All about ants

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Are they everywhere? They sure seem to be from early spring to first freeze. They’re all over our plants. They invite themselves to our cookouts. They squeeze their way into our kitchens. They can be downright annoying. It’s easy to think of them as rather useless insects, but they’re actually very beneficial. They’re predators of insect eggs. They aerate the soil, which allows water and nutrients to penetrate down to plant roots. They distribute seeds by storing them in their tunnels. And, they’re food for numerous other beneficial wildlife.

Fossil evidence shows that ants evolved from a lineage of aculeate (stinging) wasps and have existed for at least 92 million years (since the Cretaceous Period, 145.5 to 65.5 million years ago). Today, there are 12,500 species of ants classified so far, and scientists estimate there are thousands of species yet to be discovered. It’s thought that ants’ eusocial society has enabled them to adapt through the ages to environmental changes, since they’re found in nearly all terrestrial habitats and continents, except for Antarctica and a few isolated islands.

Ants live in highly organized colonies that vary in size from dozens to hundreds of thousands of individuals. A colony in Jamaica was found to have 630,000 individuals! The highest number of species inhabits the tropics. About 1,000 species inhabit North America.

The word “ant” derives from Old English æmette and the German word Ameise, plus a combination of other word forms. All put together “ant” means “the biter off,” perhaps a reference to their biting habits.

Ants belong to the taxonomic order Hymenoptera (hymen-AWP-ter-uh), along with bees, wasps and sawflies. Ants, wasps and sawflies (but not bees) are further classified into the family Formicidae (form-uh-CID-ee).¹

Physical description
The size of ants varies tremendously. They range from 0.030 to 2.0 inches long (0.75 to 52 mm). The largest ants in the world are probably a South American species in the genus Dinoponera, which measure from 1.2 to 1.6 inches long (3 to 4 cm). The smallest ants are probably in the genus Carebara, which is found worldwide. The largest ant in North America is most likely the Black Carpenter Ant, which ranges up to 1/2-inch (1.25 cm) long. Most ants are black or red. A few species are green, and some tropical species have a metallic look.

Ants are invertebrate animals. (All insects are classified in the Animal Kingdom.) They have a skeleton, but, unlike humans, their skeleton is on the outside of their body. Called an exoskeleton, it’s hard and supports and protects the body. Ants have three main body sections: head, thorax and abdomen. Between the thorax and the abdomen is a node which looks like a “waist.” Called a petiole, it aids in digestion. It may have one or two segments, depending on the species. (The petiole is used to identify ant subfamilies.)

Head
An ant’s brain contains only about 250,000 cells (humans have around 100 billion cells) and is essentially a bundle of nerves and fibers. Still, it’s capable of processing complex data and storing information. Some ants have 14 to 15 percent brain mass compared to body weight, the highest of all animals! 

Close up image of a queen ant's head.

Head of a queen ant. The two projections are her antennae. (Roverhate / Pixabay; PD)

The head contains two compound eyes and three simple eyes called ocelli (oh-CELL-ee). The compound eyes detect movement and the ocelli detect light and polarization. A few subterranean species are blind.

Two moveable antennae with elbows are attached to the front of the head. The antennae are used to smell, touch and feel, as well as to communicate with other ants.

 Ants have two strong mandibles (jaws) used for cutting, carrying food, nest construction, fighting and digging. Ants also have smaller mouthparts for chewing food. A gland called a labial gland begins at the tip of the tongue and travels partway into the thorax; it aids in digestion. Some species have a small pocket in their mouth which stores food for passing along to other ants or larvae.

Thorax
The thorax is the middle part of an ant’s body. Attached to it are six legs, three on each side. Each leg has three joints and a hooked claw at the end used for gripping surfaces and other ants. Queens and male ants have two pairs of wings. Most ants, however, are females and have no wings.

Drawing of the anatomy of an ant, with labels.

External anatomy of an ant. (Mariana Ruiz / wiki; PD)

Several long body parts originate in the ant’s head and extend into the abdomen. Among them: The labial glands, which begin at the tip of the tongue. Nerve ganglia, which extend from the brain, and supply commands to the legs and wings, as well as sensory input from the eyes. And, the esophagus, which begins in the head and connects to the forward part of the stomach. The stomach, itself, begins in the thorax and continues into the abdomen. 

A “waist,” called a petiole, connects the thorax to the abdomen. It may have one segment or two, depending on the species.

Color illustration of the internal anatomy of an ant, showing the glands.

(Cronodon.com; cc by-nc-sa 3.0)



Ants breathe through a set of tiny holes, called spiracles, located on each side of their body. The spiracles, lead to tube-shaped trachea The trachea grow narrower and narrower as they network through the entire body supplying oxygen where it’s needed. Ants lack the ability to pump air in and out through their spiracles. Their breathing occurs passively as the body moves, by a process called rhythmic tracheal compression. (It’s the lack of lungs that keeps insects from getting as big as humans.)

