
out on the hunt. A small percentage of the colony acts as foragers
though, so the majority of the colony is safely tucked away in the
nest. The foragers are sacrificing for the majority of the colony,
which means that the colony will probably survive longer. If every
ant in the colony had to forage, then every individual would be exposed
to great dangers. Interestingly, duties are often assigned according
to the age of the ant. The younger ants are closer to the queen, taking
care of her and the brood, while the older ants are usually delegated
the more dangerous tasks of foraging and defense.
Foraging
The overriding goal of every foraging ant is to spend the least amount
of energy to obtain and deliver the greatest food value to the colony.
As long as enough foragers are successful, the colony can maintain
the energy flow to continue reproducing indefinitely.
One major deciding factor as to foraging technique takes into account
the external dangers presented to ants while foraging. They are constantly
under attack by predators and competitors. When an ant is lost to
predation or injury, the whole colony loses a small energy packet.
Predators may influence whether a colony hunts with packs of ants
going off at a time or if the colony uses individuals to hunt on their
own. Will they hunt out in the open or will they hide under some sort
of canopy?
If competition is a problem, ants may expend a larger amount of energy
to retrieve a food source quickly, rather than to go slowly and allow
other colonies to take the food.
Temperature and humidity are two important environmental factors effecting
foraging. Desert ants are adapted to higher temperatures and lower
humidity, while cold weather species do better in higher humidity,
cooler temperatures. Some ants forage before sunrise and after sunset
when the temperatures are not too hot. Others forage only after sunrise
and before sunset to take advantage of the warmer temperatures. Some
ants may forage at unseemly times of the day to avoid predators. Foraging
may be controlled by the time of availability of the food source.
High humidity seems to present a higher temperature window as to when
ants will forage. Most ants will not forage during or shortly after
a rainfall.
When foraging, ants seek a variety of materials depending on the needs
of their colony. At different times, the colony may need protein or
may need carbohydrates. Ants can be predators, omnivorous, seed specialists,
or fungus gardeners. There are no known ants that actually eat leaves,
but there are ants that use leaves for different purposes.
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Symbiotic
Relationships
Relationships between ants and other organisms are numerous and varied.
- Ant/Ant: Some species of ants are extreme in their dependence
upon other ant species. For example, the ant Tuleutomyrmex schneidere
spends almost its entire life riding on the backs of host ant species.
They seem to contribute nothing to the hosts, but are tolerated
and even fed. Slavemaker ants (Formica subintegra, for example)
steal brood from other colonies and return the brood to develop
and serve the Slavemaker colony. The slaves are absolutely dependent
in that if they don't work, they don't get fed. Other ants work
together as with the Crematogaster limata parabiotica and
Monacis debilis. These ants have their nests close together
and share the same foraging trails. Camponotus has also been
seen giving food to the Monacis workers.
- Ant/Other Insect: These relationships are many and diverse,
ranging from commensual to parasitic. Aphids and ants have many
species relationships where both the ants and aphids benefit (mutualism).
Aphids secrete honeydew and amino acids through their anus. The
ants eat or store the honeydew. The ants sometimes incorporate the
aphid territory into their own territory, which allows easier access
to the aphids and affords the aphids protection by a greater number
of ants. The honeydew sometimes contains chemicals that are purposely
directed at attracting ants. The aphids sometimes release chemical
signals that warn other aphids of a predatorial attack and also
alert the ants so they can attack the invader.
- Ant/Plant: These relationships are also known to be abundant.
Some carnivorous plants allow ants to hunt herbivores on them. In
turn, the ant protects the plant from the herbivores eating their
plant tissue. Many plants have extrafloral nectaries on various
parts of the plant. These are nectar- producing structures not associated
with flowers. The ants are attracted to the plant where they can
obtain small amounts of sugar and, in turn, defend the plant from
other insects. Ants provide this same service of eliminating herbivores
to many plants. Other ants confiscate plant parts to grow fungus
on in fungus gardens deep with colonies. These leaf cutter ants
process the leaves and use the fungus grown upon the leaf material
for food. Sometimes ants live in tree hollows and have no effect
on the plant at all. Harvester ants do a great service to plants
by collecting and transporting seeds. In one case, the ants eat
a small part of the seed and leave the rest of the still- viable
seed to germinate.
