NERVOUS SYSTEM I I 7 



body for the purpose of stimulating and co-ordinating the 

 contractions of the muscles. The number of the ganglia 

 in the ventral chain differs greatly in different Insects, and 

 even in the different stages of metamorphosis of the same 

 species, but never exceeds thirteen. As this number is that 

 of the segments of the body, it has been considered that each 

 segment had primitively a single ganglion. Thirteen ganglia for 

 the ventral chain can, however, be only demonstrated in the 

 embryonic state ; in the later stages of life eleven appears to be 

 the largest number that can be distinguished, and so many as 

 this are found but rarely, and then chiefly in the larval stage. 

 The diminution in number takes place by the amalgamation or 

 coalescence of some of the ganglia, and hence those Insects in 

 which the ganglia are few are said to have a highly concentrated 

 nervous system. The modes in which these ganglia combine are 

 very various ; the most usual is perhaps that of the combination 

 of the three terminal ganglia into one body. As a rule it may be 

 said that concentration is the concomitant of a more forward posi- 

 tion of the ganglia. As a result of this it is found that in some 

 cases, as in Lamellicorn beetles, there are no ganglia situate in 

 the abdomen. In the perfect state of the higher Diptera, the 

 thoracic and abdominal ganglia are so completely concentrated 

 in the thorax as to form a sort of thoracic brain. In Fig. 64 

 we represent a very diffuse and a very concentrated ganglionic 

 chain ; A being that of the larva of Chironomus, B that of the 

 imago of Hippohosca. In both these sketches the cephalic ganglia 

 as well as those of the ventral chain are shown. 



Turning next to the cephalic masses, we find these in the 

 perfect Insect to be nearly always two in number : a very large 

 and complex one placed above the oesophagus, and therefore 

 called the supra-oesophageal ganglion ; and a smaller one, the 

 sub- or infra-oesophageal, placed below the oesophagus. The 

 latter ganglion is in many Insects so closely approximated to 

 the supra-oesophageal ganglion that it appears to be a part 

 thereof, and is sometimes spoken of as the lower brain. In 

 other Insects these two cranolia are more remote, and the infra- 

 oesophageal one then appears part of the ventral chain. In 

 the embryo it is said that the mode of development of the 

 supra-oesophageal ganglion lends support to the idea that it 

 may be the equivalent of three ganglia ; there being at one 



