THE METAMORPHOSES OF THE NERVOUS SYSTEM. 33 



do not form important parts of the insect's economy. Concen- 

 tration, growth, further development, absorption, and atrophy- 

 take place in the nervous system, then, as the insect progresses 

 from the embryonic condition within the egg to the perfect 

 state ; and these phenomena are most gradual when there is no 

 metamorphosis ; less so when it is incomplete ; and most marked 

 when it is very decided. 



When the apparatus which vivifies the muscles and all the 

 organs of insects is considered, its bulk and elaborate mechanism 

 impress us with the importance of these active and energetic 

 beings in the economy of nature. 



The nervous structures of insects constitute a series of en- 

 largements united by intervening cords, and they correspond to 

 the brain and spinal cord of the higher animals. The cords 

 are double, and extend from the tail end of the body, beneath 

 the stomach, up the middle line to the gullet ; they encircle this, 

 and re-unite above it in the head. The enlargements are placed 

 in definite order, one in front of the other, and are really double, 

 although apparently single to the superficial observer ; the largest 

 is in the head and over the gullet, and is called the brain ; whilst 

 the others are termed ganglions, or medullary centres. The 

 cords are made up of the nerve-fibres which conduct the nervous 

 force from the ganglions, where it originates, and the impressions 

 derived from without through the medium of the senses to these 

 medullary centres. The ganglions are formed principally of cells 

 intimately connected with the fibres, and they originate the nerves 

 of sensation and motion, and regulate the animal life of the insect. 

 The vegetative life — that which is beyond the will, and relates to 

 the digestive functions, for instance — is maintained through the 

 agency of an offshoot from the main nervous structures. The heads 

 of insects being formed of several segments, there is little doubt 

 that in the earliest stage of life the ganglionic cords were con- 

 tinued forwards, but the fusion of the rings occurs so soon, and 

 the coalescence of the separate ganglions is so rapid, that only 

 one pair of medullary enlargements is ever distinguishable. The 

 ganglions which form the brain are small in the larva in rela- 

 tion to the size of the inside of the head. During the condition 

 of nymph, pupa, or chrysalis, they increase in bulk ; and in the- 



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