222 



fEXT-BOOK OF ENTOMOLOGY 



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THE NERVOUS SYSTEM 

 a. The nervous system as a whole 



The nervous system of insects consists 

 of a double series or chain of ganglia con- 

 nected by nervous cords or commissures. 

 The first of these is the brain or supra- 

 oasophageal ganglion ; it is situated in the 

 upper part of the head, above the gullet 

 or oesophagus, while the rest of the sys- 

 tem, called the ventral cord, lies on the 

 floor of the body, under the digestive 

 canal. 



A ganglion or nerve-centre consists of 

 a mass of ganglion-cells, from each of 

 which a process or fibre passes off, uniting 

 with others to form a nerve ; by means of 

 these nerves the ganglia are connected 

 with other ganglia, and with the sensory 

 cells and muscle-fibres. The ganglia may 

 be simple, and arranged in pairs, corre- 

 sponding to each segment of the body, 

 or they may be compound, the result of 

 the fusion of several pairs of ganglia, 

 which in the early stages of the embryo 

 are separate. Thus the brain of insects 

 is a compound ganglion, or ganglionic 

 mass. 



The nerves are of two kinds: 1. Sen- 

 sory, which transmit sensations from the 

 peripheral sense-cells to the ganglion, or 

 brain; 2. Motor, which send stimuli from 

 the brain or any other ganglion to the 

 muscles. 



Of ganglion cells, some are tactile, and 

 others give rise to nerves of special sense, 

 being distributed to the eyes, or to the 

 organs of hearing, smell, taste, or touch. 



While the supracEsophageal ganglion, 

 or "brain," of the insect is much more 

 complex than any other ganglion, consist- 



