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 
