THE NERVOUS SYSTEM. 33 



sternal, episternal, and epimeral pieces, freely articulated 

 together, correspond in function with the sternum, the ribs, 

 and the clavicles of birds.* The thorax is expanded and con- 

 tracted at each motion of the 

 wings, as in birds and other ani- 

 mals, and becomes fixed at each 

 increased effort as a fulcrum or 

 point of resistance upon which 

 the great muscles of the wings 

 are to act, thus identifying this 

 part of the body in function as 

 in structure with that of other ani- 

 mals." (Newport.) 



THE NERVOUS SYSTEM. In its 

 simplest form the nervous system 

 consists of two longitudinal cords, 

 each with a swelling (nerve-knot, 

 or ganglion,) corresponding to 

 each segment (Fig. 43). This 

 cord lies on the ventral side of the 

 bod} r , but in the head it passes 

 upwards, sending a filament from 

 each side to surround the oesoph- 

 agus, f As in the Vertebrates, 

 the nervous cord of insects is 

 composed of two distinct columns Fi - 43 - 



of fibres placed one upon the other. "The under or external 

 column, which is nearest to the exterior of the body, is that in 

 which the ganglia, or enlargements, are situated. The upper 

 one, or that which is internal and nearest to the viscera, is 

 entirely without ganglia, and passes directly over the ganglia 

 of the under column without forming part of them, but in very 



* Bennet on the Anatomy of the Thorax in Insects, and its Function during 

 Flight. Zoological Journal, vol. i, p. :',!U. 



tThe brain of insects is formed of several pairs of ganglia, corresponding, 

 probably, to the number of primitive segments composing the head. The nervous 

 conl is thus, in the head, massed together and compacted to form a brain. 



\ '!<;. 43. Nervous System of Corydaltts cornutus. a, "cerebrum ;" b, "cere- 

 brellum;" c, thoracic ganglia, which distribute a nerve to each leg; d, eight pairs 

 of abdominal ganglia. The dotted lines represent the wings. From Leidy. 



