II.

Body Features

Most ants are between 2 and 10 mm (0.08 and 0.4 in) long. Some ants, however, are a mere 0.7 mm (0.03 in) in length, and others are nearly 3 cm (1.2 in) long. Like other insects, ants have bodies that contain three major segments: the head, the thorax, and the abdomen. Unlike other insects, however, the first segment of the ant’s abdomen is fused to the back of the thorax. For this reason, scientists use other terms to designate the second two segments of an ant’s body—alitrunk corresponds to the thorax, and gaster corresponds to the abdomen.

A.

The Head

The shape of the ant head varies among different species; it may be spherical, triangular, egg-shaped, or rectangular. In all ants, at the back of the head is an opening through which nerves, the beginning part of the digestive tract, and blood pass into the alitrunk. At the front of the head is the mouth, which is associated with three appendages, or mouthparts. The mandibles, sometimes described as jaws, are long and broad, and they are serrated, or toothed, along their inner sides. Used for digging, carrying, collecting food, building nests, fighting, and cutting, the mandibles are probably the most important work tool that ants possess. The other mouth appendages are the maxillae, or lower jaws, used for chewing foods to extract liquids, and a tongue for sucking up liquid food. The mouthparts also have two pairs of slender palpi, segmented structures resembling small antennae, which play a role in tasting food.

Most ants have two compound eyes, which are made up of light-sensitive compartments called ommatidia. These compartments work together to generate an image in the ant’s brain. Some types of ants have three simple eyes, called ocelli, at the top of their heads. Ocelli can detect light, but they do not form images. Different species of ants vary in their ability to see: Some have well-developed sight, but others are entirely blind. Sight is of little importance to those ants that spend all or much of their lives underground.

Attached to the front of the head is a pair of flexible, segmented appendages called antennae, which contain organs of taste, smell, and touch. Each antenna is shaped like a human arm that is bent at the elbow. This antennae shape is an identifying feature of ants. Antennae are an ant’s main source of information about the world. When an ant is active, its antennae are in nearly constant motion—tapping the ground or vegetation, other ants, and food sources, or sampling odors from the air.

B.

Alitrunk and Petiole

An ant’s head connects to the alitrunk. This middle part of the body bears three pairs of jointed legs, each of which ends in a claw. The legs are used not only for walking or running but also for more dexterous tasks, such as handling food. Each of an ant’s front legs contains two combs that the ant uses to clean its other legs and its antennae.

In males and young queens, the alitrunk contains two pairs of wings: a larger front pair and a smaller rear pair. During flight, the hind pair of wings is hooked to the rear edge of the front pair so that the two pairs function as a single unit.

Behind the alitrunk is the narrow petiole, a one- or two-segmented section that forms what appears like a waist. This slender body part helps ants to bend while passing through twisting underground tunnels. In all ants, the alitrunk contains a structure called the metapleural gland, which secretes an antiseptic chemical that destroys bacteria and fungi. Ants rely on this chemical to keep their moist underground nests free from microorganisms that might destroy eggs, larvae, pupae, or stored food supplies.

C.

The Gaster

The hindmost body section of the ant, called the gaster, contains the heart, most of the digestive system, the excretory system, and the reproductive system. The segments of the gaster form a series of rings of different sizes, with the largest rings in the middle. When an ant’s digestive system is filled with food, the gaster expands by spreading out, or telescoping, these rings. At other times, the smaller rings of the gaster fit compactly inside of the larger ones.

The workers of many ant species carry a stinger within the hind end of the gaster. These ants use the stinger to defend against their enemies. In some species, worker ants lack a stinger but use the tip of their gaster to squirt or dab poison at other small animals and when fighting battles with other ants, fending off predators, or killing insects or other animals that they use as food.