Abdomen
The abdomen is the back part of the ant. It contains nerves, vital organs, part of the digestive system and reproductive parts. The rearmost portion of the abdomen is called the gaster. 

The digestive system consists of the crop (also called the dry stomach because it contains no digestive juices) and part of the stomach. The crop stores liquid for regurgitation to other ants and larvae. It connects to the stomach through a valve which opens and closes to move food in one direction only — from the crop to the stomach. The stomach contains digestive juices. 

Queen ants and worker ants (which are always females) have egg tubes. The tubes run from the front end of the abdomen to the tip. Only the queen has fertilized eggs, which she stores in little sacs. 

With many ant species, but not all, the egg tubes are modified into a sting (commonly called a stinger). The sting contains a poison gland that holds formic acid,* a strong irritant — this is what gives Fire Ants and others their “fire.” Also at the tip of the abdomen is the Dufour’s gland. It secretes a fluid used as a trail for other ants to follow.

Social insects
Ants, along with various bees, wasps and termites, are called eusocial, or social, insects. They live in a cooperative group where only one (usually) female and the males reproduce and the rest of the group provides care and protection.

Eusocial insects have an advantage over non-social ones. They organize work efficiently by dividing tasks and sharing information. As an example, by sharing information, ants can convey to each other the shorter distance between two paths or to the better source of food. 

It’s commonly thought that ant colonies have only one queen, but, depending on the species, there may be more than one. Or none: Some colonies have no queens and all the females, called gamergates, can reproduce. These are called gamergate (GAM-er-gate) colonies. Colonies with queens are called queen-right. Species with more than one queen may have a queen and some workers leave to form a new colony.

Generally, there are three kinds of ants in a colony: a fertile queen, female workers and males. The queen and the males have wings, although workers are wingless. Mated queens spend their entire lives laying eggs; it’s their only role. Males have only one job, too, to mate with the queen, and they die soon afterward. Everything else in a colony is done by its workers. Workers are always sterile females and have specialized jobs. The smallest ants, called minor workers, find and gather food, clean, maintain and expand the nest, feed and care for the queen, and care for eggs and young offspring. 

Some species have additional castes of worker ants: major workers, sometimes called soldier ants, and median workers. Soldiers are larger and stronger than other workers. They have a large head with powerful jaws designed for specialized tasks, such as seed-cracking, wood-boring or combat. Median workers fit, in size, between major workers and minor workers and also have tasks to support the colony.² 

Communication
To communicate information to each other ants use pheromones, sounds and touch. Pheromones are secreted chemical substances that trigger specific responses in ants. They’re produced by several different structures within the ant’s body and have many purposes. For instance, an ant can leave a pheromone trail for others to follow to a good food source. An injured or crushed ant emits a pheromone that triggers alarm. Other pheromones are emitted to mark territories, mark the way to new nest sites, recognize nest mates and trigger sexual behavior. A single pheromone trail may consist of several different pheromones from one or more glands.

Recent research has shown that trail pheromones are remarkably sophisticated, providing information about previously used trails, current trails and unrewarding branches. Also, there is worker specialization in laying trails and detection. When ant queens stop producing a specific pheromone, it signals to the workers that it’s time to start raising new queens in the colony. 

Ants use their antennae to perceive information about distance and intensity of smells. As for hearing, ants don’t have ears, but they can “hear” by feeling vibrations.³ (It’s like when we place our hands on a loudspeaker and can feel the sound coming out, even if our ears are tightly plugged.)

Reproduction
Ants begin life by going through a process called complete metamorphosis, which means they progress from an egg to a larva to a pupa to an adult.

Workers provide care and protection through all immature stages and feed the larvae with regurgitated food (a method called trophallaxis). Later they may be fed with solid food. (This is also how adult ants share food.) Females intended to become workers are fed a different food from those destined to become queens. If the egg was fertilized, the ant will be a female. If not, it’ll be a male.

Image of a male Carpenter Ant with wings.

Winged male Carpenter Ant, Camponotus castaneus. (Katja Schulz / Wiki; cc by 3.0)

Most ants produce a new generation every year. It all begins in late spring or early summer, depending on the species. All the reproductive virgin queens and males of the same species simultaneously leave their existing colonies in great swarms called “nuptial flights.” Their wings allow the ants to relocate far away from their old colonies, where they can mate with members of other colonies and prevent inbreeding.

Queens can choose to store fertilized eggs in their “sperm pocket” (spermathecae) for use soon or in the future, or they can lay unfertilized eggs, which will become female workers. As soon as the new workers hatch, they take over for the Queen. The males die a day or so after mating, having fulfilled their only role in life. The sperm stored in a queen’s sperm pocket will last throughout her life and fertilize millions of eggs.Once the queens have mated with one or more males, they fly off to find a good nesting site.

