Showing posts tagged with "ants"

Cities at your feet: A closer look at the world of ants

by David Rolnick
MIT Department of Mathematics
Posted on January 15th, 2014
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City Life

The colony is the unit of ant society. Depending on the species, colonies can range from only a few dozen ants to millions. Within a colony, different jobs are performed by different castes of ants. Soldiers are large and powerful and focus on fighting. Workers make up the bulk of the colony and do the day-to-day tasks like gathering food and tending the young. There is generally a queen — or sometimes several, in large colonies. The job of the queen is to lay eggs, and she usually just sits and reproduces all day long, using sperm stored up from her youth. Her former partners are long dead. Soldiers, workers, and queens are all female, and the only purpose of male ants is to mate and die.

When ants do need to mate, they do it on a grand scale. Male ants and young queens emerge from their colonies in great swarms. They have wings and fly around in a frenzy looking for partners. You may have seen this happening and not realized it. Whenever it seems like there are a lot of insects flying about, but none of them are biting you, it is probably the mating day of some species of ant. Even after waiting a year, all the colonies somehow manage to synchronize it, so that the orgy is sometimes done within an hour.

The home that an ant colony builds is called the nest. Here in Massachusetts, you often find ant nests in sidewalks or under rocks. That is because the rocks and concrete help keep the nest warm on cold days. Archaeologists know to look for fossils on top of ant nests, because some ants cover their nests with rocks gathered from the surrounding land in order to collect solar radiation. Ants in the tropics have the opposite problem and generally nest underneath bark or in other places where they can keep cool.

Some ants go to great lengths to build the perfect nest. Weaver ants, for instance, make nests in trees. Dangling from one another, chains of workers pull leaves and branches into place, and then fasten them together using silk. Another species of ant (Camponotus femoratus) plants cactuses in its nest in order to make it sturdier. Yet another species has an alliance with a tree, in which the tree provides the ant food and shelter, while the ant stings all the trees’ competitors to death. Because of this arrangement, there are large bare patches inside South American rainforests, where no plants grow — except for this one species of tree.

Down on the farm

In Massachusetts, many species of ants rely on livestock farming for their food. Their cows are small, defenseless insects called aphids, which sit on plants all day, drinking sugary sap and trying not to get eaten. Ant colonies collect groups of these aphids and protect them from predators. In exchange, the aphids secrete a concentrated sugary secretion called honeydew (a bit like maple syrup), which the ants eat. The tie between farmers and cattle runs so deep that when a colony of ants moves to another nest, it carries the weak aphids along with it and installs them on another plant that they can use as pasture.

In South America, leafcutter ants carry out an even stranger form of agriculture. Every night, the colony goes out and defoliates an entire tree, chops up the leaves, and brings the pieces home. Like many ants, leafcutters nest underground in a network of subterranean chambers; a colony may excavate up to forty tons of earth. Inside the nest, different castes of successively smaller ants chop the pieces up into increasingly tiny fragments. The ants don’t eat the leaves. Instead, the bits of leaf are fed to the filaments of a gigantic fungus that the ants keep in their nest. This fungus is food for the entire colony, which may contain millions of ants.

A disciplined army

Ants go to war all the time. They are really the only animals that fight to an extent comparable to humans. Consider those ants that you get in ant farms (Pogonomyrmex californicus, to be precise). In the wild, when these ants go out looking for food, they get into so many fights that 6 percent of them die every hour! Not surprisingly, therefore, they have a lot of weapons that they can use in battle. Ants evolved from wasps and still have the ability to sting. They also have powerful jaws, and some species can spray acid from their mouths. (All you chemists, “formic acid” actually means “ant acid.”) Some carpenter ants in Malaysia even have the ability to explode, killing themselves in the process, but showering the enemy with chemicals that function both as immobilizing glue and as deadly poison.

Ants are powerful creatures, which can shape entire ecosystems because they work together in large numbers. Most species are predatory and the terror of the insect world. Ants are so formidable that one harmless insect in South America has an ant decoy growing out of its back, in an effort to scare away predators. In the Amazon, army ants sweep over the ground in giant swarms, methodically killing everything from grasshoppers to frogs. Birds, known as antbirds, follow the army ants, picking off other insects that are driven out of hiding. Butterflies, known as antbutterflies, follow the birds, feeding from the poop that they leave behind. Thus a whole community of organisms is centered upon the army ant.

There are many interesting ants that don’t share the fame of the army ant. The trapjaw ant, for instance, has spring-loaded jaws that bend open 180 degrees, like a bear trap, and snap shut if touched. (This is actually the fastest action of any animal on Earth.) Instead of bears, the ant catches tiny snow fleas, which can jump so fast that it takes superpowers to catch them. Another fascinating species is the African honeypot ant, which stores food in the form of a sweet liquid like honey. This honey is kept within the hugely swollen bodies of some of the ants, whose sole function is to be living jars of food. Apparently, these honey-filled ants are delicious and are often eaten by people in Africa.

Whether as fighters, farmers, or food, ants offer fascination and even inspiration. Their proverbial diligence has given birth to many metaphors and fables. In Greek mythology, ants are among the only creatures that were turned into men, rather than the other way around. Achilles’ followers, the fierce Myrmidons, take their name from the Greek word for “ant”, and supposedly sprang from ants at the command of Zeus. Thoreau writes, after studying a battle between rival colonies, “I was myself excited … as if they had been men.” We humans may think the Earth is ours, but the teeming cities at our feet remind us that it is also a world of ants.

A version of this article previously appeared in The Tech on October 29, 2013

About the author
David Rolnick
MIT Department of Mathematics

David Rolnick is a graduate student at the department of Mathematics at MIT