Ants. M. V. Brian
Much of the head is occupied by muscles which work the jaws and the food pump (pharynx). The biggest muscles are those that close the mandibles and they are obviously capable of exercising a strong and tight grip; the muscles are fixed on the back of the head and run to the inner edge of each jaw (fig. 3). Seated on the ventral part of the head and running to the outer part of each jaw are the muscles which open them; they are much smaller. Other small muscles retract the tongue after it has been extended by blood pressure and there are some inside which manipulate it. The pharynx is a flattened part of the gut in the front of the head which can be enlarged and diminished by means of six muscles attached to it in various ways, some from above, some from the sides and some transversely (fig. 2). It pumps liquid food up after the silt and solid particles have been taken out and passes it down the oesophagus. This latter is a very thin tube, more or less circular in cross section that runs right through the mesosoma and petiole to the gaster. The brain is a ganglion of nerve cells and fibres behind the pharynx and over the oesophagus. It collects sensory data from outside and inside the body, works out correlations and associations and stores relevant information. This enables the insect to learn and react to quite complicated patterns of stimuli. The brain also has closely associated with it a number of ductless glands that control most internal chemical processes. Underneath the oesophagus is another ganglion, connected to the brain by thick nerve trunks, which is largely responsible for controlling motor activity; it co-ordinates the movements of antennae and mouth parts. In its turn it sends nerve trunks into the mesosoma.
FIG. 1. Worker of Myrmica rubra, sagittal section showing main organs and divisions of the body (after Janet).
A gland opens on each mouth part (figs. 1, 2, 3). The mandibles have glands that lie against the outer wall of the head, one on each side between the eye and the mandible base. A single layer of cells produces a secretion which is stored in a thin-walled reservoir and conducted by tube to the base of the mandible where it opens through a pore. These glands have several functions: they are partly digestive, partly they lubricate the joints of the mouth and they also contain volatile substances that alert other ants. The maxillary glands have no reservoir and open directly at the base of the maxillae; they probably produce a digestive fluid. The labial glands are not in the head at all; they occur in the front of the mesosoma and are connected to the tip of the tongue by a long, thin tube. They supply a watery lubricant which contains some digestive juices. The most striking gland in the head, next to the mandibular, is the pharyngeal gland; this consists of about twenty finger-shaped tubes, often containing a yellow oil. It is thought that this substance is separated from the food as it travels through the pharynx; it may be digested and used as an energy supply. The watery part of the food goes down the oesophagus to the crop which is in the gaster. From here it can, nevertheless, be regurgitated to larvae and queens and as it contains most of the proteins it is of great value to them, either for body growth or egg production, as the case may be.
FIG. 2. Worker of Myrmica rubra, head sagittal section showing main organs (after Janet).
In the narrow constriction between the head and the mesosoma there are six channels: the ventral nerve cord, the duct of the labial gland, the oesophagus, two air ducts and the tubular heart. In the mesosoma the ventral nerve cord forms into three ganglia (the last a compound one) that are largely concerned with co-ordinating leg and wing movements. It contains all the muscles operating the legs and, in the sexuals, the wings. In fertile queens these wing muscles degenerate and the material from them is used to make eggs which not only give rise to a brood of larvae but are also used to feed them. Later on, when the colony is well established, the space occupied by the muscles becomes filled with storage tissue called fat-body from the opalescent oil droplets that are the principal component. Muscular action uses a lot of energy and the mesosomal organ must be well-supplied with oxygen from the three pairs of air tubes. There is also a pair of large air sacs which may be ventilated during muscular movement in some way.
FIG. 3. Worker of Myrmica rubra, head section from a. to b. of fig. 2 (after Janet).
Water-dissolved substances are carried through the long, thin oesophagus to be stored in a dilatation called the crop which lies dorsally in the front of the gaster. This can be distended to a huge size for it is used to carry nectar and honeydew back to the nest. There is a valve between the crop and the rest of the gut which prevents a good deal of loss from the crop but not all. In some ants this is held closed by muscular sphincters; in others it closes passively when the fluid pressure in the crop rises. It is thought that this second method of control is more efficient since it uses no energy. The crop contents are mostly very liquid and can be regurgitated to other adults and to larvae. This is probably done by telescoping the gastral segments inwards but perhaps by reversing the pharyngeal pump mechanism.
Once fluid has passed back into the midgut it can no longer be shared by other ants. The thick walls of the midgut (fig. 4) secrete digestive enzymes and absorb the small molecules into which the food is broken. Waste material passes back through another tube into a thin-walled hindgut which stores indigestible residues and probably extracts some water and water-soluble substances. Finally, the liquid waste is ejected in a special part of the nest some way away from the brood rearing zone, along with the refuse. Foragers probably eject their own residues outside the nest. It includes waste substances extracted from the body fluids by long, thin, hollow, thread-like tubules; they join the gut just after the midgut. Much of this excretion is synthesized into uric acid which appears as white granules. In workers the main storage tissue is in the gaster. Oils, glycogen and proteins are all stored there though most of it is oil.
FIG. 4. Queen of Myrmica rubra, gaster sagittal section showing main organs (after Janet).
A very important part of the gaster is occupied by the genital system (fig. 4). This consists of thin-walled egg tubes (many more in queens than in workers) that start just below the heart at the front of the gaster and pass backwards and downwards to the genital opening below the anus and the sting. Eggs start as small cells at the tip and move down, growing and maturing as they go. Part of the material for growth comes from nurse cells which are broken down and incorporated into the egg tissue but some is extracted from the blood by an enveloping layer of follicle cells. When the egg has been fully formed it is enclosed in a special skin (the chorion) and the egg passes into the oviduct to be fertilized. The oviduct is a simple tube with a thicker wall than the ovary. In queens it has two pouches on its dorsal side, one near the outside which received sperm during copulation and the other a bit farther in that stores the sperm alive for a decade or more. How it gets from one to the other is unknown. These pouches are not present in workers unless the ant is a very primitive species.
It remains only to mention two glands that open into the sting (fig. 4). One is the poison gland which consists of two closed tubes feeding a thin-walled storage vesicle and which contains a watery solution of mixed venoms. The other is a smaller gland called after Dufour which contains an oily material secreted by cells which surround the storage space. The poison glands vary quite considerably in different families of ant, depending upon whether they synthesize a thin liquid for squirting or injecting with a sting, or