Ant colonies > Internal structure of ants > Nervous system

Nervous system

"It is certain that there may be extraordinary activity with an extremely small absolute mass of nervous matter; thus the wonderfully diversified instincts, mental powers, and affections of ants are notorious, yet their cerebral ganglia are not so large as the quarter of a small pin's head. Under this point of view, the brain of an ant is one of the most.marvellous atoms of matter in the world, perhaps more so than the brain of man;" -Charles Darwin, " The Descent of Man."

The Nervous System

The structure of the central nervous system is best considered in connection with the primitive segmentation as this is revealed in the embryonic ant. As stated in a previous chapter, the body of the ant, like that of all other true insects (Pterygogenea), consists of a series of twenty metameres, or segments. The first and last of these are peculiar in certain respects and have been called the acron and telson respectively. In the embryo the ectoderm of the mid-ventral portion of each segment (except the telson ) thickens and gives rise to a pair of ganglia that soon split off from a thin surface layer of cells which then become the ventral integument.

The ganglia of each segment are closely approximated and connected with each other by a pair of commissures, while the ganglia of successive segments are united by pairs of connectives which therefore run longitudinally. Later these connectives lengthen, and as the body grows more rapidly than the ganglia, we find the latter forming a chain extending through the ventral region of the head, thorax and abdomen. Not only do many of the ganglia thus become rather widely separated from one another, but there is also a tendency for some of them to fuse together and make larger masses. Thus the ganglia of the first (acron), second (antennary) and third (intercalary) segments, known respectively as the proto-, deuto- and tritocerebrum of Viallanes, fuse to form the brain, or supraoesophageal ganglion. As the latter term indicates, this mass is dorsal to the oesophagus, and therefore preoral. This is true, however, only of the protocerebrum of the embryo, the two other pairs of ganglia being postoral at first, but moving forward and becoming­preoral before the hatching of the larva. The ganglia of the mandi­bular, maxillary and labial segments also unite to form a single mass, the suboesophageal ganglion, which, as its name implies, lies behind the gullet. This ganglion is united to the brain by means of a pair of circumoesophabeal connectives, The pro- and mesothoracic ganglia remain distinct and lie in their respective segments even in the adult ant. The first ( mediary ) and second abdominal ganglia, however, are drawi. up into the metathorax and fused with the metathoracic ganglion, and the ganglion of the third abdominal segment comes to lie in the petiole (second abdominal segment) (Fig. 13, ag3 ) . The fourth, fifth, sixth and seventh abdominal ganglia retain their independence, but the latter two are close together and are immediately succeeded by the fused eighth to tenth, which constitute a single ellipsoidal mass, terminating the chain and in the adult ant lying some distance in front of the pos­terior end of the gaster (Fig. 13, ag8-11). The central nervous system of the adult ant therefore presents only eleven ganglionic masses, formed by condensation of the primitive nineteen.

For convenience in descrip­tion, this system may be divided into the brain and ventral cord, and these, with their ganglia and peripheral nerves, may be briefly consid­ered before we take up the sympathetic nervous system and the sense organs.