The body of the ant, like those of other insects, is segmented, and covered with a hard chitinous external skeleton. It is separated by constrictions into three distinct parts, the head, which bears the eyes and mouth-parts; the thorax, to which the wings and legs are attached; and the abdomen, which contains most of the entrails and the sexual apparatus.

The Head, Eyes, and Mouth-parts. The head varies greatly in shape and size, but always bears a frontal plate or clypeus, just above which the two jointed antennae or feelers are attached. The antennae contain a great number of minute structures which are supposed to be connected with the sense of smell. Three small simple eyes or ocelli are set in the top of the head, and two large compound eyes are located one on either side. The eyes are always very well developed in the males, and somewhat less so in the females; the eyes of the workers are relatively small, and the ocelli are sometimes lacking altogether. The compound eyes are the principal organs of vision, while the ocelli are supposed to register only very near objects.

Just below the clypeus are the mouth-parts, consisting of the labrum or upper lip, a pair of powerful mandibles, another pair of jaws called maxillae, and the labium or lower lip. Both maxillae and labium bear little palpi or feelers, and are plentifully supplied with taste-buds containing the gustatory cells. The tongue or glossa with which the ant laps up its food is attached to the upper part of the labium.

The Thorax, Legs and Wings. The ant’s thorax consists of four segments. The first segment is known as the prothorax; it is quite small, and bears the first pair of legs. The next segment, the mesothorax, carries the second pair of legs and the front wings—when wings are present. The third segment or metathorax bears the third pair of legs and the hind wings—if there are any wings. The fourth segment is really a part of the abdomen, and is known as the epinotum. On each side of the thorax are two breathing-holes or stigmata, which communicate directly with the tracheae or windpipes which supply air to the interior tissues.

The ant has six legs, one pair attached to each of the three segments of the thorax proper. Each leg consists of five parts, the coxa, the trochanter, the femur, the tibia, and the tarsus or foot. The wings are four in number, and the venation is similar to that found in other members of the order Hymenoptera, but the wings are not much used in classification because the workers are always wingless, and the females wear wings only for a part of their lives.

The Abdomen and Its Appendages. The ant’s abdomen is divided into two parts, the slender pedicel which articulates with the last segment of the thorax, and the larger part of the abdomen called the gaster. The pedicel is provided with a file-like structure, which by rubbing against a non-striated segment produces a sound of very high pitch. In some species the females and workers bear stings and poison glands in the last segment of the gaster. The female has no ovipositor. In the male the tip of the gaster usually bears three pairs of sexual appendages; the two outer pairs are used in clasping the female during copulation, and the inner pair, when held tightly together, form a tube which functions as a penis.

The Alimentary Canal. The mouth is located between the maxillae, and is provided with a little pouch called the infrabuccal cavity, which is used to hold solid matter while the liquid nutriment is being sucked out of it. When this has been accomplished the pellet is thrown out. The liquid food passes back into the pharynx, and then on through a slender tube called the esophagus, which is lined with fine hairs. In the gaster the esophagus expands into the crop, which acts as a reservoir; no food is absorbed through its walls, but is often regurgitated to feed the young. Just back of the crop is the proventriculus or gizzard, the movements of which provide the suction by which liquid is drawn up the esophagus and into the crop, and the force by which food is regurgitated. The true stomach is rather small, and it is here that the food is both digested and absorbed. The small intestine communicates with the stomach by a valve, and is connected with a number of Malpighian tubes which act as kidneys, absorbing liquid waste from the blood and pouring it into the intestine. The large intestine or rectum receives the feces and urine from the small intestine and expels them from the body by way of the anal opening.

The Circulatory System. The blood of the ant, like that of other insects, is colorless, and contains several kinds of corpuscles.

Its function is to carry food from the stomach where it is absorbed to other parts of the body where it is needed. The blood of insects has no red corpuscles, and does not carry oxygen about. The blood is not confined in definite veins and arteries as in the higher animals, but percolates about through the entire body cavity. There is a simple heart in the dorsal part of the abdomen which pulsates and forces blood forward through an aorta into the head, from which it seeps gradually back into the abdomen, to be pumped forward through the aorta again. Thus a sluggish circulation is maintained.

Respiration. Ants have neither lungs nor gills, and the blood does not carry oxygen into the cells and carbon dioxide out as in the higher animals. As in most other insects, air is taken into the body through breathing-holes or stigmata, and brought into direct contact with the tissues. There are ten pairs of these stigmata in the ant—two pairs in the thorax and eight in the abdominal segments. Each opens through a sort of valve into a trachea or wind-pipe, which branches until its ramifications extend to all parts of the body. When certain muscles contract the size of the body increases, and air is drawn in through the stigmata; when the size of the body is decreased the air is forced out. The incoming air brings in the necessary oxygen, and the outgoing current is laden with carbon dioxide waste from the tissues.

The Nervous System. The brain proper is a mass of nerve matter in the head just above the esophagus, but the subesophageal ganglion is very close to it, and the two are connected by heavy fibers on each side of the esophagus, so that the whole thing has the appearance of a brain with the gullet running through the middle of it. The major part of the upper brain is connected with the compound eyes, but there are nerves also which supply the ocelli, the antennae, the pharynx, the labrum, and muscles in the head. The subesophageal ganglion gives off nerves to the mandibles, maxillae and labium. From the lower back part of the subesophageal ganglion the ventral nerve cord arises, and runs through the thorax and far back into the abdomen. This cord bears three large thoracic ganglia which innervate the muscles of the wings and legs. In the abdomen are eleven smaller abdominal ganglia, with nerves running out to supply all of the abdominal organs. The so-called sympathetic system consists of a few very small ganglia and nerves not directly connected with the ventral nerve cord, which function in connection with the digestive organs.

The Reproductive Organs. The ovaries of the female or queen ant are located in the upper and front part of the gaster, and each is connected by a slender oviduct with the uterus. The uterus is continuous with the vagina, the external opening of which is located near the tip of the abdomen. At the top of the uterus is a small pouch called the seminal receptacle, which receives the sperm from the male in copulation. The spermatozoa live in this pouch for several years, and meet and fertilize the eggs as they descend into the uterus from the ovaries.

The organs of the worker are similar to those of the queen, except that they are very much smaller, and are usually incapable of functioning normally. Worker ants have never been seen to copulate. The testes of the male ant are located in the front part of the gaster, and are connected by the vas deferens with the seminal vesicles. Tubes from the vesicles unite to form the ejaculatory duct, which is connected with the penis at the tip of the abdomen.