THE METAMORPHOSES OF THE NERVOUS SYSTEM. 37 



adult beetle shows how the nerves unite laterally to supply the 

 one large compound eye on each side of the head. 



The nerves just noticed are given off Irora the brain, above, 

 the gullet. 



The lower ganglion of tlve head, or the sub-cesophageal, 

 is a mass of nerve tissue, whose arms, as it were, enclose the 

 gullet, and unite above with the brain proper. Its prolongation 

 in the opposite direction is continuous with the double cord, 

 and the ganglions which run down the inside of the insect, below 

 and beneath the digestive organs. In some larva; the sub- 

 oesophageal ganglion is double, but it soon becomes single as 

 their growth proceeds. The nerves which supply the masticatory 

 organs are derived from this ganglion, and there is a twig sent 

 off on each side to the jaws, mandibles, and to the lower lip. 

 The size of the twigs depends upon that of the parts of the 

 mouth, and it diminishes during any metamorphosis which is 

 accompanied by a diminution of the masticatory apparatus. 



There are three sets or pairs of ganglions, united into three 

 enlargements in the thorax in perfect insects. One is in the seg- 

 ment called the prothorax, another in the mesothorax, and the 

 third in the metathorax. These three nervous centres remain 

 perfectly distinct and separate in all larvae ; but as the meta- 

 morphosis proceeds, the cords which connect them are absorbed, 

 and usually the second and third ganglions unite and fuse toge- 

 ther. Very frequently this takes place with regard to the first 

 also, and one thoracic medullary centre is produced. 



By comparing the number and the posizion of the thoracic 

 ganglions of the silkworm when in the caterpillar state with 

 those of the moth, a very good idea can be formed of the 

 important modifications in the nervous system which occur in 

 consequence of the alterations in the general development of 

 the insect. 



The first or prothoracic ganglion is invariably united by longer 

 or shorter nervous links to the sub-oesophageal mass, and all the 

 medullary centres or ganglions are united to those in front and 

 behind in the same manner. In the silkworm caterpillar the 

 three thoracic nervous centres are small and wide apart, and each 

 one is in a segment of the body ; but in the moth condition 



