CHAPTER IV. 



THE INTERNAL STRUCTURE OF ANTS. (CONCLUDED.) 



" 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 becoming 

 preoral before the hatching of the larva. The ganglia of the mandi-V 

 bular, maxillary and labial segments also unite to form a single mass, 

 the subcesophageal ganglion, which, as its name implies, lies behind 

 the gullet. This ganglion is united to the brain by means of a pair of 

 circumcesophageal connectives. The pro- and mesothoracic ganglia 



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