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The ants are one of the most succesful groups of insects, and are of interest because they form highly advanced colonies. They are grouped as the family Formicidae in the order Hymenoptera, and are especially close relatives of the Vespid wasps (yellojackets and such), from which they are physiologically distinguished mainly by having elbowed antennae and a bead-like pedicel formed from the first few abdominal segments. Also, most ants are wingless, although this varies between individuals in a colony rather than between species, as discussed below. The first known ants appeared sometime during the later Cretaceous.

Ant colonies are eusocial, and are very much like those found in other such Hymenopterans, though the various groups of these probably developed sociality independently through convergent evolution. Eggs are laid by one or sometimes more queens, and most of these grow up to become wingless, sterile females called workers. Periodically swarms of new queens and males are produced, usually winged, which leave to mate. The males die shortly thereafter, while the surviving queens either found new colonies or occasionally return to their old one.

Ants develop by complete metamorphosis, passing through larval and pupal stages before they become adults. The larval stage is particularly helpless - for instance it lacks legs entirely - because it does not need to care for itself. The difference between queens and workers, and between different castes of workers when they exist, is determined by feeding in the larval stage. Food is given to the larvae by a process called trophallaxis, where an ant regurgitates food held in a crop for communal storage. This is also how adults distribute food amongst themselves. Larvae and pupae need to be kept at fairly constant temperatures to ensure proper development, and so are often moved around various brood chambers within the colony.

A new worker spends the first few days of its adult life caring for the queen and young. After that it graduates to digging and other nest work, and then again to foraging and defense of the nest. These changes are fairly abrupt and define what are called temporal castes. In a few ants there are also physical castes - workers come in a spectrum of sizes, called minor, media, and major workers, the latter beginning foraging sooner. Often the larger ants will have disproportionately larger heads, and so stronger mandibles. In a few species the media workers have disappeared, so there is a sharp divide and clear physical difference between the minors and majors, sometimes called soldiers.

The most primitive ants are the army ants and driver ants, from South America and Africa respectively. These do not form permanent nests, but instead alternate between nomadic stages and stages where the workers form a temporary nest (bivouac) out of their own bodies. Most other ants form stationary colonies, usually dug into the ground or some other hollow. Colonies reproduce either through nuptial flights as described above, or by fission, where a group of workers simply dig a new hole and raise new queens. Colony members are distinguished by smell, and other intruders are usually attacked, with notable exceptions.

Ant communication is primarily through chemicals called pheromones, which because most ants spend their time in direct contact with the ground are more developed than in other Hymenopterans. So for instance when a forager finds food, on his way home (found typically through remembered landmarks and the position of the sun) he will leave a trail along the ground, which in short time other ants will follow. When they return home they will reinforce the trail, bringing other ants, until the food is exhausted, after which the trail is not reinforced and so slowly dissipates. A crushed ant will emit an alarm pheromone that in high concentration sends other ants nearby into an attack frenzy, and in lower concentration attracts them, while a few ants use what are called propaganda pheromones to confuse their enemies. And so forth.

Like other insects, ants smell with their antennae. These are fairly mobile, having as mentioned above a distinct elbow joint after an elongated first segment, and since they come in pairs provide information about direction as well as intensity. Pheromones are also exchanged as compounds mixed in with the food interchanged in trophallaxis, giving the ants information about one another's health and nutrition. Of special note, the queen produces a special pheromone without which the workers will begin raising new queens.

There is a great diversity among ants and their behaviors. Of special note:

• Many ants will raid each other's colonies, taking the coccoons, which once hatched act as members of the raider's colonies despite not being genetically related to the queen. A few species have become utterly dependent on such slaves, to the point of being otherwise unable to feed themselves.
• Some ants, called honeypot ants, have special workers called repletes who simply store food for the rest of the colony, generally becoming immobile with greatly enlarged abdomens.
• Leafcutter ants (Atta) feed exclusively on a special fungus that lives only within their colonies, and continually collect tiny pieces of leaves for it to grow on.
• Weaver ants (Oecophylla) build nest in trees by attaching leaves together, first pulling them together with bridges of workers and then sewing them together by pressing silk-producing larvae against them in alternation.

A number of insects have symbiotic relationships with ants. Most notable of these are aphids, which excrete a sweet liquid called honeydew that is normally dropped to the ground, but left in a bead for ants to collect when present. The ants in turn keep predators away and will move the aphids around to better feeding locations.