94 NERVOUS SYSTEM OF INSECTS. 



612. The Nervous system of Insects shows the general 

 arrangement, and the chief part of the modifications, which we 

 have already described in treating of the Sub-kingdom, to which 

 these animals belong ( 595). It is principally composed of a 

 double series of ganglia, which are united together by longitu- 

 dinal cords ; the number of these ganglia corresponds with that 

 of the segments ; and they are sometimes at nearly equal dis- 

 tances, and extend from one end of the body to the other, whilst 

 in other instances several of them are connected together in such 

 a manner, as to constitute a single mass. The cephalic ganglia 

 present a very high development, and give origin 



to the nerves of the Antennae and of the Eyes, &c. 

 The first pair of ganglia below the oesophagus sup- 

 plies the nerves of the mouth ; and the cords which 

 unite these nervous centres to the cephalic ganglia, 

 and which pass round the oesophagus, give off on 

 each side a nerve which goes back to the stomach ; 

 and which joining itself with that of the opposite 

 side, forms a single trunk, running on the central 

 line above the digestive canal, and having two gan- 

 glia in its course. The three pair of ganglia on the 

 ventral cord, following those which are situated im- 

 mediately beneath the oesophagus, belong to the three 

 rings of the thorax, and are the points of departure of 

 the nerves of the feet and the wings ; in general they 

 are very close together, and are much larger than 

 the succeeding pairs, which belong to the abdomen. 



613. The manner in which Insects are nourished, 



7 rlG. 313. 



varies very much ; some only live on the juices of NERVOUS s\-s- 

 plants and animals, others feed on solid food, and 

 are either carnivorous or devourers of plants ; and these differ- 

 ences correspond with remarkable modifications in the structure 

 of the mouth. Amongst gnawing insects, such as Beetles, 

 Cockchafers, Cockroaches, and Locusts, this opening is furnished 

 in front with a central piece, named labrum, or upper lip (#, 

 Figs. 314 and 315) ; and has on each side a kind of large tooth, 

 moveable and very hard, called the mandible, or upper jaw (&), 



