Ant Palaces

By Craig Anderson
Illustrated by David Besenger

Most people are familiar with the small, craterlike ant mounds that pop up in the cracks of sidewalks. But some species of ants build mounds that are so large you can hardly step over them. They could be called ant palaces. And like any good palace, they are full of vast networks of passageways, chambers and, of course, ants.

Some species of palace builders belong to the genus­or group­Formica. They are commonly called mound ants, thatcher ants or wood ants. Depending on the species, they are medium to large in size, with either all-dark or partly dark and partly red bodies.

One species called the Allegheny mound builder makes the largest mounds in Missouri and probably in eastern North America. Some of these mounds are truly spectacular. I saw one in Marion County that was about 6 feet across at the base and 3 feet tall. Other mounds have been measured at 42 inches tall and 24 feet across. Nests can be pretty common in some places, with as many as 30 to 59 nests per acre.

Just because an Allegheny mound builder nest is huge doesn't necessarily mean lots of ants live in it. The number of ants in a nest depends on many things, including the age of the colony, competition with surrounding nests, time of year and the abundance of food.

For example, an ant scientist dug up a 19-inch tall mound and found about 237,000 workers and 1,400 queens. He excavated a second mound that was 11 inches tall and counted just 41,000 workers and 40 queens. The researcher estimated there might be more than 12 million ants on his 10 acre study site. Another scientist estimated that there were only about 10,000 ants per colony where he was working.

The outside of an Allegheny mound ant nest looks simple: a dirt cone that may be covered with a thin layer of some cut-up plant stems and perhaps some plants growing on it. And of course, ants running all over it.

An ant uses its antenna to "milk" honey dew from an aphid.

At first glance, all of the activity may look chaotic, but it is actually ordered. Some ants on the outside of the nest maintain the mound by cutting vegetation or other chores. Other ants scour the vicinity for food, and others act as sentries to warn their nest mates of danger. All of these are important tasks.

Worker ants typically keep the outside of their cone-shaped nests free of living grasses, wildflowers and other plants. However, some nests may have steeper sides and be covered with vegetation. The amount of vegetation the ants allow to grow on a nest might depend on where the nest is; often, the ants tolerate fewer plants on mounds built in partial shade than those in full sunlight. In either case, the ants always keep the top of the mound bare. Usually a band of plants is allowed to grow right at the base of the mound, and that is where most of the entrances are located.

Inside this magnificent ant palace, however, is truly where the action is. The mound contains a network of tunnels and chambers used to house eggs, pupae and larvae that are the future of the colony. Ants inside the mound take care of the queens and offspring. They move the young to different levels in the mound to take advantage of the best conditions. These interior workers also clean up debris, build new tunnels and chambers and defend against intruders.

The way mounds are built helps control humidity and regulate temperature. Humidity is important because if an ant gets too dry, it dies. Temperature is also important because ants are commonly described as cold-blooded, which means their body temperatures are controlled by their environment and levels of activity.

Higher temperatures-but not too high-can help ants grow more quickly from egg to adult. A tall nest with sloping sides, like that built by the Allegheny mound builders, can catch morning light and boost the nest temperature more quickly than a flat nest would. Worker ants constantly shift their eggs and young to higher or lower levels in the mound as temperatures change.

Some ant species have only one queen that lays eggs. Others, including the Allegheny mound builders, have several to many queens producing eggs. The queens are busy during the warm seasons laying eggs that will become new workers, new queens and new males. The overwhelming majority of ants in a colony are females. Most are workers and some are queens.

Workers attend the queens as she lays eggs in her chamber.

Males do no work around the nest; their sole function is to fertilize new queens. Shortly after mating, the males die, and the new queens try to establish a colony. Queens mate only once during their lives. Sometimes a newly fertilized queen may enter and be adopted by a colony of another species of Formica, resulting in a nest with a couple of different species living in it.

If you see a mound of the Allegheny mound builder, you may not be looking at the entire ant colony. There is often a cluster of 10 or more mounds related to one another. This happens when part of the parent mound's queens and workers move to a new nearby location to set up shop. After a while, these new mounds may functionally become independent colonies.

Food is important to any animal, be it human or insect, and some ants are particular about what they eat. For example, another large mound builder called the harvester ant consumes mostly seeds and a few insects. Other ant species have a more generalized diet. Allegheny mound builders are not nearly as picky as harvester ants. Allegheny mound builders will scavenge dead insects and kill other insects, especially those that have soft bodies or poor defenses. Honeydew is another important source of food. Honeydew is a sugary byproduct that comes from the digestion process of aphids and some other insects. It forms drops at the end of the insect's abdomens, and the ants collect it like milk from a cow.

Ants are generally territorial animals, which makes sense if you think about the food supply. With a colony potentially consisting of hundreds of thousands of Allegheny mound ants, a lot of food needs to be brought to the nest. Reliable food sources, like honeydew from aphids, are valuable, and ants defend them from other ants, wasps and any other insects that might prey on their herd of aphids.

In general, when the ants detect an intruder, they will release alarm chemicals to recruit some of their nest mates. The defending ants then grab the invader with their mandibles-or jaws-and spray it with formic acid.

Even though Allegheny mound builder ants are fierce predators, they are by no means invulnerable. Ants working outside, either on the mound or wandering around looking for food, often become prey to other animals, such as birds and spiders. A few spider species even mimic ants and seem able to pick off ants without raising an alarm.

There are dangers inside the nest, too, where other insects live either part or all of their lives. Some of these interlopers scavenge dead ants or food the ants have brought in. Others steal ant eggs, larvae and pupae, or even kill adult ants. Sometimes the workers that are supposed to be taking care of the larvae eat the larvae instead!

Ants are fascinating creatures but challenging to watch because they are often small or live in well-hidden nests. That's just another reason why mound builders like the Allegheny mound builder are so great -their big, conical mounds swarming with thousands of red and black colored ants will be easy to spot the next time you're out on a prairie or open savanna.