At the new site, the queen detaches her wings, which fall to the ground, never to be used again. Her job now is to construct a nest. Depending on the species, it might be underground, in wood, walls, cracks in pavement, nearly anywhere that’s suitable for a long tunnel with a small chamber at the end. She usually digs the nest alone, but some queen species cooperate and raise the first brood together. (If this occurs in a species that has only one queen to a colony (called monogyny), ultimately the once-cooperative queens will fight to the death until only one of the remains.)

Queens seal themselves in their nests and lay eggs immediately. Or perhaps not — they sometimes wait until the following spring. Either way, they stay in total darkness for the rest of their lives.

An ant egg is about 0.02-inch (0.5 mm) in diameter and weighs about 0.0005 grams. It’s kidney-shaped and also sticky, which lets it to bond to other eggs. Adults to carry many stuck-together eggs at once, especially helpful in an emergency.

Larvae, which look like tiny maggots, emerge in 5 to 7 days. As they grow larger, they outgrow their skin and molt several times. They grow hairier with each molt. Some of the hairs are hooked at the end and can lock to other larvae — this makes it easier for workers to carry them when necessary. At the last molt, the larvae have grown too large for workers to carry as a group and are then carried singly. Larvae don’t have legs, but they can move minimally. Their food consists of juices made from solid foods brought by adults. They can also receive regurgitated food.

The queen can control the sex of her offspring. In a reproductive process called haplodiploidy, she can lay unfertilized eggs, which become males, or fertilized eggs, which becomes females. Then, depending on how larvae are fed, adults emerge as one of a particular caste. There are three female castes: worker, soldier, queen, and one male.

Foods
Ants are omnivores. They eat a wide variety of foods, which vary by species. They’re opportunistic feeders, rather than hunters, and mostly eat what they come across. In general, they feed on soft-bodied animals, such as mites and other spiders, the larvae of other insects, and carrion (dead animals). They also eat sweets found outdoors or inside — candy, honey, fruit and jam. In some parts of the world, ants in some parts of the world eat seeds or fungus. Others “farm” aphids, from which they collect a sweet secretion called honeydew. They even go so far as to protect their farm animals from predators!

Nests
Ant nests have chambers connected to each other by tunnels. There are chambers for nurseries and food storage. Nests can be found just about anywhere. Depending on the species, they may be underground, in wood, under stones, in mulch or debris, in walls or mounds of soil. Gardeners find nests under or in flowerpots or even in the open end of a water hose left unused for too long. Some ants set up their nests in houses. 

Pavement Ants (Tetramorium caespitum) typically nest in or under cracks in pavement or foundation walls. Wood ants (Formica sp.) nest within masses of pine needles they (usually) place on top of old tree stumps or on the ground. Acorn ants (Temnothorax sp.) nest inside acorns (also in walnuts, twigs and other small cavities).

Image of an army ant bivouac.

Army ant bivouac, with the ants all clinging to each other. (Geoff Gallice / Wiki; cc by 2.0)

Army Ants (over 200 species in several genera) defy custom by not building permanent nests at all. They travel continuously in search of food, and when they stop they create “bivouacs,” temporary camps made of their own bodies linked together until they’re ready to travel again. In other parts of the world are species (genus Oecophylla) that use silk to weave leaves into nests. Weaver ants nest high in treetops of rainforests.

Image of belowground structure of a Florida Harvester Ant.

Belowground nest structure of a Florida Harvester Ant. (Shaners, Becker / wiki; cc by-sa 2.0)



Some subterranean nests can be discovered by the anthills (mounds) that form as a colony carries debris to the surface during excavation. Their nests can be massive in size. A nest found in Australia in 2004 measures 62 miles (100 km) wide, most of it underground. Nests begin small and expand as the colony grows larger. The larger they get, the more complex they become, with corridors and chambers interconnected for vast distances. Some underground nests may trail down 26 feet (2.7 m).

Defense
Some ants defend themselves by stinging. Ants can also bite. Some block the entrance to their nest with their heads (a process called phragmosis) to prevent enemies from entering. The Fish-hook Ant (Polyrhachis bihamata), a rainforest species, has sharp spikes on its back which can be used to interlock with other ants to prevent one of their colony from being dragged away. They can also be used to stab enemies.

The sting of the Bullet Ant (Paraponera clavata), a large rainforest species, is worse than all other Hymenoptera. According to some victims, its sting is equivalent to being shot.

Life cycle
Queen ants can live up to 30 years, the longest span of any known insect. Workers can live from a few months up to five years.

*Top photo: cc by 2.0

²Read about specific ant species  
³In-depth information about pheromones and ant communication 
Read more about social insects     
More diagrams, information about the physical characteristics of ants

 
 
 
 
 
 
 
 
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