Physical Characteristics Common to All Ants
Most
ants are between 2 and 10 mm (0.8 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 (alitrunk), and the
abdomen (gaster).
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.
Alitrunk
The 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.
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.
Physiology
Ants have a rigid, external skeleton called an exoskeleton that gives the soft, inner body its form. The brains of ants process information from various sense organs via a nerve cord that runs along the lower side of the body. They also possess a tube-like heart, which contains muscles that contract to force ants’ colorless blood through their bodies. Instead of lungs, ants have a set of branching, air-filled tubes called tracheae that provide body tissues with oxygen.
Adult ants digest only liquid foods. Some ants obtain nutrients from solid food, such as seeds, but they must turn this food into a liquid before swallowing it. These ants mix digestive juices into the food to help dissolve it. They then use their tongues to lap up the resulting juices and semiliquid bits of food. Once inside the mouth, partially solid food enters a chamber beneath the mouth opening, where a filter prevents solid particles from entering the digestive tract. These particles are pressed into a solid pellet that the ant removes from its mouth.
Food passes from the mouth into an organ called the crop, an expandable sac in which liquids can be stored for long periods without being digested. In many species, ants share the food stored in the crop with other ants, reflecting their social way of life. A valve at the inner end of the crop allows a trickle of food to pass into the ant’s midgut, where it is digested.
Reproduction
The reproductive organs of ants are located inside the gaster. Males have a pair of sperm-producing organs called testes, which are connected by ducts to an organ called the aedeagus. During mating, the male inserts his aedeagus into an opening in the queen’s body, called the vulva, to deposit sperm. Queen ants have a pair of egg-producing organs called ovaries, a genital chamber through which an egg passes after leaving the ovary, a sperm-storing structure called the spermatheca, and an egg-laying organ known as the ovipositor.
An egg travels from the queen’s ovary to the genital chamber on its way to being laid by the ovipositor. During this journey, the egg passes near a duct that leads out of the sperm-storing spermatheca. The duct is surrounded by a muscle that, when contracted, opens the duct. If the duct is open, one or more sperm may leave the spermatheca and fertilize the egg. Fertilized eggs give rise to either sterile female workers with no wings or fertile queens—females that are larger than workers and have wings. If the duct is closed, the egg is released without being fertilized. Unfertilized eggs develop into males, which always have wings and are equipped with reproductive organs.