Chapter II: The Ant’s Body

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.