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 from the brain, above 
the gullet. 
The lower ganglion of the head, or the sub-oesophageal, 
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 larvae 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 position 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 
